Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen‐Bonded Phosphate Network

Utilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10−4 S cm−1) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three‐dimensional (3D) hydrogen‐bonded (H‐bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H‐bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10−3 S cm−1 at ≈351 K, which is the first example for organic molecular crystal. Variable‐temperature X‐ray structural analysis and solid‐state 2H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H‐bonded phosphate network. The single‐crystalline 1 : 2 salt of imidazole and phosphoric acid exhibits isotropic anhydrous superprotonic conductivity exceeding 10−3 S cm−1, which is observed for the first time in organic molecular crystals. Significant 3D molecular motion of imidazolium cations was revealed to accelerate proton conduction via the 3D hydrogen‐bonded phosphate network by the depolarization effect.