Critical Aspects in Lithium-Mediated Nitrogen Reduction Reaction for Electroproduced Ammonia

Ammonia is a fundamental building-block for fertilizers as well as for many other commodities, and its production via the Haber-Bosch (HB) process is responsible of around 1.4% of the global greenhouse gas emissions due to the severe conditions (200 atm) of these few huge, centralized plants. Finding a renewable-driven and delocalized electrochemical process for NH3 production, complementary to HB, could be a key solution for our society that is facing climate change crisis and that is demographically growing. In view of process electrification, the Li-mediated pathway represents the most promising solution in the N2 reduction reaction challenging field. Exploiting the unique reducing power of this alkali metal, this strategy achieves the highest Faradic efficiency (FE) and NH3 production rate. Different strategies, both continuous and step-by-step systems, are under evaluation in current literature. In the first, Li+ ions from the aprotic electrolyte are electrodeposited on the cathode, where N2 is reduced and protonated into NH3 directly in the same environment. On the plated Li, a solid electrolyte interphase (SEI) unavoidably forms due to electrolyte degradation on the interface, and the different diffusion rate of Li+, N2, and H+ trough the SEI layer determines the selectivity towards NH3 formation. In the latter, the formation of Li3N is the key intermediate step. The exploitation of a Li-N2 galvanic cell, inspired by lithium-air batteries, could maximize Li3N formation. Even in this case, nevertheless, practical aspects are delaying the rising of this innovative research filed, and a critical eye on interferents and impurities should be adopted for a correct NH3 quantification. Our laboratory is currently addressing these challenges within the SuN2rise project, and an overview of electrochemical and engineering-related crucial aspects will be presented in this contribution.