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Swelling‐Induced Structural Transformation Strategy: Controllable Synthesis of 2D Porous Polypyrrole/MXene Heterostructures with Tunable Pore Structures

Swelling‐Induced Structural Transformation Strategy: Controllable Synthesis of 2D Porous... IntroductionPorous polymers featuring high porosity, low density, and good physicochemical stability are attractive in the basic research and industrial application fields.[1] Particularly, 2D porous polymers, combining the advantages of 2D materials and inherent characteristics of porous polymers, exhibit huge application potentials in the fields of energy storage and conversion, especially in the field of supercapacitors.[2] It is well known that the electrochemical performances of 2D porous polymers are often influenced by many factors such as specific surface area, electronic conductivity, and pore structure.[2b,3] In this case, pore structure plays a crucial role in electrochemical performance, which is often overlooked. Up to now, the precise control of pore structure mainly depends on two methods, namely, the prefabricated template method and the molecular self‐assembly method.[1d,4] In the former scenario, the monomer is first cross‐linked on the surface of the 2D silica‐based template prepared in advance, followed by the etching of the template with corrosive acid or base to obtain a porous structure. Although various 2D porous polymers can be synthesized through this route, the pore structure is limited by the morphology of parent silica templates, and the obtained pore structure is mainly spherical mesopores.[5] Moreover, the procedure of this method is tedious, costly, as well as not suitable for mass production, which greatly hinders their practical applications.By contrast, the molecular self‐assembly method based on the cooperative organization of polymer precursors and surfactant templates on 2D substrates is a more efficient strategy.[6] Hitherto, this method has been quite mature for controlling the pore structure of materials, especially for metal oxides, silicates, carbon, and so on.[7] However, it is still a huge challenge to precisely control the pore structures of 2D porous polymers. Owing to the fact that the polymerization process of pyrrole is very violent and rapid, it is hard to accurately control the reaction kinetics of pyrrole monomers, enabling them cannot be uniformly arranged and further grow on the surface of 2D substrates. The lack of suited interaction or driving forces between polymer monomers, surfactant micelles, and 2D nanosheets results in difficulties existing in regulating the co‐assembly behavior, eventually leading to the formation of bulk or nonporous structure.[8] To the best of our knowledge, there are few reports on the synthesis of 2D porous polymers with spherical and cylindrical mesopores by using poly(ethylene oxide)‐b‐polystyrene (PEO‐b‐PS), Pluronic F127, and Pluronic P123 as templates, respectively.[9] In addition, the existing synthesis strategies to construct 2D porous polymers usually employ graphene as a 2D substrate, which inevitably results in some adverse impacts such as uneven growth of porous polymers, severe stacking of 2D substrate, and general conductivity.[3a,10] Fortunately, MXene, a new material family of 2D carbide or carbonitride, owns abundant surface functional groups, unique accordion‐like structure, and superior metallic conductivity, and maybe a satisfactory 2D substrate to replace graphene in the construction of 2D porous polymers.[11] Recent research works have shown that many kinds of porous materials can be combined with MXene nanosheets to form 2D porous heterostructures.[12] Compared with graphene, MXene has even better conductivity, which is a benefit for electrochemical performances. Furthermore, the OH groups of MXene contributed to the uniform growth of porous polymers. Therefore, it is extremely urgent and vital to develop an effective synthetic strategy for the controllable construction of 2D porous polymers and further to understand the inherent relationship between pore structures and electrochemical performances.Herein, we developed a swelling‐induced structural transformation strategy to construct 2D porous polypyrrole/MXene heterostructures (2D porous PPy/MXene) by using Pluronic P123 as only template, 1,3,5‐trimethylbenzene (TMB) as swelling agent, and MXene nanosheets (Ti3C2Tx) as 2D substrate. By adjusting the dosages of TMB, the hydrodynamic diameters of the composite micelles formed by self‐assembly P123 and TMB can be regulated from 11.7 to 58.7 nm, thus achieving precise control over pore structures (cylindrical mesostructure, spherical mesostructure, and spherical macrostructure), pore sizes (≈7.8–52 nm), and specific surface area (≈129–188 m2 g−1) of 2D porous PPy/MXene. Due to the cylindrical mesoporous structure providing a fast electrolyte transferring mode, large electrolyte/electrode contact area, and shorter ion diffusion distance, the obtained 2D cylindrical mesoporous PPy/MXene exhibits the optimal supercapacitor performances including high capacitance of 477 F g−1 at 1 A g−1, superior rate performance of ≈300 F g−1 at 5 A g−1, and favorable cyclability with tiny capacitance degradation (5.8%) after 10 000 cycles. This work not only reveals the structure‐activity relationship between pore structure and electrochemical performance but also provides new insights for the construction of novel 2D porous heterostructure materials.Results and DiscussionFigure 1 displays the schematic illustration of the synthesis process of 2D porous PPy/MXene. First, P123 copolymers were dissolved into water to form a uniform micelle solution. Second, the pyrrole monomers were introduced into the synthesis system, which can interact on the surface of micelles through H‐bonding and self‐assemble into P123/pyrrole composite micelles. Third, after adding the MXene nanosheets, the composite micelles can be anchored on the Ti3C2Tx surface by hydrogen bond and electrostatic interaction, and the pyrrole monomers were polymerized under the effect of the initiator of FeCl3 (Figure S1, Supporting Information).