Nafion-assisted cross-linking of sulfonated poly(arylene ether ketone) bearing carboxylic acid groups and their composite membranes for fuel cells

In this study, a new type of cross-linked composite membrane is prepared and considered for its potential applications in direct methanol fuel cell. Nafion and sulfonated poly(arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) are blended and subsequently cross-linked by a Friedel-Craft reaction using the carboxylic acid groups in the SPAEK-C to achieve lower methanol permeability. The perfluoroalkyl sulfonic acid groups of Nafion act as a benign solid catalyst, which assist the cross-linking of SPAEK-C. The physical and chemical characterizations of the cross-linked composite membranes are performed by varying the contents of SPAEK-C. The c-Nafion-15% membrane exhibits appropriate water uptake (10.49-25.22%), low methanol permeability (2.57 × 10⁻⁷ cm² s⁻¹), and high proton conductivity (0.179 S cm⁻¹ at 80 °C). DSC and FTIR analyze suggest the cross-linking reaction. These results show that the self-cross-linking of SPAEK-C in the Nafion membrane can effectively reduce methanol permeability while maintaining high proton conductivity.     

Synthesis and characterization of novel sulfonated poly(arylene ether ketone) copolymers with pendant carboxylic acid groups for proton exchange membranes

A series of novel side-chain-type sulfonated poly(arylene ether ketone)s with pendant carboxylic acid groups copolymers (C-SPAEKs) were synthesized by direct copolymerization of sodium 5,5′-carbonyl-bis(2-fluorobenzenesulfonate), 4,4′-difluorobenzophenone and 4,4′-bis(4-hydroxyphenyl) valeric acid (DPA). The expected structure of the sulfonated copolymers was confirmed by FT-IR and ¹H NMR. Membranes with good thermal and mechanical stability could be obtained by solvent cast process. It should be noted that the proton conductivity of these copolymers with high sulfonatation degree (DS > 0.6) was higher than 0.03 S cm⁻¹ and increased with increasing temperature. At 80 °C, the conductivity of C-SPAEK-3 (DS = 0.6) and C-SPAEK-4 (DS = 0.8) reached up to 0.12 and 0.16 S cm⁻¹, respectively, which were higher than that of Nafion 117 (0.10 S cm⁻¹). Moreover, their methanol permeability was much lower than that of Nafion 117. These results showed that the synthesized materials might have potential applications as the proton exchange membranes for DMFCs.     

Sulfonated mesoporous benzene-silica-embedded sulfonated poly(ether ether ketone) membranes for enhanced proton conduction and anti-dehydration

In polymer electrolyte fuel cell operation, a decrease in the proton conductivity of the membrane at reduced humidity is a main cause for poor cell performance at high temperature. To alleviate the dehydration of the membrane at high temperature, sulfonated mesoporous benzene-silica (sMBS) particles are embedded in sulfonated poly(ether ether ketone) (sPEEK) membranes. As the sMBS itself is highly sulfonated on both organic and inorganic moieties, the proton conductivity of composite membranes is much higher than that of the pristine sPEEK membrane, and it reaches that of Nafion 117 at a high relative humidity (RH) of 90%. The dehydration rate of the membrane is reduced significantly by the capillary condensation effect of sMBS particles with the nanometer-scale 2-D hexagonal cylindrical pores, and the proton conductivity of the composite membranes, 0.234 × 10⁻¹ S cm⁻¹, is much higher than that of pristine sPEEK membrane, 0.59 × 10⁻³ S cm⁻¹, at a relatively low humidity of 40% RH. This maintenance of high conductivity at low humidity is attributed to the high water-holding capacity of the sMBS proton conductors. The sMBS-embedded sPEEK composite membranes show a much lower methanol permeability of 2–5 × 10⁻⁷ cm² s⁻¹ compared to that of Nafion 117, which is 1.6 × 10⁻⁶ cm² s⁻¹ at room temperature.     

