Double-layer capacitors thermal properties

To apply ionic liquids as an electrolyte for double-layer capacitors, they should have wide operational temperature range and high safety. Although ionic liquids have favorable properties such as nonvolatility and nonflammability from the viewpoint of thermal stability and safety, they have the disadvantage that because of their higher viscosity, they rapidly lose their electrolytic conductivity as the temperature decreases. 

In the drive towards expanding the range of useful ionic liquids (ILs), with suitably low viscosities, sufficient stability, and conductivity for electrochemical uses, phosphonium-based ionic liquids (PILs) are exceptionally promising compounds. Due to the high thermal and electrochemical stabilities of PILs compared to ammonium-based ILs (allq l-ammonium, imidazolium, pyridinium, pyrrolidinium, and piperidinium compounds), phosphonium salts have been especially considered for applications as electrolytes. Their properties (both thermal and electrochemical stabilities) are in fact crucial to improve safety, durability, power and energy densities of electrochemical devices such as electric double layer capacitors. 

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