[13] Finally, the 2D porous PPy/MXene samples were obtained after removing the template by washing it with ethanol. Notably, by adjusting the addition amounts of TMB, three representative 2D porous PPy/MXene samples, including 2D cylindrical mesoporous polypyrrole/MXene, 2D spherical mesoporous polypyrrole/MXene, and 2D spherical macroporous polypyrrole/MXene, were synthesized, which were denoted as 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3, respectively.1FigureSchematic illustration of the fabrication of 2D porous polypyrrole/MXene heterostructures by swelling‐induced structural transformation strategy.As presented in Figure 2a, the P123 solution with different addition amounts of TMB was denoted as Micelle 1, Micelle 2, and Micelle 3, respectively. The photograph of three micelle solutions showed that the colors were transformed from transparent to milky white with increasing amounts of TMB. Meanwhile, these micelle solutions all possessed obvious Tyndall effect, indicating the formation of micelles at 40 °C. Dynamic light scattering (DLS) measurement suggested that the hydrodynamic diameters of the formed micelles were centered at ≈11.7, ≈32.6, and ≈58.7 nm, respectively (Figure 2b). Moreover, a linear fitting with an equation of y = 1.175x + 10.833 (r2 = 0.99) was acquired to illustrate that the volume of TMB is linearly related to the hydrodynamic diameters of the micelle (Figure 2c). The morphologies and pore structures of 2D porous PPy/MXene samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images of three samples all exhibited uniform 2D nanosheet morphology with observable pores on the surface (Figure 2d–f). The TEM images of 2D porous PPy/MXene also displayed a 2D layered structure with abundant pores. With increasing the addition amount of TMB, 2D porous PPy/MXene samples showed the transformation of pore structures from cylindrical mesostructures, to spherical mesostructures, and then to spherical macrostructures (Figure 2g–i). Furthermore, the average pore sizes of 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3 can be measured to be about 7.8, 29.6, and 52 nm, respectively, which were consistent with the DLS results. The high‐angle annular dark‐field‐scan TEM (HAADF‐STEM) and energy dispersive spectrometry (EDS) mapping images of 2D porous PPy/MXene‐1 revealed that the Ti, C, and N elements were distributed evenly in the sample (Figure 2j–m).2Figurea) Photograph of P123 aqueous solution mixed with different volumes of TMB (0 µL for Micelle 1, 20 µL for Micelle 2, and 40 µL for Micelle 3). b) DLS spectrum of different micelle solutions. c) The linear fitting curves of the dosages of TMB and the average micellar diameter (dmicelle) measured by DLS. SEM images of d) 2D porous PPy/MXene‐1, e) 2D porous PPy/MXene‐2, and f) 2D porous PPy/MXene‐3. TEM images of g) 2D porous PPy/MXene‐1, h) 2D porous PPy/MXene‐2, and i) 2D porous PPy/MXene‐3. j–m) HADDF‐STEM and EDS mapping images of elemental Ti, C, and N of 2D porous PPy/MXene‐1.The detailed porosities of 2D porous PPy/MXene were characterized by N2 adsorption–desorption measurements. Every adsorption isotherm of 2D porous PPy/MXene exhibits obvious hysteresis loops, implying they have large porosity (Figure 3a). Moreover, the Brunauer–Emmett–Teller surface areas (SBET) of 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3 are 188, 158, and 129 m2 g−1, respectively, which are much higher than those of reported porous PPy‐based materials (Table 1 and Table S1, Supporting Information).[3c,6c,e,8,14] The pore volume of 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3 are calculated to be 0.62, 0.34, and 0.30 cm3 g−1, respectively. These results show that the SBET and pore volumes of 2D porous PPy/MXene samples decrease linearly with the increase in pore size. Fourier transform infrared (FT‐IR) spectra of 2D porous PPy/MXene‐x all display the two typical characteristic peaks of polypyrrole at 1308 and 1700 cm−1, which can be attributed to CN and CN bonds, revealing the successful polymerization of pyrrole (Figure 3b).[3a,9a] Additionally, the disappearance of a characteristic signal at around 2870 cm−1 manifests the complete removal of P123. Raman spectra of 2D porous PPy/MXene‐x show that the characteristic peaks at ≈1160 and ≈1350 cm−1 were ascribed to the ring stretching and in‐plane CH modes of PPy, and the peaks located at ≈1595 cm−1 corresponded to the CC bond stretching mode in the pyrrole ring (Figure 3c).[15] The XRD patterns of 2D porous PPy/MXene‐x show two typical characteristic peaks at ≈24.8° and 51.2°, indicating the samples were amorphous (Figure 3d).[16] Moreover, no significant oxidation peak was observed, suggesting the Ti3C2Tx nanosheets are not oxidized during the polymerization process (Figure S2, Supporting Information).[12d] X‐ray photoelectron spectroscopy (XPS) spectra show the existence of signals of Ti, C, and N in these samples (Figure S3, Supporting Information). Four distinct energy values at 282.8, 283.7, 284.8, and 286.8 eV can be observed in C1s spectra for 2D porous PPy/MXene‐x, which can be assigned to CC, CC, CN, and disordered carbon, respectively (Figure 3e). The high‐resolution N 1s spectra contain the main NH bonds at 398.6 eV, N+ bonds at 399.2 eV, and slight N+H bonds at 400.7 eV (Figure 3f).[17] For all the 2D porous PPy/MXene samples, the proportions of the relative peak area of each group of C and N atoms are almost the same, implying the chemical states of 2D porous PPy/MXene‐x samples are quite similar (Table S2, Supporting Information). The above results demonstrate that the 2D porous PPy/MXene‐x samples own satisfactory porosities and similar chemical compositions.3FigureStructural characterizations of 2D porous PPy/MXene. a) Nitrogen adsorption–desorption isotherms, b) FTIR spectra, c) Raman spectra, d) XRD patterns of 2D porous PPy/MXene‐x. e) C 1s and f) N 1s high‐resolution XPS spectra of 2D porous PPy/MXene‐x.1TableStructural properties of samplesSamplesTemplateSBETa) [m2g−1]dTEMb) [nm]dBJHb) [nm]VTotalc) [cm3g−1]2D porous PPy/MXene‐1P1231887.87.70.622D porous PPy/MXene‐2P12315829.628.80.342D porous PPy/MXene‐3P12312952–0.302D PPy/MXene–89––0.17mPPyP12313253.40.64a)The SBET measured from the nitrogen adsorption–desorption isothermsb)The average pore diameters are calculated by TEM images and BJH method, respectivelyc)VTotal: Total pore volume calculated from adsorption isotherm at P/P0 = 0.99.Based on the above results, we propose a swelling‐induced structural transformation strategy for the synthesis of 2D porous PPy/MXene (Figure 1). First, the Pluronic P123 (PEO20–PPO70–PEO20) triblock copolymers self‐assemble into cylindrical micelles with hydrophobic PPO blocks as the cores and hydrophilic PEO blocks as the coronas in aqueous solution at 40 °C (Route 1).