Spin-coordinated mixed-valence transport via indium-induced perovskite-spinel heterointerface in oxygen electrode for reversible protonic ceramic cells

Abstract Achieving efficient oxygen electrocatalysis in protonic ceramic electrochemical cells requires more than accelerating surface reactions, which demands precise coordination of electronic and spin states across the bulk-interface continuum. Conventional strategies based on vacancy engineering or catalyst infiltration increase active sites, yet they do not intrinsically couple bulk charge transport with interfacial redox kinetics. Here, we introduce spin-coordinated heterointerface engineering as a general design principle, exemplified by In-induced self-assembly of a Co 3 O 4 spinel layer on Pr 0.5 Ba 0.5 Co 0.7 Fe 0.3 O 3-δ (PBCF), forming PBCFIn05. Indium incorporation triggers selective Co migration and interfacial spinel reconstruction, which in turn drives a bulk spin-valence reconfiguration from predominantly high-spin Co 3 ⁺ to mixed-spin Co 3 ⁺/Co 4 ⁺. This evolution establishes complementary double-exchange conduction in the perovskite bulk and small-polaron hopping within the Co 3 O 4 spinel, creating a continuous mixed-valence pathway that synergistically lowers the activation barriers for both oxygen reduction and oxygen evolution. These results demonstrate that self-assembled spin-active heterointerfaces provide a powerful route to overcome intrinsic spin-selection bottlenecks in ceramic electrocatalysis and offer a broadly applicable platform for advanced solid-state energy conversion systems.