Anomalous Voltammetric Behavior Observed for Electrodeposition of

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Anomalous Voltammetric Behavior Observed for Electrodeposition of Indium in the 1-Butyl-1-Methylpyrrolidinium Dicyanamide Ionic Liquid; A Result of the Ionic Liquid Cation Adsorption Yi-Chen Liu, Yu-Chen Chen, Yi-Ting Hsieh, and I-Wen Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01375 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017

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The Journal of Physical Chemistry

Anomalous Voltammetric Behavior Observed for Electrodeposition of Indium in the 1-Butyl-1-Methylpyrrolidinium Dicyanamide Ionic Liquid; A Result of the Ionic Liquid Cation Adsorption Yi-Chen Liu1, Yu-Chen Chen1, Yi-Ting Hsieh2,*and I-Wen Sun1,*

1

Department of Chemistry, National Cheng Kung University, Tainan, Taiwan 70101

2

Department of Chemistry, Soochow University, Taipei, Taiwan 11102

ABSTRACT.

Ionic liquids (ILs) are widely used for electrochemical studies. However, electrochemical reactions taking place in ILs could be complicated by the interfacial IL ion layers at the electrode/IL interface. In this work, such complications are revealed via the electrochemical study of indium(III)/indium redox couple in the 1-butyl-1- methylpyrrolidinium dicyanamide ([BMP]+[DCA]-) IL. Anomalous voltammetric behavior is observed with static cyclic voltammetry, convective rotating 1 ACS Paragon Plus Environment


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disk electrode voltammetry, and potential step chronoamperometry. The results indicate that the In(III) reduction process is complicated by the adsorption/desorption of the [BMP]+ cations. Meanwhile, less anomalous voltammetric behavior is observed in the 1-ethyl-3-methylimidazolium dicyanamide ([EMim]+[DCA]-) IL, in agreement with the weaker adsorption strength of the [EMim]+ cation. The impact of the cation adsorption is weakened by raising temperature.

INTRODUCTION

Ionic liquids (ILs) are a class of molten salts that exhibit low melting points (< 100oC).1 The ILs normally consist of bulky and asymmetric cations (such as di-alkyl imidazolium and di-alkyl pyrrolidinium) and/or anions (such as Al2Cl7-, BF4-, PF6-, bis(trifluoromethylsulfonyl)imide ([TFSI]-), and trifluoromethylsulfonate ([TfO]-) that reduce the electrostatic force between ions and destabilize the crystalline lattice of the salts, resulting in the low melting point. Generally, ILs possess good intrinsic ionic conductivity, nonvolatility, high thermal stability, and wide electrochemical window. Moreover, the physical properties (such as density, viscosity, polarity, and Lewis acidity) of the ILs can be tuned by selection of different combinations of cations and anions. These features led to vast interests in using ILs as promising media replacing

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traditional molecular volatile aqueous and organic solvents for various applications such as chemical synthesis,1, 2 chemical analysis,3 chemical separation,4 catalysis,5 and electrochemistry.6-8 The performance of these applications, however, can be affected strongly by the electrode/IL interface structure under applied potential.9 The electrode/IL interface is different from the molecular electrolytes by composing of an innermost interfacial layer zone which then decay through a transition zone to the bulk liquid zone.10,

11

Theoretical12-16 and experimental17-27 studies have shown a

multilayer structure of the cations and anions at the electrode/IL interface, and the arrangement as well as the confirmation of the ion layers can be altered by the applied potential. Furthermore, the electrode/IL interface structure is dependent on the cation and anion type. For example, STM/AFM measurements23-26 indicated greater strength of the [BMP]+ compared to [EMIm]+ with the electrode, resulting in larger number of layers in the ILs containing the former. Meanwhile, the interfacial structure increased from about one layer to approximately five layers when the anions [BF4]- and [PF6]were replaced by [DCA]-.18 The above literatures suggest that the interfacial IL ion layers get more compact at highly charged electrode surface, resulting in pronounced electrostatic screening (or an energetic barrier) which blocks the solute ions from approaching the electrode surface, and, therefore, impedes the charge transfer reaction rate. The effects of such potential-dependent double layer reconstruction on the 3 ACS Paragon Plus Environment


