Electrochemical Nitrate-to-Ammonia Conversion over a Broad Concentration Range via a Hollow Co3O4/CuO Catalyst

Abstract

Widespread nitrate contamination in water challenges both aquatic ecosystems and drinking water safety, and the electrocatalytic nitrate reduction to ammonia (eNRA) presents a promising sustainable way to convert pollutants to value-added products. However, most existing eNRA systems operate efficiently only under highly alkaline and concentrated conditions that are incompatible with real wastewater. Here, we report a hollow heterostructured Co3O4/CuO catalyst which permits efficient eNRA in neutral aqueous media over a wide range of environmentally relevant nitrate concentrations (1-250 mM). The catalyst achieves NH3 Faradaic efficiencies above 97% in the 10-250 mM nitrate range and maintains 86% even at 1 mM nitrate. Furthermore, the catalyst showed excellent stability over 100 hours of continuous operation. In situ Fourier transform infrared spectroscopy analysis suggests that the hollow architecture creates a spatially confined microenvironment that accelerates the hydrogenation of NOx intermediates toward NH₃, and density functional theory calculations indicate the ensemble effect of Co3O4/CuO interface in reducing the free-energy changes of key nitrate reduction steps. This work highlights a structure engineering strategy in electrocatalysts for efficient nitrate remediation and wastewater treatment.