Sulfonated poly(ether ether ketone) reinforced with polystyrene sulfonic acid functionalized micelle templated mesoporous MCM-41 for direct methanol fuel cells

A composite membrane comprising sulfonated polyether ether ketone (sPEEK) and polystyrene sulfonic acid functionalized micelle templated MCM-41 (PSSA-f-MCM-41) is realized as a potential alternative to the Nafion membrane for direct methanol fuel cells (DMFC). PSSA-f-MCM-41 incorporation in sPEEK leads to improvement in the physio-chemical properties such as ionic conductivity, methanol permeability, tensile strength, water uptake and ion exchange capacity. The methanol permeability of the composite membrane i.e. PSSA-f-MCM-41 (1.5 wt %)-sPEEK is reduced by 60% in comparison to pristine sPEEK. Moreover, the electrochemical selectivity of the aforesaid composite membrane is four times higher than sPEEK. The DMFC performance shows the peak power density of 147 mW cm^?2 for the composite membrane which is higher than the pristine sPEEK and Nafion 117 membranes. Stability test for the composite membranes confirm less degradation in OCV in comparison to pristine sPEEK.     

Cross-linked membranes based on sulfonated poly (ether ether ketone) (SPEEK)/Nafion for direct methanol fuel cells (DMFCs)

A series of cross-linked membranes based on SPEEK/Nafion have been prepared to improve methanol resistance and dimension stability of SPEEK membrane for the usage in the direct methanol fuel cells (DMFCs). Sulfonated diamine monomer is synthesized and used as cross-linker to improve the dispersion of Nafion in the composite membranes and decrease the negative effect of cross-linking on proton conductivity of membranes. FT-IR analysis shows that the cross-linking reaction is performed successfully. The effects of different contents of Nafion on the properties of cross-linked membranes are investigated in detail. All the cross-linked membranes show lower methanol permeability and better dimensional stability compared with the pristine SPEEK membrane. SPEEK-N30 with the 30 wt % Nafion shows a methanol permeability of 0.73 × 10⁻⁶ cm² s⁻¹ and a water uptake of 24.4% at 25 °C, which are lower than those of the pristine membrane. Meanwhile, the proton conductivity of SPEEK-N30 still remains at 0.041 S cm⁻¹ at 25 °C, which is comparable to that of the pristine SPEEK membrane. All the results indicate that these cross-linked membranes based on SPEEK/Nafion show good prospect for the use as proton exchange membranes.     

Novel sulfonated poly(ether ether ketone)s containing nitrile groups and their composite membranes for fuel cells

A series of novel sulfonated poly(ether ether ketone)s containing a cyanophenyl group (SPEEKCNxx) are prepared based on (4-cyano)phenylhydroquinone via nucleophilic substitution polycondensation reactions. To further improve their properties, novel composite membranes composed of sulfonated poly(ether ether ketone)s containing cyanophenyl group as an acidic component and aminated poly(aryl ether ketone) as a basic component are successfully prepared. Most of the membranes exhibit excellent thermal, oxidative and dimensional stability, low-swelling ratio, high proton conductivity, low methanol permeability and high selectivity. The proton conductivities of the membranes are close to Nafion 117 at room temperature. And especially, the values of SPEEKCN40 and its composite membranes are higher than Nafion 117 at 80 °C (0.17 S cm⁻¹ of Nafion, 0.26 S cm⁻¹ of SPEEKCN40, 0.20 S cm⁻¹ of SPEEKCN40-1, and 0.18 S cm⁻¹ of SPEEKCN40-2). Moreover, the methanol permeability is one order magnitude lower than that of Nafion 117. All the data prove that both copolymers and their composite membranes may be potential proton exchange membrane for fuel cells applications.     

Improving the proton conductivity of sulfonated poly (ether ether ketone) membranes by incorporating a crystalline nanoassembly of trimesic acid and melamine

A novel proton exchange membrane was synthesized by embedding a crystalline which was nano-assembled through trimesic acid and melamine (TMA·M) into the matrix of the sulfonated poly (ether ether ketone) (SPEEK) to enhance the proton conductivity of the SPEEK membrane. Fourier transform infrared indicated that hydrogen bonds existed between SPEEK and TMA·M. XRD and SEM indicated that TMA·M was uniformly distributed within the matrix of SPEEK, and no phase separation occurred. Thermogravimetric analysis showed that this membrane could be applied as high temperature proton exchange membrane until 250 °C. The dimensional stability and mechanical properties of the composite membranes showed that the performance of the composite membranes is superior to that of the pristine SPEEK. Since TMA·M had a highly ordered nanostructure, and contained lots of hydrogen bonds and water molecules, the proton conductivity of the SPEEK/TMA·M-20% reached 0.00513 S cm⁻¹ at 25 °C and relative humidity 100%, which was 3 times more than the pristine SPEEK membrane, and achieved 0.00994 S cm⁻¹ at 120 °C.     

