Construction of continuous proton channels via a 2D sulfonated COF for in-situ ionic self-crosslinked membranes in superior direct methanol fuel cell

High-performance proton exchange membranes (PEMs) that simultaneously possess high proton conductivity and low methanol permeability is crucial for direct methanol fuel cell (DMFCs) applications. Herein, a sulfonated covalent organic framework (COF–SO 3 H) was synthesized and incorporated into a sulfonated polybenzimidazole (SPBI) matrix to fabricate composite proton exchange membranes. Acid-base interactions between sulfonic acid and protonated imidazole groups under acidic conditions to form ionic self-crosslinked structure, effectively reducing free volume size to suppress methanol permeability. The hydrophilic COF–SO 3 H improved water uptake and facilitated continuous proton-conducting pathways through its two-dimensional structure. The optimized membrane exhibited a high proton conductivity of 106.13 mS cm −1 at 80 o C. Furthermore, the SPBI/COF–SO 3 H-1.5 % membrane with thickness of 10 μm achieved a low methanol crossover current density of 187.10 mA cm −2 and outstanding power density of 152.67 mW cm −2 under 2 M methanol concentration, significantly outperforming those of commercialized Nafion 115. This study presents a promising strategy for high-performance PEMs in DMFCs system.