Dual sulfonation engineering of cellulose acetate-based mixed-matrix membranes reinforced with sulfonated ZIF-8 toward sustainable direct methanol fuel cells

: Metal-organic frameworks (MOFs) have emerged as promising fillers for enhancing polymer electrolyte membranes in fuel cell applications. In this study, sulfonated poly(vinylidene fluoride-co-hexafluoropropylene)/cellulose acetate (SPvF/CA) composite membranes incorporating sulfonated ZIF-8 (SZ) were successfully fabricated via solution casting for direct methanol fuel cells (DMFCs). Structural, morphological, and thermal properties were comprehensively characterized by FTIR, XRD, SEM/EDS, TEM, AFM, and TGA, confirming successful membrane fabrication and uniform SZ dispersion. The optimized SPvFC 30 /SZ 5 membrane exhibited improved proton conductivity of 9.35×10 -3 S cm -1 , representing a remarkable enhancement over pristine CA (0.08×10 -3 S cm -1 ), together with improved hydrophilicity, water uptake, and oxidative stability. In addition, methanol permeability was reduced to 1.95×10 -7 cm 2 s -1, approximately 11-fold lower than Nafion®117, resulting in a 7.5-fold increase in electrochemical selectivity. Single-cell testing delivered a maximum power density of 75.58 mW cm -2 at 80 °C, approaching that of Nafion®117 (∼ 80 mW cm -2 ). These findings demonstrate that the developed SPvFC 30 /SZ 5 membrane is a cost-effective and high-performance proton exchange membrane for low-temperature DMFC applications.