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

The electrocatalytic nitrate reduction reaction (NO₃RR) is a very promising pathway for ammonia synthesis. However, achieving the simultaneous enhancement of activity, selectivity, and stability in NO₃RR is still challenging. Herein, taking nickel phosphide as an example, phase engineering (Ni₅P₄, Ni₂P, NiP₂) is verified to be an effective approach to concurrently obtain improved activity, selectivity, as well as stability for ammonia synthesis. Among Ni₅P₄, Ni₂P, and NiP₂, theoretical calculations predict that the Ni₅P₄ 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 Ni₅P₄ nanosheets experimentally exhibit improved activity and selectivity in alkaline electrolyte, slightly higher than Ni₂P nanosheets and much higher than NiP₂ nanosheets. At -0.4 V vs. RHE, 97.4% of nitrate conversion, 97.7% of NH₄ ⁺ selectivity, and 97.6% of NH₄ ⁺ Faradaic efficiency are delivered by Ni₅P₄ nanosheets. With limited surface oxidation to Ni(OH)₂, the Ni₅P₄ 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 Ni₅P₄ nanosheets also perform well in air plasma oxidation-electrocatalytic reduction cascade system.