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Review—The Design, Performance and Continuing Development of Electrochemical Reactors for Clean Electrosynthesis

Review—The Design, Performance and Continuing Development of Electrochemical Reactors for Clean... Journal of The Electrochemical Society, 2020 167 155525 Review—The Design, Performance and Continuing Development of Electrochemical Reactors for Clean Electrosynthesis Samuel C. Perry, Carlos Ponce de León, and Frank C. Walsh Electrochemical Engineering Laboratory, Department of Mechanical Engineering, Faculty of Engineering and Physical Sciences University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom A critical review of classical and improved electrodes, electrocatalysts and reactors is provided. The principles governing the selection of electrochemical flow reactor or progression of a particular design for laboratory or pilot scale are reviewed integrating the principles of electrochemistry and electrochemical engineering with practical aspects. The required performance, ease of assembly, maintenance schedule and scale-up plans must be incorporated. Reactor designs can be enhanced by decorating their surfaces with nanostructured electrocatalysts. The simple parallel plate geometry design, often in modular, filter-press format, occupies a prominent position, both in the laboratory and in industry and may incorporates porous, 3D or structured electrode surfaces and bipolar electrical connections considering the reaction environment, especially potential- and current-distributions, uniformity of flow, mass transport rates, electrode activity, side reactions and current leakage. Specialised electrode geometries include capillary gap and thin film cells, rotating cylinder electrodes, 3-D porous electrodes, fluidised bed electrodes and bipolar trickle tower reactors. Applications span inorganic, organic electrosynthesis and environmental remediation. Recent developments in cell design: 3D printing, nanostructured, templating 3D porous electrodes, microchannel flow, combinatorial electrocatalyst studies, bioelectrodes and computational modelling. Figures of merit describing electrochemical reactor performance and their use are illustrated. Future research and development needs are suggested. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/ 1945-7111/abc58e] Manuscript submitted August 4, 2020; revised manuscript received October 8, 2020. Published November 6, 2020. This paper is part of the JES Focus Issue on Organic and Inorganic Molecular Electrochemistry. List of symbols (Continued). Symbol Meaning Units Symbol Meaning Units t Time s T Temperature K A Geometrical electrode area m −1 −1 v Mean linear flow velocity of electrolyte m s A Electrode area per unit electrode volume m V Volume of reactor m B Breadth of rectangular flow channel m −3 V Overall volume of electrode m c Reactant concentration mol m E −3 V Volume of electrolyte in the reactor m c Reactant concentration in the bulk electro- mol m R V Volume of electrolyte in the tank m lyte T w Catalyst mass loading g d Equivalent (hydraulic) diameter of a rectan- m x Distance along electrode m gular flow channel 2 −1 X Fractional conversion of reactant Dimensionless D Diffusion coefficient of an aqueous species m s A z Electron stoichiometry Dimensionless E Electrode potential vs a reference electrode V U Cell potential difference V cell Greek E Equilibrium cell potential difference V e,cell E Standard electrode potential V −1 Symbol Meaning Units F Faraday constant C mol −1 G Molar Gibbs free energy J mol α Charge transfer coefficient Dimensionless I Current A ε Surface roughness of electrode m I Limiting current due to convective-diffusion A γ Limiting current enhancement factor com- Dimensionless −2 j Current density A m pared to a smooth surface −2 j Exchange current density A m η Overpotential (η = E−E)V −1 e k First order apparent rate constant s 2 −1 ν Kinematic viscosity of the electrolyte m s −1 k Mass transfer coefficient m s ω Velocity exponent Dimensionless L Length of rectangular flow channel in the m φ Current efficiency Dimensionless direction of flow ρ Electrical resistivity ohm m −1 M Molar mass g mol −3 −1 ρ Space time yield mol m s ST n Amount of a species mol τ Mean residence time in the tank (τ = V /Q)s T T T q Electrical charge C 3 −1 Q Volumetric flow rate of electrolyte m s Subscripts R Electrical resistance ohm a Anode −1 s Space velocity s act Activation (under charge transfer control) S Separation between the electrode and mem- m c Cathode brane (divided reactor) or between the conc Concentration (under mass transfer con- electrodes (undivided reactor) trol) cell Cell e At equilibrium E Electrode E-mail: f.c.walsh@soton.ac.uk Journal of The Electrochemical Society, 2020 167 155525 N Normalised simplify engineering design and result in lower capital and R Reactor running costs. T Tank (reservoir) 2. Convenience and reliability, adequate design, installation, (0) At time zero operation and maintenance and monitoring procedures. (t) At time t 3. Appropriate facilities to control and monitor concentration, Dimensionless groups potential, current density and adequate mass transport regime to Le Dimensionless length (Le = ε/d ) provide and remove reactants and products respectively, via Re Reynolds number (Re = vd /ν) suitable flow distributions. Sc Schmidt number (Sc = ν/D) 4. Simplicity and versatility are perhaps the least quantified and Sh Sherwood number (Sh = k d /D) most overlooked, yet perhaps the most important, factors for m e achieving an elegant and long-lasting design to attract users. Electrosynthesis has a proud history, which ranges from the 5. Provision for future developments by designing a modular routine, tonnage scale production of chlor-alkali chemicals and configuration that facilitates scale-up by adding unit cells or electrowinning of metals to the small scale realisation of speciality by increasing the size of each unit. products, such as pharmaceutical and fine chemicals, metal alloys, 1–4 composites, semiconductors and superconductors. Proponents Industrial applications of electrochemical reactors include the would highlight the convenience of electrochemistry, its ease of production and conversion of chemicals e.g. the chlor-alkali 7 1 8 control, the environmentally clean nature of the electron as a reagent process, aluminium metal production and adiponitrile among and the ability to produce powerful species, in situ, under near other important applications, such as energy conversion and ambient conditions. Antagonists might balance these features against storage. The reactors also offer the opportunity to use electro- limitations, including the speciality nature of the discipline and their chemical techniques for the investigation of electrode processes lack of training and support in practical electrochemistry and involving mass transfer, charge transfer and influence of cell electrochemical engineering together with the shortage of literature hydrodynamics which is typically carried out in small electroche- on successful case studies of modern electrosynthesis at an industrial mical cells at laboratory scale. scale. The design, operation and scale-up of electrochemical cells In electrosynthesis, the electrolyte is often single phase, the remains a critical challenge to the continued development and reactant(s) and/or products being sufficiently soluble to avoid undue diversification of electrosynthesis. mass transport restrictions. In order to minimise ohmic drop in the The design of electrochemical cells has often been focussed on electrolyte, a small interelectrode gap may be used and gas hold-up glass cells for laboratory benchtop use, the services of a skilled should be avoided by using a sufficient flow velocity. It is also glassblower being essential in sealing electrodes in glass and making common to add a concentrated indifferent electrolyte, the ions microporous gas bubbling/cell dividing frits or flanges to accom- carrying most of the migration current between the electrodes. The modate ion exchange membranes. The motivation for glass beaker counter electrode reaction must also be considered as gas evolution cell use is clear, as they are easy to assemble and clean, and sample or corrosion may unbalance concentration or pH and introduce exchange for manual batch analysis is facile. However, reaction evolved gas as a second phase. conditions are often poorly reproducible, especially when attempting In a two-phase liquid-liquid electrolyte, a phase transfer reagent to accelerate reactions rates through magnetic stirrer bars or gas may be needed to facilitate reactant or product transfer between the bubbling. phases. In an organic-aqueous electrolyte, quaternary ammonium This has led to a large proportion of electrochemical synthesis salts or surfactants have often been used. In special cases, the works moving away from beaker cells and into more specialist electrolyte may be a solid-liquid emulsion or suspension. Composite reactors, with flow fields, turbulence promotors and fixed electrode electrodeposition of materials utilises an agitated slurry of solid geometry and position greatly enhancing reproducibility and turn- particles or a sol to deposit materials by a combination of over rates. The danger here is that all electrosynthesis studies are electrophoresis, convective-diffusion and electrodeposition. carried out in the same reactor; the different reaction mechanisms require vastly different reaction environments in order to optimise Cell voltage and its components.—When current flows through reaction rate, charge efficiency and product conversion. Often these an electrolytic cell, the cell voltage (U) can be expressed as: three parameters cannot all be maximised, and so reactor design must focus on reaching a compromise reaction profile. UU=+ ∣∣h + ∣IR∣ [1] e åå This review provides an overview of the key reactor designs employed for electrochemical synthesis, covering initial bench-top The first term on the right hand side is set by selecting the proof of concept scale up to larger industrial standard reactors. The electrode reactions and their thermodynamics, U being directly merits of various reactor designs are discussed in terms of practical related to the Gibbs free energy change (ΔG ) for the cell reaction: cell applications, along with methods of improving their output via the complementary design of 3D electrode structures, electrolyte flow D=GzFU=-zFE [2] cell e cell profiles and mass transport distribution. where F is Faraday’s constant and z is the electron stoichiometry. E is the equilibrium cell potential difference, defined as the cell Principles of Electrochemical Reactor Design a c difference between the anode and cathode potentials (E —E ). The e e second term in Eq. 1 is governed by the electrode kinetics and mass General considerations.—Electrochemical reactors can be in- transport to and from the electrodes, as determined by the electrode herently complex and contain several controls and sensors for overpotential (η). The third term represents ohmic losses, related to specific electrosynthesis processes or can also be a simpler generic the current passed (I) and electrical resistance (R). Expanding the design suitable for a screening process at small scale. The final expression gives: design should comply with a number of essential characteristics that include: UU=+∣∣hh+∣ ∣+∣h ∣+∣h ∣+ ∣IR∣ e å c,, act a act c,conc a,conc 1. Moderate capital and running costs, require low cost compo- [] 3 nents, a low cell potential difference and a low pressure drop over the entire cell including the inlet and outlet flow manifolds The modulus of values is taken to allow for negative cathode for the electrolyte; where possible, an undivided reactor will current values indicating a reduction. By convention, positive Journal of The Electrochemical Society, 2020 167 155525 overpotentials give rise to oxidation currents and negative over- Electrosynthesis cells should always be provided with facilities for potentials give rise to reduction currents. For (non-spontaneous) incorporating a robust reference probe. electrolytic processes, Eqs. 1 and 3 predict that the cell potential For an electrosynthesis cell, minimising all potential losses in the difference, U is positive and becomes larger with increasing current reactor minimises the cell voltage required. density. The overall rate of an electrode process can be described by η and η are the charge transfer overpotentials at the Faraday’s laws of electrolysis, which may be written as a space-time c,act a,act cathode and anode, respectively, representing kinetic charge transfer yield, ρ , i.e., the amount of product per unit reactor volume per ST limitations which dominate at low current densities. Such over- unit time: potentials can be minimised using the appropriate catalysts for a 1 dn fI particular reaction and operating at a higher temperature. The r == [] 4 ST concentration overpotentials under convective-diffusion terms, V dt zFV η and η are important at high current densities. These c,conc a,conc where V is the reactor volume, V is the electrolyte volume, n is the overpotentials cause a potential loss due to mass transport limitations R amount of a species and t is the elapsed time. The current efficiency of electroactive species reaching, or leaving, the electrode surface (or charge yield),f allows for a fraction of the current being used in and can be minimised employing high surface area electrodes, high −3 −1 secondary reactions. The common units of mol m h can be mass transport flow regimes or turbulence promoters. The IR term is obtained on multiplying the right-hand side by 3600. When the sum of all electrical resistances across the reactor and the comparing electrode performance it is often useful discuss in terms electrodes as well as that of the electronic connections. These of a normalised current, either the current density (j) by normalising include all ohmic drops in the system, i.e. the external electrical current against the electrode area (A): contacts, the current collectors and electrodes, electrolyte(s) and membrane. Minimisation of these resistances lowers energy loss and can be achieved by: (a) reducing the overpotentials of the reactions j = [] 5 at the electrodes using a suitable catalyst and (b) reducing the resistances through the membrane, across the electrolyte(s) and or a specific current (I ) by normalising current against catalyst mass through the electrodes. For an electrosynthesis cell, minimising all loading (w): potential losses in the reactor minimises the cell voltage required. jA I = [] 6 Electrode kinetics.—Electrode potential is “the double-edged sword of electrochemical technology.” If the potential is well controlled, yield, selectivity and reaction rate can be high, but Normalisation is usually against the geometric area rather than when the potential distribution is poor, rate, purity and yield can all the electrochemically active area, rougher or more porous electrodes suffer. It is important to monitor electrode potential not just in being able to support much larger current densities. laboratory cells but in pilot and full scale electrochemical reactors, both to relate performance back to polarisation behaviour and act as Figures of merit.—Figures of merit (FOM) for electrochemical a powerful diagnostic probe for cell condition monitoring. reactors are convenient normalised criteria of performance for the 1,13 Table I. Typical figures of merit used to describe electrochemical reactor performance. FOM Expression Observations nn - () 0 (t) Fractional conversion n initial amount (0) X = , () 0 n amount at time t (t) product wzF Current efficiency q , charge passed to produce product product f== q Mq q, total charge M, molar mass product Selectivity n , amount of product S = product total n amount of all species produced total -zFE cell Specific energy consumption for electrolysis fM DGf Energy efficiency during electrolysis EzF cell Active electrode area per unit volume A = Mass transport coefficient I mass transport limiting current L, k = zAFc c concentration of the electroactive species kA kA L L A , electrode area per unit electrode volume Mass transport coefficient associated to the electroactive area k A = or k A = LE LE V V R E V , electrode volume Space-time R Q, volumetric flow rate. t = ST Space-velocity s = w 1 Space-time yield r =´ ST tV Normalised space-time velocity c If () 0 s = log ( ) () cc - VzF c ()tR (0) ()t Normalised space time yield c IM f () 0.9 () 0 r = log ( ) VzFX c RA ()t Journal of The Electrochemical Society, 2020 167 155525 Figure 1. The C-Flow® Lab 5 × 5 laboratory-scale flow cell. The electrodes have a projected electrode area of 25 cm with a typical electrolyte linear flow −1 velocity of 1–10 cm s . Figure courtesy of C-Tech Innovation Ltd. diverse range of electrochemical reactors that exist for different components, comprising of electrode plates, gaskets, separators and applications. The figures of merit are useful to compare different housings. Practical cells usually require even compression of flow and reactors and help the selection for a particular purpose. In addition electrode compartments to ensure adequate sealing. It is common to the selection should be based on safety and reliability characteristics facilitate uniform compression by using rigid (often insulated steel) as well as the most convenient operational mode. The most common end plates. Well-spaced threaded (often stainless steel) fastenings figures of merit are presented in Table I. apply compression through the cell, as seen in commercial cells such 1 20 It is also important to evaluate the suitability of an electro- as the Electrocell, the FM01-LC electrolyser and the Microflow chemical rector and the initial investment cost and the operational cell. Tools such as wrenches (and especially torque wrenches) are cost, including the costs of electrolysis and moving parts, i.e. rarely found in synthesis laboratories and manual, thumb screws are electrolyte pumping or electrode rotation. The evaluation should convenient and used in several designs. In the versatile laboratory cell, also consider the life time of all the components which cost could one end plate is fixed in a frame, the other being tightened and relaxed increase the initial investment and running of the process. A careful via a single thumb screw. It is useful to have the inlet and outlet flow selection of suitable materials needs to consider lifetime, sustain- tube connectors mounted on an end plate and able to be tightened and ability, recycling and environmental disposal. relaxed manually, precluding the need for hand tools. Thumb screws Figures of merit are extensively used to compare the performance are convenient and their use has been continued in contemporary of electrochemical reactors employed for waste water treatment designs, such as the C-Flow cell shown in Fig. 1. 14 15 containing phenolic compounds, ground water treatment, waste Electrochemical cell components are prime candidates for 3D water from the petrochemical industry and metal recovery or printing, either from polymer materials or stainless steel depending remediation of waters containing phenolic compounds. on whether the component should be electrically insulating or should Electrochemical reactors require high current density, energy function as a current collector. The same technologies can be used to density and energy efficiency which can only be achieved by the fabricate the electrodes themselves, or conductive steel, Ni or Ti units appropriate selection of electroactive species and selective catalyst can allow insertion and removal of anode and cathode plates. Of course, to avoid parasitic reactions as well as comparative performance to 3D printing is not a necessity; stacked components with machined quantify their capability. The explicit mathematical figures of merit pores and channels can be arranged to give relatively complex internal listed in Table I are essential to make such comparisons specially structures providing an adequate seal is obtained. Stacked cell designs those based on the normalisation considering the stack volume and require adequate separation between anode and cathode to prevent the electrolyte volume electrical short circuits or product crossover. Electrical separation can In the case of electrosynthesis of soluble redox mediators in be achieved with porous polymer meshes or crushed glass frits, or bipolar filterpress cells, many engineering aspects of cell design are chemical separation requires ion exchange membranes. in common with that of redox flow batteries, which have been The choice of solvent and electrolyte must consider the stability 17–19 treated in extensive reviews. of reaction intermediates for the full electrochemical mechanism, and also whether sufficient overpotential can be applied within the Practical features.—The most basic electrochemical testing can solvent window. Aqueous systems are cost effective and simple to be performed in simple glass beakers with anodes and cathodes work with, although the solvent window of water is narrow immersed in electrolyte. Adding glass-blown flanges facilitates compared to organic media or ionic liquids. Many electrochemical anodic and cathodic separation via a membrane or porous separator. systems show a strong pH dependence and the requirement for As reactors are scaled up and flow is introduced, more complex proton transfers often necessitates and aqueous electrolyte. It is also reactor designs are usually assembled from a series of stacked important to consider surface interactions between electrolyte ions Journal of The Electrochemical Society, 2020 167 155525 and the catalyst surface. Strongly adsorbing species can block a humidified gas stream. This has been successfully achieved for catalyst sites, which can reduce the rate of reaction or have drastic CO reduction in ionic liquids, although so far the limited water 23 – influence on product selectivity. availability has only given access to 2e formate or CO products and Organic electrolytes and room temperature ionic liquids give a cannot reach desirable C species such as ethylene or ethanol. wider solvent window than aqueous equivalents, and allow reactions Ionic liquids are especially desirable from an environmental through intermediate species that would be oxidised in an aqueous perspective as they can be recycled via solvent extraction to be environment. Ionic liquids are able to stabilise high energy radical reused for further electrochemical synthesis. Since their environ- intermediates for organic electrochemical synthesis. Proton mental standing is dependent on extraction and recycling and their transfer in organic or ionic liquid media can be facilitated by viscosity gives slow mass transport rates, their use is currently too including low concentrations of water either in the liquid phase or as costly for industrial scale-up. Figure 2. A decision tree regarding cell features. Journal of The Electrochemical Society, 2020 167 155525 Figure 3. Schematic diagrams for a number of commonly employed laboratory scale electrochemical cells. Solid arrows indicate the direction of solution flow. Dashed lines indicate a porous separator or ion conductive membrane. Electrolytes in A-C may also be mixed through additional means such as magnetic stirrers or gas bubbling. (A) Undivided beaker cell, (B) Beaker cell with the anode confined in a porous chamber, (C) Divided H-Cell with a membrane separator, (D) Undivided flow cell with an electrolyte reservoir and circulating electrolyte pump. (E) Divided flow cell with separate anolyte and catholyte reservoirs on either side of a microporous membrane separator. Decisions During Reactor Selection or Design integration so the reactor aligns with existing production processes, d) reaction engineering to optimise selectivity, production, current Strategic decisions.—In the 1960s, electrochemical engineering and potential distribution, mass transport, electroactive area, inter- principles were increasingly applied to cell design and a diverse electrode gap and low overpotentials, e) operational cost having range of electrode and cell geometries developed for laboratory and reliable and low cost cell components such as electrolyte and 6,28–31 pilot scale use in the 1970s. Many of these designs, particu- separator, if needed, f) minimisation of mechanical devices such as larly packed, fluidised and moving beds, were primarily intended electrolyte pumps or electrode agitation and g) low pressure drop for metal ion removal from dilute waste liquors although uses in over the reactor. synthesis have been considered. Such developments and their Other strategic decisions are whether the process is batch or 1,34 industrial applications have been reviewed. continuous operation and how the products will be removed from the It is generally accepted that depending on the applications the reactor, depending on their physical characteristics. Gas products are electrochemical reactor has to be designed for the particular process typically vented at lower pressure or displace them with an inert gas in order to optimise the figures of merit such as conversion, current or via a gas liquid separation unit. Liquid and solid products can be efficiency, selectivity, energy consumption and efficiency, cell separated by flotation, settlement, or solvent extraction. voltage, electroactive area, mass transport and space tie and space Figure 2 offers a simple strategy to aid selection or development velocity. Selectivity and conversion are more important than energy of a particular electrode geometry and cell design. When used efficiency in electroplating and organic electrochemistry respec- retrospectively, this approach helps to rationalise diverse cell tively, whereas energy efficiency is more important for redox flow designs by considering their major characteristics. Alternatively, batteries and industrial production of, e.g., adiponitrile, aluminium Fig. 2 can aid the selection of an available cell design. The benefits and chlor-alkali. and compromises involved in making such a choices can be briefly Independently of the application of the electrochemical reactors, considered. several principles need to be followed in order to provide an optimised design. Some basic strategic decisions for constructing an electrochemical reactor include considerations that could be Divided and undivided reactors.—One of the first decisions in conflicting and the decision on what aspects to favour on detrimental reactor design whether the reactor operates with separated cathodic to others has to be realised on their importance for the process. Some and anodic electrolyte compartments by an ion exchange membrane, of these considerations include: a) simplicity in order to reduce cost, a porous separator or a single electrolyte compartment. The single b) reliability for routine operations, cleaning and inspection, c) compartment design is simpler and avoids the cost of the ion Journal of The Electrochemical Society, 2020 167 155525 exchange membrane and the gaskets and fittings in the electrolyte membrane should maintain the material balance of the anodic and required to fit the separator (Fig. 3). Divided reactors avoid mixing cathodic reactions in order to maintain the neutrality and avoid of catholyte and anolyte electrolytes which prevents product con- drastic pH changes in both electrolyte compartments. sumption or unwanted side reactions occurring at the opposite CEMs and AEMs are designed to conduct cations and anions electrode. Although small inter-electrode distances can be realised respectively, in theory CEM repel neutral molecules and anions with a separator, as with the 2 mm inter-electrode gap of the bromide while AEM repel neutral molecules and cations. Nafion® is the most polysulfide redox flow battery, divided reactors also increase the commonly used CEM due to its high ionic conductivity and ionic resistance due to the separator. chemical resistance due to a robust fluorocarbon backbone with Cell designs without a membrane or separator have a smaller sulfonic groups as ion exchange sites; other membranes based on ohmic resistance as the impedance caused by a separator is absent sulfonated styrenes, polyimides, and arylene ethers, are less stable. and are capable of wider range of flow profiles. Furthermore, On the other hand, AEM are based on fluorinated hydrocarbons, poly degradation and material cost for membranes do not have to be (ketones), poly(ethers), and poly(ether ketones) with imidazolium, considered. For instance, the membrane in PEM fuel cells accounts quaternary amine or phosphonium as the anion-exchange groups but to 24% of the total cost. Membrane-less designs can have porous these are chemical less stable than CEM. A clear example of the electrodes, allowing flow through, fluidised bed electrodes, gas convenience of using anion membranes is the borohydride fuel diffusion electrodes and redox mediators. cell ; although CEMs have good resistance in the alkaline environ- The selection of a divided or undivided reactor is important in the ment, they produce a chemical imbalance when OH is consumed electrochemical water treatment methods to deplete anthropogenic and not replaced from the catholyte compartment, making the persistent organic pollutants. Typically, the main direct and anolyte more acidic in the long term. AEMs keep the chemical mediated oxidation (via highly oxidising radicals) occurs in the balance by replacing the OH ; unfortunately, most anionic mem- anode compartment, in which case a divided reactor will be required. branes are unstable in alkaline environments. In some instances, a two-stage remediation process involves the Bipolar membranes (BPMs) offer an alternative structure to preparation of the oxidants (e.g., persulfates, perphosphates, percar- address the challenges limitations of AEMs and CEMs. A cationic bonates) in the reactor which are then added to the wastewater. In a and anionic layer are combined to give a two-layer structure, divided reactor, direct and mediated oxidation can be carried out at preventing product crossover while still permitting the charge to + – the anode where single, and mixtures of, highly oxidising species be carried by H and OH . There are two modes of operation can be generated at a high current efficiency. determined by the reaction at the cationic-anionic phase interface; + – A more synergetic process is the combination of the anodic and facing the cationic side to the cathode generates H and OH by cathodic processes to increase the degradation efficiency, in which case water electrolysis, whereas facing it to the anode drives the opposite an undivided reactor may be used. For example, in order to increase reaction. Care still must be taken, as some undesired ion crossover the oxidation power of cathodically generated H O , an undivided may still occur and BPMs can suffer from delamination and 2 2 reactor will allow the hydrogen peroxide to couple with other reactions dehydration, particularly at large current densities. such as Electro-Fenton based processes. It can be argued that the efficiency will decrease due to the decomposition of H O at the anode Monopolar and bipolar electrical connections to electrodes.— 2 2 a trade-off exists between the increase of cell potential due to the The electrochemical characteristics of a system are typically separator and the effectiveness in the depletion of the organic pollutant determined in a single three-electrode laboratory scale cells of by the concerted anodic/cathodic treatment when using undivided ≈100 cm volume with cell voltages of approx. 