J. Cent. South Univ. Technol. (2008) 15: 617-621

DOI: 10.1007/s11771-008-0115-7

Electrochemical behavior of CoCl₂ in

ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate

ZHOU Zhou(周 舟), HE De-liang(何德良), CUI Zheng-dan(崔正丹),

ZHONG Jian-fang(钟建芳), LI Guo-xi(李国希)

(State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China)

Abstract: The electrochemical behavior of CoCl₂ in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF₆) was investigated by cyclic voltammetry. The cyclic voltammograms were obtained from electrochemical measurement under different temperatures, and the reversible behavior for Co²⁺/Co³⁺ redox couple on glassy carbon electrode in [bmim]PF₆ was confirmed by the characteristic of the peak currents. The diffusion coefficients (about 10^{-11 m2}/s) of Co²⁺ in [bmim]PF₆ under different temperatures were evaluated from the dependence of the peak current density on the potential scan rates in cyclic voltammograms. It is found that the diffusion coefficient increases with increasing temperature. Diffusion activation energy of Co²⁺ in [bmim]PF₆ is also calculated to be 23.4 kJ/mol according to the relationship between diffusion coefficient and temperature.

Key words: CoCl₂; electrochemical behavior; ionic liquid; cyclic voltammetry; diffusion coefficient; diffusion activation energy

1 Introduction

Ionic liquids(ILs) are organic melting salts at ambient temperatures^[1]. Typically they are comprised of bulky N,N-dialkylimidazolium, quaternary ammoniums, quaternary phosphonium or alkylpyridinium organic cations and a variety of anions such as chloroaluminate (AlCl₄^-), hexafluorophosphate (PF₆^-), tetrafluoroborate (BF₄^-), bis(perfluoromethyl-sulfonyl)imide ((CF₃SO₂)₂- N^-). ILs exhibit many excellent physical and chemical properties, for example, low volatility, high electrical conductivity, excellent thermal and electrochemical stability, excellent solubility properties, and they are widely studied in electrochemistry, organic synthesis and other fields^[2-5]. In comparison with early chloroaluminate-based ILs, the non-chloroaluminate- based ILs that are stable to moisture can adapt to common atmosphere environment. So the number of publications related to new type ILs grows rapidly in recent years^[6-9].

ILs are applied widely for electrochemical investigation as both supporting electrolytes and solvents^[10-11]. However, the investigations on basic electrochemical parameters in ILs are limited, and the former work mainly is focused on chloroaluminate-based ILs, for example, Fe^[12] and I^[13]. Especially, the studies on electrochemical properties and the measurements of electrochemical parameters for metal ions in non- chloroaluminate-based ILs are few^[14].

It is significant to investigate the redox of some metal ions in ILs, a new electrolyte environment. ZEIN EL ABEDIN^[15] reported the electroreduction of Se, In and Cu in ILs. Electrodeposition of cobalt and different cobalt alloys from ILs was studied in Refs.[16-18], but the major studies were correlative to the reduction of cobalt from chloroaluminate-based ILs. In this work, the electrochemical behavior of Co²⁺ on glassy carbon electrode was investigated in non-chloroaluminate-based IL i.e. 1-butyl-3-methylimidazolium hexafluoropho- sphate ([bmim]PF₆). The effects of temperature on diffusion coefficient of Co²⁺ in [bmim]PF₆ were studied by cyclic voltammetry. And diffusion activation energy of Co²⁺ in [bmim]PF₆ was derived from the relationship between diffusion coefficients and temperature.

2 Experimental

[bmim]PF₆ was prepared according to Ref.[19]. The equal molar amounts of 1-methylimidazole and chlorobutane were heated and stirred, then hexafluorophosphoric ammonium was added to the resulting viscous liquid. The mixture was allowed to proceed with continuous stirring. The product was washed with redistilled water, and then heated under vacuum for 48 h to control the water content less than 3×10^-6, as determined by Karl-Fischer titration. The result of NMR of [bmim]PF₆ was the same to that in Ref.[20] without multi-resonance peaks of impurities.

