Potential- and Time-Dependent Operando X-Ray Absorption Study of Cu2O Microcrystals Transformations during Nitrate Reduction to Ammonia

Abstract

Electrochemical nitrate reduction reaction (NO3RR) represents a sustainable, carbon-neutral alternative to the Haber–Bosch process for ammonia synthesis. Time-resolved operando X-ray absorption spectroscopy reveals the chemical states and structural evolution of copper(I) oxide microcrystals deposited on carbon fiber (Cu2O/C) across a potential range of +0.6 to −0.7 V versus reversible hydrogen electrode (VRHE), where nitrate reduction to nitrite and ammonia occurs. Without nitrate, Cu2O microcrystals are quickly reduced to metallic Cu(0) aggregates at low reduction potentials (≈0.1 VRHE). In contrast, only 29% Cu(0) is observed in 0.1 M NaNO3 at 0.1 VRHE, indicating that nitrate adsorption passivates the surface and promotes selective electron transfer to nitrate, thereby retarding the kinetics of Cu2O microcrystals transformation to Cu(0) particles. Ammonia formation initiates at −0.3 VRHE in 0.1 M NaNO3 (pH 13) solution, accompanied by the formation of metallic copper particles for the hydrogenation of the intermediates. The Faradaic efficiency (FE) of ammonia is increased with more negative potential, accompanied by the formation of metallic Cu(0) particles. The fully reduced Cu particles exhibit superior NO3RR activity to produce nitrite at lower reduction potentials and ammonia at higher reduction potentials, achieving 89.7% ammonia FE at −0.7 VRHE.