1–2 V. If the system cells. One example of the use of divided and undivided reactors was is scaled up to a large number of electrochemical cells it might need reported by Ochoa-Chavez et al. who showed small difference in the the application of larger cell voltages. Several single electrochemical degradation of 5-fluoro-1H-pyrimidine-2,4-dione (5-FU) with 75% and cells can be put together to increase the area, thus the production −1 77% for undivided and divided reactor, respectively using 50 mg l capacity, and they can be arranged as a monopolar connection as is −2 −1 FU at 150 A m ,13 lh and6hofelectrolysis. shown in Fig. 4A, where each electrode is either positive or negative. The use of divided electrochemical flow reactors has been widely This arrangement maintains the cell voltage of one individual cell explored in inorganic process such as the chlor-alkali and in energy but is able to generate high currents. In order to increase the voltage, generation and storage devices like fuel cells and redox flow the monopolar connected cells can be arranged in series, see Fig. 4B. batteries. They increase energy conversion efficiency by preventing Another strategy consists of connecting the cells in a bipolar parasitic reactions and can reduce energy losses by separate configuration as is shown in Fig. 4C, which is commonly used in optimisation of the anodic and cathode reactions. Their use in electrosynthesis cells, fuel cells and redox flow batteries. In this organic electrosynthesis and the effect of the organic material on the arrangement the voltage depends on the number of cells. The ion exchange membranes is poorly explored. The issue becomes electrodes acquire a different charge on each side, driving more complex when deciding whether a cationic (CEM) or anionic the oxidation reaction on one side and the reduction reaction on exchange membrane (AEM) can be used as the selection of the the other simultaneously. Figure 4. (A) monopolar electrode connections, (B) monopolar cell stacks connected in electrical series, (C) bipolar electrodes. Journal of The Electrochemical Society, 2020 167 155525 Figure 5. Commonly employed 3D electrode scaffolds in electrochemical reactors, offering a great range in porosity, tortuosity and active surface area. Figure adapted from Ref. 57 available under Creative Commons (CC-BY) license, published on behalf of The Electrochemical Society by IOP Publishing Limited 2020. Bipolar electrode connections allow more compact cells than applications such as electrosynthesis, oxidation of organic materials 45–48 monopolar connections because there are no electrical cables in wastewater, metal recovery, energy storage and generation. connecting each electrode. Both monopolar and bipolar configura- They can be arranged in two configurations; flow-through and tions are found in cells designs that can be easily scaled-up to flow-by, where the current and the electrolyte flow run parallel and 23,43 industrial production. The bipolar electrodes typically contain an perpendicular to each other, respectively. In both configurations the electronic conductive flat plate in the centre which has two purposes, electrodes face each other in order to ensure a uniform current a barrier for the positive and negative electrolytes and as an distribution. However, although the intricate structure of the 3D electronic connection to transfer electrons. In redox flow batteries electrodes contributes to increase the mass transport of electroactive and fuel cells the core electronic conductive is a graphite plate. In species towards their surface, it also causes different resistance fuel cells, the bipolar plates also carry the gases to and from the gas values between opposite points of the electrodes. This geometrical diffusion electrodes, i.e. they act as flow fields. differences causes changes in the potential and current distribution on the electrodes affecting their performance. The current distribu- Porous, 3-D electrode structures.—Porous materials offer large tion can be divided between primary, which depends on the surface electrode areas and are typically used in electrochemical geometry, secondary that depends on kinetic factors and tertiary flow reactors, where they are most effective, for a variety of which depends on the concentration. Journal of The Electrochemical Society, 2020 167 155525 Figure 6. Schematic diagrams of common electrode and reactor reactors. Grey shading indicates a driving electrode, brown a catalytically active material and white an insulating surround or membrane. Solid arrows show the electrolyte flow direction, dashed arrows show component rotation: (A) parallel plate flow-by electrode, (B) parallel plate flow through electrode, (C) interdigitated flow through electrode, (D) rotating disc electrode, (E) rotating cylinder electrode, (F) trickle bed electrode, (G) fluidised bed reactor, (H) thin film bipolar electrode disc stack. The geometry of the three dimensional electrodes are also able to homogeneous. Rather than use electrodeposition, catalyst materials increase the space-time yield of electrochemical reactors by pro- can be added via electroless deposition or dip coating, which can be viding effective use of the reactor volume and increase their monitored via the open circuit potential. However, there is still the efficiency compared to 2D electrodes. In flow reactors, the mass possibility that parts of the electrode are inactive due to the potential transport can be controlled and measured and correlated to the distribution if the electrode is too thick or if the concentration of the pressure drop and flow dispersion to evaluate the overall cost–benefit electroactive species is too low. of the electrodes. The most preferred techniques to characterise 3D The determination of the optimal thickness of 3D electrodes can electrodes include the limiting current and conversion rate measure- be obtained by mathematical simulation. For example Nava et al. ments because they are fast and convenient. Typical materials and suggested that, in conductive porous electrodes, a unidirectional 51 52 configurations include carbon felt, foam and reticulated vitreous potential distribution under limiting current conditions can be carbon (RVC) as a cost-effective porous electrode with large modelled assuming plug-flow conditions and that in excess of 54 55 56 surface area and porosity. Metal mesh, felt and foam electrodes supporting electrolyte, the conductivity changes during electrolysis have also been prepared from materials such as nickel, titanium and are negligible. The model assumes that only the concentration decay copper (Fig. 5). of the electroactive species within the electrode is responsible for the The importance of 3D architectures has been emphasised by the potential distribution. More complex models assume that the properties observed when structures such as nanorods, nanospheres, electrolyte flow rate and electrode thickness determine the ohmic nanoonions, networks of nanowires and nanoflowers, microflowers, drop inside the porous electrode. In practice, the potential difference nanowalls and hierarchical structures are manufactured on flat plate between the porous electrode surface and the solution should not be 58,59 electrodes or inside already three dimensional electrodes. too large in order to ensure that hydrogen and oxygen evolution do Indeed, the electrochemical properties of 3D substrates can be not occur during reduction and oxidation, respectively. improved and tailored by surface treatment or deposition of catalyst for a particular reaction. A typical example is the electrodepositon of Examples of Reactor Designs and their Performance Pt on titanium plate, felt or meshes. One of the disadvantages of 3D electrodes is that they can present uneven current and potential Vertical plates in a stirred beaker.—The number of commonly distributions, resulting in asymmetric electrodeposition of catalyst employed reactor designs is extremely broad, depending on the particles as well as uneven final operations. It is necessary to desired reaction profile and scale (Fig. 6). Vertical plate electrodes in establish the optimal electrode thickness to ensure that all the a stirred beaker are a staple in electrochemical laboratories thanks to covered electrode surface is electrochemically active. In thicker their simple design, ease of assembly and broad application scope. electrodes not all the surface of the 3D electrode is at the same Basic reactor designs hold the anode and cathode within the same potential and the distribution of the catalyst might not be beaker. Small modifications can separate these via an ion-exchange Journal of The Electrochemical Society, 2020 167 155525 Figure 7. Examples of commercially available electrochemical flow cells. (A) Exploded view schematic for the FM01-LC. Figure taken from Ref. 36, available under Creative Commons (CC-BY) license, published by ACS Publications 2018. (B) The ElectroSyn reactor for pilot and medium operations. The cutaway on the right shows the internal components, including the electrodes plates, turbulence promotors and polymer frames. Images courtesy of ElectroCell A/S, Denmark. (C) Ammonite8 microflow cell. The top image shows the individual components, bottom shows the full assembled cell. A larger model (Ammonite15) is also available for electrochemical synthesis up to the tens of grams scale. Figures taken from Ref. 75, available under Creative Commons (CC-BY) license, published by Elsevier B.V. 2016. 63 36 membrane in an H-cell configuration. Both can be operated in product conversion. Further enhancements are achieved through terms of cell potential, or a reference electrode can be inserted into turbulence promotors to encourage solution mixing, incorporating one compartment near to either the anode or cathode, defined then as porous flow-through electrodes to increase the electroactive area, or the working electrode. As well as a popular choice for electro- through combining multiple stacked cells in sequence or parallel 67,68 chemical teaching laboratories, the ability to rapidly replace elec- configurations. Stack designs are greatly simplified through trodes, electrolytes and membranes make these cells idea for batch the use of bipolar electrodes, as multiple cells can be potentiosta- testing and proof of concept work in catalysis, redox flow batteries tically or galvanostatically controlled using only two electrical 64 69 and electrochemical synthesis. connections. Although convenient for initial experimentation, the simplicity of Modifications can be made to the core flow cell design based on the cell design limits the applicability of the stirred beaker for up- the needs of the individual reaction. Separating anodic and cathodic scaling. The use of magnetic stirrer bars gives a limited mass compartments allows for different products to be produced at each. transport range with poor reproducibility, due to the variable stirrer This has most notably been achieved in the chlor-alkali industry, position, non-standardized stirrer bar size and interaction of stirred where sodium hydroxide and chlorine gas are produced at the 65 70 solution with the beaker wall. Up-scaling efforts must proceed via cathode and anode respectively. Conversion efficiencies can be intermediate-scaled reactors with volumes on the order of m , which increased with recirculating pumps so that starting materials have 71,72 offer a compromise in easy of assembly and varying parameters multiple passes over the electrode. Care should be taken for against applicability towards up-scaling to industrial specifications. electrochemically active products such as hydrogen peroxide, since a Up-scaling focuses on incorporating multiple anodes and cathodes in cyclic approach would likely decompose the product rather than the same tank, either unseparated or by confining all of one electrode produce more. Single-pass flow systems are also desirable where (e.g., all the anodes) in ionic membrane compartments, which are flow cells can be directly integrated into second-phase synthesis or submerged in a reactor tank containing unconfined cathodes. Such purification modules. designs are employed for large scale electrochemical processes, such as metal salt synthesis, electroplating and metal recovery. Examples of commercially available flow cells.—A broad range of commercial reactors exists, providing electrode areas on the order 2 2 The planar electrode in a rectangular flow channel.—The of 10 cm up to >10 m (Fig. 7). Cells such as the FM01-LC provide rectangular flow cell is ubiquitous in continuous electrochemical bench-top equivalents of industrial scale flow reactors, with the 2 2 flow processes, providing a reproducible flow profile for reactant FM01-LC itself being a 64 cm (4 × 16 cm stack) derivation of 2 20 delivery and product removal, with the same core design for bench the 2100 cm FM21-SP reactor used in the chlor-alkali industry. top and industrial scale reactors. The rapid mass transport rates and The FM01-LC uses a parallel plate design which can be operated large electrode area to electrolyte volume ratio means that flow cells with turbulence promotors and porous electrodes and with or without consistently outperform stirred beaker cells for reaction rate and a separating ion-exchange membrane. ElectroCell offer a similar Journal of The Electrochemical Society, 2020 167 155525 route to up-scaling electrochemical processes by producing similarly comparable reactors over greatly different scales, starting with the ElectroMP cell (0.01–0.2 m ) up to the ElectroProd cell (0.4– 2 1 16 m ). There are also popular commercial microflow reactors, focusing on small inter-electrode gaps and long reaction paths in order to give maximum product conversion in a single pass. Different reactors take different approaches to maximising the path length; the Syrris Asia FLUX module uses a compact serpentine 74 75 reaction path, whereas the Ammonite8 flow cell uses a spiral. Both reactors have shown impressive conversion rates over several hours of operation. 3D printed flow cells.—3D printing technology offers a cost- effective means for rapid prototype development and validation of 76–78 digital simulations. Most components in a traditional flow cell can be fabricated via 3D printing thanks to the increasingly wide range of materials that can be used as a feedstock. Traditionally, polymer materials, including but not limited to poly(lactic acid), poly(propylene) or acrylonitrile butadiene styrene have been used for non-conductive parts such as the flow cell frame or turbulence 79,80 promotors. Users must consider the nature of the electrolyte and products formed, as many polymers are unsuitable due their susceptibility to hydrolysis, particularly under extremes of pH. 3D printing technologies are also able to produce conductive structures, allowing customised 3D flow-through electrodes to maximise electrode-solution interaction and generate turbulence in flow-through cell configurations. In most cases, the first stage is to print a 3D scaffold from stainless steel, and then electrodeposit the catalyst to give the same structure at greatly reduced cost. Alternatively, incorporating conductive materials into the polymer Figure 8. Examples of commonly employed flow field designs for flow-by feed provides a conductive polymer scaffold for subsequent electro- electrochemical cells. (A) Serpentine, (B) parallel, (C) pin, (D) spiral, (E) deposition and use as an electrode. Other groups have incorporated interdigitated. Figures adapted from Ref. 99 available under Creative more complex catalysts directly into the polymer, such as Pt/C or 84 Commons (CC-BY) license, Copyright © 2014 liu, Li, Juarez-Robles, MoSe . These conductive 3D polymer electrodes have been Wang and Hernandez-Guerrer. 85,86 87 successfully employed in flow cells, electrolysers and electro- chemical sensors. This presents an opportunity to 3D print an to have two parallel electrodes. A recent paper demonstrated that entire flow cell using a low-cost desktop printer, offering an improved electrical contact was made using magnetisable electrode economic route to prototype development. particles under the influence of a magnetic field. The strategy resulted in an increase of the electrochemical conversion up to 400% compared Porous, 3D electrodes in a flow cell.—The choice of porous to the use of non-magnetised particles. material depends on the flow profile employed within the cell, primarily whether the electrolyte flows-by or -through the Flow field designs.—The challenge in flow field design is to electrode. Flow-by configurations flow electrolyte within a channel provide optimal flow conditions for reactant supply to and product over the surface of the porous structure, usually assisted by removal from the electrode surface, while simultaneously mini- turbulence promotors to maximise interaction between the electro- mising the pressure drop between inlet and outlet. To this end, a lyte and the inner porous structure. Flow-through instead fills the number of different flow field designs have been employed (Fig. 8). channel with the porous structure, requiring the electrolyte to Within these categories, multiple works have investigated varied permeate the whole structure during the flow. While maximising flow channel widths, depths and orientations, with both experimental the interaction between electrode and electrolyte, the flow-through and computational approaches probing the flow profile, pressure configuration is practically limited due to pressure drop across the drop and potential distribution across the electrode surface. porous electrode material. Flow through configurations therefore Comparison of different flow fields is challenging since varied are most often employed to very thin electrodes in microfluidic 90,91 experimental conditions affect reactor functionality alongside devices. changes in the flow field design. However, some experimental and 3D electrode materials can also be used as scaffolds to support computational works indicate that serpentine fields outperform the catalytically active particles, which may be deposited through spray 99,100,101 92,93 94 55 other related designs. coating, drop casting or electrodeposition. With flow appli- An alternative flow field design often employed for 3D porous cations in mind, decorating the conductive scaffold must find a electrodes is the interdigitated flow field (IFF), which has a series of compromise between loading sufficient catalytic material and parallel channels where each channel is blocked at alternate ends. In blocking the porous structure. The tortuosity of the porous material order to continue the flow, liquid must move through the 3D will determine how deep into the pores the catalyst can be loaded. electrode structure itself. This gives a much greater degree of Electrodeposition and drop casting exceed spray coating here, since electrolyte-electrode interaction and a faster rate of reaction. electrolyte can permeate porous structures whereas spray coating The same technique can also be applied to gas phase flow to requires a linear line of sight. encourage interaction with the catalyst at the solid-electrolyte An alternate route to high surface area electrochemical catalysts is interface. The benefit of IFFs can be thought of as providing through fluidised bed reactors. These feature solid catalyst particles flow-through activity, without the associated pressure drop, thanks to held in suspension by an upward gas or liquid flow stream, providing the shorter mass transport path length. When designing an IFF, there excellent catalyst interaction and mixing for electrochemical must be a compromise between the electrode penetration depth and reactions. Although fluidised bed reactors suffer from poor electrical the pressure drop, since greater flow-through characteristics will contact, they removed the assumption that an electrochemical cell has Journal of The Electrochemical Society, 2020 167 155525 hydrophobic and hydrophilic channels within the electrode structure. This has been shown to facilitate water transport within the electrode structure to prevent channel flooding, whilst still favouring gas 107,108 transport for O reduction. Rotating disc and cylinder electrode reactors.—Rotating disc electrodes (RDEs) are staples in electrochemical laboratories thanks to their well-defined and reproducible electrolyte flow and mass transport profiles, with the transport rate being proportional to the 109,110 square root of the rotation rate. An extension to this design is the rotating ring disc electrode (RRDE), which has a secondary ring electrode on the outside of the central disc. Electroactive species produced at the disc are detected at the ring, allowing in situ quantification of electrochemically generated products. Samples of interest can be drop-cast onto glassy carbon substrates, or affixed to the electrode surface, provided mathematical considerations are made for the sample shape and thickness. This provides an attractive option to assess the impact of flow rate on a reaction in a simplified set up, before moving to a more complex flow cell design. Rotating cylinder electrodes (RCEs) have received less attention than RDEs. Like the RDE, the rate of mass transport is defined by its rotation rate. The RCE offers a substantially larger surface area along with controlled turbulent flow conditions, making it suitable for up-scaling to industrial applications. Primarily, the large RCE surface area has been employed for electrodeposition applications, 113,114 including metal recovery and metal powder production. Thin film reactors: capillary gap and bipolar trickle tower reactors.—Trickle tower reactors use gravity flow to pass the working solution through a porous catalyst material, usually a foam, mesh or bed of particles. The small pore volume gives a thin film of electrolyte over all catalyst surface, giving unique electrochemical conditions. Specifically, the thin electrolyte layer over a large surface area catalyst give a high conversion rate and minimal Ohmic drop when working with weakly concentrated, resistive solutions. This makes trickle towers ideal for water treatment, particularly in the removal of the low concentrations of organic pollutants. The same concept is seen in capillary gap electrodes, where the electrolyte flows down a thin channel between anode and cathode. This can be done on a range of scales from microfluidic devices up to stacks of bipolar capillary electrodes. The small inter- Figure 9. Different approaches to the use of microflow reactors for electrode gap provides an additional advantage thanks to the short electrochemical synthesis based on the needs of the reaction mechanism. diffusion paths under laminar flow conditions. This gives rapid rates (A) Direct electrochemical oxidation of benzyl alcohol to the corresponding benzaldehyde at the anode. Cathode balances the charge through hydrogen of reaction without turbulence or forced convection, allowing fast evolution. Mechanism taken from Ref. 138. (B) Electrochemical conversion turnover rates than can be supported by relatively simple computa- of cubane carboxylic acid to alkoxy cubanes. Reaction intermediates are tion modelling. Microchannel reactors with short inter-electrode produced at both the anode and cathode, which subsequently react to give the distances allow for paired electrochemical processes, where the end product. Mechanism taken from Ref. 141. (C) Electrochemical synthesis anodic and cathodic diffusion layers interact to give further reactions of metal-salen complexes. A sacrificial anode acts as a source of metal between electrochemically generated products. cations for the reaction. Anodic metal dissolution and ligand reduction are performed simultaneously, which combine to give the final complex. Mechanism taken from Ref. 142. (D) Oxidising alcohols to the corre- Technological Developments in the Design, Construction and sponding ketone via the redox mediator 2,2,6,6-tetramethylpiperidine-1-oxyl Simulation of Electrodes and Cells (TEMPO). Electrochemistry at the cathode is not considered. Mechanism taken from Ref. 143. Gas diffusion electrodes (GDEs) for gaseous reactants.—Gas diffusion electrodes offer rapid rates of reaction for gas-phase electrochemical processes. The core structure consists of a hydro- phobic, gas-permeable structure with a catalyst deposited on the cause a greater pressure drop. New IFF designs tune channel size electrolyte-facing side. The keeps mass transport in the gas phase, and density in order to give a uniform gas/electrolyte distribution for 103,104 which is inherently rapid, and circumvents challenges of poor an optimum power density. Recent advances have included the solubility that hinders the rate of reaction when bubbling gas development of hierarchical IFFs, where smaller branching channels through electrolyte in a beaker or H-cell. GDEs are relatively cheap give an even mass transport distribution whilst providing a small and simple to manufacture, leading to their extensive use in PEM pressure drop. 122 123 124 fuel cells, flow cells and electrolysers. Electrochemically A key challenge with interdigitated flow fields for gas phase synthesised products may be collected in the gas or liquid phase, or electrochemistry is water management. Capillary pressure resulting multiple products may be collected from both phases simulta- within porous materials causes water to accumulate in flow channels, neously, as has been achieved for ethylene and ethanol from CO hindering gas flow and resulting in unstable cell performance. 2 reduction. This challenge lead to the development of new electrodes with Journal of The Electrochemical Society, 2020 167 155525 Many key developments in GDE technologies have focused on with a 4.2-fold increase in conversation rate for 2,2′-bis(bromo- the hydrophobicity in order to prevent GDE flooding or dehydration methyl)-1,1′-biphenyl intramolecular cyclisation. Hollow Cu of ion exchange membranes. Commercial GDEs contain hydro- fibres have also been used to create a gas diffuser that acts as the phobic components such as PTFE or Teflon to hinder this but further working electrode for CO reduction in a beaker cell. improvements are needed. A number of works have taken steps to Other works have focussed on new immobilisation strategies for improve the hydrophobicity of GDE surfaces, such as hydrophobic catalytic material on 3D electrode supports. Catalytic nanoparticles 128 129 130 oxidised carbon nanotube, PTFE or dimethyl silicon coat- can be immobilised on carbon and metallic foams, felts and meshes ings. Other groups have removed hydrophilic components from the through the in situ reduction of the corresponding metal salts, either 157 158 GDE entirely. Hydrophobic polymer GDEs or silanized nanoporous chemically or electrochemically. The chemical route allows for alumina membranes can replace traditional carbon materials, with the physical bonding of catalyst material to a 3D support, such as for one polymer-based GDE giving efficient CO reduction over gold nanoparticles on graphite felt via thiol moeties or the direct 150 h. synthesis of hierarchical ZnO nanowires on CuO nanowires on Cu It is equally important to consider the impact of the GDE surface, foam via sequential wet chemical and thermal synthesis steps. as the structure and dispersion of catalytic layers will affect the gas Electrodeposition has also been used to fabricate mixed-metal 133 160 161 dispersion and potential distribution over the GDE. Many GDEs catalysts, such as Ag particles on Al and Ni foam, Pt on Ti 162 163 164,165 have a microporous layer (MPL) at their surface, usually a felt, Fe-doped Pt/C and Mn O -coated graphite felt. x y combination of carbon black and PTFE, allowing fine control over Simpler approaches can electrodeposit the same material on a porosity and hydrophobicity. Multiple works have improved the conductive scaffold to give a more active surface, such as for MPL performance through targeting the porosity or hydrophobicity. electrodeposited high surface area Ni via liquid crystal Others have removed it entirely, using carbon nanofibers to enhance templating. The electrochemical route is particularly advanta- the electrical conductivity or microporous polymers to tune the geous as the level of doping or surface modification can be simply hydrophobicity. controlled by varying the applied potential or charge passed. Alternatively, magnetron sputtering has been used for metallic Microflow channel reactors.—The unique combination of la- particle loading on porous GDEs, which gives efficient deposition of minar flow with short diffusion path length allows for interesting small, active particles. Deposition can use a low loading while approaches to electrochemical synthesis (Fig. 9). Despite the maintaining activity, which is particularly attractive for costly noble absence of a membrane, the laminar flow profile gives limited metal materials. As a line of sight method, care must be taken not mixing between solutions at the anodic or cathodic side. This has to block the GDE pores, and particles cannot deposit as deeply allowed selective interaction of solution species with only one within the 3D structure, as in the case of electrodeposition. electrode, such as oxidations of alcohols or heterocyclic Similarly, chemical vapour deposition methods have been used to cross-coupling under mild conditions with only H as a by- create carbon structures on porous electrode scaffolds, such as for product produced at the cathode. In these cases, engineering carbon nanofibres on Ni foam or N-doped graphene on Cu solutions have to allow for H release before the solution is recycled foam. or passed onto the next reactor, as H bubbles are resistive and will slow the rate of further electrochemical steps. 3-D printing of electrodes and cell bodies.—3D printing devel- Bubble generation, usually from H or O evolution, can generate opments range from the fabrication of individual components up to 2 2 turbulence within the flow channel to deviate from the laminar total cell fabrication. Materials choices are determined by the regime, which must be considered particularly when operating at solvents and reaction conditions. Many printable polymers are large overpotentials. In other cases, bubbles within the flow susceptible to degradation, with poly(etherethylketone) (PEEK) reactor can be used as part of the synthesis strategy, such as using recently shown to outperform other materials for chemical, thermal 146 170 bubbles as an H source for hydrogenation reactions. Bubbles of and mechanical stability. Polymer materials are unsuitable for immiscible liquids can also be used for product extraction within the organic media, and can produce hotspots due to poor thermal flow channel, such as for 5-hydroxymethylfurfural synthesis in an conductivity, leading to a number of groups to use selective laser aqueous electrolyte, which then moved into organic bubbles to melting to 3D print components from stainless steel. prevent further degradation or polymerisation reactions. The intrinsic precision of 3D printing allows for complex flow Other groups have reduced the inter-electrode gap to facilitate fields and multi-channel cell designs to target specific reaction reactions between anodically and cathodically generated species. A requirements. The impact of flow field pattern (serpentine, parallel, method to produce copper-N-heterocyclic carbenes via imidazolium interdigitated, spiral, etc.) on reaction yields has been demonstrated, reduction and anodic metal dissolution has been used to produce a since it is straightforward to produce flow fields with multiple 142 172 wide range of complexes. Reducing the inter-electrode gap also arrangements and exchange them within the cell. Multiple inlets allows the electrolyte content to be reduced by a factor of ten for have also recently been used to introduce reagents stepwise for 148 173 sulfonamide electrosynthesis vs beaker reactors, with other difluoromethylation or diphenylacetonitrile. Printed cells can be 149 174 reactors removing the electrolyte entirely. An alternative ap- designed to be dismantled for working electrode exchange, or proach has been to use the electrochemical step to produce a printing can be paused and restarted to confine plate electrodes homogeneous catalyst, which drives the desired substrate reaction, within the cell structure. such as using the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) 3D printing stainless steel or titanium offers a simple route to mediator to drive alcohol oxidations. electrodes with tailored surface area, porosity and flow profile. The desired catalyst can then be added through electrodeposition, as has 176 82 3D porous electrodes.—Metal foam electrodes synthesised from been achieved for recent Ni and Pt electrodes. An interesting electroless deposition on polymer scaffolds or lost carbonate alternative is to incorporate conductive material such as carbon or sintering give random pore orientations. More ordered structures copper powder into 3D printed polymer to provide conductive 88,177 have been produced via selective laser melting, giving access to polymer electrodes. These have been used directly as 152 153 86 178 regular pore distributions and tailored electrode architectures. electrodes, after pre-treatment with solvents or electrochemical A number of groups have reduced the pore size in order to increase cycling or after electroplating to give a metal film on the 3D the active surface area (Fig. 10). Ni electrodeposition inside an printed scaffold. anodic aluminium oxide template produced a Ni nanomesh, which was then Pt doped for H evolution applications, outperforming Computational modelling of reaction environment in reac- commercial Pt/C catalysts. New metal felts have been produces tors.