Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction Gang Wang, Jialin Li, Lichao Shang, Haibo He, Tingting Cui, Shengchao Chai, Chengji Zhao, Lixin Wu and Haolong Li Gang Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Jialin Li Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Lichao Shang Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Haibo He State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Tingting Cui State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Shengchao Chai State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Chengji Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Lixin Wu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Haolong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000608 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The key challenge for the use of polymer electrolytes is to realize a high ionic conductivity without scarifying their mechanical performance. Herein, we report a facile strategy to prepare a nanostructured polymer electrolyte with both high proton conductivity and high modulus, based on the electrostatic self-assembly of polyoxometalate cluster H3PW12O40 (PW) and comb copolymer poly(ether-ether-ketone)-grafted-poly(vinyl pyrrolidone) (PEEK-g-PVP). The incorporation of protonic acid PW can enable the PEEK-g-PVP to be highly proton conductive and create flexible composite electrolyte membranes. Moreover, nanoscale phase separation between PEEK domains and PVP/PW domains spontaneously occurs in these membranes, forming a bicontinuous structure with three-dimensional (3D)-connected PW networks. Due to the dual role of PW networks as both proton transport pathways and mechanical enhancers, these membranes exhibit proton conductivities higher than 30 mS cm−1 and modulus over 4 GPa. Notably, the direct methanol fuel cells equipped with these membranes show good cell performance. Given the wide tunability of comb copolymers and polyoxometalates, this system can be extended to develop a variety of functional electrolyte materials, for example, the lithium-ion conductive electrolytes by using polyoxometalate-based lithium salts, which provides a promising platform to explore versatile electrolyte materials for energy and electronic applications. Download figure Download PowerPoint Introduction Ion-conducting polymer electrolytes have tremendous potential in a wide range of energy applications such as fuel cells, batteries, supercapacitors, and other energy storage and conversion devices.1–12 Despite these promising applications, a key issue in polymer electrolytes is maximizing the ionic conductivity while simultaneously optimizing the mechanical performance, so as to prepare high conductive and high stable electrolyte membranes for practical use.13 However, the facilitation of ion transport generally involves increasing the content of ionic groups or accelerating the segmental dynamics of polymer electrolytes, which inevitably causes the electrolyte membranes to become swollen or softened and thus sacrifice their mechanical strength. Nanostructured polymer electrolytes can provide a platform to address the trade-off between ionic conductivity and mechanical performance. In these materials, the nanoscale phase-separated domains can independently implement different tasks such as ion transport and mechanical support, which can decouple the interferent between these properties and realize their enhancement simultaneously.14–20 Among various polymer nanostructures, the bicontinuous structure shows a unique structural advantage in electrolyte materials due to its dual three-dimensional (3D) spatial connectivity. In a bicontinuous electrolyte, the 3D interconnected ion-conducting domains can reduce the energy barriers for ion diffusion and promote a highly efficient transport; meanwhile, the continuity of the mechanically robust domains can render superior mechanical properties.21–26 Thus, extensive attention has been paid to constructing bicontinuous polymer electrolytes by covalent or noncovalent modification. For example, by precisely placing the ionic groups at the periodic interval sites or the terminal sites of linear polymer backbones, Park group27 and Winey group28 constructed ordered bicontinuous structures, the gyroidal ionic phase, in homopolymers and block copolymers, respectively, achieving remarkably enhanced ionic conductivities. Lodge and co-workers29,30 presented the use of polymerization-induced