Reversible and Irreversible Cation Intercalation in NiFeO Oxygen Evolution Catalysts in Alkaline Media

For electrocatalysts with a layered structure, ion intercalation is a common phenomenon. Gaining reliable information about the intercalation of ions from the electrolyte is indispensable for a better understanding of the catalytic performance of these electrocatalysts. Here, we take a holistic approach for following intercalation processes by studying the dynamics of the catalyst, water molecules, and ions during intercalation using operando soft X-ray absorption spectroscopy (XAS). Sodium and oxygen K-edge and nickel L-edge spectra were used to investigate the Na⁺ intercalation in a Ni_0.8Fe_0.2O electrocatalyst during the oxygen evolution reaction (OER) in NaOH (0.1 M). The Na K-edge spectra show an irreversible intensity increase upon initial potential cycling and a reversible intensity increase at the intercalation potential, 1.45 V_RHE, coinciding with an increase in the Ni oxidation state. Simultaneously, the O K-edge spectra show that the Na⁺ intercalation does not significantly impact the hydration of the catalyst.