Manipulating the Spin State of Spinel Octahedral Sites via a π–π Type Orbital Coupling to Boost Water Oxidation

Abstract Spin state is often regarded as the crucial valve to release the reactivity of energy‐related catalysts, yet it is also challenging to precisely manipulate, especially for the active center ions occupied at the specific geometric sites. Herein, a π–π type orbital coupling of 3d (Co)‐2p (O)‐4f (Ce) was employed to regulate the spin state of octahedral cobalt sites (Co Oh ) in the composite of Co 3 O 4 /CeO 2 . More specifically, the equivalent high‐spin ratio of Co Oh can reach to 54.7 % via tuning the CeO 2 content, thereby triggering the average e g filling (1.094) close to the theoretical optimum value. The corresponding catalyst exhibits a superior water oxidation performance with an overpotential of 251 mV at 10 mA cm −2 , rivaling most cobalt‐based oxides state‐of‐the‐art. The π–π type coupling corroborated by the matched energy levels between Ce t 1u /t 2u ‐O and Co Oh t 2g ‐O π type bond in the calculated crystal orbital Hamilton population and partial density of states profiles, stimulates a π‐donation between O 2p and π‐symmetric Ce 4f yz 2 orbital, consequently facilitating the electrons hopping from t 2g to e g orbital of Co Oh . This work offers an in‐depth insight into understanding the 4f and 3d orbital coupling for spin state optimization in composite oxides.