Ionic liquid 의 이온 전도도 측정

신배근, 김기섭, Demberel Nymba, 이흔

화학공학의 이론과 응용 제8권 제1호 2002년

한국과학기술원 생명화학공학과

Measurement of the ionic conductivities of the ionic liquids

Bae Kun Shin, Ki Sub Kim, Demberel Nyamba, Heun Lee

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Department of Chemical and Biomolecular Engineering, KAIST

Introduction

In recent years, the room temperature molten salts, which are consist of N, N'-dialkylimidazolium salts, have been vigorously studied, because they are expected to be a new electrolyte medium. Especially when the counter anion of such imidazolium salts is exchanged from halogens to another such as tetrafluoroborate, hexafluoroarsenate, hexafluorophosphate, and bis(trifluoromethylsulfonimide), they are known to be water stable and less viscous liquid at ambient temperature. These characteristics are preferable as electrolyte solution for lithium battery, and many studied have been carried out aiming this kind of application. In most of these studies, N, N'-dialkylimidazolium salt, especially 1-ethyl-3-methyl-imidazolium salt was focused empirically because of low viscosity and correspondingly high ionic conductivity.

In the field of ionics, molten salts occupy a special position being solvent free, but otherwise possessing all of the liquid-like motional properties exhibited by sol-vent

based electrolytes, including traditional solid poly-mer electrolytes. In this state of ionic aggregation inter-ionic interactions and geometric factors become governing and isolated from solvent effects. Conceptually, the molten salt can be considered to be the end member of a continuous series of solution electrolyte states from pure solvent to pure salt. However, the mechanisms governing conduction in these solvent free systems become distinctly different than in solvent based electrolytes.

Molten salts are the most concentrated electrolytic fluids, possessing a high number of charge carriers per unit volume. When these charge carriers are mobile, very high conductivities are possible. Interest in this field of research has increased since the discovery by Wilkes et al. in 1982 of a molten salt which was liquid over a wide range of compositions and to temperatures as low as -95℃. Ionic liquids are being

investigated for many applications including electro-plating, batteries, electrochemical capacitors and photoelectrochemical (PEC) cells. In PEC cell applications, molten salts could be utilised as the electrolyte solvent only or also be involved in the redox mediation processes. The physical properties which make the systems attractive in such roles are their large electrochemical windows and high conductivities, excellent thermal and chemical stability, negligible vapour pressure at elevated temperatures and miscibility in a diverse range of solvents .

Experimental

1.Materials

Reactants for the synthesis of the ionic liquids ( BMPBr, EVPBr, BMPBF4,

C12VPBF4) were supplied from Aldrich-Sigma chemical Co. and was used without further purification. The solution was prepared using triple-distilled water. Materials for the ionic conductivit are BMPBr, EVPBr, BMPBF4 and C12VPBF4.

2. Apparatus and procedure

The ionic conductivity was measured by the complex impedance method with the frequency range from 10 Hz to 10 MHz (Schlumberger, Solartron SI 1260). The sample cell preparation and ionic conductivity measurement were carried out in a glove box filled with dry Argon gas. The temperature can be controlled within ±0.1K by the circulator. The temperature was measured by mercury-filled glass thermometer. To measure the ionic conductivities, we must make the cell, which consists of the electrodes (SUS304), the blue pack, the separator (Cellgard 2500) and the ionic liquid. The shape of the cell is shown by the Fig.1. The ionic conductivity is calculated by the following equation.

[eqn: □ = d over {R_b A}]                  (1)

where □ is the ionic conductivity, d is the thickness of the sample, A is the  electrode area and [eqn: R_b] is the resistance of the sample.

Also, the dependencies of the ionic conductivities on the temperature is described by Arrhenius equation

[eqn: □ = □_0 exp(- E_a over RT )]          (2)

where [eqn: □_0] is the limiting conductivity, [eqn: E_a] is the activation energy and R is the gas constant..

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Results and discussions

For the check of reliability of our experiment, the ionic conductivities of 10mol% NaCl solution was measured at 297.15K. The ionic conductivities of pure Aceton was good agreement with literature data. The ionic conductivity values of Ionic liquids were measured over the temperature ranging from 297.15 to 323.15K.  Each data set was repeated three times and averaged.

Since the values of the ionic conductivities are high  Figure 2 shows the ionic conductivities of pure ionic liquids. This figure is the logarithm plots and follows the Arrehnius equation (eqn. 2). In Figure 2, all ionic liquids have more values than -4. Since this values are high and good(specially, EVPBr), these ionic liquids have the potentials to alternate to the electrolytes

References

[1] H. Every, A.G. Bishop, M. Forsyth, D.R. MacFarlane, Ion diffusion in molten salt mixtures 1998 1279-1284

[2] Kaori Ito, Naoko Nishina, Hiroyuki Ohno, Enhaced ion conduction in imidazolium-type molten salts, 1999, 1295-1298

[3] D.R. McFarlane, J. Sun, J. Solding, P. Meakin, M. Forsyth, High conductivity molten salts based on the imide ion, 1998, 1271-1278

Fig 2. cell for the ionic conductivity measurement

Fig 2. ionic conductivities of the ionic liquids