Modification of sulfonated poly(ether ether ketone) proton exchange membrane for reducing methanol crossover

A drawback of sulfonated aromatic main-chain polymers such as sulfonated poly(ether ether ketone)s (SPEEKs) is their high methanol crossover when the proton conductivity is sufficient for direct methanol fuel cell (DMFC) applications. To overcome this disadvantage, in this paper, the SPEEK substrate was coated with the crosslinked chitosan (CS) barrier layer to form the two-layer composite membranes. Scanning electron microscope (SEM) micrographs showed that the CS layer was tightly adhered on the SPEEK substrate and the thickness of CS layer could be adjusted by varying the concentration of CS solution. It was noticed that with the increment of thickness of CS layer, the methanol diffusion coefficient of the composite membranes significantly dropped from 3.15 × 10⁻⁶ to 2.81 × 10⁻⁷ cm² s⁻¹ at 25 °C which was about one order of magnitude lower than those of the pure SPEEK and Nafion^® 117 membranes. In addition to the effective methanol barrier, the composite membranes possessed adequate thermal stability (the 5% weight lose temperature exceeded 240 °C) and good proton conductivity. The proton conductivity of all composite membranes was in the order of 10⁻² S cm⁻¹ and increased with the elevation of temperature. Furthermore, the composite membranes exhibited much higher selectivity (conductivity/methanol diffusion coefficient) compared with the pure SPEEK and Nafion^® 117 membranes. These results indicated that introducing the crosslinked CS layer onto the SPEEK surface was an effective method for improving the performance of the SPEEK membrane, especially for reducing the methanol crossover.     

Novel acid–base molecule-enhanced blends/copolymers for fuel cell applications

A series of acid–base molecule-enhanced composite membranes are successfully prepared. The composite membranes are composed of a sulfonated poly(aryl ether ketone) (6FSPEEK) as an acidic component, and of aminated poly(aryl ether ketone) containing a naphthyl group (AmPEEKK-NA) as a basic component. The composite membranes exhibit obviously improved thermal, oxidative and dimensional stability. Especially, these composite membranes possess excellent tensile properties both in the dry and wet state. The proton conductivities of these membranes are higher than 2.45 × 10⁻² S cm⁻¹ at room temperature and higher than 6.0 × 10⁻² S cm⁻¹ at 80 °C. The morphology of the membranes is studied in detail by SEM and AFM. All the data prove that both composite and aminated/sulfonated copolymer membranes may be potential proton exchange membrane for fuel cell applications.     

Novel modification method to prepare crosslinked sulfonated poly(ether ether ketone)/silica hybrid membranes for fuel cells

Crosslinked organic-inorganic hybrid membranes are prepared from hydroxyl-functionalized sulfonated poly(ether ether ketone) (SPEEK) and various amounts of silica with the aims to improve dimensional stability and methanol resistance. The partially hydroxyl-functionalized SPEEK is prepared by the reduction of some benzophenone moieties of SPEEK into the corresponding benzhydrol moieties which is then reacted with (3-isocyanatopropyl)triethoxysilane (ICPTES) to get a side chained polymer bearing triethoxysilyl groups. These groups are subsequently co-hydrolyzed with tetraethoxysilane (TEOS) and allow the membrane to form a crosslinked network via a sol-gel process. The obtained hybrid membranes with covalent bonds between organic and inorganic phases exhibit much lower methanol swelling ratio and water uptake. With the increase of silica content, the methanol permeability coefficient of the hybrid membranes decreases at first and then increased. At silica content of about 6 wt.%, the methanol permeability coefficient reaches a minimum of 7.15 × 10⁻⁷ cm² s⁻¹, a 5-fold decrease compared with that of the pristine SPEEK. Despite the fact that the proton conductivity is decreased to some extent as a result of introduction of the silica, the hybrid membranes with silica content of 4-8 wt.% shows higher selectivity than Nafion117.     

Sulfonated titania submicrospheres-doped sulfonated poly(ether ether ketone) hybrid membranes with enhanced proton conductivity and reduced methanol permeability

Sulfonated titania submicrospheres (TiO₂-SO₃H) prepared through a facile chelation method are incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate organic-inorganic hybrid membranes with enhanced proton conductivity and reduced methanol permeability for potential use in direct methanol fuel cells (DMFCs). The pristine titania submicrospheres (TiO₂) with a uniform particle size are synthesized through a modified sol-gel method and sulfonated using 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt as the sulfonation reagent. The sulfonation process is confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectra (XPS). The hybrid membranes are systematically characterized in terms of thermal property, mechanical property, ionic exchange capacity (IEC), swelling behavior, and microstructural features. The methanol barrier property and the proton conductivity of the SPEEK/TiO₂-SO₃H hybrid membranes are evaluated. The presence of the fillers reduces methanol crossover through the membrane. Compared with the unsulfonated TiO₂-doped membranes, the TiO₂-SO₃H-doped ones exhibit higher proton conductivity due to the additional sulfonic acid groups on the surface of TiO₂. The hybrid membrane doped with 15 wt.% TiO₂-SO₃H submicrospheres exhibits an acceptable proton conductivity of 0.053 S cm⁻¹ and a reduced methanol permeability of 4.19 × 10⁻⁷ cm² s⁻¹.     