—Broadly speaking, computational models can focus on from nanowires, giving a high surface area flow-through electrode hydrodynamics, mass transport, heat transfer, current distribution, Journal of The Electrochemical Society, 2020 167 155525 Figure 10. Recent novel 3D electrode designs for varied electrochemical applications. (A) Flexible, self-supported Pt-doped Ni nanowire mesh for hydrogen evolution reaction applications in a liquid H-cell. Scale bars represent 2 cm (left), 500 μm (centre) and 5 μm (right). Figures adapted from Ref. 154, Copyright © 2018, American Chemical Society. (B) High density Cu nanowire felt for single pass electrochemical synthesis in a flow-through liquid phase reactor. Left and centre schematics highlight large substrate conversion rate thanks to dense 3D structure. Figures adapted from Ref. 155, Copyright © 2019, American Chemical Society. (C) Cu porous hollow fibre gas diffusion electrode for electrochemical CO reduction. CO is flowed through the central void then passes through the porous walls to react at the liquid interface. Scale bars represent 50 μm (left) and 500 μm (centre). Figures adapted from Ref. 156 available under Creative Commons (CC-BY) license, published by Springer Nature 2016. or combinations two or more of these. The scale and computational cost depends on the number of parameters solved for, the number of assumptions made and the overall complexity of the model geometry. At their simplest, 1D models can be used to calculate to show concentration gradients, flow profiles or potential distributions along flow channels or electrode materials, assuming an even distribution across the second dimension. 1D models are built on analytical solutions operating within set boundary conditions. The range of expressions and conditions is extremely broad, being 66,182–186 the subject of a number of excellent reviews. In all cases it is essential to consider the boundary conditions of each model in relation to the experimental conditions in order to ensure a good fit between computational and experimental data. Moving up to 2D allows heterogeneity in catalyst, flow field and other component surfaces to be explicitly modelled. This allows models to incorporate chemical kinetics alongside flow profiles in Figure 11. Distribution of chlorine oxidation current efficiency over a order to explicitly model reactions inside a flow cell, allowing the planar electrode using a mass transport wall function model. The inlet is at user to model the impact of cell potential, feed concentration and 188–190 the bottom, with turbulent flow conditions following a turbulence promotor. flow rate. Changes to the reaction environment during The more turbulent environment close to the inlet results in a greater current operation can also be fed back into the model, such as resistive efficiency at the inlet vs the outlet. Increasing the flow rate gives a greater bubble formation during electrolysis. Modelling via finite-ele- efficiency further along the electrode as turbulence eddies are extended ment, -difference or -volume simulation provides clear visual further down the flow channel. Figure taken from Ref. 199 with permission representations of concentrations, flow rates and pressures, allowing from Elsevier (Copyright © 2018 Elsevier Science S.A. All rights reserved). Journal of The Electrochemical Society, 2020 167 155525 Figure 12. Schematic flow chart for combinatorial electrochemical synthesis via the cation flow system. S and Nu represent possible substrates and nucleophiles respectively. Changing the flow path allows multiple substrate—nucleophile combinations to be systematically produced by the same reactor. Figure adapted from Ref. 227 with permission from Wiley-VCH (Copyright © 2005 Wiley-VCH Verlag GmbH & Co. All rights reserved). reactors designs to be modified to encourage solution mixing and power output. Electron transfer rates have been improved via 192 203–205 remove stagnant zones. targeted modifications to the enzyme structure or through The added computational costs of moving to 3D are often incorporating conductive polymers into the enzyme-electrode 206,207 necessary for complex cell designs, particularly when investigating assembly. Alternatively, electron transfer can be mediated 193,194 195 the impact of 3D electrodes, parallel stacked cells, flow via conductive nanostructures, as has been achieved with 196–198 199,200 208,209 210,211 212–214 field geometry or turbulence promotors on solution nanoparticles, graphene, and carbon nanotubes. mixing and turbulent flow. Modelling the impact of turbulence Similarly, microbial fuel cells offer environmentally friendly promoters on flow profile and current distribution allows for a better opportunities for reactor designs, particularly with a focus on energy understanding of how to avoid dead zones and give an even current generation and wastewater treatment. These come with a number distribution over a large electrode surface (Fig. 11). of important environmental benefits, such as the ability to operate over a wide range of temperatures and pH on diverse types of Immobilised enzyme electrodes and biosynthesis.—The natural biomass without the need for energetically expensive aeration. specificity of enzymes towards a particular reaction offers a The core design uses microorganisms carry out a redox process at an promising route to selective electrochemical synthesis. Reactors electrode surface, such as the oxidation of an organic substrate at the are primarily based on fuel cell designs, where electrochemical anode or nitrification of ammonium to nitrate and subsequent catalysts are replaced with redox active enzymes immobilised onto denitrification to nitrogen gas. an electrode surface. Biofuel cells typically employ fuel-specific Present microbial fuel cell designs are held back by restrictive enzymes at the anode to oxidised the fuel in an aqueous environ- costs; wastewater treatment via microbial cells is currently around ment, with oxygen reducing enzymes such as multi-copper oxidase 30 times higher than treatment via the conventional activated sludge or bilirubin oxidase reducing oxygen at the cathode. process due to the need for expensive electrode, separator and 216,219 Since enzymes already offer excellent reaction specificity, much membrane materials. Advancements in more cost effective research into enzymatic reactors focuses on facilitating the electron materials along with higher power outputs to offset these costs are transfer rate between the enzyme and the electrode to improve the essential in making this technology viable for upscaling and wider Journal of The Electrochemical Society, 2020 167 155525 usage. Costs can be reduced by removing separators in favour of a 3. A wide range of electrode geometry and electrolyte flow simpler single cell design, but these risk bio-fouling at the cathode conditions is accessible in published cell designs. over extended periods of operation or electrocatalyst poisoning. 4. Parallel plate flow cells are the first choice for flow electro- chemical synthesis thanks to their simple assembly and well- Combinatorial approaches to electrosynthesis.—A drawback to defined, reproducible flow profile. Static or magnetically stirred electrochemical synthesis is the scale of parameters that must be beaker cells will still have a place in the electrochemical refined in order to maximise the reaction yield, including but not laboratory, but they are entirely unsuitable when considering limited to electrode material, surface structure, electrolyte, solvent, scale-up operations. overpotential and temperature. A combinatorial approach allows for 5. Electrochemically active surface area can be markedly increased the bath analysis of multiple parameters, which greatly accelerates the via 3D porous electrodes. There are a wealth of different 3D 222,223 optimisation process (Fig. 12). The first examples used a series electrode materials readily available, including multiple carbon, of electrodes in a well electrolysis platform to perform galvanostatic textile, metal and metal composite structures. All of these 224–226 electrosynthesis in wells containing different reagents. materials can be used as suppled or with additional catalytic Rapid parameterisation has also been achieved with microflow coatings, which can be readily tunes to the catalytic needs of a systems, thanks to their precise control over mass transport regime specific electrochemical reaction via physical, chemical or and residence time. Using a flow-switching system allows the electrochemical deposition. As well as catalytic activity, the same anodically or cathodically generated intermediate to be mass transport profile of a number of mesh, foam and felt introduced to multiple reagents in a combinatorial approach. electrodes has been characterised. Other reactor designs isolate multiple materials in individual 6. The increasing accessibility of 3D printing facilities has been of compartments to assess their activity for the same reaction. great benefit to the field of electrochemical reactor design. Reactors designs incorporate individually addressable sensors to Rapid prototyping is possible for reactor components, especially assess conversion rate, such as imaging bubble size for water for polymer-based flow fields and turbulence promotors. electrolysis or hyphenating to a secondary analysis equipment Recently, this has been extended to the 3D printing of via switchable flow channels. electrodes, either by printing with stainless steel or by incorpor- ating conductive materials into the polymer to print a conductive Electrodeposited composite materials.—Many materials, in- polymer composite material. cluding metals, alloys, ceramics, polymers and composites can be 7. The composition and architecture of GDE electrodes has been obtained by electrodeposition and/or electrophoretic deposition. greatly expanded to facilitate a broad range of electrochemical While common engineering applications have focussed on e.g., systems, including water and CO electrolysers and hydrogen PTFE or SiC in a nickel matrix for tribological use, both anodic fuel cells. Changes to the hydrophobic structure have been made and cathodic deposition may be used to synthesise a wide variety of to prevent GDE flooding in electrolysers and aid in water materials, including nanostructured and controlled phase compo- management for fuel cells. sition materials. Examples include titanates and titanium oxide 8. Microflow reactor channels, primarily based on the rectangular nanotubes uniformly embedded in a polypyrrole matrix for corrosion channel geometry have been introduced for laboratory scale protection and MoS particles embedded in a nickel matrix for electrosynthesis, particularly in the case of specialty organics. controlled wear resistance and self-lubrication. The importance of Short anode-cathode distances minimise IR drop even in low or controlling the reaction environment around the cathode to tailor the zero electrolyte solvents, and long path lengths give excellent deposit morphology and engineering properties has been conversion rates. highlighted. The choice of electrolyte composition in the field 9. Combinatorial electrochemical approaches provide a convenient of composite electrodeposition has become more constrained in route to study electrosynthesis of a wide range of reactant/ recent years by environmental concerns over perfluorinated surfac- product concentrations. In particular, the deployment of micro- tants (used to help disperse hydrophobic materials such as graphene, flow reactors accelerates this process by switching flow chan- MoS and PTFE) but sol preparation, stability, agitation and nels, allowing an electrochemically generated intermediate to enhancement of mass transport to the cathode can be facilitated by react with multiple substrates. preparation of sols using shear blade mixing and ultrasonic 238 239 agitation. Improved availability of non-ionic liquids and environmentally acceptable water miscible organic acids has Future R & D Needs extended the choice of electrolytes. Despite decades of development Moving forward, continued development of electrochemical and progress with mechanistic descriptions, a universal model reactors will depend on advancements in the following areas: able to describe the rate and composition of composite electro- deposits from a knowledge of process conditions is long awaited. 1. Characterisation of the distribution of fluid flow, potential, current and reactant/product concentrations for all electroche- Conclusions mical reactors. In particular, quantitative and comparative Electrochemical reactors offer practical solutions to numerous characterisation of 3D electrodes, specifically focussing on their challenges in electrochemical synthesis, from combinatorial ap- activity, real surface area and mass transport towards and within proaches to synthesis on a bench-top scale up to industrial produc- their structure. tion on the tonnage scale. In this review, we have addressed the 2. More use of established figures of merit to provide a quantita- following key points: tive statement of cell performance and enable comparisons of cell geometry over various operating conditions. This should 1. Electrochemical cells offer a broad range of possible electrode include studies of batch kinetics under galvanostatic control to geometries and flow conditions, depending on the needs of the examine figures of merit over a wide range of reactant user. Importantly, many electrochemical cells can be developed concentration and fractional conversion. at the laboratory scale, and then up-scaled for industrial 3. Longer term evaluation of newly introduced electrode and applications with minimal modifications to the core design. membrane materials. 2. The use of electrosynthesis cells has extended beyond tonnage 4. Studies demonstrating scale-up over a wide range of electrode scale process chemicals for commodity use to many fine size, cell size, current and production rate of various electrode chemicals used in pharmaceutical and medical products. materials and cell designs for electrosynthesis. Journal of The Electrochemical Society, 2020 167 155525 5. Accessible, international showcases to demonstrate promising 12. F. C. Walsh, L. F. Arenas, and C. Ponce de León, “Developments in electrode design: structure, decoration and applications of electrodes for electrochemical cell components, reactor designs and their performance. technology.” J. Chem. Technol. Biotechnol., 93, 3073 (2018). 6. Increased use and scale-up of microflow channel cells for both 13. G. Kreysa, “Normalized space velocity—a new figure of merit for waste water inorganic and organic electrosynthesis. electrolysis cells.” Electrochim. Acta, 26, 1693 (1981). 7. The sustained tailoring of GDEs beyond precious metal on 14. M. Mascia, A. Vacca, A. M. Polcaro, S. Palmas, J. R. Ruiz, and A. Da Pozzo, “Electrochemical treatment of phenolic waters in presence of chloride with boron- carbon electrodes intended for use in H and O /air in fuel cells 2 2 doped diamond (BDD) anodes: experimental study and mathematical model.” and batteries towards speciality electrosynthesis of materials. J. Hazard. Mater., 174, 314 (2010). 8. Uptake of electrochemical synthesis techniques by the broader 15. R. Oriol, M. d. P. Bernícola, E. Brillas, P. L. Cabot, and I. Sirés, “Paired electro- research community. It is rare to see electrosynthesis techniques oxidation of insecticide imidacloprid and electrodenitrification in simulated and real water matrices.” Electrochim. Acta, 317, 753 (2019). and their success being described in sectors outside electro- 16. E. M. Mattiusi, N. M. S. Kaminari, M. J. J. S. Ponte, and H. A. Ponte, “Behavior chemistry and electrochemical engineering. It is vital to over- analysis of a porous bed electrochemical reactor the treatment of petrochemical come this by improving education of non-electrochemists, industry wastewater contaminated by hydrogen sulfide (H S).” Chem. Eng. J., 275, training of industrial personnel and providing adequate process 305 (2015). 17. A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, and Q. Liu, experience to new practitioners. “Redox flow batteries: a review.” J. Appl. Electrochem., 41, 1137 (2011). 9. The combination of microflow cells with combinatorial techni- 18. X. Ke, J. M. Prahl, J. I. D. Alexander, J. S. Wainright, T. A. Zawodzinski, and R. ques presents exciting opportunities to screen electrocatalysts F. Savinell, “Rechargeable redox flow batteries: flow fields, stacks and design and explore the importance of reaction environment in electro- considerations.” Chem. Soc. Rev., 47, 8721 (2018). 19. L. Arenas, C. Ponce de León, and F. Walsh, “Engineering aspects of the design, synthesis. construction and performance of modular redox flow batteries for energy storage.” 10. The pressure to improve energy efficiency and lower costs J. Energy Storage, 11, 119 (2017). provides an incentive to explore regenerative cells which 20. F. F. Rivera, C. P. de León, J. L. Nava, and F. C. Walsh, “The filter-press FM01- integrate electricity production with electrosynthesis. LC laboratory flow reactor and its applications.” Electrochim. Acta, 163, 338 (2015). 11. The tailoring of electrode structure needs to be further integrated 21. L. Wu, L. F. Arenas, J. E. Graves, and F. C. Walsh, “Flow cell characterisation: with the design of selective, nanostructured electrocatalysts to flow visualisation, pressure drop and mass transport at 2d electrodes in a realise next generation multiscale electrode architecture. rectangular channel.” J. Electrochem. Soc., 167, 043505 (2020). 12. The integration of porous, 3-D electrodes with high photolysis 22. T. R. Ralph, M. L. Hitchman, J. P. Millington, and F. C. Walsh, “Evaluation of a reactor model and cathode materials for batch electrolysis of l-cystine hydro- irradiation offers the opportunity to develop high efficiency chloride.” J. Electroanal. Chem., 462, 97 (1999). hybrid tubular cells, incorporating thin film electrolytes, for 23. S. C. Perry and G. Denuault, “Transient study of the oxygen reduction reaction on rapid generation of in situ oxidants for process intensive indirect reduced Pt and Pt alloys microelectrodes: evidence for the reduction of pre- electrosynthesis. adsorbed oxygen species linked to dissolved oxygen.” Phys. Chem. Chem. Phys., 17, 30005 (2015). 13. More creative use of immobilised enzyme and microbial 24. S. Möhle, M. Zirbes, E. Rodrigo, T. Gieshoff, A. Wiebe, and S. R. Waldvogel, bioelectrodes opens up a wide range of bioelectrosynthetic “Modern electrochemical aspects for the synthesis of value-added organic routes to speciality chemicals. products.” Angew. Chem. Int. Ed., 57, 6018 (2018). 14. Recent improvements in our ability to model and scale two- 25. A. Wiebe, T. Gieshoff, S. Möhle, E. Rodrigo, M. Zirbes, and S. R. Waldvogel, “Electrifying organic synthesis.” Angew. Chem. Int. Ed., 57, 5594 (2018). phase, solid particle/liquid electrolyte systems provides a new 26. D. Faggion, W. D. G. Gonçalves, and J. Dupont, “CO electroreduction in ionic platform for studies in electrosynthesis of chemicals and liquids.” Front. Chem., 7, 102 (2019). composite electrodeposits. 27. M. Kathiresan and D. Velayutham, “Ionic liquids as an electrolyte for the electro 15. The performance vs construction/cost compromises involved in synthesis of organic compounds.” Chem. Commun., 51, 17499 (2015). 28. R. J. Marshall and F. C. Walsh, “A review of some recent electrolytic cell selecting or progressing a particular reactor design for an designs.” Surf. Technol., 24, 45 (1985). electrosynthesis are poorly documented in the literature; case 29. F. C. Walsh and D. Pletcher, “Electrochemical engineering and cell design in.” study examples of capital and running costs are rarely seen. Developments in Electrochemistry: Science Inspired by Martin Fleischmann, ed. D. Pletcher, Z. Q. Tian, and D. E. Williams (Wiley, Chichester) p. 95 (2014). 30. F. Goodridge and K. Scott, Electrochemical Process Engineering: A Guide to the ORCID Design of Electrolytic Plant (Springer, US) (2013). 31. H. Wendt and G. Kreysa, Electrochemical Engineering: Science and Technology Samuel C. Perry https://orcid.org/0000-0002-6263-6114 in Chemical and Other Industries (Springer, Berlin Heidelberg) (2013). Carlos Ponce de León https://orcid.org/0000-0002-1907-5913 32. M. Fleischmann and J. W. Oldfield, “Fluidised bed electrodes: part I. Polarisation predicted by simplified models.” J. Electroanal. Chem. Interf. Electrochem., 29, References 211 (1971). 33. F. C. Walsh and G. W. Reade, “Electrochemical techniques for the treatment of 1. D. Pletcher and F. C. Walsh, Industrial Electrochemistry (Springer, Netherlands) dilute metal-ion solutions.” Studies in Environmental Science, ed. C. A. (1990). C. Sequeira (Elsevier, Amsterdam) p. 3 (1994). 2. K. Scott, Electrochemical Reaction Engineering (Academic Press, New York) 34. T. F. Fuller and J. N. Harb, Electrochemical Engineering (Wiley, New York) (1991). (2018). 3. J. D. Genders and D. Pletcher, Electrosynthesis: From Laboratory, to Pilot, to 35. M. Fleischmann and R. E. W. Jansson, “The application of the principles of Production: 3rd International Forum on Electrolysis in the Chemical Industry: reaction engineering to electrochemical cell design.” J. Chem. Technol. Papers (Electrosynthesis Company, New York) (1990). Biotechnol., 30, 351 (1980). 4. O. Hammerich and B. Speiser, Organic Electrochemistry: Revised and Expanded 36. D. Pletcher, R. A. Green, and R. C. D. Brown, “Flow electrolysis cells for the (CRC Press, Boca Raton, FL) (2015). synthetic organic chemistry laboratory.” Chem. Rev., 118, 4573 (2018). 5. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and 37. C. Salazar, I. Sirés, C. A. Zaror, and E. Brillas, “Treatment of a mixture of Applications (Wiley Textbooks, New York) 2nd ed. (2000). chloromethoxyphenols in hypochlorite medium by electrochemical AOPs as an 6. F. C. Walsh and C. Ponce de León, “Progress in electrochemical flow reactors for alternative for the remediation of pulp and paper mill process waters.” laboratory and pilot scale processing.” Electrochim. Acta, 280, 121 (2018). Electrocatalysis, 4, 212 (2013). 7. S. Lakshmanan and T. Murugesan, “The chlor-alkali process: work in progress.” 38. A. S. Ochoa-Chavez, A. Pieczyńska, A. Fiszka Borzyszkowska, P. J. Espinoza- Clean Technol. Environ. Policy, 16, 225 (2014). Montero, and E. M. Siedlecka, “Electrochemical degradation of 5-FU using a flow 8. D. E. Blanco, P. A. Prasad, K. Dunningan, and M. A. Modestino, “Insights into reactor with BDD electrode: comparison of two electrochemical systems.” membrane-separated organic electrosynthesis: the case of adiponitrile electro- Chemosphere, 201, 816 (2018). chemical production.” React. Chem. Eng., 5, 136 (2020). 39. C. Ponce de León, F. C. Walsh, D. Pletcher, D. J. Browning, and J. B. Lakeman, 9. C. Ponce de León, G. W. Reade, I. Whyte, S. E. Male, and F. C. Walsh, “Direct borohydride fuel cells.” J. Power Sources, 155, 172 (2006). “Characterization of the reaction environment in a filter-press redox flow reactor.” 40. S. S. Daud, M. A. Norrdin, J. Jaafar, and R. Sudirman, “The effect of material on Electrochim. Acta, 52, 5815 (2007). bipolar membrane fuel cell performance: a review.” IOP Conf. Ser.: Mater. Sci. 10. R. G. A. Wills and F. C. Walsh, “7 - electroplating for protection against wear.” Eng., 736, 032003 (2020). Surface Coatings for Protection Against Wear, ed. B. G. Mellor (Woodhead 41. M. A. Blommaert, J. A. H. Verdonk, H. C. B. Blommaert, W. A. Smith, and Publishing, Cambridge) p. 226 (2006). D. A. Vermaas, “Reduced ion crossover in bipolar membrane electrolysis via 11. U. Landau, “Three-electrode measurements in industrial cells.” J. Electrochem. increased current density, molecular size, and valence.” ACS Appl. Energy Mater., Soc., 135, 396 (1988). 3, 5804 (2020). Journal of The Electrochemical Society, 2020 167 155525 42. C. Shen, R. Wycisk, and P. N. Pintauro, “High performance electrospun bipolar 72. M. A. Cataldo-Hernández, A. Bonakdarpour, J. T. English, M. Mohseni, and membrane with a 3D junction.” Energy Environ. Sci., 10, 1435 (2017). D. P. Wilkinson, “A membrane-based electrochemical flow reactor for generation 43. R. Alkire and P. K. Ng, “Studies on flow‐by porous electrodes having of ferrates at near neutral pH conditions.” React. Chem. Eng., 4, 1116 (2019). perpendicular directions of current and electrolyte flow.” J. Electrochem. Soc., 73. S. C. Perry, D. Pangotra, L. Vieira, L.-I. Csepei, V. Sieber, L. Wang, C. Ponce de 124, 1220 (1977). León, and F. C. Walsh, “Electrochemical synthesis of hydrogen peroxide from 44. C. Ponce de León, I. Whyte, G. W. Reade, S. E. Male, and F. C. Walsh, “Mass water and oxygen.” Nat. Rev. Chem., 3, 442 (2019). transport and flow dispersion in the compartments of a modular 10 cell filter-press 74. R. A. Green, R. C. D. Brown, and D. Pletcher, “Understanding the performance of stack.” Aust. J. Chem., 61, 797 (2008). a microfluidic electrolysis cell for routine organic electrosynthesis.” J. Flow 45. I. Garagounis, A. Vourros, D. Stoukides, D. Dasopoulos, and M. Stoukides, Chem., 5, 31 (2015). “Electrochemical synthesis of ammonia: recent efforts and future outlook.” 75. R. A. Green, R. C. D. Brown, D. Pletcher, and B. Harji, “An extended channel Membranes, 9, 112 (2019). length microflow electrolysis cell for convenient laboratory synthesis.” 46. R. Kas, K. Yang, D. Bohra, R. Kortlever, T. Burdyny, and W. Smith, Electrochem. Commun., 73, 63 (2016). “Electrochemical CO reduction on nanostructured metal electrodes: fact or 76. L. F. Arenas, F. C. Walsh, and C. P. de León, “3D-printing of redox flow batteries defect?” Chem. Sci., 11, 1738 (2020). for energy storage: a rapid prototype laboratory cell.” ECS J. Solid State SC, 4, 47. C. Wang et al., “Construction of a microchannel electrochemical reactor with a P3080 (2015). monolithic porous-carbon cathode for adsorption and degradation of organic 77. H. Piri, X. T. Bi, H. Li, and H. Wang, “3D-printed fuel-cell bipolar plates for pollutants in several minutes of retention time.” Environ. Sci. Technol., 54, 1920 evaluating flow-field performance.” Clean Energy, 4, 142 (2020). (2020). 78. J. R. Hudkins, D. G. Wheeler, B. Peña, and C. P. Berlinguette, “Rapid prototyping 48. M. A. Khan, H. Zhao, W. Zou, Z. Chen, W. Cao, J. Fang, J. Xu, L. Zhang, and of electrolyzer flow field plates.” Energy Environ. Sci., 9, 3417 (2016). J. Zhang, “Recent progresses in electrocatalysts for water electrolysis.” 79. T. Pérez, L. F. Arenas, D. Villalobos-Lara, N. Zhou, S. Wang, F. C. Walsh, Electrochem. Energy Rev., 1, 483 (2018). J. L. Nava, and C. P. de León, “Simulations of fluid flow, mass transport and 49. R. Alkire and B. Gracon, “Flow‐through porous electrodes.” J. Electrochem. Soc., current distribution in a parallel plate flow cell during nickel electrodeposition.” 122, 1594 (1975). J. Electroanal. Chem., 873, 114359 (2020). 50. L. F. Arenas, C. P. d. León, and F. C. Walsh, “Mass transport and active area of 80. C. Ponce de León, W. Hussey, F. Frazao, D. Jones, E. Ruggeri, S. Tzortzatos, porous Pt/Ti electrodes for the Zn-Ce redox flow battery determined from limiting R. D. Mckerracher, R. G. A. Wills, S. Yang, and F. C. Walsh, “The 3D printing of current measurements.” Electrochim. Acta, 221, 154 (2016). a polymeric electrochemical cell body and its characterisation.” Chem. Eng. 51. L. F. Castañeda, F. C. Walsh, J. L. Nava, and C. Ponce de León, “Graphite felt as a Trans., 41, 1 (2014). versatile electrode material: properties, reaction environment, performance and 81. S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, and R. Mülhaupt, “Polymers for 3D applications.” Electrochim. Acta, 258, 1115 (2017). printing and customized additive manufacturing.” Chem. Rev., 117, 10212 (2017). 52. I. Mustafa, R. Susantyoko, C.-H. Wu, F. Ahmed, R. Hashaikeh, F. Almarzooqi, 82. L. F. Arenas, N. Kaishubayeva, C. Ponce de León, and F. C. Walsh, and S. Almheiri, “Nanoscopic and macro-porous carbon nano-foam electrodes “Electrodeposition of platinum on 3D-printed titanium mesh to produce tailored, with improved mass transport for vanadium redox flow batteries.” Sci. Rep., 9, high area anodes.” Trans. Inst. Met. Finish., 98, 48 (2020). 17655 (2019). 83. J. C. Bui, J. T. Davis, and D. V. Esposito, “3D-printed electrodes for membrane- 53. L. F. Arenas, R. P. Boardman, C. Ponce de León, and F. C. Walsh, “X-ray less water electrolysis.” Sustain. Energy Fuels, 4, 213 (2020). computed micro-tomography of reticulated vitreous carbon.” Carbon, 135,85 84. J. P. Hughes, P. L. dos Santos, M. P. Down, C. W. Foster, J. A. Bonacin, E. (2018). M. Keefe, S. J. Rowley-Neale, and C. E. Banks, “Single step additive 54. L. F. Arenas, C. Ponce de León, R. P. Boardman, and F. C. Walsh, manufacturing (3D printing) of electrocatalytic anodes and cathodes for efficient “Characterisation of platinum electrodeposits on a titanium micromesh stack in water splitting.” Sustain. Energy Fuels, 4, 302 (2020). a rectangular channel flow cell.” Electrochim. Acta, 247, 994 (2017). 85. Q. Sun, J. Wang, M. Tang, L. Huang, Z. Zhang, C. Liu, X. Lu, K. W. Hunter, and 55. L. F. Arenas, C. P. de León, R. P. Boardman, and F. C. Walsh, “Editors’ choice - G. Chen, “A new electrochemical system based on a flow-field shaped solid 2+ electrodeposition of platinum on titanium felt in a rectangular channel flow cell.” electrode and 3D-printed thin-layer flow cell: detection of Pb ions by continuous J. Electrochem. Soc., 164, D57 (2016). flow accumulation square-wave anodic stripping voltammetry.” Anal. Chem., 89, 56. M.-S. Park, N.-J. Lee, S.-W. Lee, K. J. Kim, D.-J. Oh, and Y.-J. Kim, “High- 5024 (2017). energy redox-flow batteries with hybrid metal foam electrodes.” ACS Appl. Mater. 86. G. D. O’Neil, S. Ahmed, K. Halloran, J. N. Janusz, A. Rodríguez, and I. Interfaces, 6, 10729 (2014). M. Terrero Rodríguez, “Single-step fabrication of electrochemical flow cells 57. L. F. Arenas, C. Ponce de León, and F. C. Walsh, “Critical review - the versatile utilizing multi-material 3D printing.” Electrochem. Commun., 99, 56 (2019). plane parallel electrode geometry: an illustrated review.” J. Electrochem. Soc., 87. G. Chisholm, P. J. Kitson, N. D. Kirkaldy, L. G. Bloor, and L. Cronin, “3D printed 167, 023504 (2020). flow plates for the electrolysis of water: an economic and adaptable approach to 58. V. Egorov and C. O’Dwyer, “Architected porous metals in electrochemical energy device manufacture.” Energy Environ. Sci., 7, 3026 (2014). storage.” Curr. Opin. Electrochem., 21, 201 (2020). 88. H. H. Hamzah, S. A. Shafiee, A. Abdalla, and B. A. Patel, “3D printable 59. Z. Liu et al., “Three-dimensional ordered porous electrode materials for electro- conductive materials for the fabrication of electrochemical sensors: a mini chemical energy storage.” NPG Asia Mater., 11, 12 (2019). review.” Electrochem. Commun., 96, 27 (2018). 60. L. F. Arenas, C. Ponce de León, and F. C. Walsh, “Three-dimensional porous 89. D. Krishnamurthy, E. O. Johansson, J. W. Lee, and E. Kjeang, “Computational metal electrodes: fabrication, characterisation and use.” Curr. Opin. Electrochem., modeling of microfluidic fuel cells with flow-through porous electrodes.” J. Power 16, 1 (2019). Sources, 196, 10019 (2011). 61. W. Tiedemann and J. Newman, “Maximum effective capacity in an ohmically 90. E. Kjeang, R. Michel, D. A. Harrington, N. Djilali, and D. Sinton, “Amicrofluidic limited porous electrode.” J. Electrochem. Soc., 122, 1482 (1975). fuel cell with flow-through porous electrodes.” J. Am. Chem. Soc., 130, 4000 (2008). 62. J. L. Nava, M. T. Oropeza, C. Ponce de León, J. González-García, and A. J. Frías- 91. L. Li, K. Zheng, M. Ni, M. K. H. Leung, and J. Xuan, “Partial modification of Ferrer, “Determination of the effective thickness of a porous electrode in a flow- flow-through porous electrodes in microfluidic fuel cell.” Energy, 88, 563 (2015). through reactor; effect of the specific surface area of stainless steel fibres, used as a 92. T. Hong,S. Lee,P.Ohodnicki,and K. Brinkman, “A highly scalable spray coating porous cathode, during the deposition of Ag(I) ions.” Hydrometallurgy, 91,98 technique for electrode infiltration: barium carbonate infiltrated La Sr Co Fe O 0.6 0.4 0.2 0.8 3-δ (2008). perovskite structured electrocatalyst with demonstrated long term durability.” Int. J. 63. P. Lobaccaro, M. R. Singh, E. L. Clark, Y. Kwon, A. T. Bell, and J. W. Ager, Hydrogen Energy, 42, 24978 (2017). “Effects of temperature and gas–liquid mass transfer on the operation of small 93. T. Li, E. W. Lees, M. Goldman, D. A. Salvatore, D. M. Weekes, and electrochemical cells for the quantitative evaluation of CO reduction electro- C. P. Berlinguette, “Electrolytic conversion of bicarbonate into CO in a flow catalysts.” Phys. Chem. Chem. Phys., 18, 26777 (2016). cell.” Joule, 3, 1487 (2019). 