The team, led by Fengxia Zhai, Liqi Zhuang, Xinyu Wei, Fahai Cao, and Shicheng Zhao, worked with sulfonated polyimides, a family of aromatic polymers prized for their thermal stability, mechanical robustness, and film-forming ability. By attaching sulfonic acid groups to the polymer backbone, chemists create the acidic sites that attract water and enable protons to hop through the material. The catch is dosage. To reach conductivities useful in a fuel cell, sulfonated polyimides typically need a high degree of sulfonation, and that very density of acidic groups makes the polymer drink up water voraciously. The membrane swells, its dimensions drift, and over time water attacks the imide linkages themselves, degrading the film hydrolytically until it fails. This long-standing trade-off between conductivity and durability has kept sulfonated polyimides largely out of commercial fuel cells despite their attractive chemistry.

The Shanghai group’s strategy was to sidestep the trade-off rather than fight it head-on. Instead of pushing sulfonation ever higher, they kept the polymer matrices at moderate sulfonation levels and introduced an inorganic proton conductor, phosphotungstic acid, as a dispersed filler. Phosphotungstic acid belongs to the polyoxometalate family, inorganic clusters with a distinctive Keggin structure in which a central phosphorus atom is caged by twelve tungsten-oxygen octahedra. These clusters are dense with acidic protons and can conduct them efficiently, particularly when hydrated. Crucially, the acid is strong enough to contribute conductivity without requiring the polymer host to be saturated with sulfonic acid groups, which means the membrane can stay dimensionally stable and hydrolytically tough while still moving protons quickly.

Building the composite required care, because polyoxometalates tend to aggregate and can leach out of polymer films. The researchers synthesized a series of sulfonated polyimides with varying degrees of sulfonation and two different chain architectures, random copolymers in which sulfonated units are scattered along the backbone, and block copolymers in which sulfonated and unsulfonated segments are grouped into distinct blocks. Block architectures are known to encourage microphase separation, creating connected hydrophilic channels that can act as proton highways. The phosphotungstic acid was then blended into each matrix using ultrasonic-assisted mixing followed by solution casting, a processing route in which ultrasound helps disperse the inorganic clusters evenly through the polymer solution before it is cast into a film and dried.

Characterization confirmed the approach worked at the molecular level. Fourier-transform infrared spectroscopy detected the vibrational fingerprints of the Keggin clusters inside the composite films, showing that the acid’s characteristic structure survived the blending and casting process intact. Scanning electron microscopy revealed a uniform dispersion of the filler throughout the polymer matrix, with no large aggregates that would have compromised mechanical integrity or created defects. That uniformity matters because proton conduction in a composite depends on connectivity: isolated pockets of conductor do little, but a well-dispersed phase can form continuous pathways that supplement the polymer’s own sulfonic acid network.

The performance data are the heart of the study. Among the many formulations tested, one stood out: a composite built from a block copolymer designated BB (3:1)-s, loaded with ten percent phosphotungstic acid. This membrane achieved a proton conductivity of 0.161 siemens per centimeter, a figure comparable to Nafion 117, the widely used commercial reference material. For a hydrocarbon-based polymer membrane to reach that benchmark is significant, since Nafion’s perfluorinated structure and phase-separated morphology have long given it an edge in hydrated proton transport. Equally important, the conductivity gain came without sacrificing strength: the same membrane recorded a tensile strength of 15 megapascals, indicating that the filler reinforced rather than plasticized the film.

Durability results may prove even more consequential than the conductivity numbers. The optimized composite membrane withstood more than 1,600 hours of hydrolytic stability testing, an endurance figure that addresses the Achilles heel of sulfonated polyimides. Because the membrane does not rely on extreme sulfonation, it absorbs water in a controlled way, limiting the swelling stresses and backbone hydrolysis that normally destroy highly sulfonated films. The authors also report favorable methanol barrier properties, an attribute that matters for direct methanol fuel cells, where fuel crossing the membrane from anode to cathode wastes fuel and poisons the cathode reaction. A membrane that conducts protons well but resists methanol crossover addresses one of the central design tensions in liquid-fuel cell development.

The work fits into a broader international effort to replace or augment perfluorinated ionomers with cheaper, more tunable hydrocarbon membranes. Recent years have seen sulfonated polyimides, polybenzimidazoles, and polyarylene ethers modified with cross-linking, branching, acid-base blending, and a parade of inorganic fillers including graphene oxide, metal-organic frameworks, mesoporous carbon nitride, and various heteropolyacids. Phosphotungstic acid in particular has been anchored into chitosan films, trapped inside metal-organic frameworks, and covalently linked to mesoporous supports in earlier studies. What distinguishes the new report is the systematic pairing of chain architecture with filler loading: by comparing random and block copolymer matrices across a sulfonation series, the researchers could identify a formulation where polymer morphology and inorganic conduction reinforce each other rather than compete.

There remain familiar hurdles between laboratory membranes and fuel cell stacks. Polyoxometalates are water-soluble, and long-term leaching under real operating cycles, with their humidity swings and thermal excursions, will need to be demonstrated beyond the hydrolytic soaking tests reported here. Membrane-electrode assembly integration, chemical stability in the presence of peroxide radicals generated at the electrodes, and manufacturability at scale are all questions the Polymer Bulletin paper does not settle. The authors acknowledge support from the National Natural Science Foundation of China, and the study was conducted at the Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, with co-author Zhai also affiliated with Sushui Energy Technology, a Shanghai company, hinting at an interest in commercial translation.

Still, the result offers a template for how composite design can dissolve a classic trade-off. By letting an inorganic superacid carry part of the proton-transport burden, the polymer matrix can stay modestly sulfonated, and therefore tough and dimensionally stable, while the composite as a whole conducts protons at Nafion-level rates. A membrane that simultaneously delivers 0.161 siemens per centimeter, 15 megapascals of tensile strength, more than 1,600 hours of hydrolytic endurance, and methanol resistance is a rare combination in the hydrocarbon membrane literature. If follow-up work confirms that the phosphotungstic acid stays put over thousands of fuel cell cycles, sulfonated polyimide composites of this kind could move from polymer chemistry journals toward the membranes that actually sit inside hydrogen and methanol fuel cells, where the balance of properties, not any single figure of merit, decides what succeeds.

Subject of Research: Composite proton exchange membranes combining sulfonated polyimide with phosphotungstic acid for fuel cells

Article Title: Sulfonated polyimide/phosphotungstic acid composite proton exchange membranes with balanced properties for fuel cells

Article References: Zhai, F., Zhuang, L., Wei, X., Cao, F., & Zhao, S. (2026). Sulfonated polyimide/phosphotungstic acid composite proton exchange membranes with balanced properties for fuel cells. Polymer Bulletin, 83(12), Article 653. https://doi.org/10.1007/s00289-026-06725-7

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06725-7

Keywords: proton exchange membrane, sulfonated polyimide, phosphotungstic acid, fuel cells, composite membrane, proton conductivity, hydrolytic stability, block copolymer, methanol crossover, Keggin structure, hydrogen fuel, polymer electrolyte