Mechanistic Insights into Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride Nanoparticles

Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, a selective electrocatalyst is lacking. Here we show that vanadium nitride (VN) nanoparticles are active, selective, and stable ENRR catalysts with an ENRR rate and a Faradaic efficiency (FE) of 3.3 × 10^-10 mol s^-1 cm^-2 and 6.0% at -0.1 V within 1 h, respectively. ENRR with ¹⁵N₂ as the feed produces both ¹⁴NH₃ and ¹⁵NH₃, which indicates that the reaction follows a Mars-van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified VN_0.7O_0.45 as the active phase in the ENRR. Operando X-ray absorption spectroscopy and catalyst durability test results corroborate this hypothesis and indicate that the conversion of VN_0.7O_0.45 to the VN phase leads to catalyst deactivation. We hypothesize that only the surface N sites adjacent to a surface O are active in the ENRR. An ammonia production rate of 1.1 × 10^-10 mol s^-1 cm^-2 can be maintained for 116 h, with a steady-state turnover number of 431.