On the feasibility of bifunctional hydrogen oxidation on Ni and NiCu surfaces

Abstract

Nickel is a promising alternative to noble metal catalysts for hydrogen electrocatalysis in alkaline membrane fuel cells and electrolyzers. To boost its electrocatalytic activity, it is often associated with other metals, like Cu. In this work, we combine density functional theory (DFT), Monte Carlo calculations and kinetic modeling in order to shed light on the origin of the experimentally observed activity enhancement on bimetallic NiCu nanoparticles containing ca. 5% of Cu compared to pure Ni (Topics in Catalysis, 58, (2015), 1181–1192; J. Electroanal. Chem., 783, (2016), 146–151). It is shown that the bifunctional HOR mechanism, where Had and OHad recombine to form water, is not viable on the monometallic Ni electrode because of the strong hydrogen adsorption and the high activation energy barrier of the recombination reaction. On bimetallic NiCu surfaces, DFT calculations predict a significant weakening of Had adsorption energy as well as a decrease of the activation barrier of the recombination reaction for ca. 50% Cu surface coverage. Monte Carlo simulations reveal that the surface of NiCu nanoparticles is enriched in Cu atoms even for a small (ca. 5%) Cu atomic fraction in the bulk. Kinetic modeling suggests that the experimental data for NiCu nanoparticles can be well reproduced either considering fast recombination step (bifunctional mechanism) or fast (10-fold enhancement compared to pure Ni) Volmer step (Heyrovsky-Volmer mechanism).