64. J. O. M. Bockris and B. E. Conway, Modern Aspects of Electrochemistry No. 6 94. L. Fan, C. Xia, F. Yang, J. Wang, H. Wang, and Y. Lu, “Strategies in catalysts and (Springer, US) (2012). electrolyzer design for electrochemical CO( ) reduction toward C( ) products.” 2 2+ 65. G. Hilt, “Basic strategies and types of applications in organic electrochemistry.” Sci. Adv., 6, eaay3111 (2020). ChemElectroChem, 7, 395 (2020). 95. B. Tjaden, D. J. L. Brett, and P. R. Shearing, “Tortuosity in electrochemical 66. F. F. Rivera, C. P. d. León, F. C. Walsh, and J. L. Nava, “The reaction devices: a review of calculation approaches.” Int. Mater. Rev., 63, 47 (2018). environment in a filter-press laboratory reactor: the FM01-LC flow cell.” 96. S. Kumar, T. Ramamurthy, B. Subramanian, and A. Basha, “Studies on the Electrochim. Acta, 161, 436 (2015). fluidized bed electrode.” Int. J. Chem. React. Eng., 6, 1 (2008). 67. K. Scott, Sustainable and Green Electrochemical Science and Technology (Wiley, 97. A. Tschöpe, S. Heikenwälder, M. Schneider, K. Mandel, and M. Franzreb, New York) (2017). “Electrical conductivity of magnetically stabilized fluidized-bed electrodes— 68. M. Lehmann, C. C. Scarborough, E. Godineau, and C. Battilocchio, “An chronoamperometric and impedance studies.” Chem. Eng. J., 396, 125326 (2020). electrochemical flow-through cell for rapid reactions.” Ind. Eng. Chem. Res., 59, 98. A. P. Manso, F. F. Marzo, J. Barranco, X. Garikano, and M. Garmendia Mujika, 7321 (2020). “Influence of geometric parameters of the flow fields on the performance of a PEM 69. T. Noël, Y. Cao, and G. Laudadio, “The fundamentals behind the use of flow fuel cell. A review.” Int. J. Hydrogen Energy, 37, 15256 (2012). reactors in electrochemistry.” Acc. Chem. Res., 52, 2858 (2019). 99. H. Liu, P. Li, D. Juarez-Robles, K. Wang, and A. Hernandez-Guerrero, 70. R. K. B. Karlsson and A. Cornell, “Selectivity between oxygen and chlorine “Experimental study and comparison of various designs of gas flow fields to evolution in the chlor-alkali and chlorate processes.” Chem. Rev., 116, 2982 PEM fuel cells and cell stack performance.” Front. Energy Res., 2, 2 (2014). (2016). 100. C.-T. Wang, Y.-T. Ou, B.-X. Wu, S. Thangavel, S.-W. Hong, W.-T. Chung, and 71. M.-A. Goulet and E. Kjeang, “Reactant recirculation in electrochemical co- W.-M. Yan, “A modified serpentine flow slab for in proton exchange membrane laminar flow cells.” Electrochim. Acta, 140, 217 (2014). fuel cells (PEMFCs).” Energy Proc., 142, 667 (2017). Journal of The Electrochemical Society, 2020 167 155525 101. R. Gundlapalli and S. Jayanti, “Performance characteristics of several variants of 128. M. Lee and X. Huang, “Development of a hydrophobic coating for the porous gas interdigitated flow fields for flow battery applications.” J. Power Sources, 467, diffusion layer in a PEM-based electrochemical hydrogen pump to mitigate anode 228225 (2020). flooding.” Electrochem. Commun., 100, 39 (2019). 102. V. Manzi-Orezzoli, M. Siegwart, M. Cochet, T. J. Schmidt, and P. Boillat, 129. Q. Zhang, M. Zhou, G. Ren, Y. Li, Y. Li, and X. Du, “Highly efficient “Improved water management for PEFC with interdigitated flow fields using electrosynthesis of hydrogen peroxide on a superhydrophobic three-phase inter- modified gas diffusion layers.” J. Electrochem. Soc., 167, 054503 (2019). face by natural air diffusion.” Nat. Commun., 11, 1731 (2020). 103. X. You, Q. Ye, and P. Cheng, “Scale-up of high power density redox flow batteries 130. A. Xu, B. He, H. Yu, W. Han, J. Li, J. Shen, X. Sun, and L. Wang, “A facile by introducing interdigitated flow fields.” Int. Commun. Heat Mass, 75, 7 (2016). solution to mature cathode modified by hydrophobic dimethyl silicon oil (DMS) 104. M. R. Gerhardt, A. A. Wong, and M. J. Aziz, “The effect of interdigitated channel layer for electro-fenton processes: water proof and enhanced oxygen transport.” and land dimensions on flow cell performance.” J. Electrochem. Soc., 165, A2625 Electrochim. Acta, 308, 158 (2019). (2018). 131. W. V. Fernandez, R. T. Tosello, and J. L. Fernández, “Compact and efficient gas 105. Y. Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, and B. Xiang, “A hierarchical diffusion electrodes based on nanoporous alumina membranes for microfuel cells interdigitated flow field design for scale-up of high-performance redox flow and gas sensors.” Analyst, 145, 122 (2020). batteries.” Appl. Energy, 238, 435 (2019). 132. C.-T. Dinh et al., “CO electroreduction to ethylene via hydroxide-mediated 106. N. J. Cooper, A. D. Santamaria, M. K. Becton, and J. W. Park, “Investigation of copper catalysis at an abrupt interface.” Science, 360, 783 (2018). the performance improvement in decreasing aspect ratio interdigitated flow field 133. G. Chen, G. Zhang, L. Guo, and H. Liu, “Systematic study on the functions and PEMFCs.” Energy Convers. Manage., 136, 307 (2017). mechanisms of micro porous layer on water transport in proton exchange 107. V. Manzi-Orezzoli, M. Siegwart, D. Scheuble, Y.-C. Chen, T. J. Schmidt, and membrane fuel cells.” Int. J. Hydrogen Energy, 41, 5063 (2016). P. Boillat, “Impact of the microporous layer on gas diffusion layers with patterned 134. S. Park, J.-W. Lee, and B. N. Popov, “A review of gas diffusion layer in PEM fuel wettability I: material design and characterization.” J. Electrochem. Soc., 167, cells: materials and designs.” Int. J. Hydrogen Energy, 37, 5850 (2012). 064516 (2020). 135. R. Sandström, J. Ekspong, A. Annamalai, T. Sharifi, A. Klechikov, and 108. D. Niblett, A. Mularczyk, V. Niasar, J. Eller, and S. Holmes, “Two-phase flow T. Wågberg, “Fabrication of microporous layer—free hierarchical gas diffusion dynamics in a gas diffusion layer—gas channel—microporous layer system.” electrode as a low Pt-loading PEMFC cathode by direct growth of helical carbon J. Power Sources, 471, 228427 (2020). nanofibers.” RSC Adv., 8, 41566 (2018). 109. R. Saravanakumar, P. Pirabaharan, M. Abukhaled, and L. Rajendran, “Theoretical 136. S. C. Perry, S. M. Gateman, R. Malpass-Evans, N. McKeown, M. Wegener, analysis of voltammetry at a rotating disk electrode in the absence of supporting P. Nazarovs, J. Mauzeroll, L. Wang, and C. Ponce de León, “Polymers with electrolyte.” J. Phys. Chem. B, 124, 443 (2020). intrinsic microporosity (PIMs) for targeted CO reduction to ethylene.” 110. V. G. Levich and S. T. Ltd, Physicochemical Hydrodynamics (Prentice-Hall, Chemosphere, 248, 125993 (2020). Englewood Cliffs, NJ) (1962). 137. D. Horii, T. Fuchigami, and M. Atobe, “A new approach to anodic substitution 111. M. D. Pohl, S. Haschke, D. Göhl, O. Kasian, J. Bachmann, K. J. J. Mayrhofer, and reaction using parallel laminar flow in a micro-flow reactor.” J. Am. Chem. Soc., I. Katsounaros, “Extension of the rotating disk electrode method to thin samples of 129, 11692 (2007). non-disk shape.” J. Electrochem. Soc., 166, H791 (2019). 138. D. Wang, P. Wang, S. Wang, Y.-H. Chen, H. Zhang, and A. Lei, “Direct 112. M. Rosales and J. L. Nava, “Simulations of turbulent flow, mass transport, and electrochemical oxidation of alcohols with hydrogen evolution in continuous-flow tertiary current distribution on the cathode of a rotating cylinder electrode reactor reactor.” Nat. Commun., 10, 2796 (2019). in continuous operation mode during silver deposition.” J. Electrochem. Soc., 164, 139. C. Huang, X.-Y. Qian, and H.-C. Xu, “Continuous-flow electrosynthesis of E3345 (2017). benzofused S-heterocycles by dehydrogenative C−S cross-coupling.” Angew. 113. F. C. Walsh, G. Kear, A. H. Nahlé, J. A. Wharton, and L. F. Arenas, “The rotating Chem. Int. Ed., 58, 6650 (2019). cylinder electrode for studies of corrosion engineering and protection of metals— 140. B. Gleede, M. Selt, C. Gütz, A. Stenglein, and S. R. Waldvogel, “Large, highly an illustrated review.” Corros. Sci., 123, 1 (2017). modular narrow-gap electrolytic flow cell and application in dehydrogenative 114. D. P. Barkey, R. H. Muller, and C. W. Tobias, “Roughness development in metal cross-coupling of phenols.” Org. Process Res. Dev., 24, 1916 (2020). electrodeposition: I. Experimental results.” J. Electrochem. Soc., 136, 2199 141. D. E. Collin, A. A. Folgueiras-Amador, D. Pletcher, M. E. Light, B. Linclau, and (1989). R. C. D. Brown, “Cubane electrochemistry: direct conversion of cubane carboxylic 115. Z. Solomenko, Y. Haroun, M. Fourati, F. Larachi, C. Boyer, and F. Augier, acids to alkoxy cubanes using the hofer–moest reaction under flow conditions.” “Liquid spreading in trickle-bed reactors: experiments and numerical simulations Chem. Eur. J., 26, 374 (2020). using eulerian–eulerian two-fluid approach.” Chem. Eng. Sci., 126, 698 (2015). 142. M. R. Chapman, S. E. Henkelis, N. Kapur, B. N. Nguyen, and C. E. Willans, 116. P. Trinidad, F. C. Walsh, S. A. Sheppard, S. P. Gillard, and S. A. Campbell, “The “A straightforward electrochemical approach to imine- and amine-bisphenolate effect of operational parameters on the performance of a bipolar trickle tower metal complexes with facile control over metal oxidation state.” ChemistryOpen, reactor.” J. Chem. Technol. Biotechnol., 79, 954 (2004). 5, 351 (2016). 117. Y. Yavuz, A. S. Koparal, and Ü. B. Öğütveren, “Phenol degradation in a bipolar 143. A. E. Delorme, V. Sans, P. Licence, and D. A. Walsh, “Tuning the reactivity of trickle tower reactor using boron-doped diamond electrode.” J. Environ. Eng., 134, tempo during electrocatalytic alcohol oxidations in room-temperature ionic 24 (2008). liquids.” ACS Sustain. Chem. Eng., 7, 11691 (2019). 118. M. A. Islam, S. C. Lam, Y. Li, M. A. Atia, P. Mahbub, P. N. Nesterenko, B. Paull, 144. M. B. Plutschack, B. Pieber, K. Gilmore, and P. H. Seeberger, “The Hitchhiker’s and M. Macka, “Capillary gap flow cell as capillary-end electrochemical detector guide to flow chemistry.” Chem. Rev., 117, 11796 (2017). in flow-based analysis.” Electrochim. Acta, 303, 85 (2019). 145. Y. Liu, G. Chen, and J. Yue, “Manipulation of gas-liquid-liquid systems in continuous 119. A. Fankhauser, L. Ouattara, U. Griesbach, A. Fischer, H. Pütter, and flow microreactors for efficient reaction processes.” J. Flow Chem., 10, 103 (2020). C. Comninellis, “Investigation of the anodic acetoxylation of p-methylanisole 146. D. Karan and S. Khan, “Mesoscale triphasic flow reactors for metal catalyzed 2 3 (p-MA) in glacial acetic acid medium using graphite (sp ) and BDD (sp ) gas–liquid reactions.” React. Chem. Eng., 4, 1331 (2019). electrodes.” J. Electroanal. Chem., 614, 107 (2008). 147. W. Guo, H. J. Heeres, and J. Yue, “Continuous synthesis of 5-hydroxymethyl- 120. J.-I. Yoshida, H. Kim, and A. Nagaki, “‘Impossible’ chemistries based on flow and furfural from glucose using a combination of AlCl and HCl as catalyst in a micro.” J. Flow Chem., 7, 60 (2017). biphasic slug flow capillary microreactor.” Chem. Eng. J., 381, 122754 (2020). 121. C. A. Paddon, M. Atobe, T. Fuchigami, P. He, P. Watts, S. J. Haswell, 148. G. Laudadio, E. Barmpoutsis, C. Schotten, L. Struik, S. Govaerts, D. L. Browne, G. J. Pritchard, S. D. Bull, and F. Marken, “Towards paired and coupled electrode and T. Noël, “Sulfonamide synthesis through electrochemical oxidative coupling reactions for clean organic microreactor electrosyntheses.” J. Appl. Electrochem., of amines and thiols.” J. Am. Chem. Soc., 141, 5664 (2019). 36, 617 (2006). 149. S. Momeni and D. Nematollahi, “Electrosynthesis of new quinone sulfonimide 122. A. Jayakumar, S. Singamneni, M. Ramos, A. M. Al-Jumaily, and S. S. Pethaiah, derivatives using a conventional batch and a new electrolyte-free flow cell.” Green “Manufacturing the gas diffusion layer for PEM fuel cell using a novel 3D printing Chem., 20, 4036 (2018). technique and critical assessment of the challenges encountered.” Materials, 10, 150. F. García-Moreno, “Commercial applications of metal foams: their properties and 796 (2017). production.” Materials, 9, 85 (2016). 123. T. Pérez, G. Coria, I. Sirés, J. L. Nava, and A. R. Uribe, “Electrosynthesis of 151. P. Zhu and Y. Zhao, “Mass transfer performance of porous nickel manufactured by hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion lost carbonate sintering process.” Adv. Eng. Mater., 19, 1700392 (2017). cathode.” J. Electroanal. Chem., 812, 54 (2018). 152. X. Huang, S. Chang, W. S. V. Lee, J. Ding, and J. Xue, “Three-dimensional 124. M. Koj, J. Qian, and T. Turek, “Novel alkaline water electrolysis with nickel-iron printed cellular stainless steel as high-activity catalytic electrode for oxygen gas diffusion electrode for oxygen evolution.” Int. J. Hydrogen Energy, 44, 29862 evolution.” J. Mater. Chem. A, 5, 18176 (2017). (2019). 153. J. Lölsberg, O. Starck, S. Stiefel, J. Hereijgers, T. Breugelmans, and M. Wessling, 125. S. C. Perry, P.-K. Leung, L. Wang, and C. Ponce de León, “Developments on “3D-printed electrodes with improved mass transport properties.” ChemElectroChem, carbon dioxide reduction: their promise, achievements, and challenges.” Curr. 4, 3309 (2017). Opin. Electrochem., 20, 88 (2020). 154. S. P. Zankowski and P. M. Vereecken, “Combining high porosity with high 126. R. B. Ferreira, D. S. Falcão, V. B. Oliveira, and A. M. F. R. Pinto, “Experimental surface area in flexible interconnected nanowire meshes for hydrogen generation study on the membrane electrode assembly of a proton exchange membrane fuel and beyond.” ACS Appl. Mater. Interfaces, 10, 44634 (2018). cell: effects of microporous layer, membrane thickness and gas diffusion layer 155. M. J. Kim, Y. Seo, M. A. Cruz, and B. J. Wiley, “Metal nanowire felt as a flow- hydrophobic treatment.” Electrochim. Acta, 224, 337 (2017). through electrode for high-productivity electrochemistry.” ACS Nano, 13, 6998 127. R. Omrani and B. Shabani, “Gas diffusion layer modifications and treatments for (2019). improving the performance of proton exchange membrane fuel cells and 156. R. Kas, K. K. Hummadi, R. Kortlever, P. de Wit, A. Milbrat, M. W. J. Luiten- electrolysers: a review.” Int. J. Hydrogen Energy, 42, 28515 (2017). Olieman, N. E. Benes, M. T. M. Koper, and G. Mul, “Three-dimensional porous Journal of The Electrochemical Society, 2020 167 155525 hollow fibre copper electrodes for efficient and high-rate electrochemical carbon 181. L. C. Brée, M. Wessling, and A. Mitsos, “Modular modeling of electrochemical dioxide reduction.” Nat. Commun., 7, 10748 (2016). reactors: comparison of CO -electolyzers.” Comput. Chem. Eng., 139, 106890 157. R. Poupart, B. Le Droumaguet, M. Guerrouache, D. Grande, and B. Carbonnier, (2020). “Gold nanoparticles immobilized on porous monoliths obtained from disulfide- 182. L. F. Catañeda, F. F. Rivera, T. Pérez, and J. L. Nava, “Mathematical modeling based dimethacrylate: application to supported catalysis.” Polymer, 126, 455 and simulation of the reaction environment in electrochemical reactors.” Curr. (2017). Opin. Electrochem., 16, 75 (2019). 158. L. Wan, Y. Qin, and J. Xiang, “Rapid electrochemical fabrication of porous gold 183. J. N. Hakizimana, B. Gourich, M. Chafi, Y. Stiriba, C. Vial, P. Drogui, and J. Naja, nanoparticles for high-performance electrocatalysis towards oxygen reduction.” “Electrocoagulation process in water treatment: a review of electrocoagulation Electrochim. Acta, 238, 220 (2017). modeling approaches.” Desalination, 404, 1 (2017). 159. M. Zaghdoudi, L. Moreaud, P. Even-Hernandez, V. Marchi, F. Fourcade, 184. A. Roldan, “Frontiers in first principles modelling of electrochemical simulations.” A. Amrane, H. Maghraoui-Meherzi, and F. Geneste, “Immobilization of synthetic Curr. Opin. Electrochem., 10, 1 (2018). gold nanoparticles on a three-dimensional porous electrode.” Electrochem. 185. L. I. Stephens and J. Mauzeroll, “Demystifying mathematical modeling of Commun., 88, 15 (2018). electrochemical systems.” J. Chem. Educ., 96, 2217 (2019). 160. V. Vedharathinam, Z. Qi, C. Horwood, B. Bourcier, M. Stadermann, J. Biener, and 186. A. Taqieddin, R. Nazari, L. Rajic, and A. Alshawabkeh, “Review - physicochem- M. Biener, “Using a 3D porous flow-through electrode geometry for high-rate ical hydrodynamics of gas bubbles in two phase electrochemical systems.” electrochemical reduction of CO to CO in ionic liquid.” ACS Catal., 9, 10605 J. Electrochem. Soc., 164, E448 (2017). (2019). 187. M. R. Cruz-Díaz, E. P. Rivero, F. A. Rodríguez, and R. Domínguez-Bautista, 161. E. Verlato, W. He, A. Amrane, S. Barison, D. Floner, F. Fourcade, F. Geneste, “Experimental study and mathematical modeling of the electrochemical degrada- M. Musiani, and R. Seraglia, “Preparation of silver-modified nickel foams by tion of dyeing wastewaters in presence of chloride ion with dimensional stable galvanic displacement and their use as cathodes for the reductive dechlorination of anodes (DSA) of expanded meshes in a FM01-LC reactor.” Electrochim. Acta, herbicides.” ChemElectroChem, 3, 2084 (2016). 260, 726 (2018). 162. U. Rost, P. Podleschny, M. Schumacher, R. Muntean, D. T. Pascal, C. Mutascu, 188. K. Wu, E. Birgersson, B. Kim, P. J. A. Kenis, and I. A. Karimi, “Modeling and J. Koziolek, G. Marginean, and M. Brodmann, “Long-term stable electrodes based experimental validation of electrochemical reduction of CO to CO in a on platinum electrocatalysts supported on titanium sintered felt for the use in PEM microfluidic cell.” J. Electrochem. Soc., 162, F23 (2014). fuel cells.” IOP Conf. Ser.: Mater. Sci. Eng., 416, 012013 (2018). 189. M. R. Cruz-Díaz, E. P. Rivero, F. J. Almazán-Ruiz, Á. Torres-Mendoza, and 163. C. A. Martins, O. A. Ibrahim, P. Pei, and E. Kjeang, “In situ decoration of metallic I. González, “Design of a new FM01-LC reactor in parallel plate configuration catalysts in flow-through electrodes: application of Fe/Pt/C for glycerol oxidation using numerical simulation and experimental validation with residence time in a microfluidic fuel cell.” Electrochim. Acta, 305, 47 (2019). distribution (RTD).” Chem. Eng. Process., 85, 145 (2014). 164. N. Sergienko and J. Radjenovic, “Manganese oxide-based porous electrodes for 190. L. I. Stephens, S. C. Perry, S. M. Gateman, R. Lacasse, R. Schulz, and rapid and selective (electro)catalytic removal and recovery of sulfide from J. Mauzeroll, “Development of a model for experimental data treatment of wastewater.” Appl. Catal., B, 267, 118608 (2020). diffusion and activation limited polarization curves for magnesium and steel 165. C. Wang, L. Yue, S. Wang, Y. Pu, X. Zhang, X. Hao, W. Wang, and S. Chen, alloys.” J. Electrochem. Soc., 164, E3576 (2017). “Role of electric field and reactive oxygen species in enhancing antibacterial 191. A. N. Colli and H. H. Girault, “Compact and general strategy for solving current activity: a case study of 3D Cu foam electrode with branched CuO–ZnO NWs.” and potential distribution in electrochemical cells composed of massive monopolar J. Phys. Chem. C, 122, 26454 (2018). and bipolar electrodes.” J. Electrochem. Soc., 164, E3465 (2017). 166. F. A. Lowenheim and J. Davis, “Modern electroplating.” J. Electrochem. Soc., 192. S. Li and B. Sundén, “Effects of gas diffusion layer deformation on the transport 121, 314 (1974). phenomena and performance of PEM fuel cells with interdigitated flow fields.” Int. 167. G. Sievers, T. Vidakovic-Koch, C. Walter, F. Steffen, S. Jakubith, A. Kruth, J. Hydrogen Energy, 43, 16279 (2018). D. Hermsdorf, K. Sundmacher, and V. Brüser, “Ultra-low loading Pt-sputtered gas 193. R. Cervantes-Alcalá and M. Miranda-Hernández, “Flow distribution and mass diffusion electrodes for oxygen reduction reaction.” J. Appl. Electrochem., 48, 221 transport analysis in cell geometries for redox flow batteries through computa- (2018). tional fluid dynamics.” J. Appl. Electrochem., 48, 1243 (2018). 168. J. M. Roemers-van Beek, Z.-J. Wang, A. Rinaldi, M. G. Willinger, and L. Lefferts, 194. A. S. Danis, W. L. Odette, S. C. Perry, S. Canesi, H. F. Sleiman, and J. Mauzeroll, “Initiation of carbon nanofiber growth on polycrystalline nickel foam under low “Cuvette-based electrogenerated chemiluminescence detection system for the ethylene pressure.” ChemCatChem, 10, 3107 (2018). assessment of polymerizable ruthenium luminophores.” ChemElectroChem, 4, 169. R. Mao, C. Huang, X. Zhao, M. Ma, and J. Qu, “Dechlorination of triclosan by 1736 (2017). enhanced atomic hydrogen-mediated electrochemical reduction: kinetics, me- 195. M. A. Sandoval, R. Fuentes, F. C. Walsh, J. L. Nava, and C. P. de León, chanism, and toxicity assessment.” Appl. Catal., B, 241, 120 (2019). “Computational fluid dynamics simulations of single-phase flow in a filter-press 170. M. J. Harding, S. Brady, H. O’Connor, R. Lopez-Rodriguez, M. D. Edwards, flow reactor having a stack of three cells.” Electrochim. Acta, 216, 490 (2016). S. Tracy, D. Dowling, G. Gibson, K. P. Girard, and S. Ferguson, “3D printing of 196. L. F. Castañeda and J. L. Nava, “Simulations of single-phase flow in an up-flow peek reactors for flow chemistry and continuous chemical processing.” React. electrochemical reactor with parallel plate electrodes in a serpentine array.” Chem. Eng., 5, 728 (2020). J. Electroanal. Chem., 832, 31 (2019). 171. M. C. Maier et al., “Development of customized 3D printed stainless steel reactors 197. M. Movahedi, A. Ramiar, and A. A. Ranjber, “3D numerical investigation of with inline oxygen sensors for aerobic oxidation of grignard reagents in clamping pressure effect on the performance of proton exchange membrane fuel continuous flow.” React. Chem. Eng., 4, 393 (2019). cell with interdigitated flow field.” Energy, 142, 617 (2018). 172. A. Ambrosi and R. D. Webster, “3D printing for aqueous and non-aqueous redox 198. T. Noyhouzer, S. C. Perry, A. Vicente-Luis, P. L. Hayes, and J. Mauzeroll, “The flow batteries.” Curr. Opin. Electrochem., 20, 28 (2020). best of both worlds: combining ultramicroelectrode and flow cell technologies.” 173. B. Gutmann, M. Köckinger, G. Glotz, T. Ciaglia, E. Slama, M. Zadravec, J. Electrochem. Soc., 165, H10 (2018). S. Pfanner, M. C. Maier, H. Gruber-Wölfler, and C. Oliver Kappe, “Design and 199. E. P. Rivero, F. A. Rodríguez, M. R. Cruz-Díaz, and I. González, “Reactive 3D printing of a stainless steel reactor for continuous difluoromethylations using diffusion migration layer and mass transfer wall function to model active chlorine fluoroform.” React. Chem. Eng., 2, 919 (2017). generation in a filter press type electrochemical reactor for organic pollutant 174. R. M. Cardoso, D. M. H. Mendonça, W. P. Silva, M. N. T. Silva, E. Nossol, R. A. degradation.” Chem. Eng. Res. Des., 138, 533 (2018). B. da Silva, E. M. Richter, and R. A. A. Muñoz, “3D printing for electroanalysis: 200. F. F. Rivera, L. Castañeda, P. E. Hidalgo, and G. Orozco, “Study of hydro- from multiuse electrochemical cells to sensors.” Anal. Chim. Acta, 1033,49 dynamics at asahitm prototype electrochemical flow reactor, using computational (2018). fluid dynamics and experimental characterization techniques.” Electrochim. Acta, 175. C. G. W. van Melis, M. R. Penny, A. D. Garcia, A. Petti, A. P. Dobbs, S. T. Hilton, 245, 107 (2017). and K. Lam, “Supporting-electrolyte-free electrochemical methoxymethylation of 201. D. Leech, P. Kavanagh, and W. Schuhmann, “Enzymatic fuel cells: recent alcohols using a 3D-printed electrosynthesis continuous flow cell system.” progress.” Electrochim. Acta, 84, 223 (2012). ChemElectroChem, 6, 4144 (2019). 202. P. Pinyou, V. Blay, L. M. Muresan, and T. Noguer, “Enzyme-modified electrodes 176. L. F. Arenas, C. Ponce de León, and F. C. Walsh, “3D-printed porous electrodes for biosensors and biofuel cells.” Mater. Horiz., 6, 1336 (2019). for advanced electrochemical flow reactors: a Ni/stainless steel electrode and its 203. M. A. Dwyer and H. W. Hellinga, “Periplasmic binding proteins: a versatile mass transport characteristics.” Electrochem. Commun., 77, 133 (2017). superfamily for protein engineering.” Curr. Opin. Struct. Biol., 14, 495 (2004). 177. K. Fu, Y. Yao, J. Dai, and L. Hu, “Progress in 3D printing of carbon materials for 204. J. Madoz-Gúrpide, J. M. Abad, J. Fernández-Recio, M. Vélez, L. Vázquez, energy-related applications.” Adv. Mater., 29, 1603486 (2017). C. Gómez-Moreno, and V. M. Fernández, “Modulation of electroenzymatic 178. R. Gusmão, M. P. Browne, Z. Sofer, and M. Pumera, “The capacitance and NADPH oxidation through oriented immobilization of ferredoxin:NADP re- electron transfer of 3D-printed graphene electrodes are dramatically influenced by ductase onto modified gold electrodes.” J. Am. Chem. Soc., 122, 9808 (2000). the type of solvent used for pre-treatment.” Electrochem. Commun., 102,83 205. C. C. Moser, J. L. R. Anderson, and P. L. Dutton, “Guidelines for tunneling in (2019). enzymes.” Biochim. Biophys. Acta, Bioenerg., 1797, 1573 (2010). 179. P. L. dos Santos, V. Katic, H. C. Loureiro, M. F. dos Santos, D. P. dos Santos, A. 206. M. Ates, “A review study of (bio)sensor systems based on conducting polymers.” L. B. Formiga, and J. A. Bonacin, “Enhanced performance of 3D printed graphene Mater. Sci. Eng. C, 33, 1853 (2013). electrodes after electrochemical pre-treatment: role of exposed graphene sheets.” 207. M. Naseri, L. Fotouhi, and A. Ehsani, “Recent progress in the development of Sens. Actuators, B, 281, 837 (2019). conducting polymer-based nanocomposites for electrochemical biosensors appli- 180. E. Vaněčková, M. Bouša, R. Sokolová, P. Moreno-García, P. Broekmann, cations: a mini-review.” Chem. Rec., 18, 599 (2018). V. Shestivska, J. Rathouský, M. Gál, T. Sebechlebská, and V. Kolivoška, 208. A. D. Chowdhury, R. Gangopadhyay, and A. De, “Highly sensitive electroche- “Copper electroplating of 3D printed composite electrodes.” J. Electroanal. mical biosensor for glucose, DNA and protein using gold-polyaniline nanocom- Chem., 858, 113763 (2020). posites as a common matrix.” Sens. Actuators, B, 190, 348 (2014). Journal of The Electrochemical Society, 2020 167 155525 209. K. Murata, K. Kajiya, N. Nakamura, and H. Ohno, “Direct electrochemistry of 225. T. Tajima and A. Nakajima, “Parallel electrosynthesis of N-acyliminium ion bilirubin oxidase on three-dimensional gold nanoparticle electrodes and its equivalents using silica gel-supported piperidine.” Chem. Lett., 38, 160 (2009). application in a biofuel cell.” Energy Environ. Sci., 2, 1280 (2009). 226. C. Gütz, B. Klöckner, and S. R. Waldvogel, “Electrochemical screening for 210. P. Bollella, G. Fusco, C. Tortolini, G. Sanzò, G. Favero, L. Gorton, and electroorganic synthesis.” Org. Process Res. Dev., 20, 26 (2016). R. Antiochia, “Beyond graphene: electrochemical sensors and biosensors for 227. S. Suga, M. Okajima, K. Fujiwara, and J.-I. Yoshida, “Electrochemical combina- biomarkers detection.” Biosens. Bioelectron., 89, 152 (2017). torial organic syntheses using microflow systems.” QSAR Comb. Sci., 24, 728 211. H. Wang, X. Yuan, G. Zeng, Y. Wu, Y. Liu, Q. Jiang, and S. Gu, “Three (2005). dimensional graphene based materials: synthesis and applications from energy 228. M. Atobe, H. Tateno, and Y. Matsumura, “Applications of flow microreactors in storage and conversion to electrochemical sensor and environmental remediation.” electrosynthetic processes.” Chem. Rev., 118, 4541 (2018). Adv. Colloid Interface Sci., 221, 41 (2015). 229. K. Saito, K. Ueoka, K. Matsumoto, S. Suga, T. Nokami, and J.-I. Yoshida, 212. H. Zhong, R. Yuan, Y. Chai, W. Li, X. Zhong, and Y. Zhang, “In situ chemo- “Indirect cation-flow method: flash generation of alkoxycarbenium ions and synthesized multi-wall carbon nanotube-conductive polyaniline nanocomposites: studies on the stability of glycosyl cations.” Angew. Chem. Int. Ed., 50, 5153 characterization and application for a glucose amperometric biosensor.” Talanta, (2011). 85, 104 (2011). 230. C. Xiang, S. K. Suram, J. A. Haber, D. W. Guevarra, E. Soedarmadji, J. Jin, and J. 213. S. Chiashi, K. Kono, D. Matsumoto, J. Shitaba, N. Homma, A. Beniya, M. Gregoire, “High-throughput bubble screening method for combinatorial T. Yamamoto, and Y. Homma, “Adsorption effects on radial breathing mode of discovery of electrocatalysts for water splitting.” ACS Comb. Sci., 16, 47 (2014). single-walled carbon nanotubes.” Physical Review B, 91, 155415 (2015). 231. H. Hashiba, S. Yotsuhashi, M. Deguchi, and Y. Yamada, “Systematic analysis of 214. J. H. T. Luong, S. Hrapovic, D. Wang, F. Bensebaa, and B. Simard, electrochemical CO reduction with various reaction parameters using combina- “Solubilization of multiwall carbon nanotubes by 3-aminopropyltriethoxysilane torial reactors.” ACS Comb. Sci., 18, 203 (2016). towards the fabrication of electrochemical biosensors with promoted electron 232. F. C. Walsh, C. Ponce de León, D. V. Bavykin, C. T. J. Low, S. C. Wang, and transfer.” Electroanalysis, 16, 132 (2004). C. Larson, “The formation of nanostructured surfaces by electrochemical 215. G. Palanisamy, H.-Y. Jung, T. Sadhasivam, M. D. Kurkuri, S. C. Kim, and techniques: a range of emerging surface finishes. Part 2: examples of nanos- S.-H. Roh, “A comprehensive review on microbial fuel cell technologies: tructured surfaces by plating and anodising with their applications.” Trans. Inst. processes, utilization, and advanced developments in electrodes and membranes.” Met. Finish., 93, 241 (2015). J. Clean. Prod., 221, 598 (2019). 233. C. T. J. Low, R. G. A. Wills, and F. C. Walsh, “Electrodeposition of composite 216. L. He, P. Du, Y. Chen, H. Lu, X. Cheng, B. Chang, and Z. Wang, “Advances in coatings containing nanoparticles in a metal deposit.” Surf. Coat. Technol., 201, microbial fuel cells for wastewater treatment.” Renew. Sustain. Energy Rev., 71, 371 (2006). 388 (2017). 234. P. Herrasti, A. N. Kulak, D. V. Bavykin, C. P. de Léon, J. Zekonyte, and 217. Z. He, S. D. Minteer, and L. T. Angenent, “Electricity generation from artificial F. C. Walsh, “Electrodeposition of polypyrrole–titanate nanotube composites wastewater using an upflow microbial fuel cell.” Environ. Sci. Technol., 39, 5262 coatings and their corrosion resistance.” Electrochim. Acta, 56, 1323 (2011). (2005). 235. Y. He, S. C. Wang, F. C. Walsh, Y. L. Chiu, and P. A. S. Reed, “Self-lubricating 218. Y. Park, S. Park, V. K. Nguyen, J. Yu, C. I. Torres, B. E. Rittmann, and T. Lee, Ni-P-MoS composite coatings.” Surf. Coat. Technol., 307, 926 (2016). “Complete nitrogen removal by simultaneous nitrification and denitrification in 236. F. C. Walsh and C. Ponce de Leon, “A review of the electrodeposition of metal flat-panel air-cathode microbial fuel cells treating domestic wastewater.” Chem. matrix composite coatings by inclusion of particles in a metal layer: an established Eng. J., 316, 673 (2017). and diversifying technology.” Trans. Inst. Met. Finish., 92, 83 (2014). 219. V. G. Gude, “Wastewater treatment in microbial fuel cells—an overview.” 237. N. Zhou, S. Wang, and F. C. Walsh, “Effective particle dispersion via high-shear J. Clean. Prod., 122, 287 (2016). mixing of the electrolyte for electroplating a nickel-molybdenum disulphide 220. P. Clauwaert, P. Aelterman, T. H. Pham, L. De Schamphelaire, M. Carballa, composite.” Electrochim. Acta, 283, 568 (2018). K. Rabaey, and W. Verstraete, “Minimizing losses in bio-electrochemical systems: 238. I. Tudela, Y. Zhang, M. Pal, I. Kerr, and A. J. Cobley, “Ultrasound-assisted the road to applications.” Appl. Microbiol. Biotechnol., 79, 901 (2008). electrodeposition of composite coatings with particles.” Surf. Coat. Technol., 259, 221. M. J. Al Lawati, T. Jafary, M. S. Baawain, and A. Al-Mamun, “A mini review on 363 (2014). biofouling on air cathode of single chamber microbial fuel cell; prevention and 239. A. P. Abbott and K. J. McKenzie, “Application of ionic liquids to the mitigation strategies.” Biocatalysis and Agricultural Biotechnology, 22, 101370 (2019). electrodeposition of metals.” Phys. Chem. Chem. Phys., 8, 4265 (2006). 222. K. Mitsudo, Y. Kurimoto, K. Yoshioka, and S. Suga, “Combinatorial electro- 240. F. C. Walsh and C. Ponce de León, “Versatile electrochemical coatings and chemistry for organic synthesis.” Curr. Opin. Electrochem., 8, 8 (2018). surface layers from aqueous methanesulfonic acid.” Surf. Coat. Technol., 259, 676 223. K. Mitsudo, Y. Kurimoto, K. Yoshioka, and S. Suga, “Miniaturization and (2014). combinatorial approach in organic electrochemistry.” Chem. Rev., 118, 5985 (2018). 241. A. Hovestad and L. J. J. Janssen, “Electroplating of metal matrix composites by 224. T. Siu, W. Li, and A. K. Yudin, “Parallel electrosynthesis of α-alkoxycarbamates, codeposition of suspended particles.” Modern Aspects of Electrochemistry, ed. B. α-alkoxyamides, and α-alkoxysulfonamides using the spatially addressable E. Conway, C. G. Vayenas, R. E. White, and M. E. Gamboa-Adelco (Springer, electrolysis platform (SAEP).” J. Comb. Chem., 2, 545 (2000). Boston, MA) p. 475 (2005). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Electrochemical Society IOP Publishing

Review—The Design, Performance and Continuing Development of Electrochemical Reactors for Clean Electrosynthesis

