Phase Engineering of Nickel Phosphides for Simultaneous Enhancement of Activity, Selectivity, and Stability Toward Electrocatalytic Ammonia Synthesis

Abstract The electrocatalytic nitrate reduction reaction (NO 3 RR) is a very promising pathway for ammonia synthesis. However, achieving the simultaneous enhancement of activity, selectivity, and stability in NO 3 RR is still challenging. Herein, taking nickel phosphide as an example, phase engineering (Ni 5 P 4 , Ni 2 P, NiP 2 ) is verified to be an effective approach to concurrently obtain improved activity, selectivity, as well as stability for ammonia synthesis. Among Ni 5 P 4 , Ni 2 P, and NiP 2 , theoretical calculations predict that the Ni 5 P 4 phase possesses better conductivity, optimized H * adsorption energy, lower reaction barrier of the potential‐determining step (NO * to NOH * ), and favorable thermodynamic stability. Inspired by theoretical predictions, the nickel phosphides nanosheets with three phases are successfully synthesized. As expected, the Ni 5 P 4 nanosheets experimentally exhibit improved activity and selectivity in alkaline electrolyte, slightly higher than Ni 2 P nanosheets and much higher than NiP 2 nanosheets. At −0.4 V vs . RHE, 97.4% of nitrate conversion, 97.7% of NH 4 + selectivity, and 97.6% of NH 4 + Faradaic efficiency are delivered by Ni 5 P 4 nanosheets. With limited surface oxidation to Ni(OH) 2 , the Ni 5 P 4 nanosheets also present decent durability. The in situ generated Ni(OH) 2 plays a vital role in providing protons for the hydrogenation of nitrogen‐related species. Furthermore, the Ni 5 P 4 nanosheets also perform well in air plasma oxidation–electrocatalytic reduction cascade system.