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Solar-induced direct biomass-to-electricity hybrid fuel cell using polyoxometalates as photocatalyst and charge carrier

Solar-induced direct biomass-to-electricity hybrid fuel cell using polyoxometalates as... ARTICLE Received 8 Oct 2013 | Accepted 7 Jan 2014 | Published 7 Feb 2014 DOI: 10.1038/ncomms4208 Solar-induced direct biomass-to-electricity hybrid fuel cell using polyoxometalates as photocatalyst and charge carrier 1,2 1 1 3 1 1 Wei Liu , Wei Mu , Mengjie Liu , Xiaodan Zhang , Hongli Cai & Yulin Deng The current polymer-exchange membrane fuel cell technology cannot directly use biomass as fuel. Here we present a solar-induced hybrid fuel cell that is directly powered with natural polymeric biomasses, such as starch, cellulose, lignin, and even switchgrass and wood powders. The fuel cell uses polyoxometalates as the photocatalyst and charge carrier to generate electricity at low temperature. This solar-induced hybrid fuel cell combines some features of solar cells, fuel cells and redox flow batteries. The power density of the solar-induced hybrid fuel cell powered by cellulose reaches 0.72 mWcm , which is almost 100 times higher than cellulose-based microbial fuel cells and is close to that of the best microbial fuel cells reported in literature. Unlike most cell technologies that are sensitive to impurities, the cell reported in this study is inert to most organic and inorganic contaminants present in the fuels. School of Chemical & Biomolecular Engineering and IPST at Georgia Tech, Georgia Institute of Technology, 500 10th Street NW, Atlanta, Georgia 30332-0620, USA. College of Chemistry and Chemical Engineering, Key Laboratory of Chemometrics and Chemical Biological Sensing Technologies, Ministry of Education, Hunan University, Changsha 410082, China. School of Materials Science and Engineering and IPSTat Georgia Tech, Georgia Institute of Technology, 500 10th Street NW, Atlanta, Georgia 30332-0620, USA. Correspondence and requests for materials should be addressed to Y.D. (email: yulin.deng@chbe.gatech.edu). NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ith the depletion of fossil energy and growing catalysed by POMs in the solution rather than by noble metals on environmental concerns, developing renewable energy the anode electrode. Finally, the solar-induced hybrid fuel cell is Wsources becomes more and more important . Today, directly powered by unpurified polymeric biomasses that could fossil fuels still dominate the energy market, accounting for 87% significantly reduce the fuel cell cost. of global energy consumption . Solar energy and biomass energy are two important sustainable energy sources . Therefore, production of electricity to power our world from solar energy Results and biomass can reduce the dependence on fossil fuels. Electron and proton transfer under photo-irradiation. The Biomass-to-electricity conversion using natural biomass structure of the solar-induced hybrid fuel cell is shown in Fig. 1. It resources can be realized by two common technologies: solid includes a common fuel cell equipped with a membrane electrode 4–6 7,8 oxide fuel cells and microbial fuel cells . However, several assembly (MEA) and a transparent glass fuel storage vessel that critical issues exist with the current technologies. Solid oxide fuel can be pre-irradiated with light. It should be noted that the cells require very high working temperatures (500B1,000 C) for cathode electrode included Pt (60%)/C catalyst (5 mg cm ), but 9,10 gasification of biomass . Although microbial fuel cells can work the anode electrode was made of simple carbon cloth without Pt. at low temperature, very low electric power output, rigorous Experimentally, the biomass–POM electrolyte solution was either reaction conditions and limited lifetime seriously hinder their pre-irradiated before circulation in the anode cell or irradiated 11,12 applications . The polymer-exchange membrane fuel cell in situ to generate electricity. (PEMFC) is another most promising fuel cell technology. In this study, the photoredox process was investigated by Although highly effective PEMFCs powered by hydrogen or low irradiating the reaction solution under controlled temperature of molecular weight alcohols have been successfully commercialized, 25 C to exclude the thermal effect. It was observed that the polymeric biomass such as starch, lignin and cellulose have not starch–H PMo O (PMo ) aqueous solution (15 g l starch 3 12 40 12 been directly used in PEMFC because the C–C bonds cannot be and 0.3 mol l PMo ) gradually changes colour from the initial completely electro-oxidized to CO at low temperatures with a yellow to deep blue, which indicates the reduction of PMo . 2 12 13,14 noble metal catalyst . Even for the simplest C–C molecules Figure 2a shows the absorbance of the solution as a function of such as ethanol, it was reported that the fuel could only be wavelength at different irradiation times. The reduced PMo is converted to acetaldehyde ( 2e ) or acetic acid ( 4e ) with a known as molybdenum blue with a characteristic absorption at noble metal anode, suggesting that only 16.7 and 33.3% of the around 750 nm, as shown in the ultraviolet–visible spectrum. The 15,16 total 12 electrons can be actually converted to electric power . concentration of reduced PMo in the photoredox reactions was Therefore, cleavage of C–C bonds in biomass fuels is a major measured using spectrophotometry (calibration curve showed in challenge in developing high-efficiency PEMFCs. Furthermore, it Supplementary Fig. 1) at 750 nm and converted to the is well known that noble metals such as Pt, Au, Ru and so on are corresponding number of electrons accepted by PMo . The easily poisoned by impurities in the biomass. For these reasons, a results are plotted in Fig. 2b (curve 1). It can be seen that the low-temperature PEMFC that directly uses natural polymeric electron transfer rate continuously increased during the light biomasses has not been reported. irradiation period. After 80 h, 0.41 mmol electrons were trans- Here we present a solar-induced hybrid fuel cell that directly ferred from starch to PMo per ml solution, which equals 1.23 consumes natural polymeric biomass, such as starch, lignin, electrons obtained per Keggin unit. cellulose, and even switchgrass or wood powders. With this POM is not only a strong photo-oxidizing agent but also a 17–19 hybrid fuel cell technology, the biomass is oxidized by strong Brønsted acid . Under light irradiation, starch was polyoxometalates (POMs) in the solution under solar irradiation oxidized by PMo and degraded to low-molecular-weight and the reduced POM is oxidized by oxygen through an external segments, which could improve the reaction kinetics. circuit, producing electricity. The designed solar-induced biomass Meanwhile, a slight increase in pH of the reaction solution hybrid fuel cell has many advantages. First, the cell combines during this photoredox period was observed as shown in Fig. 2b. photochemical and solar–thermal biomass degradation in a single The reason is that the addition of electrons decreases the acidity chemical process, leading to high solar conversion and effective of the POM and is accompanied by protonation . Therefore, biomass degradation. Second, it does not use expensive noble POM acts as an electron and proton carrier during the metals as anode catalysts because the fuel oxidation reactions are photodegradation of starch. A similar behaviour has been hυ Figure 1 | Structure of the solar-induced hybrid fuel cell. (a) MEA (Nafion 117 polymer exchange membrane, anode made of carbon cloth and cathode loaded with Pt/C catalyst), (b) graphite bipolar plate, (c) acrylic plastic end plate, (d) transparent glass vessel with starch-H PMo O (PMo ) 3 12 40 12 solution, (e) pump, (f) oxygen inlet, and (g) water and oxygen outlet. 2 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. Load e NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ARTICLE 0.40 1.4 3.0 0.35 10, 22, 34, 46, 58, 70, 82 h 1.2 2.5 0.30 1.0 2.0 0.25 0.8 0.20 1.5 0.6 0.15 1.0 0.4 0.10 0.5 0.2 0.05 0.0 0.0 0.00 400 500 600 700 800 900 1,000 1,100 020 40 60 80 0 2468 10 Wavelength (nm) Light irradiation Discharge time (h) time (h) Figure 2 | Electron–proton transfer during photoreduction and discharge process. (a) Ultraviolet–visible spectrums of starch–H PMo O (PMo ) 3 12 40 12 solution under photo-irradiation (diluted to 1 mmol l of PMo ). (b) Number of electrons transferred and pH change plots during light irradiation and discharge period (in single cell, room temperature). observed and studied previously, in which the PMo acted as Output of the starch-based cell. As expected and shown in electron-coupled-proton buffer . Fig. 3a, when a pure starch solution (2.5 wt%) was used alone at After exposure to simulated sunlight, the starch–PMo room temperature, the power output was very small because of solution was cyclically pumped through the anode cell while the lack of catalyst on the anode to electro-oxidize the starch. oxygen was simultaneously fed to the cathode cell. Figure 2b When PMo was added without sunlight irradiation, the power (curve 2) shows the number of discharged electrons, calculated by output still did not improve because the redox reaction between integrating the electric current output curve. The number of PMo and starch could not take place at room temperature electrons discharged on the electrode was 0.375 mmol electrons without light irradiation. PMo in its original oxidation state per ml solution, which is close to the calculated number shown in cannot oxidize starch in the hybrid cell at room temperature curve 1. This indicates that the electrons stored in the reduced without irradiation. However, after the PMo was pre-reduced to PMo were almost completely transferred to the cathode via the molybdenum blue by the photo- or thermal-induced redox external circuit and captured by O . After discharging, the pH reactions with starch, power density increased to 0.28 (photo was almost recovered to the original value because the oxidation induced) and 0.44 mW cm (thermal induced). The discharge of the reduced POM led to the release of protons from the POM process could continue for B8 h at room temperature without to the solution . light until the reduced PMo in the solution was fully oxidized (evidenced by the colour change from deep blue to light yellow). After the photo-irradiated solution was fully discharged, the Electron and proton transfer at elevated temperatures. solution with excess unreacted starch and its various derivatives Molybdenum blue can strongly absorb visible-near infrared light was then exposed to simulated solar light followed by discharging (700–1,000 nm), which is caused by the intervalence charge again. The photo-irradiation discharge cycles were repeated three transfer (CT) transitions via an oxobridge . The long-wavelength times and the power outputs were measured, as illustrated in light absorption leads to the conversion of light energy to thermal Fig. 3b. Surprisingly, it was found that the output power density energy, which raises the solution temperature. As shown in increased with the number of repeated cycles. A power density of Supplementary Fig. 2, the temperature of the starch–PMo 0.65 mW cm was reached on the third cycle, even though no reaction solution can reach up to 84 C under actual sunlight extra starch or PMo was added. It is believed that the increase in irradiation (clear sky, 28 C, Atlanta, GA, USA) for 90 min, which the power density is due to the decrease in the starch molecular is 20.8% higher than the maximum temperature reached by weight during the repeated tests. The low-molecular-weight deionized (DI) water exposed to the same sunlight irradiation species from oxidation and acid hydrolysis reduced the solution condition. POMs can harvest electrons and protons from viscosity and increased the reaction rate with POM molecules, as organics by simply heating without light irradiation as reported compared with high-molecular-weight starch. 22–27 before . Therefore, it is expected that the redox reactions Different from the pre-light irradiation or pre-heating tests between starch and PMo could be further enhanced at elevated conducted above, a continuous experiment with in-situ light temperature. irradiation and PMo redox reaction simultaneous to electricity To confirm the heating effect, the starch–PMo solution was production was also conducted. The reaction solution was placed heated and kept at 95 C for 6 h without light irradiation. The under simulated sunlight irradiation (irradiation intensity of transfer of electrons and change in pH of the solution during the 100 mW cm ) and heated to 95 C while being discharged thermal-reduction experiment are shown in Supplementary simultaneously. The cell worked continuously for almost 20 h and Fig. 3, which suggest that 0.525 mmol electrons were transferred a steady current density up to 2.5 mA cm was observed, as from starch to PMo per ml solution by thermal degradation shown in Fig. 3c. Both repeated cycle tests and in-situ continuous alone (equal to 2.13 electrons per Keggin unit). The pH value experiments indicate that the POM catalyst can be re-used increased from 0.65 to 0.74, and then returned to 0.62 after without further treatment. the reduced PMo solution was discharged, which is similar to the light irradiation and discharge test described previously. The results suggest that the designed biomass–POM reaction system Output of different biomass-based cell. The solar-induced could combine photochemical and thermal biomass degradation hybrid fuel cell reported in this study can be powered by various in a single process, which means that the sunlight utilization biomasses. Figure 4a shows the power density that was produced could be extended to the near-infrared band. by cellulose, lignin, switchgrass and poplar powder. The power NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. Absorbance (a.u.) –1 Electrons transferred (mmol ml ) pH ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 0.5 0.5 0.5 0.7 0.4 0.4 0.6 0.4 0.5 0.3 0.3 0.4 0.3 0.2 0.3 0.2 PMo –starch–heat 12 0.2 0.1 PMo –starch–light 0.2 1st time 0.1 0.1 2nd time PMo –starch 3rd time 0.0 Starch solution 0.0 0.1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 –2 –2 Current density (mA cm ) Current density (mA cm ) 3.0 2.5 2.0 1.5 1.0 0 200 400 600 800 1,000 1,200 1,400 Time (min) Figure 3 | Performances of the solar-induced hybrid fuel cell using starch as fuel. (a) Voltage–current density and power–current density plots with different reaction systems used in a single solar-induced fuel cell at room temperature; (b) plots of three repeated photo-irradiation–discharge cycles with starch–H PMo O (PMo ) reaction system (discharged at room temperature); (c) current curve of continuous performance test with light 3 12 40 12 irradiation at 95 C. 0.8 0.7 0.4 0.6 0.4 0.6 0.5 0.4 0.3 0.3 0.4 0.3 0.2 0.2 0.2 0.2 Cellulose 2+ 3+ Poplar 0.1 PMo -Cu -Fe PMo -SnCI Lignin 12 4 Switch grass 0.0 PMo 0.0 0.1 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 –2 –2 Current density (mA cm ) Current density (mA cm ) Figure 4 | Performances of the solar-induced hybrid fuel cell using different biomass. (a) Voltage–current density and power–current density plots of different biomasses used in the H PMo O (PMo ) reaction system in photothermal experiments; (b) plots of cellulose and PMo degradation 3 12 40 12 12 with different metal salts as Lewis acids in photothermal experiments (discharged at room temperature). density of our solar-induced hybrid fuel cell can reach 0.65 and Consequently, as shown in Fig. 4b, when these promoters were 0.62 mW cm when fueled by poplar and switchgrass powders, used in this system, the power density obviously increased, even 2 2þ 3 þ respectively. It should be noted that all these materials are water up to 0.72 mW cm (Cu –Fe as promoters for PMo ), insoluble; hence, they were particle suspensions at the beginning, which is almost 100 times higher than that of cellulose-based but the biomass particles were degraded and dissolved in the microbial fuel cells reported in literature . reaction solution as time progressed. The results illustrate that all these biomass materials can be directly used as fuel in this solar- induced hybrid fuel cell with POM as the catalyst and charge Discussion carrier. The general principle of our solar-induced hybrid fuel cell is that It was also noted that the power density produced by PMo can oxidize biomass under solar irradiation while being 6 þ 5 þ 5þ crystalline cellulose was lower than that obtained from starch reduced from Mo to Mo , and then Mo can be oxidized 6 þ solution under similar conditions, probably because of the slow to Mo again by oxygen through a catalytic electrochemical hydrolysis of cellulose crystals. It was reported that some metal reaction, as illustrated in Fig. 5a. In a previous study, Yamase 4þ 3þ 2þ ions, such as Sn ,Fe and Cu and so on could function as proposed that electron transfer from organics to POM under 28,29 Lewis acids to help cleave glycosidic bonds in cellulose . light irradiation could result in the formation of an 4 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. Voltage (V) Voltage (V) Discharged current –2 density (mA cm ) –2 –2 Power density (mW cm ) Power density (mW cm ) Voltage (V) Voltage (V) –2 –2 Power density (mW cm ) Power density (mW cm ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ARTICLE Sun light UV Vis IR –+ Photo chemical Solar Thermal Starch 1/2O MEA Oxygen + – +2H +2e H – Starch H O –H + Water + oxygen Starch oligomers Starch –COOH, ˙OH, etc. Hydrolysis Oxidation Carbon cloth Pt/C –H PMo (VI) Reduced PMo (V) 12 12 0 h 4 h 6 h 8 h Figure 5 | Schematic illustration of this solar-induced fuel cell. (a) The working principle of this cell. In fact, there are 12 Mo elements in 1 POM cluster. To VI V simplify the drawing, oxidized Mo is represented by Mo and reduced Mo is represented by Mo , although the 12 Mo elements in the cluster may not have the same valences. (b) The colour change of starch–H PMo O (PMo ) solution after electrically discharging up to 8 h. 3 12 40 12 intermolecular charge-transfer complex. This mechanism was and thus stabilizing the reduced state of PMo . In this study, the 21,32,33 further investigated and accepted widely in other studies . formed starch–PMo complex was separated from the reaction According to this mechanism, the representative reaction in our solution and characterized by Fourier-transform (FT)–infrared, starch-PMo solution is as shown in equation (1). which provides evidence of the interaction between starch and PMo (shown in Supplementary Fig. 4 and Supplementary HO O Table 1). H O hυ VI Starch Mo (1) O Starch O Mo As light irradiation has a dual effect on the oxidation of O O O O biomass by POMs as verified before, both photo and heat reduction of PMo can happen simultaneously in the presence of The PMo has the best-known Keggin structure consisting of a starch. As a result, POM captures an electron and proton from central tetrahedral [PO ] surrounded by 12 [MoO ] octahedrons starch; thus, starch is oxidized and degraded to starch oligomers 4 6 that are photosensitive . Under short-wavelength light and glucose derivatives as shown in reaction (2). In fact, POM can irradiation, the commonly known O-Mo ligand-to-metal CT accept more than one electron per Keggin unit as the reaction occurs, which means the 2p electron in the oxygen of [MoO ]is continued progressed, which means the increase of reduction excited to the empty d orbital in Mo, changing the electron degree of POM. 0 1 configurations of Mo from d to d , and leaving a hole at the light or heat 3 3 VI VI V 21,31 2Starch-OHþ2PMo O Starch-O- HPMo Mo O 12 40 11 40 oxygen atom of the POM lattice . This hole interacts with one ! 