Electrocatalytic selectivity for nitrogen reduction vs. hydrogen evolution: a comparison of vanadium and cobalt oxynitrides at different pH values

Abstract

The electrocatalytic nitrogen reduction reaction (NRR) is of significant interest as an environmentally friendly method for NH₃ production for agricultural and clean energy applications. Selectivity of NRR vis-à-vis the hydrogen evolution reaction (HER), however, is thought to adversely impact many potential catalysts, including Earth-abundant transition metal oxynitrides. Relative HER/NRR selectivities are therefore directly compared for two transition metal oxynitrides with different metal oxophilicities—Co and V. Electrocatalytic current–potential measurements, operando fluorescence, absorption, and GC measurements of H₂ and NH₃ production, ex situ X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined to directly compare NRR and HER activities under identical reaction conditions. Results show that cobalt oxynitrides – with Co primarily in the Co(II) oxidation state – are NRR active at pH 10, with electrochemical reduction of both lattice nitrogen and dissolved N₂, the latter occurring without N incorporation into the lattice. Removal of lattice N then yields Co(II) oxide, which is still NRR active. These results are complemented by calculations showing that N₂ binding at Co(II) sites is energetically favored over binding at Co(III) sites. GC analysis demonstrates that H₂ production occurs in concert with ammonia production but at a far greater rate. In contrast, vanadium oxynitride films are HER inactive under the same (pH 10) conditions, as well as at pH 7, but are NRR active at pH 7. DFT calculations indicate that a major difference in the two materials is hindered O–H dissociation of H₂O adsorbed at O-ligated Co vs. V cation centers. The combined studies indicate significant variation in HER vs. NRR selectivity as a function of employed transition metal oxynitrides, as well as different HER mechanisms in V and Co oxynitrides.