Cation Substitution Strategy for Developing Perovskite Oxide with Rich Oxygen Vacancy-Mediated Charge Redistribution Enables Highly Efficient Nitrate Electroreduction to Ammonia

The electrocatalytic nitrate (NO₃^-) reduction reaction (eNITRR) is a promising method for ammonia synthesis. However, its efficacy is currently limited due to poor selectivity, largely caused by the inherent complexity of the multiple-electron processes involved. To address these issues, oxygen-vacancy-rich LaFe_0.9M_0.1O_3-δ (M = Co, Ni, and Cu) perovskite submicrofibers have been designed from the starting material LaFeO_3-δ (LF) by a B-site substitution strategy and used as the eNITRR electrocatalyst. Consequently, the LaFe_0.9Cu_0.1O_3-δ (LF_0.9Cu_0.1) submicrofibers with a stronger Fe-O hybridization, more oxygen vacancies, and more positive surface potential exhibit a higher ammonia yield rate of 349 ± 15 μg h^-1 mg^-1_cat. and a Faradaic efficiency of 48 ± 2% than LF submicrofibers. The COMSOL Multiphysics simulations demonstrate that the more positive surface of LF_0.9Cu_0.1 submicrofibers can induce NO₃^- enrichment and suppress the competing hydrogen evolution reaction. By combining a variety of in situ characterizations and density functional theory calculations, the eNITRR mechanism is revealed, where the first proton-electron coupling step (*NO₃ + H⁺ + e^- → *HNO₃) is the rate-determining step with a reduced energy barrier of 1.83 eV. This work highlights the positive effect of cation substitution in promoting eNITRR properties of perovskites and provides new insights into the studies of perovskite-type electrocatalytic ammonia synthesis catalysts.