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

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 (V_RHE), 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 V_RHE). In contrast, only 29% Cu(0) is observed in 0.1 M NaNO₃ at 0.1 V_RHE, 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 V_RHE 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 V_RHE.