Free-standing membrane incorporating single-atom catalysts for ultrafast electroreduction of low-concentration nitrate

Abstract

The release of wastewaters containing relatively low levels of nitrate (NO₃^-) results in sufficient contamination to induce harmful algal blooms and to elevate drinking water NO₃^- concentrations to potentially hazardous levels. In particular, the facile triggering of algal blooms by ultra-low concentrations of NO₃^- necessitates the development of efficient methods for NO₃^- destruction. However, promising electrochemical methods suffer from weak mass transport under low reactant concentrations, resulting in long treatment times (on the order of hours) for complete NO₃^- destruction. In this study, we present flow-through electrofiltration via an electrified membrane incorporating nonprecious metal single-atom catalysts for NO₃^- reduction activity enhancement and selectivity modification, achieving near-complete removal of ultra-low concentration NO₃^- (10 mg-N L^-1) with a residence time of only a few seconds (10 s). By anchoring Cu single atoms supported on N-doped carbon in a carbon nanotube interwoven framework, we fabricate a free-standing carbonaceous membrane featuring high conductivity, permeability, and flexibility. The membrane achieves over 97% NO₃^- removal with high N₂ selectivity of 86% in a single-pass electrofiltration, which is a significant improvement over flow-by operation (30% NO₃^- removal with 7% N₂ selectivity). This high NO₃^- reduction performance is attributed to the greater adsorption and transport of nitric oxide under high molecular collision frequency coupled with a balanced supply of atomic hydrogen through H₂ dissociation during electrofiltration. Overall, our findings provide a paradigm of applying a flow-through electrified membrane incorporating single-atom catalysts to improve the rate and selectivity of NO₃^- reduction for efficient water purification.