Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis

Abstract

Lithium-mediated ammonia synthesis (LiMAS) is an emerging electrochemical method for NH3 production, featuring a meticulous three-step process involving Li+ electrodeposition, Li nitridation, and Li3N protolysis. The essence lies in the electrodeposition of Li+, a critical phase demanding current oscillations to fortify the solid-electrolyte interface (SEI) and ensure voltage stability. This distinctive operational cadence orchestrates Li nitridation and Li3N protolysis, profoundly influencing the NH3 selectivity. Increasing N2 pressure enhances the NH3 faradaic efficiency (FE) up to 20 bar, beyond which proton availability controls selectivity between Li nitridation and Li3N protolysis. The proton donor, typically alcohols, is a key factor, with 1-butanol observed to yield the highest NH3 FE. Counterion in the Li salt is also observed to be significant, with larger anions (e.g., exemplified by BF4 –) improving SEI stability, directly impacting LiMAS efficacy. Notably, we report a peak NH3 FE of ∼70% and an NH3 current density of ∼−100 mA/cm2 via a delicate balance of process conditions, encompassing N2 pressure, proton donor, Li salt, and their respective concentrations. In contrast to the recent literature, we find that the theoretical maximum energy efficiency of LiMAS hinges significantly on the proton source, with LiMAS utilizing H2O calculated to have a maximum achievable energy efficiency of 27.8%. Despite inherent challenges, a technoeconomic analysis suggests high-pressure LiMAS to be more feasible than both ambient LiMAS and a modified green Haber–Bosch process. Our analysis finds that, at a 100 mA/cm2 NH3 current density and a 6 V cell voltage, LiMAS delivers green NH3 at an all-inclusive cost of $456 per ton, significantly lower than conventional cost barriers. Our economic analysis underscores high-pressure LiMAS as a potentially transformative technology that may revolutionize large-scale NH3 production, paving the way for a sustainable future.

Metal Nitride as a Mediator for the Electrochemical Synthesis of NH3

“…In contrast, considerable progress has been made in the past five years toward the Li-mediated NH 3 synthesis (Li-MAS) process. − ,,− However, fundamental aspects of the process remain poorly understood, such as the mechanism of NH 3 synthesis, formation of the solid-electrolyte interface (SEI), and transport across it. , Here, we hypothesize a mechanism illustrated in Scheme , which involves the following steps: (1) electrodeposition of Li onto the metallic substrate (e.g., Ni, Cu); (2) spontaneous reaction of Li metal with N 2 to form Li 3 N; (3) protonation of Li 3 N with a proton donor (e.g., ethanol) to form NH 3 and a surface N vacancy on the lithium nitride; (4) exergonic binding of N 2 in the N vacancy site on lithium nitride; (5) reduction of N 2 via an associative mechanism, recovering lithium nitride and closing the catalytic cycle. Note that this hypothesized mechanism does not involve concomitant corrosion of Li into solution, which should be endergonic at the applied potentials typically used …”