Electrochemical Behavior of Iron-based Imidazolium Chloride Ionic Liquids

Accepted Manuscript Title: Electrochemical Behavior of Iron-based Imidazolium Chloride Ionic Liquids Author: Zhihui Guo Tingting Zhang Murtaza Khan Shujing Gao Tiantian Liu Jiang Yu PII: DOI: Reference:

S0013-4686(14)01529-1 http://dx.doi.org/doi:10.1016/j.electacta.2014.07.110 EA 23148

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

19-2-2014 18-7-2014 18-7-2014

Please cite this article as: Z. Guo, T. Zhang, M. Khan, S. Gao, T. Liu, J. Yu, Electrochemical Behavior of Iron-based Imidazolium Chloride Ionic Liquids, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.07.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrochemical Behavior of Iron-based Imidazolium

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Chloride Ionic Liquids

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Zhihui Guo, Tingting Zhang, Murtaza Khan, Shujing Gao, Tiantian Liu, Jiang Yu 

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(Research Group of Environmental Catalysis & Separation Process, College of Chemical Engineering, Beijing

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University of Chemical Technology, Beijing 100029, China)

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Abstract: In order to study the electrochemical properties of the iron-based

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imidazolium chloride ionic liquids, two kinds of ionic liquids - trivalent (Fe(III)-IL)

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and trivalent / divalent coexisting system (Fe(III/II)-IL) - were constructed by mixing

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1-butyl-3-methylimidazolium chloride (BmimCl), FeCl3∙6H2O and FeCl2∙4H2O.

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[Bmim]+, [Bmim2Cl]+, [FeCl4]- and [Fe3Cl7]- in Fe(III/II)-IL were detected by UV-vis

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absorption spectroscopy and liquid chromatography - mass spectrometry with an

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electrospray ionization source (ESI-MS). The electrochemical behaviors of [FeCl4]-

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and [Fe3Cl7]- on a platinum electrode were investigated by cyclic voltammetry.

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Platinum microelectrode was used to reduce the effect of poor conductivity of these

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ionic liquids. Cyclic voltammetric data indicated that the electrode reaction of

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FeCl4-/[Fe3Cl7]- at platinum electrode was a quasi-reversible process. The electrode

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reaction of FeCl4- in the ionic liquids was followed by a homogeneous reaction. Both

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migration and diffusion of electroactive ions affected the electrochemical behavior of

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Fe(III)/Fe(II) on Pt microelectrode. The values of apparent diffusion coefficient,

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viscosity, and conductivity of Fe(III)-IL and Fe(III/II)-IL over the temperature range

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from 293 K to 333 K were measured, and the corresponding activation energies for

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Corresponding author. Email: [email protected], Telephone No.: 0086-10-64438933. -1-

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mass and charge transport were calculated by Arrhenius equation. The activation

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energies of Fe(III)-IL and Fe(III/II)-IL are between 20 and 35 kJ·mol-1. Moreover, the

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influence of co-solvents, 1,2-dichloroethane and acetonitrile, on Fe(III/II)-IL was

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discussed. Results showed that addition of strong polar acetonitrile has more

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pronounced effect than less polar 1,2-dichloroethane on viscosity, conductivity and

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diffusion coefficient of Fe(III/II)-IL.

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Key words: Iron-based imidazolium chloride ionic liquids; Trivalent iron; Divalent

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iron; Electrochemical behavior

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1. Introduction

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Trivalent and divalent iron complexes commonly exist in many chemical

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systems. They have been studied in many research areas because of their good redox

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properties [1-4]. Recently, some kinds of iron complexes have been introduced into

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some functional green room temperature ionic liquids. These iron-containing ionic

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liquids are one of the attractive reaction media and catalysts, not only because they

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have many advantages common to ionic liquids, such as good solubility, good thermal

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stability, low vapor pressure, high conductivity, etc.[ 5-6 ], but also have some

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oxidative, catalytic, or other special ability [ 7- 12 ]. Because of their attractive

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electrochemical properties, ionic liquids are expected to be used in various practical

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applications, such as batteries, electroplating and electrochemical syntheses [13-15].

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Nguyen et al. [16-17] have prepared a series of iron-based ionic liquids as

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catalysts by mixing [RMI]X and FeX3 (X=Cl, Br) under an inert atmosphere of -2-

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helium gas in a glovebox with the strict exclusion of air and moisture. FeCl4- and

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Fe2Cl7- were detected by liquid chromatography - mass spectrometry with an

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electrospray ionization source (ESI-MS) when reacting 1-butyl-3-methylimidazolium

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chloride (BmimCl) with two equivalents of FeCl3. While Hayashi [18] have prepared

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BmimFeCl4 more easily by using FeCl3·6H2O instead of FeCl3 at an open

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environment. BmimFeCl4 is magnetic and immiscible with water. It is a kind of Lewis

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and Brønsted acid [19] and has been used in a new system of wet oxidation

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desulfurization of hydrogen sulfide [20]. This system took three advantages on the

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iron-based imidazolium chloride ionic liquid (Fe(III)-IL), that were oxidative, acidic

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and hydrophobic properties, which finally overcome the shortcomings of the

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traditional desulfurization methods without any by-product productions of inorganic

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salts and losses of desulfurizing agent [20]. After desulfurization, the Fe(III) in the

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oxidative iron-ionic liquid has been reduced to Fe(II). Depending on the reversibility

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of redox reaction of Fe(III)/Fe(II) in the ionic liquid, the reduced Fe(II) can be

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oxidized to Fe(III) again for recycle and reuse. As reported by Mizuta [21], an

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electrochemical method can be used to oxidize Fe(II) to Fe(III) on anode and reduce

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H+ to hydrogen gas on cathode for recycle. After desulfurization and regeneration,

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hydrogen sulfide can be removed and valuable sulfur and hydrogen gas can be

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obtained. However, ionic liquids are some kinds of special liquid systems, which are

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more complex than the FeCl3 aqueous solution. So it is necessary to investigate the

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redox reaction mechanism and the interaction of Fe(III)/Fe(II) system in Fe(III)-IL for

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the recycle application of Fe(III)-IL, especially for the regeneration using

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electrochemical technology. In many cases trivalent and divalent iron complexes coexist in the ionic liquids

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especially in the processes of catalytic oxidation by iron-containing ionic liquids. So,

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in order to apply these ionic liquids for the practical electrochemical applications, it is

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necessary to investigate the speciation, interaction and electrochemical behavior of

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the iron complexes on the electrode. Yamagata et al. [22] reported that the redox

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reaction of Fe(III)/Fe(II) was quasi-reversible when Fe(III) or Fe(II) was introduced

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into BMPTFSI (1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide)

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by dissolving Fe(TFSI)2/Fe(TFSI)3. In contrast to Fe(TFSI)2, the electrode reaction of

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BMPFeBr4 in BMPTFSI presented a reversible one-electron charge transfer reaction

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as below.

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(1)

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FeBr4- + e- = FeBr42-

The diffusion coefficients of charged iron complexes such as FeCl4-/FeCl42-,

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[Fe(bpy)3]3+/[Fe(bpy)3]2+ (bpy= 2,2’-bipyridine) and [Fe(Cp)2]+/[Fe(Cp)2] (Cp- =

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cyclopentadienyl) have already been investigated by Yamagata [22] and Tachikawa et

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al [23]. Results revealed that the diffusion of the iron complexes mainly depends on

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the size of the complexes. However, the ion species of the trivalent and divalent irons

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coexisting system, the electrode reaction mechanism and the electrochemical kinetics

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of the iron complexes in the ionic liquids especially in the Fe(II)-rich ionic liquids

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need to be explored further.

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Although some kinds of ionic liquids can be used as electrolytes for fuel cells or

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as solvents for organic electrosynthesis [24-26], from the results reported by Barhdadi -4-

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et al. [26-27], the ion mobility would be limited and results in a huge ohmic drop due

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to their high viscosity. As a result the current density was very low even at high

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voltage (a few mA∙cm-2 at voltage over 40 V). Thus the system of these ionic liquids

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should be improved to adapt better to industrial electrochemical applications.

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Comminges et al. [27] have reported a series of experimental data for diffusion

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coefficients of ferrocene and conductivities for 1-butyl-3-methylimidazolium

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tetrafluoroborate

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bis(trifluoromethylsulfonyl)-imide (HMPTFSI) when diluted by dimethylformamide

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or 3-picoline. The results show that adding co-solvents can be good means for

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enhancing mass transfer and electrical conductivity. Besides, it is also advisable to

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study the influence of some co-solvents on the electrochemical behavior of iron

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complexes in iron-based ionic liquids. The results can provide theoretical guidance to

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increase regeneration efficiency of desulfurization solution of an iron-based ionic

and

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BMPTFSI,

N-methyl-N-hexylpyrrolidinium

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(BmimBF4),

liquid and also can be used in the electro-synthetic applications in ionic liquids.

