Uncovering Fast Proton Conductors Based on Lattice Dynamics

Achieving high proton conductivity in inorganic solids is key for many electrochemical technologies, such as energy-efficient fuel cells and electrolyzers, proton batteries, and low-energy nanoelectronics. 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. We have mapped the structural, chemical, and dynamic properties of solid acids and ternary metal oxides 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 as key descriptors of local proton hopping, and select rigid unit phonon modes frequencies as the key descriptor of lattice flexibility. Lattice flexibility facilitates reorganization of the lattice as the proton is transferred from the donor to the acceptor sites. In solid acids, lattice flexibility manifests itself as the extent of the polyanion group rotations, and in ternary oxides as the flexibility of the lattice to bring donor and acceptor sites (oxygen pairs) close to each other dynamically by leveraging certain phonon modes. The lattice flexibility also correlates with the superprotonic transition temperature (T_sp). Using these descriptors, we have identified potential solid acid proton conductors, which go beyond the traditionally considered monovalent alkali cations in solid acids. Overall, our results indicate to the importance of lattice flexibility in increasing proton conductivity in inorganic compounds.