Iron-Doped Nickel Phosphate as Synergistic Electrocatalyst for Water Oxidation

Electrochemical water splitting into hydrogen and oxygen has been regarded as one of the most promising approaches to produce clean hydrogen fuel using electricity generated from renewable energy sources such as solar energy, wind power, or hydropower etc. Recent findings have demonstrated significant potential of nonprecious, nickel-based electrocatalysts as efficient oxygen evolution reaction (OER) to replace traditional ruthenium (Ru) and iridium (Ir)-based precious metal catalysts. Here, for the first time, we report a novel three-dimensional iron-doped nickel phosphate catalyst by stepwise autologous hydrothermal growth of nickel phosphate (Ni:Pi) spontaneously from nickel foam (NF) followed by electrodeposition of iron hydroxide (denoted as Ni:Pi-Fe/NF). Our findings reveal that the incorporated iron could play strong synergistic effects on the OER activities of nickel phosphate in alkaline solution, delivering a current density of 10 mA cm^–2 at an extremely small overpotential (η) of 220 mV and extraordinary high current density of 500 mA cm^–2 at η = 290 mV in 1 M KOH, which is among the best Ni-based OER electrocatalysts to date. Furthermore, in a concentrated alkaline electrolyte (5 M KOH), the Ni:Pi-Fe/NF electrode can reach a high current density of 1 600 mA cm^–2 at an overpotential merely of 332 mV and shows excellent electrocatalytic stability in prolonged bulk water electrolysis, meaning it could highly meet the requirement of the industry alkaline electrolysis system. Mechanism investigations employing X-ray photoelectron spectroscopy (XPS), electrochemical polarization, contact angle measurement, and Raman spectra suggest strong interactions between Ni:Pi and Fe, with nickel oxyhydroxide (NiOOH) being the primary catalytic active site and nickel phosphate facilitating water adsorption. The iron doping changes the local Ni–O environments which synergistically enhance the Ni:Pi-Fe/NF catalytic activity toward OER