[2c,3a] According to the packing parameter (p = v/a0lc, where p is the packing parameter, v is the volume of the hydrophobic core, a0 is the contact surface area of the polar head, and lc is the length of hydrophobic chain) theory, it can be extrapolated that the packing parameter of P123 micelles is between 1/3 and 1/2 under this condition.[18] After adding a small amount of TMB, based on the similarity‐intermiscibility principle, the hydrophobic TMB can enter into the PPO segment of P123 micelles, which enables the micelles to swell and increases the exposed surface area (a0), leading to the decrease of packing parameter below 1/3 and the transformation of the morphology of composite micelles from cylindrical mesostructure to spherical mesostructure (Route 2).[19] With further increasing the dosage of TMB, the core of mesoscopic spherical micelles further swells and converts into macroscopic spherical micelles (Route 3). Subsequently, after the introduction of the pyrrole, the O in PEO coronas of micelles can interact with NH of pyrrole monomers via hydrogen bonding interaction to assemble into composite micelles. Then, the composite micelles anchor on the surface of Ti3C2Tx nanosheets by hydrogen bonding and electrostatic interactions between the OH of Ti3C2Tx and the NH of pyrrole. The polymerization of pyrrole monomers is triggered by adding FeCl3, yielding 2D composite micelles/polypyrrole/MXene hybrid materials. Finally, the 2D porous PPy/MXene samples were obtained by removing the templates with ethanol extraction. In addition, we found that the pyrrole amount, Ti3C2Tx amount, reaction temperature, and initiator will also affect the structure and morphology of the product, indicating the well‐matched ratio of pyrrole and Ti3C2Tx, appropriate reaction conditions are the prerequisites for the construction of 2D porous PPy/MXene (Figures S4–S8, Supporting Information). The 2D polypyrrole/MXene (2D PPy/MXene) and mesoporous PPy (mPPy) were formed without adding P123 and Ti3C2Tx, respectively, which manifests that P123 and Ti3C2Tx are essential to the formation of pore structure and 2D structure (Table 1 and Figure S9, Supporting Information). Therefore, this swelling‐induced structural transformation strategy not only guides the synthesis of 2D porous PPy/MXene but also provides new insight into the controllable synthesis of 2D porous polymers with adjustable porous architectures.Owing to the satisfactory reversible redox activity, favorable environmental stability, great hydrophilicity, and cost‐effectiveness, 2D porous polymers have been verified to be an ideal electrode material for aqueous supercapacitors.[2b,3c,14c,20] Thus, we appraised their supercapacitor performances by using 2D porous PPy/MXene samples as working electrodes in a three‐electrode system with 1 m H2SO4 electrolyte at the potential window from −0.2 to 0.5 V (vs Ag/AgCl electrode).[21] The cyclic voltammetry (CV) curves of 2D porous PPy/MXene‐x all contain a pair of prominent redox peaks, which can be attributed to the surface redox reactions of PPy and Ti3C2Tx (Figure 4a and Figure S10, Supporting Information).[22] The integral area of 2D porous PPy/MXene‐1 is the largest among 2D porous PPy/MXene‐x, which indicates that it possesses the highest capacitance. The specific capacitances of 2D porous PPy/MXene‐x were further calculated based on galvanostatic charge/discharge (GCD) curves. It can be observed that the GCD curves of all prepared samples are nonlinear, indicating the 2D porous PPy/MXene‐x possess typical pseudocapacitive behavior, which is highly consistent with the corresponding CV curves (Figure 4b and Figure S11, Supporting Information). Similarly, the 2D porous PPy/MXene‐1 electrode achieves the optimal specific capacitance of 477 F g−1 at a current density of 1 A g−1, which is much higher than those of 2D porous PPy/MXene‐2 (323 F g−1), 2D porous PPy/MXene‐3 (257 F g−1). Furthermore, the electrochemical performances of 2D porous PPy/MXene with larger pore sizes were also tested, the capacitances of these two samples are both lower than 250 F g−1, suggesting that the larger pore sizes will reduce the capacitances of the samples. The specific capacitances of 2D porous PPy/MXene‐1 in different current densities all exceed other 2D porous PPy/MXene samples. The 2D porous PPy/MXene‐1 can still maintain as high as 296 F g−1 at a large current density of 5 A g−1, which is about 155% and 180% higher than those of 2D porous PPy/MXene‐2 and 2D porous PPy/MXene‐3 under the same conditions, respectively, indicating it owns outstanding rate performance (Figure 4c). In contrast, the supercapacitor performances of 2D PPy/MXene, mPPy, Ti3C2Tx, and most reported 2D porous polymer materials are inferior to those of 2D porous PPy/MXene samples, which ascribes the structural superiority of our samples (Figure S12 and Table S3, Supporting Information).[21b] The lower resistance plays a key role in boosting the electrical conductivity and electrochemical performances of supercapacitor electrode materials.[23] Thus, we performed the electrochemical impedance spectroscopy (EIS) tests to study the charge transfer resistance (Rct) and ion transport ability of 2D porous PPy/MXene‐x samples (Figure 4d). The Nyquist plots of as‐prepared samples are almost parallel to the y‐axis, confirming the good diffusion characteristics and near‐ideal capacitive behaviors of 2D porous PPy/MXene‐x. Moreover, according to the fitting results, the charge transfer resistance (0.48 Ω) of 2D porous PPy/MXene‐1 is lower than those of 2D porous PPy/MXene‐2 (0.53 Ω) and 2D porous PPy/MXene‐3 (0.61 Ω) (Figure S13, Supporting Information). Compared with 2D porous PPy/MXene‐x, Ti3C2Tx possesses the lower Rct (0.36 Ω) and similar ion transport ability, but its capacitance (201 F g−1) is inferior to that of 1/2 of 2D porous PPy/MXene‐1, suggesting the high capacitance of 2D porous PPy/MXene‐1 stems from the pseudocapacitance reactions brought by PPy. Furthermore, the electrochemical performances of 2D porous PPy/MXene with larger pore sizes were also tested, the capacitances of these two samples are both lower than 250 F g−1, suggesting that the larger pore sizes will reduce the capacitances of these