Role of Iron Incorporation in the Structural and Chemical Stability of Spinel Cobalt Oxide under Oxygen Evolution Reaction Conditions in Neutral Media

Abstract

Stable, earth-abundant catalysts for the oxygen evolution reaction (OER), which is a bottleneck in water electrolysis, operating in neutral media, are essential for advancing environmentally friendly water electrolysis and CO2 electrocatalytic reduction. However, the OER remains a significant kinetic bottleneck. In this study, the surface structure and chemical composition of spinel-type FexCo3-xO4 (0 ≤ x ≤ 2) nanoparticles before and after the OER stability test under neutral conditions were investigated to systematically examine the relationship between these nanoscale changes and the OER stability. Spinel-type FexCo3-xO4 was synthesized by a liquid-phase gelation method. All as-synthesized samples were spinel-type single crystals with particle sizes of 30 – 60 nm. STEM-EDS analysis revealed that iron substitution occurred preferentially at the particle surface as the iron content increased. The pristine Co3O4 exhibits a Co4+/Co3+ redox peak at 1.54 V vs. RHE. It is reported that the formation of Co4+ induces a phase transition to cobalt oxyhydroxide, which reverts to the spinel Co3O4 phase upon removal of potential. The intensity of this peak decreased with increasing iron content up to x = 0.5 and disappeared at x ≥ 1.0, indicating that iron incorporation suppresses the formation of Co4+ species. While the initial current density for the OER decreased when x exceeded 1.5. The high OER activities observed at x = 0.5 and 1 were maintained at 1.9 V vs. RHE for over 24 h, whereas those at x = 0, 0.1, and 2 rapidly decreased within a few hours. After the stability test, the crystallinities, particle sizes, chemical compositions, and Co valence states of all samples remained essentially unchanged. These results suggest that iron appears to be selectively substituted for the Co3+ species prone to oxidation within the spinel structure. This iron incorporation could suppress the formation of Co4+ species, which are regarded as inactive layers, significantly enhancing OER stability under neutral conditions.