Exploring binary mixtures of protic ionic liquids– Interactions, dynamics, and non-ideal behavior

Doctoral thesis, 2016

Ionic liquids are organic salts that melt at low temperatures and provide a set of properties beneficial for diverse applications. These properties include good thermal stability, high ionic conductivity, low volatility and non-flammability. In this thesis protic ionic liquids have been at focus, which are of interest for use as electrolytes in next-generation proton exchange membrane fuel cells. The impact of the molecular structure of the ions as well as of the addition of a second compound on selected physicochemical properties has been investigated. Imidazolium and ammonium based protic ionic liquids have been considered as possible proton conducting materials, whereas water, imidazole and ethylene glycol were chosen as neutral additives, all being a priori capable of forming hydrogen bonds. Transport properties like self-diffusion, ionic conductivity and viscosity have been thoroughly investigated, and the observed behavior explained in terms of established intermolecular interactions. These have been probed by 1H NMR and vibrational (Raman and infrared) spectroscopy, used as powerful and complementary experimental tools. Overall both self-diffusion and ionic conductivity increase upon addition of a second compound, but the extent of this increase very much depends on the molecular structure of the cation-anion pair in the ionic liquid, and the ability of the ions to establish hydrogen bonds. For example, in the case of the protic ionic liquid ethylimidazolium bis(trifluoromethanesulfonyl)imide (C2HImTFSI) added water preferably interacts with the cation, while both cations and anions interact with added imidazole. These coordinations also results in very different phase changes and different mechanism of charge transport, with added imidazole promoting the Grotthuss mechanism of proton transfer as opposed to the case of added water. In a more hydrophilic protic ionic liquid like ethylimidazolium triflate (C2HImTfO), however, water forms bonds with both cations and anions, and a local and fast proton exchange has been probed. Nevertheless, the choice of the added compound is not straightforward since not all required properties may be enhanced at once. For instance, while ethylene glycol affects ionic conductivity by a lesser extent it can provide a wider window of thermal stability. The effect of confining an ionic liquid into nano-porous silica micro-particles has also been studied. The so called silica supported ionogels that we have considered can retain large volume fractions of the liquid and thus serve as support materials for electrolytes for use in fuel cell applications. Our results show that a strong interaction between the ammonium based protic ionic liquid (DEMA-OMs) and the pore walls of the silica nano-particles restricts the ionic mobility. As a solution to this effect the silica pore walls were functionalized with hydrophobic alkyl groups whereby a significant enhancement of the ionic conductivity was observed for the ionic liquid in the nano-sized pore domains. Our findings provide new useful insights for designing new electrolyte materials, the functionality of which will crucially depend on a careful selection of the ionic liquid, the added second compound and the surface chemistry of the support material. An optimal combination should be able to provide a fast and selective proton motion as required for use in fuel cells.