samples (Figure S14, Supporting Information).4FigureElectrochemical characterization of 2D porous PPy/MXene: a) CV curves of different 2D porous PPy/MXene‐x. b) GCD curves of different 2D porous PPy/MXene samples. c) Comparisons of gravimetric capacitances of different 2D porous PPy/MXene samples at various current densities. d) Nyquist plots of 2D porous PPy/MXene samples (inset: the electrical equivalent circuit used for fitting impedance curve). e) The relative capacitance contributions in 2D porous PPy/MXene‐1 analysis by Trasatti's method and Dunn's method. f) The CA of 2D porous PPy/MXene‐x in comparison with those of other materials. g) Cycling stability tests of 2D porous PPy/MXene‐1 and mPPy at 5 A g−1. h) Schematic illustration of the effect of structure on electrochemical performance.In order to quantify the relative capacitance contributions in 2D porous PPy/MXene‐1, two capacitance quantification methods, Trasatti's method and Dunn's method, were conducted to evaluate its capacitance.[6b,24] As a result, Trasatti's method reveals that 55.68% of CT of 2D porous PPy/MXene‐1 is derived from pseudocapacitance, and 44.32% come from electrochemical double‐layer capacitance (Figure S15, Supporting Information). Meanwhile, the relative contributions for 2D porous PPy/MXene‐1 at 1 mV s−1 were calculated to be 54.80% for pseudocapacitance and 45.20% for electrochemical double‐layer capacitance via Dunn's method, respectively (Figure 4e and Figure S16, Supporting Information). The relative capacitance contributions of 2D porous PPy/MXene‐1 calculated by two capacitance quantification methods are almost the same, indicating it has ultrahigh and controllable pseudocapacitance (Figure 3e). Benefiting from the pseudocapacitance of polypyrrole and Ti3C2Tx, the surface area normalized capacitance (CA = C/SBET) was calculated to be 254 µF cm−2 for 2D porous PPy/MXene‐1, 204 µF cm−2 for 2D porous PPy/MXene‐2, and 199 µF cm−2 for 2D porous PPy/MXene‐3, respectively, which are much higher than those of most reported porous materials (Figure 4f).[9a,20,25] The long term cycling stability of 2D porous PPy/MXene‐1 was also tested at current density of 5 A g−1 (Figure 4g). Remarkably, the capacitance of 2D porous PPy/MXene‐1 retains 94.2% of initial capacitance after 10 000 cycles, which is much better than those of mPPy and most PPy‐based electrode materials, validating its remarkable cyclic stability (Table S4, Supporting Information).[26]Based on the above‐mentioned results, we found that the 2D porous PPy/MXene with cylindrical mesostructure delivers the best supercapacitor performances. Accordingly, we consider that the electrochemical performances are related to the elaborately designed pore structure of 2D porous PPy/MXene, and summarize the inherent relationship between pore structures and electrochemical performances (Figure 4h). 1) Compared with the spherical mesostructure and spherical macrostructure, the cylindrical mesostructure has an interconnected channel, which can offer an in‐plane pathway for fast electrolyte diffusion and shorten the ionic transport pathway, greatly facilitating rapid ionic transport. 2) Moreover, the high specific surface area is also a critical factor affecting the electrochemical performances. The high surface area improves the electrolyte/electrode contact area, which allows more electrical charge to gather in the electrode, and increases the redox‐active sites to offer more pseudocapacitance, thus synergistically enhancing the specific capacitance. 3) The conducting polymer frameworks offer an electronic transmission path along the parallel direction and the heterostructures improve inner electron transfer in the vertical direction of the 2D nanosheet, which greatly reduces the charge transfer resistance and enhances electronic conductivity. Therefore, 2D porous PPy/MXene with cylindrical mesostructure displays the best supercapacitor performances including excellent specific capacitance, outstanding rate performance, distinguished surface area normalized capacitance, and remarkable cyclic stability.ConclusionIn summary, we proposed a swelling‐induced structural transformation strategy to synthesize 2D porous PPy/MXene. TMB is used as a swelling agent to adjust the self‐assembly behavior of P123 micelles. By changing the dosages of TMB, we succeed in controlling the pore structures of 2D porous PPy/MXene from cylindrical mesostructure to spherical mesostructure and spherical macrostructure. The resultant 2D porous PPy/MXene with cylindrical mesostructure can provide fast mass transfer, high electrolyte/electrode contact area, and rapid electronic transport, and thus displays superior supercapacitor performances with excellent capacitance and outstanding rate performance. Owing to their advanced structural advantages, the 2D porous PPy/MXene also exhibited ultrahigh surface area normalized capacitance and excellent cyclability, indicating the construction of the cylindrical mesostructure is an effective way to enhance the electrochemical performance of 2D porous polymers. This work reveals the inherent relationship between pore structures and electrochemical performances of 2D porous polymers, which not only offers a novel insight into the controllable synthesis of 2D porous polymers with adjustable porous architectures but also provides a clue for the reasonable design and construction of 2D porous polymers with high electrochemical performances.AcknowledgementsThis work was supported by the National Natural Science Foundation of China (22105033, 21621001, and 21671073), the “111” Project of the Ministry of Education of China (B17020), the Jilin Province Science and Technology Development Plan (YDZJ202101ZYTS137), and Program for JLU Science and Technology Innovative Research Team, Interdisciplinary Integration and Innovation Project of Jilin University. H.X. is grateful for financial support from the National Natural Science Foundation of China (22105192) and the China Postdoctoral Science Foundation (2021M693065 and 2021TQ0322).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.a) X. Zhuang, Y. Mai, D. Wu, F. Zhang, X. Feng, Adv. Mater. 2015, 27, 403;b) F. Bu, M. M. Zagho, Y. Ibrahim, B. Ma, A. Elzatahry, D. Zhao, Nano Today 2020, 30, 100803;c) L. Zu, W. Zhang, L. Qu, L. Liu, W. Li, A. Yu, D. Zhao, Adv. Energy Mater. 2020, 10, 2002152;d) C. Li, Q. Li, Y. V. Kaneti, D. Hou, Y. Yamauchi, Y. Mai, Chem. Soc. Rev. 2020, 49, 4681;e) T. Wang, F. Okejiri, Z.‐A. Qiao, S. Dai, Adv. Mater. 2020, 32, 2002475;f) Y. Meng, D. Gu, F. Q. Zhang, Y. F. Shi, H. F. Yang, Z. Li, C. Z. Yu, B. Tu, D. Y. Zhao, Angew. Chem., Int. Ed. 2005, 44, 7053.a) Y. Shi, G. Yu, Chem. Mater. 2016, 28, 2466;b) H. 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Swelling‐Induced Structural Transformation Strategy: Controllable Synthesis of 2D Porous Polypyrrole/MXene Heterostructures with Tunable Pore Structures

