Investigations on dynamic properties of ionic liquids towards their application as electrolytes

by Alexander Benedikt Rupp

Institution: Universität Freiburg
Department: Chemie und Pharmazie (seit 01.01.2013)
Degree: PhD
Year: 2015
Record ID: 1105827
Full text PDF: http://www.freidok.uni-freiburg.de/volltexte/2015/9939/


In this work, the synthesis and a thorough investigation on the transport properties of highly fluorinated ionic liquids (ILs) is presented, with the main focus on a possible application of these ILs in supercapacitors. An extensive comparative study of the dynamic viscosities, electrical conductivities and ion specific diffusion constants of known and new [NTf2]– (Tf = SO2CF3) and [Al(hfip)4]– (hfip = OCH(CF3)2) based ILs showed that [Al(hfip)4]– ILs possess at least comparable and partially even better dynamic properties than the analogous [NTf2]– ILs. An analysis of the diffusion constants with a modified Stokes-Einstein equation (SEE) allowed for a semiquantitative comparison of the interactions in both IL classes. Additionally, the applicability of the original SEE could be falsified. Hirshfeld analyses and the theoretical treatment with a modified Marcus theory proved that [Al(hfip)4]– ILs exhibit considerably lower interactions, leading to higher ionicities and better transport properties. Mixtures of several highly fluorinated ILs were prepared, and viscosities and diffusion constants were determined. Almost all mixtures yielded clear deviations from ideal behavior, which was attributed to the interplay of different kinds of interactions and mass transport via holes within the ILs. The mixtures of several [B(hfip)4]–, [Al(hfip)4]– and [NTf2]– ILs with dimethyl carbonate opened another way to further improve the dynamic properties of these substances. An attempt to use [Al(hfip)4]– ILs as electrolytes in supercapacitors was not productive due to a pronounced sieving effect at the electrodes. The utilization of macroporous electrodes lead to a relative improvement compared to [NTf2]– ILs. On an absolute scale, however, no enhancement was found. The synthesis of a new class of ILs based on the [B(tfe)4]– (tfe = OCH2CF3) anion was achieved. Viscosities and melting points of these ILs indicate a possible application as lubricants. For further optimization of the properties of the investigated highly fluorinated ILs, first attempts towards the synthesis of partially fluorinated imidazolium cations were conducted. It could be shown that such syntheses are somewhat intricate but well possible. Viscosities of ILs with such cations are higher than in the case of unfluorinated cations due to reduced entropy. Still, this field is a promising starting point for future improvements, for instance of the stability of ILs. Die vorliegende Arbeit beschäftigt sich mit der Untersuchung der Transporteigenschaften sowie der Synthese hochfluorierter Ionischer Flüssigkeiten (ionic liquids, ILs), besonders im Hinblick auf eine mögliche Anwendung dieser ILs in Superkondensatoren. Es wurde ein ausführlicher Vergleich dynamischer Viskositäten, elektrischer Leitfähigkeiten und ionenspezifischer Diffusionskonstanten bekannter und neuer ILs auf Basis von [NTf2]–- (Tf = SO2CF3) und [Al(hfip)4]–-Anionen (hfip = OCH(CF3)2) durchgeführt. Dabei wiesen die Aluminat-ILs mindestens vergleichbare und teils drastisch bessere dynamische…