Electrochemical synthesis of ammonia directly from air and water using a Li+/H+/NH4+ mixed conducting electrolyte

Abstract

Ammonia has been successfully synthesised directly from air and water using an electrochemical cell based on an H⁺/Li⁺/NH₄ ⁺ mixed conducting membrane and Pt/C electrodes. It is found that the Nafion 211 membrane exhibits mixed H⁺/Li⁺ conduction after exchanging in 0.1 M Li₂SO₄ solution. The ionic conductivity of the mixed conductor is slightly lower than that of H⁺-form Nafion 211. The introduction of Li⁺ ions to the cell did not improve the ammonia formation rates in our experiments. Reasonably higher temperature may favour ammonia formation and the highest ammonia formation rate (9.37 × 10⁻⁶ mol m⁻² s⁻¹) and Faraday efficiency (0.83%) was obtained at 80 °C when a voltage of 1.2 V was applied. The ammonia formation rate decreased when 0.1 M Li₂SO₄ solution instead of water was used in the cell. Under the applied potential, the presence of Li⁺ ions might have a blocking effect on the transfer of protons resulting in a lower current at higher applied voltage, particularly at lower temperatures.