Investigation of ionic liquids as electrolytes for carbon nanotube electrodes

Investigation of ionic liquids as electrolytes for carbon nanotube electrodes

Electrochemistry Communications 6 (2004) 22–27 www.elsevier.com/locate/elecom Investigation of ionic liquids as electrolytes for carbon nanotube elec...

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Electrochemistry Communications 6 (2004) 22–27 www.elsevier.com/locate/elecom

Investigation of ionic liquids as electrolytes for carbon nanotube electrodes J.N. Barisci

a,*

, G.G. Wallace a, D.R. MacFarlane b, R.H. Baughman

c

a

c

Department of Chemistry, University of Wollongong, Wollongong, NSW 2522, Australia b School of Chemistry, Monash University, Clayton, Vic. 3800, Australia NanoTech Institute, The University of Texas at Dallas, Richardson, TX 75083-0688, USA

Received 2 September 2003; received in revised form 25 September 2003; accepted 29 September 2003 Published online: 31 October 2003

Abstract The use of ionic liquids (IL) as electrolytes for electrochemical applications involving carbon nanotube (CNT) electrodes has been investigated in a brief initial study. The use of IL electrolytes in conjunction with CNT electrodes has proved possible and advantageous. Ionic liquids provide relatively high conductivity, wide potential window (up to 5.5 V) along with chemical stability and nonvolatile nature. While some decrease in the electrode capacitance and charging rate are observed in IL with respect to conventional electrolytes, the magnitude of the decrease is not substantial. The general well defined electrochemical behaviour of CNT electrodes in IL, coupled to the wide potential window and other advantages of these electrolytes, suggest new avenues for the design of capacitors, batteries and electromechanical actuators. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Ionic liquids; Electrodes; Electrolyte; Cyclic voltammetry

1. Introduction The use of carbon electrodes [1] is of critical importance in areas as diverse as bioelectrochemistry and electrolytic refining of metals. In both areas the unique surface chemistry of carbon is important. Since the discovery of carbon nanotubes (CNT) the use of these as electrode materials has attracted considerable interest [2–7]. Crooks and co-workers [2] fabricated electrodes comprising a single CNT. The electrochemical characteristics were as expected for an ultramicroelectrode. In other studies, forests of aligned CNT have been assembled and used as electrodes for biosensors [3]. Ajayan and co-workers have investigated the use of as-grown multiwall CNT entangled mats as electrodes and shown rapid electron transfer for oxidation–reduction of the ferro/ferricyanide couple [4]. Others indicate that acid treatment of CNT electrodes improve the *

Corresponding author. Tel.: +61-2-4221-3685; fax: +61-2-42213114. E-mail address: [email protected] (J.N. Barisci). 1388-2481/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2003.09.015

electron transfer kinetics for this couple [5]. The Ajayan group has shown that the two electron oxidation of dopamine was more rapid on CNT electrodes than on other carbon electrodes [6]. Glassy carbon electrodes have been coated with CNT by casting a CNT dispersion in a volatile solvent [7,8]. These electrodes exhibited electrocatalytic effects towards oxidation of the biomolecules dopamine, epinephrine and ascorbic acid [7] and enabled direct electron transfer to/from cytochrome c [8]. Stand-alone CNT mats have been shown to have good electrochemical properties and high specific capacitance (up to 40 F/ g) in a range of electrolytes [9–12]. These electrochemical properties have been put to use in a number of areas including sensors [3], electrocatalysis [12], supercapacitors [13,14] and electromechanical actuators [15,16]. The ability to store charge determines performance capabilities in the latter two applications. This ability is dependent on the inert electrochemical potential window available. Given the inert nature of carbon, it is usually the case that the potential window available is limited by the electrolyte used. Recently, it has been shown that

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ionic liquids (IL) are highly effective electrolytes with a wide electrochemical potential window [17,18]. This capability combined with the environmental stability of IL has led to their use in photoelectrochemical cells [19], capacitors [20–22] and electromechanical actuators based on conducting polymers [21]. Building on our previously established knowledge of CNT [9–11,15] and IL [18], we aimed in this work to investigate the basic electrochemical properties of a range of IL and establish their suitability as electrolytes for applications involving CNT electrodes.

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otherwise specified. Electrochemical impedance data were collected using a 10 mV amplitude sinusoidal wave with a frequency range between 5 kHz and 50 mHz. Resistance data was obtained from the Nyquist plot. Capacitance values were extracted from the diameter of the complex capacitance plots provided by the PowerSuite software (EG&G).