microphase separation to obtain irregular bicontinuous electrolytes based on cross-linked block copolymers. Besides covalent modification, small nanoparticles like polyhedral oligomeric silsesquioxane can also be employed as nanoadditives to adjust the phase behavior of block copolymers, which can form bicontinuous composite polymer electrolytes, as reported by the Balsara group.31 These advances demonstrate that topological control and nanocomposite formation are both useful strategies toward bicontinuous polymer electrolytes. Polyoxometalates (POMs), the molecularly well-defined clusters of metal oxides,32 are widely used as functional inorganic nanobuilding blocks for the preparation of hybrid polymer nanocomposites.33–47 Moreover, the multicharged anionic characteristic and the high ion-dissociation ability of POMs make them promising molecular electrolytes to transport cations such as protons and lithium ions for energy applications.48–50 Recently, the excellent proton conductivity, electrochemical stability, and water-retention ability of POMs have attracted special interest to develop POM-polymer composite electrolytes for proton conduction.51–61 However, it is difficult to manipulate the distribution of POMs in polymer electrolytes to form well-connected proton conductive pathways, as the strong electrostatic interaction easily induces the disordered aggregation of POMs. Recently, Guiver and co-workers reported a breakthrough they obtained: one-dimensional (1D) POM-based proton-conducting channels in a homopolymer electrolyte matrix through an external magnetic field alignment.57 Nonetheless, a facile, large-area, and cost-efficient strategy to construct highly continuous POM-based channels, for example, networks, in polymer electrolyte matrices, is still needed. Herein, we report the fabrication of high modulus and high proton conductive polymer composite electrolyte membranes that integrate both a bicontinuous structure and a 3D-connected POM-based network by a facile electrostatic self-assembly method, as shown in Figure 1. This whole idea takes full account of the topological structure and mechanical strength of matrix polymers, as well as the complexation feature between matrix polymers and POMs. To facilitate the formation of a bicontinuous structure, an amphiphilic comb-shaped copolymer, poly(ether-ether-ketone)-grafted-poly(vinyl pyrrolidone) (PEEK-g-PVP, abbreviated as PGP) is designed as the matrix polymer, as the hydrophobic main chains and the hydrophilic side chains in such comb copolymers tend to form a continuously structured and microphase-separated morphology.62,63 PEEK, a well-known engineering plastic with high strength and durability, is used to impart mechanical stability; as for the PVP component, it is easily wettened and softened by water, and thus can efficiently promote the dissociation and conduction of protons. Besides, PVP performs a relatively weak electrostatic interaction with POMs, which can suppress the aggregation of POMs and favor the self-assembly of POMs into uniform networks in PVP domains. As expected, the 3D-connected POM networks remarkably enhance the proton conductivity and the mechanical strength of the resultant composite electrolytes. Figure 1 | (a) Synthetic route of PEEK-g-PVP. (b) Preparation strategy of PGP/PW nanocomposites electrolytes. Download figure Download PowerPoint Experimental Methods Synthesis of PGP Here, a general synthetic procedure of PGP is present, and the details are provided in the Supporting Information. Methyl-PEEK with a degree of polymerization of ca. 80, an Mw of ca. 25,000, and a polydispersity index (PDI) of ca. 3.3 was synthesized as the main chain, according to our previous work.64 The benzylmethyl groups on PEEK backbones were used as the grafting sites. By brominating the benzylmethyl groups and a subsequent nucleophilic substitution reaction, ethylxanthate groups were modified onto PEEK, leading to a PEEK-based macromolecular chain transfer agent (PEEK-CTA). Then, different lengths of PVP side chains were grafted from the PEEK-CTA by reversible addition-fragmentation chain transfer (RAFT) polymerization. Moreover, a shuttle CTA was added to regulate the RAFT polymerization to achieve a narrow distribution of PVP side chains.65 In this procedure, N-vinyl pyrrolidone monomer, PEEK-CTA, the shuttle CTA, and 2,2'-Azobis(2-methylpropionitrile) (AIBN) were mixed with N-methyl-2-pyrrolidinone solvent in an