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The Feasibility of Electrochemical Ammonia Synthesis in Molten LiCl–KCl Eutectics
Author(s) -
McPherson Ian J.,
Sudmeier Tim,
Fellowes Joshua P.,
Wilkinson Ian,
Hughes Tim,
Tsang S. C. Edman
Publication year - 2019
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201909831
Subject(s) - disproportionation , electrochemistry , chemistry , electrolyte , stoichiometry , eutectic system , inorganic chemistry , faraday efficiency , redox , catalysis , ammonia , oxidation state , electrode , organic chemistry , alloy
Abstract Molten LiCl and related eutectic electrolytes are known to permit direct electrochemical reduction of N 2 to N 3− with high efficiency. It had been proposed that this could be coupled with H 2 oxidation in an electrolytic cell to produce NH 3 at ambient pressure. Here, this proposal is tested in a LiCl–KCl–Li 3 N cell and is found not to be the case, as the previous assumption of the direct electrochemical oxidation of N 3− to NH 3 is grossly over‐simplified. We find that Li 3 N added to the molten electrolyte promotes the spontaneous and simultaneous chemical disproportionation of H 2 (H oxidation state 0) into H − (H oxidation state −1) and H + in the form of NH 2− /NH 2 − /NH 3 (H oxidation state +1) in the absence of applied current, resulting in non‐Faradaic release of NH 3 . It is further observed that NH 2− and NH 2 − possess their own redox chemistry. However, these spontaneous reactions allow us to propose an alternative, truly catalytic cycle. By adding LiH, rather than Li 3 N, N 2 can be reduced to N 3− while stoichiometric amounts of H − are oxidised to H 2 . The H 2 can then react spontaneously with N 3− to form NH 3 , regenerating H − and closing the catalytic cycle. Initial tests show a peak NH 3 synthesis rate of 2.4×10 −8 mol cm −2 s −1 at a maximum current efficiency of 4.2 %. Isotopic labelling with 15 N 2 confirms the resulting NH 3 is from catalytic N 2 reduction.