Hard X‑ray\nPhotoelectron Spectroscopy Probing\nFe Segregation during the Oxygen Evolution Reaction

NiFe electrocatalysts are among the most active phases\nfor water\nsplitting with regard to the alkaline oxygen evolution reaction (OER).\nThe interplay between Ni and Fe, both at the surface and in the subsurface\nof the catalyst, is crucial to understanding such outstanding properties\nand remains a subject of debate. Various phenomena, ranging from the\nformation of oxides/(oxy)hydroxides to the associated segregation\nof certain species, occur during the electrochemical reactions and\nadd another dimension of complexity that hinders the rational design\nof electrodes for water splitting. In this work, we have developed\nthe procedure for the quantification of chemical depth profiling by\nXPS/HAXPES measurements and applied it to two NiFe electrodes with\ndifferent porosities. The main outcome of this study is related to\nthe surface reconstruction of the electrodes during the OER, followed\nat two different depths by means of X-ray photoelectron spectroscopy.\nWe find that Fe initially segregates at the surface when exposed to\nambient conditions, resulting in the formation of an inactive FeO<sub><i>x</i></sub> phase. In addition, the porosity of the\ncatalyst plays a significant role in the segregation process and thus\nin the performance of the electrode. In particular, the higher porosity\nof the nanostructured sample is responsible for a more pronounced\ndiffusion of Fe from the subsurface to the surface with a more effective\nsuppression of the activity of the Ni<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>OOH phase. These results highlight\nthe importance of the fact that the chemical state of the surface\nof a multielement system is a snapshot in time, dependent on both\nexternal parameters, such as the applied potential and the adjacent\nelectrolyte, and the underlying bulk properties accessible with HAXPES.