Excellent Overall Water Splitting Catalyst of Single Phase 6H Triple Perovskite Ba ₃ Co(Co 0.25 Ru 0.75 ) ₂ O ₉ at 1 A cm ⁻² for 1000 h

Developing efficient bifunctional electrocatalysts for overall water splitting (OWS) remains a critical challenge due to the divergent reaction mechanisms required for oxygen and hydrogen evolution reactions (OER and HER), especially in a single–phase material. Here, we present a novel self‐adaptive 6H triple perovskite oxide, Ba 3 Co(Co 0.25 Ru 0.75 ) 2 O 9 (BCCRO), as a highly active and durable alkaline OWS catalyst. BCCRO exhibits low overpotentials of 178 and 32 mV for the OER and HER at 10 mA cm −2 in 1.0 M KOH, respectively. A symmetric electrolyzer maintains stable performance for 1000 h at 1 A cm −2 . Operando X‐ray absorption spectroscopy reveals its dynamic surface restructuring processes: under OER conditions, Co 3+ and Ru 5+ are oxidized to Co 4+ and Ru 6+ , while during HER, they transform into metallic CoRu nanoparticles. Density functional theory calculations indicate that its high activity of OWS originates from synergy between high valence Ru 6+ and Co 4+ for OER, and between metallic Co and Ru for HER. This study presents a rare single‐phase oxide catalyst that achieves exceptional OWS performance through operando adaptation, providing a transformative framework for developing practical water‐splitting technologies.