Proton conducting ionic liquids-Binary mixtures and structural modifications

Abstract: Proton conduction is a phenomenon of fundamental importance for biological and chemical systems, such as in photosynthesis and aerobic respiration. Proton conduction is also key in the proton exchange membrane fuel cell, a clean electrochemical device that converts chemical into electrical energy. The archetypical proton exchange membrane is currently a perfluorinated polymer with pending acidic groups. The fuel cell technology is presently at renewed focus in the commercial sector, and consequently also in the scope of research and development programs. In order to enable the development of more sustainable proton exchange membrane fuel cells, new requirements have been set that for instance target operation at higher temperatures and anhydrous conditions. In this context, protic ionic liquids have been highlighted as interesting next-generation electrolytes with the potential to assist proton conduction. Protic ionic liquids are a subset of ionic liquids, also known as low temperature molten salts, entirely composed of ions and displaying very low vapor pressures. Moreover, a protic ionic liquid has an exchangeable proton and is able to establish extended networks of hydrogen bonds. Understanding the dynamics of this local structures is fundamental to design new protic ionic liquids able to sustain a fast proton motion decoupled from the diffusion of the ionic species. To achieve such a dynamical behavior we have investigated two different approaches, that is i) mixing a protic ionic liquid candidate with a co-solvent (here ethylene glycol) able to participate in the hydrogen bonds and ii) modifying the molecular structure of the cation (here by alkyl chains of length (n) varying from ethyl (n=2) to dodecyl (n=12)) in a series of imidazolium based protic ionic liquids. The local intermolecular inter- actions, the nano-structuration as well as ionic and protonic dynamics have been investigated by suitable techniques that include vibrational spectroscopy, NMR spectroscopy, diffusion NMR, impedance spectroscopy and X-ray scattering.