Selectivity of Nitrate Reduction and Hydrogen Evolution Reaction Modulated by Aggregation of Carbon-Supported Fe Species

Fe-species distribution plays an important role in electrochemical nitrate reduction, yet its influence on the competing hydrogen evolution reaction (HER) remains insufficiently understood. Herein, carbon-supported Fe catalysts with distinct Fe-species distributions were prepared by controlling the Fe precursor concentration. FeAC consists of highly dispersed atomic-scale Fe species and FeOx nanoclusters, whereas Fe2O3/FeAC additionally contains crystalline α-Fe2O3 nanoparticles. X-ray absorption spectroscopy, X-ray diffraction, and electron microscopy confirm the distinct Fe dispersion states in the two catalysts. Electrochemical measurements reveal that FeAC achieves significantly higher NH3 Faradaic efficiency (79.6 ± 2.2%) than Fe2O3/FeAC (37.8 ± 3.1%) at −0.8 V versus RHE, whereas Fe2O3/FeAC exhibits a higher NH3 production rate accompanied by enhanced HER. Comparable electrochemically accessible surface areas and interfacial charge-transfer characteristics indicate that the observed catalytic differences are not primarily governed by electrochemical surface area or charge-transfer properties. Operando Raman spectroscopy reveals distinct surface reaction environments, while operando ATR-SEIRAS together with EPR measurements indicates enhanced formation of hydrogen-related Fe–H intermediates and hydrogen radicals on Fe2O3/FeAC, consistent with its stronger HER activity. In contrast, the absence of detectable Fe–H species on FeAC is associated with suppressed hydrogen adsorption and improved nitrate reduction selectivity. These results demonstrate that controlling Fe-species distribution provides an effective strategy for balancing activity and selectivity in carbon-supported Fe electrocatalysts for electrochemical nitrate reduction.