Supercapacitors Based on Mixture of Room Temperature Ionic Liquids Containing Specifically Adsorbed Iodide Anions

Applicability of room-temperature ionic liquids (RTILs) as electrolytes for electrical double layer capacitors (EDLCs), so-called supercapacitors and batteries has been discussed in many papers. RTILs have lower conductivity, higher viscosity and thus, narrower low temperature operation limit compared to aqueous and non-aqueous electrolytes. However, compared with volatile aqueous and organic electrolytes, RTILs are much safer in EDLCs applications. In order to improve the power and energy densities of EDLCs, various mixtures of ionic liquids have been investigated including systems of EMImBF₄with EMImBr [1].

In the present studies 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) was chosen due to high bulk conductivity (k=13.6 mS cm^-1). To increase the capacitance, EMImI was added because the noticeable increase in capacitance had been established within the potential region of I^- ion specific adsorption for Bi(111)„ EMImBF₄+ EMImI interface [2]. The electrochemical characteristics of this RTIL mixture have been characterized by cyclic voltammetry, electrochemical impedance spectroscopy, constant current charge/discharge and constant power discharge methods. Carbon used in these studies was synthesized from D-glucose by the hydrothermal carbonization method followed by additional pyrolysis and carbon dioxide activation steps (GDAC - D-glucose derived activated carbon) as described by Thomberg et al. [3].

Capacitance peaks in cyclic voltammetry curves (Fig. A) for EMImBF₄ + EMImI mixture within the cell potential region from 0.8 to 1.2 V were established differently from the data for pure EMImBF₄ and about 50% higher discharging charge Q values have been calculated at lower cell potential scan rate v≤10 mV s^-1 in comparison with pure EMImBF₄. According to the experimental energy and power relationships data, i. e Ragone plots (Fig. B), the addition of EMImI into EMImBF₄increases noticeably the specific energy and specific power values of EDLC.

Acknowledgments

This work was supported by European Regional Development Fund Project 3.2.0501.10–0015 and Estonian Center of Excellence in Research Project 3.2.0101–0030 “High-technology Materials for Sustainable Development”.

 

References:

1. S. Yamazaki, T. Ito, M. Yamagata and M. Ishikawa, Electrochim. Acta. 86, 294 (2012).

2. L. Siinor, C. Siimenson, K. Lust and E. Lust, Electrochem. Commun., 35, 5 (2013).

3. T. Thomberg, T. Tooming, T. Romann, R. Palm, A. Jänes and E. Lust, J. Electrochem. Soc., 160(10), A1834 (2013).