Unlocking the potential of sub-nanometer-scale copper via confinement engineering: A remarkable approach for electrochemical nitrate-to-ammonia conversion in wastewater treatment

Electrochemical-nitrate-reduction-reaction (eNitRR) ammonia synthesis shows the vision of green ammonia synthesis and is also an effective way to purify nitrate wastewater. In this study, the complex of 1,10-phenanthroline and Cu2+ was adsorbed on the surface of polydopamine-derived nitrogen-doped-hollow-carbon-spheres, and sub-nanometer-scale copper species (CuCN/NHCSs) were obtained after heat treatment. Using CuCN/NHCSs as catalyst, the ammonia yield rate and Faradaic efficiency of eNitRR reached 21413.5 mu g h-1 mgcat. - 1 (-1.1 V vs RHE) and 91.08% (-0.9 V vs RHE), respectively. CuCN/NHCSs still have high catalytic activity for eNitRR even in complex water environments (such as acidity, alkalinity, coexisting ions). In the face of low concentration of nitrate in natural sewage, CuCN/NHCSs as a catalyst can reduce the concentration of NO3- to below the limits set by WHO and US EPA within 10 h, and achieve effective purification of nitrate sewage. DFT calculations show that both single-atom Cu sites and Cu atom cluster sites on NHCSs can promote the electrochemical conversion of NO3- to ammonia, so minimizing copper sites can maximize the unit activity. Furthermore, the simultaneous anchoring of Cu atom clusters and single-atom Cu in the carbon matrix is beneficial to lower the energy barrier of the potential-determining step of eNitRR.