Thickness-dependent proton transport in Nafion-based nanofiber composite membranes: Hydration, chain mobility, and structural reorganization

Thin polymer electrolyte membranes (PEMs) can reduce ohmic resistance in fuel cells, but excessive thinning may adversely affect hydration and proton transport. Here, thickness-dependent structure–property relationships were investigated in Nafion-based composite membranes reinforced with phytic-acid-doped polybenzimidazole nanofiber mats (8–30 μm). The nanofiber framework enhanced mechanical strength and reduced oxygen permeability while maintaining water-vapor permeance comparable to commercial Nafion. Proton conductivity remained high for the 30 and 18 μm membranes but decreased at 13 μm and markedly at 8 μm, accompanied by reduced water uptake and suppressed swelling. Dynamic mechanical analysis showed a pronounced increase in the α-relaxation temperature for the 13 and 8 μm membranes, indicating restricted Nafion chain mobility. Small-angle X-ray scattering showed little thickness dependence under ambient conditions, whereas water immersion shifted the ionomer peak to lower q and enhanced low- q scattering in thinner membranes. The fitted FWHM showed no systematic thickness dependence, whereas enhanced low- q scattering suggested an increased contribution from larger-scale structures in the thinner membranes. These results indicate that proton-conductivity loss in very thin membranes arises from reduced water uptake together with restricted chain mobility and nonuniform hydration-induced structural reorganization.