Mechanistic Insights into Hydration of Solid Oxides

Some of the solid oxide materials, used in solid oxide fuel and electrolysis cells, are known to conduct protons once they are hydrated. However, the mechanisms by which solid oxide materials get hydrated is not clear. By performing detailed density functional theory calculations, we investigate hydration of two typical solid oxides with a single-crystal structure - a proton-conducting yttrium-doped strontium zirconate (SZY) and an oxide ion-conducting yttria-stabilized zirconia (YSZ). We suggest a four-step process to understand the hydration of solid oxides - water adsorption on the surface, proton migration from the surface to bulk, proton migration in the bulk, and oxide ion vacancy migration in the bulk. The hydroxide ion migration with a lower energy barrier, compared to the proton hopping mechanism, is proposed for the conduction of proton between the surface and subsurface of the perovskite oxide. Our analysis provides mechanistic insights into the hydration of single-crystal SZY and nonhydration of single-crystal YSZ. The study presented here not only explains the hydration of materials but also provides the importance of structural rearrangement when a proton is incorporated into the bulk of the solid oxide material.