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

Abstract

Fe 1−x Co 2 O 4 , 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 FeCo 2 O 4 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 Zn 0.4 Fe 0.6 Co 2 O 4 /NF is discovered to exhibit the highest OER performance, reaching an overvoltage of 330 mV at a current density of 50 mA cm 2 in 1 M NaOH. Zn 0.4 Fe 0.6 Co 2 O 4 / CNT/NF has a charge transfer resistance of 2.549 Ω and an active surface area of 526 cm 2 . 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 CoO 2 (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.