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Copyright © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited
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1945-7111
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10.1149/1945-7111/abc58e
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Journal of The Electrochemical Society, 2020 167 155525 Review—The Design, Performance and Continuing Development of Electrochemical Reactors for Clean Electrosynthesis Samuel C. Perry, Carlos Ponce de León, and Frank C. Walsh Electrochemical Engineering Laboratory, Department of Mechanical Engineering, Faculty of Engineering and Physical Sciences University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom A critical review of classical and improved electrodes, electrocatalysts and reactors is provided. The principles governing the selection of electrochemical flow reactor or progression of a particular design for laboratory or pilot scale are reviewed integrating the principles of electrochemistry and electrochemical engineering with practical aspects. The required performance, ease of assembly, maintenance schedule and scale-up plans must be incorporated. Reactor designs can be enhanced by decorating their surfaces with nanostructured electrocatalysts. The simple parallel plate geometry design, often in modular, filter-press format, occupies a prominent position, both in the laboratory and in industry and may incorporates porous, 3D or structured electrode surfaces and bipolar electrical connections considering the reaction environment, especially potential- and current-distributions, uniformity of flow, mass transport rates, electrode activity, side reactions and current leakage. Specialised electrode geometries include capillary gap and thin film cells, rotating cylinder electrodes, 3-D porous electrodes, fluidised bed electrodes and bipolar trickle tower reactors. Applications span inorganic, organic electrosynthesis and environmental remediation. Recent developments in cell design: 3D printing, nanostructured, templating 3D porous electrodes, microchannel flow, combinatorial electrocatalyst studies, bioelectrodes and computational modelling. Figures of merit describing electrochemical reactor performance and their use are illustrated. Future research and development needs are suggested. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/ 1945-7111/abc58e] Manuscript submitted August 4, 2020; revised manuscript received October 8, 2020. Published November 6, 2020. This paper is part of the JES Focus Issue on Organic and Inorganic Molecular Electrochemistry. List of symbols (Continued). Symbol Meaning Units Symbol Meaning Units t Time s T Temperature K A Geometrical electrode area m −1 −1 v Mean linear flow velocity of electrolyte m s A Electrode area per unit electrode volume m V Volume of reactor m B Breadth of rectangular flow channel m −3 V Overall volume of electrode m c Reactant concentration mol m E −3 V Volume of electrolyte in the reactor m c Reactant concentration in the bulk electro- mol m R V Volume of electrolyte in the tank m lyte T w Catalyst mass loading g d Equivalent (hydraulic) diameter of a rectan- m x Distance along electrode m gular flow channel 2 −1 X Fractional conversion of reactant Dimensionless D Diffusion coefficient of an aqueous species m s A z Electron stoichiometry Dimensionless E Electrode potential vs a reference electrode V U Cell potential difference V cell Greek E Equilibrium cell potential difference V e,cell E Standard electrode potential V −1 Symbol Meaning Units F Faraday constant C mol −1 G Molar Gibbs free energy J mol α Charge transfer coefficient Dimensionless I Current A ε Surface roughness of electrode m I Limiting current due to convective-diffusion A γ Limiting current enhancement factor com- Dimensionless −2 j Current density A m pared to a smooth surface −2 j Exchange current density A m η Overpotential (η = E−E)V −1 e k First order apparent rate constant s 2 −1 ν Kinematic viscosity of the electrolyte m s −1 k Mass transfer coefficient m s ω Velocity exponent Dimensionless L Length of rectangular flow channel in the m φ Current efficiency Dimensionless direction of flow ρ Electrical resistivity ohm m −1 M Molar mass g mol −3 −1 ρ Space time yield mol m s ST n Amount of a species mol τ Mean residence time in the tank (τ = V /Q)s T T T q Electrical charge C 3 −1 Q Volumetric flow rate of electrolyte m s Subscripts R Electrical resistance ohm a Anode −1 s Space velocity s act Activation (under charge transfer control) S Separation between the electrode and mem- m c Cathode brane (divided reactor) or between the conc Concentration (under mass transfer con- electrodes (undivided reactor) trol) cell Cell e At equilibrium E Electrode E-mail: f.c.walsh@soton.ac.uk Journal of The Electrochemical Society, 2020 167 155525 N Normalised simplify engineering design and result in lower capital and R Reactor running costs. T Tank (reservoir) 2. Convenience and reliability, adequate design, installation, (0) At time zero operation and maintenance and monitoring procedures. (t) At time t 3. Appropriate facilities to control and monitor concentration, Dimensionless groups potential, current density and adequate mass transport regime to Le Dimensionless length (Le = ε/d ) provide and remove reactants and products respectively, via Re Reynolds number (Re = vd /ν) suitable flow distributions. Sc Schmidt number (Sc = ν/D) 4. Simplicity and versatility are perhaps the least quantified and Sh Sherwood number (Sh = k d /D) most overlooked, yet perhaps the most important, factors for m e achieving an elegant and long-lasting design to attract users. Electrosynthesis has a proud history, which ranges from the 5. Provision for future developments by designing a modular routine, tonnage scale production of chlor-alkali chemicals and configuration that facilitates scale-up by adding unit cells or electrowinning of metals to the small scale realisation of speciality by increasing the size of each unit. products, such as pharmaceutical and fine chemicals, metal alloys, 1–4 composites, semiconductors and superconductors. Proponents Industrial applications of electrochemical reactors include the would highlight the convenience of electrochemistry, its ease of production and conversion of chemicals e.g. the chlor-alkali 7 1 8 control, the environmentally clean nature of the electron as a reagent process, aluminium metal production and adiponitrile among and the ability to produce powerful species, in situ, under near other important applications, such as energy conversion and ambient conditions. Antagonists might balance these features against storage. The reactors also offer the opportunity to use electro- limitations, including the speciality nature of the discipline and their chemical techniques for the investigation of electrode processes lack of training and support in practical electrochemistry and involving mass transfer, charge transfer and influence of cell electrochemical engineering together with the shortage of literature hydrodynamics which is typically carried out in small electroche- on successful case studies of modern electrosynthesis at an industrial mical cells at laboratory scale. scale. The design, operation and scale-up of electrochemical cells In electrosynthesis, the electrolyte is often single phase, the remains a critical challenge to the continued development and reactant(s) and/or products being sufficiently soluble to avoid undue diversification of electrosynthesis. mass transport restrictions. In order to minimise ohmic drop in the The design of electrochemical cells has often been focussed on electrolyte, a small interelectrode gap may be used and gas hold-up glass cells for laboratory benchtop use, the services of a skilled should be avoided by using a sufficient flow velocity. It is also glassblower being essential in sealing electrodes in glass and making common to add a concentrated indifferent electrolyte, the ions microporous gas bubbling/cell dividing frits or flanges to accom- carrying most of the migration current between the electrodes. The modate ion exchange membranes. The motivation for glass beaker counter electrode reaction must also be considered as gas evolution cell use is clear, as they are easy to assemble and clean, and sample or corrosion may unbalance concentration or pH and introduce exchange for manual batch analysis is facile. However, reaction evolved gas as a second phase. conditions are often poorly reproducible, especially when attempting In a two-phase liquid-liquid electrolyte, a phase transfer reagent to accelerate reactions rates through magnetic stirrer bars or gas may be needed to facilitate reactant or product transfer between the bubbling. phases. In an organic-aqueous electrolyte, quaternary ammonium This has led to a large proportion of electrochemical synthesis salts or surfactants have often been used. In special cases, the works moving away from beaker cells and into more specialist electrolyte may be a solid-liquid emulsion or suspension. Composite reactors, with flow fields, turbulence promotors and fixed electrode electrodeposition of materials utilises an agitated slurry of solid geometry and position greatly enhancing reproducibility and turn- particles or a sol to deposit materials by a combination of over rates. The danger here is that all electrosynthesis studies are electrophoresis, convective-diffusion and electrodeposition. carried out in the same reactor; the different reaction mechanisms require vastly different reaction environments in order to optimise Cell voltage and its components.—When current flows through reaction rate, charge efficiency and product conversion. Often these an electrolytic cell, the cell voltage (U) can be expressed as: three parameters cannot all be maximised, and so reactor design must focus on reaching a compromise reaction profile. UU=+ ∣∣h + ∣IR∣ [1] e åå This review provides an overview of the key reactor designs employed for electrochemical synthesis, covering initial bench-top The first term on the right hand side is set by selecting the proof of concept scale up to larger industrial standard reactors. The electrode reactions and their thermodynamics, U being directly merits of various reactor designs are discussed in terms of practical related to the Gibbs free energy change (ΔG ) for the cell reaction: cell applications, along with methods of improving their output via the complementary design of 3D electrode structures, electrolyte flow D=GzFU=-zFE [2] cell e cell profiles and mass transport distribution. where F is Faraday’s constant and z is the electron stoichiometry. E is the equilibrium cell potential difference, defined as the cell Principles of Electrochemical Reactor Design a c difference between the anode and cathode potentials (E —E ). The e e second term in Eq. 1 is governed by the electrode kinetics and mass General considerations.—Electrochemical reactors can be in- transport to and from the electrodes, as determined by the electrode herently complex and contain several controls and sensors for overpotential (η). The third term represents ohmic losses, related to specific electrosynthesis processes or can also be a simpler generic the current passed (I) and electrical resistance (R). Expanding the design suitable for a screening process at small scale. The final expression gives: design should comply with a number of essential characteristics that include: UU=+∣∣hh+∣ ∣+∣h ∣+∣h ∣+ ∣IR∣ e å c,, act a act c,conc a,conc 1. Moderate capital and running costs, require low cost compo- [] 3 nents, a low cell potential difference and a low pressure drop over the entire cell including the inlet and outlet flow manifolds The modulus of values is taken to allow for negative cathode for the electrolyte; where possible, an undivided reactor will current values indicating a reduction. By convention, positive Journal of The Electrochemical Society, 2020 167 155525 overpotentials give rise to oxidation currents and negative over- Electrosynthesis cells should always be provided with facilities for potentials give rise to reduction currents. For (non-spontaneous) incorporating a robust reference probe. electrolytic processes, Eqs. 1 and 3 predict that the cell potential For an electrosynthesis cell, minimising all potential losses in the difference, U is positive and becomes larger with increasing current reactor minimises the cell voltage required. density. The overall rate of an electrode process can be described by η and η are the charge transfer overpotentials at the Faraday’s laws of electrolysis, which may be written as a space-time c,act a,act cathode and anode, respectively, representing kinetic charge transfer yield, ρ , i.e., the amount of product per unit reactor volume per ST limitations which dominate at low current densities. Such over- unit time: potentials can be minimised using the appropriate catalysts for a 1 dn fI particular reaction and operating at a higher temperature. The r == [] 4 ST concentration overpotentials under convective-diffusion terms, V dt zFV η and η are important at high current densities. These c,conc a,conc where V is the reactor volume, V is the electrolyte volume, n is the overpotentials cause a potential loss due to mass transport limitations R amount of a species and t is the elapsed time. The current efficiency of electroactive species reaching, or leaving, the electrode surface (or charge yield),f allows for a fraction of the current being used in and can be minimised employing high surface area electrodes, high −3 −1 secondary reactions. The common units of mol m h can be mass transport flow regimes or turbulence promoters. The IR term is obtained on multiplying the right-hand side by 3600. When the sum of all electrical resistances across the reactor and the comparing electrode performance it is often useful discuss in terms electrodes as well as that of the electronic connections. These of a normalised current, either the current density (j) by normalising include all ohmic drops in the system, i.e. the external electrical current against the electrode area (A): contacts, the current collectors and electrodes, electrolyte(s) and membrane. Minimisation of these resistances lowers energy loss and can be achieved by: (a) reducing the overpotentials of the reactions j = [] 5 at the electrodes using a suitable catalyst and (b) reducing the resistances through the membrane, across the electrolyte(s) and or a specific current (I ) by normalising current against catalyst mass through the electrodes. For an electrosynthesis cell, minimising all loading (w): potential losses in the reactor minimises the cell voltage required. jA I = [] 6 Electrode kinetics.—Electrode potential is “the double-edged sword of electrochemical technology.” If the potential is well controlled, yield, selectivity and reaction rate can be high, but Normalisation is usually against the geometric area rather than when the potential distribution is poor, rate, purity and yield can all the electrochemically active area, rougher or more porous electrodes suffer. It is important to monitor electrode potential not just in being able to support much larger current densities. laboratory cells but in pilot and full scale electrochemical reactors, both to relate performance back to polarisation behaviour and act as Figures of merit.—Figures of merit (FOM) for electrochemical a powerful diagnostic probe for cell condition monitoring. reactors are convenient normalised criteria of performance for the 1,13 Table I. Typical figures of merit used to describe electrochemical reactor performance. FOM Expression Observations nn - () 0 (t) Fractional conversion n initial amount (0) X = , () 0 n amount at time t (t) product wzF Current efficiency q , charge passed to produce product product f== q Mq q, total charge M, molar mass product Selectivity n , amount of product S = product total n amount of all species produced total -zFE cell Specific energy consumption for electrolysis fM DGf Energy efficiency during electrolysis EzF cell Active electrode area per unit volume A = Mass transport coefficient I mass transport limiting current L, k = zAFc c concentration of the electroactive species kA kA L L A , electrode area per unit electrode volume Mass transport coefficient associated to the electroactive area k A = or k A = LE LE V V R E V , electrode volume Space-time R Q, volumetric flow rate. t = ST Space-velocity s = w 1 Space-time yield r =´ ST tV Normalised space-time velocity c If () 0 s = log ( ) () cc - VzF c ()tR (0) ()t Normalised space time yield c IM f () 0.9 () 0 r = log ( ) VzFX c RA ()t Journal of The Electrochemical Society, 2020 167 155525 Figure 1. The C-Flow® Lab 5 × 5 laboratory-scale flow cell. The electrodes have a projected electrode area of 25 cm with a typical electrolyte linear flow −1 velocity of 1–10 cm s . Figure courtesy of C-Tech Innovation Ltd. diverse range of electrochemical reactors that exist for different components, comprising of electrode plates, gaskets, separators and applications. The figures of merit are useful to compare different housings. Practical cells usually require even compression of flow and reactors and help the selection for a particular purpose. In addition electrode compartments to ensure adequate sealing. It is common to the selection should be based on safety and reliability characteristics facilitate uniform compression by using rigid (often insulated steel) as well as the most convenient operational mode. The most common end plates. Well-spaced threaded (often stainless steel) fastenings figures of merit are presented in Table I. apply compression through the cell, as seen in commercial cells such 1 20 It is also important to evaluate the suitability of an electro- as the Electrocell, the FM01-LC electrolyser and the Microflow chemical rector and the initial investment cost and the operational cell. Tools such as wrenches (and especially torque wrenches) are cost, including the costs of electrolysis and moving parts, i.e. rarely found in synthesis laboratories and manual, thumb screws are electrolyte pumping or electrode rotation. The evaluation should convenient and used in several designs. In the versatile laboratory cell, also consider the life time of all the components which cost could one end plate is fixed in a frame, the other being tightened and relaxed increase the initial investment and running of the process. A careful via a single thumb screw. It is useful to have the inlet and outlet flow selection of suitable materials needs to consider lifetime, sustain- tube connectors mounted on an end plate and able to be tightened and ability, recycling and environmental disposal. relaxed manually, precluding the need for hand tools. Thumb screws Figures of merit are extensively used to compare the performance are convenient and their use has been continued in contemporary of electrochemical reactors employed for waste water treatment designs, such as the C-Flow cell shown in Fig. 1. 14 15 containing phenolic compounds, ground water treatment, waste Electrochemical cell components are prime candidates for 3D water from the petrochemical industry and metal recovery or printing, either from polymer materials or stainless steel depending remediation of waters containing phenolic compounds. on whether the component should be electrically insulating or should Electrochemical reactors require high current density, energy function as a current collector. The same technologies can be used to density and energy efficiency which can only be achieved by the fabricate the electrodes themselves, or conductive steel, Ni or Ti units appropriate selection of electroactive species and selective catalyst can allow insertion and removal of anode and cathode plates. Of course, to avoid parasitic reactions as well as comparative performance to 3D printing is not a necessity; stacked components with machined quantify their capability. The explicit mathematical figures of merit pores and channels can be arranged to give relatively complex internal listed in Table I are essential to make such comparisons specially structures providing an adequate seal is obtained. Stacked cell designs those based on the normalisation considering the stack volume and require adequate separation between anode and cathode to prevent the electrolyte volume electrical short circuits or product crossover. Electrical separation can In the case of electrosynthesis of soluble redox mediators in be achieved with porous polymer meshes or crushed glass frits, or bipolar filterpress cells, many engineering aspects of cell design are chemical separation requires ion exchange membranes. in common with that of redox flow batteries, which have been The choice of solvent and electrolyte must consider the stability 17–19 treated in extensive reviews. of reaction intermediates for the full electrochemical mechanism, and also whether sufficient overpotential can be applied within the Practical features.—The most basic electrochemical testing can solvent window. Aqueous systems are cost effective and simple to be performed in simple glass beakers with anodes and cathodes work with, although the solvent window of water is narrow immersed in electrolyte. Adding glass-blown flanges facilitates compared to organic media or ionic liquids. Many electrochemical anodic and cathodic separation via a membrane or porous separator. systems show a strong pH dependence and the requirement for As reactors are scaled up and flow is introduced, more complex proton transfers often necessitates and aqueous electrolyte. It is also reactor designs are usually assembled from a series of stacked important to consider surface interactions between electrolyte ions Journal of The Electrochemical Society, 2020 167 155525 and the catalyst surface. Strongly adsorbing species can block a humidified gas stream. This has been successfully achieved for catalyst sites, which can reduce the rate of reaction or have drastic CO reduction in ionic liquids, although so far the limited water 23 – influence on product selectivity. availability has only given access to 2e formate or CO products and Organic electrolytes and room temperature ionic liquids give a cannot reach desirable C species such as ethylene or ethanol. wider solvent window than aqueous equivalents, and allow reactions Ionic liquids are especially desirable from an environmental through intermediate species that would be oxidised in an aqueous perspective as they can be recycled via solvent extraction to be environment. Ionic liquids are able to stabilise high energy radical reused for further electrochemical synthesis. Since their environ- intermediates for organic electrochemical synthesis. Proton mental standing is dependent on extraction and recycling and their transfer in organic or ionic liquid media can be facilitated by viscosity gives slow mass transport rates, their use is currently too including low concentrations of water either in the liquid phase or as costly for industrial scale-up. Figure 2. A decision tree regarding cell features. Journal of The Electrochemical Society, 2020 167 155525 Figure 3. Schematic diagrams for a number of commonly employed laboratory scale electrochemical cells. Solid arrows indicate the direction of solution flow. Dashed lines indicate a porous separator or ion conductive membrane. Electrolytes in A-C may also be mixed through additional means such as magnetic stirrers or gas bubbling. (A) Undivided beaker cell, (B) Beaker cell with the anode confined in a porous chamber, (C) Divided H-Cell with a membrane separator, (D) Undivided flow cell with an electrolyte reservoir and circulating electrolyte pump. (E) Divided flow cell with separate anolyte and catholyte reservoirs on either side of a microporous membrane separator. Decisions During Reactor Selection or Design integration so the reactor aligns with existing production processes, d) reaction engineering to optimise selectivity, production, current Strategic decisions.—In the 1960s, electrochemical engineering and potential distribution, mass transport, electroactive area, inter- principles were increasingly applied to cell design and a diverse electrode gap and low overpotentials, e) operational cost having range of electrode and cell geometries developed for laboratory and reliable and low cost cell components such as electrolyte and 6,28–31 pilot scale use in the 1970s. Many of these designs, particu- separator, if needed, f) minimisation of mechanical devices such as larly packed, fluidised and moving beds, were primarily intended electrolyte pumps or electrode agitation and g) low pressure drop for metal ion removal from dilute waste liquors although uses in over the reactor. synthesis have been considered. Such developments and their Other strategic decisions are whether the process is batch or 1,34 industrial applications have been reviewed. continuous operation and how the products will be removed from the It is generally accepted that depending on the applications the reactor, depending on their physical characteristics. Gas products are electrochemical reactor has to be designed for the particular process typically vented at lower pressure or displace them with an inert gas in order to optimise the figures of merit such as conversion, current or via a gas liquid separation unit. Liquid and solid products can be efficiency, selectivity, energy consumption and efficiency, cell separated by flotation, settlement, or solvent extraction. voltage, electroactive area, mass transport and space tie and space Figure 2 offers a simple strategy to aid selection or development velocity. Selectivity and conversion are more important than energy of a particular electrode geometry and cell design. When used efficiency in electroplating and organic electrochemistry respec- retrospectively, this approach helps to rationalise diverse cell tively, whereas energy efficiency is more important for redox flow designs by considering their major characteristics. Alternatively, batteries and industrial production of, e.g., adiponitrile, aluminium Fig. 2 can aid the selection of an available cell design. The benefits and chlor-alkali. and compromises involved in making such a choices can be briefly Independently of the application of the electrochemical reactors, considered. several principles need to be followed in order to provide an optimised design. Some basic strategic decisions for constructing an electrochemical reactor include considerations that could be Divided and undivided reactors.—One of the first decisions in conflicting and the decision on what aspects to favour on detrimental reactor design whether the reactor operates with separated cathodic to others has to be realised on their importance for the process. Some and anodic electrolyte compartments by an ion exchange membrane, of these considerations include: a) simplicity in order to reduce cost, a porous separator or a single electrolyte compartment. The single b) reliability for routine operations, cleaning and inspection, c) compartment design is simpler and avoids the cost of the ion Journal of The Electrochemical Society, 2020 167 155525 exchange membrane and the gaskets and fittings in the electrolyte membrane should maintain the material balance of the anodic and required to fit the separator (Fig. 3). Divided reactors avoid mixing cathodic reactions in order to maintain the neutrality and avoid of catholyte and anolyte electrolytes which prevents product con- drastic pH changes in both electrolyte compartments. sumption or unwanted side reactions occurring at the opposite CEMs and AEMs are designed to conduct cations and anions electrode. Although small inter-electrode distances can be realised respectively, in theory CEM repel neutral molecules and anions with a separator, as with the 2 mm inter-electrode gap of the bromide while AEM repel neutral molecules and cations. Nafion® is the most polysulfide redox flow battery, divided reactors also increase the commonly used CEM due to its high ionic conductivity and ionic resistance due to the separator. chemical resistance due to a robust fluorocarbon backbone with Cell designs without a membrane or separator have a smaller sulfonic groups as ion exchange sites; other membranes based on ohmic resistance as the impedance caused by a separator is absent sulfonated styrenes, polyimides, and arylene ethers, are less stable. and are capable of wider range of flow profiles. Furthermore, On the other hand, AEM are based on fluorinated hydrocarbons, poly degradation and material cost for membranes do not have to be (ketones), poly(ethers), and poly(ether ketones) with imidazolium, considered. For instance, the membrane in PEM fuel cells accounts quaternary amine or phosphonium as the anion-exchange groups but to 24% of the total cost. Membrane-less designs can have porous these are chemical less stable than CEM. A clear example of the electrodes, allowing flow through, fluidised bed electrodes, gas convenience of using anion membranes is the borohydride fuel diffusion electrodes and redox mediators. cell ; although CEMs have good resistance in the alkaline environ- The selection of a divided or undivided reactor is important in the ment, they produce a chemical imbalance when OH is consumed electrochemical water treatment methods to deplete anthropogenic and not replaced from the catholyte compartment, making the persistent organic pollutants. Typically, the main direct and anolyte more acidic in the long term. AEMs keep the chemical mediated oxidation (via highly oxidising radicals) occurs in the balance by replacing the OH ; unfortunately, most anionic mem- anode compartment, in which case a divided reactor will be required. branes are unstable in alkaline environments. In some instances, a two-stage remediation process involves the Bipolar membranes (BPMs) offer an alternative structure to preparation of the oxidants (e.g., persulfates, perphosphates, percar- address the challenges limitations of AEMs and CEMs. A cationic bonates) in the reactor which are then added to the wastewater. In a and anionic layer are combined to give a two-layer structure, divided reactor, direct and mediated oxidation can be carried out at preventing product crossover while still permitting the charge to + – the anode where single, and mixtures of, highly oxidising species be carried by H and OH . There are two modes of operation can be generated at a high current efficiency. determined by the reaction at the cationic-anionic phase interface; + – A more synergetic process is the combination of the anodic and facing the cationic side to the cathode generates H and OH by cathodic processes to increase the degradation efficiency, in which case water electrolysis, whereas facing it to the anode drives the opposite an undivided reactor may be used. For example, in order to increase reaction. Care still must be taken, as some undesired ion crossover the oxidation power of cathodically generated H O , an undivided may still occur and BPMs can suffer from delamination and 2 2 reactor will allow the hydrogen peroxide to couple with other reactions dehydration, particularly at large current densities. such as Electro-Fenton based processes. It can be argued that the efficiency will decrease due to the decomposition of H O at the anode Monopolar and bipolar electrical connections to electrodes.— 2 2 a trade-off exists between the increase of cell potential due to the The electrochemical characteristics of a system are typically separator and the effectiveness in the depletion of the organic pollutant determined in a single three-electrode laboratory scale cells of by the concerted anodic/cathodic treatment when using undivided ≈100 cm volume with cell voltages of approx. 