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electrochemical deposition of metals has been recognized in the Zn(II)/Zn27-33 and Co(II)/Co34 systems. In these systems, cyclic voltammograms (CVs) displayed unusual behavior including a cathodic current split in the forward cathodic potential scan (i.e. the initial cathodic peak is suppressed as the potential is scanned negatively, but resumes at a more negative potential), and the occurrence of a cathodic peak during the backward anodic scan. The overall appearance of the CV depends on the cations and anions of the ILs, the metal ions, and the electrode materials. Because the solvation power of the IL anions dominates the speciation, the reduction potential of the complexed metal ions is dominated by the IL anions. On the other hand, the cations dominate the potential range where the blocking effect is substantial to suppress the reduction reaction. Several studies on the electrochemistry of indium in various ILs have been reported,35-38 but the effects of the interfacial IL ion layers on the electrochemical behavior of In(III) were neither observed nor discussed in these literatures. In view of the fact that voltammetric studies related to the impact of potential-dependent double layer reconstruction on the electrochemical deposition of metals has only been briefly described27-34, we study in detail the redox behavior of In(III) in the [BMP]+[DCA]- IL using InCl3 as the In(III) source in order to further explore the influence of the potential-dependent double layer reconstruction on the 4 ACS Paragon Plus Environment

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electrodeposition of metals. The [BMP]+[DCA]- IL is chosen because [BMP]+ cation has strong interaction with the electrode to give more compact interfacial IL layers,23-26 whereas [DCA]- is a stronger ligand than [TFSI]- to complex with In(III).39,40 Indium(III) is chosen for its intrinsically relatively negative reduction potential. In this way, the reduction potential of the In(III) species is expected to overlap with the potential range that adsorbed compact cation layers can form. Therefore, the effects of the cation layers can be clearly realized.

EXPERIMENTAL SECTION [BMP]+[DCA]- RTIL was prepared and purified by following a previously reported procedure.41 A slight excess of sodium dicyanamide (NaDCA, 96%, Fluka) was mixed with solid 1-butyl-1-methylpyrrolidinium chloride ([BMP]+Cl-) into a beaker and gently heated (70 °C) and stirred for approximately one day. After cooling, the solution was sent out from the glove box and dichloromethane was added to the solution. The solid NaCl precipitate was filtered off, and the solvent was removed via rotary evaporation. [BMP]+[DCA]- RTIL was then dried under vacuum for 12 h at 100 °C. After cooling, the RTIL was stored in a glove box for further study. Electrochemical measurements were carried out in a standard three-electrode cell using an Autolab 302 N potentiostat/galvanostat controlled with GPES software. All 5 ACS Paragon Plus Environment


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deposition experiments were carried out at 35 °C under a purified nitrogen atmosphere in a glove box (Vacuum Atmospheres Co.). A Pt wire (99.9%, Aldrich) placed in a separate fritted glass tube containing pure [BMP]+[TFSI]- IL was used as the counter electrode. The reference electrode was an In wire placed in a fritted glass tube containing 25 mM InCl3 (99.99%, Alfa Aesar) in [BMP]+[DCA]- RTIL. For static voltammetry and chronoamperometry experiments, GC electrode (area = 0.0707 cm2) was used as the working electrode. All substrates were cleaned with acetone, 2 M HNO3, and deionized water, and then dried in a vacuum before use. Indium were electrodeposited on Ni substrate and examined using environmental scanning electron microscopy (ESEM, FEI Quanta 400 F, Phillips), and high-resolution scanning electron microscopy (HRSEM, SU 8000, HITACHI). The crystal structure of the indium deposits was analyzed using powder X-ray diffractometer (XRD, XRD-7000, Shimadzu).

