Complementary Surface Motifs Enhance NO 3 RR Performance in NiFe Alloys

Elemental first row transition metal electrocatalysts typically exhibit a tradeoff between Faradaic efficiency (FE) for the nitrate reduction reaction (NO 3 RR) and selectivity toward NH 4 + . Here, we find that NiFe alloys have high NO 3 RR FE and substantially higher NH 4 + selectivity than Ni or Fe. We introduce “relative nitrate adsorption,” a simple descriptor of the difference in NO 3 * and H* binding strength that rationalizes experimental trends in reaction rate order. This descriptor is consistent with competitive adsorption demonstrated in a microkinetic model that shows Fe inclusion promotes NO 3 * adsorption and increased NO 3 RR FE, but cannot describe the higher NH 4 + selectivity observed for NiFe alloys. In fact, calculated activation energies of subsequent reduction steps illustrate that no one active site motif can explain both improved FE and NH 4 + selectivity. Instead, our experimental and computational findings indicate NO 2 * deoxygenation is promoted by Ni‐rich active sites, whereas NO* dissociation is promoted by both surface Fe atoms and an underlying Fe lattice. These findings suggest that NiFe alloys leverage local site diversity via a spillover mechanism, explaining why the performance enhancements are similar regardless of the specific Ni/Fe ratio.