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Therefore, in the present study, divalent and trivalent irons are introduced into

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the imidazolium chloride-ionic liquids to construct a trivalent and divalent irons

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coexisting system (Fe(III/II)-IL) to simulate Fe(III)-IL after desulfurization. The ion

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species, electrochemical behavior and the influence of co-solvents on the

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electrochemical kinetics of Fe(III)/Fe(II) in Fe(III)-IL and Fe(III/II)-IL are

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investigated. The results can guide the design of the electrochemical regeneration

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method of Fe(II)-rich imidazolium chloride ionic liquid after desulfurization. It also

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can provide a valuable means for electrochemical application of iron-based ionic

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liquids in the desulfurization, catalysis, electro-synthesis, etc. A microelectrode is

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used to reduce the influence of the solution resistance.

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2. Experimental Section

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2.1 synthesis of Ionic Liquids

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Butyl-3-methylimidazolium chloride (BmimCl) is synthesized by mixing butyl

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chloride (Beijing Yili Fine Chemicals Co. Ltd., purity >99%) and N-methylimidazole

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(Shanghai Sanwei Industry Co. Ltd., purity ≥99％) with a molar ratio of 1.2:1 and

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stirring at 343 K for 72 h. The yellow product is washed by ethyl acetate (Beijing

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Beihua Fine Chemicals Co. Ltd., purity >99%) 3-5 times, and then purified via rotary

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evaporation method at 353 K. The trivalent iron-based ionic liquid (Fe(III)-IL) is

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prepared by the reaction of purified BmimCl with FeCl3·6H2O (Tianjin Fuchen

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Chemical Reagent Factory, purity >99%) in the molar ratio of 1:2, stirred at room

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temperature for 24 h, then separated as light liquid from water phase by

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centrifugation.

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The ionic liquid containing trivalent and divalent iron (Fe(III/II)-IL) is prepared

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by mixing of purified BmimCl, FeCl3·6H2O and FeCl2∙4H2O (Tianjin Fuchen

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Chemical Reagent Factory, purity >99%) with a molar ratio of 1:0.5:0.7, stirred at 343

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K for 48 h under nitrogen atmosphere, then separated as light liquid from water phase

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by centrifugation. Both of the two liquids are dried overnight at 353 K under vacuum.

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2.2 Measurements

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The structure of as-synthesized iron-based ionic liquids is characterized by

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UV-VIS spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co.) and -6-

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Liquid chromatography-Mass spectrometry with an electrospray ionization source

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(ESI-MS) (LCMS-2010, SHIMADZU). The samples are dissolved and diluted with

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anhydrous methanol to a concentration lower than 10 ng·ml-1. All the electrochemical measurements are performed using a computer

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controlled RST3100 Electrochemical Workstation (Suzhou Risetest Instrument Co.

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Ltd) with a three-electrode cell. A 10 µm diameter platinum microelectrode is used as

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working electrode with a bright platinum sheet as the counter electrode and an

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Ag/AgCl electrode connected by a homemade salt bridge as a quasi-reference

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electrode. The salt bridge is filled with a saturated KCl solution and solidified by agar,

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and the side near the Pt microelectrode is designed as Luggin capillary. All the

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electrochemical measurements are performed using this working electrode unless

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otherwise stated. The platinum working electrode is polished with 0.5 μm alumina

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slurries on lapping pads and washed by water and ethanol with ultrasonic treatment

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before used. A faraday cage is used to reduce electromagnetic interference.

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The electrochemical impedance spectra are obtained at the open circuit potentials,

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in the frequency range from 0.01 Hz to 1 MHz and with amplitude of 5 mV. The

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resistance values (R) of the systems are obtained by analyzing the electrochemical

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impedance data with ZSimpWin (Princeton Applied Research, Rev. 3.10). Values of

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the conductivity and viscosity are measured using a DDS-IIA Conductivity Meter

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(INESA Scientific Instrument Co.，Ltd) and a DV-1 viscometer (Shanghai Pingxuan

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scientific instrument Co. Ltd). All the experiments are performed in a CH1015

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thermostat box (Shanghai Laboratory Instrument Works Co. Ltd) with a temperature

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accurate to ±1 K.

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2.3 Microdisc Chronoamperometric Experiments Potential steps chronoamperometry is carried out in order to calculate the

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diffusion coefficients of iron species in the ionic liquids. The samples are pre-

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equilibrated for 10 s at the open circuit potentials. Then the potentials are stepped

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from the open circuit potentials to the corresponding over potentials and the current is

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measured for 10 s. The technique is undertaken using a sample time of 0.001 s. The

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over potentials when calculating the diffusion coefficients of Fe(III) and Fe(II) are

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-1.5 V and 2 V (Vs. Ag/AgCl) respectively. Diffusion coefficient can be obtained from

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steady-state limited currents via the equations (1) [28] and (2) [29]:

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I d ,ss = 4nFDjC *jr0

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I d = I (1 

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nt j zj

)

(1) (2)

where Id,ss is diffusion steady-state current, Id is diffusion current of iron species, I is

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total current, n is the number of electrons transferred, F is the Faraday constant, Dj is

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the diffusion coefficient of the diffusing species j, Cj* is the initial concentration of j,

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r0 is the radius of the disk electrode (5 µm), tj is transference number of ion j, and zj is

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electric charge of ion j by the dimension of electron charge. The minus (-) is used in

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cathode current and the plus (+) is used in anode current.

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The concentration of the divalent iron species in iron-based ionic liquid is

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measured by potassium dichromate oxidation method with automatic potentiometric

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titration (877 Titrino plus, Metrohm AG) in a sulfuric-phosphoric acid solution

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(volume ratio of concentrated sulfuric acid, concentrated phosphoric acid and water is -8-

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1.5:1.5:7). The concentration of the total iron in iron-based ionic liquids was

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measured by UV -VIS spectrophotometer according to Karamanev [ 30 ].

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Concentration of trivalent iron species is calculated from the difference of total iron

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and divalent iron concentration. The software package OriginPro 8.5 (Microcal

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Software Inc.) is used to fit the experimental data.

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3. Results and discussions

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3.1 The structure of the iron-based ionic liquids

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The visible and ultra-violet absorption spectra of as-synthesized iron-based

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ionic liquids diluted by anhydrous methanol are shown in Fig. 1. The corresponding

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UV-vis spectrum of BmimCl diluted by anhydrous methanol showed no absorption

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between 240 nm-900 nm. The bands at 249 nm and 367 nm are due to tetrahedral

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FeCl4- originated from ligand (Cl) to metal (Fe(III)) charge transfer transitions

187

6

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nm of trivalent iron-based ionic liquid (b2, agreed with the literature [18]) are

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characteristic peaks of FeCl4- which can be assigned to the d5 electron transition [22]

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(530nm, 6A1→4A1, 4E; 617 nm, 6A1→4T2; 686 nm, 6A1→4T1 [5]). There is little

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difference between Fe(III)-IL and Fe(III/II)-IL, except the broad absorption peak from

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580 nm to 650 nm. The maximum at 601 nm with a shoulder at 617 nm appear in

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Fe(III/II)-IL, while the maximum at 617 nm appear in Fe(III)-IL. This indicates the

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existence of d-d electron transition of metal Fe(II) (5E→5T2 [5]) in Fe(III/II)-IL.

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A1→6T2 [5,31] (a1 and b1). The three absorption peaks in the range of 500 nm - 800

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Fig. 1 As reported by Lee [12], Hayashi [18] and Wang [32], the structural formula of -9-

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the Fe(III)-IL is [Bmim]+∙[FeCl4]-, which is a little different from the ionic liquid

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synthesized by FeCl3 and BmimCl with excess FeCl3 in the literature [16]. Fig. 2

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shows the ESI-MS spectra of the cation and anion of the as-synthesized Fe(III/II)-IL.

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The major cationic species are [Bmim]+ (m/z: 139.1 Da) and [Bmim2Cl]+ (m/z: 313.2

201

Da), while the major anionic species are [FeCl4]- (m/z: 197.7 Da) and [Fe3Cl7]- (m/z:

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413.7 Da). No FeCl42- (m/z: 99 Da) is detected.

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Fig. 2

3.2 Cyclic voltammetry of the trivalent and divalent iron species in iron-based

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ionic liquids

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Cyclic voltammograms of as-synthesized iron-based ionic liquids at a 2 mm

207

diameter platinum electrode at the rate of 50 mV∙s-1 and 303 K are shown in Fig. 3.

208

The peak potentials of Fe(II)/Fe(III) couple in Fe(III)-IL are 0.464 V and -0.05 V,

209

while that in Fe(III/II)-IL are 0.815 V and -0.163 V respectively (vs. Ag/AgCl). The

210

large peak potential separation is due to the ohmic drop [33] of ionic liquids. The

211

difference in peak potentials of Fe(III)-IL is smaller than that of Fe(III/II)-IL because

212

the conductivity of Fe(III)-IL is larger than that of Fe(III/II)-IL (8 mS∙cm-1 vs. 3.4

213

mS∙cm-1 at 303 K). One irreversible oxidation peak at about +1.5 V is appeared in

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Fe(III/II)-IL. Both Fe3Cl7- and Bmim2Cl+ can be oxidized on electrode, but

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Fe(III)/Fe(II) showed higher redox reversibility than Cl [22, 34 ]. So the small

216

irreversible oxidation wave is due to the oxidation of Bmim2Cl+. The possible

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reaction mechanism of Bmim2Cl+ on Pt electrode is: Bmim2Cl+ → Cl(a) + 2Bmim+

218

+ e-, 2Cl(a) ↔ Cl2, where Cl(a) is a chlorine adatom [34].

