Selective Electrocatalytic Reduction of Nitrate to Ammonia with Nickel Phosphide

Abstract

Liquid ammonia is considered a sustainable liquid fuel and an easily transportable carrier of hydrogen energy; however, its synthesis processes are energy-consuming, high cost, and low yield rate. Herein, we report the electrocatalytic reduction of nitrate (NO₃^-) (ERN) to ammonia (NH₃) with nickel phosphide (Ni₂P) used as a noble metal-free cathode. Ni₂P with (111) facet was grown in situ on nickel foam (NFP), which was regarded as a self-supporting cathode for ERN to synthesis NH₃ with high yield rate (0.056 mmol h^-1 mg^-1) and superior faradaic efficiency of 99.23%. The derived atomic H (*H), verified by a quenching experiment and an electron spin resonance (ESR) technique, effectively enhanced the high selectivity for NH₃ generation. DFT calculations indicated that *NO₃ was deoxygenated to *NO₂ and *NO, and *NO was subsequently hydrogenated with *H to generate NH₃ with an energy releasing process (ΔG < 0). OLEMS also proved that NO was the merely gas intermediate. NFP exhibited the unique superhydrophilic surface, metallic properties, low impedance, and abundant surface sites, favorable for adsorption of NO₃^-, generation of *H, and then hydrogenation of NO₃^-. Hence, NFP cathode showed high selectivity for NH₃ (89.1%) in ERN. NFP with long-term stability and low energy consumption provides a facile strategy for synthesis of NH₃ and elimination of NO₃^- contamination.