Ionic liquid is a kind of liquid material composed entirely of ions due to the asymmetry of anion and cation and spatial obstruction, resulting in low electrostatic potential of ions. Aluminum trichloride and ethyl pyridine halide ionic liquid are the first generation of room temperature ionic liquid. After S. John et al. synthesized dialkyl imidazole cationic salt with better electrochemical stability, ionic liquid quickly became a research hotspot. The specific energy of supercapacitors is lower than that of lithium-ion batteries and it is an urgent problem to improve the specific energy while maintaining high specific power. To improve the specific energy of supercapacitor, it is necessary to increase the working voltage and the specific capacitance. The operating voltage is related to the decomposition voltage of the electrolyte. At present, there are two kinds of electrolytes for ultracapacitors: water system and organic system. The water electrolyte is sulfuric acid solution or potassium hydroxide solution, which is highly corrosive, and the working voltage of the prepared monomer supercapacitor is low. The organic electrolyte is an organic solution of tetraethyl ammonium salt of tetrafluoroborate, and the working voltage of the monomer supercapacitor is above 2.5V. However, there are some problems, such as volatile organic solvent, difficult to improve the conductivity and working voltage, potential safety risks and environmental impact. Ionic liquids can be directly used as liquid electrolytes for supercapacitors, and can also be dissolved in organic solvents as electrolyte salts. Solid polymer electrolytes can also be introduced to improve the related properties.
Figure 1. An example of ionic liquids used as liquid electrolyte for supercapacitors.
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