21,31 ð2Þ electron on the oxygen atom of a hydroxyl group of starch . VI V þ HPMo Mo O 11 40 Meanwhile, the hydrogen atom of the hydroxyl group shifts to the þ Oxidized starch oligomers 1 21,31 POM lattice to interact with the d electron , which is a thermally activated delocalization within the polyanion It should be noted that although one Mo is reduced from VI to molecule . Thus, the intermolecular CT complex is formed, V valence by forming a starch–POM complex, the total charge of VI V 3 leading to the separation of photo-excited electrons and holes, the polyanion (–[HPMo Mo O ] ) will not change because 11 40 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. Mo Mo Mo Mo VI Mo VI V ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 a proton is also transferred from starch to the POM complex at The power density of the cell is also affected by the redox the same time. As the standard redox potential of oxygen is potential of the POMs. As the charge carrier, POM is first 5þ higher than that of the reduced POM, the Mo in the reduced reduced by oxidizing the biomass, and then the reduced POM is PMo can be oxidized at the anode by connecting to the O oxidized by oxygen. Obviously, if the standard redox potential of 12 2 electrode through an external circuit to produce electricity. POM is high, it has greater tendency to oxidize the biomass but 5þ As a result, the reduced POM (Mo ) gives one electron to the lower tendency to be oxidized by oxygen. Therefore, POMs with carbon anode and simultaneously releases a proton to the higher standard redox potentials are not always advantageous. In solution with the gradual colour change from deep blue to light this study, three types of POMs (PMo , [PW O ] and 12 12 40 6 41 yellow as shown in Fig. 5b. The electron passes through the [PV Mo O ] ) with different redox potentials were used as 3 9 40 external circuit and is captured by oxygen to form water at the the charge and proton carriers (Supplementary Fig. 9). No simple cathode. At the same time, the starch molecules associated with relationship between standard redox potential of the POM and PMo are released into solution. The net effect of the above power output was observed. 5 þ 6þ reaction is that Mo is oxidized back to Mo at the anode, Faradic efficiency is considered as one important part of and the starch is oxidized through dehydrogenation by POM discharge efficiency. Faradic efficiency is defined as a ratio of the catalysis. Finally, the proton diffuses to the cathode side through actual discharge capacity to the total electron charge transferred the proton-exchange membrane and combines with oxygen to from the organics in the POM electrolyte solution. In this form water. The entire discharge process is represented by study, Faradic efficiency was as high as 91% and 94% in the reactions (3) and (4), discharge of the starch–PMo system for photoreduction and thermal reduction, respectively (Supplementary Fig. 10 and VI V Starch-O- HPMo Mo O 11 40 Supplementary Note 1). 3 3 VI V VI anode The solar-induced hybrid fuel cell designed in this study is a þ HPMo Mo O 2 PMo O 11 40 12 40 ð3Þ combination of solar cells, fuel cells and redox flow batteries, but þ Oxidized starch oligmers has distinct differences from each. For a traditional solar cell, light energy is converted to electricity directly via the photovoltaic þ 2e þ 2H effect when the semiconductor or dye on the semiconductor is exposed to light. However, for our hybrid fuel cell, short-wave cathode 1=2O þ 2e þ 2H ! H O ð4Þ 2 2 light excites POMs to the excited state in the presence of biomass and is stored in the form of the reduced POMs (that is, chemical Starch, which acts as the electron and proton donor, can be energy) via the photochromic reaction. Moreover, visible and directly oxidized under light radiation or hydrolysed to small near-infrared light are absorbed by the solution and converted to oligomers by the POM, and then continuously oxidized to a series heat, which can also promote the redox reaction between biomass of glucose derivatives such as aldehyde, ketones and acids, as and POM. The cell reported in this study also has a similar shown in reaction (5): feature to redox flow batteries in that electrolyte solutions with POM Re Starch ox different valence states are used in the electrode cells. It is still POM H ,POM + different from redox flow batteries because the cathode side in H ,POM POM Starch Re Oligomer Glucose CO (5) ox ox our cell uses oxygen gas rather than an electrolyte solution. In POM addition, organic fuel is consumed in our hybrid fuel cell but no Oligomer organic fuel is used in a traditional redox flow battery. The overall The direct oxidation of starch in the photochromic reaction reaction, as discussed above, is the oxidation of organic fuel by was confirmed by FT–infrared (Supplementary Fig. 4), and the oxygen, which is the fundamental basis of traditional fuel cells but degradation and further oxidation of starch glucose derivatives not redox flow batteries. However, this cell is different from a was verified by gel permeation chromatography (Supplementary traditional fuel cell where catalytic reactions happen on the 1 13 Fig. 5 and Supplementary Table 2) and H, C NMR analysis precious metal-loaded anode. The POM functions as a photo and (Supplementary Fig. 6). In addition, CO is recognized as the thermal catalyst and charge carrier that takes electrons from 5 þ final product in many photodegradation reactions of organic biomass while reducing its own valence state to Mo under 36–38 compounds by POMs , and the produced CO was light irradiation and thermal degradation. The electrons in the actually detected during the operation of the solar-induced POM are then transferred through the external circuit and the 6 þ hybrid fuel cell in this study (Supplementary Fig. 7 and POM reverts to its original Mo valence state. Supplementary Table 3). As the charge carrier in this study, In this study, the catalytic reactions are mediated by POM, but PMo is very stable in a solution acidified by H PO , which was not on the surface of a noble metal electrode. In fact, most 12 3 4 verified during our repeated cycle test using P NMR analysis biomass fuels and even some artificial polymers and organic (Supplementary Fig. 8). wastes can be directly degraded by the solar-induced hybrid fuel The performance of this solar-induced fuel cell is associated cell discussed in this study to provide electricity without using a with the reduction degree and redox potential of POMs. As precious metal anode. The organic impurities in crude biomass described before, the power density gradually increased after could also be oxidized as fuels, and inorganic impurities will not three repeated light irradiation–discharge cycles. Correspond- poison the whole process because POMs are robust and self- ingly, it was found that the reduction degree of the POM healing . As a result, the fuels used in our solar-induced hybrid increased from 1.23 to 2.4 electrons per Keggin unit after three fuel cell do not have to be pure, which would significantly reduce repeated tests. The reason for the increase in reduction degree is the cost of current fuel cell technology. In essence, this solar- that low-molecular-weight oligomers and oxidized derivatives induced fuel cell is different from any cells or batteries reported (mainly aldehydes) were formed during the photocatalytic previously, and it greatly widens the range of biomass fuels for reaction, and they have greater reduction power and higher electric power production. reactivity with the POM. Therefore, with repeated light In summary, we demonstrated the feasibility of a new concept irradiation–discharge cycles, more electrons were captured by for fabricating a solar-induced fuel cell that directly consumes each Keggin unit, but without major changes in their chemical biomass fuels in the presence of a POM photocatalyst under light 39,40 structures . irradiation. Although the operation parameters have not been 6 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ARTICLE optimized, the power density of our solar-induced hybrid fuel can Cycling test for starch–PMo system. To investigate the reproducibility of this cell, three photoreduction–discharge cycles were conducted. In each cycle, the reach 0.72 mW cm when cellulose was used as the fuel. discharged fuel–PMo solution was exposed to AM-1.5 simulated sunlight and kept at constant 25 C until use. The solution was then pumped into the anode of Methods the solar-induced fuel cell to electrically discharge for several hours over a load of Experimental details for preparation of electrolyte solution. Phosphomolybdic 1.6O resistance until the current fell below 0.1 mA. In the continuous experiments, acid (H [PMo O ], PMo ) with a-Keggin structure was purchased from TCI the transparent glass vessel was placed under irradiation of AM 1.5-type simulated 3 12 40 12 America. Potato starch solution (4 wt%) was obtained by cooking starch suspen- sunlight and heated to 95 C during a continuous discharge experiment. The glass sion for 20 min at 95 C. Calculated amount of phosphomolybdic acid was added to (sodium lime glass) was 2 mm thick, with transmission 495% for wavelengths the required volume of starch solution and then diluted to 20 ml total. The con- from 320 to 1,100 nm and transmission 480% for wavelengths from 270 to 320 nm 1  1 centrations of PMo and starch were 0.3 mol l and 15 g l , respectively. The (air as blank sample, measured on Agilent Technologies 8453 ultraviolet–visible starting pH of this solution was adjusted to 0.3 using concentrated phosphoric acid spectrophotometer). (85%, Alfa Aesar); thus, a clear yellow solution was obtained. Biomasses used in the solar-induced fuel cell. Crystalline cellulose was pur- Photoreducing experiment. The obtained yellow solution (20 ml) was kept at a chased from Alfa Aesar. The cellulose suspension was homogenized for 40 min to constant temperature of 25 C using a recirculating water bath (RM6 Lauda, form a small particle suspension in the solution before used. Lignin was isolated Brinkmann Instruments Service Inc.). Next, the reaction solution was irradiated from a commercial USA softwood kraft pulping liquor. Switchgrass was from with AM 1.5-type simulated sunlight (SoLux Solar Simulator, New York, USA; Ceres, Inc. (Thousand Oaks, CA, USA) and poplar was provided by Michigan State 50 W) for a certain time with a distance of 10 cm from the solution surface. The University. These samples were washed with DI water and then dried at 45 Cover colour of the solution gradually turned from yellow to deep blue. At regular night. The dry samples were then milled in a Wiley mill through a 0.8-mm screen, intervals, a very small amount of solution was taken out as a sample and diluted to yielding the biomass powder directly used in this study without any pretreatment. 