Amelioration in physicochemical properties and single cell performance of sulfonated poly(ether ether ketone) block copolymer composite membrane using sulfonated carbon nanotubes for intermediate humidity fuel cells

A composite membrane composed of a sulfonated diblock copolymer (SDBC) based on poly(ether ether ketone) blocks copolymerized with partially fluorinated poly(arylene ether sulfone) and sulfonated carbon nanotubes (SCNTs) was fabricated by simple solution casting. Addition of the SCNT filler enhanced the water absorption and proton conductivity of membranes because of the increased per‐cluster volume of sulfonic acid groups, at the same time reinforced the membranes' thermal and mechanical properties. The SDBC/SCNT‐1.5 membrane exhibited the most improved physicochemical properties among all materials. It obtained a proton conductivity of 10.1 mS/cm at 120°C under 20% relative humidity (RH) which was 2.6 times more improved than the pristine membrane (3.9 mS/cm). Moreover, the single cell performance of the SDBC/SCNT‐1.5 membrane at 60°C and 60% RH at ambient pressure exhibited a peak power density of 171 mW/cm² at a load current density of 378 mA/cm², while the pristine membrane exhibited 119 mW/cm² at a load current density of 294 mA/cm². Overall, the composite membrane exhibited very promising characteristics to be used as polymer electrolyte membrane in fuel cells operated at intermediate RH.     

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.     

Balancing dimensional stability and performance of proton exchange membrane using hydrophilic nanofibers as the supports

In this work, we developed a novel composite membrane by anchoring perfluorosulfonic acid into the hydrophilic poly(lactic-co-glycolic acid) (PLGA) nanofibrous network which was synthesized by electrospinning method. It was clear that the PLGA/Nafion composite membranes possessed high Nafion loading, excellent dimensional stability and proton transport capacity. When the humidity of the membrane changed from soaking in water to 25 RH% at 90 °C, the PLGA fiber network effectively controlled the swelling of Nafion resin and reduced the humidity-generated shrinkage stress from 2.2 MPa (Nafion211 membranes) to 0.5 MPa (PLGA/Nafion composite membranes). The proportion of humidity-induced stress to the yield strength was also reduced to 4.4%, in comparison to 21.2% of that of Nafion211 membrane. The area proton conductivity of the PLGA/Nafion composite membrane achieved 48.2 S cm⁻², compared with 36.0 S cm⁻² of Nafion211 membranes in the same condition. The excellent proton transport capacity greatly improved the performance of fuel cell assembled with PLGA/Nafion composite membranes and effectively reduced the dynamic response time from 22 s (Nafion211 membranes) to 7 s (PLGA/Nafion composite membranes).     

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.     

Highly branched poly(arylene ether)/surface functionalized fullerene‐based composite membrane electrolyte for DMFC applications

Sulfonated branched polymer membranes have been gaining immense attention as the separator in energy‐related applications especially in fuel cells and flow batteries. Utilization of this branched polymer membranes in direct methanol fuel cell (DMFC) is limited because of large free volume and high methanol permeation. In the present work, sulfonated fullerene is used to improve the methanol barrier property of the highly branched sulfonated poly(ether ether ketone sulfone)s membrane without sacrificing its high proton conductivity. The existence of sulfonated fullerene with larger size and the usage of small quantity in the branched polymer matrix effectively prevent the methanol transportation channel across the membrane. The composite membrane with an optimized loading of sulfonated fullerene displays the highest proton conductivity of 0.332 S cm^?1 at 80°C. Radical scavenging property of the fullerene improves the oxidative stability of the composite membrane. Composite membrane exhibits the peak power density of 74.38 mW cm^?2 at 60°C, which is 30% larger than the commercial Nafion 212 membrane (51.78 mW cm^?2) at the same condition. From these results, it clearly depicts that sulfonated fullerene‐incorporated branched polymer electrolyte membrane emerges as a promising candidate 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.     

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.     