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Abstract

IntroductionPorous polymers featuring high porosity, low density, and good physicochemical stability are attractive in the basic research and industrial application fields.[1] Particularly, 2D porous polymers, combining the advantages of 2D materials and inherent characteristics of porous polymers, exhibit huge application potentials in the fields of energy storage and conversion, especially in the field of supercapacitors.[2] It is well known that the electrochemical performances of 2D porous polymers are often influenced by many factors such as specific surface area, electronic conductivity, and pore structure.[2b,3] In this case, pore structure plays a crucial role in electrochemical performance, which is often overlooked. Up to now, the precise control of pore structure mainly depends on two methods, namely, the prefabricated template method and the molecular self‐assembly method.[1d,4] In the former scenario, the monomer is first cross‐linked on the surface of the 2D silica‐based template prepared in advance, followed by the etching of the template with corrosive acid or base to obtain a porous structure. Although various 2D porous polymers can be synthesized through this route, the pore structure is limited by the morphology of parent silica templates, and the obtained pore structure is mainly spherical mesopores.[5] Moreover, the procedure of this method is tedious, costly, as well as not suitable for mass production, which greatly hinders their practical applications.By contrast, the molecular self‐assembly method based on the cooperative organization of polymer precursors and surfactant templates on 2D substrates is a more efficient strategy.[6] Hitherto, this method has been quite mature for controlling the pore structure of materials, especially for metal oxides, silicates, carbon, and so on.[7] However, it is still a huge challenge to precisely control the pore structures of 2D porous polymers. Owing to the fact that the polymerization process of pyrrole is very violent and rapid, it is hard to accurately control the reaction kinetics of pyrrole monomers, enabling them cannot be uniformly arranged and further grow on the surface of 2D substrates. The lack of suited interaction or driving forces between polymer monomers, surfactant micelles, and 2D nanosheets results in difficulties existing in regulating the co‐assembly behavior, eventually leading to the formation of bulk or nonporous structure.[8] To the best of our knowledge, there are few reports on the synthesis of 2D porous polymers with spherical and cylindrical mesopores by using poly(ethylene oxide)‐b‐polystyrene (PEO‐b‐PS), Pluronic F127, and Pluronic P123 as templates, respectively.[9] In addition, the existing synthesis strategies to construct 2D porous polymers usually employ graphene as a 2D substrate, which inevitably results in some adverse impacts such as uneven growth of porous polymers, severe stacking of 2D substrate, and general conductivity.[3a,10] Fortunately, MXene, a new material family of 2D carbide or carbonitride, owns abundant surface functional groups, unique accordion‐like structure, and superior metallic conductivity, and maybe a satisfactory 2D substrate to replace graphene in the construction of 2D porous polymers.[11] Recent research works have shown that many kinds of porous materials can be combined with MXene nanosheets to form 2D porous heterostructures.[12] Compared with graphene, MXene has even better conductivity, which is a benefit for electrochemical performances. Furthermore, the OH groups of MXene contributed to the uniform growth of porous polymers. Therefore, it is extremely urgent and vital to develop an effective synthetic strategy for the controllable construction of 2D porous polymers and further to understand the inherent relationship between pore structures and electrochemical performances.Herein, we developed a swelling‐induced structural transformation strategy to construct 2D porous polypyrrole/MXene heterostructures (2D porous PPy/MXene) by using Pluronic P123 as only template, 1,3,5‐trimethylbenzene (TMB) as swelling agent, and MXene nanosheets (Ti3C2Tx) as 2D substrate. By adjusting the dosages of TMB, the hydrodynamic diameters of the composite micelles formed by self‐assembly P123 and TMB can be regulated from 11.7 to 58.7 nm, thus achieving precise control over pore structures (cylindrical mesostructure, spherical mesostructure, and spherical macrostructure), pore sizes (≈7.8–52 nm), and specific surface area (≈129–188 m2 g−1) of 2D porous PPy/MXene. Due to the cylindrical mesoporous structure providing a fast electrolyte transferring mode, large electrolyte/electrode contact area, and shorter ion diffusion distance, the obtained 2D cylindrical mesoporous PPy/MXene exhibits the optimal supercapacitor performances including high capacitance of 477 F g−1 at 1 A g−1, superior rate performance of ≈300 F g−1 at 5 A g−1, and favorable cyclability with tiny capacitance degradation (5.8%) after 10 000 cycles. This work not only reveals the structure‐activity relationship between pore structure and electrochemical performance but also provides new insights for the construction of novel 2D porous heterostructure materials.Results and DiscussionFigure 1 displays the schematic illustration of the synthesis process of 2D porous PPy/MXene. First, P123 copolymers were dissolved into water to form a uniform micelle solution. Second, the pyrrole monomers were introduced into the synthesis system, which can interact on the surface of micelles through H‐bonding and self‐assemble into P123/pyrrole composite micelles. Third, after adding the MXene nanosheets, the composite micelles can be anchored on the Ti3C2Tx surface by hydrogen bond and electrostatic interaction, and the pyrrole monomers were polymerized under the effect of the initiator of FeCl3 (Figure S1, Supporting Information).