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

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.