Simultaneous Activation of Different Coordination Sites in Single-Phase FeCoMo₃O₈ for the Oxygen Evolution Reaction

ABMo(3)O(8) is an emerging oxygen evolution reaction (OER) electrocatalyst, exhibiting dual coordination sites for transition metals with good conductivity. However, it is unclear which sites are active for the OER and how to activate them both. Herein, we demonstrated experimentally that only tetrahedral (Td) Co sites are highly active in Co2Mo3O8, and octahedral (Oh) sites are activated by introducing Fe. Various synchrotron X-ray based spectroscopies confirmed the allocation of Co2+ at Td and Fe2+ at Oh sites. The dual activation of different sites improved the OER efficiency with overpotentials of 308 mV@10 mA cm(-2) and 361 mV@100 mA cm(-2). This unique structure with corner-shared T-d Co and O-h Fe in high spin states increases the active site numbers, produces synergistic effects, optimizes the adsorption of intermediates, and creates an unobstructed spin channel for electron transfer. This work provides an effective strategy to design a pair of OER catalysts by coordination engineering.