A certain mass of CoCl₂·6H₂O was dehydrated to turn blue by vacuum drying, and then added into [bmim]PF₆ quickly. The concentration of Co²⁺ was controlled to be 0.03 mol/L. All chemicals were analytical grade. The glass apparatus was washed by dilute nitric acid and redistilled water, and then dried in vacuum.

The electrochemical characteristics of CoCl₂ in [bmim]PF₆ were investigated in a one-compartment cell by using CHI 660B electrochemical working station (CH Instrument Inc). The working electrode was a 3 mm- diameter glassy carbon disc (CH Instrument Inc). Before the electrochemical measurements, the surface of the working electrode was polished with 0.5 μm alumina powders and then washed with redistilled water, ethanol and acetone. Pt coil was used as the counter electrode. The reference electrode was a self-designed Ag/AgCl electrode. Cyclic voltammograms were scanned at a series of potential scan rates of 0.050, 0.100, 0.200, 0.300, 0.400 and 0.500 V/s. All electrochemical experiments were performed in a nitrogen-filled constant temperature box.

3 Results and discussion

3.1 Cyclic voltammogram of Co²⁺ in [bmim]PF₆

Cyclic voltammgram of pure [bmim]PF₆ on glassy carbon electrode is shown in Fig.1(a). It can be found that the electrochemical window of [bmim]PF₆ is 3.7 V at 308.15 K, which is in agreement with the result in Ref.[21]. The limits of redox correspond to the decomposition of PF₆^- and the reduction of [bmim]⁺. Fig.1(b) shows the cyclic voltammogram of Co²⁺ in [bmim]PF₆ on glassy carbon electrode at 308.15 K. The oxidation peak (0.750 V) and reduction peak (-0.245 V) can be observed obviously when potential scan rate is 0.500 V/s.

Fig.1 Cyclic voltammograms of glassy carbon electrode in [bmim]PF₆ at 308.15 K: (a) Pure [bmim]PF₆, 0.005 V/s;     (b) 0.03 mol/L Co²⁺ in [bmim]PF₆, 0.500 V/s

3.2 Electrochemical oxidation of Co²⁺ in [bmim]PF₆

It is well known that some metal ions, such as Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺, are reversible redox couples in aqueous solutions. However, the reversibility is greatly affected by the medium^[12]. Here, it is necessary to study the reversibility of Co²⁺/Co³⁺ redox couple in  [bmim]PF₆. Cyclic voltammgrams of 0.03 mol/L Co²⁺ on glassy carbon electrode in [bmim]PF₆ were measured at 318.15 K. The results are shown in Fig.2. The potential scan rates are 0.050, 0.100, 0.200, 0.300, 0.400 and 0.500 V/s, respectively. The detailed data at various potential scan rates are listed in Table 1. It can be found that the distance between the anodic and cathodic peak potential is above 59 mV. The anodic peak moves more positively, and the cathodic peak moves negatively when the potential scan rate increases. The potential shift may be attributed to the depressing reversibility of Co²⁺/Co³⁺ redox couple and/or ohmic resistance of the ionic liquid [bmim]PF₆. Nevertheless, the anodic and cathodic peak currents (I_{p, a}, I_p,c) are almost symmetrical. The ratios of I_p,a to I_p,c are approximately equal to 1, and I_p,a and I_p,c can fit well with the square root of scan rate (v^1/2) to linear relationships (R_a=0.997, R_c=0.991). Therefore, it can be presumed that the Co²⁺/Co³⁺ redox couple in [bmim]PF₆ is a reversible electrochemical reaction according to Ref.[22].

