Tailored Catalytic Microenvironments Enable Efficient Electrochemical Ammonia Production

Multi-electron hydrogenation reactions are central to sustainable energy and nitrogen cycling but require catalysts with precise proton-coupled electron transfer (PCET) and intermediate control. Here we report an integrated catalytic microenvironment (ICM) in a Co-substituted inverse spinel oxide (FeFeCoO₄) that enables efficient nitrate-to-ammonia reduction (NO₃RR). Guided by crystal field and Lewis acid-base principles, Co² ⁺ substitution induces Jahn-Teller distortions that activate otherwise inert tetrahedral Fe³ ⁺ sites as H* donors. Concurrently, hard-soft acid-base interactions establish decoupled dual-metal sites: Co² ⁺ stabilizes nitrogen intermediates, while Fe³ ⁺ forms Fe─O antibonding interactions that facilitate PCET. This ICM directs electron flow, promotes hydrogenation, and suppresses hydrogen evolution. Consequently, FeFeCoO₄ achieves a peak NH₃ yield of 1.89 × 10⁻ ⁶ mol s⁻ ¹ cm⁻ ² with 96.6% Faradaic efficiency and 30.5% energy efficiency, and stable operation at 0.5 A cm⁻ ² for 265 h. This strategy offers a blueprint for catalysts for multi-electron hydrogenations beyond nitrate reduction.