32 Veroutis, Emmanouil Granwehr, Josef Klankermayer, Jürgen Wiegand, Thomas 155520 150000 RWTH Aachen University Aachen English 1 Online-Ressource : Illustrationen, Diagramme The thesis exploits the application of choline acetate ([Ch]+[OAc]−) and choline trifluoroacetate ([Ch]+[TFA]−) as IL electrolytes for Zn-air batteries. In particular, NMR has been used extensively in order to elucidate the intra- and inter-molecular interactions in the solution mixtures. These interactions are found to be of importance in the physiochemical properties, e.g. viscosity, melting point, conductivity etc. of the IL electrolyte. Furthermore, the interactions of the ILs with polyacrylonitrile (PAN) derived fiber membranes, which are proposed as alternatives to gas diffusion electrodes in metal-air batteries, have been elucidated. For the mixtures with 30 wt.% H2O proton exchange kinetics with lifetimes on the order of 4 ms were observed for the –OH of choline and H2O. From the corresponding enthalpies and entropies of activation it is further deduced that the proton exchange reaction proceeds via unobservable intermediate states, which may correspond to protonation of the acetate. From relaxation and NOESY measurements it is further suggested that the ions and water exhibit collective reorientation for temperatures up to about 306 K. These observations support the existence of a solvent shared ion pair (SIP) configuration. At the same time no significant correlation was observed in the translational dynamics of the ions, which allows for high ionic conductivity and charge transport in the electrolyte. From titration experiments in solutions of [Ch]+[OAc]− in DMSO-d6, dissociation constants and corresponding Gibbs energies reveal the existence of solvent separated ion pairs (2SIP) at low concentrations (<300 mM), whereas at high concentrations solvent shared ion pairs (SIP) were more favorable. A transition from free to collective ion translational dynamics at about 300 mM was also assigned from the diffusion coefficients. It is further deduced that the viscosity of the solutions at high concentrations is dominated by local ion-solvent interactions and is not attributed to the viscosity of the solvent. The extent of ion association found in solvents such as DMSO is not expected to significantly limit the charge transport in choline-based electrolytes. Nonetheless, association effects in analogous solutions need to be taken into consideration for rational design of electrolytes. For the ILs adsorbed on PAN carbon fiber (CPAN) membranes 1H downfield shifts indicate that the ions are located at distances of about 7 Å from the surface. In particular for [Ch]+[TFA]−, the slow exchange kinetics observed for the –OH of choline and H2O, as well as the negative intra-molecular NOEs, suggest a change in water solvation of the ions and a surface induce dynamic hindrance. These observations are in accordance with an increase in the viscosity of the surface layer and support the existence of a cation-water-anion layered structure on the negatively charged surface of CPAN. It is proposed that for low IL/electrolyte concentrations on carbonized PAN fiber membranes the physiochemical properties of the electrolyte e.g. viscosity, density, ionic conductivity etc. are dominated by ion-surface interactions. This can be a contributing factor for the performance of Zn-air batteries when low aqueous electrolyte loadings in carbon fiber-based gas diffusion electrodes (GDEs) are used. Veröffentlicht auf dem Publikationsserver der RWTH Aachen University ; Dissertation, RWTH Aachen University, 2022 ; 2, ; PUB:(DE-HGF)11, ; Dissertation / PhD Thesis Dissertation RWTH Aachen University 2022 2022 RWTH-2022-10182 2022 https://publications.rwth-aachen.de/record/855365