Imidazolium ionic liquids nano-confined in mesoporous silica

Licentiatavhandling, 2020

Protic ionic liquids (PILs), showing unique proton conductive properties, are considered to be suitable electrolytes for low and intermediate temperature fuel cells. To form solid-like electrolyte, one approach is to confine PILs in the pores of a solid matrix, thus forming a functional hybrid material in which the PIL constitutes a proton conductive phase and the matrix ensures mechanical stability. In this work, proton transport mechanisms and correlated local structures in the binary liquid mixture of HC8ImTFSI (1-octylimidazolium bis(trifluromethanesulfonyl)imide) and imidazole was studied while nano-confined in hydrophobized silica pores. For this, series of hybrid materials with varying liquid-to-silica ratios were investigated. One aspect considered in this approach was the effect of downsizing the pores on transport and structural properties. In these materials proton conduction properties were characterized by impedance spectroscopy and PFG NMR (pulse-field gradient nuclear magnetic resonance spectroscopy), under the assumption that the flipped-ion effect occurs, when the alkyl chains of the cations are oriented towards the hydrophobic silica pore walls. In such structuration, there are weak cation-pore wall interactions that favor unrestricted diffusion. Ionic liquids (ILs) are also extremely interesting as multi-functional softtemplates for the synthesis of mesoporous materials and IL-based functional hybrid materials. Here, C16MIMCl (1-hexadecyl-3-methylimidazolium chloride), was used as soft-template for the formation of vertically aligned, uniform, channel-like pores, running through the entire thickness of the film, with a well defined pore width of 2.5 nm in silica thin films deposited with the electrochemically assisted self-assembly (EASA) method. Furthermore, the mechanism of pore formation is explained; unlike the mechanisms reported for short chain imidazolium IL templates, in the case of C16MIMCl the dominating so-called cooperative interaction is the electrostatic attraction between the C16MIM+ cation and the network-forming negatively charged silicate oligomers.