Effects of ionic liquid-functionalized mesoporous silica on the proton conductivity of acid-doped poly(2,5-benzimidazole) composite membranes for high-temperature fuel cells

This work involves the comparative study of the effect of pore diameter and size of the unmodified and ionic liquid (IL)-functionalized mesoporous silica on the proton conductivity of phosphoric acid (PA)-doped poly(2,5-benzimidazole) (ABPBI) membranes. Two varieties of silica have been selected for this purpose: MCM-41 (UMC, pore diameters of 3.40nm and size 90-150nm) and SBA-15 (USB, pore diameters of 9.60nm and size 800-1100nm). The high degree of dispersion of silica particles in ABPBI matrix improved the storage modulus of the membrane comprising 5wt% IL-functionalized USB (dA5SIL) by 213% compared to that of the PA-doped ABPBI membrane at 250°C. The PA doping amounts of the USB-based composites are lower than those of the UMC-based composites, in spite of the larger pore diameter of USB. This is possibly due to the surface tension of PA, the larger size of USB compared to UMC and the inferior dispersion of USB in the ABPBI matrix. A maximum proton conductivity of 6.74 × 10^-2 S cm^-1 (e.g., 0.0674 S cm^-1) has been obtained for the PA-doped composite membrane comprising 5wt% functionalized UMC (dA5MIL) at 150°C, which is similar to the room-temperature conductivity of Nafion at 100% RH.