The rate and selectivity of the electrocatalytic reduction of nitrate relies on activation and cleavage of NO bonds. Here we investigate the role of oxygen-deficient metal oxide supports on the rate and selectivity of nitrate reduction on copper at environmentally relevant concentrations and pH, finding ceria supports demonstrate enhanced nitrate affinity and correspondingly enhanced electrocatalytic activity.

Abstract

The electrochemical nitrate reduction reaction (NO₃RR) has the potential for distributed water treatment and renewable chemical synthesis. Cu is an active monometallic electrocatalyst for the NO₃RR in acidic and alkaline electrolytes, where activity is limited by the reduction of adsorbed nitrate to nitrite. Oxygen-vacancy forming metal-oxide supports provide sites for N-O bond activation in thermal reduction, impacting product distribution as well. Here we compare the electrochemical NO₃RR activity of Cu deposited on two metal-oxide supports (cerium dioxide [Cu/CeO_2-δ] and fluorine-doped tin dioxide [Cu/FTO]) to a Cu foil benchmark. Considering activity in phosphate-buffered neutral media, nitrate and adsorbed hydrogen compete for surface sites under NO₃RR conditions. The less-cathodic overpotential on Cu/CeO_2-δ compared to Cu/FTO is attributed to stronger nitrate adsorption, similar to thermal nitrate reduction. Utilization of CeO_2-δ as an electrocatalyst support slightly shifting product distribution toward more oxidized products, either by enhancing nitrate affinity or by a more dynamic process involving the formation and healing of oxygen vacancies (𝑣_O ^••). These results suggest supporting catalysts on metal oxides may enhance activity by promoting the adsorption of anionic reactants on cathodic electrocatalysts.