Electrochemical Characterization of Iodide Ions Adsorption Kinetics at Bi(111) Electrode from Three-Component Ionic Liquids Mixtures

The electrochemical behavior of RTIL mixtures containing 1-ethyl-3-methylimidazolium tetraﬂuoroborate, 1-ethyl-3- methylimidazolium triﬂuoromethanesulfonate and 1-ethyl-3-methylimidazolium iodide were characterized by using cyclic voltammetry and electrochemical impedance spectroscopy methods. The capacitance values of the system at potentials more positive than − 0.6 V (vs. Ag | AgCl | mixture of RTIL) strongly depend on the chemical nature of the anions in the mixture. Also the strong adsorption of I − at Bi(111) (well-known in aqueous and classical organic solvents) has been demonstrated in RTIL media. The capacitance values are noticeably higher for EMImOTF + EMImBF 4 + 1 wt% EMImI than that for EMImOTF + 1 wt% EMImI. permits unrestricted reuse of the work in any medium,

Ionic liquids offer a unique properties and a number of various energy technology related applications. RTIL properties are ideal for designing novel electrolytes for batteries, supercapacitors, dye sensitized solar cells, fuel cells, thermo-electrochemical cells, etc. [1][2][3] First room temperature ionic liquid (RTIL) was synthesized at the beginning of XX century, but still there are many things to investigate and develop further. 2 The main disadvantage of RTILs in practical aspect is the high cost of RTILs. Also purity issues, high viscosity and some specific properties (H 2 O and O 2 sensibility) of pure RTIL and their mixtures are important to study. 3 By mixing two or more ionic liquids with different properties (viscosity, dielectric permeability, polarity) it is possible to maximize the advantage of RTIL as an electrolyte. [4][5][6][7][8] Our recent studies have shown that the addition of specifically adsorbed iodide ions strongly influences the electrochemical characteristics of RTILs systems increasing the specific capacitance and decreasing the series resistance values. [9][10][11][12] For the further investigation of the specific adsorption of iodide ions from RTIL mixtures the 1ethyl-3-methylimidazolium trifluoromethanesulfonate (EMImOTF) was added to the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF 4 ) + 1-ethyl-3-methylimidazolium iodide (EMImI) mixture. It is important that the imidazolium based ionic liquids with stable anions such as BF 4 − , PF 6 − and CF 3 SO 3 − exhibit air and moisture stability. These properties become important when electrochemical devices will be used outdoors. [13][14][15][16] The main aim of this work was to decrease the viscosity and reduce the price of the mixed electrolytes for possible practical use as an electrolyte for supercapacitors as well as different batteries.

