Electrochemically Selective Ammonia Extraction from Nitrate by Coupling Electron- and Phase-Transfer Reactions at a Three-Phase Interface

Abstract

As an attractive alternative to the Haber-Bosch process, an electrochemical process for nitrate (NO₃^-) reduction to ammonia (NH₃) has made great strides in the development of advanced electrocatalysts to suppress the unavoidable H₂ evolution reaction (HER) and side production of N₂. However, isochronous NH₃ separation and recovery from the mother liquor, especially wastewaters, are awfully neglected in state-of-the-art electrochemical systems. Here, we designed electrochemical three-phase interfaces constructed by a CoP cathode and a flat-sheet gas membrane to achieve NO₃^- reduction to ammonia and simultaneous NH₃ recovery in the form of (NH₄)₂SO₄ from wastewaters. The partial current density for ammonia yield and its recovery rate were 37.3 mA cm^-2 and 306 g NH₃-N m^-2 day^-1, respectively, accompanying 100% NO₃^- removal and 99.7% NH₃ extraction. By favoring the originally unfavored side reaction HER, it served as the driving force for NH₃ separation from the wastewater through gas stripping and membrane separation at the three-phase interfaces. Unexpectedly, the timely NH₃ separation could also promote the reduction of NO₃^- to ammonia due to the release of much more active sites. From these, we envision that the present electrochemical process can be routinely employed as an effective strategy to address energy and environmental issues with NH₃ recovery from NO₃^- wastewater.