High Voltage AC/AC Capacitors Using the Carbon/Iodine Interface in Ionic Liquid Electrolyte

Lately we have demonstrated that huge pseudo-capacitive contributions can be originated from the electrolyte when using high surface area activated carbon electrodes, for example by using iodide based solutions in water [1]. Although the use of such concept allows very high capacitance values to be reached, the energy of the AC/AC capacitors based on these electrolytes is limited by the relatively low value of voltage inherent to use of water. Therefore, in this work, we have investigated the performance of the carbon/iodine interface in ionic liquid electrolytes, both protic (PIL) and aprotic (AIL).

The PILs selected were triethylammonium bis(tetrafluoromethylsulfonyl)imide [Et₃NH]-[TFSI] and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide [Emim]-[FSI]. Organic iodides - triethylammonium iodide [Et₃NH]-[I] and tetraethylammonium iodide [Et₄N]-[I] at a concentration of 0.25 mol·L^-1 and inorganic iodides (LiI and KI) at a concentration of 0.2 mol·L^-1 were introduced in the ionic liquid. 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) activated by KOH (4:1) with high microporosity ( S_BET = 2180 m²·g^-1, V_micro = 0.96 cm³·g^-1, L₀ = 1.55 nm)  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 with reference electrode (Figure 1B) demonstrates a remarkable increase of capacitance when KI is added to [Et₃NH]-[TFSI], from 122 to 184 F·g^-1. The voltammograms of figure 1A show clearly that the increase of capacitance is caused by the positive electrode where faradaic processes occur. The capacitance values of the positive electrode for 0.25 mol·L^-1 [Et₃NH]-[I] and 0.2 mol·L^-1 LiI dissolved in [Et₃NH]-[TFSI] reached 199 and 238 F·g^-1_, respectively; for the negative electrode the capacitance reached 163 and 121 F·g^-1_, respectively. The addition of 0.2 mol·L^-1 inorganic iodide to aprotic IL also enhanced the capacitance from 116 F·g^-1 for the pure AIL to 129 and 153 F·g^-1for the KI and LiI solutions, respectively. Regardless of the different type of iodide derivatives dissolved in IL medium, faradaic processes occur at the positive electrode greatly increasing the capacitance of the whole capacitor system. However, iodide solutions in PILs seem to be more effective than the AILs ones.

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.

References

[1] G. Lota, E. Frackowiak, Electrochem. Comm. 11 (2009) 87-90