Novel Cu(200)/Ti cathode for the enhancement of N2 selectivity in direct ammonia electrolysis: The controls of Cu cathode facet orientation

Abstract

Direct electrochemical ammonia oxidation reaction (AOR) using NiCu-based anodes is effective in removing ammonia from wastewater. However, this type of anode frequently produces undesired byproducts NO₃⁻. Here, we investigated three configurations of novel Cu/Ti cathodes (Cu(1 1 1)/Ti, Cu(2 0 0)/Ti, Cu(2 2 0)/Ti) coupled with Cu/Ni foam (Cu/NF) anode to enhance N₂ selectivity (SN₂) in a direct ammonia electrolysis cell. Cu(2 0 0)/Ti cathode improved SN₂ from 35 % (bare Ti cathode) to 60 % and it achieved 10–15 % higher SN₂ compared to Cu(1 1 1)/Ti and Cu(2 2 0)/Ti. The improvement of SN₂ on Cu(2 0 0) facet was ascribed to the high nitrate electroreduction activity and its conversion to N₂. In real wastewater, Cu/NF anode-Cu(2 0 0)/Ti cathode paired electrolysis system demonstrated its excellent capability of 88 % NH₃ removal with 95 % SN₂. Our electrolysis system was capable to maintain the residual NH₃-N and the NO₃-N below 8 mg L⁻¹, meeting effluent discharge standards. Our findings highlighted the importance of the control of Cu cathode facet orientations for an efficient elimination of NO₃^– and improvement of N₂ production during direct ammonia electrolysis.