ampule bottle, subjected to three freeze–pump–thaw cycles, backfilled with nitrogen gas, and then reacted at 70 °C in an oil bath for 10 h. The solution was dropped into methanol to remove PVP homopolymers and the unreacted monomers. This procedure was repeated twice. The resulting copolymer was washed with water and methanol and then dried under vacuum at 60 °C for 24 h. Preparation of nanocomposite electrolytes H3PW12O40 (PW) was codissolved with PGP in N-methyl pyrrolidone (NMP) to get a homogeneous solution. Then after casting and being dried in air at 60 °C for 48 h, composite electrolyte membranes were readily obtained. Characterizations Proton nuclear magnetic resonance (1H NMR) spectra were measured on a Bruker UltraShield 500 MHz spectrometer (Bruker, Switzerland) in CDCl3 at room temperature. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectra were obtained on a Bruker Autoflex MALDI-TOF mass spectrometer (Bruker, Germany). Fourier transform infrared (FTIR) spectra were measured on a Bruker VERTEX 80 V FTIR spectrometer (Bruker, Germany) using KBr pellets in the wavelength range of 4000–400 cm−1. X-ray diffraction (XRD) patterns were obtained on a Rigaku X-ray diffractometer (D/max 2550; Rigaku, Japan) using an X-ray wavelength of 1.542 Å. XRD samples were prepared by PGP/PW nanocomposite films and PW powder. Small-angle X-ray scattering (SAXS) data were obtained on an Anton Paar instrument SAXSess mc2 (Anton Paar instrument, Austria) using an X-ray wavelength of 1.542 Å. Transmission electron microscopy (TEM) was recorded on a JEM-2100F electron microscope (JEOL, Japan). The TEM samples of films were embedded in resin (SPI-Pon 812) and cured at 60 °C overnight. A Leica EM UC7 microtome (Leica Microsystems, Germany) was used to prepare the ultrathin sections of the PGP/PW composite membranes with a thickness of about 80 nm. Mechanical measurements The tensile moduli of the samples were obtained on a tensile tester (SHIMADZU AG-I 1KN; SHIMADZU, Japan) at room temperature at a speed of 2 mm min−1. The compression moduli of the samples were obtained using the continuous stiffness measurement (CSM) method and XP-style actuator on an Agilent Nano Indenter G200 (Agilent Technologies Inc., United States) at room temperature. Ionic conductivity measurement The resistance value (R) was measured by alternating current (AC) impedance spectroscopy on a Solartron 1260A (AMETEK Scientific Ins., United States) impedance analyzer (1 Hz to 10 MHz, two-probe method). The proton conductivity (σ) of the composite membranes was calculated according to σ = L/(R × A), where L is the membrane thickness between the two electrodes, and A is the membrane area in contact with the electrodes. Direct methanol fuel cells The catalyst and the Nafion solution were mixed at room temperature and then sprayed evenly on both sides of the membranes. After drying, the catalyst-coated membranes (CCMs) were obtained. Then the gas diffusion layers (GDLs) were pressed on both sides of the CCMs. The cathode side was provided humidified oxygen at a flow rate of 30 mL min−1, and the anode side was provided methanol solution (2 M) at a flow rate of 2 mL min−1 when testing at 80 °C. Results and Discussion The synthetic route of PEEK-g-PVP is shown in Figure 1a, through grafting the PVP side chains onto a main chain of Methyl-PEEK (Mw of ca. 25,000) by RAFT polymerization. In the resultant PEEK-g-PVP, the polymerization degree of PVP side chains was controlled at 8, 17, 23, and 43, with the same grafting density of 0.1, which means that approximately 10% benzylmethyl groups on PEEK were grafted by PVP. Detailed synthetic procedures and characterizations like NMR are shown in Supporting Information Figures S1–S3. The four comb copolymers were named PGP8, PGP17, PGP23, and PGP43, respectively. Note that the molecular weights of PVP side chains have a narrow distribution, as exemplified by the MALDI-TOF results of PGP23, which displays a PDI of ca. 1.1 ( Supporting Information Figure S4). For the preparation of proton-conducting composite electrolytes, we selected a Keggin-type POM, PW, as the inorganic component, which is a well-known high proton conductor.50 The composite electrolyte membranes were prepared by simply casting the NMP solution of PW and PGP (Figure 1b). The content of PW and PVP in the membranes was optimized to realize both a high proton conduction and a good mechanical property. For polymer electrolytes, their