Synergistic Interplay of Acceptor and Isovalent Co-Doping on BaZrO₃-Based Proton Conducting Oxides: A...

Acceptor-doped BaZrO₃ (BZO) with an ABO3-type structure is a promising proton-conducting oxide (PCO) for fuel cells and electrolyzers. However, dehydration at elevated temperatures significantly reduces proton concentration, hindering commercialization. While recent studies have shown that isovalent co-doping in acceptor-doped systems improves hydration performance, the dehydration remains unresolved due to a lack of fundamental understanding of these co-doping effects. In this study, first-principles density functional theory calculations were employed to systematically investigate how acceptor and isovalent co-doping affects the hydration performance of BZO at the atomic scale. Analysis of trivalent acceptors at the B-site ( $M_{Zr}^{\text{'}}$ ) shows that acceptors suppress dehydration by inhibiting oxygen vacancy formation through reduced M─O bonding lengths, with $M_{Zr}^{\text{'}}$ = Yb and Tm showing the most favorable hydration energy. Isovalent co-dopings at either the B-site ( $N_{Zr}^{\times }$ ) or A-site ( $N_{Ba}^{\times }$ ) further enhance hydration by stabilizing protons via increased distance between A-site cations and protons, with $N_{Zr}^X$ = Th and $N_{Ba}^X$ = Ca showing the most effective proton stabilization. The synergistic interaction of acceptors inhibiting oxygen vacancy formation and isovalent dopants stabilizing protons enables precise tailoring of hydration energy. Based on these complementary effects, we suggest four promising co-doping combinations (Yb─Ca, Yb─Th, Tm─Ca, and Tm─Th) as superior alternatives to conventional Y─Ce pairs. Our findings provide co-doping design guidelines for developing high-performance PCOs.