DFT Investigation on Pure and Doped Fe-Oxyhydroxide Clusters for Homogeneous, Electrocatalytic Oxygen Evolution Reaction

Fe(III) oxyhydroxides are promising electrocatalysts due to their redox properties, enhanced surface reactivity, and natural abundance as minerals, combined with their abilities to modulate the catalytic activities in the presence of dopant transition metal atoms, specifically Ni and Co. In this study, we investigate the electrocatalytic oxygen evolution reaction (OER) using Fe-oxyhydroxide clusters in a homogeneous aqueous medium by employing density functional theory (DFT). A water molecule coordinated to the Fe center of the Fe(III) oxyhydroxide cluster was converted to oxygen in a few sequential stepwise electron-transfer reactions analogous to the conventional OER. We first studied the free energy changes of the OER using small Fe(III) oxyhydroxide clusters and evaluated the effects of doping with Co, Ni, and Mn on their OER activity. Our investigations show that the OER activity improves in Fe2 dimers with more terminal −OH ligands compared to cationic cases, and the neutral FeCo dimer exhibited the best performance, indicating better OER activities in neutral to alkaline medium. The OER activities of larger δ-Fe13 Keggin clusters showed reduced OER activities compared to the Fe2 dimers and Fe3 trimers; however, Co(III) doping enhanced the OER activity, showing an overpotential of ∼1.07 V. The better performance of the Co(III) doped Fe(III) oxyhydroxide clusters was then rationalized in terms of the stabilities of oxyl radical species centered on the Co(III) center compared to the Fe(III) center of the oxyhydroxide clusters. These findings may offer insights into optimizing Fe-oxyhydroxide catalysts from natural minerals or sustainable hydrogen production in homogeneous media.