Achieving enhanced ionic mobility in nanoporous silica by controlled surface interactions

Abstract

We report a strategy to enhance the ionic mobility in an emerging class of gels, based on robust nanoporous silica micro-particles, by chemical functionalization of the silica surface. Two very different ionic liquids are used to fill the nano-pores of silica at varying pore filling factors, namely one aprotic imidazolium based (1-methyl-3-hexylimidazolium bis(trifluoromethanesulfonyl)imide, CCImTFSI), and one protic ammonium based (diethylmethylammonium methanesulfonate, DEMAOMs) ionic liquid. Both these ionic liquids display higher ionic mobility when confined in functionalized silica as compared to untreated silica nano-pores, an improvement that is more pronounced at low pore filling factors (i.e. in the nano-sized pore domains) and observed in the whole temperature window investigated (i.e. from -10 to 140 °C). Solid-state NMR, diffusion NMR and dielectric spectroscopy concomitantly demonstrate this effect. The origin of this enhancement is explained in terms of weaker intermolecular interactions and a consequent flipped-ion effect at the silica interface strongly supported by 2D solid-state NMR experiments. The possibility to significantly enhance the ionic mobility by controlling the nature of surface interactions is extremely important in the field of materials science and highlights these structurally tunable gels as promising solid-like electrolytes for use in energy relevant devices. These include, but are not limited to, Li-ion batteries and proton exchange membrane (PEM) fuel cells.

Comparison of the Ionic Conductivity of Pure Imidazolium Nitrate and That Doped with a Lithium Nitrate Salt in Liquid and Gel Forms as Potential Electrolytes

“…An increase in conductivity for the lowest studied concentration of salt addition in gel state can be clearly seen in Figure 2, which shows the conductivity vs. molality curves for different temperatures. This behavior agrees with Garaga et al [4], who found an enhancement of ionic mobility after gelation using silica micro-particles, although to our knowledge, studies about the behavior of conductivity with salt addition in gel form have not been previously performed.…”

“…A main procedure for the sol-gel route in order to prepare gel samples was carried out, using an adaptation of the method described by Garaga et al [4]. A brief description of the selected method is the following:…”

Physicochemical study of diethylmethylammonium methanesulfonate under anhydrous conditions

“…17 In addition, [DEMA][OMs] was employed to fill nano-pores of silica to enhance the ionic mobility. 19 (3) Studies of the σdc and proton conduction mechanisms. 19−21 Martinelli et al found that the σdc of bulk [DEMA][OMs] obeys a VFT dependence, and that the charge transport mechanism is strongly associated with the vehicle mechanism.…”

“…19−21 Martinelli et al found that the σdc of bulk [DEMA][OMs] obeys a VFT dependence, and that the charge transport mechanism is strongly associated with the vehicle mechanism. 19 Yarger et al measured the ionicity and transport properties of a series of diethylmethylamine based PILs by nuclear magnetic resonance (NMR) spectroscopy, and concluded that the gas phase proton affinity is a better predictor of ionicity than the commonly used aqueous ∆pKa. 20 18 ) as received without further purification while other reports also indicated significant water content (e.g.…”

Hydrothermal stability of water sorption ionogels

“…However, in immobilizing an IL for gas-solid applications, a relatively weak physical interaction between support and the IL may be adequate owing the very low volatility of the IL [43]. A number of ionogels have been prepared using silica and physical immobilization [39,42,44,45], and their features such as ionic transport and viscoelastic properties have been studied [46,47]. Ionogels incorporating highly hygroscopic ILs can be potentially used as adsorbents for AD but there have been few studies on the hydrothermal stability and adsorption behavior of ionogels.…”