Cobalt-iron (oxides) water oxidation catalysts: Tracking catalyst redox states and reaction dynamic mechanism

Abstract

Developing earth-abundant materials to replace the traditional noble metals in water splitting to meet industrial requirements remains a challenge. Cobalt-iron (oxides) have been widely studied as electrocatalysts for the oxygen evolution reaction (OER), yet our understanding of the OER dynamic reactivity related to the oxidation state changes as well the adsorption energies of surface species on the metal surface linked to the water oxidation are not well-documented. In this work, a facile chemical reduction process is developed for preparation of Co-only, Co3Fe7 alloy, and Fe-only catalysts. We use X-ray photoelectron spectroscopy (XPS) and in-situ X-ray absorption spectroscopy (XAS) to evaluate metal valences and the dynamics of the oxidation state changes of the electrocatalysts in 0.1 M KOH solution, which disclose that about 20% of the Co centers get oxidized in Co-only from the oxidation state of +2 to +3/+4, while only 1% reach to +3 valence for the Co3Fe7 catalyst under cyclic voltammetry (CV) operation. The small edge changes of Fe centers in Fe-only result in negligible changing the oxidation state. Density-functional theory (DFT) calculation predicts the mechanism of OER performance, which indicates that the OER activity largely relies on the metal oxidation states on the surface of catalysts. Co3O4 on the surface of Co-only catalyst presenting the most positive d-band center and the fewest eg electron contributes to the highest OER activity. Fe-only coated by γ-Fe2O3 shows the lowest OER performance due to the weakest oxygen adsorption energy of γ-Fe2O3 as well as the poor electrical conductivity of FeOOH evolved after operation. Co3Fe7 exhibiting medium OER activity is aroused by the co-existence of CoO and γ-Fe2O3, wherein Co2+ is less active than Co3+. Introducing Fe in Co matrix could depress the formation of Co cations with high oxidation state in as-prepared catalysts, which is not favorable for oxygen production.