Optimization of ionic-liquid based electrolyte concentration for high-energy density graphene supercapacitors

Abstract

Electrolyte properties play an important role to determine the capacitive performance of electric double-layer capacitors (EDLCs), especially the specific capacitance and energy density. In general, electrolytes with a high electrochemical stability window (ESW) can offer both higher specific capacitance and energy density, which explains why ionic liquid (IL)-based electrolytes have been extensively studied. The concentration of IL is a critical parameter to control its viscosity, ionic conductivity, and potential window that is reflected in EDLC working voltage, which has yet to be systematically studied. In this paper, we presented a systematic approach to determine the optimum IL concentration for graphene-based EDLCs by measuring the viscosity and ionic conductivity of IL electrolyte containing different amounts of organic solvent and the corresponding maximum working voltages (MWV) of EDLCs via cyclic voltammetry (CV), as well as the associated specific capacitances. Such a systematic study fills in the missing knowledge on the optimum ionic liquid-based electrolyte concentration for graphene-based supercapacitors applications. We found that the electrolyte viscosity increases exponentially with increasing IL concentration, while the ionic conductivity decreases with an increase in IL concentration beyond its maximum at 2 M EMIMBF4/IL. The specific capacitance shows a strong dependence not only on the electrolyte viscosity and ionic conductivity, but also the MWV where electrode specific capacitance increases with the MWV of EDLCs. Therefore, despite the highest viscosity and the lowest ionic conductivity, the neat IL (i.e., EMIMBF4 in this work) offers the largest specific capacitance and energy density among all IL concentrations for graphene-based EDLCs due to the largest MWV offered by the neat ILs. However, if the EDLC is not required to operate at the ESW of the IL, diluted IL electrolytes with an optimized concentration/IL viscosity can be used instead to achieve the most economic EDLC performance. In conclusion, the concentration of IL electrolyte should be optimized according to the working voltage required.