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Anion effects in imidazolium ionic liquids on the performance of IPMCs
Sensors and Actuators B 137 (2009) 539–546
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Anion effects in imidazolium ionic liquids on the performance of IPMCs Jang-Woo Lee a,b , Young-Tai Yoo a,b,∗ a b
Artificial Muscle Research Center, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea Department of Materials Chemistry and Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 August 2008 Received in revised form 16 January 2009 Accepted 22 January 2009 Available online 31 January 2009 Keywords: Ionic polymer–metal composite Actuators Ionic liquids Nafion
a b s t r a c t Four ionic liquids were explored for as inner solvents of IPMC to overcome the shortcomings of water, especially its high volatility and low electrolysis potential. The imidazolium salts were composed of 1-ethyl-3-methylimidazolium [EMIm] cation and anions including bromide [Br], nitrate [NO3 ], acetate [AcO], and trifluoroacetate [TA]. The 1 H NMR studies confirmed the structures of the four ionic liquids and indicated that [EMIm][AcO] and [EMIm][TA] had stronger interactions between anion and C(2) hydrogen in imidazolium cation compared with [EMIm][NO3 ] and [EMIm][Br]. [EMIm][Br] and [EMIm][NO3 ] exhibited enormously low volatility with initial weight loss above ca. 290 ◦ C. Two-probe linear sweep voltammetry (LSV) revealed that the electrolytic stability decreased in the order of [EMIm][TA] > [EMIm][AcO] > [EMIm][NO3 ] ∼ [EMIm][Br], whereas current density in IPMCs was in the reverse order. Thus, [EMIm][Br]- and [EMIm][NO3 ]-based IPMCs exhibited a superior performance in terms of tip displacement and response speed compared to [EMIm][AcO]- and [EMIm][TA]-based ones. These results corresponded with the size and ionic mobility of anions in imidazolium salts. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A typical IPMC is constructed with an ionic polymer membrane and two platinum electrode layers. A polymeric membrane contains mobile metal cations and water as an inner solvent. Under an electric potential of 1–3 V, hydrated metal cations in the ionic membrane are attracted toward the negatively charged electrode, which subsequently triggers the electro-osmotic flow of the inner solvent. As a result of the mass transport from one side to another, volume difference takes place between the two sides of the IPMC strip, resulting in a bending deformation of the composite film [1,2]. The IPMCs can be considered as viable actuators, producing a large bending deformation, a fast response, and a low operational voltage; however, their practical application is limited due to major drawbacks. Some of these include a back relaxation under dc potentials, a relatively low actuation force, a low reproducibility, and, most importantly, a rapid loss of water through natural evaporation and electrolysis at over 1.23 V [3]. In particular, the loss of water causes a reduced flux of metal cations and inner solvent and damage to the electrode. Abundant studies in the literature have discussed these aspects of IPMC [4–14] and many attempts have been made to replace water, the conventional inner solvent in IPMC, with less
∗ Corresponding author at: Artificial Muscle Research Center, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea. Tel.: +82 2 450 3207 fax: +82 2 444 0711. E-mail address: [email protected] (Y.-T. Yoo). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.01.041
volatile and stable organic compounds. Nasser used mixtures of ethylene glycol and glycerol as inner solvents for IPMCs [12,13]. However, the use of organic solvents alone resulted in a limited and sluggish actuation performance. Lee et al. reported that mixtures of polar organic solvents and deuterated water could improve the electrolytic stability [14]. Recently, ionic liquids have been recognized as alternatives to some conventional organic solvents as they exhibited low vapor pressure, a wide electrochemical window, high electrical conductivity, and low viscosity [5,15–19]. Bennett and Leo chose 1-ethyl-3methylimidazolium trifluoromethanesulfonate ([EMIm][TfO]) and 1-ethyl-3-methylimidazolium bromide ([EMIm][Br]) ionic liquids as inner solvents of IPMC actuators. They observed enormously improved stabilities but slow responses with the IPMCs containing the ionic liquids compared to water-based IPMCs [5]. Kothera and Leo reported the linearity of the voltage-to-current relationship of IPMCs with [EMIm][TfO] ionic liquid [15]. Akle et al. found that the charge accumulation at the polymer–electrode interface of IPMCs with ionic liquids is the key to producing high strain in ionic polymer transducers [16]. Fukushima et al. reported the bucky-gel-based bimorph actuator, having an electrolyte consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF(HFP)) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4 ]), with two electrodes consisting of single-walled carbon nanotubes (SWNTs), [BMIm][BF4 ], and PVdF(HFP) [17]. Vidal et al. presented a conducting semi-IPN/ionic liquid-based actuator, and Lim et al. a multi-walled carbon nanotube (MWNT)/ionic liquid-based IPMC [18,19].
