Investigation of Zn-Substituted FeCo₂O₄ for the Oxygen Evolution Reaction and Reaction Mechanism Monitoring through In Situ Near-Ambient-Pressure X-ray Photoelectron Spectroscopy

Zn-substituted iron cobaltite spinel (ZnxFe1–xCo2O4, 0 < x < 0.6 in intervals of 0.2) on nickel foam (NF) is synthesized through a hydrothermal process, and carbon nanotubes (CNTs) are embedded in NF to provide additional conductivity and nucleation sites for the catalyst. Zn ions are used as a substitute for Fe in FeCo2O4 to increase the material’s electrochemical surface area and provide more active sites for electron transport at the electrode–electrolyte interface. Sufficient Zn substitution greatly promotes oxygen evolution reaction (OER) activity, and Zn0.4Fe0.6Co2O4/NF is discovered to exhibit the highest OER performance, reaching an overvoltage of 330 mV at a current density of 50 mA cm2 in 1 M NaOH. Zn0.4Fe0.6Co2O4/CNT/NF has a charge transfer resistance of 2.549 Ω and an active surface area of 526 cm2. In situ near-ambient-pressure X-ray photoelectron spectroscopy directly confirms the variation of the electrode surface composition during OER and shows that the highest percentage of CoO2 (about 51%) grows on the catalyst surface, resulting in increased surface oxygen adsorption. We identify Co(IV) as an intermediate in the OER adsorption, forming a superoxide species that is the key intermediate in oxygen gas generation.