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

•ABPBI-mesoporous silica composite membranes were synthesized.•Ionic liquid functionalization of silica improved physical properties of nanocomposites.•Despite low acid doping ionic liquid improved the proton conductivity of the membranes.•Proton conductivity of 6.74 × 10-2 S cm−1 (e.g., 0.0674 S cm−1) obtained at 150 °C which is similar to that of Nafion at 30 °C.

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 3.40 nm and size 90–150 nm) and SBA-15 (USB, pore diameters of 9.60 nm and size 800–1100 nm). The high degree of dispersion of silica particles in ABPBI matrix improved the storage modulus of the membrane comprising 5 wt% 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 5 wt% functionalized UMC (dA5MIL) at 150 °C, which is similar to the room-temperature conductivity of Nafion at 100% RH.

Graphical abstractThe presence of ionic liquid on the surface and pore of the mesoporous silica modifies the proton conduction mechanism of the phosphoric acid doped ABPBI membrane thereby increasing the proton conductivity of the membrane despite low phosphoric acid doping amount.Figure optionsDownload full-size imageDownload high-quality image (206 K)Download as PowerPoint slide