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Fig. 3 To reduce the impact of the uncompensated resistance on the cyclic voltammetry,

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a 10 μm diameter microelectrode is used. The resistance values (R) of the systems

222

containing as-synthetized ionic liquids are obtained by electrochemical impedance

223

spectroscopy [29]. Typical Nyquist plots of Fe(III)-IL and Fe(III/II)-IL on the Pt

224

microelectrode are shown in Fig. 4. The resistance value of Fe(III)-IL and

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Fe(III/II)-IL on the Pt microelectrode at 303 K are 27.3 kΩ and 29.5 kΩ respectively.

226

Fig. 4

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Cyclic voltammograms of BmimCl and the iron-based ionic liquids at a 10 μm

228

diameter microelectrode at the scan rate of 50 mV∙s-1 and 303 K are shown in Fig. 5.

229

The data is corrected for the uncompensated resistance by plotting the current I(t)

230

versus E(t) – RI(t). However, because of the low current on the Pt microelectrode, the

231

ohmic polarization (IR drop) of Fe(III)-IL and Fe(III/II)-IL is very small. IR at the

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oxidization peak and reduction peak is only 0.002 V. The results confirm that the

233

impact of ohmic polarization (IR drop) on the voltammograms can be reduced by

234

using microelectrode. As shown in Fig. 5, BmimCl exhibits an electrochemical

235

window of 3.14 V (-2.14 V - +1.00 V). Cyclic voltammogram of Fe(III)-IL showed an

236

onset of reduction current at +0.5 V (vs. Ag/AgCl) with a maximum at +0.193 V (vs.

237

Ag/AgCl), which is due to the reduction of FeCl4-. An associated oxidation wave is

238

appeared at 0 V (vs. Ag/AgCl) with a maximum at +0.284 V (vs. Ag/AgCl) when the

239

scan is reversed. The peak potential separation at 50 mV·s-1 and 303 K is

240

approximately 0.091 V (A theoretical value of 0.060 V is expected for a one electron

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reversible charge transfer at 303 K [29]). The result coincide with the result of the

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Fe(III)/Fe(II) electrode reaction in the room temperature molten salt of aluminum

243

chloride and N-(n-butyl)pyridinium chloride with molar ratio of 2:1 [35]. But in

244

contrast to FeCl4- in the Fe(III)-IL, the electrode reaction of FeBr4- in BMPTFSI is

245

found to be reversible [22]. All the values of ΔEp are greater than 0.060 V as shown in

246

Table 1, which illustrates that the electrode reactions at different concentration of

247

FeCl4- in BmimCl on Pt microelectrode are quasi-reversible processes. Even at the

248

same concentration as the FeBr4- in BMPTFSI (0.1mol·dm-3) [22] (the effect of

249

migration can be ignored at this concentration), the peak potential separation of

250

Fe(III)/Fe(II) in this system, ΔEp, is still much greater than 0.060 V at 303 K (Table 1,

251

0.108 V, 4% Fe(III)-IL in the mixture of Fe(III)-IL and BmimCl. The mixture is a

252

viscous solution at 303 K.). So the electrode reaction of FeCl4- at Pt electrode is a

253

quasi-reversible process, which is controlled not only by diffusion but also by charge

254

transfer kinetics. The steric hindrance effects of FeBr4- and FeCl4- are similar since

255

both of them have tetrahedron structure [16, 33, 35-37], but the electron density of Br

256

element is higher than that of Cl element. So electron transfer from Pt electrode to

257

redox center Fe(III) is easier for Fe(III) being surrounded by Br than by Cl.

258

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The cyclic voltammogram of Fe(III/II)-IL at the scan rate of 50 mV·s-1 and 303

259

K in Fig. 5 shows a reduction wave of Fe(III) occurring at the peak potential of

260

+0.227 V (vs. Ag/AgCl) and an oxidation wave of Fe(II) occurring at the peak

261

potential of +0.335 V (vs. Ag/AgCl). The peak potential separation is approximately

262

0.108 V, which is larger than that of Fe(III)-IL. The possible reason is that the - 12 -

Page 12 of 57

structure of Fe3Cl7- in Fe(III/II)-IL is a little complex. The steric hindrance and the

264

complex structure may affect the reversibility of Fe(III)/Fe(II) electrode reaction.

265

However, there is no obvious oxidation peak distinguishing Fe3Cl7- from FeCl42-

266

(FeCl42- is produced by reduction of FeCl4- on the electrode). Perhaps because the

267

oxidization of the Fe3Cl7- on the anode is comparable to that of the FeCl42- and the

268

oxidative potentials of Fe3Cl7- and FeCl42- are too similar to be distinguished. A small

269

change in oxidation and reduction peaks of Fe(III)-IL and Fe(III/II)-IL is observed

270

(Table 1) when the scan region is shorted, because the structures of iron species is

271

changed slightly with the oxidation of Cl- at more positive potential. The oxidation

272

wave at higher magnitude observed in Fig. 5 is possibly due to the oxidation of Cl

273

element on Pt electrode. The mechanism of the chlorine reaction on Pt electrode is Cl-

274

→ Cl(a) + e-, 2Cl(a) ↔ Cl2, where Cl(a) is a chlorine adatom [34]. In contrast to

275

BmimCl, the oxidation potentials of Cl for Fe(III)-IL and Fe(III/II)-IL is shifted

276

toward more positive value. Because FeCl4- exists in Fe(III)-IL while FeCl4-, Fe3Cl7-

277

and Bmim2Cl- exist in Fe(III/II)-IL, the oxidization peak of Cl in Fe(III)-IL and

278

Fe(III/II)-IL are due to Fe-Cl complexes or Bmim2Cl+. So the free chlorine in

279

BmimCl is easier to be oxidized than that in Fe-Cl complexes and Bmim2Cl+. Thus,

280

no free chlorine ions exist in Fe(III)-IL and Fe(III/II)-IL.

281

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ip t

263

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 5

282

To further explore the electrode reaction mechanism, the influence of Cl-

283

concentration on the redox cyclic voltammetry peaks of Fe(III)-IL and Fe(III/II)-IL

284

are studied. The voltammetric data of Fe(III)/Fe(II) system in the Fe(III)-IL + - 13 -

Page 13 of 57

BmimCl mixture and Fe(III/II)-IL + BmimCl mixture at different ratios are listed in

286

Table 1. Ipa and Ipc are measured from the corresponding baselines. The baselines are

287

assumed to be the current that would be obtained if the forward sweep is continued

288

for the same amount of time that it could take to reach the reverse peak [29, 38]. As

289

shown in Table 1, all the values of |Ipa / Ipc| are greater than 1, indicating that there are

290

some homogeneous reactions or other complexity in the electrode reaction process

291

[29]. Combined with the results obtained by EIS-MS, Fe3Cl7- rather than FeCl42- is

292

found in Fe(III/II)-IL. Because of the low valence states of Fe(II), FeCl 42- was less

293

stable than FeCl4-. So the coordination number of the redox species maybe changed

294

slightly during the reaction. We presume that FeCl42- produced by the reduction of

295

FeCl4- on the electrode may transform to Fe3Cl7-. At high concentration of

296

electroactive ions, both migration and diffusion of electroactive ions can affect the

297

experimental results. The current (i) was actually the sum total of diffusion current (id)

298

and migration current (im):

cr

us

an

M

ed

ce pt

299

ip t

285

i = id + im

(3)

300

where, the directions of id and im may be the same or not, which depend on the electric

301

field direction and the charges of the electroactive ions [29]. Because the negative ion

302

Fe3Cl7- (FeCl42-) is easier to migrate to the anode to lose electrons than the negative

303

ion FeCl4- to migrate to the cathode to accept electrons under electric field, the

304

directions of id and im are the same for the oxidization of Fe3Cl7- (FeCl42-), while that

305

is contrary for the reduction of FeCl4-. So the values of |Ipa / Ipc| are greater than 1.

306

Fe3Cl7- also has certain electrochemical activity and can be oxidized to FeCl4-

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 14 -

Page 14 of 57

combining with Cl- on the anode. The oxidation of Fe3Cl7- can be enhanced when Cl-

308

or Fe(II) are present in the system. So the value of |Ipa / Ipc| increases as the

309

concentration of Cl- increases. All the values of |Ipa / Ipc| of Fe(III)-IL are smaller than

310

that of Fe(III/II)-IL at the same condition. For better understanding the transformation

311

between Fe(III) and Fe(II) in the as-synthetized ionic liquids, the whole reaction

312

process is simplified to the EC mechanism as follows:

cr

ip t

307

313

FeCl4- + e- = FeCl42-

314

3FeCl42- = Fe3Cl7- + 5Cl-

us

(2)

(3)

An increase in content of Cl- or Fe(II) is likely to shift the equilibrium (3) toward the

316

left side and enhance the oxidation of FeCl42-. It is consistent with the experimental

317

results at different concentration of Cl- in Fe(III)-IL and in Fe(III/II)-IL . According to

318

an EC mechanism, the reaction (3) may lead a small shift of the reduction peak

319

towards more positive values with respect to the case where only the reaction (2) is

320

taking place. But, in our opinion, the reaction (3) can take place easily at high

321

concentration of Fe(III)-IL. The potential shift may be not obvious with the increase

322

of Cl- concentration. As shown in Table 1, Epc shows no obvious changes (just only

323

few mV). So the small potential shift may not be caused by Cl-. The cyclic

324

voltammetric data of the Pt microelectrode in BmimCl containing 0.1mol·dm-3

325

Fe(III)-IL are shown in Table 1. At low concentration of FeCl4-, the value of |Ipa / Ipc|

326

is near to 1. In this system, the migration of BmimCl at high concentration can

327

suppress FeCl4- migration, so the current observed is only diffusion current. Besides,

328

in this system, a large concentration of BmimCl surrounding FeCl42- may depress the

ce pt

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Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 15 -

Page 15 of 57

329

reaction (3).

