Electrolyte and Catalyst Cooperatively Control Proton Supply in Nitrate-to-Ammonia Electroreduction on CuRu at Positive Potentials

Abstract

Above 0 V versus the reversible hydrogen electrode, the hydrogen evolution reaction is thermodynamically suppressed, making this potential region attractive for nitrate-to-ammonia conversion. However, in neutral and alkaline media typically used for the nitrate reduction reaction, protons for successive hydrogenation steps derive mainly from water dissociation, which becomes increasingly sluggish at more positive potentials. Proton availability thus governs the hydrogenation kinetics of nitrate and its intermediates. Using CuRu nanorods as a model catalyst, we resolve how the electrolyte and the catalyst jointly control the proton supply during NO3RR at positive potentials. Macroelectrode measurements reveal that the NO3– reduction kinetics depend on OH– concentration, and single-particle electrochemistry on a single CuRu nanorod, where ultrafast mass transport avoids local OH– accumulation, assigns this dependence to the bulk. Moreover, in situ scanning electrochemical microscopy shows that bulk OH– governs water dissociation kinetics; capacitance and potential-of-zero-charge measurements trace this to a strengthened, cation-enriched interfacial electric field, which accelerates water dissociation and sustains the proton supply at positive potentials. Building on the electrolyte-enhanced proton supply, Cu incorporation tunes the competitive adsorption between H2O-derived species and NO3– on the CuRu surface, thereby avoiding a proton oversupply that compromises the conversion kinetics. These results provide systematic insight into how bulk OH– concentration and catalyst composition cooperatively modulate proton supply, enhancing nitrate-to-ammonia conversion at HER-free potentials.