Promoting Mn3+ Spin-State Transitions from t2g to eg through Ni Doping in Antiperovskite CuNMn3 for Highly Efficient Ammonia Synthesis

Abstract

The electrochemical nitrogen reduction reaction (e-NRR) offers a sustainable approach to ammonia synthesis under ambient conditions, with the potential to replace the energy-intensive Haber–Bosch process. Despite significant progress in this promising field, the low NH₃ yield rate and limited Faradaic efficiency (FE) remain formidable challenges. Here, we introduce antiperovskite Cu_1–xNi_xNMn₃, where partial substitution of Cu by Ni in CuNMn₃ is developed as an effective and robust e-NRR electrocatalyst. Notably, Cu_0.7Ni_0.3NMn₃ demonstrates outstanding e-NRR performance, achieving an NH₃ yield rate of 33.9 ± 1.1 μg h^–1 mg^–1, an FE of 19.2 ± 0.62% at −0.4 V versus RHE, and excellent long-term stability over 50 h of electrolysis. In-depth mechanistic studies reveal that the Ni/Cu exchange process in Cu_1–xNi_xNMn₃ maintains structural integrity and stabilizes the valence states. Ni atoms at the corner sites interact with adjacent Mn atoms at the face centers via antiferromagnetic interactions, altering the original magnetic exchange interactions. This modification triggers a spin-state transition of some Mn³⁺ ions from a low-spin (t_2g⁴e_g⁰) to a high-spin (t_2g³e_g¹) configuration. Density functional theory (DFT) calculations confirm that the improved e_g orbital electronic configuration enhances N₂ adsorption energy at Mn catalytic sites and promotes the hydrogenation of N₂ to form *NNH intermediates, thereby accounting for the high activity of Cu_0.3Ni_0.7NMn₃ in the e-NRR.