Tailoring local free volume to establish continuous hydrophilic proton transport channels enabling high-performance direct methanol fuel cells

Sulfonated polybenzimidazole (SPBI) is a promising proton exchange membrane for direct methanol fuel cells (DMFCs) owing to its excellent methanol resistance. However, conventional strategies to regulate its intrinsic free volume rely on random pore generation, which lacks precise cavity control and fails to construct long-range continuous hydrophilic transport pathways, thus limiting cell performance. Herein, we report a “grafting–template molecule removal” strategy to precisely tailor the local free volume of SPBI membranes. Amine templates with varying chain lengths are covalently grafted onto sulfonic acid side groups and then selectively removed, creating controllable sub-nanoscale cavities without altering the SPBI chemical structure. This approach increases the fractional free volume while avoiding excessive cavity enlargement, effectively enhancing water uptake and hydration number, which in turn builds continuous proton-conducting hydrophilic networks. The optimized TR-DMOA-SPBI membrane simultaneously achieves high proton conductivity (0.124 S cm −1 ), low methanol permeability (15.17 × 10 −7 cm 2 s −1 ), and good dimensional stability, yielding a peak power density of 137.6 mW cm −2 in single-cell DMFC tests—significantly outperforming pristine SPBI and commercial Nafion 115. Our work demonstrates that molecular-level free-volume engineering via removable templates offers a versatile, additive-free route to overcome the classic conductivity-methanol permeability trade-off in sulfonated polymer membranes.