Modulating the valence electronic structure of Co3O4 to improve catalytic activity of electrochemical nitrate-to-ammonia conversion

Electrochemical conversion of NO3− to NH3via the nitrate reduction reaction (NO3−RR) is a promising approach for ammonia production and storage of “green hydrogen”. Co3O4 has shown satisfactory Faradaic efficiency toward $${\rm{N}}{{\rm{H}}_3}\,({\rm{F}}{{\rm{E}}_{{\rm{N}}{{\rm{H}}_3}}})$$ and stability, making it a potential electrocatalyst for the NO3−-to-NH3 conversion. However, the high overpotential required for triggering the NO3−RR on Co3O4 limits its conversion efficiency. In this study, we synthesized Cu-doped Co3O4 porous hollow nanospheres (Cu−Co3O4 PHNSs) for NO3−RR. Cu-doping effectively reduced the required overpotential and improved the NH3 yield rate on the Co3O4 matrix without reducing $${\rm{F}}{{\rm{E}}_{{\rm{N}}{{\rm{H}}_3}}}$$ and stability. Both experimental and theoretical analyses demonstrated that Cu-doping up-shifted the highest occupied state (HOS) of Co3O4, narrowed the energy barrier between the HOS of Co3O4 and the lowest unoccupied molecular orbital of NO3−, and thus reduced the overpotential required for triggering the electron transfer from Co3O4 to NO3−, thereby endowing the as-prepared Cu−Co3O4 PHNSs with outstanding electrocatalytic activity and durability for the NO3−-to-NH3 conversion. This study provides a novel theoretical perspective on the regulation of electrochemical performance.