Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes

Jianan Wang; Joshua J Buzolic; Jesse W Mullen; Paul A Fitzgerald; Zachary M Aman; Maria Forsyth; Hua Li; Debbie S Silvester; Gregory G Warr; Rob Atkin

doi:10.1021/acsnano.3c06609

Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes

ACS Nano. 2023 Nov 14;17(21):21567-21584. doi: 10.1021/acsnano.3c06609. Epub 2023 Oct 26.

Authors

Affiliations

¹ School of Molecular Sciences, The University of Western Australia, Perth 6009, Australia.
² School of Molecular and Life Sciences, Curtin University, Perth 6102, Australia.
³ Sydney Analytical, Core Research Facilities, The University of Sydney, Sydney 2050, Australia.
⁴ Department of Chemical Engineering, The University of Western Australia, Perth 6009, Australia.
⁵ Institute for Frontier Materials and the ARC Centre of Excellence for Electromaterials Science, Deakin University, Geelong 3220, Australia.
⁶ Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth 6009, Australia.
⁷ School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney 2050, Australia.

PMID: 37883191
DOI: 10.1021/acsnano.3c06609

Abstract

The physical properties of ionic liquids (ILs) have led to intense research interest, but for many applications, high viscosity is problematic. Mixing the IL with a diluent that lowers viscosity offers a solution if the favorable IL physical properties are not compromised. Here we show that mixing an IL or IL electrolyte (ILE, an IL with dissolved metal ions) with a nonsolvating fluorous diluent produces a low viscosity mixture in which the local ion arrangements, and therefore key physical properties, are retained or enhanced. The locally concentrated ionic liquids (LCILs) examined are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM TFSI), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), or 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (BMIM FAP) mixed with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1, 1:1, and 1:2 (w/w) IL:TFTFE, as well as the locally concentrated ILEs (LCILEs) formed from 2:1 (w/w) HMIM TFSI-TFTFE with 0.25, 0.5, and 0.75 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). Rheology and conductivity measurements reveal that the added TFTFE significantly reduces viscosity and increases ionic conductivity, and cyclic voltammetry (CV) reveals minimal reductions in electrochemical windows on gold and carbon electrodes. This is explained by the small- and wide-angle X-ray scattering (S/WAXS) and atomic force microscopy (AFM) data, which show that the local ion nanostructures are largely retained in LCILs and LCILEs in bulk and at gold and graphite electrodes for all potentials investigated.

Keywords: diluent; electrochemical window; electrolyte; ionic liquid; nanostructure; viscosity.