RESULTS AND DISCUSSION Indium chloride, InCl3, dissolves in the [BMP]+[DCA]- via coordinating with [DCA]- anions to form soluble complex anions. From previous literatures reported on metal ions in various ILs, 28, 30, 34, 42 it can be concluded that the [InCl3(DCA)2]2complex anion is the predominant In(III) species in this solution. The Nernst plot 6 ACS Paragon Plus Environment

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constructed by measuring the equilibrium potential, Eeq, as a function of the [In(III)]/[In(0)] concentration ratio was linear with a slope of 0.02 V, which is in good agreement with the theoretical value expected for a three-electron redox couple at this temperature.43 Furthermore, in accordance with the literature,44 the linear Nernst plot indicates that the In(III) formed an ideal solution in this IL. In this article, the [InCl3(DCA)2]2- is expressed as In(III) for short. The voltammetric behavior of the In(III) complex anion was first studied at an In wire electrode in the [BMP]+[DCA]- IL containing 50 mM InCl3 at 35°C. A typical cyclic voltammogram scanned initially toward negative direction from the open-circuit potential (OCP) is presented in Fig. 1(i). As can be seen, a cathodic peak, c1, appears at about -0.25 V. After this peak, the reduction current decreases first, but rises again as the potential reaches -1.2 V to give a second cathodic peak, c2. When reverse the potential scan towards positive direction, the current first declines but then increases again, giving a cathodic peak, c1’, at a potential close to peak c1. The potential scan was stopped at 0 V to avoid the anodization of the indium electrode. Indium metal deposits were obtained when constant potential electrolysis experiments were performed at both c1 and c2, indicating that both peaks are attributed to the reduction of In(III) complex anions to indium metal. The reduction peak c1 is in consistent with the expected low overpotential required for the nucleation and growth 7 ACS Paragon Plus Environment


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of indium on the indium substrate. On the other hand, the occurrence of reduction peak c2 is unusual because it is at a potential far negative than the formal potential of the In(III)/In(0) redox couple, indicating an unexpected large overpotential is associated with the deposition of indium at this peak. Another unusual phenomenon is the appearing of reduction peak c1’ during the anodic scan because the cathodic current should normally continuously declines with the diffusion layer thickening until the potential, where oxidation would occur, is reached. The unusual In(III)/In(0) redox behavior is further studied on a GC disc electrode as shown in Fig. 1(ii). In contrast to the CV recorded on the indium electrode, the reduction peak c1 does not appear on the GC, and the main reduction of In(III) to In(0) only occurs at c2 when the potential is scanned cathodically from the OCP. However, a reduction peak c1’ due to the deposition of indium appears at about -0.25V when the potential scan was reversed from -1.3V toward 1.5 V. In addition, two anodic stripping peaks, a1 and a2, are observed at 0.1V and 1.2V. While the anodic peak a1 can be attributed to the stripping of the deposited In(0), the peak a2 is unclear, but according the literature45 it could be the generation of Cl2 which cleans the GC surface. The exact identity of the anodic peaks needs be further studied. The absence of c1 on the GC is in consistent with the fact that an overpotential is usually required for the deposition of a metal (indium in here) on a foreign substrate (GC in here) so that the deposition of indium 8 ACS Paragon Plus Environment

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occurs at peak c2 where a substantial over-potential has been applied. The interesting feature is, however, the current declines substantially when the potential scan is reversed until the occurrence of the cathodic peak c1’. The unusual CV behavior was also studied with multi-scan CVs, and the results are given in the supporting information S1. Referring to the literature,27-34 the above described anomalous CV processes, including the current split (co-existence of peak c1 and c2 during the cathodic potential scan) and the occurrence of peak c1’ during the anodic potential scan, can be rationalized by the unique double layer structure at the electrode/IL interface. As has been demonstrated in the literature, the electrode/IL interface is constructed with multiple ion layers at the OCP. Upon applied with a negative electrode potential, the electrode/IL interface becomes cation-rich, and the number of adsorbed interfacial cations as well as their interaction force with the electrode surface increases as the electrode potential is made more negative. These adsorbed cation ions separate the anionic species from coming into direct contact with the electrode, and therefore create an extra barrier for the electron transfer to the In(III) complex anions. Previous reports indicated that [BMP]+ cation is especially easy to adsorb on the electrode23-26, and can complicate the electrochemical behavior of species at the electrode/IL interface. In the present case, it is believed that the adsorption of [BMP]+ cation layers 9 ACS Paragon Plus Environment