ionic conductivity mainly depends on two factors: the ion-exchange capacity (IEC) and the connectivity of ion-conductive pathways. In our system, the protons from PW clusters contributed to the IEC, and thus a high content of PW can lead to a high IEC. On the other hand, the connectivity of proton transport pathways depends on the percolating degree of PVP/PW hybrid domains. A high volume fraction of PVP improves this connectivity. Based on these discussions, we optimized the composition of PGP/PW nanocomposites by gradually loading more PW or using the PGP with longer PVP side chains. However, we found that the composite membranes become fragile when the content of PW is higher than 30 wt % or the polymerization degree of PVP is more than 43 (Figure 2a and Supporting Information Figure S5a). Thus, PGP23-30 (where 30 represents the PW content of 30 wt %) is the sample with the optimal composition. For a systematical investigation of the influence of PVP length on the resultant composites, we also prepared PGP8-30 and PGP17-30. The three samples are all flexible transparent membranes, implying that PW is compatible with the PGP matrices (Figure 2a). Moreover, XRD patterns of these samples showed no characteristic peaks corresponding to crystalline PW aggregates, which further supports that PW clusters are homogeneously dispersed in the PVP domains (Figure 2b). To evaluate the contribution of the comb polymeric structure of PGP on the composite formation, we also prepared a ternary composite membrane of PW, PEEK, and PVP. Unfortunately, a serious macrophase separation occurred in this sample, which reflects that covalently anchoring PVP onto PEEK is necessary to improve the macroscopic compatibility of different components in this system ( Supporting Information Figure S5b). Figure 2 | (a) Photographs of PGP/PW nanocomposites. (b) XRD patterns of PGP8-30, PGP17-30, PGP23-30 and PW. (c) FTIR spectra of PW, PGP8-30, PGP17-30, and PGP23-30. (d) SAXS profiles of PGP8-30, PGP17-30, and PGP23-30. The inset of (d) corresponds to the high q region. Download figure Download PowerPoint In the composite electrolytes, the hydrophilic PVP chains that contain N-heterocycles can electrostatically interact with the acidic PW clusters through protonation, forming proton conductive domains; meanwhile, the hydrophobic PEEK chains provide mechanical support. The complexation between PVP and PW was studied by using FTIR spectroscopy (Figure 2c). In the IR spectrum of pristine PW, four kinds of oxygen atoms in PW exhibit the characteristic vibration bands at 1080 cm−1 νas(P–Oa), 984 cm−1 νas(W=Od), 890 cm−1 νas(W–Ob–W), and 808 cm−1 νas(W–Oc–W), (Oa, oxygen in PO4 tetrahedron; Ob, corner-sharing oxygen; Oc, edge-sharing oxygen; Od, terminal oxygen). These vibration bands were all displayed in the IR spectra of PGP8-30, PGP17-30, and PGP23-30, indicating that the PW clusters retained their structural integrity in the nanocomposites. In contrast to pristine PW, slight shifts were observed in these vibration bands for all three nanocomposites. The νas(P–Oa) and νas(W=Od) peaks shifted to the low-frequency region, appearing at 1079 and 977 cm−1, whereas the νas(W–Ob–W) and νas(W–Oc–W) peaks shifted to the high-frequency region, appearing at 897 and 818 cm−1, respectively. Similar phenomena were also found in the IR spectra of other PVP/POM composites in the literature, which were supposed to be due to the varied electrostatic environment of POM anions.52,66 The nanoscale morphologies of the composite membranes of PGP8-30, PGP17-30, and PGP23-30 were studied by using the SAXS method and the TEM. Under the same SAXS testing conditions, no obvious scattering signal was observed for PGP8-30. However, PGP17-30 and PGP23-30 displayed scattering peaks at the wave vector q of 0.18 and 0.20 nm−1, corresponding to the d-spacing of ca. 34.9 and 31.4 nm, respectively, given by 2π/q, which indicated that PGP17-30 and PGP23-30 both possess typical microphase-separated morphologies (Figure 2d). Notably, the SAXS profiles of PGP8-30, PGP17-30, and PGP23-30 at the high q region show the scattering peaks at 5.01, 4.44, and 3.04 nm−1, corresponding to the d-spacing of 1.3, 1.4, and 2.1 nm, respectively (the inset of Figure 2d). These peaks reflect the distribution distance between PW clusters in polymer matrices.58 In these samples, the contents of POMs are the same, but the volume fractions of PVP are increased. The increasing