[13] Finally, the 2D porous PPy/MXene samples were obtained after removing the template by washing it with ethanol. Notably, by adjusting the addition amounts of TMB, three representative 2D porous PPy/MXene samples, including 2D cylindrical mesoporous polypyrrole/MXene, 2D spherical mesoporous polypyrrole/MXene, and 2D spherical macroporous polypyrrole/MXene, were synthesized, which were denoted as 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3, respectively.1FigureSchematic illustration of the fabrication of 2D porous polypyrrole/MXene heterostructures by swelling‐induced structural transformation strategy.As presented in Figure 2a, the P123 solution with different addition amounts of TMB was denoted as Micelle 1, Micelle 2, and Micelle 3, respectively. The photograph of three micelle solutions showed that the colors were transformed from transparent to milky white with increasing amounts of TMB. Meanwhile, these micelle solutions all possessed obvious Tyndall effect, indicating the formation of micelles at 40 °C. Dynamic light scattering (DLS) measurement suggested that the hydrodynamic diameters of the formed micelles were centered at ≈11.7, ≈32.6, and ≈58.7 nm, respectively (Figure 2b). Moreover, a linear fitting with an equation of y = 1.175x + 10.833 (r2 = 0.99) was acquired to illustrate that the volume of TMB is linearly related to the hydrodynamic diameters of the micelle (Figure 2c). The morphologies and pore structures of 2D porous PPy/MXene samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images of three samples all exhibited uniform 2D nanosheet morphology with observable pores on the surface (Figure 2d–f). The TEM images of 2D porous PPy/MXene also displayed a 2D layered structure with abundant pores. With increasing the addition amount of TMB, 2D porous PPy/MXene samples showed the transformation of pore structures from cylindrical mesostructures, to spherical mesostructures, and then to spherical macrostructures (Figure 2g–i). Furthermore, the average pore sizes of 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3 can be measured to be about 7.8, 29.6, and 52 nm, respectively, which were consistent with the DLS results. The high‐angle annular dark‐field‐scan TEM (HAADF‐STEM) and energy dispersive spectrometry (EDS) mapping images of 2D porous PPy/MXene‐1 revealed that the Ti, C, and N elements were distributed evenly in the sample (Figure 2j–m).2Figurea) Photograph of P123 aqueous solution mixed with different volumes of TMB (0 µL for Micelle 1, 20 µL for Micelle 2, and 40 µL for Micelle 3). b) DLS spectrum of different micelle solutions. c) The linear fitting curves of the dosages of TMB and the average micellar diameter (dmicelle) measured by DLS. SEM images of d) 2D porous PPy/MXene‐1, e) 2D porous PPy/MXene‐2, and f) 2D porous PPy/MXene‐3. TEM images of g) 2D porous PPy/MXene‐1, h) 2D porous PPy/MXene‐2, and i) 2D porous PPy/MXene‐3. j–m) HADDF‐STEM and EDS mapping images of elemental Ti, C, and N of 2D porous PPy/MXene‐1.The detailed porosities of 2D porous PPy/MXene were characterized by N2 adsorption–desorption measurements. Every adsorption isotherm of 2D porous PPy/MXene exhibits obvious hysteresis loops, implying they have large porosity (Figure 3a). Moreover, the Brunauer–Emmett–Teller surface areas (SBET) of 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3 are 188, 158, and 129 m2 g−1, respectively, which are much higher than those of reported porous PPy‐based materials (Table 1 and Table S1, Supporting Information).[3c,6c,e,8,14] The pore volume of 2D porous PPy/MXene‐1, 2D porous PPy/MXene‐2, and 2D porous PPy/MXene‐3 are calculated to be 0.62, 0.34, and 0.30 cm3 g−1, respectively. These results show that the SBET and pore volumes of 2D porous PPy/MXene samples decrease linearly with the increase in pore size. Fourier transform infrared (FT‐IR) spectra of 2D porous PPy/MXene‐x all display the two typical characteristic peaks of polypyrrole at 1308 and 1700 cm−1, which can be attributed to CN and CN bonds, revealing the successful polymerization of pyrrole (Figure 3b).[3a,9a] Additionally, the disappearance of a characteristic signal at around 2870 cm−1 manifests the complete removal of P123. Raman spectra of 2D porous PPy/MXene‐x show that the characteristic peaks at ≈1160 and ≈1350 cm−1 were ascribed to the ring stretching and in‐plane CH modes of PPy, and the peaks located at ≈1595 cm−1 corresponded to the CC bond stretching mode in the pyrrole ring (Figure 3c).[15] The XRD patterns of 2D porous PPy/MXene‐x show two typical characteristic peaks at ≈24.8° and 51.2°, indicating the samples were amorphous (Figure 3d).[16] Moreover, no significant oxidation peak was observed, suggesting the Ti3C2Tx nanosheets are not oxidized during the polymerization process (Figure S2, Supporting Information).[12d] X‐ray photoelectron spectroscopy (XPS) spectra show the existence of signals of Ti, C, and N in these samples (Figure S3, Supporting Information). Four distinct energy values at 282.8, 283.7, 284.8, and 286.8 eV can be observed in C1s spectra for 2D porous PPy/MXene‐x, which can be assigned to CC, CC, CN, and disordered carbon, respectively (Figure 3e). The high‐resolution N 1s spectra contain the main NH bonds at 398.6 eV, N+ bonds at 399.2 eV, and slight N+H bonds at 400.7 eV (Figure 3f).[17] For all the 2D porous PPy/MXene samples, the proportions of the relative peak area of each group of C and N atoms are almost the same, implying the chemical states of 2D porous PPy/MXene‐x samples are quite similar (Table S2, Supporting Information). The above results demonstrate that the 2D porous PPy/MXene‐x samples own satisfactory porosities and similar chemical compositions.3FigureStructural characterizations of 2D porous PPy/MXene. a) Nitrogen adsorption–desorption isotherms, b) FTIR spectra, c) Raman spectra, d) XRD patterns of 2D porous PPy/MXene‐x. e) C 1s and f) N 1s high‐resolution XPS spectra of 2D porous PPy/MXene‐x.1TableStructural properties of samplesSamplesTemplateSBETa) [m2g−1]dTEMb) [nm]dBJHb) [nm]VTotalc) [cm3g−1]2D porous PPy/MXene‐1P1231887.87.70.622D porous PPy/MXene‐2P12315829.628.80.342D porous PPy/MXene‐3P12312952–0.302D PPy/MXene–89––0.17mPPyP12313253.40.64a)The SBET measured from the nitrogen adsorption–desorption isothermsb)The average pore diameters are calculated by TEM images and BJH method, respectivelyc)VTotal: Total pore volume calculated from adsorption isotherm at P/P0 = 0.99.Based on the above results, we propose a swelling‐induced structural transformation strategy for the synthesis of 2D porous PPy/MXene (Figure 1). First, the Pluronic P123 (PEO20–PPO70–PEO20) triblock copolymers self‐assemble into cylindrical micelles with hydrophobic PPO blocks as the cores and hydrophilic PEO blocks as the coronas in aqueous solution at 40 °C (Route 1).