3. Results and discussion 3.1. Characterisation of ionic liquids

2. Experimental 2.1. Chemicals and materials The single-wall CNT were HiPco obtained from Carbon Nanotechnologies Inc. (USA). The IL were synthesised at Monash University. All other chemicals were supplied by Sigma. 2.2. Instrumentation A conventional 3-electrode cell was used for all electrochemical experiments. The working electrode was a 1.5-mm diameter platinum disc electrode or a piece (0.3 cm2 ) of CNT paper attached to a thin platinum wire clip. The reference electrode was Ag/Agþ (0.01 M AgClO4 /0.1 M tetrabutylammonium perchlorate/acetonitrile) for nonaqueous solutions or a Ag wire for IL. The auxiliary electrode consisted of a piece of CNT paper several times larger than the working electrode. The potentiostat for cyclic voltammetry and chronoamperometry consisted of PAR 174/175 modules. A computer operated MacLab 4e (ADInstruments) interface driven by Chart software (ADInstruments) were used to control the potentiostat and to record its output. Electrochemical impedance data were obtained with a computer controlled EG&G BES potentiostat operated by PowerSuite software (EG&G).

An initial characterisation was carried out to establish the electrochemical window and stability of selected IL. The name and structure of the IL ions described are given in Table 1. The cyclic voltammograms presented in Fig. 1 show the differences existing between the IL investigated. The limits of the useful potential range are indicated by a sharp and steady rise in the current. These limits and the width of the stable potential windows (uncorrected for iR drop) are listed in Table 2. The potential window covers in each case a somewhat different region of the potential spectrum. Excursions beyond the described stable potential limits tend to induce redox processes, some at intermediate potentials, as exemplified by the strong reduction peak at 0.1 V for P13 TFSA. Presumably, these responses arise as a result of IL electrolysis or from the reaction of associated electroactive decomposition products. The best overall stability is delivered by P13 TFSA which exhibits similar, and wide, useful limits at both positive and negative potentials. This IL could provide outstanding capabilities for actuators and capacitors, both of which rely on large potential differences to enhance their performance. Despite the chemical stability and hydrophobicity of the TFSA-containing IL, it was found that the presence Table 1 Name and structure of ionic liquid ions Chemical name

Structure

Abbreviationa

R,R0 -Imidazolium

IRR0

R,R0 -Pyrrolidinium

PRR0

2.3. Procedures The basic fabrication procedure for CNT paper has been previously described [9–11,15]. It involved vacuum filtration of a single-wall CNT dispersion in 1% Triton X-100 on a membrane filter, washing with water and methanol, air drying, and removal of the formed film from the filter. All CNT paper electrodes were cycled in the electrolyte between 1.0 V for 3–5 cycles at 50 mV/s before any electrochemical measurements were made. Solutions were not deoxygenated unless otherwise indicated. All studies were carried out at room temperature unless

Bis(trifluoromethanesulphonyl)amide Dicyanamide Hexafluorophosphate p-tolunesulphonate

(CF3 SO2 )2 N

TFSA

N(CN) 2 PF 6 C7 H7 –SO 3

DCA HFP PTS

a Subscripts (R,R0 ) indicate length of alkyl chains attached to Natoms.

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(Fig. 1), capacitance and potential window were similar to those described for room temperature IL, suggesting that in molten state, the general electrochemical behaviour of these salts should be expected to be similar regardless of melting point, provided their conductivity in the molten state is also similar.

6 I 21 DCA

i/µA

4 I 21 TFSA

2

0

3.2. CNT electrodes in ionic liquids

-2

-4 -2

-1

0

1

2

E/V 20

10

P 13 TFSA

i/µA

0

-10 P 13 PTS -20

-30 -3

-2

-1

0

1

2

3

Fig. 1. Cyclic voltammograms for selected IL, platinum disc electrode, scan rate 100 mV/s.

of small amounts of water, absorbed from the ambient, affected the electrochemical stability of the electrolyte. For instance, under dry-box conditions a potential window 200 mV wider than in ambient conditions was observed for I21 TFSA. The amount of absorbed water varies with the electrolyte, reaching values of around 1% for the more hydrophilic HPF and DCA-containing IL. Some tests were also carried out to examine the behaviour of IL with melting point above room temperature. One example of this type was P13 PTS which melts at around 95 °C. The observed voltammetric curves

