Selectivity of metal oxynitrides for electrocatalytic reduction of nitrogen to ammonia vs. hydrogen evolution: A comparison of titanium, vanadium and cobalt oxynitrides at different pH values

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-a-vis the hydrogen evolution reaction (HER), however, is thought to adversely impact many potential catalysts, including Earth-abundant transition metal oxynitrides. Herein we compare the selectivity of three metal oxynitrides with different metal oxophilicities— Ti, V and Co. 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. Our results show that cobalt oxynitrides are NRR active at pH 10, with electrochemical reduction of both lattice nitrogen and dissolved N₂ but even more active for HER. In contrast, vanadium oxynitride films are HER inactive at pH 7 & 10 but are NRR active at pH 7 while titanium oxynitrides are active at pH 3.2 but inactive at neutral and basic pH. The combined studies demonstrate significant variation in HER vs. NRR selectivity as a function of transition metal oxophilicity/azaphilicity, indicative of different NRR & HER mechanisms.