Annealing-Induced Local Structure and Molecular Dynamics Effects Enhance the Proton Conductivity of a Bisimidazolium Diphosphonate Salt

To address current energy challenges, solid-state proton conductors for proton exchange membrane fuel cells have attracted significant attention. Understanding and improving proton conduction requires insights into both overall and local structure/molecular dynamics. However, the local structure and molecular dynamics of molecular crystal-based materials have not been explored sufficiently. In this study, we synthesized a novel proton-conducting molecular crystal with a stoichiometric acid–base pair, viz. bisimidazolium 1,5-pentylenediphosphonate (5DPA-2Im), which is a typical composition for molecular crystal-based proton conductors. We also investigated the effect of local structure and molecular dynamics on proton conduction by this molecular crystal. Upon heating, the proton conductivity of 5DPA-2Im increased and continued to increase over time when its temperature was maintained at 413 K. Thermogravimetric (TG) and nuclear magnetic resonance (NMR) analyses revealed that one imidazole molecule is removed from the 5DPA-2Im structure upon heating, which accelerates the molecular motion of other imidazole and 5DPA molecules. Furthermore, proton exchange between two phosphonic acid moieties in 5DPA occurs due to the generation of HPO3− and H2PO3 in the presence of substoichiometric amounts of imidazole, which are expected to play crucial roles in enhancing proton conductivity. These results suggested that, similar to polymer proton conductors, the local structures and molecular dynamics are also important in molecular crystalline proton conductors.