Impact of Acid–Base Amounts on Proton Conductivity and Molecular Dynamics of Phosphonic Acid-Modified Mesoporous Silica/Imidazole Composites

Proton-exchange membrane fuel cells (PEMFCs) have attracted considerable attention, owing to their potential to generate energy without pollutants. In this study, to achieve higher proton conductivity, a phosphonic acid-modified mesoporous silica (SBA-15)/imidazole composite was prepared, and the relationship between the proton conductivity, phosphonic acid-imidazole content, and the molecular motion of imidazole was examined. The composites were characterized by X-ray diffraction, N2 gas adsorption, solid-state 13C and 31P nuclear magnetic resonance (NMR) spectroscopy, and thermogravimetric analysis. Proton conductivity measurements using the same amount of phosphonic acid indicated that a higher imidazole concentration was favorable for achieving a higher proton conductivity. However, excessive phosphonic acid led to a decrease in the proton conductivity due to restriction of the molecular motion of imidazole (on average), as indicated by solid-state 2H NMR.