1 mmol l PMo for analysis of the concentration of reduced PMo and the pH 12 12 value of the solution. Different metal salts used as Lewis acids. In these experiments, the reaction solutions consisted of 0.25 mol l PMo and 0.1 g biomass with a total solution Thermal-reducing experiment. The concentration of starch–PMo solution used volume of 20 ml and pH 0.5. The solution was irradiated by simulated sunlight 1  1 in this experiment was 15 g l starch and 0.25 mol l PMo at pH 0.65. It was  2 (100 mW cm ) and heated on a hotplate up to 95 C and kept for 6 h (photo- continuously heated under reflux at 95 C with magnetic stirring in the dark for 6 h. thermal experiment). The discharge condition was the same as the starch–PMo The sample collection procedure was the same as that of photoreducing experi- system. To improve the reduction degree of PMo in cellulose–PMo reaction 12 12 ment. The prepared heteropoly blue solution was directly circulated in the designed system, some metal salts as Lewis acids were added. The Lewis acids used in this fuel cell to generate electricity. As the phosphomolybdic blue is very stable in  1 3þ 2þ study include SnCl (Alfa Aesar, 0.188 mol l )and Fe -Cu (Fe (SO ) (Alfa 4 2 4 3 solution under air, this prepared solution can be stored for a long time.  1  1 Aesar, 0.15 mol l ); CuSO (Alfa Aesar, 0.15 mol l ). Analysis of electron transfer using spectrophotometry. To obtain the cali- Different POMs used in this hybrid fuel cell. Three different POMs with various bration curve, pure PMo solution without starch was reduced by electrochemical electrode potentials were used in this study: PMo , phosphotungstic acid and 5þ 12 reduction treatment at 3 V for different times. The concentration of Mo in the vanadium substituted phosphomolybdic acid. Phosphotungstic acid 5þ 6þ PMo solution (Mo and Mo mixture) was determined by titration with (H [PW O ]) was purchased from Sigma-Aldrich, and vanadium-substituted 3 12 40 potassium permanganate solution (calibrated by standard sodium oxalate). Thus, phosphomolybdic acid (H [PMo V O ]) was synthesized by the method in 6 9 3 40 the calibration curve for 1 mmol l PMo at different reduction degrees was 12 41 reported literature . In comparison experiments, the reaction solutions consisted 5þ obtained. The concentration of Mo in the starch–PMo reaction solution was 12  1 of 0.25 mol l POMs, 0.3 g starch and a total solution volume of 20 ml at pH 0.5. measured using the ultraviolet–visible spectrophotometer (Agilent Technologies The solution was irradiated by simulated sunlight and heated at 95 C for 6 h (that Inc., Santa Clara, CA, USA, 8453) to calculate the amount of electrons transferred is, photothermal experiment). For comparison, phosphotungstic acid was fully from starch to PMo during light irradiation or heating. The samples taken in reduced by ascorbic acid (1:1 molar ratio) in solution under light irradiation and photoreducing or thermal-reducing experiments were diluted to a concentration of heating at 95 C. The discharge conditions were the same as described above. 1 mmol l of total PMo and then used for ultraviolet–visible measurement. Product analysis of starch degradation. The starch–PMo complex was sepa- Investigation of solar thermal energy transfer. The experimental apparatus rated through precipitation from the photoredox reaction system by salting out consisted of a polymer foam insulation system, transparent test tubes (d¼ 10 mm, (excess Na SO ). The sample was then filtered and dried in a vacuum drying 2 4 h¼ 175 mm) and a simulated sunlight source (SoLux Solar Simulator; 50 W) as chamber at 30 C overnight. The changes in the starch that was associated with shown in Supplementary Fig. 2b. The heteropoly blue reaction solutions were 1 PMo were characterized by FT–infrared. FT–infrared spectra were obtained by diluted to 1 and 10 mmol l , and then transferred into test tubes with the same 1  1 averaging 64 scans from 4,000 to 650 cm with 4 cm resolution. Gel per- liquid levels. The simulated sunlight source was placed above the foam insulation meation chromatography was used to determine the changes in molecular weight in which the test tubes were laid side by side. The light energy density that irra- 2 of starch. Samples were taken from the reaction solution at different times and the diated the surface of the tubes was 100 mw cm , and the temperature of the proper amount of NaOH solution was added. The PMo degraded to MoO in 12 4 solution inside the test tubes was collected by thermocouples. In addition, the 1 alkaline solution and the final concentration of organic substance was around actual reaction solution (250 mmol l PMo ) and DI water were used in this 1mgml with pH 10. The analysis was performed on an Agilent 1,200 HPLC photo-to-thermal test under actual sunlight. System (Agilent Technologies Inc.) equipped with ultrahydrogel columns (Waters Corporation, Milford, MA, USA) and an Agilent RI detector using DI water as the 1 1 13 Assembly of solar-induced fuel cell and test methods. The MEA was purchased mobile phase (0.5 ml min ) with injection volumes of 25ml. H NMR and C as a commercialized product from Fuelcells Etc, TX, USA. The MEA was com- NMR were used to analyse the final products of starch in the solution. To increase posed of a Nafion 117 membrane with 5 layers of carbon cloth on one side (used as the signal intensity, light irradiation and discharge cycles were repeated four times. the anode electrode) and 5-layer carbon cloth plus 60% Pt/C catalyst loaded at The starch–POM solution after repeated irradiation and discharge was dried at 2 1 13 5mg cm on the other side (used as cathode). The grain size of the active carbon 80 C in vacuum, then resolved in D O for the NMR. H and C NMR spectral used as support for the Pt/C catalyst was 74mm and its specific surface area was data reported in this study were recorded with a Bruker Avance/DMX 400 MHz 2  1 13 90 m g . The crystal size of the nano-Pt particles in the Pt/C catalyst was 4.0– NMR spectrometer. Quantitative C NMR employed an inverse gated decoupling 2  1 5.5 nm and the specific surface area was 60 m g . The bipolar plates of the cell pulse sequence, 90 pulse angle, a pulse delay of 5 s and 6,000 scans. Quantitative were made of high-density graphite plates with a straight flow channel 2 mm wide, H NMR was acquired with 16 scans and 1 s pulse delay. To collect and analyse the 2 mm deep and 5 cm long (a total active area of 1 cm ). The MEA was sandwiched emission gas in the hybrid fuel cell, an airtight glass vessel was connected with an between two graphite flow-field plates, which were clamped between two acrylic emission gas outlet and a valve (shown in Supplementary Fig. 7). The starch– 1  1 plastic end plates, as shown in Fig. 1. Rubber gaskets were included on the cir- PMo solution (15 g l starch and 0.3 mol l PMo ) was stored in the airtight 12 12 cumference of the graphite flow-field plates to prevent any leakage. In our glass vessel and exposed to simulated sunlight (100 mW cm ) and kept at 95 C. experiments, the prepared molybdenum blue solution was pumped through the Meanwhile, the starch–PMo solution was pumped to the anode chamber of the anode reaction cell, and oxygen was flowed through the cathode cell using a fuel cell and discharged for 3 days. A gas collection bag was connected to the gas compressed oxygen cylinder. The temperature of the liquid in the cell was B25 C outlet through a rubber tube. The composition of gas was analysed on Varian 490 (room temperature). The solution flow rate through the anode graphite plate was micro-GC equipped with a Pora Plot U (10 m) column. P NMR was used to 1  1 12 ml min and the oxygen flow rate through the cathode was 75 ml min at analyse the stability of POM. After several repeated irradiation and discharge 1 atm. A DS345 30 MHz (Stanford Research Systems) and a 4,200-SCS (Semi- cycles, the reaction solution was dried at 95 C then resolved in D O for P NMR. conductor Characterization system, Keithley Instruments Inc.) were used to The spectral data were collected on a Bruker Avance/DMX 400 MHz NMR spec- examine the I–V curves using the controlled potentiostatic method. trometer with 8 s pulse delay. NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 References 27. Hill, C. L. Homogeneous catalysis: controlled green oxidation. Nature 401, 1. Ru¨hl, C., Appleby, P., Fennema, J., Naumov, A. & Schaffer, M. 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W.L. performed the 5, 403–409 (2013). photo and thermal experiments. W.L., M.L. and H.C. fabricated the solar-induced fuel 21. He, T. & Yao, J. Photochromism in composite and hybrid materials based on cell. W.L. and M.L. tested the performance under a variety of conditions. W.M. per- transition-metal oxides and polyoxometalates. Prog. Mater. Sci. 51, 810–879 formed NMR analyses. W.L. performed gel permeation chromatography, infrared, (2006). ultraviolet–visible and GC analysis. X.Z. and M.L. helped in paper preparation. 22. Gaspar, A. R., Gamelas, J. A. F., Evtuguin, D. V. & Pascoal Neto, C. Alternatives for lignocellulosic pulp delignification using polyoxometalates and oxygen: a review. Green Chem. 9, 717–730 (2007). Additional information 23. Weinstock, I. A. et al. Equilibrating metal-oxide cluster ensembles for oxidation Supplementary Information accompanies this paper at http://www.nature.com/ naturecommunications reactions using oxygen in water. Nature 414, 191–195 (2001). 24. Weinstock, I. A. Homogeneous-phase electron-transfer reactions of Competing financial interests: The authors declare no competing financial interests. polyoxometalates. Chem. Rev. 98, 113–170 (1998). 25. Hill, C. L. & Weinstock, I. A. Homogeneous catalysis: on the trail of dioxygen Reprints and permission information is available online at http://npg.nature.com/ activation. Nature 388, 332–333 (1997). reprintsandpermissions/ 26. Weinstock Ira, A., Atalla Rajai, H., Reiner Richard, S., Houtman Carl, J. & Hill Craig, L. Selective transition-metal catalysis of oxygen delignification using How to cite this article: Liu, W. et al. Solar-induced direct biomass-to-electricity hybrid water-soluble salts of polyoxometalate (POM) Anions. Part I. Chemical fuel cell using polyoxometalates as photo-catalyst and charge carrier. Nat. Commun. principles and process concepts. Holzforschung 52, 304–310 (1998). 5:3208 doi: 10.1038/ncomms4208 (2014). 8 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Solar-induced direct biomass-to-electricity hybrid fuel cell using polyoxometalates as photocatalyst and charge carrier

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Copyright © 2014 by Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.
