Ionic liquid electrolytes in electric double layer capacitors

Abstract

Electric double layer capacitors (EDLCs), which store free charges on the electrode surface via non-Faradaic process, balanced by the electric double layer on the electrolyte side, exhibit excellent cycle stability and high power density. Though EDLCs are considered as promising energy storage devices, the charges stored on the electrode surface in EDLCs are much lower than those in batteries. Ionic liquids (ILs), as a new type of electrolytes in EDLCs, are capable to deliver high energy density, due to their excellent physicochemical properties and wide electrochemical window. In this review, we focus on the widely studied IL electrolytes for EDLCs, including pure ILs, IL/IL binary electrolytes, IL/organic solvent mixtures, as well as functionalized ILs, with attention on the relationship between the structures of different IL-based electrolytes and the energy storage properties in EDLCs. For imidazolium- and ammonium-based IL electrolytes which are most widely studied in EDLCs, the former generally have higher gravimetric specific capacitance, while the latter exhibit wider electrochemical window. The modifications of functional group substituted can be an effective strategy to enhance the gravimetric specific capacitance of the latter and thus improve the energy density of EDLCs.