Direct Detection of Fe VI Water Oxidation Intermediates in an Aqueous Solution.

In situ detection of highly-oxidized metal intermediates is the key to identifying the active center of an oxygen evolution reaction (OER) catalyst, but it remains challenging for NiFe-based catalysts in an aqueous solution under working conditions. Here, by utilizing the dynamic stability of the Fe VI O 4 2- intermediates in a self-healing water oxidation cycle of NiFe-based catalyst, the highly-oxidized Fe VI intermediates leached into the electrolyte are directly detected by simple spectroelectrochemistry. Our results provide direct evidence that Fe is the active center in NiFe-based OER catalysts. Furthermore, it is revealed that the incorporation of Co into NiFe-based catalyst facilitates the formation of Fe VI active species, thus enhancing the OER activity of NiCoFe-based catalyst. The insights into the mechanisms for the sustainable generation of Fe VI active species in these NiFe-based catalysts lay the foundation for the design of more efficient and stable OER catalysts.