Different strategies in the lithium-mediated nitrogen reduction reaction into ammonia: classification of present achievements and future possibilities

The lithium-mediated (Li-m) nitrogen reduction reaction (NRR) represents the most promising electrochemical process for renewable-driven and delocalized NH3 production. Finding a complementary pathway to the Haber-Bosh (HB) process allows a step forward to the net-zero carbon emission policy, essential to contrast the climate crisis. Indeed, HB causes a global average of 2.86 tons of CO2 emitted per ton of NH3. The reducing power of lithium has been applied in different strategies; it is possible to distinguish between continuous processes and step-by-step systems. In the first case, N2 is reduced simultaneously to the protonation into NH3, while, in the second option, the Li nitridation is conducted in the absence of H+ to avoid the competitive hydrogen evolution reaction (HER). Among the continuous processes, the most promising one employed a batch cell at 20 bar of N2 with a fluorinated Li salt in tetrahydrofuran and with ethanol addition as the proton donor. Indeed, these systems reached a Faradaic efficiency approaching 100%, as well as a commercially relevant ammonia production rate of 153.28 μg/h*cm2 at a current density of 1 A/cm2geo. The step-by-step technology presents the intrinsic advantage of water exploitation as the proton donor in a separate environment and or time of the process, ensuring greater stability. Therefore, this pathway avoids organic molecule degradation, as well as H2 feedstock need and consumption. Moreover, the Li–N2 reaction in a completely aprotic environment could maximize Li exploitation, enhancing scalability. Indeed, Li reduction, essential for the mediator recirculation, is the most energy-requiring step. Li nitridation has been studied both in a direct thermochemical reaction and with promising Li–N2 galvanic cells. In similarity with metallic Li–gaseous batteries (e.g. Li-O2 devices), Li-N2 devices have been recently tested both for NH3 production and for energy storage. Even if this technology is still in its infancy, a proof-of-concept of Li3N formation has been verified.