1–2 V. If the system cells. One example of the use of divided and undivided reactors was is scaled up to a large number of electrochemical cells it might need reported by Ochoa-Chavez et al. who showed small difference in the the application of larger cell voltages. Several single electrochemical degradation of 5-fluoro-1H-pyrimidine-2,4-dione (5-FU) with 75% and cells can be put together to increase the area, thus the production −1 77% for undivided and divided reactor, respectively using 50 mg l capacity, and they can be arranged as a monopolar connection as is −2 −1 FU at 150 A m ,13 lh and6hofelectrolysis. shown in Fig. 4A, where each electrode is either positive or negative. The use of divided electrochemical flow reactors has been widely This arrangement maintains the cell voltage of one individual cell explored in inorganic process such as the chlor-alkali and in energy but is able to generate high currents. In order to increase the voltage, generation and storage devices like fuel cells and redox flow the monopolar connected cells can be arranged in series, see Fig. 4B. batteries. They increase energy conversion efficiency by preventing Another strategy consists of connecting the cells in a bipolar parasitic reactions and can reduce energy losses by separate configuration as is shown in Fig. 4C, which is commonly used in optimisation of the anodic and cathode reactions. Their use in electrosynthesis cells, fuel cells and redox flow batteries. In this organic electrosynthesis and the effect of the organic material on the arrangement the voltage depends on the number of cells. The ion exchange membranes is poorly explored. The issue becomes electrodes acquire a different charge on each side, driving more complex when deciding whether a cationic (CEM) or anionic the oxidation reaction on one side and the reduction reaction on exchange membrane (AEM) can be used as the selection of the the other simultaneously. Figure 4. (A) monopolar electrode connections, (B) monopolar cell stacks connected in electrical series, (C) bipolar electrodes. Journal of The Electrochemical Society, 2020 167 155525 Figure 5. Commonly employed 3D electrode scaffolds in electrochemical reactors, offering a great range in porosity, tortuosity and active surface area. Figure adapted from Ref. 57 available under Creative Commons (CC-BY) license, published on behalf of The Electrochemical Society by IOP Publishing Limited 2020. Bipolar electrode connections allow more compact cells than applications such as electrosynthesis, oxidation of organic materials 45–48 monopolar connections because there are no electrical cables in wastewater, metal recovery, energy storage and generation. connecting each electrode. Both monopolar and bipolar configura- They can be arranged in two configurations; flow-through and tions are found in cells designs that can be easily scaled-up to flow-by, where the current and the electrolyte flow run parallel and 23,43 industrial production. The bipolar electrodes typically contain an perpendicular to each other, respectively. In both configurations the electronic conductive flat plate in the centre which has two purposes, electrodes face each other in order to ensure a uniform current a barrier for the positive and negative electrolytes and as an distribution. However, although the intricate structure of the 3D electronic connection to transfer electrons. In redox flow batteries electrodes contributes to increase the mass transport of electroactive and fuel cells the core electronic conductive is a graphite plate. In species towards their surface, it also causes different resistance fuel cells, the bipolar plates also carry the gases to and from the gas values between opposite points of the electrodes. This geometrical diffusion electrodes, i.e. they act as flow fields. differences causes changes in the potential and current distribution on the electrodes affecting their performance. The current distribu- Porous, 3-D electrode structures.—Porous materials offer large tion can be divided between primary, which depends on the surface electrode areas and are typically used in electrochemical geometry, secondary that depends on kinetic factors and tertiary flow reactors, where they are most effective, for a variety of which depends on the concentration. Journal of The Electrochemical Society, 2020 167 155525 Figure 6. Schematic diagrams of common electrode and reactor reactors. Grey shading indicates a driving electrode, brown a catalytically active material and white an insulating surround or membrane. Solid arrows show the electrolyte flow direction, dashed arrows show component rotation: (A) parallel plate flow-by electrode, (B) parallel plate flow through electrode, (C) interdigitated flow through electrode, (D) rotating disc electrode, (E) rotating cylinder electrode, (F) trickle bed electrode, (G) fluidised bed reactor, (H) thin film bipolar electrode disc stack. The geometry of the three dimensional electrodes are also able to homogeneous. Rather than use electrodeposition, catalyst materials increase the space-time yield of electrochemical reactors by pro- can be added via electroless deposition or dip coating, which can be viding effective use of the reactor volume and increase their monitored via the open circuit potential. However, there is still the efficiency compared to 2D electrodes. In flow reactors, the mass possibility that parts of the electrode are inactive due to the potential transport can be controlled and measured and correlated to the distribution if the electrode is too thick or if the concentration of the pressure drop and flow dispersion to evaluate the overall cost–benefit electroactive species is too low. of the electrodes. The most preferred techniques to characterise 3D The determination of the optimal thickness of 3D electrodes can electrodes include the limiting current and conversion rate measure- be obtained by mathematical simulation. For example Nava et al. ments because they are fast and convenient. Typical materials and suggested that, in conductive porous electrodes, a unidirectional 51 52 configurations include carbon felt, foam and reticulated vitreous potential distribution under limiting current conditions can be carbon (RVC) as a cost-effective porous electrode with large modelled assuming plug-flow conditions and that in excess of 54 55 56 surface area and porosity. Metal mesh, felt and foam electrodes supporting electrolyte, the conductivity changes during electrolysis have also been prepared from materials such as nickel, titanium and are negligible. The model assumes that only the concentration decay copper (Fig. 5). of the electroactive species within the electrode is responsible for the The importance of 3D architectures has been emphasised by the potential distribution. More complex models assume that the properties observed when structures such as nanorods, nanospheres, electrolyte flow rate and electrode thickness determine the ohmic nanoonions, networks of nanowires and nanoflowers, microflowers, drop inside the porous electrode. In practice, the potential difference nanowalls and hierarchical structures are manufactured on flat plate between the porous electrode surface and the solution should not be 58,59 electrodes or inside already three dimensional electrodes. too large in order to ensure that hydrogen and oxygen evolution do Indeed, the electrochemical properties of 3D substrates can be not occur during reduction and oxidation, respectively. improved and tailored by surface treatment or deposition of catalyst for a particular reaction. A typical example is the electrodepositon of Examples of Reactor Designs and their Performance Pt on titanium plate, felt or meshes. One of the disadvantages of 3D electrodes is that they can present uneven current and potential Vertical plates in a stirred beaker.—The number of commonly distributions, resulting in asymmetric electrodeposition of catalyst employed reactor designs is extremely broad, depending on the particles as well as uneven final operations. It is necessary to desired reaction profile and scale (Fig. 6). Vertical plate electrodes in establish the optimal electrode thickness to ensure that all the a stirred beaker are a staple in electrochemical laboratories thanks to covered electrode surface is electrochemically active. In thicker their simple design, ease of assembly and broad application scope. electrodes not all the surface of the 3D electrode is at the same Basic reactor designs hold the anode and cathode within the same potential and the distribution of the catalyst might not be beaker. Small modifications can separate these via an ion-exchange Journal of The Electrochemical Society, 2020 167 155525 Figure 7. Examples of commercially available electrochemical flow cells. (A) Exploded view schematic for the FM01-LC. Figure taken from Ref. 36, available under Creative Commons (CC-BY) license, published by ACS Publications 2018. (B) The ElectroSyn reactor for pilot and medium operations. The cutaway on the right shows the internal components, including the electrodes plates, turbulence promotors and polymer frames. Images courtesy of ElectroCell A/S, Denmark. (C) Ammonite8 microflow cell. The top image shows the individual components, bottom shows the full assembled cell. A larger model (Ammonite15) is also available for electrochemical synthesis up to the tens of grams scale. Figures taken from Ref. 75, available under Creative Commons (CC-BY) license, published by Elsevier B.V. 2016. 63 36 membrane in an H-cell configuration. Both can be operated in product conversion. Further enhancements are achieved through terms of cell potential, or a reference electrode can be inserted into turbulence promotors to encourage solution mixing, incorporating one compartment near to either the anode or cathode, defined then as porous flow-through electrodes to increase the electroactive area, or the working electrode. As well as a popular choice for electro- through combining multiple stacked cells in sequence or parallel 67,68 chemical teaching laboratories, the ability to rapidly replace elec- configurations. Stack designs are greatly simplified through trodes, electrolytes and membranes make these cells idea for batch the use of bipolar electrodes, as multiple cells can be potentiosta- testing and proof of concept work in catalysis, redox flow batteries tically or galvanostatically controlled using only two electrical 64 69 and electrochemical synthesis. connections. Although convenient for initial experimentation, the simplicity of Modifications can be made to the core flow cell design based on the cell design limits the applicability of the stirred beaker for up- the needs of the individual reaction. Separating anodic and cathodic scaling. The use of magnetic stirrer bars gives a limited mass compartments allows for different products to be produced at each. transport range with poor reproducibility, due to the variable stirrer This has most notably been achieved in the chlor-alkali industry, position, non-standardized stirrer bar size and interaction of stirred where sodium hydroxide and chlorine gas are produced at the 65 70 solution with the beaker wall. Up-scaling efforts must proceed via cathode and anode respectively. Conversion efficiencies can be intermediate-scaled reactors with volumes on the order of m , which increased with recirculating pumps so that starting materials have 71,72 offer a compromise in easy of assembly and varying parameters multiple passes over the electrode. Care should be taken for against applicability towards up-scaling to industrial specifications. electrochemically active products such as hydrogen peroxide, since a Up-scaling focuses on incorporating multiple anodes and cathodes in cyclic approach would likely decompose the product rather than the same tank, either unseparated or by confining all of one electrode produce more. Single-pass flow systems are also desirable where (e.g., all the anodes) in ionic membrane compartments, which are flow cells can be directly integrated into second-phase synthesis or submerged in a reactor tank containing unconfined cathodes. Such purification modules. designs are employed for large scale electrochemical processes, such as metal salt synthesis, electroplating and metal recovery. Examples of commercially available flow cells.—A broad range of commercial reactors exists, providing electrode areas on the order 2 2 The planar electrode in a rectangular flow channel.—The of 10 cm up to >10 m (Fig. 7). Cells such as the FM01-LC provide rectangular flow cell is ubiquitous in continuous electrochemical bench-top equivalents of industrial scale flow reactors, with the 2 2 flow processes, providing a reproducible flow profile for reactant FM01-LC itself being a 64 cm (4 × 16 cm stack) derivation of 2 20 delivery and product removal, with the same core design for bench the 2100 cm FM21-SP reactor used in the chlor-alkali industry. top and industrial scale reactors. The rapid mass transport rates and The FM01-LC uses a parallel plate design which can be operated large electrode area to electrolyte volume ratio means that flow cells with turbulence promotors and porous electrodes and with or without consistently outperform stirred beaker cells for reaction rate and a separating ion-exchange membrane. ElectroCell offer a similar Journal of The Electrochemical Society, 2020 167 155525 route to up-scaling electrochemical processes by producing similarly comparable reactors over greatly different scales, starting with the ElectroMP cell (0.01–0.2 m ) up to the ElectroProd cell (0.4– 2 1 16 m ). There are also popular commercial microflow reactors, focusing on small inter-electrode gaps and long reaction paths in order to give maximum product conversion in a single pass. Different reactors take different approaches to maximising the path length; the Syrris Asia FLUX module uses a compact serpentine 74 75 reaction path, whereas the Ammonite8 flow cell uses a spiral. Both reactors have shown impressive conversion rates over several hours of operation. 3D printed flow cells.—3D printing technology offers a cost- effective means for rapid prototype development and validation of 76–78 digital simulations. Most components in a traditional flow cell can be fabricated via 3D printing thanks to the increasingly wide range of materials that can be used as a feedstock. Traditionally, polymer materials, including but not limited to poly(lactic acid), poly(propylene) or acrylonitrile butadiene styrene have been used for non-conductive parts such as the flow cell frame or turbulence 79,80 promotors. Users must consider the nature of the electrolyte and products formed, as many polymers are unsuitable due their susceptibility to hydrolysis, particularly under extremes of pH. 3D printing technologies are also able to produce conductive structures, allowing customised 3D flow-through electrodes to maximise electrode-solution interaction and generate turbulence in flow-through cell configurations. In most cases, the first stage is to print a 3D scaffold from stainless steel, and then electrodeposit the catalyst to give the same structure at greatly reduced cost. Alternatively, incorporating conductive materials into the polymer Figure 8. Examples of commonly employed flow field designs for flow-by feed provides a conductive polymer scaffold for subsequent electro- electrochemical cells. (A) Serpentine, (B) parallel, (C) pin, (D) spiral, (E) deposition and use as an electrode. Other groups have incorporated interdigitated. Figures adapted from Ref. 99 available under Creative more complex catalysts directly into the polymer, such as Pt/C or 84 Commons (CC-BY) license, Copyright © 2014 liu, Li, Juarez-Robles, MoSe . These conductive 3D polymer electrodes have been Wang and Hernandez-Guerrer. 85,86 87 successfully employed in flow cells, electrolysers and electro- chemical sensors. This presents an opportunity to 3D print an to have two parallel electrodes. A recent paper demonstrated that entire flow cell using a low-cost desktop printer, offering an improved electrical contact was made using magnetisable electrode economic route to prototype development. particles under the influence of a magnetic field. The strategy resulted in an increase of the electrochemical conversion up to 400% compared Porous, 3D electrodes in a flow cell.—The choice of porous to the use of non-magnetised particles. material depends on the flow profile employed within the cell, primarily whether the electrolyte flows-by or -through the Flow field designs.—The challenge in flow field design is to electrode. Flow-by configurations flow electrolyte within a channel provide optimal flow conditions for reactant supply to and product over the surface of the porous structure, usually assisted by removal from the electrode surface, while simultaneously mini- turbulence promotors to maximise interaction between the electro- mising the pressure drop between inlet and outlet. To this end, a lyte and the inner porous structure. Flow-through instead fills the number of different flow field designs have been employed (Fig. 8). channel with the porous structure, requiring the electrolyte to Within these categories, multiple works have investigated varied permeate the whole structure during the flow. While maximising flow channel widths, depths and orientations, with both experimental the interaction between electrode and electrolyte, the flow-through and computational approaches probing the flow profile, pressure configuration is practically limited due to pressure drop across the drop and potential distribution across the electrode surface. porous electrode material. Flow through configurations therefore Comparison of different flow fields is challenging since varied are most often employed to very thin electrodes in microfluidic 90,91 experimental conditions affect reactor functionality alongside devices. changes in the flow field design. However, some experimental and 3D electrode materials can also be used as scaffolds to support computational works indicate that serpentine fields outperform the catalytically active particles, which may be deposited through spray 99,100,101 92,93 94 55 other related designs. coating, drop casting or electrodeposition. With flow appli- An alternative flow field design often employed for 3D porous cations in mind, decorating the conductive scaffold must find a electrodes is the interdigitated flow field (IFF), which has a series of compromise between loading sufficient catalytic material and parallel channels where each channel is blocked at alternate ends. In blocking the porous structure. The tortuosity of the porous material order to continue the flow, liquid must move through the 3D will determine how deep into the pores the catalyst can be loaded. electrode structure itself. This gives a much greater degree of Electrodeposition and drop casting exceed spray coating here, since electrolyte-electrode interaction and a faster rate of reaction. electrolyte can permeate porous structures whereas spray coating The same technique can also be applied to gas phase flow to requires a linear line of sight. encourage interaction with the catalyst at the solid-electrolyte An alternate route to high surface area electrochemical catalysts is interface. The benefit of IFFs can be thought of as providing through fluidised bed reactors. These feature solid catalyst particles flow-through activity, without the associated pressure drop, thanks to held in suspension by an upward gas or liquid flow stream, providing the shorter mass transport path length. When designing an IFF, there excellent catalyst interaction and mixing for electrochemical must be a compromise between the electrode penetration depth and reactions. Although fluidised bed reactors suffer from poor electrical the pressure drop, since greater flow-through characteristics will contact, they removed the assumption that an electrochemical cell has Journal of The Electrochemical Society, 2020 167 155525 hydrophobic and hydrophilic channels within the electrode structure. This has been shown to facilitate water transport within the electrode structure to prevent channel flooding, whilst still favouring gas 107,108 transport for O reduction. Rotating disc and cylinder electrode reactors.—Rotating disc electrodes (RDEs) are staples in electrochemical laboratories thanks to their well-defined and reproducible electrolyte flow and mass transport profiles, with the transport rate being proportional to the 109,110 square root of the rotation rate. An extension to this design is the rotating ring disc electrode (RRDE), which has a secondary ring electrode on the outside of the central disc. Electroactive species produced at the disc are detected at the ring, allowing in situ quantification of electrochemically generated products. Samples of interest can be drop-cast onto glassy carbon substrates, or affixed to the electrode surface, provided mathematical considerations are made for the sample shape and thickness. This provides an attractive option to assess the impact of flow rate on a reaction in a simplified set up, before moving to a more complex flow cell design. Rotating cylinder electrodes (RCEs) have received less attention than RDEs. Like the RDE, the rate of mass transport is defined by its rotation rate. The RCE offers a substantially larger surface area along with controlled turbulent flow conditions, making it suitable for up-scaling to industrial applications. Primarily, the large RCE surface area has been employed for electrodeposition applications, 113,114 including metal recovery and metal powder production. Thin film reactors: capillary gap and bipolar trickle tower reactors.—Trickle tower reactors use gravity flow to pass the working solution through a porous catalyst material, usually a foam, mesh or bed of particles. The small pore volume gives a thin film of electrolyte over all catalyst surface, giving unique electrochemical conditions. Specifically, the thin electrolyte layer over a large surface area catalyst give a high conversion rate and minimal Ohmic drop when working with weakly concentrated, resistive solutions. This makes trickle towers ideal for water treatment, particularly in the removal of the low concentrations of organic pollutants. The same concept is seen in capillary gap electrodes, where the electrolyte flows down a thin channel between anode and cathode. This can be done on a range of scales from microfluidic devices up to stacks of bipolar capillary electrodes. The small inter- Figure 9. Different approaches to the use of microflow reactors for electrode gap provides an additional advantage thanks to the short electrochemical synthesis based on the needs of the reaction mechanism. diffusion paths under laminar flow conditions. This gives rapid rates (A) Direct electrochemical oxidation of benzyl alcohol to the corresponding benzaldehyde at the anode. Cathode balances the charge through hydrogen of reaction without turbulence or forced convection, allowing fast evolution. Mechanism taken from Ref. 138. (B) Electrochemical conversion turnover rates than can be supported by relatively simple computa- of cubane carboxylic acid to alkoxy cubanes. Reaction intermediates are tion modelling. Microchannel reactors with short inter-electrode produced at both the anode and cathode, which subsequently react to give the distances allow for paired electrochemical processes, where the end product. Mechanism taken from Ref. 141. (C) Electrochemical synthesis anodic and cathodic diffusion layers interact to give further reactions of metal-salen complexes. A sacrificial anode acts as a source of metal between electrochemically generated products. cations for the reaction. Anodic metal dissolution and ligand reduction are performed simultaneously, which combine to give the final complex. Mechanism taken from Ref. 142. (D) Oxidising alcohols to the corre- Technological Developments in the Design, Construction and sponding ketone via the redox mediator 2,2,6,6-tetramethylpiperidine-1-oxyl Simulation of Electrodes and Cells (TEMPO). Electrochemistry at the cathode is not considered. Mechanism taken from Ref. 143. Gas diffusion electrodes (GDEs) for gaseous reactants.—Gas diffusion electrodes offer rapid rates of reaction for gas-phase electrochemical processes. The core structure consists of a hydro- phobic, gas-permeable structure with a catalyst deposited on the cause a greater pressure drop. New IFF designs tune channel size electrolyte-facing side. The keeps mass transport in the gas phase, and density in order to give a uniform gas/electrolyte distribution for 103,104 which is inherently rapid, and circumvents challenges of poor an optimum power density. Recent advances have included the solubility that hinders the rate of reaction when bubbling gas development of hierarchical IFFs, where smaller branching channels through electrolyte in a beaker or H-cell. GDEs are relatively cheap give an even mass transport distribution whilst providing a small and simple to manufacture, leading to their extensive use in PEM pressure drop. 122 123 124 fuel cells, flow cells and electrolysers. Electrochemically A key challenge with interdigitated flow fields for gas phase synthesised products may be collected in the gas or liquid phase, or electrochemistry is water management. Capillary pressure resulting multiple products may be collected from both phases simulta- within porous materials causes water to accumulate in flow channels, neously, as has been achieved for ethylene and ethanol from CO hindering gas flow and resulting in unstable cell performance. 2 reduction. This challenge lead to the development of new electrodes with Journal of The Electrochemical Society, 2020 167 155525 Many key developments in GDE technologies have focused on with a 4.2-fold increase in conversation rate for 2,2′-bis(bromo- the hydrophobicity in order to prevent GDE flooding or dehydration methyl)-1,1′-biphenyl intramolecular cyclisation. Hollow Cu of ion exchange membranes. Commercial GDEs contain hydro- fibres have also been used to create a gas diffuser that acts as the phobic components such as PTFE or Teflon to hinder this but further working electrode for CO reduction in a beaker cell. improvements are needed. A number of works have taken steps to Other works have focussed on new immobilisation strategies for improve the hydrophobicity of GDE surfaces, such as hydrophobic catalytic material on 3D electrode supports. Catalytic nanoparticles 128 129 130 oxidised carbon nanotube, PTFE or dimethyl silicon coat- can be immobilised on carbon and metallic foams, felts and meshes ings. Other groups have removed hydrophilic components from the through the in situ reduction of the corresponding metal salts, either 157 158 GDE entirely. Hydrophobic polymer GDEs or silanized nanoporous chemically or electrochemically. The chemical route allows for alumina membranes can replace traditional carbon materials, with the physical bonding of catalyst material to a 3D support, such as for one polymer-based GDE giving efficient CO reduction over gold nanoparticles on graphite felt via thiol moeties or the direct 150 h. synthesis of hierarchical ZnO nanowires on CuO nanowires on Cu It is equally important to consider the impact of the GDE surface, foam via sequential wet chemical and thermal synthesis steps. as the structure and dispersion of catalytic layers will affect the gas Electrodeposition has also been used to fabricate mixed-metal 133 160 161 dispersion and potential distribution over the GDE. Many GDEs catalysts, such as Ag particles on Al and Ni foam, Pt on Ti 162 163 164,165 have a microporous layer (MPL) at their surface, usually a felt, Fe-doped Pt/C and Mn O -coated graphite felt. x y combination of carbon black and PTFE, allowing fine control over Simpler approaches can electrodeposit the same material on a porosity and hydrophobicity. Multiple works have improved the conductive scaffold to give a more active surface, such as for MPL performance through targeting the porosity or hydrophobicity. electrodeposited high surface area Ni via liquid crystal Others have removed it entirely, using carbon nanofibers to enhance templating. The electrochemical route is particularly advanta- the electrical conductivity or microporous polymers to tune the geous as the level of doping or surface modification can be simply hydrophobicity. controlled by varying the applied potential or charge passed. Alternatively, magnetron sputtering has been used for metallic Microflow channel reactors.—The unique combination of la- particle loading on porous GDEs, which gives efficient deposition of minar flow with short diffusion path length allows for interesting small, active particles. Deposition can use a low loading while approaches to electrochemical synthesis (Fig. 9). Despite the maintaining activity, which is particularly attractive for costly noble absence of a membrane, the laminar flow profile gives limited metal materials. As a line of sight method, care must be taken not mixing between solutions at the anodic or cathodic side. This has to block the GDE pores, and particles cannot deposit as deeply allowed selective interaction of solution species with only one within the 3D structure, as in the case of electrodeposition. electrode, such as oxidations of alcohols or heterocyclic Similarly, chemical vapour deposition methods have been used to cross-coupling under mild conditions with only H as a by- create carbon structures on porous electrode scaffolds, such as for product produced at the cathode. In these cases, engineering carbon nanofibres on Ni foam or N-doped graphene on Cu solutions have to allow for H release before the solution is recycled foam. or passed onto the next reactor, as H bubbles are resistive and will slow the rate of further electrochemical steps. 3-D printing of electrodes and cell bodies.—3D printing devel- Bubble generation, usually from H or O evolution, can generate opments range from the fabrication of individual components up to 2 2 turbulence within the flow channel to deviate from the laminar total cell fabrication. Materials choices are determined by the regime, which must be considered particularly when operating at solvents and reaction conditions. Many printable polymers are large overpotentials. In other cases, bubbles within the flow susceptible to degradation, with poly(etherethylketone) (PEEK) reactor can be used as part of the synthesis strategy, such as using recently shown to outperform other materials for chemical, thermal 146 170 bubbles as an H source for hydrogenation reactions. Bubbles of and mechanical stability. Polymer materials are unsuitable for immiscible liquids can also be used for product extraction within the organic media, and can produce hotspots due to poor thermal flow channel, such as for 5-hydroxymethylfurfural synthesis in an conductivity, leading to a number of groups to use selective laser aqueous electrolyte, which then moved into organic bubbles to melting to 3D print components from stainless steel. prevent further degradation or polymerisation reactions. The intrinsic precision of 3D printing allows for complex flow Other groups have reduced the inter-electrode gap to facilitate fields and multi-channel cell designs to target specific reaction reactions between anodically and cathodically generated species. A requirements. The impact of flow field pattern (serpentine, parallel, method to produce copper-N-heterocyclic carbenes via imidazolium interdigitated, spiral, etc.) on reaction yields has been demonstrated, reduction and anodic metal dissolution has been used to produce a since it is straightforward to produce flow fields with multiple 142 172 wide range of complexes. Reducing the inter-electrode gap also arrangements and exchange them within the cell. Multiple inlets allows the electrolyte content to be reduced by a factor of ten for have also recently been used to introduce reagents stepwise for 148 173 sulfonamide electrosynthesis vs beaker reactors, with other difluoromethylation or diphenylacetonitrile. Printed cells can be 149 174 reactors removing the electrolyte entirely. An alternative ap- designed to be dismantled for working electrode exchange, or proach has been to use the electrochemical step to produce a printing can be paused and restarted to confine plate electrodes homogeneous catalyst, which drives the desired substrate reaction, within the cell structure. such as using the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) 3D printing stainless steel or titanium offers a simple route to mediator to drive alcohol oxidations. electrodes with tailored surface area, porosity and flow profile. The desired catalyst can then be added through electrodeposition, as has 176 82 3D porous electrodes.—Metal foam electrodes synthesised from been achieved for recent Ni and Pt electrodes. An interesting electroless deposition on polymer scaffolds or lost carbonate alternative is to incorporate conductive material such as carbon or sintering give random pore orientations. More ordered structures copper powder into 3D printed polymer to provide conductive 88,177 have been produced via selective laser melting, giving access to polymer electrodes. These have been used directly as 152 153 86 178 regular pore distributions and tailored electrode architectures. electrodes, after pre-treatment with solvents or electrochemical A number of groups have reduced the pore size in order to increase cycling or after electroplating to give a metal film on the 3D the active surface area (Fig. 10). Ni electrodeposition inside an printed scaffold. anodic aluminium oxide template produced a Ni nanomesh, which was then Pt doped for H evolution applications, outperforming Computational modelling of reaction environment in reac- commercial Pt/C catalysts. New metal felts have been produces tors.