Organic acid–base salts have attracted increasing attention as a promising candidate of anhydrous proton conductors. In this study, we have successfully disclosed the relationship between proton conductivity and hydrogen-bond (H-bond) interactions in such kinds of organic salts, composed of dicarboxylic acid and imidazole. We have grown high-quality single crystals of imidazolium succinate (Im-Suc) or glutarate (Im-Glu) with two-dimensional H-bonding networks and measured the proton conductivity within and perpendicular to the networks. On the basis of the observed “intrinsic” proton conductivities without grain boundary contributions, their relationship to the crystal structure and molecular arrangement was investigated in detail. Importantly, in both materials, the proton conductivities within the H-bonding networks are almost 2 orders of magnitude higher than those perpendicular to the networks, demonstrating that the proton conduction is highly mediated by the H-bonds. In addition, a suitable combination and arrangement of acid and base molecules for realizing high proton conductivity is discussed in terms of their proton donating/accepting abilities (pK a) and molecular motions. These results provide important insights into the effects of H-bonds on proton conductivity in this kind of anhydrous organic acid–base salts.

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Anhydrous organic crystalline materials incorporating imidazolium hydrogen succinate (Im-Suc), which exhibit high proton conduction even at temperatures above 100 °C, are attractive for elucidating proton conduction mechanisms toward the development of solid electrolytes for fuel cells. Herein, quantum chemical calculations were used to investigate the proton conduction mechanism in terms of hydrogenbonding (H-bonding) changes and restricted molecular rotation in Im-Suc. The local H-bond structures for proton conduction were characterized by vibrational frequency analysis and compared with corresponding experimental data. The calculated potential energy surface involving proton transfer (PT) and imidazole (Im) rotational motion showed that PT between Im and succinic acid was a rate-limiting step for proton transport in Im-Suc and that proton conduction proceeded via the successive coupling of PT and Im rotational motion based on a Grotthusstype mechanism. These findings provide molecular-level insights into proton conduction mechanisms for Im-based (or -incorporated) H-bonding organic proton conductors.

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