Magnetic‐Field Control of Surface States in CoFe ₂ O ₄ Thin Films for Nitrate Electroreduction to Ammonia

ABSTRACT Magnetic‐field‐assisted material synthesis enables more control over spin polarization and electronic structure in electrocatalysts, offering new opportunities for complex electrochemical reactions such as nitrate reduction, which underpins sustainable ammonia synthesis and nitrate remediation. Here, we report a magnetic‐field‐assisted chemical vapor deposition approach to engineer CoFe 2 O 4 electrocatalysts, where cobalt serves as the active and selective site while an external magnetic field modulates cation‐redistribution and surface morphology. Field‐induced structural modification under 1 T delivers a nitrate‐to‐ammonia production rate of 133 ± 38 µmol cm −2 h −1 with a Faradaic efficiency of 96 ± 3% at −0.5 V vs RHE. This corresponds to a 288% enhancement relative to field‐free synthesis and a 2078% increase compared with Fe 3 O 4 , yielding an overall performance improvement of 5989%. Density functional theory calculations reveal that the CoFe 2 O 4 (311) facet preferentially stabilizes nitrate adsorption while suppressing hydrogen evolution, enabled by a higher work function (6.98 eV) and stronger thermodynamic affinity for NO 3 − (−5.58 eV) relative to isostructural Fe 3 O 4 .