High-Rate, High-Selectivity Electrochemical Oxidation of Ammonia to Nitrite with a Silver-Based Catalyst

Effective operation of electrocatalysts for the ammonia oxidation reaction (AOR) is typically constrained by poisoning, rapid corrosion, and low Faradaic efficiencies (FEs). Aiming to address these challenges, we present herein a robust silver oxide (AgO_x) catalyst generated by in situ anodic electrodeposition from a solution of Tollen’s reagent ([Ag(NH₃)₂]OH). When tested in 1 M KOH and 0.1 M NH₃ at 1.1 V vs standard hydrogen electrode, AgO_x catalyzes the AOR to nitrite at an FE of 70 ± 6% and a yield rate of 42 ± 7 nmol s^-1 cm^-2, on a time scale of 33 h, which is the highest performance reported so far. Our data suggest that the mechanism of the AgO_x-catalyzed AOR is defined by pH, with the strongly alkaline conditions (>0.1 M KOH) favoring the heterogeneous electrocatalytic pathway producing nitrite. In turn, operation at <0.1 M KOH promotes the formation of N₂ via a homogeneous reaction mediated by the Ag^3+/2+ redox couple. Electrochemical and physical characterization, including by in situ X-ray absorption and Raman spectroscopy, suggests that the active sites within AgO_x promoting nitrite production are Ag³⁺-based.