Ammonia (NH3) is a primary form of reactive nitrogen (Nr), which is an essential nutrient for all lives on the earth. Over the past century, the man-made N2-fixing process in the industry (i.e., the Haber-Bosch process) has contributed to a significant portion of NH3 production, and has also led to the continued accumulation of Nr in our ecosystems that has caused alarming and profound damages to the environments as well as human welfare, as the rate of Nr generation is not balanced by the natural nitrification-denitrification process for its elimination. Therefore, global nitrogen-cycle has become a grand engineering challenge faced to humanity. In this presentation, we report high-performance NO3−-to-NH3 conversion in a scalable and cost-effective membrane-free alkaline electrolyzer (MFAEL). A high NH3 partial current density of 4.22 ± 0.25 A cm−2 with a faradaic efficiency of 84.5 ± 4.9% was achieved. The unique configuration of MFAEL allows for the continuous production of fertilizers and chemicals (i.e. (NH4)2SO4, NH3 solution, NH4HCO3, etc) 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− concentrating. In addition, pairing NO3− reduction with the oxidation of organic Nr compounds at the anode of 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 exciting, economically viable solution to the growing trend of Nr buildup and increasing demand for sustainable NH3 with reduced carbon footprints.

Membrane-free alkaline electrolyzer for high-performance ammonia electrosynthesis from waste nitrogen sources

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