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.
Keywords: DFT calculations; ammonia; atomic H; nickel phosphide; nitrate electroreduction.