3.2.1. Electrode charging The ability of the CNT electrodes to store charge and the rate at which the charge could be injected were investigated. These parameters are of significance in the design of electrochemical capacitors or electromechanical actuators [15]. Examination of the data from Table 2 indicates that a broad positive correlation exists between the electrolyte conductivity and electrode capacitance. With respect to the conventional nonaqueous electrolyte (1 M TBAHFP/acetonitrile), the decrease in capacitance varies with the IL. P13 TFSA exhibits the largest drop in capacitance (40%) although its conductivity is in relative terms much lower (around 10 times) than that of 1 M TBAHFP/acetonitrile. The values of electrode charging time constant (RC) for I41 HFP indicate that this electrolyte is the least useful for high currents or fast processes. I21 DCA provides the best charging performance assisted clearly by its relatively large conductivity. In all cases, however, well defined, undistorted charging curves, similar to that for the 1 M TBAHFP/acetonitrile electrolyte, are observed (Fig. 2). 3.2.2. Voltammetry Typical cyclic voltammograms for a CNT sheet electrode in IL are shown in Fig. 3. In all cases the curves are basically featureless with no signs of redox processes. I41 HFP is the least effective electrolyte of the group investigated, as is apparent from the lower charging currents generated and the slower transient response at both ends of the voltammogram. This is consistent with the lower charging rate associated with this electrolyte. Clearly, I21 TFSA and I21 DCA provide

Table 2 Electrochemical data for selected electrolytes at room temperature Electrolyte

Conductivitya (mS/cm)

Potential limitsc (V)

Potential window (V)

C d (F/g)

RCe (ms)

1 M TBAHFP I21 DCA I21 TFSA I41 HFP P13 TFSA

10 18 8.6 1.5b 1.4

)1.5/1.2 )1.6/1.4 )2.0/2.0 )1.1/2.1 )2.5/2.8

2.7 3.0 4.0 3.2 5.3

24.3 24.3 18.2 17.8 14.3

290 331 607 >5000 628

a

Measured using a microconductivity probe and ac impedance [18]. From [23]. c Measured using a platinum electrode (vs Ag wire). d Measured by cyclic voltammetry for a CNT sheet electrode at 0.0 V (vs Ag) for ionic liquids or 0.0 V (vs Ag/Agþ ) for 1 M TBAHFP/acetonitrile. e Measured by applying a 0.2 V potential step at 0.0 V. b

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25

30 1.0

20 0.8

i/Ag -1

i

10 0.6 I 21 TFSA 0.4 I 21 DCA

0.2

P13 TFSA

I 21 DCA

-30 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 2. Chronoamperograms showing normalised current for a CNT sheet electrode in I21 DCA and I21 TFSA obtained after application of a 0.2 V potential step at 0.0 V.

6 TBAHFP 4

2

i/Ag -1

-10 -20

0.0

0

I 41 HFP

-2

-4

I 21 DCA

-6 -1.0

-0.5

0.0

0.5

1.0

E/V 4

2

i/Ag -1

0

0

-2

-1

0

1

2

Fig. 4. Cyclic voltammograms for a CNT sheet electrode in P13 TFSA and I21 DCA, scan rate 50 mV/s.

3.2.3. Impedance spectroscopy The electrochemical impedance behaviour of the CNT electrodes in IL was similar to that observed previously in both aqueous and nonaqueous electrolytes [24]. The Nyquist plots (Fig. 5) exhibit the typical features of a porous electrode with a slopping (approaching 45°) linear region at high-medium frequencies and an almost vertical line at low frequencies, where the behaviour becomes mainly capacitive. As also observed in other electrolytes, a decrease in the slope and increase in the length of the 45° region of the Nyquist plot, features associated with electrode pore resistance, is observed as the resistivity of the electrolyte increases [24]. Interestingly, a change in the electronic resistance of the electrode occurs as the applied potential is varied (Fig. 6), with a maximum at 0.0 V vs Ag. This behaviour is similar to that described previously for CNT electrodes in aqueous and nonaqueous electrolytes [24].

P 13 TFSA

-2 I 21 TFSA

250

-4

I 21 TFSA -0.5

0.0

0.5

1.0

Fig. 3. Cyclic voltammograms for a CNT sheet electrode in selected ionic liquids and in 1 M TBAHFP/acetonitrile, scan rate 50 mV/s.

sufficient ionic mobility to prevent degradation of the CNT electrode performance and compare very well with 1 M TBAHFP/acetonitrile. Extending the potential scan to 2.0 V produces no redox processes except for I21 DCA. In this case, broad responses and increased background currents are observed, presumably as a result of the decomposition of the DCA anion at positive potentials (Fig. 4). Significantly larger background currents (not shown) were observed when 1 M TBAHFP/ acetonitrile was used.

200

-Im Z /Ohm

-1.0

150 P13 TFSA

I 41 HPF

100

50

0 100

200

300

400

500

Fig. 5. Nyquist plots for a CNT sheet electrode in I21 TFSA, P13 TFSA and I41 HFP at 0.0 V.

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45 60

35

55

30

R/Ohm

C/Fg -1

40

50 25 20

45

15 -1.5

-1

-0.5

0

0.5

1

1.5

Fig. 6. Resistance (j) and capacitance (d) for a CNT sheet electrode measured by EIS as a function of applied potential in P13 TFSA electrolyte.