[2c,3a] According to the packing parameter (p = v/a0lc, where p is the packing parameter, v is the volume of the hydrophobic core, a0 is the contact surface area of the polar head, and lc is the length of hydrophobic chain) theory, it can be extrapolated that the packing parameter of P123 micelles is between 1/3 and 1/2 under this condition.[18] After adding a small amount of TMB, based on the similarity‐intermiscibility principle, the hydrophobic TMB can enter into the PPO segment of P123 micelles, which enables the micelles to swell and increases the exposed surface area (a0), leading to the decrease of packing parameter below 1/3 and the transformation of the morphology of composite micelles from cylindrical mesostructure to spherical mesostructure (Route 2).[19] With further increasing the dosage of TMB, the core of mesoscopic spherical micelles further swells and converts into macroscopic spherical micelles (Route 3). Subsequently, after the introduction of the pyrrole, the O in PEO coronas of micelles can interact with NH of pyrrole monomers via hydrogen bonding interaction to assemble into composite micelles. Then, the composite micelles anchor on the surface of Ti3C2Tx nanosheets by hydrogen bonding and electrostatic interactions between the OH of Ti3C2Tx and the NH of pyrrole. The polymerization of pyrrole monomers is triggered by adding FeCl3, yielding 2D composite micelles/polypyrrole/MXene hybrid materials. Finally, the 2D porous PPy/MXene samples were obtained by removing the templates with ethanol extraction. In addition, we found that the pyrrole amount, Ti3C2Tx amount, reaction temperature, and initiator will also affect the structure and morphology of the product, indicating the well‐matched ratio of pyrrole and Ti3C2Tx, appropriate reaction conditions are the prerequisites for the construction of 2D porous PPy/MXene (Figures S4–S8, Supporting Information). The 2D polypyrrole/MXene (2D PPy/MXene) and mesoporous PPy (mPPy) were formed without adding P123 and Ti3C2Tx, respectively, which manifests that P123 and Ti3C2Tx are essential to the formation of pore structure and 2D structure (Table 1 and Figure S9, Supporting Information). Therefore, this swelling‐induced structural transformation strategy not only guides the synthesis of 2D porous PPy/MXene but also provides new insight into the controllable synthesis of 2D porous polymers with adjustable porous architectures.Owing to the satisfactory reversible redox activity, favorable environmental stability, great hydrophilicity, and cost‐effectiveness, 2D porous polymers have been verified to be an ideal electrode material for aqueous supercapacitors.[2b,3c,14c,20] Thus, we appraised their supercapacitor performances by using 2D porous PPy/MXene samples as working electrodes in a three‐electrode system with 1 m H2SO4 electrolyte at the potential window from −0.2 to 0.5 V (vs Ag/AgCl electrode).[21] The cyclic voltammetry (CV) curves of 2D porous PPy/MXene‐x all contain a pair of prominent redox peaks, which can be attributed to the surface redox reactions of PPy and Ti3C2Tx (Figure 4a and Figure S10, Supporting Information).[22] The integral area of 2D porous PPy/MXene‐1 is the largest among 2D porous PPy/MXene‐x, which indicates that it possesses the highest capacitance. The specific capacitances of 2D porous PPy/MXene‐x were further calculated based on galvanostatic charge/discharge (GCD) curves. It can be observed that the GCD curves of all prepared samples are nonlinear, indicating the 2D porous PPy/MXene‐x possess typical pseudocapacitive behavior, which is highly consistent with the corresponding CV curves (Figure 4b and Figure S11, Supporting Information). Similarly, the 2D porous PPy/MXene‐1 electrode achieves the optimal specific capacitance of 477 F g−1 at a current density of 1 A g−1, which is much higher than those of 2D porous PPy/MXene‐2 (323 F g−1), 2D porous PPy/MXene‐3 (257 F g−1). Furthermore, the electrochemical performances of 2D porous PPy/MXene with larger pore sizes were also tested, the capacitances of these two samples are both lower than 250 F g−1, suggesting that the larger pore sizes will reduce the capacitances of the samples. The specific capacitances of 2D porous PPy/MXene‐1 in different current densities all exceed other 2D porous PPy/MXene samples. The 2D porous PPy/MXene‐1 can still maintain as high as 296 F g−1 at a large current density of 5 A g−1, which is about 155% and 180% higher than those of 2D porous PPy/MXene‐2 and 2D porous PPy/MXene‐3 under the same conditions, respectively, indicating it owns outstanding rate performance (Figure 4c). In contrast, the supercapacitor performances of 2D PPy/MXene, mPPy, Ti3C2Tx, and most reported 2D porous polymer materials are inferior to those of 2D porous PPy/MXene samples, which ascribes the structural superiority of our samples (Figure S12 and Table S3, Supporting Information).[21b] The lower resistance plays a key role in boosting the electrical conductivity and electrochemical performances of supercapacitor electrode materials.