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Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
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Abstract

ARTICLE Received 8 Oct 2013 | Accepted 7 Jan 2014 | Published 7 Feb 2014 DOI: 10.1038/ncomms4208 Solar-induced direct biomass-to-electricity hybrid fuel cell using polyoxometalates as photocatalyst and charge carrier 1,2 1 1 3 1 1 Wei Liu , Wei Mu , Mengjie Liu , Xiaodan Zhang , Hongli Cai & Yulin Deng The current polymer-exchange membrane fuel cell technology cannot directly use biomass as fuel. Here we present a solar-induced hybrid fuel cell that is directly powered with natural polymeric biomasses, such as starch, cellulose, lignin, and even switchgrass and wood powders. The fuel cell uses polyoxometalates as the photocatalyst and charge carrier to generate electricity at low temperature. This solar-induced hybrid fuel cell combines some features of solar cells, fuel cells and redox flow batteries. The power density of the solar-induced hybrid fuel cell powered by cellulose reaches 0.72 mWcm , which is almost 100 times higher than cellulose-based microbial fuel cells and is close to that of the best microbial fuel cells reported in literature. Unlike most cell technologies that are sensitive to impurities, the cell reported in this study is inert to most organic and inorganic contaminants present in the fuels. School of Chemical & Biomolecular Engineering and IPST at Georgia Tech, Georgia Institute of Technology, 500 10th Street NW, Atlanta, Georgia 30332-0620, USA. College of Chemistry and Chemical Engineering, Key Laboratory of Chemometrics and Chemical Biological Sensing Technologies, Ministry of Education, Hunan University, Changsha 410082, China. School of Materials Science and Engineering and IPSTat Georgia Tech, Georgia Institute of Technology, 500 10th Street NW, Atlanta, Georgia 30332-0620, USA. Correspondence and requests for materials should be addressed to Y.D. (email: yulin.deng@chbe.gatech.edu). NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ith the depletion of fossil energy and growing catalysed by POMs in the solution rather than by noble metals on environmental concerns, developing renewable energy the anode electrode. Finally, the solar-induced hybrid fuel cell is Wsources becomes more and more important . Today, directly powered by unpurified polymeric biomasses that could fossil fuels still dominate the energy market, accounting for 87% significantly reduce the fuel cell cost. of global energy consumption . Solar energy and biomass energy are two important sustainable energy sources . Therefore, production of electricity to power our world from solar energy Results and biomass can reduce the dependence on fossil fuels. Electron and proton transfer under photo-irradiation. The Biomass-to-electricity conversion using natural biomass structure of the solar-induced hybrid fuel cell is shown in Fig. 1. It resources can be realized by two common technologies: solid includes a common fuel cell equipped with a membrane electrode 4–6 7,8 oxide fuel cells and microbial fuel cells . However, several assembly (MEA) and a transparent glass fuel storage vessel that critical issues exist with the current technologies. Solid oxide fuel can be pre-irradiated with light. It should be noted that the cells require very high working temperatures (500B1,000 C) for cathode electrode included Pt (60%)/C catalyst (5 mg cm ), but 9,10 gasification of biomass . Although microbial fuel cells can work the anode electrode was made of simple carbon cloth without Pt. at low temperature, very low electric power output, rigorous Experimentally, the biomass–POM electrolyte solution was either reaction conditions and limited lifetime seriously hinder their pre-irradiated before circulation in the anode cell or irradiated 11,12 applications . The polymer-exchange membrane fuel cell in situ to generate electricity. (PEMFC) is another most promising fuel cell technology. In this study, the photoredox process was investigated by Although highly effective PEMFCs powered by hydrogen or low irradiating the reaction solution under controlled temperature of molecular weight alcohols have been successfully commercialized, 25 C to exclude the thermal effect. It was observed that the polymeric biomass such as starch, lignin and cellulose have not starch–H PMo O (PMo ) aqueous solution (15 g l starch 3 12 40 12 been directly used in PEMFC because the C–C bonds cannot be and 0.3 mol l PMo ) gradually changes colour from the initial completely electro-oxidized to CO at low temperatures with a yellow to deep blue, which indicates the reduction of PMo . 2 12 13,14 noble metal catalyst . Even for the simplest C–C molecules Figure 2a shows the absorbance of the solution as a function of such as ethanol, it was reported that the fuel could only be wavelength at different irradiation times. The reduced PMo is converted to acetaldehyde ( 2e ) or acetic acid ( 4e ) with a known as molybdenum blue with a characteristic absorption at noble metal anode, suggesting that only 16.7 and 33.3% of the around 750 nm, as shown in the ultraviolet–visible spectrum. The 15,16 total 12 electrons can be actually converted to electric power . concentration of reduced PMo in the photoredox reactions was Therefore, cleavage of C–C bonds in biomass fuels is a major measured using spectrophotometry (calibration curve showed in challenge in developing high-efficiency PEMFCs. Furthermore, it Supplementary Fig. 1) at 750 nm and converted to the is well known that noble metals such as Pt, Au, Ru and so on are corresponding number of electrons accepted by PMo . The easily poisoned by impurities in the biomass. For these reasons, a results are plotted in Fig. 2b (curve 1). It can be seen that the low-temperature PEMFC that directly uses natural polymeric electron transfer rate continuously increased during the light biomasses has not been reported. irradiation period. After 80 h, 0.41 mmol electrons were trans- Here we present a solar-induced hybrid fuel cell that directly ferred from starch to PMo per ml solution, which equals 1.23 consumes natural polymeric biomass, such as starch, lignin, electrons obtained per Keggin unit. cellulose, and even switchgrass or wood powders. With this POM is not only a strong photo-oxidizing agent but also a 17–19 hybrid fuel cell technology, the biomass is oxidized by strong Brønsted acid . Under light irradiation, starch was polyoxometalates (POMs) in the solution under solar irradiation oxidized by PMo and degraded to low-molecular-weight and the reduced POM is oxidized by oxygen through an external segments, which could improve the reaction kinetics. circuit, producing electricity. The designed solar-induced biomass Meanwhile, a slight increase in pH of the reaction solution hybrid fuel cell has many advantages. First, the cell combines during this photoredox period was observed as shown in Fig. 2b. photochemical and solar–thermal biomass degradation in a single The reason is that the addition of electrons decreases the acidity chemical process, leading to high solar conversion and effective of the POM and is accompanied by protonation . Therefore, biomass degradation. Second, it does not use expensive noble POM acts as an electron and proton carrier during the metals as anode catalysts because the fuel oxidation reactions are photodegradation of starch. A similar behaviour has been hυ Figure 1 | Structure of the solar-induced hybrid fuel cell. (a) MEA (Nafion 117 polymer exchange membrane, anode made of carbon cloth and cathode loaded with Pt/C catalyst), (b) graphite bipolar plate, (c) acrylic plastic end plate, (d) transparent glass vessel with starch-H PMo O (PMo ) 3 12 40 12 solution, (e) pump, (f) oxygen inlet, and (g) water and oxygen outlet. 2 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. Load e NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ARTICLE 0.40 1.4 3.0 0.35 10, 22, 34, 46, 58, 70, 82 h 1.2 2.5 0.30 1.0 2.0 0.25 0.8 0.20 1.5 0.6 0.15 1.0 0.4 0.10 0.5 0.2 0.05 0.0 0.0 0.00 400 500 600 700 800 900 1,000 1,100 020 40 60 80 0 2468 10 Wavelength (nm) Light irradiation Discharge time (h) time (h) Figure 2 | Electron–proton transfer during photoreduction and discharge process. (a) Ultraviolet–visible spectrums of starch–H PMo O (PMo ) 3 12 40 12 solution under photo-irradiation (diluted to 1 mmol l of PMo ). (b) Number of electrons transferred and pH change plots during light irradiation and discharge period (in single cell, room temperature). observed and studied previously, in which the PMo acted as Output of the starch-based cell. As expected and shown in electron-coupled-proton buffer . Fig. 3a, when a pure starch solution (2.5 wt%) was used alone at After exposure to simulated sunlight, the starch–PMo room temperature, the power output was very small because of solution was cyclically pumped through the anode cell while the lack of catalyst on the anode to electro-oxidize the starch. oxygen was simultaneously fed to the cathode cell. Figure 2b When PMo was added without sunlight irradiation, the power (curve 2) shows the number of discharged electrons, calculated by output still did not improve because the redox reaction between integrating the electric current output curve. The number of PMo and starch could not take place at room temperature electrons discharged on the electrode was 0.375 mmol electrons without light irradiation. PMo in its original oxidation state per ml solution, which is close to the calculated number shown in cannot oxidize starch in the hybrid cell at room temperature curve 1. This indicates that the electrons stored in the reduced without irradiation. However, after the PMo was pre-reduced to PMo were almost completely transferred to the cathode via the molybdenum blue by the photo- or thermal-induced redox external circuit and captured by O . After discharging, the pH reactions with starch, power density increased to 0.28 (photo was almost recovered to the original value because the oxidation induced) and 0.44 mW cm (thermal induced). The discharge of the reduced POM led to the release of protons from the POM process could continue for B8 h at room temperature without to the solution . light until the reduced PMo in the solution was fully oxidized (evidenced by the colour change from