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Final Activity and Management Report Summary - UOXSSDWA (Ultrafast optical and X-ray studies of solvation dynamics in water and alcohol)

Electrolytes are usually conducting media, in which charged ions move freely in a solvent. Now, experiments led by Dr Verner Thorsmølle and Prof. Jacques-E. Moser revealed a new mechanism for the electrical conduction in ionic liquid electrolytes. The existence of a new conduction mechanism that did not involve the transport of ions was demonstrated. T%:%: his discovery was made while looking for ways to improve the conductivity of ionic liquids, which were used as electrolytes in dye-sensitised solar cells. The peculiarity of these liquids was that they only contained ions and no neutral solvent molecule. Evaporation of the liquid and its degradation were, thus, prevented. Iodine added to iodide-based ionic liquids led to extraordinarily efficient charge transport, vastly exceeding the one expected for such viscous systems. Using terahertz time-domain spectroscopy, in conjunction with dc conductivity, diffusivity and viscosity measurements, conductivity pathways in such an ionic melt were unravelled. The resulting measurements enabled the detection of the vibrations of the ions, as well as their way to associate with each other. This achievement was made possible thanks to ultra-short laser pulses that generated THz radiation, which lay between the infrared and the microwave frequency range. This radiation had the property of interacting with mobile electrical charges and low-frequency vibrations of matter. It was thus possible to observe in an ionic liquid, containing iodide and triiodide ions, a bond-exchange phenomenon. In this scenario, an iodide ion (I-) bound to a neighbouring triiodide (I3-) ion, while at the other end of the chain a bond was broken and a new I- ion was liberated. This mechanism governing the displacement of electrical charges only implied back and forth movements over very short distances, of the order of 3 angstroms, and did not require the transport of ions over longer ranges. Thanks to the bond exchange between ions, electrical conduction remained efficient, even in viscous liquids and at very low temperature. A similar mechanism was suggested by Theodor de Grotthuss at the beginning of the 19th century to explain the conductivity of water, but had never been demonstrated for an ionic liquid.

Temperature-dependent terahertz time domain spectroscopy measurements were also carried out on some ionic liquid mixtures containing the iodide-iodine redox couple. It was discovered that beyond a threshold concentration of iodine, new phases appeared between liquid and solid phases that allowed for increased conduction. These phases were found to be consistent with complementary X-ray studies of the melts. These novel results were very important for the fundamental understanding of conduction in room temperature molten salts and for applications where ionic liquids were used as charge transporting media such as in batteries and dye-sensitised solar cells.

Reported by

ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE
LAUSANNE
Switzerland