Complementary Surface Motifs Enhance NO₃RR Performance in NiFe Alloys

Elemental first row transition metal electrocatalysts typically exhibit a tradeoff between Faradaic efficiency (FE) for the nitrate reduction reaction (NO₃RR) and selectivity toward NH₄ ⁺. Here, we find that NiFe alloys have high NO₃RR FE and substantially higher NH₄ ⁺ selectivity than Ni or Fe. We introduce "relative nitrate adsorption," a simple descriptor of the difference in NO₃* 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 NO₃* adsorption and increased NO₃RR FE, but cannot describe the higher NH₄ ⁺ 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 NH₄ ⁺ selectivity. Instead, our experimental and computational findings indicate NO₂* 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.