[23] Thus, we performed the electrochemical impedance spectroscopy (EIS) tests to study the charge transfer resistance (Rct) and ion transport ability of 2D porous PPy/MXene‐x samples (Figure 4d). The Nyquist plots of as‐prepared samples are almost parallel to the y‐axis, confirming the good diffusion characteristics and near‐ideal capacitive behaviors of 2D porous PPy/MXene‐x. Moreover, according to the fitting results, the charge transfer resistance (0.48 Ω) of 2D porous PPy/MXene‐1 is lower than those of 2D porous PPy/MXene‐2 (0.53 Ω) and 2D porous PPy/MXene‐3 (0.61 Ω) (Figure S13, Supporting Information). Compared with 2D porous PPy/MXene‐x, Ti3C2Tx possesses the lower Rct (0.36 Ω) and similar ion transport ability, but its capacitance (201 F g−1) is inferior to that of 1/2 of 2D porous PPy/MXene‐1, suggesting the high capacitance of 2D porous PPy/MXene‐1 stems from the pseudocapacitance reactions brought by PPy. Furthermore, the electrochemical performances of 2D porous PPy/MXene with larger pore sizes were also tested, the capacitances of these two samples are both lower than 250 F g−1, suggesting that the larger pore sizes will reduce the capacitances of these samples (Figure S14, Supporting Information).4FigureElectrochemical characterization of 2D porous PPy/MXene: a) CV curves of different 2D porous PPy/MXene‐x. b) GCD curves of different 2D porous PPy/MXene samples. c) Comparisons of gravimetric capacitances of different 2D porous PPy/MXene samples at various current densities. d) Nyquist plots of 2D porous PPy/MXene samples (inset: the electrical equivalent circuit used for fitting impedance curve). e) The relative capacitance contributions in 2D porous PPy/MXene‐1 analysis by Trasatti's method and Dunn's method. f) The CA of 2D porous PPy/MXene‐x in comparison with those of other materials. g) Cycling stability tests of 2D porous PPy/MXene‐1 and mPPy at 5 A g−1. h) Schematic illustration of the effect of structure on electrochemical performance.In order to quantify the relative capacitance contributions in 2D porous PPy/MXene‐1, two capacitance quantification methods, Trasatti's method and Dunn's method, were conducted to evaluate its capacitance.[6b,24] As a result, Trasatti's method reveals that 55.68% of CT of 2D porous PPy/MXene‐1 is derived from pseudocapacitance, and 44.32% come from electrochemical double‐layer capacitance (Figure S15, Supporting Information). Meanwhile, the relative contributions for 2D porous PPy/MXene‐1 at 1 mV s−1 were calculated to be 54.80% for pseudocapacitance and 45.20% for electrochemical double‐layer capacitance via Dunn's method, respectively (Figure 4e and Figure S16, Supporting Information). The relative capacitance contributions of 2D porous PPy/MXene‐1 calculated by two capacitance quantification methods are almost the same, indicating it has ultrahigh and controllable pseudocapacitance (Figure 3e). Benefiting from the pseudocapacitance of polypyrrole and Ti3C2Tx, the surface area normalized capacitance (CA = C/SBET) was calculated to be 254 µF cm−2 for 2D porous PPy/MXene‐1, 204 µF cm−2 for 2D porous PPy/MXene‐2, and 199 µF cm−2 for 2D porous PPy/MXene‐3, respectively, which are much higher than those of most reported porous materials (Figure 4f).[9a,20,25] The long term cycling stability of 2D porous PPy/MXene‐1 was also tested at current density of 5 A g−1 (Figure 4g). Remarkably, the capacitance of 2D porous PPy/MXene‐1 retains 94.2% of initial capacitance after 10 000 cycles, which is much better than those of mPPy and most PPy‐based electrode materials, validating its remarkable cyclic stability (Table S4, Supporting Information).[26]Based on the above‐mentioned results, we found that the 2D porous PPy/MXene with cylindrical mesostructure delivers the best supercapacitor performances. Accordingly, we consider that the electrochemical performances are related to the elaborately designed pore structure of 2D porous PPy/MXene, and summarize the inherent relationship between pore structures and electrochemical performances (Figure 4h). 1) Compared with the spherical mesostructure and spherical macrostructure, the cylindrical mesostructure has an interconnected channel, which can offer an in‐plane pathway for fast electrolyte diffusion and shorten the ionic transport pathway, greatly facilitating rapid ionic transport. 2) Moreover, the high specific surface area is also a critical factor affecting the electrochemical performances. The high surface area improves the electrolyte/electrode contact area, which allows more electrical charge to gather in the electrode, and increases the redox‐active sites to offer more pseudocapacitance, thus synergistically enhancing the specific capacitance. 3) The conducting polymer frameworks offer an electronic transmission path along the parallel direction and the heterostructures improve inner electron transfer in the vertical direction of the 2D nanosheet, which greatly reduces the charge transfer resistance and enhances electronic conductivity. Therefore, 2D porous PPy/MXene with cylindrical mesostructure displays the best supercapacitor performances including excellent specific capacitance, outstanding rate performance, distinguished surface area normalized capacitance, and remarkable cyclic stability.ConclusionIn summary, we proposed a swelling‐induced structural transformation strategy to synthesize 2D porous PPy/MXene. TMB is used as a swelling agent to adjust the self‐assembly behavior of P123 micelles. By changing the dosages of TMB, we succeed in controlling the pore structures of 2D porous PPy/MXene from cylindrical mesostructure to spherical mesostructure and spherical macrostructure. The resultant 2D porous PPy/MXene with cylindrical mesostructure can provide fast mass transfer, high electrolyte/electrode contact area, and rapid electronic transport, and thus displays superior supercapacitor performances with excellent capacitance and outstanding rate performance. Owing to their advanced structural advantages, the 2D porous PPy/MXene also exhibited ultrahigh surface area normalized capacitance and excellent cyclability, indicating the construction of the cylindrical mesostructure is an effective way to enhance the electrochemical performance of 2D porous polymers. This work reveals the inherent relationship between pore structures and electrochemical performances of 2D porous polymers, which not only offers a novel insight into the controllable synthesis of 2D porous polymers with adjustable porous architectures but also provides a clue for the reasonable design and construction of 2D porous polymers with high electrochemical performances.AcknowledgementsThis work was supported by the National Natural Science Foundation of China (22105033, 21621001, and 21671073), the “111” Project of the Ministry of Education of China (B17020), the Jilin Province Science and Technology Development Plan (YDZJ202101ZYTS137), and Program for JLU Science and Technology Innovative Research Team, Interdisciplinary Integration and Innovation Project of Jilin University. 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Journal

Advanced Materials Interfaces – Wiley

Published: Apr 1, 2023

Keywords: 2D porous polymers; mesoporous materials; supercapacitor; swelling‐induced structural transformation

References