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.