It has been suggested [24–26] that the origin of this phenomenon is the change in the number of free carriers resulting from electron depletion or filling of specific energy bands of the CNT as the Fermi level is shifted. As also observed previously in conventional electrolytes [24], a minimum in the capacitance–potential curves exists for CNT electrodes in IL electrolytes. The potential for this minimum coincides with the potential for the resistance maximum. It has been proposed [24,27,28] that the minimum in the capacitance–potential curve at high electrolyte concentration, as would be the case in IL, is due to the presence of a space-charge region within the CNT electrode. In contrast to the behaviour in conventional electrolytes, however, the parabolic curves in Fig. 6 are sharper, suggesting a different double layer structure. This would be expected considering the absence of solvent in the electrolyte.

possible. The total thickness of the device was 121 lm with the CNT layer accounting for 5 lm. A strip of the CNT/membrane/Pt ‘‘sandwich’’ (7 mm  2.2 mm) was immersed in IL which percolated through the outer layers and into the separating membrane. The device, consisting effectively of two electrodes separated by an electrolyte layer, and better described as CNT/Pt//membrane (IL)//Pt, could then be tested. This arrangement could be viewed as an embryonic capacitor and/or as an electromechanical actuator. To determine the electrochemical properties of the CNT layer, it was independently connected via a metallic clip and the working electrode lead to a potentiostat to perform standard 3-electrode cyclic voltammetry and capacitance measurements in the IL (I21 TFSA). The voltammetric curves were found to be similar to those for CNT sheets. The measured capacitance was around 19 F/g, also very similar to that of free-standing CNT sheets [9–11]. Once the normal behaviour of the CNT layer was established, studies were next carried out with a 2-electrode system using the CNT and platinum layers deposited on each side of the membrane as the electrodes. Initially, the CNT/Pt//membrane//Pt assembly was immersed in the liquid electrolyte (I21 TFSA) but in a second stage it was completely removed from the electrolyte and operated in air. In this latter case, operation relied on the IL trapped inside both the porous separating membrane and overlying electrodes. A comparison of cyclic voltammograms of the device within and outside the liquid electrolyte is presented in Fig. 7. It can be seen that, remarkably, no difference is observed between the curves, suggesting that no degradation of performance occurs in the absence of electrolyte outside the multilayer assembly. Chronoamperometry also revealed that the multilayer device could be charged very rapidly. These results highlight the efficiency of the thinlayer approach as well as the advantage of using liquid electrolytes like IL, which are highly conducting, nonvolatile and very stable.

3.3. CNT-IL membrane electrode 2

i/Ag -1

To investigate the use and possible advantages of IL in practical devices, a flexible, thin-layer assembly was constructed using a porous membrane (PVDF, 0.22 lm pore size) as the basic component. First, both sides of the membrane were sputter-coated with a thin layer of platinum. On one side, a thin film of CNT was next deposited by filtration of a CNT dispersion through the membrane. The underlying platinum layer was necessary to provide adhesion for the CNT film which would otherwise peel off spontaneously. At this stage, deposition of a CNT layer by filtration on the other side of the platinum coated membrane was not

1

0 -1

-1.0

-0.5

0.0

0.5

1.0

Fig. 7. Cyclic voltammograms for a CNT/Pt//membrane//Pt assembly obtained, first immersed in electrolyte (I21 TFSA), and next in air, 2electrode system, scan rate 50 mV/s.

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4. Conclusions The anticipated advantages of IL as electrolytes for electrochemical applications have been mostly confirmed by this initial limited study. The relatively high conductivity and wide potential window (up to 5.5 V) observed, added to the already known chemical stability and nonvolatile nature of these materials, promise significant advances in the development of electrochemical devices. The use of IL electrolytes in conjunction with CNT electrodes has proved possible and advantageous. While some loss of capacitance and charging rate are observed with respect to conventional electrolytes, the magnitude of the decrease is overall not substantial. The general good electrochemical behaviour of CNT electrodes in IL, coupled to the wider potential window and nonvolatility of these electrolytes, suggest new avenues for the design of capacitors, batteries and electromechanical actuators.

Acknowledgements This work was partially supported by US DARPA grant MDA972-02-C-0005. G.G. Wallace acknowledges the continued support of the Australian Research Council. R.H. Baughman thanks the Robert A. Welch Foundation for partial financial support. References [1] K. Kinoshita, Carbon: Electrochemical and Physical Properties, Wiley, New York, 1998. [2] J.K. Campbell, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 121 (1999) 3779. [3] M. Gao, L. Dai, G.G. Wallace, Electroanalysis, in press. [4] J.M. Nugent, K.S.V. Santhanan, A. Rubio, P.M. Ajayan, NanoLett. 1 (2001) 87. [5] J. Li, A. Cassell, L. Delzeit, J. Han, M. Mevyappon, J. Phys. Chem. B 106 (2002) 9299.

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