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creates a substantial energy barrier (charge transfer resistance) for the reduction of the In(III) complex anions on the GC during the initial cathodic potential scan so that meaningful In(III) reduction does not take place until the potential is scanned to about -1.0 V where the current resumes again. This can be understood as the quantum tunneling length of the electron is proportional to the overpotential. At low overpotential the electron tunneling length is so short that it cannot reach the In(III) ions that are kept away from the electrode by the adsorbed [BMP]+ cations. As the overpotential increases the electron tunneling strength increases, and eventually the In(III) complex ions can be reduced via the electron tunneling despite the blocking cation layer, and gives the c2 peak. When the potential scan is reversed toward positive direction, the applied overpotential diminishes so that the reduction of In(III) is again suppressed by the adsorbed [BMP]+ cations, and the In(III) complex anions can only accumulate in the electrode surface vicinity. However, the number of adsorbed [BMP]+ cations is also getting less as the potential is made more positive, and essentially the energy barrier resulted from the adsorbed [BMP]+ layers turns out to be so low that the In(III) complex anions that have accumulated near the electrode surface vicinity can be reduced to give peak c1’ on GC electrode because the GC electrode has been deposited with a layer of indium at c2 during the cathodic scan, and a substantial less overpotential is required for the reduction of In(III) complex anions 10 ACS Paragon Plus Environment

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to take place on the deposited indium film. To have additional understanding on the reduction peak c1’, CVs with the potential scan reversed at various switching potentials, Es, were recorded on the GC electrode and displayed in Fig. 2. Interestingly, peak c1’ appears when Es is set at -0.5V, even though there is no appreciable reduction current observed during the cathodic scan. The peak current of c1’ increased as Es is extended to -0.6 V. It is noted that the c1’ peak exhibits a sharp symmetric shape typical for that of a surface confined electrode reaction. More careful examination of the CV reveals that, although not obvious, a small reduction current does occur during the cathodic potential scan prior to the bulk deposition of indium at peak c2, indicating that regardless the hindrance from the adsorbed [BMP]+ cations, a small number of indium nuclei do form slowly during the cathodic scan. Upon reverse the potential scan, these indium nuclei catalyze the reduction of the In(III) complex anions to give the sharp c1’. Because it takes a longer time for the potential scan to reach -0.6 V than to -0.5 V, more indium nuclei and In(III) complex anions are accumulated near the electrode vicinity. Therefore, higher c1’ peak current is associated with Es = -0.6V with respect to that with Es = -0.5V. Peak c1’ was further studied by recording the CVs at various scan rates in the range of -0.6 to 1.5 V on the GC electrode as shown in the inset of Fig. 2. This inset figure shows that the peak current of c1’ decreases as the scan rate is increased from 10 to 70 mV·s-1 because 11 ACS Paragon Plus Environment


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faster scan rate gives less time for the indium nuclei to form. Moreover, Fig. 2 shows that a shoulder gradually evolves on peak c1’ when Es is further extended to more negative potentials where peak c2 occurs. The charges, Qc1’ and Qc2 that were obtained by integrating the current under peaks c1’ and c2, respectively, are collected in Table 1. This table reveals that Qc1’ increases as Es shifts from -0.5V to -1.1V but decreases as Es shifts to potentials more negative then peak c2. Meanwhile, Qc2 continues to increase with negative shifting of Es. Such behavior can be attributed to the facts that during the cathodic scan to peak c2, the In(III) complex anions within the diffusion layer are consumed, and the [DCA]- and Cl- anions are released from the In(III) complex anions; the consumption of In(III) complex anions at peak c2 makes less In(III) anions available for reduction at c1’, whereas the released [DCA]- and Clanions change the local environment at the electrode surface, shifting the peak c1’ to more negative potential during the reversed potential scan. It is noted that ratio of the stripping peak to reduction peak is good at low overpotentials, but as the potential becomes more negative the stripping peak does not increase significantly, indicating that the In(0) was formed during the cathodic scan but lost from the electrode surface likely as nanoparticles. This fact implies that some of the reduction does not occur at the electrode surface but in the solution via the electron tunneling.


