Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction

Abstract

Electrochemical nitrogen reduction to ammonia is studied as a distributed and renewable alternative to Haber-Bosch. Most nitrogen reduction chemistries are performed in aqueous media, which suffer from low rates and selectivities. We present a lithium-mediated approach for producing ammonia in a non-aqueous medium that demonstrates high rates and yields. A coupled kinetic-transport model is developed to describe observed behaviors, which suggests that the process is limited by nitrogen diffusion to the electrode. HIGHLIGHTSLithium-metal-mediated approach for nitrogen electroreduction to ammonia is studied Ammonia production rate of (7.9 G 1.6) 3 10 À9 mol cm À2 s À1 is achieved Faradaic efficiency of 18.5% G 2.9% is obtained A kinetic-transport model incorporating observed diffusion limitations is developed Lazouski et al., SUMMARYAmmonia is a large-scale commodity chemical that is crucial for producing nitrogen-containing fertilizers. Electrochemical methods have been proposed as renewable and distributed alternatives to the incumbent Haber-Bosch process, which utilizes fossil fuels for ammonia production. Herein, we report a mechanistic study of lithium-mediated electrochemical nitrogen reduction to ammonia in a non-aqueous system. The rate laws of the main reactions in the system were determined. At high current densities, nitrogen transport limitations begin to affect the nitrogen reduction process. Based on these observations, we developed a coupled kinetic-transport model of the process, which we used to optimize operating conditions for ammonia production. The highest Faradaic efficiency observed was 18.5% G 2.9%, while the highest production rate obtained was (7.9 G 1.6) 3 10 À9 mol cm À2 s À1 . Our understanding of the reaction network and the influence of transport provides foundational knowledge for future improvements in continuous lithium-mediated ammonia synthesis.

Single-atom catalysts boost nitrogen electroreduction reaction

“…[83] Likewise, Li-mediated NERR was achieved because at the given potential Li + could be reduced to Li, which spontaneously reacts with N2 to form Li3N, which was then protonated to form NH3 when using ethanol as a proton carrier. [84] Moreover, Li + was demonstrated to retard the HER process because of the formation of active O-Li+ site, and afford a larger potential window for NERR with higher selectivity.…”

Ambient Lithium-Mediated Ammonia Synthesis

“…5 In the absence of a proton source, here ethanol, the lithium metal is unreactive. 7 Ethanol is used as a model proton source, which is useful from the standpoint that ethanol is a renewable feedstock, although eventual work will need to transition towards more cost-effective hydrogen sources such as water. Only once a critical ethanol concentration is reached does the lithium react with nitrogen and ethanol (Steps I, II).…”

“…Only once a critical ethanol concentration is reached does the lithium react with nitrogen and ethanol (Steps I, II). 7 Beyond this critical concentration, ethanol competes with nitrogen for available lithium (Step II), such that ammonia yields decrease rapidly with increasing ethanol concentration due to generation of hydrogen and lithium ethoxide. Reduction of nitrogen (Step I) and ethanol (Step II) exhibit different orders with respect to lithium: nitrogen is third order in lithium concentration, while ethanol is second order; the orders correspond to the number of lithium atoms required to produce a molecule of ammonia and hydrogen, respectively.…”

Report of the Basic Energy Sciences Roundtable on Liquid Solar Fuels

“…There are a number of examples in the literature using hydrogen derived from electrocatalytic water splitting to form ammonia using Haber-Bosch catalysis. There is also a growing literature using lithium-mediated nitrogen reduction, where, for example, lithium metal is used stoichiometrically to react with nitrogen to form lithium nitride (Li 3 N), which can be protonated to ammonia while the lithium ions are reduced back to lithium metal electrochemically 83,84 . Unlike the CO 2 RR, however, there are no viable electrochemical pathways for direct reduction of N 2 (N 2 RR).…”