Fig.2 Cyclic voltammograms of 0.03 mol/L Co²⁺ in [bmim]PF₆ at different potential scan rates and 318.15 K: (a) 0.050 V/s;   (b) 0.100 V/s; (c) 0.200 V/s; (d) 0.300 V/s; (e) 0.400 V/s;     (f) 0.500 V/s

When the plot of ln[(I_p-I)/I] vs potential(E) is a line, the corresponding oxidation or reduction process is very reversible^[23-24], where I_p is the peak current, I is the instantaneous current relevant to potentials. The data of

Table 1 Parameters of cyclic voltammograms of 0.03 mol/L Co²⁺ in [bmim]PF₆ at various potential scan rates and 318.15 K

I_p, I and potential are gained from the cyclic voltammograms in Fig.2 when the scan rate is 0.050 V/s, then the plot of ln[(I_p-I)/I] vs E is made and shown in Fig.3. It is obvious that they can be fitted to a line as Eqn.(1). This also proves that the oxidation of Co²⁺ in [bmim]PF₆ is reversible.

ln[(I_p-I)/I]=16.4-36.0E (R₁=0.998)               (1)

where  R₁ is correlation coefficient of the linear fitting.

Fig.3 Linear plot of ln[(I_p-I)/I ] vs potential at 318.15 K and scan rate of 0.050 V/s

3.3 Diffusion coefficient of Co²⁺ in [bmim]PF₆

The relationship between peak current and potential scan rate for reversible electrochemical process accords with the Randles-Sevcik equation:

I_p=0.446 3(nF)^3/2(RT)^-1/2Ac₀D^1/2v^1/2               (2)

where  n is the Faraday constant per mole of substrate electrolyzed; A is the surface area of the electrode; c₀ is the bulk concentration of reactant and D is the diffusion coefficient of reactant. The cyclic voltammograms of 0.03 mol/L Co²⁺ in [bmim]PF₆ were measured under different temperatures, and the plots of the anodic peak current vs square root of scan rate are shown in Fig.4. These plots exhibit excellent linear relationship. From the slopes (k) of the fitting line observed between I_p and v^1/2, the values of D (the diffusion coefficient of Co²⁺ in [bmim]PF₆) can be calculated and are listed in Table 2. The results are similar to those of Fe³⁺ in IL^[25], but are less than those of Co²⁺ in dimethylsolfoxide (about  10^{-9 m2}/s)^[26]. This may be attributed to the viscosity of the medium and/or the inhibition of electron transfer between the glassy carbon electrode and the redox center due to bulkiness of PF₆^-. So it is surmised that Co²⁺ and Co³⁺ are surrounded by PF₆^- to form [Co(PF₆)_n]^2-n and [Co(PF₆)_n]^3-n that will transform reciprocally as follows:

[Co(PF₆)_n]^2-n-e=[Co(PF₆)_n]^3-n                   (3)

Table 2 Diffusion coefficient of Co²⁺ in [bmim]PF₆ at c(Co²⁺)= 0.03 mol/L and different temperature

Fig.4 Plots of I_p,a vs v^1/2 at different temperatures and c(Co²⁺)=  0.03 mol/L

It can also be found that the diffusion coefficient increases with the increase of temperature. This may be attributed to a decrease in viscosity of solvent ([bmim]PF₆) and an increase in kinetic energy of reactant because of temperature rising.

3.4 Diffusion activation energy of Co²⁺ in [bmim]PF₆

The relationship between diffusion coefficient and temperature follows Arrenius law:

D=D₀exp[-E_D/(RT)]                           (4)

where  D is the diffusion coefficient relevant to T, and D₀ is preexponential factor. Eqn.(4) is given below in logarithmic form:

ln D=ln D₀-E_D/(RT)                           (5)

A proportional relationship between ln D and 1/T can be observed from Eqn.(5). Fig.5 shows the dependence of ln D on 1/T for 0.03 mol/L Co²⁺ in [bmim]PF₆. This line can be fitted as Eqn.(6) from the data listed in Table 2. The slope (k′) is equal to -E_D/R. The value of diffusion activation energy E_D of Co²⁺ in [bmim]PF₆ can be calculated to be 23.4 kJ/mol from the slope.