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To further verify that the peculiar reduction behavior observed in the CV of In(III) is resulted from the adsorption of [BMP]+ cations, cyclic voltammograms of the InCl3 in [BMP]+[DCA]- was recorded under a convective condition using a glassy carbon rotating disc electrode (GCRDE). A typical GCRDE CV recorded with a rotating rate of 1200 rpm at a scan rate of 5 mV/cm is presented in Fig. 3(i). This figure shows that the reduction of In(III) complex anions is suppressed by the adsorption of [BMP]+ cations. Nonetheless, when the potential is scanned to -0.75 V where the applied overpotential is sufficiently large to compensate the resistance created by the adsorbed [BMP]+ cations, the reduction current appears and increases to approach the limiting current as the potential is scanned to -1.4 V. When the potential scan is reversed towards positive direction, the current drops to a minimum value at about -0.6 V because the lowered overpotential cannot overcome the resistance created by the adsorbed [BMP]+ layers. However, as the adsorption of [BMP]+ cations is weakened when the potential is scanned positively, the reduction of In(III) complex anions on the previous deposited indium becomes possible again so that the current raises up again to give a peak at about -0.2 V, and then crosses to the anodic side at about 0.03 V. If the electrochemical reduction was not suppressed by the adsorption of the [BMP]+ cations, the reduction current would not have decayed but remained at the limiting value when the potential scan is reversed to the positive direction, and would 13 ACS Paragon Plus Environment


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follow the dashed trace line depicted in Fig. 3(i). For comparison, a GCRDE CV was also recorded for a solution of [EMI]+[DCA]- containing InCl3 and displayed in Fig. 3(ii). In this IL, the In(III) should also present as the [InCl3(DCA)2]2- complex anions. As can be seen that in accordance with the fact that [EMI]+ cations do not adsorb on the electrode as strongly as [BMP]+ cations,23-26 a lower energy barrier for the reduction of In(III) is formed, and the reduction of In(III) complex anions in the [EMI]+[DCA]- IL does not require an overpotential as large as in the [BMP]+[DCA]so that reduction peak c1 is observed during the cathodic potential scan. Nevertheless, as the cathodic potential scan is scanned further negatively, current split (i.e. the appearing of peak c2) is still observed, and the cathodic peak c1’ also appears during the anodic potential scan. Note that the cathodic scan in the voltammogram shown in Fig. 3(ii) is limited at -1.0 V to avoid the reduction of [EMI]+ cations. It is noted that the amount of cathodic charge is much more than the anodic charge, again indicates that In(0) was formed but lost as was observed in Fig. 2. It is possible that as the reduction of In(III) occurs, Cl- is released which might associate other InCl3 to form species, e.g. [InCl5]2- that have a different reduction potential, and could have altered the voltammetry slightly. However, this does not explain the hysteresis on the reverse scan, so double layer structuring is still the dominant feature.


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The impact of the adsorption of [BMP]+ cations on the electrodeposition behavior of In(III) was further studied using chronoamperometry on a GC electrode in the [BMP]+[DCA]- IL containing 50 mM InCl3. Typical current-time transients are shown in Figure 4. In each of these experiments, the potential was stepped from OCP to negative values and the current was recorded as a function of time. This figure shows that when the potential is stepped to -0.2V, which corresponding to the value of peak c1 observed in Fig. 1, the deposition of indium does not take place until a fairly long induction time of about 70s has passed. After the induction time, the current increases due to the nucleation and growth of the indium to reach a maximum value, thereafter the current decays as the diffusion layer growing thicker. The induction time is shortened along with an increased current maximum when the deposition overpotential is increased by stepping the potential to -0.3V. However, the reduction current starts to decrease when the potential is stepped to further negative values between -0.4 to -0.9V because the number of adsorbed [BMP]+ cations increases at more negative potentials, and the reduction of In(III) is accordingly suppressed more severely. Nevertheless, at potentials more negative than -0.9V, where the applied overpotential gradually becomes sufficiently large to overcome the resistance that is developed by the adsorbed [BMP]+ cations, the current maximum increases again with increasing overpotential, and the current-time transients resume to the normal 15 ACS Paragon Plus Environment


