Enhanced Electrocatalytic Oxygen Evolution Reaction through Inductive Effects of Cobalt in Nickel Orthophosphates

Abstract

Efficient oxygen evolution reaction (OER) electrocatalysts are crucial for various electrochemical processes, including water splitting. In this study, we investigated the effect of cobalt substitution on the OER activity of nickel orthophosphate by employing a straightforward coprecipitation synthesis route. A series of Ni/Co orthophosphate catalysts, Ni_3–xCo_x(PO₄)₂·8H₂O (x = 0, 1, 1.5, 2, and 3), were prepared. Among them, NC15 (Ni/Co = 1.5) exhibited the lowest overpotential of 321 mV for achieving 10 mA/cm² and a turnover frequency (TOF) of 605.9 s^–1 at 600 mV vs RHE, with a Tafel slope of 100.15 mV dec^–1. This enhanced performance can be attributed to the synergistic interplay of the highest TOF; disorder in the cobalt and nickel cations in the crystallographic sites within the crystal structure; and, importantly, a cathodic shift in the Ni oxidation peaks observed in the Co-containing catalysts. Here, we report the novel observation of an inductive effect of cobalt within the nickel phosphate structure, which influences the Ni redox behavior and contributes significantly to improved OER activity. XPS analysis revealed shifts in the binding energies of Co 2p, O 1s, P 2p, and Ni 2p, indicating a change in the electronic structure of the elements. DFT calculations suggest that Co substitution leads to band gap narrowing and charge redistribution, with excess charge accumulating at the Co site and transferring to the neighboring oxygen atoms.