The practical application of ${\mathrm{BaCeO}}_3$-based electrolytes is limited by their poor chemical stability in proton-conducting solid oxide fuel cells. Commonly employed $B$-site doping strategies typically improve proton transport with limited improvement in stability. Recent experiments show that $A$-site Ca doping can simultaneously enhance both properties. Here, through first-principles calculations and mechanistic analysis of Ca-doped ${\mathrm{BaCeO}}_3$, we identify the synergistic roles of small-radius, high-electronegativity $A$-site dopants in governing proton transport and chemical stability in perovskite electrolytes. We show that the higher electronegativity of an $A$-site dopant weakens the $A$-O ionic bonding, facilitating oxygen-vacancy formation and enhancing proton uptake by increasing the basicity. This weakened A-O interaction also suppresses the formation of impurity phases and reduces the adsorption strength of acidic gases such as ${\mathrm{CO}}_2$ and ${\mathrm{SO}}_2$. The lattice contraction induced by the smaller ionic radius improves thermal stability and can enhance proton diffusion in systems where proton transfer is the rate-limiting step. Furthermore, we find that Ca surface segregation can mitigate grain-boundary resistance effects. Our results demonstrate the advantages of $A$-site Ca doping in Ba-based electrolytes, clarify the mechanisms by which small-radius, high-electronegativity dopants influence proton transport and chemical stability, and provide guidance for the design of high-performance proton-conducting electrolytes.

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• Defects
• Doping effects
• Structural properties
• Energy materials
• First-principles calculations