—Broadly speaking, computational models can focus on from nanowires, giving a high surface area flow-through electrode hydrodynamics, mass transport, heat transfer, current distribution, Journal of The Electrochemical Society, 2020 167 155525 Figure 10. Recent novel 3D electrode designs for varied electrochemical applications. (A) Flexible, self-supported Pt-doped Ni nanowire mesh for hydrogen evolution reaction applications in a liquid H-cell. Scale bars represent 2 cm (left), 500 μm (centre) and 5 μm (right). Figures adapted from Ref. 154, Copyright © 2018, American Chemical Society. (B) High density Cu nanowire felt for single pass electrochemical synthesis in a flow-through liquid phase reactor. Left and centre schematics highlight large substrate conversion rate thanks to dense 3D structure. Figures adapted from Ref. 155, Copyright © 2019, American Chemical Society. (C) Cu porous hollow fibre gas diffusion electrode for electrochemical CO reduction. CO is flowed through the central void then passes through the porous walls to react at the liquid interface. Scale bars represent 50 μm (left) and 500 μm (centre). Figures adapted from Ref. 156 available under Creative Commons (CC-BY) license, published by Springer Nature 2016. or combinations two or more of these. The scale and computational cost depends on the number of parameters solved for, the number of assumptions made and the overall complexity of the model geometry. At their simplest, 1D models can be used to calculate to show concentration gradients, flow profiles or potential distributions along flow channels or electrode materials, assuming an even distribution across the second dimension. 1D models are built on analytical solutions operating within set boundary conditions. The range of expressions and conditions is extremely broad, being 66,182–186 the subject of a number of excellent reviews. In all cases it is essential to consider the boundary conditions of each model in relation to the experimental conditions in order to ensure a good fit between computational and experimental data. Moving up to 2D allows heterogeneity in catalyst, flow field and other component surfaces to be explicitly modelled. This allows models to incorporate chemical kinetics alongside flow profiles in Figure 11. Distribution of chlorine oxidation current efficiency over a order to explicitly model reactions inside a flow cell, allowing the planar electrode using a mass transport wall function model. The inlet is at user to model the impact of cell potential, feed concentration and 188–190 the bottom, with turbulent flow conditions following a turbulence promotor. flow rate. Changes to the reaction environment during The more turbulent environment close to the inlet results in a greater current operation can also be fed back into the model, such as resistive efficiency at the inlet vs the outlet. Increasing the flow rate gives a greater bubble formation during electrolysis. Modelling via finite-ele- efficiency further along the electrode as turbulence eddies are extended ment, -difference or -volume simulation provides clear visual further down the flow channel. Figure taken from Ref. 199 with permission representations of concentrations, flow rates and pressures, allowing from Elsevier (Copyright © 2018 Elsevier Science S.A. All rights reserved). Journal of The Electrochemical Society, 2020 167 155525 Figure 12. Schematic flow chart for combinatorial electrochemical synthesis via the cation flow system. S and Nu represent possible substrates and nucleophiles respectively. Changing the flow path allows multiple substrate—nucleophile combinations to be systematically produced by the same reactor. Figure adapted from Ref. 227 with permission from Wiley-VCH (Copyright © 2005 Wiley-VCH Verlag GmbH & Co. All rights reserved). reactors designs to be modified to encourage solution mixing and power output. Electron transfer rates have been improved via 192 203–205 remove stagnant zones. targeted modifications to the enzyme structure or through The added computational costs of moving to 3D are often incorporating conductive polymers into the enzyme-electrode 206,207 necessary for complex cell designs, particularly when investigating assembly. Alternatively, electron transfer can be mediated 193,194 195 the impact of 3D electrodes, parallel stacked cells, flow via conductive nanostructures, as has been achieved with 196–198 199,200 208,209 210,211 212–214 field geometry or turbulence promotors on solution nanoparticles, graphene, and carbon nanotubes. mixing and turbulent flow. Modelling the impact of turbulence Similarly, microbial fuel cells offer environmentally friendly promoters on flow profile and current distribution allows for a better opportunities for reactor designs, particularly with a focus on energy understanding of how to avoid dead zones and give an even current generation and wastewater treatment. These come with a number distribution over a large electrode surface (Fig. 11). of important environmental benefits, such as the ability to operate over a wide range of temperatures and pH on diverse types of Immobilised enzyme electrodes and biosynthesis.—The natural biomass without the need for energetically expensive aeration. specificity of enzymes towards a particular reaction offers a The core design uses microorganisms carry out a redox process at an promising route to selective electrochemical synthesis. Reactors electrode surface, such as the oxidation of an organic substrate at the are primarily based on fuel cell designs, where electrochemical anode or nitrification of ammonium to nitrate and subsequent catalysts are replaced with redox active enzymes immobilised onto denitrification to nitrogen gas. an electrode surface. Biofuel cells typically employ fuel-specific Present microbial fuel cell designs are held back by restrictive enzymes at the anode to oxidised the fuel in an aqueous environ- costs; wastewater treatment via microbial cells is currently around ment, with oxygen reducing enzymes such as multi-copper oxidase 30 times higher than treatment via the conventional activated sludge or bilirubin oxidase reducing oxygen at the cathode. process due to the need for expensive electrode, separator and 216,219 Since enzymes already offer excellent reaction specificity, much membrane materials. Advancements in more cost effective research into enzymatic reactors focuses on facilitating the electron materials along with higher power outputs to offset these costs are transfer rate between the enzyme and the electrode to improve the essential in making this technology viable for upscaling and wider Journal of The Electrochemical Society, 2020 167 155525 usage. Costs can be reduced by removing separators in favour of a 3. A wide range of electrode geometry and electrolyte flow simpler single cell design, but these risk bio-fouling at the cathode conditions is accessible in published cell designs. over extended periods of operation or electrocatalyst poisoning. 4. Parallel plate flow cells are the first choice for flow electro- chemical synthesis thanks to their simple assembly and well- Combinatorial approaches to electrosynthesis.—A drawback to defined, reproducible flow profile. Static or magnetically stirred electrochemical synthesis is the scale of parameters that must be beaker cells will still have a place in the electrochemical refined in order to maximise the reaction yield, including but not laboratory, but they are entirely unsuitable when considering limited to electrode material, surface structure, electrolyte, solvent, scale-up operations. overpotential and temperature. A combinatorial approach allows for 5. Electrochemically active surface area can be markedly increased the bath analysis of multiple parameters, which greatly accelerates the via 3D porous electrodes. There are a wealth of different 3D 222,223 optimisation process (Fig. 12). The first examples used a series electrode materials readily available, including multiple carbon, of electrodes in a well electrolysis platform to perform galvanostatic textile, metal and metal composite structures. All of these 224–226 electrosynthesis in wells containing different reagents. materials can be used as suppled or with additional catalytic Rapid parameterisation has also been achieved with microflow coatings, which can be readily tunes to the catalytic needs of a systems, thanks to their precise control over mass transport regime specific electrochemical reaction via physical, chemical or and residence time. Using a flow-switching system allows the electrochemical deposition. As well as catalytic activity, the same anodically or cathodically generated intermediate to be mass transport profile of a number of mesh, foam and felt introduced to multiple reagents in a combinatorial approach. electrodes has been characterised. Other reactor designs isolate multiple materials in individual 6. The increasing accessibility of 3D printing facilities has been of compartments to assess their activity for the same reaction. great benefit to the field of electrochemical reactor design. Reactors designs incorporate individually addressable sensors to Rapid prototyping is possible for reactor components, especially assess conversion rate, such as imaging bubble size for water for polymer-based flow fields and turbulence promotors. electrolysis or hyphenating to a secondary analysis equipment Recently, this has been extended to the 3D printing of via switchable flow channels. electrodes, either by printing with stainless steel or by incorpor- ating conductive materials into the polymer to print a conductive Electrodeposited composite materials.—Many materials, in- polymer composite material. cluding metals, alloys, ceramics, polymers and composites can be 7. The composition and architecture of GDE electrodes has been obtained by electrodeposition and/or electrophoretic deposition. greatly expanded to facilitate a broad range of electrochemical While common engineering applications have focussed on e.g., systems, including water and CO electrolysers and hydrogen PTFE or SiC in a nickel matrix for tribological use, both anodic fuel cells. Changes to the hydrophobic structure have been made and cathodic deposition may be used to synthesise a wide variety of to prevent GDE flooding in electrolysers and aid in water materials, including nanostructured and controlled phase compo- management for fuel cells. sition materials. Examples include titanates and titanium oxide 8. Microflow reactor channels, primarily based on the rectangular nanotubes uniformly embedded in a polypyrrole matrix for corrosion channel geometry have been introduced for laboratory scale protection and MoS particles embedded in a nickel matrix for electrosynthesis, particularly in the case of specialty organics. controlled wear resistance and self-lubrication. The importance of Short anode-cathode distances minimise IR drop even in low or controlling the reaction environment around the cathode to tailor the zero electrolyte solvents, and long path lengths give excellent deposit morphology and engineering properties has been conversion rates. highlighted. The choice of electrolyte composition in the field 9. Combinatorial electrochemical approaches provide a convenient of composite electrodeposition has become more constrained in route to study electrosynthesis of a wide range of reactant/ recent years by environmental concerns over perfluorinated surfac- product concentrations. In particular, the deployment of micro- tants (used to help disperse hydrophobic materials such as graphene, flow reactors accelerates this process by switching flow chan- MoS and PTFE) but sol preparation, stability, agitation and nels, allowing an electrochemically generated intermediate to enhancement of mass transport to the cathode can be facilitated by react with multiple substrates. preparation of sols using shear blade mixing and ultrasonic 238 239 agitation. Improved availability of non-ionic liquids and environmentally acceptable water miscible organic acids has Future R & D Needs extended the choice of electrolytes. Despite decades of development Moving forward, continued development of electrochemical and progress with mechanistic descriptions, a universal model reactors will depend on advancements in the following areas: able to describe the rate and composition of composite electro- deposits from a knowledge of process conditions is long awaited. 1. Characterisation of the distribution of fluid flow, potential, current and reactant/product concentrations for all electroche- Conclusions mical reactors. In particular, quantitative and comparative Electrochemical reactors offer practical solutions to numerous characterisation of 3D electrodes, specifically focussing on their challenges in electrochemical synthesis, from combinatorial ap- activity, real surface area and mass transport towards and within proaches to synthesis on a bench-top scale up to industrial produc- their structure. tion on the tonnage scale. In this review, we have addressed the 2. More use of established figures of merit to provide a quantita- following key points: tive statement of cell performance and enable comparisons of cell geometry over various operating conditions. This should 1. Electrochemical cells offer a broad range of possible electrode include studies of batch kinetics under galvanostatic control to geometries and flow conditions, depending on the needs of the examine figures of merit over a wide range of reactant user. Importantly, many electrochemical cells can be developed concentration and fractional conversion. at the laboratory scale, and then up-scaled for industrial 3. Longer term evaluation of newly introduced electrode and applications with minimal modifications to the core design. membrane materials. 2. The use of electrosynthesis cells has extended beyond tonnage 4. Studies demonstrating scale-up over a wide range of electrode scale process chemicals for commodity use to many fine size, cell size, current and production rate of various electrode chemicals used in pharmaceutical and medical products. materials and cell designs for electrosynthesis. Journal of The Electrochemical Society, 2020 167 155525 5. Accessible, international showcases to demonstrate promising 12. F. C. Walsh, L. F. Arenas, and C. Ponce de León, “Developments in electrode design: structure, decoration and applications of electrodes for electrochemical cell components, reactor designs and their performance. technology.” J. Chem. Technol. Biotechnol., 93, 3073 (2018). 6. Increased use and scale-up of microflow channel cells for both 13. G. Kreysa, “Normalized space velocity—a new figure of merit for waste water inorganic and organic electrosynthesis. electrolysis cells.” Electrochim. Acta, 26, 1693 (1981). 7. The sustained tailoring of GDEs beyond precious metal on 14. M. Mascia, A. Vacca, A. M. Polcaro, S. Palmas, J. R. Ruiz, and A. Da Pozzo, “Electrochemical treatment of phenolic waters in presence of chloride with boron- carbon electrodes intended for use in H and O /air in fuel cells 2 2 doped diamond (BDD) anodes: experimental study and mathematical model.” and batteries towards speciality electrosynthesis of materials. J. Hazard. Mater., 174, 314 (2010). 8. Uptake of electrochemical synthesis techniques by the broader 15. R. Oriol, M. d. P. Bernícola, E. Brillas, P. L. Cabot, and I. Sirés, “Paired electro- research community. It is rare to see electrosynthesis techniques oxidation of insecticide imidacloprid and electrodenitrification in simulated and real water matrices.” Electrochim. Acta, 317, 753 (2019). and their success being described in sectors outside electro- 16. E. M. Mattiusi, N. M. S. Kaminari, M. J. J. S. Ponte, and H. A. Ponte, “Behavior chemistry and electrochemical engineering. It is vital to over- analysis of a porous bed electrochemical reactor the treatment of petrochemical come this by improving education of non-electrochemists, industry wastewater contaminated by hydrogen sulfide (H S).” Chem. Eng. J., 275, training of industrial personnel and providing adequate process 305 (2015). 17. A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, and Q. Liu, experience to new practitioners. “Redox flow batteries: a review.” J. Appl. Electrochem., 41, 1137 (2011). 9. The combination of microflow cells with combinatorial techni- 18. X. Ke, J. M. Prahl, J. I. D. Alexander, J. S. Wainright, T. A. Zawodzinski, and R. ques presents exciting opportunities to screen electrocatalysts F. Savinell, “Rechargeable redox flow batteries: flow fields, stacks and design and explore the importance of reaction environment in electro- considerations.” Chem. Soc. Rev., 47, 8721 (2018). 19. L. Arenas, C. Ponce de León, and F. Walsh, “Engineering aspects of the design, synthesis. construction and performance of modular redox flow batteries for energy storage.” 10. The pressure to improve energy efficiency and lower costs J. Energy Storage, 11, 119 (2017). provides an incentive to explore regenerative cells which 20. F. F. Rivera, C. P. de León, J. L. Nava, and F. C. Walsh, “The filter-press FM01- integrate electricity production with electrosynthesis. LC laboratory flow reactor and its applications.” Electrochim. Acta, 163, 338 (2015). 11. The tailoring of electrode structure needs to be further integrated 21. L. Wu, L. F. Arenas, J. E. Graves, and F. C. Walsh, “Flow cell characterisation: with the design of selective, nanostructured electrocatalysts to flow visualisation, pressure drop and mass transport at 2d electrodes in a realise next generation multiscale electrode architecture. rectangular channel.” J. Electrochem. Soc., 167, 043505 (2020). 12. The integration of porous, 3-D electrodes with high photolysis 22. T. R. Ralph, M. L. Hitchman, J. P. Millington, and F. C. Walsh, “Evaluation of a reactor model and cathode materials for batch electrolysis of l-cystine hydro- irradiation offers the opportunity to develop high efficiency chloride.” J. Electroanal. Chem., 462, 97 (1999). hybrid tubular cells, incorporating thin film electrolytes, for 23. S. C. Perry and G. Denuault, “Transient study of the oxygen reduction reaction on rapid generation of in situ oxidants for process intensive indirect reduced Pt and Pt alloys microelectrodes: evidence for the reduction of pre- electrosynthesis. adsorbed oxygen species linked to dissolved oxygen.” Phys. Chem. Chem. Phys., 17, 30005 (2015). 13. More creative use of immobilised enzyme and microbial 24. S. Möhle, M. Zirbes, E. Rodrigo, T. Gieshoff, A. Wiebe, and S. R. Waldvogel, bioelectrodes opens up a wide range of bioelectrosynthetic “Modern electrochemical aspects for the synthesis of value-added organic routes to speciality chemicals. products.” Angew. Chem. Int. Ed., 57, 6018 (2018). 14. Recent improvements in our ability to model and scale two- 25. A. Wiebe, T. Gieshoff, S. Möhle, E. Rodrigo, M. Zirbes, and S. R. Waldvogel, “Electrifying organic synthesis.” Angew. Chem. Int. Ed., 57, 5594 (2018). phase, solid particle/liquid electrolyte systems provides a new 26. D. Faggion, W. D. G. Gonçalves, and J. Dupont, “CO electroreduction in ionic platform for studies in electrosynthesis of chemicals and liquids.” Front. Chem., 7, 102 (2019). composite electrodeposits. 27. M. Kathiresan and D. Velayutham, “Ionic liquids as an electrolyte for the electro 15. The performance vs construction/cost compromises involved in synthesis of organic compounds.” Chem. Commun., 51, 17499 (2015). 28. R. J. Marshall and F. C. Walsh, “A review of some recent electrolytic cell selecting or progressing a particular reactor design for an designs.” Surf. Technol., 24, 45 (1985). electrosynthesis are poorly documented in the literature; case 29. F. C. Walsh and D. Pletcher, “Electrochemical engineering and cell design in.” study examples of capital and running costs are rarely seen. Developments in Electrochemistry: Science Inspired by Martin Fleischmann, ed. D. Pletcher, Z. Q. Tian, and D. E. Williams (Wiley, Chichester) p. 95 (2014). 30. F. Goodridge and K. Scott, Electrochemical Process Engineering: A Guide to the ORCID Design of Electrolytic Plant (Springer, US) (2013). 31. H. Wendt and G. Kreysa, Electrochemical Engineering: Science and Technology Samuel C. Perry https://orcid.org/0000-0002-6263-6114 in Chemical and Other Industries (Springer, Berlin Heidelberg) (2013). Carlos Ponce de León https://orcid.org/0000-0002-1907-5913 32. M. Fleischmann and J. W. Oldfield, “Fluidised bed electrodes: part I. Polarisation predicted by simplified models.” J. Electroanal. Chem. Interf. Electrochem., 29, References 211 (1971). 33. F. C. Walsh and G. W. Reade, “Electrochemical techniques for the treatment of 1. D. Pletcher and F. C. Walsh, Industrial Electrochemistry (Springer, Netherlands) dilute metal-ion solutions.” Studies in Environmental Science, ed. C. A. (1990). C. Sequeira (Elsevier, Amsterdam) p. 3 (1994). 2. K. Scott, Electrochemical Reaction Engineering (Academic Press, New York) 34. T. F. Fuller and J. N. Harb, Electrochemical Engineering (Wiley, New York) (1991). (2018). 3. J. D. Genders and D. Pletcher, Electrosynthesis: From Laboratory, to Pilot, to 35. M. Fleischmann and R. E. W. Jansson, “The application of the principles of Production: 3rd International Forum on Electrolysis in the Chemical Industry: reaction engineering to electrochemical cell design.” J. Chem. Technol. Papers (Electrosynthesis Company, New York) (1990). Biotechnol., 30, 351 (1980). 4. O. Hammerich and B. Speiser, Organic Electrochemistry: Revised and Expanded 36. D. Pletcher, R. A. Green, and R. C. D. Brown, “Flow electrolysis cells for the (CRC Press, Boca Raton, FL) (2015). synthetic organic chemistry laboratory.” Chem. Rev., 118, 4573 (2018). 5. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and 37. C. Salazar, I. Sirés, C. A. Zaror, and E. Brillas, “Treatment of a mixture of Applications (Wiley Textbooks, New York) 2nd ed. (2000). chloromethoxyphenols in hypochlorite medium by electrochemical AOPs as an 6. F. C. Walsh and C. Ponce de León, “Progress in electrochemical flow reactors for alternative for the remediation of pulp and paper mill process waters.” laboratory and pilot scale processing.” Electrochim. Acta, 280, 121 (2018). Electrocatalysis, 4, 212 (2013). 7. S. Lakshmanan and T. Murugesan, “The chlor-alkali process: work in progress.” 38. A. S. Ochoa-Chavez, A. Pieczyńska, A. Fiszka Borzyszkowska, P. J. Espinoza- Clean Technol. Environ. Policy, 16, 225 (2014). Montero, and E. M. Siedlecka, “Electrochemical degradation of 5-FU using a flow 8. D. E. Blanco, P. A. Prasad, K. Dunningan, and M. A. Modestino, “Insights into reactor with BDD electrode: comparison of two electrochemical systems.” membrane-separated organic electrosynthesis: the case of adiponitrile electro- Chemosphere, 201, 816 (2018). chemical production.” React. Chem. Eng., 5, 136 (2020). 39. C. Ponce de León, F. C. Walsh, D. Pletcher, D. J. Browning, and J. B. Lakeman, 9. C. Ponce de León, G. W. Reade, I. Whyte, S. E. Male, and F. C. Walsh, “Direct borohydride fuel cells.” J. Power Sources, 155, 172 (2006). “Characterization of the reaction environment in a filter-press redox flow reactor.” 40. S. S. Daud, M. A. Norrdin, J. Jaafar, and R. Sudirman, “The effect of material on Electrochim. Acta, 52, 5815 (2007). bipolar membrane fuel cell performance: a review.” IOP Conf. Ser.: Mater. Sci. 10. R. G. A. Wills and F. C. Walsh, “7 - electroplating for protection against wear.” Eng., 736, 032003 (2020). Surface Coatings for Protection Against Wear, ed. B. G. Mellor (Woodhead 41. M. A. Blommaert, J. A. H. Verdonk, H. C. B. Blommaert, W. A. Smith, and Publishing, Cambridge) p. 226 (2006). D. A. Vermaas, “Reduced ion crossover in bipolar membrane electrolysis via 11. U. Landau, “Three-electrode measurements in industrial cells.” J. Electrochem. increased current density, molecular size, and valence.” ACS Appl. Energy Mater., Soc., 135, 396 (1988). 3, 5804 (2020). Journal of The Electrochemical Society, 2020 167 155525 42. C. Shen, R. Wycisk, and P. N. Pintauro, “High performance electrospun bipolar 72. M. A. Cataldo-Hernández, A. Bonakdarpour, J. T. English, M. Mohseni, and membrane with a 3D junction.” Energy Environ. Sci., 10, 1435 (2017). D. P. Wilkinson, “A membrane-based electrochemical flow reactor for generation 43. R. Alkire and P. K. Ng, “Studies on flow‐by porous electrodes having of ferrates at near neutral pH conditions.” React. Chem. Eng., 4, 1116 (2019). perpendicular directions of current and electrolyte flow.” J. Electrochem. Soc., 73. S. C. Perry, D. Pangotra, L. Vieira, L.-I. Csepei, V. Sieber, L. Wang, C. Ponce de 124, 1220 (1977). León, and F. C. Walsh, “Electrochemical synthesis of hydrogen peroxide from 44. C. Ponce de León, I. Whyte, G. W. Reade, S. E. Male, and F. C. Walsh, “Mass water and oxygen.” Nat. Rev. Chem., 3, 442 (2019). transport and flow dispersion in the compartments of a modular 10 cell filter-press 74. R. A. Green, R. C. D. Brown, and D. Pletcher, “Understanding the performance of stack.” Aust. J. Chem., 61, 797 (2008). a microfluidic electrolysis cell for routine organic electrosynthesis.” J. Flow 45. I. Garagounis, A. Vourros, D. Stoukides, D. Dasopoulos, and M. Stoukides, Chem., 5, 31 (2015). “Electrochemical synthesis of ammonia: recent efforts and future outlook.” 75. R. A. Green, R. C. D. Brown, D. Pletcher, and B. Harji, “An extended channel Membranes, 9, 112 (2019). length microflow electrolysis cell for convenient laboratory synthesis.” 46. R. Kas, K. Yang, D. Bohra, R. Kortlever, T. Burdyny, and W. Smith, Electrochem. Commun., 73, 63 (2016). “Electrochemical CO reduction on nanostructured metal electrodes: fact or 76. L. F. Arenas, F. C. Walsh, and C. P. de León, “3D-printing of redox flow batteries defect?” Chem. Sci., 11, 1738 (2020). for energy storage: a rapid prototype laboratory cell.” ECS J. Solid State SC, 4, 47. C. Wang et al., “Construction of a microchannel electrochemical reactor with a P3080 (2015). monolithic porous-carbon cathode for adsorption and degradation of organic 77. H. Piri, X. T. Bi, H. Li, and H. Wang, “3D-printed fuel-cell bipolar plates for pollutants in several minutes of retention time.” Environ. Sci. Technol., 54, 1920 evaluating flow-field performance.” Clean Energy, 4, 142 (2020). (2020). 78. J. R. Hudkins, D. G. Wheeler, B. Peña, and C. P. Berlinguette, “Rapid prototyping 48. M. A. Khan, H. Zhao, W. Zou, Z. Chen, W. Cao, J. Fang, J. Xu, L. Zhang, and of electrolyzer flow field plates.” Energy Environ. Sci., 9, 3417 (2016). J. Zhang, “Recent progresses in electrocatalysts for water electrolysis.” 79. T. Pérez, L. F. Arenas, D. Villalobos-Lara, N. Zhou, S. Wang, F. C. Walsh, Electrochem. Energy Rev., 1, 483 (2018). J. L. Nava, and C. P. de León, “Simulations of fluid flow, mass transport and 49. R. Alkire and B. Gracon, “Flow‐through porous electrodes.” J. Electrochem. Soc., current distribution in a parallel plate flow cell during nickel electrodeposition.” 122, 1594 (1975). J. Electroanal. Chem., 873, 114359 (2020). 50. L. F. Arenas, C. P. d. León, and F. C. Walsh, “Mass transport and active area of 80. C. Ponce de León, W. Hussey, F. Frazao, D. Jones, E. Ruggeri, S. Tzortzatos, porous Pt/Ti electrodes for the Zn-Ce redox flow battery determined from limiting R. D. Mckerracher, R. G. A. Wills, S. Yang, and F. C. Walsh, “The 3D printing of current measurements.” Electrochim. Acta, 221, 154 (2016). a polymeric electrochemical cell body and its characterisation.” Chem. Eng. 51. L. F. Castañeda, F. C. Walsh, J. L. Nava, and C. Ponce de León, “Graphite felt as a Trans., 41, 1 (2014). versatile electrode material: properties, reaction environment, performance and 81. S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, and R. Mülhaupt, “Polymers for 3D applications.” Electrochim. Acta, 258, 1115 (2017). printing and customized additive manufacturing.” Chem. Rev., 117, 10212 (2017). 52. I. Mustafa, R. Susantyoko, C.-H. Wu, F. Ahmed, R. Hashaikeh, F. Almarzooqi, 82. L. F. Arenas, N. Kaishubayeva, C. Ponce de León, and F. C. Walsh, and S. Almheiri, “Nanoscopic and macro-porous carbon nano-foam electrodes “Electrodeposition of platinum on 3D-printed titanium mesh to produce tailored, with improved mass transport for vanadium redox flow batteries.” Sci. Rep., 9, high area anodes.” Trans. Inst. Met. Finish., 98, 48 (2020). 17655 (2019). 83. J. C. Bui, J. T. Davis, and D. V. Esposito, “3D-printed electrodes for membrane- 53. L. F. Arenas, R. P. Boardman, C. Ponce de León, and F. C. Walsh, “X-ray less water electrolysis.” Sustain. Energy Fuels, 4, 213 (2020). computed micro-tomography of reticulated vitreous carbon.” Carbon, 135,85 84. J. P. Hughes, P. L. dos Santos, M. P. Down, C. W. Foster, J. A. Bonacin, E. (2018). M. Keefe, S. J. Rowley-Neale, and C. E. Banks, “Single step additive 54. L. F. Arenas, C. Ponce de León, R. P. Boardman, and F. C. Walsh, manufacturing (3D printing) of electrocatalytic anodes and cathodes for efficient “Characterisation of platinum electrodeposits on a titanium micromesh stack in water splitting.” Sustain. Energy Fuels, 4, 302 (2020). a rectangular channel flow cell.” Electrochim. Acta, 247, 994 (2017). 85. Q. Sun, J. Wang, M. Tang, L. Huang, Z. Zhang, C. Liu, X. Lu, K. W. Hunter, and 55. L. F. Arenas, C. P. de León, R. P. Boardman, and F. C. Walsh, “Editors’ choice - G. Chen, “A new electrochemical system based on a flow-field shaped solid 2+ electrodeposition of platinum on titanium felt in a rectangular channel flow cell.” electrode and 3D-printed thin-layer flow cell: detection of Pb ions by continuous J. Electrochem. Soc., 164, D57 (2016). flow accumulation square-wave anodic stripping voltammetry.” Anal. Chem., 89, 56. M.-S. Park, N.-J. Lee, S.-W. Lee, K. J. Kim, D.-J. Oh, and Y.-J. Kim, “High- 5024 (2017). energy redox-flow batteries with hybrid metal foam electrodes.” ACS Appl. Mater. 86. G. D. O’Neil, S. Ahmed, K. Halloran, J. N. Janusz, A. Rodríguez, and I. Interfaces, 6, 10729 (2014). M. Terrero Rodríguez, “Single-step fabrication of electrochemical flow cells 57. L. F. Arenas, C. Ponce de León, and F. C. Walsh, “Critical review - the versatile utilizing multi-material 3D printing.” Electrochem. Commun., 99, 56 (2019). plane parallel electrode geometry: an illustrated review.” J. Electrochem. Soc., 87. G. Chisholm, P. J. Kitson, N. D. Kirkaldy, L. G. Bloor, and L. Cronin, “3D printed 167, 023504 (2020). flow plates for the electrolysis of water: an economic and adaptable approach to 58. V. Egorov and C. O’Dwyer, “Architected porous metals in electrochemical energy device manufacture.” Energy Environ. Sci., 7, 3026 (2014). storage.” Curr. Opin. Electrochem., 21, 201 (2020). 88. H. H. Hamzah, S. A. Shafiee, A. Abdalla, and B. A. Patel, “3D printable 59. Z. Liu et al., “Three-dimensional ordered porous electrode materials for electro- conductive materials for the fabrication of electrochemical sensors: a mini chemical energy storage.” NPG Asia Mater., 11, 12 (2019). review.” Electrochem. Commun., 96, 27 (2018). 60. L. F. Arenas, C. Ponce de León, and F. C. Walsh, “Three-dimensional porous 89. D. Krishnamurthy, E. O. Johansson, J. W. Lee, and E. Kjeang, “Computational metal electrodes: fabrication, characterisation and use.” Curr. Opin. Electrochem., modeling of microfluidic fuel cells with flow-through porous electrodes.” J. Power 16, 1 (2019). Sources, 196, 10019 (2011). 61. W. Tiedemann and J. Newman, “Maximum effective capacity in an ohmically 90. E. Kjeang, R. Michel, D. A. Harrington, N. Djilali, and D. Sinton, “Amicrofluidic limited porous electrode.” J. Electrochem. Soc., 122, 1482 (1975). fuel cell with flow-through porous electrodes.” J. Am. Chem. Soc., 130, 4000 (2008). 62. J. L. Nava, M. T. Oropeza, C. Ponce de León, J. González-García, and A. J. Frías- 91. L. Li, K. Zheng, M. Ni, M. K. H. Leung, and J. Xuan, “Partial modification of Ferrer, “Determination of the effective thickness of a porous electrode in a flow- flow-through porous electrodes in microfluidic fuel cell.” Energy, 88, 563 (2015). through reactor; effect of the specific surface area of stainless steel fibres, used as a 92. T. Hong,S. Lee,P.Ohodnicki,and K. Brinkman, “A highly scalable spray coating porous cathode, during the deposition of Ag(I) ions.” Hydrometallurgy, 91,98 technique for electrode infiltration: barium carbonate infiltrated La Sr Co Fe O 0.6 0.4 0.2 0.8 3-δ (2008). perovskite structured electrocatalyst with demonstrated long term durability.” Int. J. 63. P. Lobaccaro, M. R. Singh, E. L. Clark, Y. Kwon, A. T. Bell, and J. W. Ager, Hydrogen Energy, 42, 24978 (2017). “Effects of temperature and gas–liquid mass transfer on the operation of small 93. T. Li, E. W. Lees, M. Goldman, D. A. Salvatore, D. M. Weekes, and electrochemical cells for the quantitative evaluation of CO reduction electro- C. P. Berlinguette, “Electrolytic conversion of bicarbonate into CO in a flow catalysts.” Phys. Chem. Chem. Phys., 18, 26777 (2016). cell.” Joule, 3, 1487 (2019). 