Nitrogen Electrochemical Reduction into Ammonia: The Li-Mediated Pathway with Lights and Shadows

Abstract

The lithium-mediated (Li-m) nitrogen reduction reaction (NRR) represents the most promising electrochemical process for renewable-driven and delocalized NH3 production. To move a step forward to the net-zero carbon emission policy and contrast the climate crisis, it is essential to find a complementary pathway to the Haber-Bosh (HB) process, independent from “grey” or “blue” H2 and combinable with renewables. Indeed, HB causes a global average of 2.86 tons of CO2 emitted per ton of NH3 [1].The outstanding reducing power of Li 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).The continuous processes have nowadays reached a Faradaic efficiency approaching 100% [2], as well as a commercially relevant ammonia production rate of 153.28 μg/h*cm2 geo at a current density of 1 A/cm2 geo [3] in a batch cell at 20 bars of N2. These systems present some intrinsic drawbacks, such as the proton donor (e.g. ethanol) consumption and the system degradation, that still limit the scalability of the process. To overcome these limits, the E-NRR is recently studied in combination with H2 oxidation reaction and with proton carriers [4], [5].The step-by-step technology could ensure greater stability to the process exploiting the spontaneous chemical reaction between Li3N and H2O. Therefore, this pathway avoids organic molecule degradation, as well as H2 feedstock need and consumption [6]. Moreover, the Li–N2 reaction in a completely aprotic environment could maximize Li exploitation, enhancing scalability. Indeed, the Li reduction step is essential for the mediator recirculation, but it is the most energy-requiring step [6].Li nitridation has been studied both in a direct thermochemical reaction [6] and with promising Li–N2 galvanic cells [7]. 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 [7] and our laboratory is currently addressing this challenge within the SuN2rise project.