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Although many have reported on ionic liquid-based actuators, very few have discussed the effects of structural factors of ionic liquids, particularly the anions, affecting the properties of ionic liquids and the performance of IPMCs containing them. This paper will focus on the thermal and electrochemical properties of imidazolium-based ionic liquids with various anions of Br− , NO3 − , AcO− , and TA− , and the IPMCs impregnated with these ionic liquids. The effect of the anions in the ionic liquids on the performance of IPMCs will also be investigated. 2. Experimental
2.5. Actuation test An IPMC strip for the bending performance was cut to the size of 0.5 cm × 2.5 cm. The strip was supported vertically in air and fixed to 0.5 cm length on both sides; thus, the actual effective length of the samples was 2 cm. Electric power was applied with an ac source/analyzer (Model 6811B, Agilent). The displacement measurement was performed using a laser beam device (LB-11, Keyence) and a video camera which were connected to a computer by a data acquisition (DAQ) system (SCB-68, National Instrument). The deformation response was measured under a square wave input.
2.1. Preparation of IPMC IPMCs containing lithium ions (Li+ ) were produced from NafionTM -117 membrane, and prepared via a well-known electroless plating method of platinum [20]. 2.2. Synthesis and characterization of ionic liquids Four imidazolium salts including [EMIm][Br], [EMIm][NO3 ], [EMIm][AcO], and [EMIm][TA] were prepared via metathesis reactions [21–23] and dried before use. Scheme 1 illustrates the metathesis reaction procedure for the synthesis of ionic liquids. 1 H NMR (Spectrospin400 UltraShieldTM , Bruker) and FT-IR (Genesis II FTIR Spectrometer, Mattson) spectroscopy were used to identify the chemical structure of the ionic liquids prepared in this study. Thermal analysis was performed using a differential scanning calorimeter (DSC 2010, TA Instruments) and thermogravimetric analysis (TGA 2050, TA Instruments) with a heating rate of 10 ◦ C min−1 under nitrogen atmosphere. The electrolytic stability of each ionic liquid was estimated using a source meter (Model 2400, Keithley). X-ray diffraction (D/Max-2200, Rigaku Denki) patterns were obtained under 40 kV and 30 mA of Cu K␣ radiation with a characteristic wavelength of 0.154 nm, in a 2 range of 1–80◦ with a scanning rate of 4◦ min−1 in order to identify the size distribution of ionic clusters in the Li-NafionTM matrices containing the ionic liquids. 2.3. Preparation of ionic liquid-based IPMCs In order to impregnate the IPMCs with the ionic liquids, each IPMC strip was immersed in a mixture of methanol (50 wt.%) and a dry ionic liquid at 70 ◦ C for 2 h. Then, the swollen IPMC samples were dried to remove methanol. 2.4. Characterization of ionic liquid-based IPMCs Electrical properties of the IPMC samples were measured with a complex impedance analyzer (IM6ex, Zahner) and a multichannel potentiostat/galvanostat (WMPG1000, Wonatech).
3. Results and discussion 3.1. Properties of 1,3-dialkylimidazolium salt ionic liquids Ionic liquids exhibit a relatively high conductivity, a wide window of electrochemical stability, and a low viscosity as well as an extremely low volatility [22,24]. Especially, the high electrochemical stability and low volatility of the inner solvent are essential to the stable operation of the IPMC in a dry environment. 1,3-Dialkylimidazolium salts were chosen since they met many important criteria of inner solvents in IPMC. In Fig. 1, proton NMR spectra are shown with the assignments of the protons in the 1-ethyl-3-methylimidazolium salts. The structure of the four imidazolium salts was confirmed by matching the integrated area of each peak with the number of the corresponding hydrogen. The four 1-ethyl-3-methylimidazolium salts have different anions with varying degrees of basicity. In imidazolium salts, the basic anions strongly associated with the electron deficient C(2) hydrogens via hydrogen bonds. Bonhôte et al. reported that degree of hydrogen bonding could be compared by the chemical shift of the C(2) hydrogen in the 1 H NMR spectrum of ionic liquid as an indicator of the electron donating capability of the anions [21]. According to the chemical shift of the hydrogen shown in Fig. 1(b) using [EMIm][Br] as a reference, the degree of hydrogen bonding, i.e., the basicity of the anions, decreases in the order of [EMIm][AcO] > [EMIm][TA] ∼ [EMIm][NO3 ] > [EMIm][Br]. Rapid loss of water in IPMC occurs primarily from its high volatility. Fig. 2 shows the TGA measurements of weight loss vs. temperature for the 1-ethyl-3-methylimidazolium-based ionic liquids. In particular, [EMIm][Br] and [EMIm][NO3 ] exhibited higher thermal stability with the initial weight loss above ca. 290 ◦ C compared to [EMIm][AcO] and [EMIm][TA]. Ngo et al. reported that the substitution of the abstractable acidic C(2) hydrogen of imidazolium cations with alkyl groups increases the thermal stability of imidazolium salts [22]. Fredlake et al. [25] postulated that imidazolium ionic liquids with anions of lower basicity possess higher thermal stabilities. These studies correspond with the observation that [EMIm][Br] and [EMIm][NO3 ] with anions less basic
Scheme 1. Metathesis reaction of imidazolium salt type ionic liquid.
