Electrolytes for
Supercapacitors

Challenges

The energy storage mechanism of supercapacitors is based on the formation of double layers rather than redox reactions (as in batteries). Since the double-layer formation occurs in seconds, supercapacitors can be charged / discharged very quickly in seconds, enabling high power. Their extended cycle life (> 500.000 cycles) originates from the high reversibility of the physical storage process. The double-layer formation takes place at the electrode / electrolyte interface, which requires tailoring of both the active material and the electrolyte. High surface area activated carbons are typically applied as active material for the positive and negative electrodes to form a large electrode / electrolyte interface.

During the energy storage process, the electrolyte ions are attracted and adsorbed to the electrodes with the opposite charge. The operating voltage is limited by the electrochemical stability of the supercapacitor components, especially to that of the electrolyte. In addition to EDLCs, there are also hybrid capacitors being developed. A hybrid capacitor poses two electrodes, one having similar characteristics to that of a lithium-ion battery and one similar to that of an EDLC. The EDLC electrolytes are composed of salts and solvents that enable high ionic conductivity and low viscosity, which is crucial for the rapid formation of the double-layer. Recently, many research efforts have been made to increase the operation voltage of the supercapacitors from around 2.8 V to above 3.0V with the aid of novel salt / solvent combinations.