Incorporation of Iron into Nickel-Based Phosphide as a Forward-Looking Electrocatalyst for the Oxygen Evolution Reaction in Metal-Air Batteries

Abstract

Electrolyzers as well as metal-air batteries are struggling with the slow kinetics of the oxygen evolution reaction during the charging process. Researchers all over the world expend a lot of effort to find appropriate materials catalyzing this reaction in an efficient way. Efficiency is hereby described by overpotential, particularly by the activation overpotential which is obliged to be as low as possible. Oxides of precious metals like iridium and ruthenium are commonly known for their exceptional ability to reduce the OER’s overpotential in polymer electrolyte membrane electrolyzers. Unfortunately, these metals are very rare and thus expensive. To leverage a common use of metal-air batteries in future, OER-catalysts based on inexpensive materials like transition metals are in the focus of today’s research. Especially, nickel and iron are abundant and relatively inexpensive metals. Previous studies have demonstrated that double layered hydroxides containing both nickel and iron are more active than the same compounds containing only one transition metal. Iron is supposed to be the active site in such catalytic materials. Hydroxides of nickel/iron are already well-known for their good performance in alkaline medium, whereas phosphides are just catching attention more and more. Nickel phosphide achieves a very low overpotential of 290 mV at a current density of 10 mA cm-2 in 1 M KOH at 25 °C which is roughly on a level with iridium(IV) oxide (IrO2). Starting from a simple and harmless synthesis route leading to nickel phosphide we added an iron precursor to obtain a nickel-iron phosphide. For this purpose we adjusted the sintering temperature affecting the final composition and morphology. The obtained performance exceeds state-of-the-art iridium black catalysts. To this end we investigated the modified morphology and the kinetics of the synthesized OER-catalyst. These results from physical characterization and electrochemical measurements by use of a rotating disk electrode will be shown.