Mechanistic insights into nitrate electroreduction on CuNi alloys: d-band center modulation of key intermediates

Abstract

Electrochemical nitrate reduction reaction to ammonia (NO₃RR) is a promising alternative to the traditional Haber-Bosch process for NH₃ synthesis. The development of efficient electrocatalysts is crucial for this process. Copper-nickel alloy (CuNi) catalysts demonstrate excellent catalytic performance over pristine copper (Cu) and pristine nickel (Ni) catalysts, and it is noted that Ni incorporation significantly improves nitrate removal efficiency while suppressing undesirable nitrite formation for NO₃RR. However, the reaction mechanism of NO₃RR on CuNi catalysts remains unclear, particularly regarding the role of Ni incorporation. In this study, constant-potential method calculations were applied to CuNi(1 1 1) catalysts (Cu₃Ni₁(1 1 1), Cu₂Ni₂(1 1 1), and Cu₁Ni₃(1 1 1) with Cu:Ni atomic ratios of 3:1, 1:1, and 1:3, respectively) to understand their enhanced NO₃RR performance compared to pristine Cu(1 1 1) and pristine Ni(1 1 1) catalysts. The potential-dependent adsorption energetics reveal the enhanced co-adsorption of NO₃* and H* facilitates nitrate deoxidation-protonation and improves Faradic efficiency, while the strengthened NO₂* binding simultaneously promotes its further reduction rather than desorption. Microkinetic analyses identify the dominant surface intermediates, and the simulated NH₃ formation current densities exhibit highly consistent with experimental trends. Furthermore, the electronic structure analyses elucidate that the d-band center, optimized near Cu:Ni = 1:1, modulates the binding strengths of key intermediates and reduces activation barriers, thereby suppressing nitrite formation while promoting selective ammonia formation. These fundamental insights not only rationalize previously reported experimental phenomena but also provide design principles for developing efficient NO₃RR electrocatalysts.