Tuning the electrocatalytic nitric oxide reduction activity of copper through alloying with nickel for NH3 production at low overpotentials

Abstract

Green ammonia synthesis from the electrochemical nitric oxide reduction reaction (NORR) has evolved as an alternative to the energy-intensive and environmentally polluting Haber–Bosch process. However, it is crucial to develop efficient electrocatalysts to achieve satisfactory green NH₃ production via the NORR at low overpotentials with a high selectivity for NH₃. Amidst transition metals, copper (Cu) shows ideal N* adsorption free energy to facilitate NH₃ production selectively. However, Cu needs a higher over-potential to facilitate multi-protonation steps. In an alkaline medium, protonation hindrance is more severe due to sluggish water dissociation kinetics. Thus, the surface reengineering of Cu with a foreign metal having optimum H* adsorption free energy, such as Ni, could boost the reaction rate at lower overpotentials. In this report, a series of electrocatalysts with different Cu and Ni compositions Cu_xNi_100−x@NC (x = 0–100) supported on N-doped carbon nanostructures are synthesized and their physico–chemical properties and electrochemical NORR performance in 1 M KOH are evaluated. The investigation of NORR performance revealed that CuNi@NC alloys facilitate ammonia production with high faradaic efficiency (FE_NH3) at lower overpotentials than that of pristine Cu₁₀₀@NC. The optimized alloy, Cu₇₅Ni₂₅@NC, has achieved a remarkable FE_NH3 of about 79% with a reasonable ammonia yield rate of 3.6 μmol cm⁻² h⁻¹ at an overpotential of 610 mV. The improved NORR to NH₃ activity could be attributed to the facile reaction kinetics enabled by the ideal adsorption energies for the NORR intermediates (*N and *H) over the CuNi alloy. Furthermore, we have constructed a Zn–NO battery using a Cu₇₅Ni₂₅@NC cathode for NH₃ production. The Zn–NO battery exhibited a high-power density of 3.8 mW cm⁻² with 67.33 μg cm⁻² h⁻¹ of NH₃ yield rate at a discharge potential of 0.6 V vs. Zn.