First-principles evaluation of proton hopping descriptors in oxides

Proton-conducting oxides (PCOs) are key materials used as ionic conductors for energy conversion technologies. Existing research efforts on PCO optimization and discovery generally focus on complex perovskite oxides that require doping and alloying to engineer oxygen deficiency and high proton conductivity. However, the variety of chemical compositions and coordination environments in oxides pose challenges for efficient materials design.

In this computational study, we build a database of simplified motifs to elucidate the relationship between fundamental materials chemistry and proton kinetics. Specifically, we focus on the zincblende crystal structure as a proxy for tetrahedral metal-oxide (M-O) coordination environments. We systematically quantified the effects of cation type, oxidation states, and M-O bond lengths on the proton hopping barrier, and found strong M-O bonds and metal cations with large and variable oxidation states (e.g., Mo⁶⁺, V⁵⁺) lead to smaller proton hopping barriers. By mapping the candidate cations and their preferred bond geometries onto the materials databases like the Inorganic Crystal Structure Database and Materials Project, we identified real materials containing the corresponding metal-oxide units.

In general, we observed good agreement between the calculated proton hopping barriers obtained in real crystal structures and those predicted by our motif database. We also discuss limitations of our model and possible future extensions to improve its predictive capabilities. Overall, our model serves as a first step for rational design and quick screening of energy efficient PCOs.