(119b) Nickel-Iron Alloys As Catalysts for Electrochemical Conversion of Nitrate to Ammonium

The electrochemical nitrate reduction reaction (NO₃RR) offers a scalable, cost-effective way to convert toxic nitrate from agricultural and industrial runoff into dinitrogen gas or value-added chemicals like ammonium or hydroxylamine. Targeting ammonium as the primary product, NO₃RR mitigates the environmental impact of ammonia-based fertilizers at both ends of the nitrogen cycle by (1) removing toxic nitrate from water and (2) reducing reliance on energy-intensive Haber-Bosch ammonia synthesis. Previous studies have shown that cobalt performs exceptionally well amongst various pure transition metals as cathodes for NO₃RR , which typically demonstrate a tradeoff between their selectivity to ammonium (S_NH4) and their Faradaic efficiency (FE) – their ability to reduce nitrate without losing energy to the competing hydrogen evolution reaction. In this combined experimental/computational study, we investigate whether NiFe alloys can approximate the performance of Co when used as a NO₃RR cathode. S_NH4 and FE for different NiFe alloy compositions were evaluated through electrochemical conversion measurements, in which product distributions were quantified using anion chromatography and UV-VIS spectrophotometry. Density functional theory (DFT) calculations were used to determine activation and reaction energies of selectivity-determining reaction steps on Ni(111), Fe(110), and a series of mixed NiFe surfaces. The experimentally measured product distributions show that the NiFe alloys achieve significantly greater S_NH4 and FE than either pure metal. DFT calculation results suggest that S_NH4 is enhanced on NiFe alloys due to a dual-site mechanism in which nitrate ions preferentially adsorb to Fe sites, and protons preferentially adsorb to Ni sites.