deep blue to light yellow). After the photo-irradiated solution was fully discharged, the Electron and proton transfer at elevated temperatures. solution with excess unreacted starch and its various derivatives Molybdenum blue can strongly absorb visible-near infrared light was then exposed to simulated solar light followed by discharging (700–1,000 nm), which is caused by the intervalence charge again. The photo-irradiation discharge cycles were repeated three transfer (CT) transitions via an oxobridge . The long-wavelength times and the power outputs were measured, as illustrated in light absorption leads to the conversion of light energy to thermal Fig. 3b. Surprisingly, it was found that the output power density energy, which raises the solution temperature. As shown in increased with the number of repeated cycles. A power density of Supplementary Fig. 2, the temperature of the starch–PMo 0.65 mW cm was reached on the third cycle, even though no reaction solution can reach up to 84 C under actual sunlight extra starch or PMo was added. It is believed that the increase in irradiation (clear sky, 28 C, Atlanta, GA, USA) for 90 min, which the power density is due to the decrease in the starch molecular is 20.8% higher than the maximum temperature reached by weight during the repeated tests. The low-molecular-weight deionized (DI) water exposed to the same sunlight irradiation species from oxidation and acid hydrolysis reduced the solution condition. POMs can harvest electrons and protons from viscosity and increased the reaction rate with POM molecules, as organics by simply heating without light irradiation as reported compared with high-molecular-weight starch. 22–27 before . Therefore, it is expected that the redox reactions Different from the pre-light irradiation or pre-heating tests between starch and PMo could be further enhanced at elevated conducted above, a continuous experiment with in-situ light temperature. irradiation and PMo redox reaction simultaneous to electricity To confirm the heating effect, the starch–PMo solution was production was also conducted. The reaction solution was placed heated and kept at 95 C for 6 h without light irradiation. The under simulated sunlight irradiation (irradiation intensity of transfer of electrons and change in pH of the solution during the 100 mW cm ) and heated to 95 C while being discharged thermal-reduction experiment are shown in Supplementary simultaneously. The cell worked continuously for almost 20 h and Fig. 3, which suggest that 0.525 mmol electrons were transferred a steady current density up to 2.5 mA cm was observed, as from starch to PMo per ml solution by thermal degradation shown in Fig. 3c. Both repeated cycle tests and in-situ continuous alone (equal to 2.13 electrons per Keggin unit). The pH value experiments indicate that the POM catalyst can be re-used increased from 0.65 to 0.74, and then returned to 0.62 after without further treatment. the reduced PMo solution was discharged, which is similar to the light irradiation and discharge test described previously. The results suggest that the designed biomass–POM reaction system Output of different biomass-based cell. The solar-induced could combine photochemical and thermal biomass degradation hybrid fuel cell reported in this study can be powered by various in a single process, which means that the sunlight utilization biomasses. Figure 4a shows the power density that was produced could be extended to the near-infrared band. by cellulose, lignin, switchgrass and poplar powder. The power NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. Absorbance (a.u.) –1 Electrons transferred (mmol ml ) pH ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 0.5 0.5 0.5 0.7 0.4 0.4 0.6 0.4 0.5 0.3 0.3 0.4 0.3 0.2 0.3 0.2 PMo –starch–heat 12 0.2 0.1 PMo –starch–light 0.2 1st time 0.1 0.1 2nd time PMo –starch 3rd time 0.0 Starch solution 0.0 0.1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 –2 –2 Current density (mA cm ) Current density (mA cm ) 3.0 2.5 2.0 1.5 1.0 0 200 400 600 800 1,000 1,200 1,400 Time (min) Figure 3 | Performances of the solar-induced hybrid fuel cell using starch as fuel. (a) Voltage–current density and power–current density plots with different reaction systems used in a single solar-induced fuel cell at room temperature; (b) plots of three repeated photo-irradiation–discharge cycles with starch–H PMo O (PMo ) reaction system (discharged at room temperature); (c) current curve of continuous performance test with light 3 12 40 12 irradiation at 95 C. 0.8 0.7 0.4 0.6 0.4 0.6 0.5 0.4 0.3 0.3 0.4 0.3 0.2 0.2 0.2 0.2 Cellulose 2+ 3+ Poplar 0.1 PMo -Cu -Fe PMo -SnCI Lignin 12 4 Switch grass 0.0 PMo 0.0 0.1 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 –2 –2 Current density (mA cm ) Current density (mA cm ) Figure 4 | Performances of the solar-induced hybrid fuel cell using different biomass. (a) Voltage–current density and power–current density plots of different biomasses used in the H PMo O (PMo ) reaction system in photothermal experiments; (b) plots of cellulose and PMo degradation 3 12 40 12 12 with different metal salts as Lewis acids in photothermal experiments (discharged at room temperature). density of our solar-induced hybrid fuel cell can reach 0.65 and Consequently, as shown in Fig. 4b, when these promoters were 0.62 mW cm when fueled by poplar and switchgrass powders, used in this system, the power density obviously increased, even 2 2þ 3 þ respectively. It should be noted that all these materials are water up to 0.72 mW cm (Cu –Fe as promoters for PMo ), insoluble; hence, they were particle suspensions at the beginning, which is almost 100 times higher than that of cellulose-based but the biomass particles were degraded and dissolved in the microbial fuel cells reported in literature . reaction solution as time progressed. The results illustrate that all these biomass materials can be directly used as fuel in this solar- induced hybrid fuel cell with POM as the catalyst and charge Discussion carrier. The general principle of our solar-induced hybrid fuel cell is that It was also noted that the power density produced by PMo can oxidize biomass under solar irradiation while being 6 þ 5 þ 5þ crystalline cellulose was lower than that obtained from starch reduced from Mo to Mo , and then Mo can be oxidized 6 þ solution under similar conditions, probably because of the slow to Mo again by oxygen through a catalytic electrochemical hydrolysis of cellulose crystals. It was reported that some metal reaction, as illustrated in Fig. 5a. In a previous study, Yamase 4þ 3þ 2þ ions, such as Sn ,Fe and Cu and so on could function as proposed that electron transfer from organics to POM under 28,29 Lewis acids to help cleave glycosidic bonds in cellulose . light irradiation could result in the formation of an 4 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. Voltage (V) Voltage (V) Discharged current –2 density (mA cm ) –2 –2 Power density (mW cm ) Power density (mW cm ) Voltage (V) Voltage (V) –2 –2 Power density (mW cm ) Power density (mW cm ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ARTICLE Sun light UV Vis IR –+ Photo chemical Solar Thermal Starch 1/2O MEA Oxygen + – +2H +2e H – Starch H O –H + Water + oxygen Starch oligomers Starch –COOH, ˙OH, etc. Hydrolysis Oxidation Carbon cloth Pt/C –H PMo (VI) Reduced PMo (V) 12 12 0 h 4 h 6 h 8 h Figure 5 | Schematic illustration of this solar-induced fuel cell. (a) The working principle of this cell. In fact, there are 12 Mo elements in 1 POM cluster. To VI V simplify the drawing, oxidized Mo is represented by Mo and reduced Mo is represented by Mo , although the 12 Mo elements in the cluster may not have the same valences. (b) The colour change of starch–H PMo O (PMo ) solution after electrically discharging up to 8 h. 3 12 40 12 intermolecular charge-transfer complex. This mechanism was and thus stabilizing the reduced state of PMo . In this study, the 21,32,33 further investigated and accepted widely in other studies . formed starch–PMo complex was separated from the reaction According to this mechanism, the representative reaction in our solution and characterized by Fourier-transform (FT)–infrared, starch-PMo solution is as shown in equation (1). which provides evidence of the interaction between starch and PMo (shown in Supplementary Fig. 4 and Supplementary HO O Table 1). H O hυ VI Starch Mo (1) O Starch O Mo As light irradiation has a dual effect on the oxidation of O O O O biomass by POMs as verified before, both photo and heat reduction of PMo can happen simultaneously in the presence of The PMo has the best-known Keggin structure consisting of a starch. As a result, POM captures an electron and proton from central tetrahedral [PO ] surrounded by 12 [MoO ] octahedrons starch; thus, starch is oxidized and degraded to starch oligomers 4 6 that are photosensitive . Under short-wavelength light and glucose derivatives as shown in reaction (2). In fact, POM can irradiation, the commonly known O-Mo ligand-to-metal CT accept more than one electron per Keggin unit as the reaction occurs, which means the 2p electron in the oxygen of [MoO ]is continued progressed, which means the increase of reduction excited to the empty d orbital in Mo, changing the electron degree of POM. 0 1 configurations of Mo from d to d , and leaving a hole at the light or heat 3 3 VI VI V 21,31 2Starch-OHþ2PMo O Starch-O- HPMo Mo O 12 40 11 40 oxygen atom of the POM lattice . This hole interacts with one ! 