Proton Tautomerism for Anhydrous Superprotonic Conduction in 1,2,3-Triazolium Dihydrogen Phosphate Crystal

Proton dynamics within molecular organic solids are crucial for energy-related technologies. Proton conductors for use as solid electrolytes in hydrogen fuel cells have been developed, elucidating the higher proton transport mechanism and establishing design guidelines for higher conduction. Many anhydrous proton conductors for proton transport utilizing molecular motion in solids have been studied; however, low-barrier conduction is challenging. In this study, we addressed proton tautomerism as a new guideline for proton conduction, rather than molecular motion. The key to facilitating low-barrier conduction is proton transport without molecular motion via dynamic interconversion between multiple tautomers. We demonstrated the effectiveness of proton-tautomerism strategy in 1,2,3-triazole dihydrogen phosphate crystal, which exhibited low-barrier, isotropic superprotonic conductivity exceeding 10^-3 S cm^-1. Both theoretical and experimental results confirmed that superprotonic conduction originates from proton tautomerism, demonstrating for the first time that proton tautomerism can serve as a design guide for highly efficient anhydrous proton conductors.