330

Table 1 The cyclic voltammograms of Fe(III)-IL and Fe(III/II)-IL at different scan rates

332

are shown in Fig. 6. With the increase of the scan rate, the peak separation potentials

333

of Fe(III)-IL and Fe(III/II)-IL varied from 60 mV to 185 mV and 92 mV to 258 mV

334

respectively. Because the data for the uncompensated resistance have been corrected,

335

the results indicate that the electrochemical behavior of the [FeCl4]-/[Fe3Cl7]- is a

336

quasi-reversible process.

337

an

3.3 Diffusion coefficients, viscosity and conductivity

M

338

Fig. 6

us

cr

ip t

331

Potential steps are carried out to calculate the diffusion coefficients of iron

340

species in the ionic liquids. The chronoamperometric curves of Fe(III)-IL at various

341

temperatures are shown in Fig. 7. The current increases with increasing temperature

342

because of the diffusion effect. The concentration of Fe(III) in Fe(III)-IL is 3.97

343

mol·dm-3, while in Fe(III/II)-IL, the concentration of Fe(III) and Fe(II) (Fe(II) existed

344

as the complex of Fe3Cl7-) are 2.43 and 0.29 mol·dm-3, respectively.

345 346

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339

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 7

The chronoamperometric data of iron species in Fe(III)-IL and Fe(III/II)-IL at the

347

platinum microelectrode are fitted to equations (4), (5), and (6) proposed by Shoup

348

and Szabo [39] below.

349

I d = 4nFD j C *j r0 f ( τ )

350

f ( τ ) = 0.7854 + 0.8862τ -1 / 2 + 0.2146e-0.7823τ

(4) -1 / 2

(5)

- 16 -

Page 16 of 57

351

τ = 4 D j t / r02

(6)

where, Id is the diffusion current intensity, t is the time and the other terms have their

353

usual electrochemical meanings. However, during the first second, the current is very

354

high and shows obvious discrepancy with the simulation. It may be attribute to some

355

more complex processes, such as the charging current for the double layer capacitance,

356

which require some time to decay away in a resistive electrolyte. Because of the

357

migration effects the data from t>1 also do not fit the equations well. As reported by

358

Bard [29] and Morris [40], the mass transport process of species can be expressed by

359

Nernst-Planck equation as follows:

Ci ( x) zi F  ( x)  Di Ci  Ci v( x) x RT x

(7)

M

J i ( x)   Di

an

360

us

cr

ip t

352

where, -Di(əCi/əx), -[ziF/(RT)]DiCi[əø(x)/əx] and Civ(x) account for diffusion effect,

362

migration effect and convection effect, respectively. In these ionic liquids systems, the

363

convection effect can be ignored under static condition. But the migration effect

364

cannot be ignored because of high concentration of electroactive species without any

365

supporting electrolytes. So the electrochemical mass transport process is controlled by

366

both migration and diffusion. However, the equations (4), (5) and (6) derived from the

367

Fick’s second law are only suitable for the diffusion controlled process, so these

368

equations are not suitable for obtaining diffusion coefficient directly in the ionic

369

liquids systems.

370 371

372

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361

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Similarly,

an

equation

for

the

diffusion

current

(Id)

obtained

in

chronoamperometry in quiescent solutions can be expressed by [28]

nFAD 1/ 2C0* Id = + 4nFDC0* r0 1/ 2 1/ 2 π t

(8)

- 17 -

Page 17 of 57

where, A is the electrode area and the other terms have their usual electrochemical

374

meanings. Thus, a linear regression of the current vs. t-0.5 should give a straight line

375

with an intercept which can give the steady-state term. However, because of the high

376

concentration of electroactive species and the migration effects, or other complex

377

processes, the current here is total current containing diffusion and migration. So the

378

plotting current versus t-0.5 is not a linear function, as shown in Fig. 8 as follow (Short

379

time data were disregarded because of charging current). Fig. 8

us

380

cr

ip t

373

As shown in Fig. 7, the currents are near steady-state at t=10 s, so Dj /(1  nt j / z j ) ,

382

defined as apparent diffusion coefficient (Dj’) of electroactive species, can be

383

calculated by the equations (1) and (2) from steady-state limited currents. The

384

advantage of using the apparent diffusion coefficient is that the migration effect has

385

also been taken into account. As shown in Table 2, the values of D’FeCl4- and D’Fe3Cl7-

386

increase as the temperature rises from 293 K to 333 K, and at the same temperature,

387

D’Fe3Cl7- (Fe(III/II)-IL) > D’FeCl4- (Fe(III)-IL) > D’FeCl4- (Fe(III/II)-IL). The reason is

388

that the diffusing complexes move against the frictional force that is proportional to

389

the size of particles and the viscosity of the complexes [23]. Fe(III/II)-IL is more

390

viscous than Fe(III)-IL (68.0 mPa∙s vs. 21.4 mPa∙s at 303 K), and the steric hindrance

391

effects of Fe3Cl7- is larger than FeCl4-. So the diffusion of FeCl4- in Fe(III)-IL is

392

higher than in Fe(III/II)-IL. In Fe(III/II)-IL, the diffusion of Fe3Cl7- should be lower

393

than FeCl4-. But the apparent diffusion coefficient of Fe3Cl7- is higher than FeCl4-,

394

because the oxidation of Fe3Cl7- occurs on the anode and the reduction of FeCl4-

ce pt

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an

381

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 18 -

Page 18 of 57

occurs on the cathode, and it is easier for the negative ion Fe3Cl7- to diffuse to the

396

anode than for the negative ion FeCl4- to diffuse to the cathode in electric field. From

397

the simplified electrode reaction (2) and (3), nFeCl4- = 1 and nFe3Cl7- = 3, and the

398

diffusion coefficients (Dj) can be calculated when transference numbers of iron

399

species (tj) are obtained by Hittorf or moving boundary methods [41]. This work is

400

underway recently.

401

us

403

Table 2

The apparent diffusion coefficients are analyzed in terms of the Arrhenius equation to obtain the apparent diffusion activation energy,

404

- E' a, D / RT

(9)

M

D' = D'0 e

an

402

cr

ip t

395

where, E’a,D is the activation energy for diffusion of the substrate of interest in the

406

pertinent solvent, D’0 is a constant, T is the absolute temperature and R is the

407

universal gas constant. Fig. 9 shows the Arrhenius plots of lnD’ against T-1 for FeCl4-

408

and Fe3Cl7-. The values of E’a,D for the diffusion of FeCl4- in Fe(III)-IL and

409

Fe(III/II)-IL are 33.5±0.2 kJ·mol-1 and 33.6±1.3 kJ·mol-1 respectively, while the value

410

for Fe3Cl7- in Fe(III/II)-IL is 29.9±1.1kJ·mol-1. The Ea,D value of FeCl4- can be

411

compared with that reported in literature [23], 24 kJ·mol-1.

412

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405

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 9

413

As shown in Table 2, the viscosity of Fe(III/II)-IL (68.0 mPa∙s, 303 K) is higher

414

than Fe(III)-IL (21.4 mPa∙s, 303 K), which is due to the steric hindrance effects of

415

Fe3Cl7- and Bmim2Cl+. The viscosity of Fe(III)-IL and Fe(III/II) decreases three times

416

and six times respectively as the temperature rises from 293 K to 333 K. Ionic - 19 -

Page 19 of 57

conductivity (σ) is another electrochemical parameter that has a pronounced effect on

418

the ohmic drop observed during electrolysis. In this work, the ionic conductivity of

419

Fe(III)-IL and Fe(III/II)-IL has been investigated. As reported by Comminges [27], the

420

ionic conductivity mainly depends on the viscosity of the ionic liquids and

421

temperature. Heating the solution is an effective way to enhance mass transfer and

422

improve electrical conductivity. As shown in Table 2, the conductivities of Fe(III)-IL

423

and Fe(III/II)-IL increase from 6 mS∙cm-1 to 15.7 mS∙cm-1 and from 2.7 mS∙cm-1 to

424

9.7 mS∙cm-1 respectively as the temperature rises from 293 K to 333 K. The ionic

425

conductivity of Fe(III)-IL is higher than Fe(III/II)-IL at the same temperature since the

426

lower viscosity of Fe(III)-IL than Fe(III/II)-IL. The molar conductivities (Λ) of

427

Fe(III)-IL and Fe(III/II)-IL are 1511.33 mS∙cm2·mol-1 and 992.65 mS∙cm2·mol-1 at

428

293 K, respectively. As reported by Yoshida [5], the conductivities of [EMI][FeIIICl4],

429

[EMI][FeIIICl4]0.5[GaIIICl4]0.5 (EMI = 1-ethyl-3-methylimidazolium), etc. lie between

430

18-20 mS·cm-1 at 293 K. But that of Fe(III)-IL and Fe(III/II)-IL are only 6 and 2.7

431

mS·cm-1. Silvester et al. [42] have reported that the conductivity and viscosity were

432

affected by the type of cation of ionic liquids. Because the steric hindrance effects of

433

Bmim+ is more pronounced than EMI+, the viscosity of ionic liquids containing

434

Bmim+ is higher than that containing EMI+ with the same type of anions, and the

435

conductivity of ionic liquids containing Bmim+ is lower than that containing EMI+.

