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

The release of wastewaters containing relatively low levels of nitrate (NO) results in sufficient contamination to induce harmful algal blooms and to elevate drinking water NO concentrations to potentially hazardous levels. In particular, the facile triggering of algal blooms by ultra-low concentrations of NO necessitates the development of efficient methods for NO 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 NO destruction. In this study, we present flow-through electrofiltration via an electrified membrane incorporating nonprecious metal single-atom catalysts for NO reduction activity enhancement and selectivity modification, achieving near-complete removal of ultra-low concentration NO (10 mg-N L) 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% NO removal with high N selectivity of 86% in a single-pass electrofiltration, which is a significant improvement over flow-by operation (30% NO removal with 7% N selectivity). This high NO 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 H 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 NO reduction for efficient water purification.