The effective methanol-blocking and proton conductivity membranes based on sulfonated poly (ether ether ketone ketone) and polyorganosilicon with functional groups

To solve the conflict between high proton conductivity and low methanol crossover of pristine sulfonated aromatic polymer membranes, the polyorganosilicon doped sulfonated poly (ether ether ketone ketone) (SPEEKK) composite membranes were prepared by introducing polyorganosilicon additive with various functional groups into SPEEKK in this study. Scanning electron microcopy (SEM) images showed the obtained membranes were compact. No apparent agglomerations, cracks and pinholes were observed in the SEM images of composite membranes. The good compatibility between polymer and additive led to the interconnection, thus producing new materials with great characteristics and enhanced performance. Besides, the dual crosslinked structure could be formed in composite membranes through the condensation of silanols and the strong interaction between matrix and additive. The formation of dual crosslinked structure optimized the water absorption, enhanced the hydrolytic stability and oxidative stability of membranes. Especially, the incorporation of additive improved the strength and flexibility of composite membranes at the same time, meaning that the life of the composite membranes might be extended during the fuel cell operation. Meanwhile, the proton conductivity improved with increasing additive content due to the loading of more available acidic groups. It is noteworthy that at 25% additive loading, the proton conductivity reached a maximum value of 5.4 × 10⁻² S cm⁻¹ at 25 °C, which exceeded the corresponding value of Nafion^@ 117 (5.0 × 10⁻² S cm⁻¹) under same experimental conditions. The composite membrane with 20 wt% additive was found to produce the highest selectivity (1.22 × 10⁵ S cm⁻³) with proton conductivity of 4.70 × 10⁻² S cm⁻¹ and methanol diffusion coefficient of 3.85 × 10⁻⁷ cm² s⁻¹, suggesting its best potential as proton exchange membrane for direct methanol fuel cell application. The main novelty of our work is providing a feasible and environment-friendly way to prepare the self-made polyorganosilicon with various functional groups and introducing it into SPEEKK to fabricate the dual crosslinked membranes. This design produces new materials with outstanding performance.     

Sulfonated poly(ether ether ketone)/poly(ether sulfone) composite membranes as an alternative proton exchange membrane in microbial fuel cells

Custom-made proton exchange membranes (PEM) are synthesized by incorporating sulfonated poly(ether ether ketone) (SPEEK) in poly(ether sulfone) (PES) for electricity generation in microbial fuel cells (MFCs). The composite PES/SPEEK membranes at various composition of SPEEK are prepared by the phase inversion method. The membranes are characterized by measuring roughness, proton conductivity, oxygen diffusion, water crossover and electrochemical impedance. The conductivity of hydrophobic PES membrane increases when a small amount (3–5%) of hydrophilic SPEEK is added. The electrochemical impedance spectra shows that the conductivity and capacitance of PES/SPEEK composite membranes during MFC operation are reduced from 6.15 × 10⁻⁷ to 6.93 × 10⁻⁵ (3197 Ω–162 Ω) and from 3.00 × 10⁻⁷ to 1.56 × 10⁻³ F, respectively when 5% of SPEEK added into PES membrane. The PES/SPEEK 5% membrane has the highest performance compared to other membranes with a maximum power density of 170 mW m⁻² at the maximum current density of 340 mA m⁻². However, the interfacial reaction between the membrane and the cathode with Pt catalyst indicates moderate reaction efficiency compared to other membranes. The COD removal efficiency of MFCs with composite membrane PES/SPEEK 5% is nearly 26-fold and 2-fold higher than that of MFCs with Nafion 112 and Nafion 117 membranes respectively. The results suggest that the PES/SPEEK composite membrane is a promising alternative to the costly perfluorosulfonate membranes presently used as separators in MFC systems.     

Preparation and performance of nano silica/Nafion composite membrane for proton exchange membrane fuel cells

Composite membranes made from Nafion ionomer with nano phosphonic acid-functionalised silica and colloidal silica were prepared and evaluated for proton exchange membrane fuel cells (PEMFCs) operating at elevated temperature and low relative humidity (RH). The phosphonic acid-functionalised silica additive obtained from a sol–gel process was well incorporated into Nafion membrane. The particle size determined using transmission electron microscope (TEM) had a narrow distribution with an average value of approximately 11 nm and a standard deviation of ±4 nm. The phosphonic acid-functionalised silica additive enhanced proton conductivity and water retention by introducing both acidic groups and porous silica. The proton conductivity of the composite membrane with the acid-functionalised silica was 0.026 S cm⁻¹, 24% higher than that of the unmodified Nafion membrane at 85 °C and 50% RH. Compared with the Nafion membrane, the phosphonic acid-functionalised silica (10% loading level) composite membrane exhibited 60 mV higher fuel cell performance at 1 A cm⁻², 95 °C and 35% RH, and 80 mV higher at 0.8 A cm⁻², 120 °C and 35% RH. The fuel cell performance of composite membrane made with 6% colloidal silica without acidic group was also higher than unmodified Nafion membrane, however, its performance was lower than the acid-functionalised silica additive composite membrane.     

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.     

