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.