436 437 438

ce pt

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ip t

417

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

The activation energies of Fe(III)-IL and Fe(III/II)-IL for viscosity (η) and conductivity (σ) are obtained by Arrhenius equation：

η = η0e

Ea, η / RT

(10)

- 20 -

Page 20 of 57

439

σ = σ 0e

-Ea, σ / RT

(11)

where η0 is a constant and Ea,η is the activation energy for viscous flow. Fig. 10 shows

441

the Arrhenius plots of log (η-1) and log σ against T-1 for Fe(III)-IL and Fe(III/II)-IL.

442

Values of Ea,η for Fe(III)-IL and Fe(III/II)-IL are 21.7 ± 0.7 kJ·mol-1 and 35.0 ± 0.7

443

kJ·mol-1, while values of Ea,σ for Fe(III)-IL and Fe(III/II)-IL are 19.6 ± 0.6 kJ·mol-1

444

and 27.4 ± 0.9 kJ·mol-1 respectively. The experiments are operated in a small

445

temperature range near room temperature. So Arrhenius equation can be used in these

446

systems to calculate the activation energy. As reported by W. Xu, et al. [43], VFT

447

equation is more appropriate for these ionic liquids. So VFT equation has also been

448

used to describe the relation between temperature and viscosity.

M

an

us

cr

ip t

440

η = η0e Β /(T -T0 )

449

(12)

where η0, B and T0 are constants, (T0 = 188 K used in these systems [43]). Fig. 11

451

shows the relation between T and η for Fe(III)-IL and Fe(III/II)-IL fitted by VFT

452

equation. R-square is increased from 0.9917 (Fe(III)-IL, Arrhenius equation) and

453

0.9963 (Fe(III/II)-IL, Arrhenius equation) to 0.9965 (Fe(III)-IL, VFT equation) and

454

0.9983 (Fe(III/II)-IL, VFT equation) respectively.

455 456 457

ce pt

ed

450

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 10 Fig. 11

3.4 The effect of co-solvents on the electrochemical behavior

458

Adding co-solvents with certain molar fractions can decrease the viscosity and

459

increase the conductivity of ionic liquids [27,44-47]. Therefore, it is advisable to

460

study the effect of co-solvents on the electrochemical behavior of Fe(III/II)-IL. The - 21 -

Page 21 of 57

resistance of Fe(III)-IL + acetonitrile, Fe(III)-IL + 1,2-dichloroethane, Fe(III/II)-IL +

462

acetonitrile, and Fe(III/II)-IL + 1,2-dichloroethane (mass ratio of Fe-IL and

463

co-solvents are 7:3) are 16.15 KΩ, 17.63 KΩ, 16.92 KΩ, and 17.80 KΩ respectively.

464

Fig.12, Fig 13, and Table 3 shows the cyclic voltammograms of ionic liquids +

465

co-solvents and the influence of Cl- concentration on the redox peaks of the cyclic

466

voltammograms. The electrode reaction of FeCl4- at Pt electrode is a quasi-reversible

467

process due to the large peak potential separation (more than 60 mV at 303 K). The

468

redox peaks shift to more negative potentials compared with the pure ionic liquids

469

when the co-solvents are added into the systems. Because a salt bridge filled with a

470

saturated KCl solution and solidified by agar is used to decrease the different junction

471

potentials, and the same reference electrode filled with the same electrolyte is used in

472

all the different systems, the effect of the junction potentials and the electrode can be

473

ignored. So there may be some interaction between the co-solvents and the

474

Fe(III)/Fe(II), which may cause the redox peaks shift. The peak potential separations

475

(ΔEp) for Fe(III)-IL + acetonitrile and Fe(III/II)-IL + acetonitrile at 50 mV·s-1 scan

476

rate and 303 K are larger than that for Fe(III)-IL / Fe(III/II)-IL + 1,2-dichloroethane

477

systems. ΔEp of Fe(III)/Fe(II) in Fe(III)-IL / Fe(III/II)-IL + co-solvents systems

478

decrease obviously as the concentration of Cl- increase (Table 3). It may be caused by

479

some kind of solvation effect as some interaction between the ionic groups

480

Fe(III)/Fe(II) and the surrounding co-solvents may affect the kinetic electrode

481

reaction of the electroactive species. As the concentration of BmimCl increases, the

482

interaction may be changed. Higher values of current for ionic liquids + co-solvents

ce pt

ed

M

an

us

cr

ip t

461

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 22 -

Page 22 of 57

compared with pure Fe(III)-IL / Fe(III/II)-IL are observed. The possible reason is that

484

co-solvents can increase the diffusion of Fe(III)/Fe(II), which leads more electroactive

485

species diffuse to electrode to react. The co-solvents can decrease the viscosity of the

486

system and intensify the diffusion of the ionic groups. So the current intensity is

487

noticeably increased by adding the co-solvents.

488

Fig. 12

489

Fig. 13

us

cr

ip t

483

As shown in Table 3, after the co-solvents are added into the systems, the values

491

of |Ipa / Ipc| are near to 1, while the values of |Ipa / Ipc| are greater than 1 without

492

co-solvents (Table 1). With increasing the concentration of Cl-, the values of |Ipa / Ipc|

493

of Fe(III)-IL + 1,2-dichloroethane and Fe(III/II)-IL+ 1,2-dichloroethane increase.

494

They are 1.3 and 1.2 at 20% mass fraction of BmimCl, respectively. While in the

495

Fe(III)-IL / Fe(III/II)-IL + acetonitrile, the values of |Ipa / Ipc| are still near to 1 with

496

increasing the concentration of Cl-. The possible reason is that polar solvent can

497

decrease the migration effect of electroactive species by increasing the conductivity of

498

systems obviously and decreasing the concentration of electroactive species (Table 4).

499

So the mass transfer process of iron species in the polar co-solvents systems is mainly

500

influenced and controlled by diffusion. However, the interaction between

501

Fe(III)-IL/Fe(II)-IL and co-solvents are not clear now. Further study is needed to

502

explore the effect of co-solvents on Fe(III)-IL/Fe(III/II)-IL electrochemical reaction

503

mechanism.

504

ce pt

ed

M

an

490

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Table 3 - 23 -

Page 23 of 57

505 506

Table 4 and Fig. 14 present the apparent diffusion coefficients (D’), viscosity (η) and conductivity (σ) of the mixtures of Fe(III/II)-IL and the co-solvents.

507

Table 4 It can be noted in Table 4 that the apparent diffusion coefficient D’FeCl4- and

509

D’Fe3Cl7- increase exponentially when the mole fractions of 1,2-dichloroethane and

510

acetonitrile increase in the mixtures of Fe(III/II)-IL and co-solvents. D’FeCl4- and

511

D’Fe3Cl7- in Fe(III/II)-IL containing 90% mass fraction of 1,2-dichloroethane are about

512

20 to 6 times of the pure Fe(III/II)-IL, and those in Fe(III/II)-IL containing 90% mass

513

fraction of acetonitrile are about 82 to 36 times of the pure Fe(III/II)-IL. This may be

514

attributed to the decrease in viscosity of Fe(III/II)-IL by addition of co-solvent. It is

515

further noted that acetonitrile has greater impact on it. This may provide a way to

516

improve the current density and regenerate efficiency of Fe(III)-IL. Notably, 10%

517

mass fraction of acetonitrile dosage can reduce viscosity nearly ten times, while only

518

double diffusion rate and triple conductivity can be increased. These results coincide

519

with the results reported by Wang [48] and Lewandowski [49]. According to the

520

literatures, the viscosity of the BmimBF4-acetonitrile mixture were nearly ten times

521

lower than the pure BmimBF4, and the conductivity of the EMITFSI-acetonitrile

522

mixture were seven times higher than the pure EMITFSI with 10% mass fraction of

523

acetonitrile dosage.

