Spin-State Modulation of Octahedral Co Sites for Enhancing Nitrate-to-Ammonia Conversion

Ammonia is vital for agriculture and a promising carbon-free energy carrier, but its industrial production via the Haber-Bosch process is energy-intensive and emits substantial CO₂. Electrocatalytic nitrogen oxide reduction to ammonia offers a green alternative, by using renewable electricity under ambient conditions while enabling nitrogen resource utilization. Herein, we report a metal-organic framework (MOF)-derived Co₃O₄ electrocatalyst (MD-Co₃O₄) prepared by controlled pyrolysis of Co-MOF-74, which achieved a high Faradaic efficiency (FE) of 96.7% and excellent stability for the electrosynthesis of NH₃ from nitrate, along with robust activity over a wide pH and nitrate concentration range. Comprehensive spectroscopic characterizations and density functional theory calculations revealed that the oxygen vacancies generated during MOF pyrolysis induce coordination distortion of octahedral Co sites, leading to a high-spin state transition with an electron-rich configuration that synergistically enhances nitrate adsorption and interfacial electron transfer, thereby boosting the intrinsic activity. Moreover, this MOF-based strategy is applicable to other M₁Co₂O₄ spinels, which provides an effective approach for spin engineering at the atomic scale and sustainable ammonia synthesis.