Efficient Electrocatalytic Nitrate-to-Ammonia Enabled by Reversible Lattice-Oxygen Control

Understanding the fundamentals governing reactivity and leveraging this knowledge to achieve optimal catalytic performance have long been a core objective in catalysis study. This challenge is particularly pressing for sustainable nitrogen cycle via nitrate reduction (NO₃^-RR) due to its inherent trade-off between high Faradaic efficiency (FE) and low overpotential. Here, we propose a novel strategy to enhance the NO₃^-RR performance by quantitatively regulating surface oxygen activity of transition metal oxides (TMOs) via tuning the metal-oxygen covalency. Using a series of A-site-substituted La_1-xSr_xCoO₃ perovskites, we conduct comprehensive experimental and modeling studies, revealing that NH₃ yield rate and Faradaic efficiency exhibit distinct "volcano" and "W-shaped" dependencies on surface oxygen activity. Notably, La_0.5Sr_0.5CoO₃, characterized by balanced metal-oxygen covalency, achieves exceptional activity and selectivity for NO₃^-RR. Mechanistic studies uncover a switchable active site that transitions from a lattice-oxygen vacancy to a nonstoichiometric Co on La_1-xSr_xCoO₃ during NO₃^-RR, accompanied by a dynamic and reversible lattice-oxygen refilling process. This mechanism circumvents the potential-limiting step (PLS) and blocks byproduct formation, driving superior catalytic performance. Our discoveries provide insights for designing advanced TMOs for not only NO₃^-RR but also other oxygen-sensitive reactions, while deepening the understanding of surface dynamics during electrocatalysis.