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

The electrochemical nitrate reduction reaction (NO₃RR) offers a pathway to produce NH₃ for fuel and fertilizer from waste NO₃ ^-. In this work, a polycrystalline vanadium sulfide (VS _<i>x</i> ), which is derived from solvothermally grown and annealed VS₂, is shown to exhibit excellent NO₃RR activity (2.3 ± 0.6 mg·cm^-2 geo.·h^-1 @ -0.92 V_RHE) and Faradaic efficiency to NH₄ ⁺ (69 ± 6% at -0.69 V_RHE) in buffered neutral pH electrolyte containing 0.1 M NO₃ ^-. A variety of characterization techniques are leveraged to support the VS _<i>x</i> assignment, including X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, selected area electron diffraction, and X-ray diffraction measurements. The VS₂ annealing step reduces the oxide character and generates VS _<i>x</i> , which, based on the improved NO₃RR activity, results in the creation of active sites for NO₃ ^- binding. To help shed light on NO₃RR on VS _<i>x</i> , VS₂ is used as a model system, and a grand-canonical density functional theory (GC-DFT) investigation of VS₂ shows strong evidence that S vacancies are active sites for NO₃RR, where NO₃ ^- outcompetes H⁺ for adsorption at the S-vacancy sites. Moreover, GC-DFT results highlight a thermodynamically favorable reaction to generate NH₄ ⁺ in an aqueous electrolyte at relevant cathodic potentials. As an annealed material, VS _<i>x</i> may contain undersaturated V sites, which show an electronic structure similar to the theoretically calculated S-vacancy site of VS₂, and these sites may contribute to the observed increase in NO₃RR activity and selectivity for NH₄ ⁺ on VS _<i>x</i> versus unannealed VS₂. Finally, kinetic isotope effect measurements suggest that the kinetic rate-limiting step of the NO₃RR on VS _<i>x</i> is not proton-coupled, indicating it may be the first electron transfer to adsorbed NO₃*.