ln D=-2.81/T-15.9, R₁=0.995                   (6)

Fig.5 Dependence of ln D on 1/T at c(Co²⁺)=0.03 mol/L

4 Conclusions

1) The electrochemical studies of the Co²⁺/Co³⁺ redox couple in [bmim]PF₆ are processed. It is confirmed that the oxidation of Co²⁺ on glassy carbon electrode in [bmim]PF₆ is reversible.

2) The diffusion coefficients and diffusion activation energy of Co²⁺ in [bmim]PF₆ are conveniently determined. The diffusion coefficient of about 10^{-11 m2}/s and diffusion activation energy of 23.4 kJ/mol approximate to those of other metal ions in ILs.

3) The results of the research lay the foundation for metal ions’ electrochemical redox behavior in ILs, and supply basic electrochemical parameters for Co²⁺ in [bmim]PF₆. Based on the results, it can be concluded that IL [bmim]PF₆ can be used extensively as solvent material for electrochemical investigations.

References

[1] Kubisa P. Application of ionic liquids as solvents for polymerization processes [J]. Prog Polym Sci, 2004, 29(1): 3-12.

[2] Robinson J, Osteryoung R A. The electrochemical behavior of Te(IV) in sodium tetrachloroaluminate melts [J]. J Electrochem Soc, 1978, 125(11): 1784-1789.

[3] Hussey C L, King L A, Carpio R A. The electrochemistry of copper in a room temperature acidic chloroaluminate melt [J]. J Electrochem Soc, 1979, 126(6): 1029-1034.

[4] Fry S E, Pienta N J. Effects of molten salts on reactions: Nucleophilic aromatic substitution by halide ions in molten dodecyltributylphosphonium salts [J]. J Am Chem Soc, 1985, 107(22): 6399-6400.

[5] Boon J A, Levisky J A, Pflug J L, Wilkes J S. Friedel-Crafts reactions in room temperature ionic liquids [J]. J Org Chem, 1986, 51(4): 480-483.

[6] Chen Wei-ping, Xu Li-jin, CHATTERTON C, XIAO Jian-liang. Palladium catalysed allylation reactions in ionic liquids [J]. Chem Commun, 1999(13): 1247-1248.

[7] Chen P Y, Sun I W. Electrochemical study of copper in a basic 1-ethyl-3-methylimidazolium tetrafluoroborate room temperature molten salt[J]. Electrochim Acta, 1999, 45(3): 441-450.

[8] Huddleston J G, Willauer H D, Swatlosk R P, Visser A E, Rogers R D. Room temperature ionic liquids as novel media for ‘clean’ liquid-liquid extraction [J]. Chem Commun, 1998(16): 1765-1766.

[9] Earle W J, McCormac P B, Seddon K R. Diels-Alder reactions in ionic liquids [J]. Green Chem, 1999, 1(1): 23-25.

[10] Sekiguchi K, Atobe M, Fuchigami T. Electropolymerization of pyrrole in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate room temperature ionic liquid [J]. Electrochem Commun, 2002, 4(11): 881-885.

[11] Doherty A P, Brooks C A. Electrosynthesis in room- temperature ionic liquids: Benzaldehyde reduction [J]. Electrochim Acta, 2004, 49(22/23): 3821-3826.

[12] Nanjundiah C, Shimizu K, Osteryoung R A. Electrochemical studies of Fe(II) and Fe(III) in an aluminum chloride-butylpyridinium chloride ionic liquid [J]. J Electrochem Soc, 1982, 129(11): 2474-2480.

[13] Marassi R, Chambers J Q, Mamantov G. Electrochemistry of iodine and iodide in chloroaluminate melts [J]. Journal of Electroanal Chem, 1976, 69(3): 345-359.

