Beyond Nafion with Fluorine-Free sPSU–sNIM Membranes: Nanostructured Proton Pathways for Harsh Fuel Cell Environments

In this study, nanocomposite membranes were developed by incorporating sulfonated nanoscale ionic materials (sNIMs) into a sulfonated polysulfone (sPSU) matrix for high-temperature polymer electrolyte membrane fuel cells (PEMFCs), with a focus on direct hydrogen fuel cell (DHFC) applications. The sNIMs, composed of silica nanoparticles functionalized with tethered sulfonic acid groups, were uniformly dispersed within the polymer matrix via solution casting. Structural and electrochemical characterizations demonstrated that the resulting membranes exhibit improved thermal and mechanical stability, enhanced hydration retention, and superior proton conductivity compared to pristine sPSU and recast Nafion. Remarkably, the optimized sNIM-3 formulation achieved 18 mS cm–1 conductivity at 120 °C and 30% RH, outperforming Nafion under identical conditions. Diffusion NMR and impedance spectroscopy revealed that the nanostructured ionic domains introduced by the sNIMs enable efficient proton transport predominantly via a Grotthuss-type hopping mechanism, even at low humidity and elevated temperatures. Fuel cell tests confirmed the exceptional performance of sNIM-3, making these membranes highly attractive fluorine-free candidates for next-generation PEMFCs.