21,31 ð2Þ electron on the oxygen atom of a hydroxyl group of starch . VI V þ HPMo Mo O 11 40 Meanwhile, the hydrogen atom of the hydroxyl group shifts to the þ Oxidized starch oligomers 1 21,31 POM lattice to interact with the d electron , which is a thermally activated delocalization within the polyanion It should be noted that although one Mo is reduced from VI to molecule . Thus, the intermolecular CT complex is formed, V valence by forming a starch–POM complex, the total charge of VI V 3 leading to the separation of photo-excited electrons and holes, the polyanion (–[HPMo Mo O ] ) will not change because 11 40 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. Mo Mo Mo Mo VI Mo VI V ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 a proton is also transferred from starch to the POM complex at The power density of the cell is also affected by the redox the same time. As the standard redox potential of oxygen is potential of the POMs. As the charge carrier, POM is first 5þ higher than that of the reduced POM, the Mo in the reduced reduced by oxidizing the biomass, and then the reduced POM is PMo can be oxidized at the anode by connecting to the O oxidized by oxygen. Obviously, if the standard redox potential of 12 2 electrode through an external circuit to produce electricity. POM is high, it has greater tendency to oxidize the biomass but 5þ As a result, the reduced POM (Mo ) gives one electron to the lower tendency to be oxidized by oxygen. Therefore, POMs with carbon anode and simultaneously releases a proton to the higher standard redox potentials are not always advantageous. In solution with the gradual colour change from deep blue to light this study, three types of POMs (PMo , [PW O ] and 12 12 40 6 41 yellow as shown in Fig. 5b. The electron passes through the [PV Mo O ] ) with different redox potentials were used as 3 9 40 external circuit and is captured by oxygen to form water at the the charge and proton carriers (Supplementary Fig. 9). No simple cathode. At the same time, the starch molecules associated with relationship between standard redox potential of the POM and PMo are released into solution. The net effect of the above power output was observed. 5 þ 6þ reaction is that Mo is oxidized back to Mo at the anode, Faradic efficiency is considered as one important part of and the starch is oxidized through dehydrogenation by POM discharge efficiency. Faradic efficiency is defined as a ratio of the catalysis. Finally, the proton diffuses to the cathode side through actual discharge capacity to the total electron charge transferred the proton-exchange membrane and combines with oxygen to from the organics in the POM electrolyte solution. In this form water. The entire discharge process is represented by study, Faradic efficiency was as high as 91% and 94% in the reactions (3) and (4), discharge of the starch–PMo system for photoreduction and thermal reduction, respectively (Supplementary Fig. 10 and VI V Starch-O- HPMo Mo O 11 40 Supplementary Note 1). 3 3 VI V VI anode The solar-induced hybrid fuel cell designed in this study is a þ HPMo Mo O 2 PMo O 11 40 12 40 ð3Þ combination of solar cells, fuel cells and redox flow batteries, but þ Oxidized starch oligmers has distinct differences from each. For a traditional solar cell, light energy is converted to electricity directly via the photovoltaic þ 2e þ 2H effect when the semiconductor or dye on the semiconductor is exposed to light. However, for our hybrid fuel cell, short-wave cathode 1=2O þ 2e þ 2H ! H O ð4Þ 2 2 light excites POMs to the excited state in the presence of biomass and is stored in the form of the reduced POMs (that is, chemical Starch, which acts as the electron and proton donor, can be energy) via the photochromic reaction. Moreover, visible and directly oxidized under light radiation or hydrolysed to small near-infrared light are absorbed by the solution and converted to oligomers by the POM, and then continuously oxidized to a series heat, which can also promote the redox reaction between biomass of glucose derivatives such as aldehyde, ketones and acids, as and POM. The cell reported in this study also has a similar shown in reaction (5): feature to redox flow batteries in that electrolyte solutions with POM Re Starch ox different valence states are used in the electrode cells. It is still POM H ,POM + different from redox flow batteries because the cathode side in H ,POM POM Starch Re Oligomer Glucose CO (5) ox ox our cell uses oxygen gas rather than an electrolyte solution. In POM addition, organic fuel is consumed in our hybrid fuel cell but no Oligomer organic fuel is used in a traditional redox flow battery. The overall The direct oxidation of starch in the photochromic reaction reaction, as discussed above, is the oxidation of organic fuel by was confirmed by FT–infrared (Supplementary Fig. 4), and the oxygen, which is the fundamental basis of traditional fuel cells but degradation and further oxidation of starch glucose derivatives not redox flow batteries. However, this cell is different from a was verified by gel permeation chromatography (Supplementary traditional fuel cell where catalytic reactions happen on the 1 13 Fig. 5 and Supplementary Table 2) and H, C NMR analysis precious metal-loaded anode. The POM functions as a photo and (Supplementary Fig. 6). In addition, CO is recognized as the thermal catalyst and charge carrier that takes electrons from 5 þ final product in many photodegradation reactions of organic biomass while reducing its own valence state to Mo under 36–38 compounds by POMs , and the produced CO was light irradiation and thermal degradation. The electrons in the actually detected during the operation of the solar-induced POM are then transferred through the external circuit and the 6 þ hybrid fuel cell in this study (Supplementary Fig. 7 and POM reverts to its original Mo valence state. Supplementary Table 3). As the charge carrier in this study, In this study, the catalytic reactions are mediated by POM, but PMo is very stable in a solution acidified by H PO , which was not on the surface of a noble metal electrode. In fact, most 12 3 4 verified during our repeated cycle test using P NMR analysis biomass fuels and even some artificial polymers and organic (Supplementary Fig. 8). wastes can be directly degraded by the solar-induced hybrid fuel The performance of this solar-induced fuel cell is associated cell discussed in this study to provide electricity without using a with the reduction degree and redox potential of POMs. As precious metal anode. The organic impurities in crude biomass described before, the power density gradually increased after could also be oxidized as fuels, and inorganic impurities will not three repeated light irradiation–discharge cycles. Correspond- poison the whole process because POMs are robust and self- ingly, it was found that the reduction degree of the POM healing . As a result, the fuels used in our solar-induced hybrid increased from 1.23 to 2.4 electrons per Keggin unit after three fuel cell do not have to be pure, which would significantly reduce repeated tests. The reason for the increase in reduction degree is the cost of current fuel cell technology. In essence, this solar- that low-molecular-weight oligomers and oxidized derivatives induced fuel cell is different from any cells or batteries reported (mainly aldehydes) were formed during the photocatalytic previously, and it greatly widens the range of biomass fuels for reaction, and they have greater reduction power and higher electric power production. reactivity with the POM. Therefore, with repeated light In summary, we demonstrated the feasibility of a new concept irradiation–discharge cycles, more electrons were captured by for fabricating a solar-induced fuel cell that directly consumes each Keggin unit, but without major changes in their chemical biomass fuels in the presence of a POM photocatalyst under light 39,40 structures . irradiation. Although the operation parameters have not been 6 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 ARTICLE optimized, the power density of our solar-induced hybrid fuel can Cycling test for starch–PMo system. To investigate the reproducibility of this cell, three photoreduction–discharge cycles were conducted. In each cycle, the reach 0.72 mW cm when cellulose was used as the fuel. discharged fuel–PMo solution was exposed to AM-1.5 simulated sunlight and kept at constant 25 C until use. The solution was then pumped into the anode of Methods the solar-induced fuel cell to electrically discharge for several hours over a load of Experimental details for preparation of electrolyte solution. Phosphomolybdic 1.6O resistance until the current fell below 0.1 mA. In the continuous experiments, acid (H [PMo O ], PMo ) with a-Keggin structure was purchased from TCI the transparent glass vessel was placed under irradiation of AM 1.5-type simulated 3 12 40 12 America. Potato starch solution (4 wt%) was obtained by cooking starch suspen- sunlight and heated to 95 C during a continuous discharge experiment. The glass sion for 20 min at 95 C. Calculated amount of phosphomolybdic acid was added to (sodium lime glass) was 2 mm thick, with transmission 495% for wavelengths the required volume of starch solution and then diluted to 20 ml total. The con- from 320 to 1,100 nm and transmission 480% for wavelengths from 270 to 320 nm 1  1 centrations of PMo and starch were 0.3 mol l and 15 g l , respectively. The (air as blank sample, measured on Agilent Technologies 8453 ultraviolet–visible starting pH of this solution was adjusted to 0.3 using concentrated phosphoric acid spectrophotometer). (85%, Alfa Aesar); thus, a clear yellow solution was obtained. Biomasses used in the solar-induced fuel cell. Crystalline cellulose was pur- Photoreducing experiment. The obtained yellow solution (20 ml) was kept at a chased from Alfa Aesar. The cellulose suspension was homogenized for 40 min to constant temperature of 25 C using a recirculating water bath (RM6 Lauda, form a small particle suspension in the solution before used. Lignin was isolated Brinkmann Instruments Service Inc.). Next, the reaction solution was irradiated from a commercial USA softwood kraft pulping liquor. Switchgrass was from with AM 1.5-type simulated sunlight (SoLux Solar Simulator, New York, USA; Ceres, Inc. (Thousand Oaks, CA, USA) and poplar was provided by Michigan State 50 W) for a certain time with a distance of 10 cm from the solution surface. The University. These samples were washed with DI water and then dried at 45 Cover colour of the solution gradually turned from yellow to deep blue. At regular night. The dry samples were then milled in a Wiley mill through a 0.8-mm screen, intervals, a very small amount of solution was taken out as a sample and diluted to yielding the biomass powder directly used in this study without any pretreatment. 