Engineering a dual-site CuO/Cu2O/CeO2 heterostructure: synergistic Cu+/Cu2+ and CeO2 modulation for tandem ammonia electrosynthesis from nitrate

Abstract

The electrochemical nitrate reduction reaction (eNO₃RR) offers a sustainable strategy for ammonia (NH₃) synthesis while simultaneously addressing nitrate pollution. However, its reaction kinetics and selectivity remain constrained, especially on Cu-based electrocatalysts, owing to the premature desorption of the nitrite intermediate (*NO₂), which undermines both the NH₃ production rate and faradaic efficiency (FE). In this work, a CuO/Cu₂O/CeO₂ heterostructure is fabricated on nickel foam via in situ growth and subsequent calcination. By precisely tuning the calcination temperature, the Cu⁺/Cu²⁺ ratio is effectively modulated. The incorporation of CeO₂ induces abundant oxygen vacancies and Lewis acid sites, which cooperatively stabilize Cu^δ+ (1 < δ < 2) species. This synergistic effect enhances the adsorption of nitrogen-containing intermediates and facilitates hydrogen spillover from CeO₂ to CuO/Cu₂O through water dissociation, thereby enabling a [2 + 6]-electron tandem pathway: NO₃⁻ is first reduced to NO₂⁻ (2-electron step) over CuO/Cu₂O, followed by conversion of NO₂⁻ to NH₃ (6-electron step) over CuO/Cu₂O/CeO₂. Consequently, the catalyst markedly suppresses the hydrogen evolution reaction and the accumulation of *NO₂. Under optimized conditions (1 M PBS with 0.1 M KNO₃, applied potential of −0.65 V vs. RHE) in a neutral electrolyte, the catalyst achieves a record faradaic efficiency of 99.85% and an NH₃ yield of 43.37 mg h⁻¹ cm⁻², along with outstanding stability over 120 hours. Furthermore, a proof-of-concept Zn–NO₃⁻ battery incorporating this catalyst delivers a power density of 7.10 mW cm⁻². This study underscores the critical role of copper pair sites and their synergy with CeO₂ in advancing nitrate-to-ammonia electrocatalysis.