distribution distance of PW reflects that PW clusters are dispersed more and more sparsely when the PVP domains become larger. Such an increasable spacing between PW further supports that PW clusters are homogeneously distributed in the PVP domains, without forming crystalline aggregation in which the positions of PW should be fixed. To further clarify the microphase structures of the composite membranes, we microtomed the membranes into ultrathin sections with a thickness of ca. 80 nm for TEM characterizations. As the existence of heavy element tungsten increases the electron density, the PVP/PW domains are darker than the PEEK domains, and thus they are easily distinguished under the TEM observation. The TEM image of PGP8-30 shows a structure similar to the disordered spherical phase, in which most of the dark-colored PVP/PW domains are separated by the light-colored PEEK domains, like isolated islands with irregular shapes (Figures 3a and 3d). Thus, the PW-based proton-conducting domains are not continuous in PGP8-30. For the cases of PGP17-30 and PGP23-30, they both exhibit a bicontinuous structure in which the PVP/PW domains and the PEEK domains are highly intertwined (Figures 3b and 3c). Note that the PVP/PW and the PEEK domains in PGP23-30 are narrower than those in PGP17-30. This tendency is consistent with the decreased d-spacing from PGP17-30 to PGP23-30 in the SAXS results. Under higher magnification, the PVP/PW domains in PGP17-30 and PGP23-30 are observed as highly interconnected networks, with the average mesh diameters concentrating at around 20 and 24 nm, respectively (Figures 3e and 3f). Figure 3 | Bright-field TEM images of the ultrathin microtomed sections of the nanocomposite membranes. (a and d) PGP8-30, (b and e) PGP17-30, and (c and f) PGP23-30. Download figure Download PowerPoint Upon focusing on a mesh in PGP23-30 by TEM, we can reveal that the networks are formed by the nanosized PW clusters (Figure 4a). Because the PGP23-30 sections have a certain thickness, the PW clusters at different vertical heights in the thin sections are all projected into the TEM image, and their projections are overlapped. As a consequence, the observed dark dots are 2–5 nm, larger than the actual diameter of an individual PW, which is 1 nm. Even so, several dots still display the crystalline lattices of PW clusters (inset of Figure 4a). To further confirm that the networks are formed by PW, we employed high-angle annular dark-field scanning TEM (HAADF-STEM) to observe a mesh of the networks. In the HAADF-STEM mode, the heavier components appeared brighter against the polymer matrices. We found that the mesh backbone is brighter than the surrounding region, which confirmed that it is formed by PW (Figure 4b). Also, the elemental mapping by energy-dispersive X-ray spectroscopy (EDXS) confirmed the distribution of tungsten in the mesh backbone (Figure 4c). These results demonstrated the formation of PW-based 3D networks in PGP17-30 and PGP23-30. To get an insight into the forming conditions of bicontinuous structure in PGP17-30 and PGP23-30, we analyzed the volume fractions of PVP/PW domains (fPVP/PW) which is the ionic phase (detailed calculations are shown in the Supporting Information). The fPVP/PW of PGP17-30 and PGP23-30 was 43% and 49%, respectively. In contrast, the fPVP/PW of PGP8-30 was only 28% (Table 1). Therefore, a similar volume fraction of the ionic phase and the neutral phase in the nanocomposites was beneficial to the formation of bicontinuous microphase structures. Figure 4 | A magnified mesh in the ultrathin microtomed sections of PGP23-30, imaged by (a) high-resolution bright-field TEM, (b) HAADF-STEM, and (c) EDXS mapping of W. Note that (b) and (c) correspond to the same region. The inset of (a) shows the crystalline lattice phase of PW clusters in the mesh. Download figure Download PowerPoint Table 1 | Properties of the PGP/PW Nanocomposites Sample fPVP/PW (%)a σ (mS cm−1)b Ea (eV)c Tensile Modulus (Mpa) Compression Modulus (Gpa) PGP8-30 28 7.6 0.16 723 3.2 PGP17-30 43 18.2 0.13 861 4.0 PGP23-30 49 32.7 0.12 942 4.5 aVolume fraction of PVP/PW domains. bProton conductivity at 353 K. cActivation energy for proton conduction. The PW networks endow the PGP/PW nanocomposites with enhanced mechanical properties. The tensile modulus of PGP8-30, PGP17-23, and PGP23-30 is 723, 861, and 942 MPa, respectively, exhibiting an increasing tendency (Table 1). When assembling fuel cells, the