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

Molten LiCl and related eutectic electrolytes are known to permit direct electrochemical reduction of N₂ to N^3- 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-Li₃ N cell and is found not to be the case, as the previous assumption of the direct electrochemical oxidation of N^3- to NH₃ is grossly over-simplified. We find that Li₃ N 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 NH^2- /NH₂ ^- /NH₃ (H oxidation state +1) in the absence of applied current, resulting in non-Faradaic release of NH₃ . It is further observed that NH^2- 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 Li₃ N, N₂ can be reduced to N^3- while stoichiometric amounts of H^- are oxidised to H₂ . The H₂ can then react spontaneously with N^3- 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 ¹⁵ N₂ confirms the resulting NH₃ is from catalytic N₂ reduction.