64. J. O. M. Bockris and B. E. Conway, Modern Aspects of Electrochemistry No. 6 94. L. Fan, C. Xia, F. Yang, J. Wang, H. Wang, and Y. Lu, “Strategies in catalysts and (Springer, US) (2012). electrolyzer design for electrochemical CO( ) reduction toward C( ) products.” 2 2+ 65. G. Hilt, “Basic strategies and types of applications in organic electrochemistry.” Sci. Adv., 6, eaay3111 (2020). ChemElectroChem, 7, 395 (2020). 95. B. Tjaden, D. J. L. Brett, and P. R. Shearing, “Tortuosity in electrochemical 66. F. F. Rivera, C. P. d. León, F. C. Walsh, and J. L. Nava, “The reaction devices: a review of calculation approaches.” Int. Mater. Rev., 63, 47 (2018). environment in a filter-press laboratory reactor: the FM01-LC flow cell.” 96. S. Kumar, T. Ramamurthy, B. Subramanian, and A. Basha, “Studies on the Electrochim. Acta, 161, 436 (2015). fluidized bed electrode.” Int. J. Chem. React. Eng., 6, 1 (2008). 67. K. Scott, Sustainable and Green Electrochemical Science and Technology (Wiley, 97. A. Tschöpe, S. Heikenwälder, M. Schneider, K. Mandel, and M. Franzreb, New York) (2017). “Electrical conductivity of magnetically stabilized fluidized-bed electrodes— 68. M. Lehmann, C. C. Scarborough, E. Godineau, and C. Battilocchio, “An chronoamperometric and impedance studies.” Chem. Eng. J., 396, 125326 (2020). electrochemical flow-through cell for rapid reactions.” Ind. Eng. Chem. Res., 59, 98. A. P. Manso, F. F. Marzo, J. Barranco, X. Garikano, and M. Garmendia Mujika, 7321 (2020). “Influence of geometric parameters of the flow fields on the performance of a PEM 69. T. Noël, Y. Cao, and G. Laudadio, “The fundamentals behind the use of flow fuel cell. A review.” Int. J. Hydrogen Energy, 37, 15256 (2012). reactors in electrochemistry.” Acc. Chem. Res., 52, 2858 (2019). 99. H. Liu, P. Li, D. Juarez-Robles, K. Wang, and A. Hernandez-Guerrero, 70. R. K. B. Karlsson and A. Cornell, “Selectivity between oxygen and chlorine “Experimental study and comparison of various designs of gas flow fields to evolution in the chlor-alkali and chlorate processes.” Chem. Rev., 116, 2982 PEM fuel cells and cell stack performance.” Front. Energy Res., 2, 2 (2014). (2016). 100. C.-T. Wang, Y.-T. Ou, B.-X. Wu, S. Thangavel, S.-W. Hong, W.-T. Chung, and 71. M.-A. Goulet and E. Kjeang, “Reactant recirculation in electrochemical co- W.-M. Yan, “A modified serpentine flow slab for in proton exchange membrane laminar flow cells.” Electrochim. Acta, 140, 217 (2014). fuel cells (PEMFCs).” Energy Proc., 142, 667 (2017). Journal of The Electrochemical Society, 2020 167 155525 101. R. Gundlapalli and S. Jayanti, “Performance characteristics of several variants of 128. M. Lee and X. Huang, “Development of a hydrophobic coating for the porous gas interdigitated flow fields for flow battery applications.” J. Power Sources, 467, diffusion layer in a PEM-based electrochemical hydrogen pump to mitigate anode 228225 (2020). flooding.” Electrochem. Commun., 100, 39 (2019). 102. V. Manzi-Orezzoli, M. Siegwart, M. Cochet, T. J. Schmidt, and P. Boillat, 129. Q. Zhang, M. Zhou, G. Ren, Y. Li, Y. Li, and X. Du, “Highly efficient “Improved water management for PEFC with interdigitated flow fields using electrosynthesis of hydrogen peroxide on a superhydrophobic three-phase inter- modified gas diffusion layers.” J. Electrochem. Soc., 167, 054503 (2019). face by natural air diffusion.” Nat. Commun., 11, 1731 (2020). 103. X. You, Q. Ye, and P. Cheng, “Scale-up of high power density redox flow batteries 130. A. Xu, B. He, H. Yu, W. Han, J. Li, J. Shen, X. Sun, and L. Wang, “A facile by introducing interdigitated flow fields.” Int. Commun. Heat Mass, 75, 7 (2016). solution to mature cathode modified by hydrophobic dimethyl silicon oil (DMS) 104. M. R. Gerhardt, A. A. Wong, and M. J. Aziz, “The effect of interdigitated channel layer for electro-fenton processes: water proof and enhanced oxygen transport.” and land dimensions on flow cell performance.” J. Electrochem. Soc., 165, A2625 Electrochim. Acta, 308, 158 (2019). (2018). 131. W. V. Fernandez, R. T. Tosello, and J. L. Fernández, “Compact and efficient gas 105. Y. Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, and B. Xiang, “A hierarchical diffusion electrodes based on nanoporous alumina membranes for microfuel cells interdigitated flow field design for scale-up of high-performance redox flow and gas sensors.” Analyst, 145, 122 (2020). batteries.” Appl. Energy, 238, 435 (2019). 132. C.-T. Dinh et al., “CO electroreduction to ethylene via hydroxide-mediated 106. N. J. Cooper, A. D. Santamaria, M. K. Becton, and J. W. Park, “Investigation of copper catalysis at an abrupt interface.” Science, 360, 783 (2018). the performance improvement in decreasing aspect ratio interdigitated flow field 133. G. Chen, G. Zhang, L. Guo, and H. Liu, “Systematic study on the functions and PEMFCs.” Energy Convers. Manage., 136, 307 (2017). mechanisms of micro porous layer on water transport in proton exchange 107. V. Manzi-Orezzoli, M. Siegwart, D. Scheuble, Y.-C. Chen, T. J. Schmidt, and membrane fuel cells.” Int. J. Hydrogen Energy, 41, 5063 (2016). P. Boillat, “Impact of the microporous layer on gas diffusion layers with patterned 134. S. Park, J.-W. Lee, and B. N. Popov, “A review of gas diffusion layer in PEM fuel wettability I: material design and characterization.” J. Electrochem. Soc., 167, cells: materials and designs.” Int. J. Hydrogen Energy, 37, 5850 (2012). 064516 (2020). 135. R. Sandström, J. Ekspong, A. Annamalai, T. Sharifi, A. Klechikov, and 108. D. Niblett, A. Mularczyk, V. Niasar, J. Eller, and S. Holmes, “Two-phase flow T. Wågberg, “Fabrication of microporous layer—free hierarchical gas diffusion dynamics in a gas diffusion layer—gas channel—microporous layer system.” electrode as a low Pt-loading PEMFC cathode by direct growth of helical carbon J. Power Sources, 471, 228427 (2020). nanofibers.” RSC Adv., 8, 41566 (2018). 109. R. Saravanakumar, P. Pirabaharan, M. Abukhaled, and L. Rajendran, “Theoretical 136. S. C. Perry, S. M. Gateman, R. Malpass-Evans, N. McKeown, M. Wegener, analysis of voltammetry at a rotating disk electrode in the absence of supporting P. Nazarovs, J. Mauzeroll, L. Wang, and C. Ponce de León, “Polymers with electrolyte.” J. Phys. Chem. B, 124, 443 (2020). intrinsic microporosity (PIMs) for targeted CO reduction to ethylene.” 110. V. G. Levich and S. T. Ltd, Physicochemical Hydrodynamics (Prentice-Hall, Chemosphere, 248, 125993 (2020). Englewood Cliffs, NJ) (1962). 137. D. Horii, T. Fuchigami, and M. Atobe, “A new approach to anodic substitution 111. M. D. Pohl, S. Haschke, D. Göhl, O. Kasian, J. Bachmann, K. J. J. Mayrhofer, and reaction using parallel laminar flow in a micro-flow reactor.” J. Am. Chem. Soc., I. Katsounaros, “Extension of the rotating disk electrode method to thin samples of 129, 11692 (2007). non-disk shape.” J. Electrochem. Soc., 166, H791 (2019). 138. D. Wang, P. Wang, S. Wang, Y.-H. Chen, H. Zhang, and A. Lei, “Direct 112. M. Rosales and J. L. Nava, “Simulations of turbulent flow, mass transport, and electrochemical oxidation of alcohols with hydrogen evolution in continuous-flow tertiary current distribution on the cathode of a rotating cylinder electrode reactor reactor.” Nat. Commun., 10, 2796 (2019). in continuous operation mode during silver deposition.” J. Electrochem. Soc., 164, 139. C. Huang, X.-Y. Qian, and H.-C. Xu, “Continuous-flow electrosynthesis of E3345 (2017). benzofused S-heterocycles by dehydrogenative C−S cross-coupling.” Angew. 113. F. C. Walsh, G. Kear, A. H. Nahlé, J. A. Wharton, and L. F. Arenas, “The rotating Chem. Int. Ed., 58, 6650 (2019). cylinder electrode for studies of corrosion engineering and protection of metals— 140. B. Gleede, M. Selt, C. Gütz, A. Stenglein, and S. R. Waldvogel, “Large, highly an illustrated review.” Corros. Sci., 123, 1 (2017). modular narrow-gap electrolytic flow cell and application in dehydrogenative 114. D. P. Barkey, R. H. Muller, and C. W. Tobias, “Roughness development in metal cross-coupling of phenols.” Org. Process Res. Dev., 24, 1916 (2020). electrodeposition: I. Experimental results.” J. Electrochem. Soc., 136, 2199 141. D. E. Collin, A. A. Folgueiras-Amador, D. Pletcher, M. E. Light, B. Linclau, and (1989). R. C. D. Brown, “Cubane electrochemistry: direct conversion of cubane carboxylic 115. Z. Solomenko, Y. Haroun, M. Fourati, F. Larachi, C. Boyer, and F. Augier, acids to alkoxy cubanes using the hofer–moest reaction under flow conditions.” “Liquid spreading in trickle-bed reactors: experiments and numerical simulations Chem. Eur. J., 26, 374 (2020). using eulerian–eulerian two-fluid approach.” Chem. Eng. Sci., 126, 698 (2015). 142. M. R. Chapman, S. E. Henkelis, N. Kapur, B. N. Nguyen, and C. E. Willans, 116. P. Trinidad, F. C. Walsh, S. A. Sheppard, S. P. Gillard, and S. A. Campbell, “The “A straightforward electrochemical approach to imine- and amine-bisphenolate effect of operational parameters on the performance of a bipolar trickle tower metal complexes with facile control over metal oxidation state.” ChemistryOpen, reactor.” J. Chem. Technol. Biotechnol., 79, 954 (2004). 5, 351 (2016). 117. Y. Yavuz, A. S. Koparal, and Ü. B. Öğütveren, “Phenol degradation in a bipolar 143. A. E. Delorme, V. Sans, P. Licence, and D. A. Walsh, “Tuning the reactivity of trickle tower reactor using boron-doped diamond electrode.” J. Environ. Eng., 134, tempo during electrocatalytic alcohol oxidations in room-temperature ionic 24 (2008). liquids.” ACS Sustain. Chem. Eng., 7, 11691 (2019). 118. M. A. Islam, S. C. Lam, Y. Li, M. A. Atia, P. Mahbub, P. N. Nesterenko, B. Paull, 144. M. B. Plutschack, B. Pieber, K. Gilmore, and P. H. Seeberger, “The Hitchhiker’s and M. Macka, “Capillary gap flow cell as capillary-end electrochemical detector guide to flow chemistry.” Chem. Rev., 117, 11796 (2017). in flow-based analysis.” Electrochim. Acta, 303, 85 (2019). 145. Y. Liu, G. Chen, and J. Yue, “Manipulation of gas-liquid-liquid systems in continuous 119. A. Fankhauser, L. Ouattara, U. Griesbach, A. Fischer, H. Pütter, and flow microreactors for efficient reaction processes.” J. Flow Chem., 10, 103 (2020). C. Comninellis, “Investigation of the anodic acetoxylation of p-methylanisole 146. D. Karan and S. Khan, “Mesoscale triphasic flow reactors for metal catalyzed 2 3 (p-MA) in glacial acetic acid medium using graphite (sp ) and BDD (sp ) gas–liquid reactions.” React. Chem. Eng., 4, 1331 (2019). electrodes.” J. Electroanal. Chem., 614, 107 (2008). 147. W. Guo, H. J. Heeres, and J. Yue, “Continuous synthesis of 5-hydroxymethyl- 120. J.-I. Yoshida, H. Kim, and A. Nagaki, “‘Impossible’ chemistries based on flow and furfural from glucose using a combination of AlCl and HCl as catalyst in a micro.” J. Flow Chem., 7, 60 (2017). biphasic slug flow capillary microreactor.” Chem. Eng. J., 381, 122754 (2020). 121. C. A. Paddon, M. Atobe, T. Fuchigami, P. He, P. Watts, S. J. Haswell, 148. G. Laudadio, E. Barmpoutsis, C. Schotten, L. Struik, S. Govaerts, D. L. Browne, G. J. Pritchard, S. D. Bull, and F. Marken, “Towards paired and coupled electrode and T. Noël, “Sulfonamide synthesis through electrochemical oxidative coupling reactions for clean organic microreactor electrosyntheses.” J. Appl. Electrochem., of amines and thiols.” J. Am. Chem. Soc., 141, 5664 (2019). 36, 617 (2006). 149. S. Momeni and D. Nematollahi, “Electrosynthesis of new quinone sulfonimide 122. A. Jayakumar, S. Singamneni, M. Ramos, A. M. Al-Jumaily, and S. S. Pethaiah, derivatives using a conventional batch and a new electrolyte-free flow cell.” Green “Manufacturing the gas diffusion layer for PEM fuel cell using a novel 3D printing Chem., 20, 4036 (2018). technique and critical assessment of the challenges encountered.” Materials, 10, 150. F. García-Moreno, “Commercial applications of metal foams: their properties and 796 (2017). production.” Materials, 9, 85 (2016). 123. T. Pérez, G. Coria, I. Sirés, J. L. Nava, and A. R. Uribe, “Electrosynthesis of 151. P. Zhu and Y. Zhao, “Mass transfer performance of porous nickel manufactured by hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion lost carbonate sintering process.” Adv. Eng. Mater., 19, 1700392 (2017). cathode.” J. Electroanal. Chem., 812, 54 (2018). 152. X. Huang, S. Chang, W. S. V. Lee, J. Ding, and J. Xue, “Three-dimensional 124. M. Koj, J. Qian, and T. Turek, “Novel alkaline water electrolysis with nickel-iron printed cellular stainless steel as high-activity catalytic electrode for oxygen gas diffusion electrode for oxygen evolution.” Int. J. Hydrogen Energy, 44, 29862 evolution.” J. Mater. Chem. A, 5, 18176 (2017). (2019). 153. J. Lölsberg, O. Starck, S. Stiefel, J. Hereijgers, T. Breugelmans, and M. Wessling, 125. S. C. Perry, P.-K. Leung, L. Wang, and C. Ponce de León, “Developments on “3D-printed electrodes with improved mass transport properties.” ChemElectroChem, carbon dioxide reduction: their promise, achievements, and challenges.” Curr. 4, 3309 (2017). Opin. Electrochem., 20, 88 (2020). 154. S. P. Zankowski and P. M. Vereecken, “Combining high porosity with high 126. R. B. Ferreira, D. S. Falcão, V. B. Oliveira, and A. M. F. R. Pinto, “Experimental surface area in flexible interconnected nanowire meshes for hydrogen generation study on the membrane electrode assembly of a proton exchange membrane fuel and beyond.” ACS Appl. Mater. Interfaces, 10, 44634 (2018). cell: effects of microporous layer, membrane thickness and gas diffusion layer 155. M. J. Kim, Y. Seo, M. A. Cruz, and B. J. Wiley, “Metal nanowire felt as a flow- hydrophobic treatment.” Electrochim. Acta, 224, 337 (2017). through electrode for high-productivity electrochemistry.” ACS Nano, 13, 6998 127. R. Omrani and B. Shabani, “Gas diffusion layer modifications and treatments for (2019). improving the performance of proton exchange membrane fuel cells and 156. R. Kas, K. K. Hummadi, R. Kortlever, P. de Wit, A. Milbrat, M. W. J. Luiten- electrolysers: a review.” Int. J. Hydrogen Energy, 42, 28515 (2017). Olieman, N. E. Benes, M. T. M. Koper, and G. Mul, “Three-dimensional porous Journal of The Electrochemical Society, 2020 167 155525 hollow fibre copper electrodes for efficient and high-rate electrochemical carbon 181. L. C. Brée, M. Wessling, and A. Mitsos, “Modular modeling of electrochemical dioxide reduction.” Nat. Commun., 7, 10748 (2016). reactors: comparison of CO -electolyzers.” Comput. Chem. Eng., 139, 106890 157. R. Poupart, B. Le Droumaguet, M. Guerrouache, D. Grande, and B. Carbonnier, (2020). “Gold nanoparticles immobilized on porous monoliths obtained from disulfide- 182. L. F. Catañeda, F. F. Rivera, T. Pérez, and J. L. Nava, “Mathematical modeling based dimethacrylate: application to supported catalysis.” Polymer, 126, 455 and simulation of the reaction environment in electrochemical reactors.” Curr. (2017). Opin. Electrochem., 16, 75 (2019). 158. L. Wan, Y. Qin, and J. Xiang, “Rapid electrochemical fabrication of porous gold 183. J. N. Hakizimana, B. Gourich, M. Chafi, Y. Stiriba, C. Vial, P. Drogui, and J. Naja, nanoparticles for high-performance electrocatalysis towards oxygen reduction.” “Electrocoagulation process in water treatment: a review of electrocoagulation Electrochim. Acta, 238, 220 (2017). modeling approaches.” Desalination, 404, 1 (2017). 159. M. Zaghdoudi, L. Moreaud, P. Even-Hernandez, V. Marchi, F. Fourcade, 184. A. Roldan, “Frontiers in first principles modelling of electrochemical simulations.” A. Amrane, H. Maghraoui-Meherzi, and F. Geneste, “Immobilization of synthetic Curr. Opin. Electrochem., 10, 1 (2018). gold nanoparticles on a three-dimensional porous electrode.” Electrochem. 185. L. I. Stephens and J. Mauzeroll, “Demystifying mathematical modeling of Commun., 88, 15 (2018). electrochemical systems.” J. Chem. Educ., 96, 2217 (2019). 160. V. Vedharathinam, Z. Qi, C. Horwood, B. Bourcier, M. Stadermann, J. Biener, and 186. A. Taqieddin, R. Nazari, L. Rajic, and A. Alshawabkeh, “Review - physicochem- M. Biener, “Using a 3D porous flow-through electrode geometry for high-rate ical hydrodynamics of gas bubbles in two phase electrochemical systems.” electrochemical reduction of CO to CO in ionic liquid.” ACS Catal., 9, 10605 J. Electrochem. Soc., 164, E448 (2017). (2019). 187. M. R. Cruz-Díaz, E. P. Rivero, F. A. Rodríguez, and R. Domínguez-Bautista, 161. E. Verlato, W. He, A. Amrane, S. Barison, D. Floner, F. Fourcade, F. Geneste, “Experimental study and mathematical modeling of the electrochemical degrada- M. Musiani, and R. Seraglia, “Preparation of silver-modified nickel foams by tion of dyeing wastewaters in presence of chloride ion with dimensional stable galvanic displacement and their use as cathodes for the reductive dechlorination of anodes (DSA) of expanded meshes in a FM01-LC reactor.” Electrochim. Acta, herbicides.” ChemElectroChem, 3, 2084 (2016). 260, 726 (2018). 162. U. Rost, P. Podleschny, M. Schumacher, R. Muntean, D. T. Pascal, C. Mutascu, 188. K. Wu, E. Birgersson, B. Kim, P. J. A. Kenis, and I. A. Karimi, “Modeling and J. Koziolek, G. Marginean, and M. Brodmann, “Long-term stable electrodes based experimental validation of electrochemical reduction of CO to CO in a on platinum electrocatalysts supported on titanium sintered felt for the use in PEM microfluidic cell.” J. Electrochem. Soc., 162, F23 (2014). fuel cells.” IOP Conf. Ser.: Mater. Sci. Eng., 416, 012013 (2018). 189. M. R. Cruz-Díaz, E. P. Rivero, F. J. Almazán-Ruiz, Á. Torres-Mendoza, and 163. C. A. Martins, O. A. Ibrahim, P. Pei, and E. Kjeang, “In situ decoration of metallic I. González, “Design of a new FM01-LC reactor in parallel plate configuration catalysts in flow-through electrodes: application of Fe/Pt/C for glycerol oxidation using numerical simulation and experimental validation with residence time in a microfluidic fuel cell.” Electrochim. Acta, 305, 47 (2019). distribution (RTD).” Chem. Eng. Process., 85, 145 (2014). 164. N. Sergienko and J. Radjenovic, “Manganese oxide-based porous electrodes for 190. L. I. Stephens, S. C. Perry, S. M. Gateman, R. Lacasse, R. Schulz, and rapid and selective (electro)catalytic removal and recovery of sulfide from J. Mauzeroll, “Development of a model for experimental data treatment of wastewater.” Appl. Catal., B, 267, 118608 (2020). diffusion and activation limited polarization curves for magnesium and steel 165. C. Wang, L. Yue, S. Wang, Y. Pu, X. Zhang, X. Hao, W. Wang, and S. Chen, alloys.” J. Electrochem. Soc., 164, E3576 (2017). “Role of electric field and reactive oxygen species in enhancing antibacterial 191. A. N. Colli and H. H. Girault, “Compact and general strategy for solving current activity: a case study of 3D Cu foam electrode with branched CuO–ZnO NWs.” and potential distribution in electrochemical cells composed of massive monopolar J. Phys. Chem. C, 122, 26454 (2018). and bipolar electrodes.” J. Electrochem. Soc., 164, E3465 (2017). 166. F. A. Lowenheim and J. Davis, “Modern electroplating.” J. Electrochem. Soc., 192. S. Li and B. Sundén, “Effects of gas diffusion layer deformation on the transport 121, 314 (1974). phenomena and performance of PEM fuel cells with interdigitated flow fields.” Int. 167. G. Sievers, T. Vidakovic-Koch, C. Walter, F. Steffen, S. Jakubith, A. Kruth, J. Hydrogen Energy, 43, 16279 (2018). D. Hermsdorf, K. Sundmacher, and V. Brüser, “Ultra-low loading Pt-sputtered gas 193. R. Cervantes-Alcalá and M. Miranda-Hernández, “Flow distribution and mass diffusion electrodes for oxygen reduction reaction.” J. Appl. Electrochem., 48, 221 transport analysis in cell geometries for redox flow batteries through computa- (2018). tional fluid dynamics.” J. Appl. Electrochem., 48, 1243 (2018). 168. J. M. Roemers-van Beek, Z.-J. Wang, A. Rinaldi, M. G. Willinger, and L. Lefferts, 194. A. S. Danis, W. L. Odette, S. C. Perry, S. Canesi, H. F. Sleiman, and J. Mauzeroll, “Initiation of carbon nanofiber growth on polycrystalline nickel foam under low “Cuvette-based electrogenerated chemiluminescence detection system for the ethylene pressure.” ChemCatChem, 10, 3107 (2018). assessment of polymerizable ruthenium luminophores.” ChemElectroChem, 4, 169. R. Mao, C. Huang, X. Zhao, M. Ma, and J. Qu, “Dechlorination of triclosan by 1736 (2017). enhanced atomic hydrogen-mediated electrochemical reduction: kinetics, me- 195. M. A. Sandoval, R. Fuentes, F. C. Walsh, J. L. Nava, and C. P. de León, chanism, and toxicity assessment.” Appl. Catal., B, 241, 120 (2019). “Computational fluid dynamics simulations of single-phase flow in a filter-press 170. M. J. Harding, S. Brady, H. O’Connor, R. Lopez-Rodriguez, M. D. Edwards, flow reactor having a stack of three cells.” Electrochim. Acta, 216, 490 (2016). S. Tracy, D. Dowling, G. Gibson, K. P. Girard, and S. Ferguson, “3D printing of 196. L. F. Castañeda and J. L. Nava, “Simulations of single-phase flow in an up-flow peek reactors for flow chemistry and continuous chemical processing.” React. electrochemical reactor with parallel plate electrodes in a serpentine array.” Chem. Eng., 5, 728 (2020). J. Electroanal. Chem., 832, 31 (2019). 171. M. C. Maier et al., “Development of customized 3D printed stainless steel reactors 197. M. Movahedi, A. Ramiar, and A. A. Ranjber, “3D numerical investigation of with inline oxygen sensors for aerobic oxidation of grignard reagents in clamping pressure effect on the performance of proton exchange membrane fuel continuous flow.” React. Chem. Eng., 4, 393 (2019). cell with interdigitated flow field.” Energy, 142, 617 (2018). 172. A. Ambrosi and R. D. Webster, “3D printing for aqueous and non-aqueous redox 198. T. Noyhouzer, S. C. Perry, A. Vicente-Luis, P. L. Hayes, and J. Mauzeroll, “The flow batteries.” Curr. Opin. Electrochem., 20, 28 (2020). best of both worlds: combining ultramicroelectrode and flow cell technologies.” 173. B. Gutmann, M. Köckinger, G. Glotz, T. Ciaglia, E. Slama, M. Zadravec, J. Electrochem. Soc., 165, H10 (2018). S. Pfanner, M. C. Maier, H. Gruber-Wölfler, and C. Oliver Kappe, “Design and 199. E. P. Rivero, F. A. Rodríguez, M. R. Cruz-Díaz, and I. González, “Reactive 3D printing of a stainless steel reactor for continuous difluoromethylations using diffusion migration layer and mass transfer wall function to model active chlorine fluoroform.” React. Chem. Eng., 2, 919 (2017). generation in a filter press type electrochemical reactor for organic pollutant 174. R. M. Cardoso, D. M. H. Mendonça, W. P. Silva, M. N. T. Silva, E. Nossol, R. A. degradation.” Chem. Eng. Res. Des., 138, 533 (2018). B. da Silva, E. M. Richter, and R. A. A. Muñoz, “3D printing for electroanalysis: 200. F. F. Rivera, L. Castañeda, P. E. Hidalgo, and G. Orozco, “Study of hydro- from multiuse electrochemical cells to sensors.” Anal. Chim. Acta, 1033,49 dynamics at asahitm prototype electrochemical flow reactor, using computational (2018). fluid dynamics and experimental characterization techniques.” Electrochim. Acta, 175. C. G. W. van Melis, M. R. Penny, A. D. Garcia, A. Petti, A. P. Dobbs, S. T. Hilton, 245, 107 (2017). and K. Lam, “Supporting-electrolyte-free electrochemical methoxymethylation of 201. D. Leech, P. Kavanagh, and W. Schuhmann, “Enzymatic fuel cells: recent alcohols using a 3D-printed electrosynthesis continuous flow cell system.” progress.” Electrochim. Acta, 84, 223 (2012). ChemElectroChem, 6, 4144 (2019). 202. P. Pinyou, V. Blay, L. M. Muresan, and T. Noguer, “Enzyme-modified electrodes 176. L. F. Arenas, C. Ponce de León, and F. C. Walsh, “3D-printed porous electrodes for biosensors and biofuel cells.” Mater. Horiz., 6, 1336 (2019). for advanced electrochemical flow reactors: a Ni/stainless steel electrode and its 203. M. A. Dwyer and H. W. Hellinga, “Periplasmic binding proteins: a versatile mass transport characteristics.” Electrochem. Commun., 77, 133 (2017). superfamily for protein engineering.” Curr. Opin. Struct. Biol., 14, 495 (2004). 177. K. Fu, Y. Yao, J. Dai, and L. Hu, “Progress in 3D printing of carbon materials for 204. J. Madoz-Gúrpide, J. M. Abad, J. Fernández-Recio, M. Vélez, L. Vázquez, energy-related applications.” Adv. Mater., 29, 1603486 (2017). C. Gómez-Moreno, and V. M. Fernández, “Modulation of electroenzymatic 178. R. Gusmão, M. P. Browne, Z. Sofer, and M. Pumera, “The capacitance and NADPH oxidation through oriented immobilization of ferredoxin:NADP re- electron transfer of 3D-printed graphene electrodes are dramatically influenced by ductase onto modified gold electrodes.” J. Am. Chem. Soc., 122, 9808 (2000). the type of solvent used for pre-treatment.” Electrochem. Commun., 102,83 205. C. C. Moser, J. L. R. Anderson, and P. L. Dutton, “Guidelines for tunneling in (2019). enzymes.” Biochim. Biophys. Acta, Bioenerg., 1797, 1573 (2010). 179. P. L. dos Santos, V. Katic, H. C. Loureiro, M. F. dos Santos, D. P. dos Santos, A. 206. M. Ates, “A review study of (bio)sensor systems based on conducting polymers.” L. B. Formiga, and J. A. Bonacin, “Enhanced performance of 3D printed graphene Mater. Sci. Eng. C, 33, 1853 (2013). electrodes after electrochemical pre-treatment: role of exposed graphene sheets.” 207. M. Naseri, L. Fotouhi, and A. Ehsani, “Recent progress in the development of Sens. Actuators, B, 281, 837 (2019). conducting polymer-based nanocomposites for electrochemical biosensors appli- 180. E. Vaněčková, M. Bouša, R. Sokolová, P. Moreno-García, P. Broekmann, cations: a mini-review.” Chem. Rec., 18, 599 (2018). V. Shestivska, J. Rathouský, M. Gál, T. Sebechlebská, and V. Kolivoška, 208. A. D. Chowdhury, R. Gangopadhyay, and A. De, “Highly sensitive electroche- “Copper electroplating of 3D printed composite electrodes.” J. Electroanal. mical biosensor for glucose, DNA and protein using gold-polyaniline nanocom- Chem., 858, 113763 (2020). posites as a common matrix.” Sens. Actuators, B, 190, 348 (2014). Journal of The Electrochemical Society, 2020 167 155525 209. K. Murata, K. Kajiya, N. Nakamura, and H. Ohno, “Direct electrochemistry of 225. T. Tajima and A. Nakajima, “Parallel electrosynthesis of N-acyliminium ion bilirubin oxidase on three-dimensional gold nanoparticle electrodes and its equivalents using silica gel-supported piperidine.” Chem. Lett., 38, 160 (2009). application in a biofuel cell.” Energy Environ. Sci., 2, 1280 (2009). 226. C. Gütz, B. Klöckner, and S. R. Waldvogel, “Electrochemical screening for 210. P. Bollella, G. Fusco, C. Tortolini, G. Sanzò, G. Favero, L. Gorton, and electroorganic synthesis.” Org. Process Res. Dev., 20, 26 (2016). R. Antiochia, “Beyond graphene: electrochemical sensors and biosensors for 227. S. Suga, M. Okajima, K. Fujiwara, and J.-I. Yoshida, “Electrochemical combina- biomarkers detection.” Biosens. Bioelectron., 89, 152 (2017). torial organic syntheses using microflow systems.” QSAR Comb. Sci., 24, 728 211. H. Wang, X. Yuan, G. Zeng, Y. Wu, Y. Liu, Q. Jiang, and S. Gu, “Three (2005). dimensional graphene based materials: synthesis and applications from energy 228. M. Atobe, H. Tateno, and Y. Matsumura, “Applications of flow microreactors in storage and conversion to electrochemical sensor and environmental remediation.” electrosynthetic processes.” Chem. Rev., 118, 4541 (2018). Adv. Colloid Interface Sci., 221, 41 (2015). 229. K. Saito, K. Ueoka, K. Matsumoto, S. Suga, T. Nokami, and J.-I. Yoshida, 212. H. Zhong, R. Yuan, Y. Chai, W. Li, X. Zhong, and Y. Zhang, “In situ chemo- “Indirect cation-flow method: flash generation of alkoxycarbenium ions and synthesized multi-wall carbon nanotube-conductive polyaniline nanocomposites: studies on the stability of glycosyl cations.” Angew. Chem. Int. Ed., 50, 5153 characterization and application for a glucose amperometric biosensor.” Talanta, (2011). 85, 104 (2011). 230. C. Xiang, S. K. Suram, J. A. Haber, D. W. Guevarra, E. Soedarmadji, J. Jin, and J. 213. S. Chiashi, K. Kono, D. Matsumoto, J. Shitaba, N. Homma, A. Beniya, M. Gregoire, “High-throughput bubble screening method for combinatorial T. Yamamoto, and Y. Homma, “Adsorption effects on radial breathing mode of discovery of electrocatalysts for water splitting.” ACS Comb. Sci., 16, 47 (2014). single-walled carbon nanotubes.” Physical Review B, 91, 155415 (2015). 231. H. Hashiba, S. Yotsuhashi, M. Deguchi, and Y. Yamada, “Systematic analysis of 214. J. H. T. Luong, S. Hrapovic, D. Wang, F. Bensebaa, and B. Simard, electrochemical CO reduction with various reaction parameters using combina- “Solubilization of multiwall carbon nanotubes by 3-aminopropyltriethoxysilane torial reactors.” ACS Comb. Sci., 18, 203 (2016). towards the fabrication of electrochemical biosensors with promoted electron 232. F. C. Walsh, C. Ponce de León, D. V. Bavykin, C. T. J. Low, S. C. Wang, and transfer.” Electroanalysis, 16, 132 (2004). C. Larson, “The formation of nanostructured surfaces by electrochemical 215. G. Palanisamy, H.-Y. Jung, T. Sadhasivam, M. D. Kurkuri, S. C. Kim, and techniques: a range of emerging surface finishes. Part 2: examples of nanos- S.-H. Roh, “A comprehensive review on microbial fuel cell technologies: tructured surfaces by plating and anodising with their applications.” Trans. Inst. processes, utilization, and advanced developments in electrodes and membranes.” Met. Finish., 93, 241 (2015). J. Clean. Prod., 221, 598 (2019). 233. C. T. J. Low, R. G. A. Wills, and F. C. Walsh, “Electrodeposition of composite 216. L. He, P. Du, Y. Chen, H. Lu, X. Cheng, B. Chang, and Z. Wang, “Advances in coatings containing nanoparticles in a metal deposit.” Surf. Coat. Technol., 201, microbial fuel cells for wastewater treatment.” Renew. Sustain. Energy Rev., 71, 371 (2006). 388 (2017). 234. P. Herrasti, A. N. Kulak, D. V. Bavykin, C. P. de Léon, J. Zekonyte, and 217. Z. He, S. D. Minteer, and L. T. Angenent, “Electricity generation from artificial F. C. Walsh, “Electrodeposition of polypyrrole–titanate nanotube composites wastewater using an upflow microbial fuel cell.” Environ. Sci. Technol., 39, 5262 coatings and their corrosion resistance.” Electrochim. Acta, 56, 1323 (2011). (2005). 235. Y. He, S. C. Wang, F. C. Walsh, Y. L. Chiu, and P. A. S. Reed, “Self-lubricating 218. Y. Park, S. Park, V. K. Nguyen, J. Yu, C. I. Torres, B. E. Rittmann, and T. Lee, Ni-P-MoS composite coatings.” Surf. Coat. Technol., 307, 926 (2016). “Complete nitrogen removal by simultaneous nitrification and denitrification in 236. F. C. Walsh and C. Ponce de Leon, “A review of the electrodeposition of metal flat-panel air-cathode microbial fuel cells treating domestic wastewater.” Chem. matrix composite coatings by inclusion of particles in a metal layer: an established Eng. J., 316, 673 (2017). and diversifying technology.” Trans. Inst. Met. Finish., 92, 83 (2014). 219. V. G. Gude, “Wastewater treatment in microbial fuel cells—an overview.” 237. N. Zhou, S. Wang, and F. C. Walsh, “Effective particle dispersion via high-shear J. Clean. Prod., 122, 287 (2016). mixing of the electrolyte for electroplating a nickel-molybdenum disulphide 220. P. Clauwaert, P. Aelterman, T. H. Pham, L. De Schamphelaire, M. Carballa, composite.” Electrochim. Acta, 283, 568 (2018). K. Rabaey, and W. Verstraete, “Minimizing losses in bio-electrochemical systems: 238. I. Tudela, Y. Zhang, M. Pal, I. Kerr, and A. J. Cobley, “Ultrasound-assisted the road to applications.” Appl. Microbiol. Biotechnol., 79, 901 (2008). electrodeposition of composite coatings with particles.” Surf. Coat. Technol., 259, 221. M. J. Al Lawati, T. Jafary, M. S. Baawain, and A. Al-Mamun, “A mini review on 363 (2014). biofouling on air cathode of single chamber microbial fuel cell; prevention and 239. A. P. Abbott and K. J. McKenzie, “Application of ionic liquids to the mitigation strategies.” Biocatalysis and Agricultural Biotechnology, 22, 101370 (2019). electrodeposition of metals.” Phys. Chem. Chem. Phys., 8, 4265 (2006). 222. K. Mitsudo, Y. Kurimoto, K. Yoshioka, and S. Suga, “Combinatorial electro- 240. F. C. Walsh and C. Ponce de León, “Versatile electrochemical coatings and chemistry for organic synthesis.” Curr. Opin. Electrochem., 8, 8 (2018). surface layers from aqueous methanesulfonic acid.” Surf. Coat. Technol., 259, 676 223. K. Mitsudo, Y. Kurimoto, K. Yoshioka, and S. Suga, “Miniaturization and (2014). combinatorial approach in organic electrochemistry.” Chem. Rev., 118, 5985 (2018). 241. A. Hovestad and L. J. J. Janssen, “Electroplating of metal matrix composites by 224. T. Siu, W. Li, and A. K. Yudin, “Parallel electrosynthesis of α-alkoxycarbamates, codeposition of suspended particles.” Modern Aspects of Electrochemistry, ed. B. α-alkoxyamides, and α-alkoxysulfonamides using the spatially addressable E. Conway, C. G. Vayenas, R. E. White, and M. E. Gamboa-Adelco (Springer, electrolysis platform (SAEP).” J. Comb. Chem., 2, 545 (2000). Boston, MA) p. 475 (2005).

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