Spin-Matched Hydrogenation via High-Spin Co2+ Sites for Robust pH-Universal Ammonia Electrosynthesis

Abstract Electrocatalytic nitrate reduction reaction (NO 3 − RR) offers a sustainable route for NO 3 − remediation and ammonia (NH 3 ) electrosynthesis, yet selective NH 3 formation is frequently limited by the sluggish hydrogenation of deeply reduced nitrogen intermediates, particularly under neutral conditions. Herein, inverse-spinel CoFe 2 O 4 containing high-spin octahedral ( O h )-Co 2+ centers is developed as a spin-defined electrocatalyst for pH-universal NO 3 − RR. The high-spin O h -Co 2+ centers enable spin-matched interaction with the singly occupied N 2p-derived frontier orbital of open-shell *NH, thereby promoting spin-allowed electron transfer and lowering the key *NH → *NH 2 hydrogenation barrier. In neutral electrolyte, CoFe 2 O 4 delivers an NH 3 Faradaic efficiency (FE) of 95.1% at − 0.9 V RHE and a remarkable NH 3 yield rate of 2.2 mmol h − 1 cm − 2 at − 1.0 V RHE , ranking among the best reported neutral NO 3 − RR electrocatalysts. It maintains high NH 3 selectivity and durability in acidic and alkaline media, with NH 3 FEs exceeding 90% over a broad alkaline potential window and stable operation for over 200 h. In situ characterization reveals attenuated *NH accumulation and accelerated downstream hydrogenation, consistent with the proposed spin-matched pathway. These findings identify late-stage *NH hydrogenation as a spin-sensitive kinetic bottleneck and establish metal–adsorbate spin matching as a design principle for pH-universal NO 3 − RR.