Ionic liquid is salt that composed of ions, mainly characterized by large and asymmetric cations and small anions. The usual ionic compounds are generally solid at room temperature because of the strong ionic bonds that make the ions and ions on the lattice cannot rotate or translate but only vibrate. However, compared to conventional liquid and solid molecules, the volume of anions and cations of ionic liquids becomes large and asymmetrical, then the strong electrostatic force and spatial obstacles make it impossible for the anions and cations to form dense accumulation at the micro level. In this structure, anions and cations can not only rotate, but also rotate and translate, which leads to the destruction of the entire crystal structure, the lattice energy becomes smaller, so that the melting point of this ion is reduced, and it can be liquid at room temperature. Ionic liquids have a unique significance for electrochemistry because of their high conductivity and wide electrochemical window, which makes ionic liquids attract the attention of researchers all over the world. Electrolyte plays an important role in the composition of fuel cells, and ionic liquids are very suitable for the development of new fuel cells because of their excellent ionic conductivity and rich designability. In addition, ionic liquids can also be used to modify the proton exchange membrane.
Figure 1. An example of ionic liquid used as electrolyte for fuel cell.
Figure 2. An example of ionic liquid modified proton exchange membrane.