New Insights into the Electrochemical Nitrate Reduction Reaction on Cu(111) from Theoretical Calculations

Ammonia production via renewable electricity-driven electrochemical processes at ambient conditions is preferable to traditional energy- and carbon-intensive Haber–Bosch processes. Nitrate (NO3–), a common water pollutant with mild N–O bond strength, is an ideal nitrogen precursor for the generation of NH3. An atomistic understanding of the reaction mechanisms of the electrochemical nitrate reduction reaction (NO3RR) on model catalytic surfaces is needed for the bottom-up design of efficient catalysts to develop NO3– to NH3 conversion technologies. Here, we perform first-principles density functional theory (DFT) calculations to get an atomistic understanding of the NO3RR on Cu(111) under conditions similar to experiments. Our calculations provide a deeper mechanistic insight into the NO3RR pathways and show that the NO3RR proceeds via the formation of *NO3 → *NO2-*O → *NO2-*OH → *NO2 → *NO-*O → *NO-*OH → *NO → *NHO → *NHOH → *NH-*OH → *NH → *NH2 → *NH3 → * + NH3 intermediates with *NHO formation identified as the potential determining step (PDS). The charge extrapolated-constant potential (CE-CP) and grand canonical-density functional theory (GC-DFT) methods are implemented to compute the potential-dependent reaction energetics of the NO3RR along the most favorable pathway. The results show that the computational hydrogen electrode (CHE) model overestimates the free energy change (ΔG) of electrochemical steps in the anodic region (U higher than 0 V vs URHE) and underestimates it in the cathodic region (U lower than 0 V vs URHE) for electrochemical reduction reaction steps. Furthermore, in general, GC-DFT calculated free energy changes are lower compared with values obtained using CHE and CE-CP methods. Importantly, our constant potential (U) calculations illustrate that in alkaline conditions, the NO3RR is facilitated at U above −0.49 V, and the HER is favorable at U below −0.49 V, providing a key insight into the selection of the U and pH range suitable for selective NO3RR. Thus, our extensive DFT calculations provide new insights into the reaction pathways and key steps of the NO3RR. The U-dependent reaction energetics outline a rational selection of operating conditions (pH, U range) to selectively convert NO3– to NH3, minimizing the undesired side products. These fundamental key insights will be critical in designing Cu-based catalysts for the selective NO3RR.