Uncovering Fast Solid-Acid Proton Conductors Based on Dynamics of Polyanion Groups and Proton Bonding Strength

Achieving high proton conductivity in inorganic solids is key for many electrochemical technologies, such as low-energy nanoelectronics and energy-efficient fuel cells and electrolyzers. A quantitative understanding of the physical traits of a material that regulate the Grotthuss mechanism of proton diffusion is necessary for accelerating the discovery of new proton conductors in these technologies. In this work, we have mapped the structural, chemical, and dynamic properties of solid acids to the mechanistic steps of proton diffusion, by performing ab initio molecular dynamics, phonon spectra and atomic structure calculations. We have identified the donor-hydrogen bond lengths and the acidity of donor/acceptor groups as key descriptors of local proton hopping, and the vibrational frequencies of the cation framework as the key descriptor of lattice flexibility to facilitate rotations of polyanion groups and long-range proton migration in solid acid proton conductors. The lattice flexibility also correlates with the superprotonic transition temperature (T_sp). Using these descriptors, we have identified promising solid acid proton conductors with monovalent as well as divalent and trivalent cations, such as Ag⁺, Sr²⁺, Ba²⁺ and Er³⁺ cations, which go beyond the traditionally considered monovalent alkali cations (Cs⁺, Rb⁺, K⁺, NH₄⁺) in solid acids.