Specific charge transport in ionic liquids and ion gels and the importance in material science

Abstract

Ionic liquids have been recognized as ideal electrolytes for electrochemical systems owing to their unique properties such as non-volatility, non-flammability, high ionic conductivity, and gel-forming property with polymers. The examples include lithium batteries, fuel cells, double-layer capacitors, actuators, and dye-sensitized solar cells (DSSCs). Here, we revealed the details of charge transport in conventional ionic liquids, proton-conducting protic ionic liquids, and electron-transporting ionic liquids. The latter two examples were task-specific ionic liquids, potentially applicable as fuel cell electrolytes under non-humidifying conditions and also as non-volatile electrolytes in DSSCs, where charge transport occurs via the so-called Grotthuss-like mechanism. To realize polymer electrolytes with high ionic conductivity, we exploited the high ionic conductivity of ionic liquids. In situ free radical polymerization of compatible vinyl monomers in an ionic liquid afforded a novel series of polymer electrolytes, which we named "ion gels". The ionic conductivity at ambient temperature reached a value close to 10-2 S cm-1. The number of carrier ions, calculated from the Nernst-Einstein equation, was found to increase in the ion gels from the corresponding value for the ionic liquid itself, which seems to be caused by the interaction between the polymer matrix and the ionic liquid.