Ion Dynamics and Transport Properties of Lewis-Acidic Imidazolium Chloroaluminate Ionic Liquids. Issue 6 (8th June 2021)

• Record Type: Journal Article
• Authors: Cosby, Tyler; Schnabel, Michael J.; Durkin, David P.; Mantz, Robert A.; Trulove, Paul C.
• Is Part Of: Journal of the Electrochemical Society. Volume 168:Issue 6(2021)
• Publication Date: 2021-06-08
• Subjects: Ionic Liquids -- Chloroaluminate -- Speciation -- Dielectric spectroscopy -- Ion Dynamics -- Structure Property Relationships; Electrochemistry -- Periodicals; 541.3705
• DOI: 10.1149/1945-7111/ac0653
• Languages: English
• ISSNs: 0013-4651

Abstract

Abstract : Ion dynamics and charge transport are investigated in a series of Lewis-acidic 1-alkyl-3-methylimidazolium chloroaluminate ionic liquids (2:1 mole ratio AlCl3 :C n MIm Cl, n = 2, 4, 6, 8) and the chloride-based IL starting materials 1-hexyl-3-methylimidazolium chloride (C6 MIm Cl) and 1-octyl-3-methylimidazolium chloride (C8 MIm Cl) via broadband dielectric spectroscopy, shear rheology, and differential scanning calorimetry. A detailed analysis of the dielectric and mechanical spectra reveals the emergence of mesoscale polar/non-polar aggregate dynamics at longer alkyl chain lengths. The observed influence of cation and anion chemical structure on the dielectric signature of mesoscale aggregate dynamics is interpreted according to a recently discovered link between these dynamics and the local shape of non-polar domains. Furthermore, conductivity relaxation and dipole reorientation processes are probed by employing a simultaneous analysis of the dielectric spectra in the complex conductivity, permittivity, and electric modulus representations. The temperature dependent rates of the observed dielectric relaxation processes are contrasted with the rates of structural relaxation evaluated by shear rheology. The results confirm the close coupling of charge transport and structural rearrangement typical of aprotic ionic liquids; however, notable separation in the rates probed by the various dielectric representations indicate ion dynamics may be significantly more heterogeneous in the chloroaluminate ILs. In particular, the rates attributed to cation translational and rotational motion are found to be considerably slower than the rate of structural relaxation. Identifying this dynamic heterogeneity and elucidating its molecular origin is critical to understanding the transport properties of this IL class and promoting rational design of future ILs.