524

ce pt

ed

M

an

us

cr

ip t

508

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 14

525

The conductivity (σ) of co-solvents + Fe(III/II)-IL mixtures has been shown in

526

Table 4 and Fig. 14. The conductivity is changed regularly as the mass fractions of - 24 -

Page 24 of 57

co-solvents increase. The highest values of conductivity are obtained with 50% mass

528

fraction of acetonitrile and 1,2-dichloroethane at 40 mS∙cm-1 and 10.1 mS∙cm-1

529

respectively. The increase in conductivity is more obvious with a polar solvent like

530

acetonitrile than with a less polar solvent like 1,2-dichloroethane. As reported by

531

Comminges [27], the conductivity dependency on the viscosity of the medium and the

532

co-solvent polarity. The addition of 1,2-dichloroethane and acetonitrile can cause a

533

decrease in viscosity. Moreover, the strong polar co-solvent can enhance the

534

dissociation of ions pairs and increase the amount of charge carriers. But the

535

concentrations of ions pairs decrease as the mass fraction of co-solvents increases. So

536

the maxima of conductivity at the mass fraction of 50% of acetonitrile /

537

1,2-dichloroethane is appeared. The molar conductivities (Λ) of Fe(III/II)-IL +

538

1,2-dichloroethane

539

mS∙cm2·mol-1 to 11851.85 mS∙cm2·mol-1 and from 1102.94 mS∙cm2·mol-1 to

540

97222.22 mS∙cm2·mol-1 as the concentration of co-solvent increases, respectively

541

(Table 4). Evidently, with addition of co-solvents, especially polar and low viscosity

542

co-solvents, the molar conductivity of ionic liquids can be improved and promoted

543

significantly.

544

4. Conclusion

Fe(III/II)-IL +

acetonitrile

increase

from

1102.94

ce pt

ed

and

M

an

us

cr

ip t

527

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

545

Two kinds of iron-based imidazolium chloride ionic liquids Fe(III)-IL and

546

Fe(III/II)-IL have been constructed with mixing certain ratio of BmimCl, FeCl3∙6H2O

547

and FeCl2∙4H2O. [Bmim]+, [Bmim2Cl]+, [FeCl4]- and [Fe3Cl7]- have been detected in

548

Fe(III/II)-IL. The electrode reaction of [FeCl4]-/[Fe3Cl7]- on Pt microelectrode in the - 25 -

Page 25 of 57

ionic liquids is a quasi-reversible process. Both migration and diffusion of

550

electroactive ions affect the electrochemical behavior of Fe(III)/Fe(II) on Pt

551

microelectrode in these pure ionic liquids. The viscosity and conductivity of Fe(III)-IL

552

and Fe(III/II)-IL are 21.4 mPa∙s-1, 8.00 mS∙cm-1 and 68.00 mPa∙s-1, 3.40 mS∙cm-1

553

respectively at 303 K. With addition of 10% mass fraction of strong polar acetonitrile,

554

the viscosity decreases ten times, and the conductivity and the apparent diffusion

555

coefficient increase three times and two times respectively compared with the pure

556

Fe(III/II)-IL.

us

cr

ip t

549

The apparent diffusion coefficients containing diffusion coefficient and

558

transference number of the active species in these complex systems are obtained from

559

the steady-state limited currents. The diffusion coefficients of the iron species at high

560

concentration in the ionic liquids could not be obtained directly by common methods.

561

Some suitable methods are expected to be developed for the determination of the

562

diffusion coefficients or the transference numbers of the active species. Besides,

563

further researches are needed to explore the electrochemical reaction mechanism of

564

the iron-based imidazolium chloride ionic liquids and the effect of solvents on their

565

reaction mechanism in detail. All of these are expected to improve some potential

566

industrial applications of the iron-based ionic liquids on desulfurization, etc.

567

Acknowledgment

ce pt

ed

M

an

557

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

568

This research is supported by National High Technology Research and

569

Development Program 863 (No. 2007AA06Z115), National Natural Science

570

Foundation of China (No. 21076019), and the Fundamental Research Funds for the - 26 -

Page 26 of 57

Accepted Manuscript Title: Electrochemical Behavior of Iron-based Imidazolium Chloride Ionic Liquids Author: Zhihui Guo Tingting Zhang Murtaza Khan Shujing Gao Tiantian Liu Jiang Yu PII: DOI: Reference:

S0013-4686(14)01529-1 http://dx.doi.org/doi:10.1016/j.electacta.2014.07.110 EA 23148

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

19-2-2014 18-7-2014 18-7-2014

Please cite this article as: Z. Guo, T. Zhang, M. Khan, S. Gao, T. Liu, J. Yu, Electrochemical Behavior of Iron-based Imidazolium Chloride Ionic Liquids, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.07.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

extractive desulfurization of gasoline with xEt3NHCl·FeCl3 ionic liquids. Energy Fuels 24 (2010) 4285-4289. [9]M. Y. Xie, P. P. Li, H. F. Guo, L. X. Gao, J. Yu. Ternary system of Fe-based ionic liquid, ethanol and water for wet flue gas desulfurization. Chin. J. Chem. Eng. 1

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cr

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bicyclo[2.2.1]hepta-2.5-diene. J. Catal. 258 (2008) 5-13. [17]Y. Yoshida, G. Saito. Influence of structural variations in 1-alkyl-3-methylimidazolium cation and tetrahalogenoferrate(III) anion on the physical properties of the paramagnetic ionic liquids. J. Mater. Chem. 16 (2006) 1254-1262. [18]S. Hayashi, H.Hamaguchi. Discovery of a magnetic ionic liquid [bmim]FeCl4. - 28 -

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135-139. [20]Y. He, J. Yu, L. B. Chen. Wet oxidation desulfurization of hydrogen sulfide with

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[21] S.Mizuta, W. Kondo, K. Fujii, H. Iida, S. Isshiki, H. Noguchi, T. Kikuchi, H.Sue, K. Sakai. Hydrogen production from hydrogen sulfide by the iron-chlorine hybrid

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process. Ind. Eng. Chem. Res. 30 (1991) 1601-1608.

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of several iron complexes in hydrophobic room-temperature ionic liquids. Electrochim. Acta. 52 (2007) 3317-3322.

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[23]N. Tachikawa, Y. Katayama, T. Miura. Electrode kinetics of some iorn

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complexes in an imide-type room-temperature ionic liquid. J. Electrochem. Soc. 154 (2007) 211-216.

[24]P. C. Howlett, D. R. MacFarlane, A. F. Hollenkamp. High lithium metal cycling efficiency in a room-temperature ionic liquid. Electrochem. Solid-State Lett. 7 (2004) A97-A101

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[25]R. F. Souza, J. C. Padilha, R. S. Goncalves, J. Dupont. Room temperature dialkylimidazolium ionic liquid-based fuel cells. Electrochem. Commun. 5 (2003) 728-731. [26]R. Barhdadi, C. Courtinard, J. Y. Nédélec, M. Troupel. Room-temperature ionic liquids as new solvents for organic electrosynthesis. The first examples of direct or nickel-catalysed electroreductive coupling involving organic halides. Chem. Commun. - 29 -

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[28]D. Pletcher. Microelectrodes: theory and applications. Why microelectrodes?

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cr

[29]A. J. Bard, L. R. Faulkner. Electrochemical methods: fundamentals and

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quantitative determination of ferric and ferrous ions in drainage waters and similar

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[31]W. H. Liu, B. Etschmann, J. Brugger, L. Spiccia, G. Foran, B. McInnes. UV-Vis spectrophotometric and XAFS studies of ferric chloride complexes in hyper-saline

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LiCl solutions at 25-90 ºC. Chem. Geol. 231 (2006) 326-349.

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[32]J. H. Wang, L. Zhu. Catalytic oxidation of hydrogen sulfide via [bmim]FeCl4 ionic liquid. Nat. Gas Chem. Ind. 37 (2012) 29-32. [33]P. Hapiot, C. Lagrost. Electrochemical reactivity in room-temperature ionic liquids. Chem. Rev. 108 (2008) 2238-2264. [34]T. Yokoyama, M. Enyo. Mechanism of the chlorine electrode reaction on

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(26DAPH)FeBr4 and (26DA35DBPH)2FeBr4∙Br. Cryst. Growth Des. 10 (2010) 158-164.

cr

[38]K. D. Gosser. Cyclic voltammetry: simulation and analysis of reaction

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mechanism. VCH Publishers, Inc. New York, 1994

[39]D. Shoup, A. Szabo. Chronoamperometric current at finite disk electrodes. J.

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Electroanal. Chem. 140 (1982) 237-245.

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Liquids. Reduction of 4-Cyanopyridine. J. Phys. Chem. 92 (1988) 5306-5313. [41] C. J. Dymek, L.A. King. Transport numbers in molten aluminum

ce pt

(1985) 1375-1380

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chloride-1-methyl-3-ethylimidazolium chloride mixtures. J. Electrochem. Soc. 132

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[43]W. Xu, E. I. Cooper, C. A. Angell. Ionic liquids: ion mobilities, glass

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temperatures, and fragilities. J. Phys. Chem. B. 107 (2003) 6170-6178. [44]K. R. Seddon, A. Stark, M. J. Torres. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl. Chem. 72 (2000) 2275-2287. [45]K. N. Marsh, J. A. Boxall, R. Lichtenthaler. Room temperature ionic liquids and their mixtures- a review. Fluid Phase Equilib. 219 (2004) 93-98. - 31 -

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ip t

ionic liquids in mixtures. J. Mol. Liq. 123 (2006) 43-50. [48]J. J. Wang, Y. Tian, Y. Zhao, K. L. Zhuo. A volumetric and viscosity study for

cr

the mixtures of 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid with

us

acetonitrile, dichloromethane, 2-butanone and N, N – dimethylformamide. Green Chem. 5 (2003) 618-622.

