The Carbon/Iodide Interface in Protic Ionic Liquid Medium for Application in Supercapacitors

Among various energy storage systems, great interest is focused on electrochemical capacitors (ECs) because of their high power and long cycle life. The most typical devices operate through the charge/discharge of the electrical double-layer at the electrode/electrolyte interface in aqueous or organic medium and as well as in ionic liquid. The capacitance can be significantly enhanced by quick redox reactions originating from the electrolyte species at the electrode/electrolyte interface, for example by using iodide salt aqueous solutions [1]. However, in the later case, the voltage of the system is limited to 1 V due do the low thermodynamic window of water. Therefore, we propose to investigate a possible extension of this window by using solutions of iodide salt in ionic liquids (ILs).

The ILs selected were triethylammonium bis(tetrafluoromethylsulfonyl)imide [Et₃NH]-[TFSI] and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide [Emim]-[FSI]. Two iodides, triethylammonium iodide [Et₃NH]-[I] or tetraethylammonium iodide [Et₄N]-[I], were introduced in the ionic liquid at a concentration of 0.25 mol L^-1. The electrochemical experiments as cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy were conducted using a two-electrode cell construction. In some experiments, a silver wire was added as pseudo-reference to monitor the potential of individual electrodes. Activated carbon (AC) with high microporosity coated on aluminum foil from Batscap (France) was used as working and counter electrode. Glass microfiber paper (Whatman) was utilized as separator. Galvanostatic (0.2 A·g^-1) cycling up to 1.8 V in two-electrode cell (Figure 1B) demonstrates a remarkable increase of capacitance when [Et₃NH]-[I] is added to [Et₃NH]-[TFSI], from 78 to 157 F·g^-1. The voltammogram of figure 1A shows clearly that the increase of capacitance is caused by the positive electrode where faradaic processes occur.

Acknowledgements:The Foundation for Polish Science is acknowledged for supporting the ECOLCAP Project realized within the WELCOME Program, co-financed from European Union Regional Development Fund.

[1] G. Lota, E. Frąckowiak, Electrochemistry Communication, 11 (2009) 87-90