Development of Chitosan–Sulfonated Polystyrene–Sulfonated Nanosilica Nanocomposite Proton Exchange Membranes for Direct Methanol Fuel Cells

Abstract In this study, proton exchange membranes (PEMs) based on chitosan, sulfonated polystyrene (SPS), and pristine or modified nanosilica were fabricated for direct methanol fuel cell (DMFC) applications. Membranes containing different SPS contents were prepared, and polystyrene was sulfonated to an optimal degree of sulfonation determined by evaluating its hydrolytic stability. Proton conductivity and methanol permeability were measured to determine the membrane selectivity parameter, defined as the ratio of proton conductivity to methanol permeability. Among the prepared membranes, the membrane containing 40 wt.% SPS exhibited the highest selectivity parameter (39,908 S. s. cm − 3 ) and was therefore identified as the optimal composition. To improve membrane performance, nanocomposite membranes containing 2 wt.% silica nanoparticles (SiO 2 ) or sulfonated silica nanoparticles (S-SiO 2 ) were separately prepared using the optimal chitosan/SPS blend. The incorporation of SPS increased the glass transition temperature and decreased the crystallinity of the chitosan matrix due to electrostatic interactions between the –SO 3 H groups of SPS and the –NH 2 groups of chitosan. Furthermore, the nanocomposite membranes exhibited higher proton conductivity and lower methanol permeability compared with the optimal blend membrane. The membrane containing S-SiO 2 nanoparticles showed the highest selectivity (54,010 S. s. cm − 3 ), indicating its strong potential as an alternative proton exchange membrane for DMFC applications.