The Feasibility of Electrochemical Ammonia Synthesis in Molten LiCl–KCl Eutectics

Abstract

Molten LiCl and related eutectic electrolytes are known to permit direct electrochemical reduction of N2 to N3− with high efficiency. It had been proposed that this could be coupled with H2 oxidation in an electrolytic cell to produce NH3 at ambient pressure. Here, this proposal is tested in a LiCl–KCl–Li3N cell and is found not to be the case, as the previous assumption of the direct electrochemical oxidation of N3− to NH3 is grossly over‐simplified. We find that Li3N added to the molten electrolyte promotes the spontaneous and simultaneous chemical disproportionation of H2 (H oxidation state 0) into H− (H oxidation state −1) and H+ in the form of NH2−/NH2−/NH3 (H oxidation state +1) in the absence of applied current, resulting in non‐Faradaic release of NH3. It is further observed that NH2− and NH2− possess their own redox chemistry. However, these spontaneous reactions allow us to propose an alternative, truly catalytic cycle. By adding LiH, rather than Li3N, N2 can be reduced to N3− while stoichiometric amounts of H− are oxidised to H2. The H2 can then react spontaneously with N3− to form NH3, regenerating H− and closing the catalytic cycle. Initial tests show a peak NH3 synthesis rate of 2.4×10−8 mol cm−2 s−1 at a maximum current efficiency of 4.2 %. Isotopic labelling with 15N2 confirms the resulting NH3 is from catalytic N2 reduction.

Energy Decarbonization via Green H2 or NH3?

“…Detailed investigation into the electrochemical mechanism has revealed the existence of a number of rapid non-electrochemical reactions of N 3− with H 2 to form NH 3 along with NH 2− and NH 2 − , which are shown to be electrochemically active by cyclic voltammetry. 62 An alternative strategy was recently demonstrated in which Li metal was used to reduce N 2 . 63 So far, several thousand electro-and photocatalysts were screened worldwide in the development of non-thermal NH 3 synthesis.…”

“…However, we have recently demonstrated that the previous assumption of a direct electrochemical reaction of N 2 and H 2 in molten LiCl–KCl to form NH 3 with high efficiency is not correct. Detailed investigation into the electrochemical mechanism has revealed the existence of a number of rapid non-electrochemical reactions of N 3– with H 2 to form NH 3 along with NH 2– and NH 2 – , which are shown to be electrochemically active by cyclic voltammetry . An alternative strategy was recently demonstrated in which Li metal was used to reduce N 2 .…”

Preliminary economics for green ammonia synthesis via lithium mediated pathway

“…The nitrogen generation section of the process was adapted from Gomez et al 22 [Colour figure can be viewed at wileyonlinelibrary.com] via electrolyte variation. We keep in mind current experimental rates 24 for ammonia formation up to 10 −8 mol s −1 cm −2 compared to the DOE target of 90% with 9.33 × 10 −7 mol s −1 cm −2 . This brings to bear that higher FE presently comes at the expense of ammonia formation rate.…”

Indirect electrosynthesis of ammonia from nitrogen and water by a magnesium chloride cycle at atmospheric pressure

“…19,20 It was reported that the electrosynthesis of NH 3 from N 2 and H 2 using molten chloride electrolytes at elevated temperatures could be faster ($10 À8 mol , cm À2 s À1 ), 21 but the result may be flawed, according to a recent investigation. 22 The claim of NH 3 synthesis from N 2 of only a very small amount should be carefully verified by 15 N tracing to avoid false positives. 23,24 Recently, it was reported that an indirect electrochemical NH 3 synthesis, based on electrolyzing molten LiOH into Li, O 2 , and H 2 O, and using Li to fix N 2 into Li 3 N, shows potential advantages of both high speed ($10 À7 mol , cm À2 s À1 ) and coulombic efficiency (88.5%).…”