Novel sulfonated poly(ether ether ketone ketone)s for direct methanol fuel cells usage: Synthesis,water uptake,methanol diffusion coefficient and proton conductivity

A novel series of sulfonated poly(ether ether ketone ketone)s (SPEEKKs) with different degrees of sulfonation (Ds) were synthesized from 1,3-bis(3-sodium sulfonate-4-fluorobenzoyl)benzene (1,3-SFBB-Na), 1,3-bis(4-fluorobenzoyl)benzene (1,3-FBB) and 3,3′,5,5′-tetramethyl-4,4′-biphenol (TMBP) by aromatic nucleophilic polycondensation. The chemical structures of SPEEKKs were confirmed by FT-IR spectroscopy and the Ds values of the polymers were calculated by ¹H NMR and titration methods, respectively. The thermal stabilities of the SPEEKKs in acid and sodium forms were characterized by thermogravimetric analysis (TGA), which showed that SPEEKKs had excellent thermal properties at high temperatures. All the SPEEKK polymers were easily solution cast into tough membranes. Water uptakes, proton conductivities and methanol diffusion coefficients of the SPEEKK membranes were measured. Water uptake increased with Ds and temperature. Compared to Nafion, the SPEEKK-60, -70 and -80 membranes showed higher proton conductivities at 80 °C, while the other SPEEKK membranes showed relatively lower proton conductivities. This may be due to the different distribution of ion-conducting domains in membrane. However, these membranes showed lower methanol diffusions in the range of 8.32 × 10⁻⁹ to 1.14 × 10⁻⁷ cm² s⁻¹ compared with that of Nafion (2 × 10⁻⁶ cm² s⁻¹) at the same temperature. The membranes also showed excellent mechanical properties (with a Young's modulus > 1 GPa and a tensile strength > 40 MPa). These results indicate that the SPEEKK membranes are promising materials for use in direct methanol fuel cell (DMFC) applications.     

Novel branched sulfonated poly(ether ether ketone)s membranes for direct methanol fuel cells

In this article, novel branched sulfonated poly(ether ether ketone)s (Br-SPEEK) containing various amounts of 1,3,5-tris(4-fluorobenzoyl)benzene as the branching agent have been successfully prepared. Compared with the traditional linear polymer membranes, the membranes prepared by Br-SPEEK showed improved mechanical strength, excellent dimensional stability and superior oxidative stability with similar proton conductivity. Notably, the Br-SPEEK-10 membrane began to break after 267 min in Fenton's reagent at 80 °C, which was 4 times longer than that of the L-SPEEK. Although the proton conductivity decreased with the addition of the branching agent, satisfying methanol permeability value was observed (down to 6.3 × 10⁻⁷ cm² s⁻¹), which was much lower than Nafion 117 (15.5 × 10⁻⁷ cm² s⁻¹). All the results indicated that the novel branched sulfonated poly(ether ether ketone)s membrane was potential candidate as proton conductive membranes for application in fuel cells.     

Preparation of main-chain-type and side-chain-type sulfonated poly(ether ether ketone) membranes for direct methanol fuel cell applications

Novel main-chain-type and side-chain-type sulphonated poly(ether ether ketone)s (MS-SPEEKs) are synthesised by reacting the sulphonic acid groups of pristine SPEEKs with 2-aminoethanesulphonic acid to improve the nano-phase separated morphology of the material. ¹H NMR and FT-IR spectroscopy are employed to determine the structure and composition of main-chain-type and side-chain-type sulphonated polymers. Flexible and tough membranes with reasonable thermal properties are obtained. The MS-SPEEKs show good hydrolytic stability, and water uptake values ranging from 15% to 30% are observed. Compared to Nafion 117^®, the methanol permeability of the MS-SPEEKs is dramatically reduced to 8.83 × 10⁻⁸ cm² s⁻¹ to 3.31 × 10⁻⁷ cm² s⁻¹. The proton conductivity increases with increasing temperature, reaching 0.013-0.182 S cm⁻¹. A maximum power density and open circuit voltage of 115 mW cm⁻² and 0.830 V are obtained at 80 °C, respectively, which is significantly greater than the values generated with Nafion 117^®. The introduction of pendent side-chain-type sulphonic acid groups increases the single-cell performance by more than approximately 20%; thus, the lower water diffusivity, methanol permeability, electro-osmotic drag coefficient and high cell performance indicated that MS-SPEEK is a promising candidate for DMFC applications.     

