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.
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The data that support the plots in this paper and other findings of this study are available from the corresponding author on request.
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This material is based on work supported by the National Science Foundation under grant no. 1944007 and the MIT Energy Initiative (MITEI) seed fund. N.L. acknowledges support by the National Science Foundation Graduate Research Fellowship under grant no. 1122374. We thank M. Wolski of Daramic for providing us with polyporous separator samples.
The authors declare no competing interests.
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a, A comparison between the Faradaic efficiency toward ammonia when various gases are fed to the cell. When using N2 with different isotopic compositions, the ammonia yields are practically identical, which is a sign that N2 reduction is responsible for ammonia formation21. There is little to no ammonia formed when Ar is used as the feed gas and in the absence of current. Vertical error bars represent the uncertainty in Faradaic efficiency quantification of a single experiment. b, The amount of ammonia quantified in the base and acid traps used to clean the inlet gas, and the concentration of ammonia in a post-cell acid trap for comparison. c, Unscaled NMR spectra of electrolyte and acid trap solutions. When 14N2 is used as the feed gas, only a triplet from 14NH4+ is detected in both the trap and solution, while both 15NH4+ and 14NH4+ are detected when 15N2 is fed. ~92% of the NH4+ is 15NH4, which suggests some 14N2 contamination in the experiment, as the nominal isotopic content of the 15N2 is 98%. The peaks shift slightly due to differences in solvent composition (THF-water mixtures). The peak at ~6.87 is from butylated hydroxytoluene (BHT) found in the THF. The 25 mA experiments were performed by using a 3-compartment cell with a platinum foil anode, while the 20 mA experiments used a cell with no separator between electrolyte compartments and a Pt/SSC anode.
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Lazouski, N., Chung, M., Williams, K. et al. Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen. Nat Catal 3, 463–469 (2020). https://doi.org/10.1038/s41929-020-0455-8
Nature Catalysis (2020)