Tracking the dynamic and interactive metal oxidation changes in CoFe, CoNi, and NiFe bimetallic hydroxides for electrocatalytic oxygen evolution

Bimetallic hydroxides based on iron group metals (i.e., Fe, Co, and Ni) are well-known electrocatalysts for the oxygen evolution reaction (OER). Tremendous efforts have been devoted to investigating their structural and electronic changes under operating conditions to build the structure-activity relationship. It is recognized that the metal oxidation states in these bimetallic hydroxides play important roles in governing the electrocatalytic performance. Therefore, operando hard X-ray absorption spectroscopy (hXAS) is applied to track the dynamic and interactive metal oxidation changes in iron group bimetallic hydroxides under different chemical and electrochemical conditions. Observations of the energy shift at metal-K edges indicate metal oxidation changes in CoFeO_xH_y are chemically triggered and potential-driven prior to the OER onset. This contrasts with CoNiO_xH_y and NiFeO_xH_y, where metal oxidation changes mainly coincide with the OER. After the activation, the Co and Fe oxidation are relatively equal and synchronized in CoFeO_xH_y. Conversely, for NiFeO_xH_y and CoNiO_xH_y, Ni oxidation outweighs Fe oxidation and Co outweighs Ni oxidation, respectively. These findings highlight the functional roles of iron group metals in different bimetallic hydroxides and correlate their dynamic and interactive oxidation changes to their dynamic structural transformation and OER performance.