Results and Discussion
The current density (j) vs. electrode potential (E) curves ( Fig. 1a) show that the Bi(111) electrode is nearly ideally polarizable within the potential range from −1 to −0.2 V for EMImBF 4 + 1% EMImI 11 and EMImOTF + EMImBF 4 + 1% EMImI mixtures, and −1 to 0.1 V for the EMImOTF + 1% EMImI mixture. The increase of j at more negative potentials is caused by the beginning of irreversible cathodic charge transfer processes including desorption of strongly adsorbed I − (decomposition of residual H 2 O and other contaminations). The narrow ideal polarizability region, compared to carbon electrodes, is caused by the sp-metallic nature of the bismuth electrode. 21 Experimental differential capacitance (C s ) vs. E dependencies ( Fig. 1) were measured at the fixed ac frequency f = 210 Hz under dc potential scanning conditions (5 mVs −1 ). 22 The shape of the C s ,E curves depends strongly on the composition of the mixture studied (Fig. 1b). There is a clear maximum in the C s ,E curve at less negative electrode potentials for RTIL mixtures, where BF 4 − and I − anions present, explained by the strong adsorption of iodide ions at the electrode surface and is not hindered by BF 4 − anions in electrical double layer (EDL). However, similarly to the pure EMImOTF, EMImOTF + 1% EMImI mixture has noticeably lower capacitance values at less negative electrode potentials. Thus, the closest approach of anions onto the Bi(111) surface depends strongly on the chemical composition and geometrical structure of the anions in a RTIL mixture. The adsorption of OTF − is still competitive with I − and the strong adsorption of I − cannot be seen.
On the basis of the practical implementation of the electrolyte and the reversibility of the system, it is very important that there is only a weak hysteresis between capacitance curves measured first toward positive and thereafter negative potential scan directions (Fig. 1b inset.). Thus, the adsorption of I − seems to be a reversible process at the Bi (111)   According to the literature data and the producer information the dynamic viscosity of EMImOTF varies between 50 to 70 cP at room temperature due to the impurities and decreases noticeably with increasing temperature. 23,24 The viscosity of EMImBF 4 is 38 cP and the viscosity of EMImOTF + EMImBF 4 + 1% EMImI mixture is 41 cP (measured with Anton Paar rheometer MCR101, at 22 • C). [25][26][27] This is promising information and probably the three-component mixture can be used as an electrolyte in electrochemical devices.
The shape of the complex impedance plane, i.e. Nyquist plots (Fig. 2a) depends noticeably on the potential applied as well as on the composition of the mixture of RTILs (Z is an imaginary component (Z = −1/ωC s , were C s is a series capacitance and ω = 2πf) and Z is a real component of the impedance equal to the series resistance). The phase angle vs. f plots for EMImBF 4 + EMImI and EMImOTF + EMImBF 4 + EMImI (Figs. 2b, 2c)  only a weak deviation from adsorption limited process at lower frequencies (1 < f < 1000 Hz). Both mixtures, containing BF 4 − , have adsorption limited behavior also in the very low frequency area (f < 1 Hz). Thus, there is a minor deviation from adsorption limited process at less negative E (from −0.7 to −0.3 V) for moderate and low frequency area. This could be explained by the strong specific adsorption of I − ions. However, for EMImOTF + 1% EMImI mixture, the mixed kinetic processes take place at low f ≤ 10 Hz. Also, for less negative E (Fig. 2c) the maximum in phase angle vs. ac f curves is shifted toward lower frequencies because the slow adsorption of I − with partial charges transfer is the main limiting process in this f region.
Form log (|Z |) vs. f plots (Fig. 3) it can be seen that in a wide range of frequencies the graph is a straight line and only in very low and high frequency areas there is a small deviation from the ideal capacitive behavior. [25][26][27] The C s values calculated from the values of Z (C s (ω) = −(Z (ω)2πf) −1 ), depend on f and E (Figs. 3b, 3c). 22,28 C s does not depend much on E at higher frequencies. C s increases in both ends of ideal polarizability region and depends also on the chemical composition of RTILs mixtures studied. These phenomena could be explained by specific adsorption of the anions at less negative electrode potentials and by faradaic processes with the reductive capacitive (desorption of I − ) behavior at more negative potentials. 9 Surprisingly for EMImOTF + 1% EMImI mixture, there is no increase in the capacitance at less negative electrode potentials (probably due to the blocking effect of OTF − at the Bi(111) | RTIL interface). The EMImBF 4 + EMImOTF + 1% EMImI mixture shows nearly similar behavior with EMImBF 4 + 1% EMImI mixture used successfully in supercapacitor applica-tions already. Capacitance values are comparable even at f = 0.2 Hz (Fig. 3a). 27 The same phenomena are also seen in the complex power vs. log f plots, i.e. the normalized real part of complex power (|P(ω|/|S(ω)|) and imaginary part of complex power (|Q(ω|/|S(ω)|) vs. f plots (calculated from electrochemical impedance data) (Fig. 4). Equilibrium adsorption times are much longer for the BF 4 − + I − containing systems. Thus, taking into account the very similar behavior of impedance data for the three-component RTIL mixture, this mixture could be used successfully in supercapacitor as well as in other electrochemical applications.

Conclusions
Cyclic voltammetry and electrochemical impedance spectroscopy have been used for characterization of the Bi(111) | mixture of three RTILs interface. Analysis of the experimental and calculated parameters shows the noticeable dependence of the EDL structure on the chemical nature and geometric structure of the anions forming EDL. At less negative electrode potentials the capacitance for EMImOTF + EMImBF 4 + EMImI and EMImBF 4 + EMImI is much higher compared to that for EMImOTF + EMImI mixture. Good potential cyclability and reversibility of the capacitance vs. potential curves measured toward positive and negative scan directions have been demonstrated. It should be stressed that the cyclability and reversibility are the key parameters for the practical application of RTIL mixtures in supercapacitor. Thus, taking into account that the EMImOTF addition into the EMImBF 4 + EMImI mixture 11,27 reduces the overall price of the electrolyte, this three-component mixture can be tested as an electrolyte for supercapacitors.