Electrochemical denitrification by a recyclable cobalt oxide cathode: Rapid recovery and selective catalysis

Abstract

Cathodic aging and fouling have presented significant challenges in the realm of electrochemical denitrification for engineering applications. This study focused on the development of an economical and recyclable nanoporous Co3O4/Co cathode through anodization for nitrate reduction. What distinguished our cathode was its exceptional sustainability. Cobalt from the inactive catalyst could be reclaimed onto the substrate, enabling the regeneration of a new Co3O4 layer. This innovative approach resulted in an exceptionally low Co catalyst consumption, a mere 1.936 g/1 kg N, making it the most cost-effective choice among all Co-based cathodes. The Co3O4 catalyst exhibited a truncated octahedron structure, primarily composed of surface Co2+ ions. Density functional theory calculations confirmed that the bonding between the O atom in NO3- ions and the Co atom in Co3O4 was thermodynamically favorable, with a free energy of − 0.89 eV. Co2+ ions acted as "electron porters" facilitating electron transfer through a redox circle Co2+–Co3+–Co2+. However, the presence of two energy barriers (*NH2NO to *N2 and *N2 to N2) with respective heights of 0.83 eV and 1.17 eV resulted in a N2 selectivity of 9.84% and an NH3 selectivity of 90.02%. In actual wastewater treatment, approximately 78% of TN and 93% of NO3- were successfully removed after 3 h, consistent with the prediction kinetic model. This anodization-based strategy offers a significant advantage in terms of long-term cost and presents a new paradigm for electrode sustainability.