Ionic Liquids For Fuel Cells

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.

Applications:

• Electrolyte for Fuel cell: Ionic liquids can be used as electrolytes for fuel cells because of their good electrical properties. Studies have shown that 1-butyl-3-methylimidazole tetrafluoroborate and 1-butyl-3-methylimidazole hexafluorophosphate can improve the performance of hydrogen fuel cells. Moreover, the performance of methane fuel cells can be improved by 1-butyl-3-methylimidazole bromide, 1-butyl-3-methylimidazole bisulfate and 1-butyl-3-methylimidazole chloride. For example, in 2003, a researcher group proposed a new type of ionic liquid fuel cell that uses a hydrophilic 1-butyl-3-methyl-imidazolium tetrafluoroborate ([Bmim]BF₄) and a hydrophobic 1-butyl-3-methyl-imidazolium hexafluorophosphonate ([Bmim]PF₆) instead of the KOH electrolyte. The obtained fuel cell has a cell efficiency of 67% and an open circuit voltage of 1.0V, especially using [Bmim]BF₄ to reduce its operating temperature to room temperature.

Figure 1. An example of ionic liquid used as electrolyte for fuel cell.

• Modified proton exchange membrane: Because ionic liquid is composed of organic cation and inorganic anion, it is suitable for proton exchange membrane, so the introduction of ionic liquid into proton exchange membrane is a very active research topic in recent years. Ionic liquid can be combined with sulfonated polyimide to prepare modified sulfonated polyimide (SPI) composite membrane, which can greatly improve the proton conductivity of proton exchange membrane. In addition, ionic liquid can also interact with inorganic nanoparticles to modify the proton exchange membrane. For example, a proton conductive composite membrane can be prepared by SiO and the proton ionic liquid, which is used for proton conduction under anhydrous conditions.

Figure 2. An example of ionic liquid modified proton exchange membrane.

Reference

• Péter Bakonyi, László Koók, Tamás Rózsenberszki, et al. Development and Application of Supported Ionic Liquid Membranes in Microbial Fuel Cell Technology: A Concise Overview[J]. Membranes, 2020, 10(1), 16.