Insights into Hydration Enthalpies of Mixed Proton-Electron Conductors

Developing mixed proton-electron conductors (MPECs) is essential to accelerate the sluggish air electrode reaction kinetics of protonic ceramic electrochemical cells, for their unique conduction behaviors can amazingly extend the active reaction zone to the whole electrode surface. Hydration enthalpy plays a key role in determining the conduction behaviors of MPECs, but its underlying factors and modulation mechanisms are still unclear. In this work, an efficient and reliable strategy to theoretically predict the hydration enthalpies of MPECs is first proposed, and then, affecting factors of hydration enthalpies are investigated based on the calculation results on M-doped BaFeO_3-δ and Ba_0.5Sr_0.5FeO_3-δ series (M = Zr⁴⁺, Sn⁴⁺, Ti⁴⁺, and Nb⁵⁺). Remarkably, the volume per atom (VPA) is found as the decisive factor of hydration enthalpies. This finding prompts new thinking about the rational design of novel MPECs.