Lewandowski, A. Olejniczak, M. Galinski, I. Stepniak. Performance of

an

[49]A.

carbon–carbon supercapacitors based on organic, aqueous and ionic liquid electrolytes.

ce pt

ed

M

J. Power Sources. 195 (2010) 5814-5819

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 32 -

Page 32 of 57

Tables and Figures Captions

574

Table 1 Voltammetric data (vs. Ag/AgCl) for Fe(III)/Fe(II) in the mixtures of

575

iron-based ionic liquids and BmimCl at 303 K. (Scan rate: 50 mV∙s-1. Scan region:

576

-0.5 V - +0.9 V.)

577

Table 2 Apparent diffusion coefficients (D’) for FeCl4- and Fe3Cl7-, viscosity (η) and

578

conductivity (σ) of Fe(III)-IL and Fe(III/II)-IL at different temperatures (T)

579

Table 3 Voltammetric data (vs. Ag/AgCl) for Fe(III)/Fe(II) in the mixtures of

580

iron-based ionic liquids, co-solvents and BmimCl at 303 K. (Scan rate: 50 mV∙s-1.

581

Scan region: -0.5 V - +0.9 V.)

582

Table 4 Apparent diffusion coefficients (D’) for FeCl4- and Fe3Cl7-, viscosity (η),

583

conductivity (σ) and molar conductivity (Λ) of Fe(III/II)-IL with different mass

584

fraction of co-solvents at 298 K

585

Fig. 1 UV-vis absorption spectra of iron-based ionic liquids diluted by anhydrous

586

methanol with different volume ratio: Fe(III/II)-IL to methyl alcohol 1:100 (a1), 1:4

587

(a2); Fe(III)-IL to methyl alcohol 1:100 (b1), 1:4 (b2).

588

Fig. 2 ESI-MS spectra of (a) cations and (b) anions of Fe(III/II)-IL

589

Fig. 3 Cyclic voltammograms of (a) Fe(III)-IL (b) Fe(III/II)-IL on a 2 mm diameter

590

platinum electrode at 303 K. (Scan rate: 50 mV∙s-1. Dashed horizontal lines show zero

591

currents.)

592

Fig. 4 Nyquist plots of (a) Fe(III)-IL and (b) Fe(III/II)-IL on a 10 μm diameter

ce pt

ed

M

an

us

cr

ip t

573

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 33 -

Page 33 of 57

platinum microelectrode at 303 K.

594

Fig. 5 Cyclic voltammograms of (a) BmimCl (b) Fe(III)-IL (c) Fe(III/II)-IL on a 10

595

μm diameter platinum microelectrode at 303 K. (Scan rate: 50 mV∙s-1. Dashed

596

horizontal lines show zero currents.)

597

Fig. 6 Cyclic voltammograms of (a) Fe(III)-IL and (b) Fe(III/II)-IL on a 10 μm

598

diameter platinum microelectrode at 303 K. (Scan rate: 10, 30, 50, 70, 90 mV∙s-1)

599

Fig. 7 Chronoamperometric curves of Fe(III) in Fe(III)-IL at 293 K, 298 K, 303 K,

600

308 K, 313 K, 318 K, 323 K, 328 K, and 333 K. (The potential was stepped from +0.3

601

V to -1.5 V.)

602

Fig. 8 Chronoamperometric current as a function of t-0.5 for the redox of Fe(III) and

603

Fe(II) in Fe(III/II)-IL. (Potential was stepped from +0.3 V to -1.5 V (Fe(III)) and from

604

+0.3 V to +2 V (Fe(II)) respectively on a 10 μm diameter platinum microelectrode at

605

298 K)

606

Fig. 9 Arrhenius plots of log D’ vs. T-1 for ■, FeCl4- in Fe(III)-IL; ●, FeCl4- and ▲,

607

Fe3Cl7- in Fe(III/II)-IL

608

Fig. 10 Arrhenius plots of (a) log (η-1) and (b) log σ vs. T-1 for ●, Fe(III)-IL; ■,

609

Fe(III/II)-IL

610

Fig. 11 The relation between T and η fitted by VFT equation for ●, Fe(III)-IL; ■,

611

Fe(III/II)-IL

612

Fig. 12 Cyclic voltammograms of (a) Fe(III)-IL (b) Fe(III)-IL + acetonitrile (c)

ce pt

ed

M

an

us

cr

ip t

593

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 34 -

Page 34 of 57

Fe(III)-IL + 1,2-dichloroethane on a 10 μm diameter platinum microelectrode at 303

614

K. (Scan rate: 50 mV∙s-1 . Dashed horizontal lines show zero currents.)

615

Fig. 13 Cyclic voltammograms of (a) Fe(III/II)-IL (b) Fe(III/II)-IL + acetonitrile (c)

616

Fe(III/II)-IL + 1,2-dichloroethane on a 10 μm diameter platinum microelectrode at

617

303 K. (Scan rate: 50 mV∙s-1 . Dashed horizontal lines show zero currents.)

618

Fig. 14 Conductivity (σ) of Fe(III/II)-IL as function of mass fraction of ■, acetonitrile

619

and ●, 1,2-dichloroethane at 298 K.

ce pt

ed

M

an

us

cr

ip t

613

Ac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

- 35 -

Page 35 of 57

Table(s)

Tables Captions

Table 1 Voltammetric data (vs. Ag/AgCl) for Fe(III)/Fe(II) in the mixtures of

ip t

iron-based ionic liquids and BmimCl at 303 K. (Scan rate: 50 mV∙s-1. Scan region:

cr

-0.5 V - +0.9 V.)

us

Table 2 Apparent diffusion coefficients (D’) for FeCl4- and Fe3Cl7-, viscosity (η) and

an

conductivity (σ) of Fe(III)-IL and Fe(III/II)-IL at different temperatures (T)

M

Table 3 Voltammetric data (vs. Ag/AgCl) for Fe(III)/Fe(II) in the mixtures of iron-based ionic liquids, co-solvents and BmimCl at 303 K. (Scan rate: 50 mV∙s-1.

ed

Scan region: -0.5 V - +0.9 V)

ce pt

Table 4 Apparent diffusion coefficients (D’) for FeCl4- and Fe3Cl7-, viscosity (η), conductivity (σ) and molar conductivity (Λ) of Fe(III/II)-IL with different mass

Ac

fraction of co-solvents at 298 K

Page 36 of 57

Epa / V

Ipa / nA

Epc / V

Ipc / nA

ΔEp / V

|Ipa / Ipc|

4

0.345

-1.000

0.237

0.967

0.108

1.034

80

0.303

-44.069

0.204

34.491

0.099

1.278

Fe(III)-IL +

85

0.300

-48.110

0.206

38.163

0.094

1.261

BmimCl

90

0.303

-50.999

0.210

42.874

0.093

1.190

95

0.301

-55.976

0.213

49.363

0.088

1.134

100

0.295

-68.140

0.202

61.918

0.093

1.100

80

0.350

-41.778

0.225

29.810

0.125

1.401

85

0.356

-45.318

0.233

33.979

0.123

1.334

90

0.359

-51.970

0.247

40.452

0.112

1.285

95

0.353

-55.957

0.244

44.084

0.109

1.269

100

0.338

-66.001

0.232

57.856

0.106

1.141

M

+ BmimCl

an

Fe(III/II)-IL

us

Solutions

ip t

w% a

cr

Table 1

Ac

ce pt

ed

a: w% = mFe-IL / (mFe-IL + mBmimCl), where m = mass of component in system and Fe-IL is Fe(III)-IL or Fe(III/II)-IL.

Page 37 of 57

Table 2

DFeCl 4

4

7

3

× 108

7

η/ mPa∙s

σ/ -1

mS∙cm-1

/ cm2∙s-1

293

3.30 (±0.10)

-

31.36

6.00

298

4.03 (±0.04)

-

24.87

7.20

303

5.08 (±0.04)

-

21.40

8.00

308

6.38 (±0.04)

-

313

7.75 (±0.10)

-

318

9.64 (±0.03)

-

323

11.64 (±0.03)

-

328

14.10 (±0.03)

-

333

16.92 (±0.03)

cr

9.40

16.16

10.80

us

18.49

-

14.66

11.90

13.19

13.20

11.47

14.70

10.26

15.70

21.7 ±

19.6 ±

0.7

0.6

293

3.08 (±0.12)

10.73 (±0.54)

123.80

2.70

298

3.49 (±0.17)

13.91 (±0.45)

92.60

3.00

4.97 (±0.18)

19.02 (±0.33)

68.00

3.40

308

ed

kJ·mol-1

33.5±0.2

ip t

/cm2∙s-1

Ea /

5.23 (±0.24)

19.88 (±0.93)

57.31

4.45

313

6.78 (±0.06)

24.61 (±0.16)

46.13

5.05

318

8.83 (±0.23)

29.67 (±0.69)

37.62

6.30

323

11.16 (±0.11)

33.98 (±0.46)

30.70

7.35

328

12.87 (±0.18)

43.46 (±0.47)

25.33

8.45

15.13 (±0.20)

48.82 (±0.52)

