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 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 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 0.95 Ba 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+δ , p‐ PBSCF) and Ba/Sr‐deficient (Pr(Ba 0.5 Sr 0.5 ) 0.95 Co 1.5 Fe 0.5 O 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.