High proton-conducting polymer electrolytes based on pendent poly(arylene ether ketone) with H-bond for proton exchange membranes

A novel side-chain poly(arylene ether ketone) functionalized with 1,4-butane-sultone (SQNPAEK-x, “x” refers to the number of the attaching sulfobutyl groups in one unit) were synthesized for proton exchange membrane fuel cell. SQNPAEK-x showed high proton conductivity up to 0.317 S cm⁻¹ at 80 °C and excellent dimensional stability due to the remaining hydroxyl groups. Morphological investigations revealed that SQNPAEK-x contained more uniform ionic clusters than the main-chain sulfonated poly(arylene ether ketone), which increased the proton conductivity. Other properties such as water uptake, mechanical properties were also investigated. It should be noted that SQNPAEK-1.5 possesses the best overall performance with proton conductivity (0.204 S cm⁻¹ at 80 °C) higher that Nafion 117, while the swelling ratio was equivalent (13.2% at 80 °C). All the results indicate that the SQNPAEK-x membranes are promising candidates for proton exchange membrane fuel cells.     

Synthesis of poly(arylene ether sulfone)s with locally and densely sulfonated pentiptycene pendants as highly conductive polymer electrolyte membranes

Poly(aryl ether sulfone)s containing sulfonated pentiptycene groups SPES-x-PPD are firstly synthesized through nucleophilic aromatic substitution polycondensation by using pentiptycene-6,13-diol, bis(4-hydroxyphenyl) sulfone and 4,4′-difluorodiphenyl sulfone, followed by postsulfonation with concentrated sulfuric acid at room temperature. The structures of SPES-x-PPD are characterized by IR, ¹H NMR and ¹³C NMR spectra. These ionomers generally show high thermal stability. Transmission electron microscopic observations reveal that SPES-x-PPD membranes form well-defined microphase separated structures. SPES-40-PPD with the IEC value 2.36 mmol g⁻¹ shows conductivity of 2.6 × 10⁻¹ S cm⁻¹ which is much higher than that of perfluorinated Nafion 117 membrane (1.1 × 10⁻¹ S cm⁻¹) at 80 °C and 94% RH. At 80 °C and 34% RH, SPES-40-PPD membrane displays the conductivity of 2.7 × 10⁻³ S cm⁻¹ which is comparable with that of Nafion 117 membrane (3.0 × 10⁻³ S cm⁻¹).     

Composite membranes based on sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety and poly(amic acid) for proton exchange membrane fuel cell application

Composite membranes based on sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety and poly(amic acid) with oligoaniline in the main chain have been prepared and immersed in H₃PO₄ to obtain acid-doped composite films. As expected, the water uptake values and methanol permeability of the composite membranes decrease with the increase of the weight fraction of PAA in the membrane matrix. Notably, the SPEEK-6F/PAA-15 shows a water uptake of 13.2% and a methanol permeability of 0.9 × 10⁻⁷ cm² s⁻¹, which are much lower than those of the Nafion (28.6% and 15.5 × 10⁻⁷ cm² s⁻¹, respectively). Although the proton conductivities decrease after the addition of PAA, higher selectivity values are obtained with the composite membranes. Therefore, the SPEEK-6F/PAA blend membranes, with the improved proton conductivity, methanol resistance and good thermal stability, can be used as a good alternative for proton conductive membranes with potential application in proton exchange membrane fuel cells (PEMFCs).     

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.     

Effect of crosslinking on the durability and electrochemical performance of sulfonated aromatic polymer membranes at elevated temperatures

End-group crosslinked sulfonated poly(arylene sulfide nitrile) (XESPSN) membranes are prepared to investigate the effect of crosslinking on the properties of sulfonated aromatic polymer membranes at elevated temperatures (>100 °C). The morphological transformation during annealing and crosslinking is confirmed by atomic force microscopy. The XESPSN membranes show outstanding thermal and mechanical properties compared to pristine and non-crosslinked ESPSN and Nafion^® up to 200 °C. In addition, the XESPSN membranes exhibit higher proton conductivities (0.011–0.023 S cm⁻¹) than the as-prepared pristine ESPSN (0.004 S cm⁻¹), particularly at elevated temperature (120 °C) and low relative humidity (35%) conditions due to its well-ordered hydrophilic morphology after crosslinking. Therefore, the XESPSN membranes demonstrate significantly improved maximum power densities (415–485 mW cm⁻²) compared to the ESPSN (281 mW cm⁻²) and Nafion^® (314 mW cm⁻²) membranes in single cell performance tests conducted at 120 °C and 35% relative humidity. Furthermore, the XESPSN membrane exhibits a much longer duration than the ESPSN membrane during fuel cell operation under a constant current load as a result of its improved mechanical and thermal stabilities.