Mobility and phase transitions of the [EMIm+][FSI-] ionic liquid confined in micro- and mesoporous carbons
[ 1 ] Instytut Chemii i Elektrochemii Technicznej, Wydział Technologii Chemicznej, Politechnika Poznańska | [ P ] employee | [ D ] phd student | [ SzD ] doctoral school student
EN Recently, it was shown that the applicability of ionic liquids (ILs) can be extended to lower temperatures by their confinement in porous matrices. However, when considering the high-power output required for energy storage systems as electrical double-layer capacitors, it is very important to obtain information about the low temperature mobility of ions confined in porous electrodes. DSC and 1 H NMR are herein applied to analyze the phase transitions and mobility of [EMIm+][FSI] confined in a microporous carbon and two silica templated mesoporous carbons of different mesopore size. In the microporous carbon, due to the strong interactions of the IL with the pore walls, the mobility of ions is already reduced at room temperature, and it only slightly declines upon cooling. The IL confined in mesopores appears in a state closer to the bulk, exhibiting freezing/melting phase transitions downshifted by about 30 K as compared to the neat IL. As referred to the neat IL, freezing in mesopores occurs over a temperature range of 10 K, whereas melting is extended to more than 20 K for Lmeso ~10.5 nm and only 5 K for Lmeso ~9 nm. The broadened freezing/melting temperature range of the IL confined in the mesoporous carbons, as compared to the neat IL, is related to the existence of two populations of ions: (i) located in the vicinity of the pore walls (interacting both with the pore walls and other molecules) and (ii) surrounded by other ions in the proximity of the pore center, thus being in a bulk-like state. Upon cooling slightly below the freezing point, the ions interacting with the mesopore walls remain relatively mobile, although the bulk-like molecules are already frozen. Hence, the combination of 1 H NMR and DSC studies allows determining if, at a considered low temperature, all the confined molecules become immobilized/mobilized and, at the same time, whether the mobility reduction/recovery is related to freezing/melting. Overall, the combination of the two approaches is a powerful tool to predict if an applied porous carbon–IL pair is eligible for the realization of a high-power energy storage system at a selected low temperature.
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