Crosslinked SPEEK/AMPS blend membranes with high proton conductivity and low methanol diffusion coefficient for DMFC applications

The crosslinked sulfonated poly (ether ether ketone)/2-acrylamido-2-methyl-1-propanesulfonic acid (SPEEK/AMPS) blend membranres were prepared and evaluated as proton exchange membranes for direct methanol fuel cell (DMFC) applications. The structure and morphology of SPEEK/AMPS membranes were characterized by FTIR and SEM, respectively. The effects of crosslinking and AMPS content on the performance of membranes were studied and discussed in detail. The proton conductivity and methanol diffusion coefficient of SPEEK/AMPS membranes increased gradually with the increase of AMPS content. Most SPEEK/AMPS membranes exhibited higher proton conductivity than Nafion^® 117 (0.05 S cm⁻¹ at 25 °C). However, all the membranes possessed much lower methanol diffusion coefficient compared with Nafion^® 117 (2.38 × 10⁻⁶ cm² s⁻¹) under the same measuring conditions. Even the methanol diffusion coefficient (8.89 × 10⁻⁷ cm² s⁻¹) of SPEEK/AMPS 30 sample with the highest proton conductivity (0.084 S cm⁻¹ at 25 °C) was only about one third of that of Nafion^® 117. The selectivity of all the SPEEK/AMPS membranes was much higher in comparison with Nafion^® 117 (2.8 × 10⁴ S s cm⁻³). In addition, the SPEEK/AMPS membranes possessed relatively good thermal and hydrolytic stability. These results suggested that the SPEEK/AMPS membranes were particularly promising to be used as proton exchange membranes in DMFCs, and the high proton conductivity, low methanol diffusion coefficient and high selectivity were their primary advantages for DMFC applications.     

Semi-interpenetrating polymer network electrolyte membranes composed of sulfonated poly(ether ether ketone) and organosiloxane-based hybrid network

A semi-interpenetrating polymer network (semi-IPN) proton exchange membrane is prepared from the sulfonated poly(ether ether ketone) (sPEEK) and organosiloxane-based organic/inorganic hybrid network (organosiloxane network). The organosiloxane network is synthesized from 3-glycidyloxypropyltrimethoxysiane and 1-hydroxyethane-1,1-diphosphonic acid. The semi-IPN membranes prepared were stable up to 300 °C without any degradation. The methanol permeability is much lower than Nafion^® 117 under addition of the organosiloxane network. The proton conductivity of semi-IPN membranes increases with an increase the organosiloxane network content; the membrane containing the 20-24 wt% organosiloxane network shows higher conductivity than Nafion^® 117. The power density of the MEA fabricated with the semi-IPN membrane with 24 wt% organosiloxane network is 135 mW cm⁻², much better than that of the pristine sPEEK membrane, 85 mW cm⁻². Chemical synthesis of the semi-IPN membranes is identified using FTIR, and its ion cluster dimension examined using SAXS. The dimensional stability associated with water swelling and dissolution is investigated at different temperatures, and the semi IPN membranes dimensionally stable in water at elevated temperature.     

Sulfonated poly(ether ether ketone)/polybenzimidazole oligomer/epoxy resin composite membranes in situ polymerization for direct methanol fuel cell usages

A diamine-terminated polybenzimidazole oligomer (o-PBI) has been synthesized for introducing the benzimidazole groups (BI) into sulfonated poly(ether ether ketone) (SPEEK) membranes. SPEEK/o-PBI/4,4′-diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP) composite membranes in situ polymerization has been prepared for the purpose of improving the performance of SPEEK with high ion-exchange capacities (IEC) for the usage in the direct methanol fuel cells (DMFCs). The composite membranes with three-dimensional network structure are obtained through a cross-linking reaction between PBI oligomer and TMBP and the acid-base interaction between sulfonic acid groups and benzimidazole groups. Resulting membranes show a significantly increasing of all of the properties, such as high proton conductivity (0.14 S cm⁻¹ at 80 °C), low methanol permeability (2.38 × 10⁻⁸ cm² s⁻¹), low water uptake (25.66% at 80 °C) and swelling ratio (4.11% at 80 °C), strong thermal and oxidative stability, and mechanical properties. Higher selectivity has been found for the composite membranes in comparison with SPEEK. Therefore, the SPEEK/o-PBI/TMBP composite membranes show a good potential in DMFCs usages.     