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Cottrell behavior at potentials more negative than -1.1V. The chronoamperometric results confirm that the deposition of indium is strongly affected by the adsorption of [BMP]+ cations. Taken together, the results of the above static cyclic voltammograms, convective RDE cyclic voltammograms and potential step chronoamperograms offer evidences showing that the electrochemical reduction of In(III) species in the [BMP]+[DCA]- IL is complicated by the potential-dependent adsorption of the [BMP]+ cations. While the adsorption of [BMP]+ cations significantly complicated the voltammetric behavior of In(III) in the [BMP]+[DCA]- IL at room temperature, the adsorption effect could be weakened by increasing the temperature. As demonstrated by the CVs shown in Fig. 5 that were taken on a GC electrode at various temperatures, the bulk deposition peak c2 shifted positively and peak c1’ diminished as the temperature increased from 35°C to 90°C. It is likely that increasing the temperature increases the kinetic energy of the cations and anions, rendering it more difficult to form a compact adlayer of [BMP]+ cations at the electrode surface. The diminishing of the cation ion blocking effect by increasing temperature has been reported in the deposition of Zn from a deep eutectic solvent.46 The diffusion coefficient and Stokes-Einstein product of the dissolved In(III) species in the [BMP]+[DCA]- IL solution are given in Table S1 of the supporting 16 ACS Paragon Plus Environment

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information. Typical SEM images of the indium electrodeposits on Ni substrate are given in Fig. S2 of the supporting information. The XRD patterns (Figure S3) recorded for the electrodeposits showing the presence of pure crystalline indium. Taken together the above data, we propose that such CV behavior can be understood with Scheme 1. As illustrated in this scheme, when the anionic metal complex ions are initially reduced at a less negative potential region I, where the cations are not strongly adsorbed, the current increases first and then decreases due to the thickening of diffusion layer, however, as the potential is further scanned negatively to region II, the number of IL cations forming the compact interfacial layers on the surface increases to a level that the charge transfer process is blocked so that the reduction current drops substantially, because the metal complex anions cannot be reduced but are accumulated near the electrode surface. Subsequently, at sufficiently negative potential, the applied overvoltage in region III turns out to be so large that the charge transfer resistance (or the barrier) created by the adsorbed cations is overwhelmed, making the adsorbed cations layer thinner, and thus, further reduction of the accumulated metal complex anions can take place through electron quantum tunneling to recommence the reduction peak (the so called cathodic current split34). On the other hand, when the potential scan is reversed positively to region II the reduction is again suppressed. But as the potential is scanned to region IV, the 17 ACS Paragon Plus Environment


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number of the adsorbed IL cation layers again decreases so that the blocking is released and the accumulated metal complex anions are reduced to give a cathodic peak during the backward anodic potential scan. CONCLUSION

In this work, the impact of the potential-dependent interfacial IL ion layers on electrochemical reaction was investigated with In(III)/In(0) couple in the [BMP]+[DCA]- RTILs using cyclic voltammetry, rotating disk electrode voltammetry, and potential step chronoamperometry. Several electrochemical parameters, such as formal potential, diffusion coefficient, and Stokes-Einstein product of the indium (III) species were determined. SEM images and XRD patterns indicate that polygonal indium crystals were obtained from the electrolyte. The voltammetric results show that the potential-dependant adsorption of the [BMP]+ cations on the electrode surface may produce an energy barrier that blocks the electron transfer process at the electrode surface, leading to unusual electrochemical behaviors including current split during cathodic potential scan and appearance of a reduction peak during anodic potential scan. The cation adsorption effect, however, can be weakened by increasing the temperature. This study indicates that, in addition to the AFM and STM techniques, voltammetry can be a complementary approach for studying the electrode/IL interface structure under applied potential. In view of the 18 ACS Paragon Plus Environment

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fact that ILs are employed as the electrolyte for many electrochemical applications such as electrodeposition, batteries and supercapacitors, the effects of the adsorption of various IL cations on the electrochemical behavior of various metal ions deserve more detailed investigation.

