Proton-Donor Identity Shapes Electrochemical Nitrate Reduction on Silver

Abstract Electrocatalytic nitrate reduction is a key reaction in nitrogen cycling and water remediation, yet the mechanistic interpretation of its voltammetric response remains poorly understood. On silver electrodes, cyclic voltammograms often display multiple cathodic features assigned to different reaction steps, although a consistent mechanistic interpretation remains elusive. Here, we combine cyclic voltammetry analysis with in situ product detection using scanning electrochemical microscopy (SECM), and hydrodynamic analysis using rotating disk electrode (RDE), to investigate nitrate-to-nitrite electroreduction on silver. We show that the cathodic response changes from a single peak to two distinct features with increasing nitrate concentration when using phosphate buffer as a supporting electrolyte. Analysis of concentration, scan rate, and pH dependencies, together with systematic variation of phosphate electrolyte composition, indicates that these features arise from distinct interfacial proton-transfer regimes associated with the availability of H2PO4– as a proton donor. In addition, SECM enables the assignment of each feature to local nitrite formation, ruling out changes in product identity as the origin of each feature. These findings show that phosphate electrolyte modulates proton-donor availability, and, consequently, the voltammetric response of nitrate reduction on silver, providing a framework for interpreting the complex electrocatalytic behavior observed in this system.