Highly conductive, methanol resistant fuel cell membranes fabricated by layer-by-layer self-assembly of inorganic heteropolyacid

Layer-by-layer (LBL) self-assembly is a simple and elegant method of constructing organic-inorganic composite thin films from environmentally benign aqueous solutions. In this paper, we utilize this method to develop proton-exchange membranes for fuel cells. The multilayer film is constructed onto the surface of sulfonated poly(arylene ether ketone) (SPAEK-COOH) membrane by LBL self-assembly of polycation chitosan (CTS) and negatively charged inorganic particle phosphotungstic acid (PTA). The highly conductive inorganic nanoparticles ensure SPAEK-COOH-(CTS/PTA)_n membranes to maintain high proton conductivity values up to 0.086 S cm⁻¹ at 25 °C and 0.24 S cm⁻¹ at 80 °C, which are superior than previous LBL assembled electrolyte systems. These multilayer systems also exhibit extremely low water swelling ratio and methanol permeability. The selectivity of SPAEK-COOH-(CTS/PTA)₈ is 2 orders of more than Nafion^® 117, which is attractive in direct methanol fuel cells (DMFCs).     

Novel cross-linked sulfonated poly (arylene ether ketone) membranes for direct methanol fuel cell

To prepare a cross-linked proton exchange membrane with low methanol permeability and high proton conductivity, poly (vinyl alcohol) is first blended with sulfonated poly (arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) and then heated to induce a cross-linking reaction between the carboxyl groups in SPAEK-C and the hydroxyl groups in PVA. Fourier transform infrared spectroscopy is used to characterize and confirm the structure of SPAEK-C and the cross-linked membranes. The proton conductivity of the cross-linked membrane with 15% PVA in weight reaches up to 0.18 S cm⁻¹ at 80 °C (100% relative humidity), which is higher than that of Nafion membrane, while the methanol permeability is nearly five times lower than Nafion. The ion-exchange capacity, water uptake and thermal stability are investigated to confirm their applicability in fuel cells.     

Poly(arylene ether)s with pendant sulfoalkoxy groups prepared by direct copolymerization method for proton exchange membranes

A novel sulfonated monomer sodium-3-(4-(2,6-difluorobenzoyl)phenoxy)propane-1-sulfonate was designed and synthesized. Based on above monomer, a series of sulfonated poly(arylene ether) copolymers containing aliphatic acid groups between aromatic backbones and sulfonic acid groups (PSOA-SPAE) were successfully prepared by direct copolymerization. Ion exchange capacity (IEC) of the copolymers could be mediated in the range of 1.07–1.61 meq g⁻¹ by the monomer ratios used in the copolymerization. These copolymers exhibited good oxidative and dimensional stability. The proton conductivities of copolymer films increased with the increase of IEC and temperature. The conductivity of PSOA-SPAE-80 was 4.94 × 10⁻² S cm⁻¹ at room temperature, and was up to 1.35 × 10⁻¹ S cm⁻¹ at 100 °C, which was close to that of Nafion 117. These copolymers may be promising proton exchange membranes (PEMs) due to their high proton conductivity and good oxidative and dimensional stability.     

