Sustainable waste-nitrogen upcycling enabled by low-concentration nitrate electrodialysis and high-performance ammonia electrosynthesis

Abstract

Reactive nitrogen (Nr) is an essential nutrient to life on earth, but its mismanagement in waste has emerged as a major problem in water pollution to our ecosystems, causing severe eutrophication and health concerns. Sustainably recovering Nr [such as nitrate (NO3-)–N] and converting it into ammonia (NH3) could mitigate the environmental impacts of Nr, while reducing the NH3 demand from the carbon-intensive Haber–Bosch process. In this work, high-performance NO3--to-NH3 conversion was achieved in a scalable, versatile, and cost-effective membrane-free alkaline electrolyzer (MFAEL): a remarkable NH3 partial current density of 4.22 ± 0.25 A cm-2 with a faradaic efficiency of 84.5 ± 4.9%. The unique configuration of MFAEL allows for the continuous production of pure NH3-based chemicals (NH3 solution and solid NH4HCO3) without the need for additional separation procedures. A comprehensive techno-economic analysis (TEA) revealed the economic competitiveness of upcycling waste N from dilute sources by combining NO3- reduction in MFAEL and a low-energy cost electrodialysis process for efficient NO3- concentration. In addition, pairing NO3- reduction with the oxidation of organic Nr compounds in MFAEL enables the convergent transformation of N–O and C–N bonds into NH3 as the sole N-containing product. Such an electricity-driven process offers an economically viable solution to the growing trend of regional and seasonal Nr buildup and increasing demand for sustainable NH3 with a reduced carbon footprint.