Proton Conduction Mechanisms in Spinel Compounds: A First-Principles Study of Structural and Elemental Effects

Proton-conducting oxides are vital for environmentally friendly electrochemical devices, such as protonic ceramic fuel cells. However, high proton conductivity in oxides has been almost exclusively observed in perovskite structures. In this study, we have performed systematic first-principles calculations to elucidate proton conduction mechanisms in normal spinel compounds AB₂O₄, which are promising candidates for proton-conducting oxides. Our results reveal that in the spinel structure, protons occupy the octahedral interstices of the oxygen sublattice and diffuse three-dimensionally via a combination of proton rotation and hopping. The hopping energy barrier increases with the volume of the octahedral interstice, whereas the rotation energy barrier depends on the A-site cation displacement during proton migration. Notably, these energy barriers are reversed in A²⁺B₂³⁺O₄^2– and A⁴⁺B₂²⁺O₄^2– compounds. The magnitudes of these barriers are comparable to those of cubic perovskites, and our analysis suggests that the activation energy of proton diffusion is minimized when the tolerance factors for spinels lie between 0.9 and 1.0. These findings provide design guidelines for the development of proton-conducting spinel oxides.