proton-conducting membranes are tightly pressed together with the bipolar plates, which are used to ensure a sealed system and prevent the leakage of fuels. Considering this point, we also tested the compression modulus of the membranes on silicon wafers. Due to the influence of substrates, the obtained compression moduli were higher than the tensile modulus. We also observed an increasing tendency of the compression moduli from 3.2, 4.0 to 4.5 GPa, for the cases of PGP8-30, PGP17-23, and PGP23-30. This tendency is in agreement with the tensile testing results (Table 1 and Supporting Information Figure S6), which reflects that the presence of connective and compact PW networks can remarkably enhance the mechanical strength of PGP/PW nanocomposites. The role of POMs as nanoenhancers in polymer nanocomposites has also been reported elsewhere.37,41 Particularly in our previous works, we found that POMs can offer a remarkable mechanical enhancement when they self-assemble into certain percolated structures in polymer matrices, for example, networks.41 The present work belongs to such a case. The proton conductivities of the wet membranes were measured in the temperature range of 303–353 K by AC impedance measurements (Figures 5a and 5b). The detailed conductivity values are shown in Supporting Information Table S1. We found that the conductivities of PGP8-30, PGP17-23, and PGP23-30 at 353 K are increased from 7.6, 18.2 to 32.7 mS cm−1, accompanied by a decreased activation energy from 0.16, 0.13 to 0.12 eV by Arrhenius fitting (Figure 5c and Table 1), respectively, which corresponds to a Grotthuss-type proton conduction mechanism.60 The contents of PW in PGP8-30, PGP17-23, and PGP23-30 are all 30 wt % and thus lead to the same IEC of 0.31 mmol g−1. Given this situation, the enhanced conductivities and the decreased Ea from PGP8-30, PGP17-23 to PGP23-30 should be ascribed to the improved connectivity of PW networks but not the IEC. It should be mentioned that the proton conductivity of PGP23-30 is 16.9 mS cm−1 at 303 K. This value is 100-fold higher than the bicontinously structured poly(styrene-block-2-vinylpyridine)/H4SiW12O40 nanocomposite (0.13 mS cm−1 at 303 K) reported in our previous work,54 although the content of H4SiW12O40 in the latter is higher than 45 wt %. We believe that the enhanced proton conductivity is due to the better water absorption of PVP than that of poly(2-vinylpyridine), which introduces more water into the system to facilitate proton transport. Therefore, when designing polymer/POM composite electrolytes for proton conduction, it is necessary to select the polymer component with strong hydrophilicity to construct the ionic phase region. Figure 5 | The Nyquist plots of PGP/PW nanocomposites at (a) 303 and (b) 353 K. (c) Temperature-dependent conductivity of PGP8-30, PGP17-30, and PGP23-30 from 303 to 353 K. (d) Polarization curves and power density curves of the DMFCs using PGP23-30 membranes. Download figure Download PowerPoint Since PGP23-30 shows the best performance in both mechanical strength and proton conductivity among all the samples, we chose it to assemble the membrane electrodes for direct methanol fuel cells (DMFCs). The cell performance was evaluated at 80 °C under the operating conditions by using a 2 M aqueous solution of methanol on the anode side and oxygen gas on the cathode side. The open-circuit voltage and the maximum power density reached 0.82 V and 10.2 mW cm−2, respectively (Figure 5d). It is worthy of note that such a cell performance is obtained at a relatively low IEC value. The IEC of PGP23-30 is only 0.31 mmol g−1. For the Nafion-based DMFCs, the maximum power densities at 80 °C are generally 25–50 mW cm−2,67,68 but Nafion has a high IEC of ca. 0.9 mmol g−1 which is threefold the IEC of our samples.63 Thus, our PGP/PW composite electrolytes still have the potential to be improved. Conclusion We demonstrate the preparation of bicontinuously structured polymer composite electrolytes containing 3D-connected POM-based nanonetworks, through the electrostatic self-assembly of PW and a comb copolymer PEEK-g-PVP. The resultant nanocomposites can effectively overcome the trade-off between ionic conductivity and mechanical strength, exhibiting a high proton conductivity over 30 mS cm−1 and a modulus higher than 4 GPa, wherein the 3D-connected PW networks contribute a dual role of both proton-conducting pathways and mechanical reinforcers. The D