Ionic liquids, consisting entirely of ions, now refer mostly to molten salts that assume a liquid state at or near room temperature. Ionic liquids have the characteristics of small steam pressure, non-flammable, good stability, large heat capacity, good electrical conductivity, designability and others. As an important type of ionic liquids, pyrrolidinium salt ionic liquids contain no unsaturated bonds and have good chemical stability. The cation of pyrrolidinium salt ionic liquid is N-alkyl-n-methyl pyrrolidine, and the alkyl group can be ethyl, propyl, butyl, hexyl, octyl and others. The anions of pyrrolidinium salt ionic liquid include bromine, tetrafluoroborate acid, hexafluorophosphate and others.
- Lithium air battery: In general, lithium air batteries are made up of four parts including a lithium metal anode, an electrolyte, a diaphragm and an air cathode. Li and oxygen are mainly transferred to the surface of the active substance through the electrolyte in the pores of the catalyst layer, and then the discharge products are formed by electrochemical reaction. Therefore, electrolyte is considered to be one of the key elements to improve the performance of lithium air batteries. The traditional organic electrolyte is volatile and generates lithium carbonate or alkyl lithium carbonate through oxygen electroreduction reaction, which will deteriorate the performance of lithium air battery. In contrast, pyrrolidinium salt ionic liquid has the characteristics of smaller volatility, higher electron conductivity, wider electrochemical window and higher solubility. As an electrolyte of lithium air battery, it is beneficial to improve battery performance.
Figure 1. An example of pyrrolidinium salt ionic liquids used in lithium air battery.
- Dye-sensitized solar cell: As a kind of semiconductor optoelectronic device which can directly convert solar energy into electric energy, solar cell is widely concerned at home and abroad and has been developed rapidly. Dye-sensitized solar cells based on titanium dioxide nanofilm have become a hot topic in the research of nanostructured photovoltaic cells due to their simple fabrication process, low cost and high conversion efficiency. In order to solve the problem of slow etching of flexible dye-sensitized solar cells plastic substrate by organic solvent electrolyte, pyrrolidinium salt ionic liquid has attracted the attention of researchers. For example, some researchers prepared three new pyrrolidinium salt ionic liquids by using allyl group as the substituent of pyrrolidine cation and combining iodide ion, thiocyanate ion and dicyandiamide ion, and then constructed two ionic liquid electrolytes with high iodide ion concentration and applied them in dye-sensitized solar cells. This study lays a foundation for designing new ionic liquid electrolytes and preparing high performance dye-sensitized solar cells in the future.
- Others: In addition to the above applications, pyrrolidinium salt ionic liquids can also be used as electrolyte in supercapacitors, lithium ion secondary batteries and others. The safety of these devices can be greatly improved by replacing traditional organic electrolytes with pyrrolidinium salt ionic liquids.
- Kerner, Manfred, Johansson, et al. Pyrrolidinium FSI and TFSI-Based Polymerized Ionic Liquids as Electrolytes for High-Temperature Lithium-Ion Batteries[J]. Batteries, 2018.