Nitrate-to-Ammonia Electroconversion at Neutral pH on Polycrystalline Vanadium Sulfide Derived from Vanadium Disulfide.

The electrochemical nitrate reduction reaction (NORR) offers a pathway to produce NH for fuel and fertilizer from waste NO . In this work, a polycrystalline vanadium sulfide (VS ), which is derived from solvothermally grown and annealed VS, is shown to exhibit excellent NORR activity (2.3 ± 0.6 mg·cm geo.·h @ -0.92 V) and Faradaic efficiency to NH (69 ± 6% at -0.69 V) in buffered neutral pH electrolyte containing 0.1 M NO . A variety of characterization techniques are leveraged to support the VS assignment, including X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, selected area electron diffraction, and X-ray diffraction measurements. The VS annealing step reduces the oxide character and generates VS , which, based on the improved NORR activity, results in the creation of active sites for NO binding. To help shed light on NORR on VS , VS is used as a model system, and a grand-canonical density functional theory (GC-DFT) investigation of VS shows strong evidence that S vacancies are active sites for NORR, where NO outcompetes H for adsorption at the S-vacancy sites. Moreover, GC-DFT results highlight a thermodynamically favorable reaction to generate NH in an aqueous electrolyte at relevant cathodic potentials. As an annealed material, VS may contain undersaturated V sites, which show an electronic structure similar to the theoretically calculated S-vacancy site of VS, and these sites may contribute to the observed increase in NORR activity and selectivity for NH on VS versus unannealed VS. Finally, kinetic isotope effect measurements suggest that the kinetic rate-limiting step of the NORR on VS is not proton-coupled, indicating it may be the first electron transfer to adsorbed NO*.