Operando Tracking the Interactions between CoOx and CeO2 during Oxygen Evolution Reaction

Abstract CeO 2 greatly enhances the electrocatalytic oxygen evolution reaction (OER) activity of CoO x , though the enhancement mechanism beyond this synergy is yet to be understood. Here, operando hard X‐ray absorption spectroscopy (hXAS) is applied to monitor the Co K edge and Ce L 3 edge in CoO x /CeO 2 to shed light on the evolution of the Co and Ce oxidation states during OER. In addition, ex situ soft XAS (sXAS) characterizations provide information on the irreversible surface‐specific transformations of the Co L 3 edge as well as of the O K edge. Combining the operando and ex situ spectroscopic characterizations with comprehensive electrochemical analyses, it is confirmed that CeO 2 is not the active center for the OER. However, coupling CeO 2 with CoO x introduces significant modifications in the Co and O species at the CoO x surface and alters the flat band potential (E fb ), leading to more favorable Co oxidation state transformations during OER and possibly modifying the preferential reaction pathway. This work establishes the connections between electronic structures, Co oxidation state and the OER reaction mechanism for CoO x /CeO 2 composites electrodes.