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

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.