Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen

Abstract

Electrochemical transformations in non-aqueous solvents are important for synthetic and energy storage applications. Use of non-polar gaseous reactants such as nitrogen and hydrogen in non-aqueous solvents is limited by their low solubility and slow transport. Conventional gas diffusion electrodes improve the transport of gaseous species in aqueous electrolytes by facilitating efficient gas–liquid contacting in the vicinity of the electrode. Their use with non-aqueous solvents is hampered by the absence of hydrophobic repulsion between the liquid phase and carbon fibre support. Herein we report a method to overcome transport limitations in tetrahydrofuran using a stainless steel cloth-based support for ammonia synthesis paired with hydrogen oxidation. An ammonia partial current density of 8.8 ± 1.4 mA cm−2 and a Faradaic efficiency of 35 ± 6% are obtained using a lithium-mediated approach. Hydrogen oxidation current densities of up to 25 mA cm−2 are obtained in two non-aqueous solvents with near-unity Faradaic efficiency. The approach is then applied to produce ammonia from nitrogen and water-splitting-derived hydrogen. Non-polar gaseous reactants such as N2 and H2 exhibit low solubility and slow transport in non-aqueous solvents and conventional gas diffusion electrodes cannot avoid non-aqueous electrolyte penetration. Here, transport limitations and catalyst flooding in tetrahydrofuran are overcome by using a stainless steel cloth-based support for lithium-mediated ammonia synthesis paired with H2 oxidation.