Isotropic Proton Conduction in an Anisotropic Crystal: The Role of Molecular Rotational Dynamics in Imidazolium Dihydrogen Phosphate

Abstract

Understanding the intrinsic anhydrous proton conductivity and conduction mechanisms of solid electrolytes is of fundamental academic importance for their application in next-generation, non-humidified fuel cells. In this study, we investigated the anhydrous proton conductivity of single crystals of imidazolium dihydrogen phosphate (1), a material in which rotational motion of both the acid and base components is expected to contribute to the conduction mechanism. By controlling the crystallization conditions, we successfully and selectively prepared two polymorphs: a thermodynamically stable, non-centrosymmetric orthorhombic form (1A) and a metastable, centrosymmetric monoclinic form (1B). Single-crystal X-ray diffraction analysis of the stable phase 1A revealed an anisotropic crystal structure with one-dimensional hydrogen-bonding phosphate chains connected with N–H···O hydrogen bonds with imidazolium. Nevertheless, the intrinsic anhydrous proton conductivity, evaluated using single crystals, exhibited nearly isotropic behavior with minimal directional dependence. Solid-state ²H NMR measurements revealed that the imidazolium cations undergo active libration motion even at room temperature. We conclude that the dynamic motion of the imidazolium cations, in addition to the known rotational motion of the H₂PO₄⁻ anions, effectively connects the proton conduction pathways, resulting in isotropic conductivity despite the anisotropic hydrogen-bond network. These findings demonstrate that the cooperative rotational dynamics of both acid and base components is a valid strategy for designing high-performance anhydrous proton-conducting materials.