Intercalation of ions from the electrolyte is a common phenomenon in electrocatalytic systems. Although catalytically inactive, literature suggests that electrolyte ions may impede or enhance the rate of electrode reactions. In Ni-Fe oxyhydroxide, an attractive catalytic material for the oxygen evolution reaction (OER) in alkaline media, the catalytic activity increases with the size of cation of the electrolyte (Li⁺<Na⁺<Cs⁺). The intercalation of electrolyte ions into the lamellar structure of the catalyst has been confirmed through microgravimetric studies based on quartz crystal microbalance. However, mass changes during such measurements may be caused by cations, anions, and/or water molecules and therefore microgravimetric measurements do not specially probe a particular electrolyte constituent. Here, we studied this intercalation process by simultaneously monitoring the Ni oxidation state and the Na+ and water content in the catalyst layer while bringing the catalyst to its active state via operando soft X-ray absorption spectroscopy. This allows us to probe specifically which species intercalate in NiFeO_x oxygen evolution catalysts in NaOH (0.1 M). The intensity of the sodium K-edge absorption spectrum can be directly related to the concentration of Na⁺ ions in the probed volume. The spectral shape also reveals information about the hydration shell of the Na⁺ cation. 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. This combination of reversible and irreversible intercalation could be of significant interest for catalyst design, as it suggests that pre-treatment of the electrode in a different electrolyte than the one used in the electrolysis process could be used as a tuning parameter for optimizing the electrocatalytic performance. Ni L-edge XAS showed that Ni²⁺ dominates at 1.0 V_RHE, whereas a mixture of Ni^2+/3+/4+ is formed at 1.7 V_RHEin the Ni_0.8Fe_0.2O_x electrocatalyst. The reversible increase in intercalated Na⁺ does not lead to an increase in in the water content of the catalyst layer, suggesting that the Ni redox and accompanying Na⁺ intercalation have little effect on the catalyst morphology.

Hanna Trzesniowski

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