Dynamic Evolution of Cu2O-CuO/Cu Electrocatalyst for Ammonia Synthesis by Electrocatalytic Nitrate Reduction

Abstract Electrocatalytic nitrate reduction (eNO3RR) is a promising alternative to the Haber–Bosch process for ammonia production. Besides, it can transform and utilize the nitrate in waste water. Copper catalysts are widely employed; however, they exhibit weak binding affinity for most reaction intermediates, and their weak hydrogen binding energy limits the hydrogenation of these intermediates, thereby affecting both the reaction activity and ammonia selectivity. In this study, a two-step electrochemical synthesis method was used to sequentially load CuO and Cu2O onto the Cu foam surface to make a Cu2O-CuO/Cu heterostructure. Combination of in situ FTIR and Raman spectra confirmed that the synergistic effect of Cu2O and CuO promoted the eNO3RR efficiency. This synergy effectively enhanced the binding affinity of reaction intermediates, where Cu2O facilitated NO3– adsorption and catalyzed its conversion to *NO2, while CuO aided in water splitting to enhance active hydrogen availability. Density functional theory (DFT) calculations showed that Cu2O-CuO/Cu efficiently lowered the energy barrier for the steps from *NO2H to *NO and from *NH2OH to *NH2. Moreover, the reaction involved a significant dynamic evolution among CuII, CuI, and Cu0, which enhanced the generation and consumption of reaction intermediates. At –0.7 V vs RHE, Cu2O-CuO/Cu resulted in a high ammonia yield of 60.5 mg h–1 cm–2 with a Faradaic efficiency (FE) of 98.7% and sustained a current density reaching 1.14 A cm–2. This work reveals the dynamic evolution of electrocatalysts and presents an efficient strategy for construction of electrocatalysts with synergistic Cu-based heterostructures.