Ammonia-production electrification: is the lithium-mediated pathway a key solution for the nitrogen electrochemical reduction reaction?

To move a step forward to the net-zero carbon emission policy, it is essential to find a pathway combinable with renewables and complementary to the Haber-Bosch (HB) process, that causes more than 2 tons of CO2 emitted per ton of NH3 produced. The lithium-mediated nitrogen reduction reaction (NRR) represents the most promising electrochemical process for renewable-driven and delocalized NH3 production. The outstanding reducing power of Li has been applied in different strategies, both continuous processes (with protons in the same cell used for N2 activation) and in step-by-step systems. In the latter, the formation of Li3N is the key intermediate step. The separation of the different steps in different times or environments could allow the application of H2O as the proton source. Indeed, the presence of a large amount of H+ near the cathode during the electrochemical reaction (the NRR) should be avoided, otherwise the competitive H2 evolution reaction (HER) would drastically limit the Faradaic efficiency (FE). The Li3N formation from solid Li and gaseous N2 is thermodynamically favored, nonetheless this specie is highly unstable and this process is nowadays the most argued by the scientific community. The continuous processes have nowadays reached a FE approaching 100%, and an integrated process with the simultaneous H2 oxidation reaction at the anode has also been proved in view of a scalable process. This anodic reaction would be necessary to provide a controlled amount of proton in the process, and let it possible to couple NRR to H2O splitting. However, these continuous systems are presenting some intrinsic weaknesses to be addressed, such as the organic electrolyte degradation that still limits the stability and scalability of the process. 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.