An overview on Li-N2 cells for ammonia production

Electrochemical N2 reduction reaction (E-NRR) is studied as a sustainable and renewable energy-based process to replace the well-established Haber-Bosch process (HB) for NH3 production, one of the top-3 chemicals produced worldwide, at the base of fertilizers synthesis and a viable green energy carrier, since it is easier transportable and safer than H2. Operating in harsh conditions (at least 450 °C - 200 atm), the HB process is responsible of about 1% of the global greenhouse gas emissions (about 400 Mtons/year of CO2 are released). Even using “green” or “blue” H2, the HB process is viable only in huge centralized plant and would reach no more than 40% energy efficiency, remaining highly energy-consuming. Thus, to reach independence from fossil energy sources, it is quite urgent to find a sustainable process working in mild conditions and driven by abundant even if discontinuous renewable energy sources. Li3N is the only stable nitride of alkali metals and its formation is thermodynamically favored also in ambient conditions. Li-mediated NRR was demonstrated in 1994 and it has recently demonstrated promising results, both in continuous systems with Li+ ions and a proton donor in the same cell, and in discontinuous processes involving Li3N formation and subsequent protonolysis into NH3. In the first setup, E-NRR carried out in aqueous or ethanol environments has been proposed and a recent study demonstrated a Faradaic efficiency (FE) of around 30%, far greater than that of standard E-NRR. In the second approach, cell design is similar to that of Li-air batteries and shows an intriguing possibility, i.e. to avoid the main competitive reaction (hydrogen reduction reaction); in this way, it is possible to reach FE higher than 60%. This poster gives an up-to-date overview of Li-N2 electrochemical systems for NH3 synthesis.