21.41

9.70

33.6±1.3

29.9±1.1

35.0 ±

27.4 ±

0.7

0.9

ce pt

303

Ac

Fe(III/II)-IL

3

1－3t Fe Cl -

an

Fe(III)-IL

T/K

DFe Cl -

M

ionic liquid

1 + t FeCl -

× 108

333 Ea /

kJ·mol-1

Page 38 of 57

w% b

Epa / V

Ipa / nA

Epc / V

Ipc / nA

80

0.281

-122.142

0.184

122.055

0.097

1.001

Fe(III)-IL /

85

0.260

-142.306

0.150

148.249

0.110

0.960

acetonitrile +

90

0.274

-180.797

0.110

190.373

0.164

0.950

BmimCl

95

0.289

-219.937

0.104

226.760

0.185

0.970

100

0.312

-292.615

0.067

298.647

ip t

Table 3

0.245

0.980

80

0.322

-70.841

0.211

53.729

0.111

1.318

Fe(III)-IL /

85

0.318

-78.424

0.207

63.068

0.111

1.243

1,2-dichloroethane

90

0.319

-82.790

0.214

69.992

0.105

1.183

+BmimCl

95

0.287

-88.667

0.182

81.674

0.105

1.086

100

0.214

-87.846

0.110

90.365

0.104

0.972

80

0.293

-111.884

0.164

112.939

0.129

0.991

Fe(III/II)-IL /

85

0.306

-142.325

0.152

146.686

0.154

0.970

acetonitrile +

90

0.342

-177.920

0.152

182.512

0.190

0.975

BmimCl

95

0.378

-223.181

0.146

224.860

0.232

0.993

0.359

-301.597

0.104

298.857

0.255

1.009

80

0.311

-46.014

0.210

39.308

0.101

1.171

Fe(III/II)-IL /

85

0.310

-51.110

0.207

45.757

0.103

1.117

1,2-dichloroethane

90

0.320

-61.912

0.217

59.573

0.103

1.039

+BmimCl

95

0.317

-70.756

0.199

70.656

0.118

1.001

100

0.315

-89.868

0.183

94.852

0.132

0.947

Ac

ce pt

100

cr

ΔEp / V |Ipa / Ipc|

us

an

M

ed

Solutions a

a: mFe-IL / mco-solvent = 7:3, where m = mass of component in system and Fe-IL is Fe(III)-IL or Fe(III/II)-IL. b: w% = (mFe-IL + mco-solvent) / mtotal, where mtotal = mFe-IL + mBmimCl + mco-solvent

Page 39 of 57

Table 4 4

w% a

3

-

1  3t Fe Cl 3

10

7

8

C / mol·dm-3

ηb/

Λ/

σ/

7

-1

mS∙cm2·

Fe3Cl7-

mPa∙s

13.91 (±0.45)

2.43

0.29

68.00

3.00

1102.94

3.92 (±0.05)

14.30 (±0.12)

2.23

0.26

46.00

4.30

1726.91

30

9.72 (±0.16)

21.90 (±0.28)

1.74

0.20

11.32

8.17

4211.34

50

23.06 (±0.11)

38.73 (±0.62)

1.23

0.15

4.85

10.10

7318.84

70

37.81 (±0.42)

56.06 (±1.16)

0.73

0.09

90

60.96 (±1.62)

79.19 (±3.16)

0.24

0.03

0

3.49 (±0.17)

13.91 (±0.45)

2.43

10

9.53 (±0.15)

34.81 (±0.14)

2.12

30

53.26 (±0.19)

127.86 (±0.54)

50

122.38 (±0.13)

208.67 (±0.33)

70

180.29 (±0.31)

303.45 (±1.18)

90

285.55 (±0.48)

503.76 (±3.86)

10

ip t

3.49 (±0.17)

cr

0

mol-1

2.20

8.60

10487.80

<1

3.20

11851.85

us

/ cm2∙s-1

0.29

68.00

3.00

1102.94

0.25

8.15

10.50

4430.38

1.47

0.18

2.36

30.00

18181.82

0.94

0.11

1.25

40.00

38095.24

0.51

0.06

<1

34.80

61052.63

0.16

0.02

<1

17.50

97222.22

an

/ cm2∙s-1

mS∙cm

-1

ce pt

ed

a: w% = mco-solvent / (mFe(III/II)-IL + mco-solvent) where m = mass of component in system. b: Temperature is 303 K

Ac

1,2-dichloroethane

4

DFe Cl

FeCl4-

solvents

acetonitrile

1 + t FeCl -

× 10

8

M

co-

DFeCl -

Page 40 of 57

Figure(s)

Figures Captions

Figure 1 UV-vis absorption spectra of iron-based ionic liquids diluted by anhydrous

ip t

methanol with different volume ratio: Fe(III/II)-IL to methyl alcohol 1:100 (a1), 1:4

cr

(a2); Fe(III)-IL to methyl alcohol 1:100 (b1), 1:4 (b2).

us

Figure 2 ESI-MS spectra of (a) cations and (b) anions of Fe(III/II)-IL

an

Figure 3 Cyclic voltammograms of (a) Fe(III)-IL (b) Fe(III/II)-IL on a 2 mm diameter

M

platinum electrode at 303 K. (Scan rate: 50 mV∙s-1. Dashed horizontal lines show zero

ed

currents.)

Figure 4 Nyquist plots of (a) Fe(III)-IL and (b) Fe(III/II)-IL on a 10 μm diameter

ce pt

platinum microelectrode at 303 K.

Ac

Figure 5 Cyclic voltammograms of (a) BmimCl (b) Fe(III)-IL (c) Fe(III/II)-IL on a 10 μm diameter platinum microelectrode at 303 K. (Scan rate: 50 mV∙s-1. Dashed horizontal lines show zero currents.)

Figure 6 Cyclic voltammograms of (a) Fe(III)-IL and (b) Fe(III/II)-IL on a 10 μm diameter platinum microelectrode at 303 K. (Scan rate: 10, 30, 50, 70, 90 mV∙s-1)

Page 41 of 57

Figure 7 Chronoamperometric curves of Fe(III) in Fe(III)-IL at 293 K, 298 K, 303 K, 308 K, 313 K, 318 K, 323 K, 328 K, and 333 K. (The potential was stepped from +0.3

ip t

V to -1.5 V.)

Figure 8 Chronoamperometric current as a function of t-0.5 for the redox of Fe(III) and

cr

Fe(II) in Fe(III/II)-IL. (Potential was stepped from +0.3 V to -1.5 V (Fe(III)) and from

us

+0.3 V to +2 V (Fe(II)) respectively on a 10 μm diameter platinum microelectrode at

an

298 K)

M

Figure 9 Arrhenius plots of log D’ vs. T-1 for ■, FeCl4- in Fe(III)-IL; ●, FeCl4- and ▲,

ed

Fe3Cl7- in Fe(III/II)-IL

Fe(III/II)-IL

ce pt

Figure 10 Arrhenius plots of (a) log (η-1) and (b) log σ vs. T-1 for ●, Fe(III)-IL; ■,

Ac

Figure 11 The relation between T and η fitted by VFT equation for ●, Fe(III)-IL; ■, Fe(III/II)-IL

Figure 12 Cyclic voltammograms of (a) Fe(III)-IL (b) Fe(III)-IL + acetonitrile (c) Fe(III)-IL + 1,2-dichloroethane on a 10 μm diameter platinum microelectrode at 303 K. (Scan rate: 50 mV∙s-1. Dashed horizontal lines show zero currents.)

Page 42 of 57

Figure 13 Cyclic voltammograms of (a) Fe(III/II)-IL (b) Fe(III/II)-IL + acetonitrile (c) Fe(III/II)-IL + 1,2-dichloroethane on a 10 μm diameter platinum microelectrode at

ip t

303 K. (Scan rate: 50 mV∙s-1. Dashed horizontal lines show zero currents.)

Figure 14 Conductivity (σ) of Fe(III/II)-IL as function of mass fraction of ■,

Ac

ce pt

ed

M

an

us

cr

acetonitrile and ●, 1,2-dichloroethane at 298 K

Page 43 of 57

4

2.0

Absorbance

Fe(II/III)-based ionic liquid

2 580

590

600

610

620

630

Wavelength /nm

(a1)

(a2)

(b1)

(b2)

500

600

us

400

650

700

800

0.0

900

an

300

1.0

0.5

1

0 200

640

ip t

Fe(III)-based ionic liquid

Absorbance

1.5

cr

Absorbance

3

M

Wavelength / nm

Ac

ce pt

ed

Figure 1

Page 44 of 57

an

us

cr

ip t

(a)

Ac

ce pt

ed

M

(b)

Figure 2

Page 45 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 3

Page 46 of 57

an

us

cr

ip t

(a)

Ac

ce pt

ed

M

(b)

Figure 4

Page 47 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 5

Page 48 of 57

an

us

cr

ip t

(a)

Ac

ce pt

ed

M

(b)

Figure 6

Page 49 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 7

Page 50 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 8

Page 51 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 9

Page 52 of 57

an

us

cr

ip t

(a)

Ac

ce pt

ed

M

(b)

Figure 10

Page 53 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 11

Page 54 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 12

Page 55 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 13

Page 56 of 57

ip t cr us an

Ac

ce pt

ed

M

Figure 14

Page 57 of 57