Direct electrosynthesis and separation of ammonia and chlorine from waste streams via a stacked membrane-free electrolyzer

Abstract Electrosynthesis, a viable path to decarbonize the chemical industry, has been harnessed to generate valuable chemicals under ambient conditions. Here, we present a membrane-free flow electrolyzer for paired electrocatalytic upcycling of nitrate (NO 3 − ) and chloride (Cl − ) to ammonia (NH 3 ) and chlorine (Cl 2 ) gases by utilizing waste streams as substitutes for traditional electrolytes. The electrolyzer concurrently couples electrosynthesis and gaseous-product separation, which minimizes the undesired redox reaction between NH 3 and Cl 2 and thus prevents products loss. Using a three-stacked-modules electrolyzer system, we efficiently processed a reverse osmosis retentate waste stream. This yielded high concentrations of (NH 4 ) 2 SO 4 (83.8 mM) and NaClO (243.4 mM) at an electrical cost of 7.1 kWh per kilogram of solid products, while residual NH 3 /NH 4 + (0.3 mM), NO 2 − (0.2 mM), and Cl 2 /HClO/ClO − (0.1 mM) pollutants in the waste stream could meet the wastewater discharge regulations for nitrogen- and chlorine-species. This study underscores the value of pairing appropriate half-reactions, utilizing waste streams to replace traditional electrolytes, and merging product synthesis with separation to refine electrosynthesis platforms.