1 mmol l PMo for analysis of the concentration of reduced PMo and the pH 12 12 value of the solution. Different metal salts used as Lewis acids. In these experiments, the reaction solutions consisted of 0.25 mol l PMo and 0.1 g biomass with a total solution Thermal-reducing experiment. The concentration of starch–PMo solution used volume of 20 ml and pH 0.5. The solution was irradiated by simulated sunlight 1  1 in this experiment was 15 g l starch and 0.25 mol l PMo at pH 0.65. It was  2 (100 mW cm ) and heated on a hotplate up to 95 C and kept for 6 h (photo- continuously heated under reflux at 95 C with magnetic stirring in the dark for 6 h. thermal experiment). The discharge condition was the same as the starch–PMo The sample collection procedure was the same as that of photoreducing experi- system. To improve the reduction degree of PMo in cellulose–PMo reaction 12 12 ment. The prepared heteropoly blue solution was directly circulated in the designed system, some metal salts as Lewis acids were added. The Lewis acids used in this fuel cell to generate electricity. As the phosphomolybdic blue is very stable in  1 3þ 2þ study include SnCl (Alfa Aesar, 0.188 mol l )and Fe -Cu (Fe (SO ) (Alfa 4 2 4 3 solution under air, this prepared solution can be stored for a long time.  1  1 Aesar, 0.15 mol l ); CuSO (Alfa Aesar, 0.15 mol l ). Analysis of electron transfer using spectrophotometry. To obtain the cali- Different POMs used in this hybrid fuel cell. Three different POMs with various bration curve, pure PMo solution without starch was reduced by electrochemical electrode potentials were used in this study: PMo , phosphotungstic acid and 5þ 12 reduction treatment at 3 V for different times. The concentration of Mo in the vanadium substituted phosphomolybdic acid. Phosphotungstic acid 5þ 6þ PMo solution (Mo and Mo mixture) was determined by titration with (H [PW O ]) was purchased from Sigma-Aldrich, and vanadium-substituted 3 12 40 potassium permanganate solution (calibrated by standard sodium oxalate). Thus, phosphomolybdic acid (H [PMo V O ]) was synthesized by the method in 6 9 3 40 the calibration curve for 1 mmol l PMo at different reduction degrees was 12 41 reported literature . In comparison experiments, the reaction solutions consisted 5þ obtained. The concentration of Mo in the starch–PMo reaction solution was 12  1 of 0.25 mol l POMs, 0.3 g starch and a total solution volume of 20 ml at pH 0.5. measured using the ultraviolet–visible spectrophotometer (Agilent Technologies The solution was irradiated by simulated sunlight and heated at 95 C for 6 h (that Inc., Santa Clara, CA, USA, 8453) to calculate the amount of electrons transferred is, photothermal experiment). For comparison, phosphotungstic acid was fully from starch to PMo during light irradiation or heating. The samples taken in reduced by ascorbic acid (1:1 molar ratio) in solution under light irradiation and photoreducing or thermal-reducing experiments were diluted to a concentration of heating at 95 C. The discharge conditions were the same as described above. 1 mmol l of total PMo and then used for ultraviolet–visible measurement. Product analysis of starch degradation. The starch–PMo complex was sepa- Investigation of solar thermal energy transfer. The experimental apparatus rated through precipitation from the photoredox reaction system by salting out consisted of a polymer foam insulation system, transparent test tubes (d¼ 10 mm, (excess Na SO ). The sample was then filtered and dried in a vacuum drying 2 4 h¼ 175 mm) and a simulated sunlight source (SoLux Solar Simulator; 50 W) as chamber at 30 C overnight. The changes in the starch that was associated with shown in Supplementary Fig. 2b. The heteropoly blue reaction solutions were 1 PMo were characterized by FT–infrared. FT–infrared spectra were obtained by diluted to 1 and 10 mmol l , and then transferred into test tubes with the same 1  1 averaging 64 scans from 4,000 to 650 cm with 4 cm resolution. Gel per- liquid levels. The simulated sunlight source was placed above the foam insulation meation chromatography was used to determine the changes in molecular weight in which the test tubes were laid side by side. The light energy density that irra- 2 of starch. Samples were taken from the reaction solution at different times and the diated the surface of the tubes was 100 mw cm , and the temperature of the proper amount of NaOH solution was added. The PMo degraded to MoO in 12 4 solution inside the test tubes was collected by thermocouples. In addition, the 1 alkaline solution and the final concentration of organic substance was around actual reaction solution (250 mmol l PMo ) and DI water were used in this 1mgml with pH 10. The analysis was performed on an Agilent 1,200 HPLC photo-to-thermal test under actual sunlight. System (Agilent Technologies Inc.) equipped with ultrahydrogel columns (Waters Corporation, Milford, MA, USA) and an Agilent RI detector using DI water as the 1 1 13 Assembly of solar-induced fuel cell and test methods. The MEA was purchased mobile phase (0.5 ml min ) with injection volumes of 25ml. H NMR and C as a commercialized product from Fuelcells Etc, TX, USA. The MEA was com- NMR were used to analyse the final products of starch in the solution. To increase posed of a Nafion 117 membrane with 5 layers of carbon cloth on one side (used as the signal intensity, light irradiation and discharge cycles were repeated four times. the anode electrode) and 5-layer carbon cloth plus 60% Pt/C catalyst loaded at The starch–POM solution after repeated irradiation and discharge was dried at 2 1 13 5mg cm on the other side (used as cathode). The grain size of the active carbon 80 C in vacuum, then resolved in D O for the NMR. H and C NMR spectral used as support for the Pt/C catalyst was 74mm and its specific surface area was data reported in this study were recorded with a Bruker Avance/DMX 400 MHz 2  1 13 90 m g . The crystal size of the nano-Pt particles in the Pt/C catalyst was 4.0– NMR spectrometer. Quantitative C NMR employed an inverse gated decoupling 2  1 5.5 nm and the specific surface area was 60 m g . The bipolar plates of the cell pulse sequence, 90 pulse angle, a pulse delay of 5 s and 6,000 scans. Quantitative were made of high-density graphite plates with a straight flow channel 2 mm wide, H NMR was acquired with 16 scans and 1 s pulse delay. To collect and analyse the 2 mm deep and 5 cm long (a total active area of 1 cm ). The MEA was sandwiched emission gas in the hybrid fuel cell, an airtight glass vessel was connected with an between two graphite flow-field plates, which were clamped between two acrylic emission gas outlet and a valve (shown in Supplementary Fig. 7). The starch– 1  1 plastic end plates, as shown in Fig. 1. Rubber gaskets were included on the cir- PMo solution (15 g l starch and 0.3 mol l PMo ) was stored in the airtight 12 12 cumference of the graphite flow-field plates to prevent any leakage. In our glass vessel and exposed to simulated sunlight (100 mW cm ) and kept at 95 C. experiments, the prepared molybdenum blue solution was pumped through the Meanwhile, the starch–PMo solution was pumped to the anode chamber of the anode reaction cell, and oxygen was flowed through the cathode cell using a fuel cell and discharged for 3 days. A gas collection bag was connected to the gas compressed oxygen cylinder. The temperature of the liquid in the cell was B25 C outlet through a rubber tube. The composition of gas was analysed on Varian 490 (room temperature). The solution flow rate through the anode graphite plate was micro-GC equipped with a Pora Plot U (10 m) column. P NMR was used to 1  1 12 ml min and the oxygen flow rate through the cathode was 75 ml min at analyse the stability of POM. After several repeated irradiation and discharge 1 atm. A DS345 30 MHz (Stanford Research Systems) and a 4,200-SCS (Semi- cycles, the reaction solution was dried at 95 C then resolved in D O for P NMR. conductor Characterization system, Keithley Instruments Inc.) were used to The spectral data were collected on a Bruker Avance/DMX 400 MHz NMR spec- examine the I–V curves using the controlled potentiostatic method. trometer with 8 s pulse delay. NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4208 References 27. Hill, C. L. Homogeneous catalysis: controlled green oxidation. Nature 401, 1. Ru¨hl, C., Appleby, P., Fennema, J., Naumov, A. & Schaffer, M. 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W.L. performed the 5, 403–409 (2013). photo and thermal experiments. W.L., M.L. and H.C. fabricated the solar-induced fuel 21. He, T. & Yao, J. Photochromism in composite and hybrid materials based on cell. W.L. and M.L. tested the performance under a variety of conditions. W.M. per- transition-metal oxides and polyoxometalates. Prog. Mater. Sci. 51, 810–879 formed NMR analyses. W.L. performed gel permeation chromatography, infrared, (2006). ultraviolet–visible and GC analysis. X.Z. and M.L. helped in paper preparation. 22. Gaspar, A. R., Gamelas, J. A. F., Evtuguin, D. V. & Pascoal Neto, C. Alternatives for lignocellulosic pulp delignification using polyoxometalates and oxygen: a review. Green Chem. 9, 717–730 (2007). Additional information 23. Weinstock, I. A. et al. Equilibrating metal-oxide cluster ensembles for oxidation Supplementary Information accompanies this paper at http://www.nature.com/ naturecommunications reactions using oxygen in water. Nature 414, 191–195 (2001). 24. Weinstock, I. A. Homogeneous-phase electron-transfer reactions of Competing financial interests: The authors declare no competing financial interests. polyoxometalates. Chem. Rev. 98, 113–170 (1998). 25. Hill, C. L. & Weinstock, I. A. Homogeneous catalysis: on the trail of dioxygen Reprints and permission information is available online at http://npg.nature.com/ activation. Nature 388, 332–333 (1997). reprintsandpermissions/ 26. Weinstock Ira, A., Atalla Rajai, H., Reiner Richard, S., Houtman Carl, J. & Hill Craig, L. Selective transition-metal catalysis of oxygen delignification using How to cite this article: Liu, W. et al. Solar-induced direct biomass-to-electricity hybrid water-soluble salts of polyoxometalate (POM) Anions. Part I. Chemical fuel cell using polyoxometalates as photo-catalyst and charge carrier. Nat. Commun. principles and process concepts. Holzforschung 52, 304–310 (1998). 5:3208 doi: 10.1038/ncomms4208 (2014). 8 NATURE COMMUNICATIONS | 5:3208 | DOI: 10.1038/ncomms4208 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved.

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