Direct and Indirect Effects of Fe-Incorporation in Nickel(oxy)hydroxide Materials for the Electrocatalytic Oxygen Evolution Reaction - Employing Constant pH/U Models for Deeper Insights.

In this study, we seek to deepen the understanding of the Fe effect in Ni-oxyhydroxide-mediated oxygen evolution reaction (OER) electrocatalysis in alkaline conditions, where extremely small amounts of Fe can have a dramatic impact on catalytic performance. For this purpose, Density Functional Theory (DFT) electronic structure calculations with implicit solvation description is employed in a constant pH/potential simulation framework. Nanoparticle models are considered for the nickel-based oxyhydroxide material with different degrees of Fe incorporation, and the pH/U-dependent interface structure is studied. It can be seen that Fe incorporation influences the total extent of oxidation and deprotonation, stabilizing oxo species at early states of the reaction even at lower potentials.  From the resting state models, we derive reaction energy profiles and O-O coupling barriers for three different OER mechanisms: water nucleophilic attack (WNA), intramolecular coupling (IMC), and the lattice oxygen mechanism (LOM). Each species is derived taking into account explicit change in protonation state and charge as a function of pH and potential. The results suggest direct and indirect modifications in Ni-oxyhydroxide reactivity and in the preferred OER pathway, which changes with Ni/Fe ratio. The results we present imply that synergy between Ni and Fe acid-base and redox properties is essential for efficient water oxidation/deprotonation and O-O bond formation.