Hybrid proton conducting membranes based on sulfonated cross-linked polysiloxane network for direct methanol fuel cell

A series of novel hybrid membranes based on sulfonated poly(arylene ether ketone)s (SNPAEKs), polysiloxane (KH-560) and sulfonated curing agent (BDSA) has been prepared by sol-gel and cross-linking reaction for direct methanol fuel cells (DMFCs). All the hybrid membranes (SKB-xx) show high thermal properties and improved oxidative stability compared with the pristine SNPAEK membrane. The sulfonated cross-linked polysiloxanes networks in the hybrid membranes enhance the mechanical properties and reduce the swelling ratio. The swelling ratio of SKB-20 is 22%, which is much lower than that of the pristine SNPAEK (37%) at 80 °C. Meanwhile, SKB-xx membranes with greatly reduced methanol permeabilities show comparative proton conductivities to pristine SNPAEK membranes. Notably, the proton conductivities of SKB-5 and SKB-10 reach to 0.192 S cm⁻¹ and 0.179 S cm⁻¹ at 80 °C, respectively, which are even higher than the 0.175 S cm⁻¹ of SNPAEK.     

Preparation and properties of crosslinked sulphonated poly(arylene ether sulphone) blend s for direct methanol fuel cell applications

HMS-based sulphonated poly(arylene ether sulphone) (HMSSH) is synthesised using 4,4′-dihydroxy-α-methylstilbene (HMS) monomer to introduce an interesting stilbene core as crosslinkable group. Crosslinked blend membranes are obtained by blending the BPA-based sulphonated poly(arylene ether sulphone) (BPASH) with crosslinkable HMS-based sulphonated poly(arylene ether sulphone) by UV irradiation of the blend membrane. Compared to the native BPASH with crosslinked BPASH/HMSSH blend membranes, the crosslinked blend membranes greatly reduce the water uptake and methanol permeability with only a slight reduction in proton conductivity. The crosslinked blend membrane, which has a 6% HMSSH content, has a water uptake of 59%, methanol permeability of 0.75 × 10⁻⁶ cm² s⁻¹, and proton conductivity of 0.08 S cm⁻¹. A membrane-electrode assembly is used to investigate single-cell performance and durability test for DMFC applications. Both the power density and open circuit voltage are higher than those of Nafion^® 117. A maximum power density of 32 mW cm⁻² at 0.2 V is obtained at 80 °C, which is higher than that of Nafion^® 117 (25 mW cm⁻²).     

Role of post-sulfonation of poly(ether ether sulfone) in proton conductivity and chemical stability of its proton exchange membranes for fuel cell

Commercially available poly(ether ether sulfone), PEES, was directly sulfonated using concentrated sulfuric acid at low temperatures by minimizing degradation during sulfonation. The sulfonation reaction was performed in the temperature range of 5–25 °C. Sulfonated polymers were characterized by FTIR, ¹H NMR spectroscopy and ion exchange capacity (IEC) measurements. Degradation during sulfonation was investigated by measuring intrinsic viscosity, glass transition temperature and thermal decomposition temperature of sulfonated polymers. Sulfonated PEES, SPEES, membranes were prepared by solvent casting method and characterized in terms of IEC, proton conductivity and water uptake. The effect of sulfonation conditions on chemical stability of membranes was also investigated via Fenton test. Optimum sulfonation condition was determined to be 10 °C with conc. H₂SO₄ based on the characteristics of sulfonated polymers and also the chemical stability of their membranes. SPEES membranes exhibited proton conductivity up to 185.8 mS cm⁻¹ which is higher than that of Nafion 117 (133.3 mS cm⁻¹) measured at 80 °C and relative humidity 100%.     

Cross-linked proton exchange membranes for direct methanol fuel cells: Effects of the cross-linker structure on the performances

A highly sulfonated poly (ether ether ketone) with pendant amino groups (Am-SPEEK) has been synthesized. The resulting copolymer showed good solubility in common organic solvents. Then the pendant amino groups of the Am-SPEEK were utilized to react with two kinds of cross-linker, epoxy resin and dibromide, to yield various cross-linked membranes. After cross-linking, the membranes could not dissolve in common solvents, just became swollen. In generally, all the cross-linked membranes showed improved mechanical properties and high dimensional stabilities, whereas the uncross-linked membranes highly swollen or even dissolved in water at high temperature. The proton conductivity of the membranes increased with an increase in temperature. At 80 °C, all the cross-linked membranes showed high proton conductivity, in the range of 0.111–0.140 S cm⁻¹. Especially, the TMBP cross-linked membrane showed a proton conductivity of 0.140 S cm⁻¹, which was higher than that of Nafion 117 (0.125 S cm⁻¹). The membranes with high proton conductivity, good oxidative stability, and improved methanol residence have been successfully developed. Furthermore, the influence of different main chain of the cross-linker on the performance of the cross-linked membranes was also investigated.