Corresponding Author *1 I-Wen Sun

Department of Chemistry, National Cheng Kung University, Tainan

701, Taiwan

Tel.:

+886-6-2757575

ext:

65355;

Fax

:

+886-6-2740552;

E-mail:

[email protected]

*2

Yi-Ting Hsieh Department of Chemistry, Soochow University, Taipei City 111,

Taiwan

Tel.:

+886-2-28819471

ext:

6805;

Fax

:

+886-2-28811053;E-mail:

[email protected]

ACKNOWLEDGMENT

This work was supported by the Ministry of Science and Technology, Taiwan. (Grant no. 105-2113-M-006 -013 -MY2, no. 105-2113-M-031 -003 -MY2,)



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Table 1. Summary of charge, Qc1’ and Qc2, integrated under deposition peaks c1’ and c2 of the cyclic voltammograms in which the potential scan is reversed at various switching potentials, Es, for 50 mM InCl3 in [BMP]+[DCA]- on GC electrodes at 35 °C and scan rate of 50 mV·s−1. Es (V)

-0.5

-0.6

-1.1

-1.2

-1.5

Qc2 (mC)

/

/

0.73

1.05

1.72

Qc1’ (mC)

0.14

0.39

0.56

0.49

0.42

SCHEME

Scheme 1. Draft of the electrode/IL interface structure transitions associated with a cyclic voltammetry cycle. 29 ACS Paragon Plus Environment


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Figure captions

Figure 1. Cyclic voltammograms of 50 mM InCl3 in [BMP]+[DCA]- recorded on (i) indium wire electrode, and (ii) GC electrode at 35 °C. Scan rate was 50 mV·s-1.

Figure 2. Cyclic voltammograms of 50 mM InCl3 in [BMP]+[DCA]- recorded on GC electrode at 35 °C. Potential scan was reversed at -1.5, -1.2, -1.1, -0.6, and -0.5 V. Scan rate was 50 mV·s-1. Inset: CVs between -0.5 V to 1.5 V with various scan rates of 50 mM InCl3 in [BMP]+[DCA]- recorded on GC electrode.

Figure 3. Cyclic voltammograms recorded on GCRDE of 50 mM InCl3 in (i) [BMP]+[DCA]- and (ii) [EMI]+[DCA]- at 1200 rpm. Scan rate was 5 mV·s-1. Note the dashed line is expected if adsorption of BMP+ cations did not occur.

Figure 4. Current- time transients resulting from chronoamperometry experiments on GC electrode in [BMP]+[DCA]- IL containing 50 mM InCl3 at 35 °C with various applied potentials.

Figure 5. Cyclic voltammograms of 50 mM InCl3 in [BMP]+[DCA]- recorded on GC electrode at 35 and 90 °C. Scan rate was 50 mV·s-1.


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FIGURES

Figure 1. Cyclic voltammograms of 50 mM InCl3 in [BMP]+[DCA]- recorded on (i) indium wire electrode, and (ii) GC electrode at 35 °C. Scan rate was 50 mV·s-1.



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Figure 2. Cyclic voltammograms of 50 mM InCl3 in [BMP]+[DCA]- recorded on GC electrode at 35 °C. Potential scan was reversed at -1.5, -1.2, -1.1, -0.6, and -0.5 V. Scan rate was 50 mV·s-1. Inset: CVs between -0.5 V to 1.5 V with various scan rates of 50 mM InCl3 in [BMP]+[DCA]- recorded on GC electrode.


Page 33 of 35

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Figure 3. Cyclic voltammograms recorded on GCRDE of 50 mM InCl3 in (i) [BMP]+[DCA]- and (ii) [EMI]+[DCA]- at 1200 rpm. Scan rate was 5 mV·s-1. Note the dashed line is expected if adsorption of BMP+ cations did not occur.



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Figure 4. Current- time transients resulting from chronoamperometry experiments on GC electrode in [BMP]+[DCA]- IL containing 50 mM InCl3 at 35 °C with various applied potentials.


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Figure 5. Cyclic voltammograms of 50 mM InCl3 in [BMP]+[DCA]- recorded on GC electrode at 35 and 90 °C. Scan rate was 50 mV·s-1.

TOC:


Anomalous Voltammetric Behavior Observed for Electrodeposition of

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