[14] Wadhawan J D, Schroder U, Neudeck A, Wilkins S J, Compton R G, Marken F, Conserti C S, Souza R F, Dupont J J. Ionic liquid modified electrodes. Unusual partitioning and diffusion effects of Fe(CN)₆^4-/3- in droplet and thin layer deposits of 1-methyl-3-(2,6-(S)-dimethylocten-2-yl)-imidazolium tetra- fluoroborate [J]. Electroanal Chem, 2000, 493(1/2): 75-83.

[15] Zein El Abedin S, Saad A Y, Farag H K, Borisenko N, Liu Q X, Endres F. Electrodeposition of selenium, indium and copper in an air- and water-stable ionic liquid at variable temperatures [J]. Electrochim Acta, 2007, 52(8): 2746-2754.

[16] Mitchell J A, Pitner W R, Hussey C L, Stafford G R. Electrodeposition of cobalt and cobait-alumium alloys from a room temperature chloroaluminate molten salt [J]. J Electrochem Soc, 1996, 143(11): 3448-3455.

[17] Chen P Y, Sun I W. Electrodeposition of cobalt and zinc-cobalt alloys from a lewis acidic zinc chloride-1-ethyl-3-methylimidazolium chloride molten salt [J]. Electrochim Acta, 2001, 46(8): 1169-1177.

[18] Zell C A, Freyland W. In situ STM and STS study of Co and Co-Al alloy electrodeposition from an ionic liquid [J]. Langmuir, 2003, 19(18): 7445-7450.

[19] Ramón A, Laureano C, Pedro G J, Pedro P M. Air oxidation of ethylbenzene catalysed by bis(acetylacetonate)nickel(II) and 1-n-butyl-3-methylimidazolium hexafluorophosphate [J]. Applied Catalysis A: General, 2001, 218: 269-279.

[20] Yagupolskii Y L, Sokolenko T M, Petko K I, Yagupolskii L M. Novel ionic liquids—Imidazolium salts with a difluoromethylene fragment directly bonded to the nitrogen atom [J]. Journal of Fluorine Chemistry, 2005, 126(4): 669-670.

[21] ZHAO Guo-ying, JIANG Tao, HAN Bu-xing, LI Zhong-hao, ZHANG Jian-min, LIU Zhi-min, HE Jun, WU Wei-ze. Electrochemical reduction of supercritical carbon dioxide in ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate [J]. Journal of Supercritical Fluids, 2004, 32(1/3): 287-291.

[22] DUAN Shu-zhen. In molten salt chemistry—Principle and application [M]. Beijing: Metallurgical Industry Press, 1989: 220. (in Chinese)

[23] Nicholson R S, Shain I. Theory of stationary electrode polarography: Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems [J]. Anal Chem, 1964, 36(4): 706-723.

[24] Nicholson R S. Theory and application of cyclic voltammetry for measurement of electrode reaction kinetics [J]. Anal Chem, 1965, 37(11): 1351-1355.

[25] YANG Jia-zhen, JIN Yi, PAN Wei, ZANG Shu-liang. Measurement of diffusion coefficient of Fe³⁺ in ionic liquid BPBF₄ by chronoampermetry [J]. Chemical Journal of Chinese Universities, 2005, 26(6): 1146-1148. (in Chinese)

[26] HE Feng-rong, WANG Yu, LIU Guan-kun, SA Li-chang. Electrochemical behavior of dysprosium and cobalt in dimethylsolfoxide [J]. Acta Scientiarum Naturalium Universitatis Sunyatseni, 2001, 40(6): 42-45. (in Chinese)

(Edited by CHEN Wei-ping)

Foundation item: Project(2005-383) supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education, China

Received date: 2008-03-06; Accepted date: 2008-05-12

Corresponding author: HE De-liang, Professor, PhD; Tel: +86-731-8821449; E-mail: delianghe@163.com