Modulating the Spin State of Co ³⁺ to Promote Ampere‐Level Electrosynthesis of Ammonia

ABSTRACT

Ammonia electrosynthesis via nitrate reduction offers a sustainable alternative to Haber–Bosch process, and this reaction involves an initial deoxygenation step from NO ₃ ⁻ to *NO ₂ , followed by successive hydrogenation steps to NH ₃ . Previous studies have mainly focused on the hydrogenation steps due to the competitive hydrogen evolution reaction, and overlooked that early‐stage adsorption and activation govern the overall reaction rate. Here we show that modulating the spin state of octahedral Co in Ni‐doped cobalt spinel markedly accelerates nitrate adsorption and activation. The high‐spin HS‐(Co _0.75 Ni _0.25 )Co ₂ O _x delivers a current density of 1.1 A cm ⁻² and achieves an ammonia yield rate of 83.64 mg·h ⁻¹ ·cm ⁻² with a Faradaic efficiency of 97.8% at −0.32 V versus RHE, substantially outperforming its low‐spin counterpart (52.53 mg·h ⁻¹ ·cm ⁻² , 94.8%). Operando characterizations combined with theoretical calculations reveal that high‐spin octahedral Co sites optimize orbital hybridization with nitrate and facilitate subsequent reduction steps. This electronic modulation lowers the Gibbs free energy change of the NO ₃ ⁻ to *NO ₂ step by 1.55 eV, substantially reducing the energetic penalty for the initial nitrate activation. This work elucidates a spin‐state‐dependent mechanism for accelerated nitrate deoxygenation kinetics and establishes spin engineering as an effective strategy to promote rate‐limiting steps in tandem electrocatalysis, enabling industrially relevant ammonia electrosynthesis.