Pre-treated carbon additive enables reduction of metal loading in CoNiFe oxide-based OER electrocatalysts while maintaining performance

Abstract

Mixed oxides (MO) of Co, Ni, and Fe have emerged as promising catalysts for the oxygen evolution reaction (OER) in alkaline water electrolysis (AWE). Although they are significantly cheaper than the precious metal catalysts used for proton exchange membrane (PEM) water splitting, the limited availability of Co and the exceptionally poor environmental footprint of Co and Ni mining remain challenges for scaling up AWE capacities. In this context, we present a promising approach toward reducing the metal content while preserving the catalytic performance. Based on a recent work on CoNiFeO_x OER catalysts from our group, admixture of pre-treated Vulcan carbon XC-72R (VC) at different synthesis stages in various loadings was evaluated in view of material structure and electrocatalytic activity. Attractive performance characteristics were obtained for the optimized MO-VC composite material, e.g., a mass activity of 370 A $g_{CoNiFe}^{-1}$ at 1.56 V vs. RHE and a Tafel slope of 39.0 mV dec⁻¹. Stable long-term performance under practical AWE conditions was confirmed in an anion exchange membrane (AEM) electrolyzer. An understanding of the effect of carbon admixture was achieved by combining in situ X-ray absorption spectroscopy, analysis of the electrochemically active surface area, as well as electrochemical impedance spectroscopy. The results point toward a benign impact of carbon admixture on the electronic structure of the MO domains under potential bias.