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 CuxNi100−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 (FENH) at lower overpotentials than that of pristine Cu100@NC. The optimized alloy, Cu75Ni25@NC, has achieved a remarkable FENH 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. © 2024 The Royal Society of Chemistry.