Electrochemical Synthesis of Ammonia from Water and Nitrogen: A Lithium‐Mediated Approach Using Lithium‐Ion Conducting Glass Ceramics

Abstract

Lithium-mediated reduction of dinitrogen is ap romising methodt oe vade electron-stealing hydrogen evolution, ac ritical challenge which limits faradaic efficiency (FE) and thus hinders the success of traditional protic-solvent-based ammonia electro-synthesis. Av iable implementation of the lithium-mediated pathway using lithium-ion conducting glass ceramics involves i) lithium deposition, ii)nitridation,a nd iii)ammonia formation. Ammonia was successfully synthesized from molecular nitrogen and water,y ielding am aximum FE of 52.3 %. Witha n ammonia synthesis rate comparable to previously reported approaches, the fairly high FE demonstrates the possibility of using this nitrogen fixation strategya sasubstitute for firmly established, yet exceedingly complicated and expensivet echnology,a nd in so doing represents an ext-generation energy storagesystem.The Haber-Bosch process is aw ell-established ammonia (NH 3 ) synthesis technology that converts more than 120 million tons of N 2 into NH 3 annually. [1,2] However,this process is exceedingly energy intensiveo wing to harsh operating conditions, and exhibits extensive carbon emission. [1] Therefore, the development of more environmentally benign alternatives is crucial.Electrochemical synthesisi sa na ttractive candidatef or N 2 fixation ase nergy consumption, if optimized (i.e. with low overvoltage and high faradaic efficiency), could potentially be reduced as compared to the Haber-Bosch process while avoiding CO 2 emission. [3,4] If the required electricity for the process is supplied from renewable sources, this approach offers a meanstostore excess renewable energy in agreen energy carrier with an energy density comparable to fossil fuels. [5] True sustainability,h owever,w ill only be realized if the electrons required for the N 2 reduction originate from water rather than H 2 .Over the last three decades, such electrochemical processes have been developed, with state-of-the art technologiess ummarized in Ta ble S1 in the Supporting Information. [6][7][8][9][10][11][12][13][14][15][16] Unfortunately,t he faradaic efficiencies (FEs) in most cases were unbearably low (< 5%)a st he competing hydrogen evolution from water or ap rotonw as preferred over the intended N 2 reduction, which was particularly sluggish owing to the strong triple bond of N 2 (941 kJ mol À1 ). [3,17,18] Innovative strategies are thus indispensable to overcome such obstacles and achieve excellent selectivity in the N 2 reduction.One promisingw ay is to employ Li as amediator in the electro-synthesis. In contact with N 2 ,m etallicL i, on accounto fi ts strong reducing power,c auses dissociation to form lithium nitride (Li 3 N) even under ambient conditions. Thus-formed Li 3 N is ah igh-energyi ntermediate for NH 3 synthesis that is readily transformed into NH 3 upon reaction with protons or water. [17] This so-called "Li-mediated pathway" was firstly applied to synthesizing NH 3 in studies by Ts unetoe tal.,w hich reported an impressively high maximum FE of 59 %a t5 0atm of N 2 (1 atm = 0....

Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction

“…chemistries have been proposed for electrochemical nitrogen reduction, including structured noble metals, 10 metal oxides, 11 metal nitrides, 12 metal sulfides, 13 nitrogen-and boron-doped carbon, 14 and lithium metal. [15][16][17][18][19] Despite the large variety of reported catalysts, many of them exhibit low Faradaic efficiencies (FEs) and rates for producing ammonia, often below 10% and 10 À10 mol cm À2 s À1 , respectively (Table S1). 19,20 Among these chemistries, lithium-metal-based methods report some of the highest FEs (Table S1).…”

“…19,20 Among these chemistries, lithium-metal-based methods report some of the highest FEs (Table S1). [15][16][17][18] Lithium metal is unique in that it can undergo a bulk reaction in which it spontaneously splits the nitrogen triple bond at ambient conditions. 21 In lithium-mediated nitrogen reduction, lithium ions are reduced to lithium metal, which spontaneously reacts with nitrogen to form lithium nitride (Li 3 N).…”

“…The Li 3 N then reacts with a proton source to form ammonia. High FEs in lithium-mediated nitrogen reduction were often achieved by running the process in a batchwise fashion, [16][17][18] at elevated pressures, 15 or at elevated temperatures at certain steps. 16,18 It would be attractive to produce ammonia continuously at ambient temperatures and pressures.…”

“…High FEs in lithium-mediated nitrogen reduction were often achieved by running the process in a batchwise fashion, [16][17][18] at elevated pressures, 15 or at elevated temperatures at certain steps. 16,18 It would be attractive to produce ammonia continuously at ambient temperatures and pressures. In this vein, a continuous method has been reported in which lithium metal is plated onto metal electrodes from a 0.2 M lithium perchlorate, 1% ethanol in tetrahydrofuran (THF) solution in the presence of nitrogen to produce ammonia.…”

Ambient Lithium-Mediated Ammonia Synthesis

“…3 Lithium-mediated electrochemical approaches have been shown to produce ammonia both continuously 4 and in a batch-wise fashion. 5,6…”