Comparative study of inorganic and hybrid ionogels as electrolyte for supercapacitors

Due to limitations of liquid electrolytes including toxicity, flammability and possibility of leakage, demand for a non-flammable solid electrolyte substitute for application in energy storage devices has increased... [ view full abstract ]

Due to limitations of liquid electrolytes including toxicity, flammability and possibility of leakage, demand for a non-flammable solid electrolyte substitute for application in energy storage devices has increased drastically. Low temperature (<100°C) molten salts, also known as ionic liquids (ILs), have drawn attention of electrochemistry experts due to their great electrochemical properties and low volatility. In order to eliminate the possibility of leakage, ILs can be immobilized inside a solid/quasi-solid matrix. Such structure is referred to as ionogel. This work elucidates the physical, thermal and electrochemical characteristics of ionogels synthesized by different silica-based precursors with the aim of stressing how effective the choice of precursor can be. The herein reported ionogels have been synthesized through a straight forward in-situ sol-gel process. 1-ethyl-3- methylimidazolium trifluoromethanesulfonate, formic acid and various silica alkoxides were used as reactants. The prepared ionogels are categorized as either inorganic group or hybrid group.
After completion of the gelation process, the ionogels were characterized using SEM, FTIR, Raman, and TGA. Their electrochemical performance for application in supercapacitors was also investigated using a combination of cycling voltammetry and EIS techniques. The findings evidenced a great influence of gel precursor on the morphology, pore structure and electrochemical behaviour of the gels. The SEM images, provided in Figure 1, show presence of a dense "protective" layer on the top surface of the hybrid gels while the morphology of the inorganic ionogels is shown to be homogeneous throughout their matrix. Furthermore, based on EIS analysis, inorganic ionogels have higher ionic conductivity ranging from 2 to 5 mS/cm while lower values are achieved with hybrid gel electrolytes (1-0.6 mS/cm). This difference in ionic conductivity resulted in capacitance values ranging from 60 to 20 F/g after 1000 voltammetric cycles while a capacitance of 70 F/g is achieved for a cell with pristine IL. Considerable difference in ionic conductivity can be attributed to diffusional barriers, poorly-connected pore structure and surface-hydrophobicity of hybrid electrolytes.