Cation‐Selective Defects Engineering in A‐Site Ordered Layered Perovskites for High‐Performance Reversible Protonic Ceramic Cells

Reversible protonic ceramic cells facilitate efficient chemical-electrical energy interconversion, advancing renewable energy utilization. Commercial viability, however, demands intermediate-to-low temperatures (ILT, 400-600 °C) operation, currently constrained by air electrode performance. A-site ordered layered perovskite PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) promises, yet faces activity and stability issues at ILT. Cation defects effectively tune defect structures in simple perovskites, boosting electrochemical performance, but their specific effects in A-site ordered perovskites with dual A-site environments remain unexplored. Here, A-site cation-selective defects are engineered to tune PBSCF's performance, with Pr-deficient (Pr_0.95Ba_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ, p-PBSCF) and Ba/Sr-deficient (Pr(Ba_0.5Sr_0.5)_0.95Co_1.5Fe_0.5O_5+δ, bs-PBSCF) variants revealing distinct defects-performance relationships. Pr defects weaken Co─O covalency to activate Co sites, enhancing oxygen electrocatalytic activity. Concurrently, it lowers oxygen vacancy concentration, inhibiting hydration-induced lattice expansion. This stabilizes Ba─O/Sr─O bonds and mitigates Ba/Sr segregation, enhancing stability. However, the reduced oxygen vacancy concentration inhibits the material's hydration, lowering proton conduction and thus restricting activity enhancement. In contrast, Ba/Sr defects not only weaken Co─O covalency to activate Co sites, but also increase oxygen vacancy concentration, promoting proton and oxygen-ion transport, thereby significantly enhancing electrode activity. Furthermore, despite increased hydration, bs-PBSCF's larger-radius cation defects yield a smaller unit cell versus p-PBSCF, further strengthening Ba─O/Sr─O bonds and inhibiting Ba/Sr segregation, thus leading to superior stability.