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Fig. 3. DSC curves of [EMIm][Br] ionic liquid in bulk and IPMC. An IPMC sample containing the ionic liquid of 21 wt.% was used.
Fig. 1. (a) 1 H NMR peaks of ionic liquids and (b) chemical shift of C(2) protons (peak e) of ionic liquids on zero basis of [EMIm][Br]. Deuterated chloroform (CDCl3 ) was used as a 1 H NMR solvent.
and thus less capable of abstracting C(2) hydrogen from imidazolium cation exhibited the stability higher than the [EMIm][AcO] and [EMIm][TA] ionic liquids. Initial weight loss over 100–140 ◦ C was attributed to the evaporation of absorbed water. A relatively larger weight loss of [EMIm][AcO] was attributed to the extremely hygroscopic nature of this compound. It was found that dried
Fig. 2. Thermal stability of 1-ethyl-3-methylimidazolium-based ionic liquids. The ionic liquids were dried before the test.
[EMIm][AcO] absorbed moisture of up to 46% of its weight within an hour. Among the four 1,3-dialkylimidazolium salts, [EMIm][Br] shows a melting phase transition above room temperature, m.p. of 75 ◦ C. Thermograms of [EMIm][Br] run in neat and in IPMC (21 wt.% in IPMC) were compared in Fig. 3. Crystallization and melting phase transition of [EMIm][Br] were not observed when it was impregnated in IPMC. The structure of the ionic liquid is viewed as a loose ionic interaction between anions and cations with delocalized charge. It is expected that the anions and cations of ionic liquid are easily associated with the respective counter ions in NafionTM , retarding the crystallization. Another important feature of ionic liquids was characterized by their electrolytic stability and wider electrochemical window. Under an electrical potential, the reduction of EMIm+ ion occurs at the cathode side and the oxidation of anion at the anode side [23,26]. Two-probe linear sweep voltammetry (LSV) was used to assess the electrolytic stability of the imidazolium type ionic liquids with various anions during the operation. As shown in Fig. 4, the electrolytic onset voltage of the ionic liquids, which is the starting voltage of the oxidation of each anion at the anode, inter-correlated to the reduction of EMIm+ at the cathode, decreasing in the order of [EMIm][TA] > [EMIm][AcO] > [EMIm][NO3 ] ∼ [EMIm][Br]. These results indicated that the electrolytic stability of the imidazolium ionic liquids varied with the stability of the anion; the electrolytic
Fig. 4. Electrolytic stability of 1-ethyl-3-methylimidazolium-based ionic liquids. The ionic liquids were dried before the test, and the data were recorded between the tips of a pair of gold probes (diameter = 1 mm) immersed in the ionic liquid, with a scan rate of 100 mV s−1 .
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Table 1 Properties and characteristics of 1,3-dialkylimidazolium salts. Species [EMIm][Br] [EMIm][NO3 ] [EMIm][AcO] [EMIm][TA]
Radius of Hydrated Anion (nm) a
0.30 0.30a (0.33b ) 0.45a nad , >0.45
Anionic Mobility (in water) (10−8 m2 s−1 V−1 ) c
8.09 7.40c 4.24c nad , <4.24
Electrolytic Onset Voltage (V)
Decomposition Temperature (◦ C)
3.0 3.1 3.6 4.1
299.3 287.8 216.1 171.8
a
From Ref. [26]. From Ref. [27]. c From Ref. [28]. d No data are reported but it is expected that with a larger radius of hydrated anion of CF3 COO− than that of CH3 COO− , the ionic mobility of CF3 COO− in water is lower than that of CH3 COO− due to the fact that the electro-negativity of fluorine is higher than hydrogen. b
onset voltage decreases with the increasing oxidation tendency of the anion at the anode, combining the reduction capability of EMIm+ ion at the cathode. The highest electrochemical stability of [EMIm][TA] was attributed to the high thermodynamic formationstability of the CF3 COO− ion due to the strong electron-withdrawing influence of fluorine. The electrochemical decomposition occurs at the interface between electrolytes and contact electrodes, where an electrical double layer is formed. Thus, high mobility of Br− and NO3 − ions could have contributed to their rapid electrochemical decomposition. Table 1 summarizes the properties and characteristics of each ionic liquid [27–29]. A proper level of solvent uptake is required to exhibit a high ionic conductivity in IPMC, although an excessive uptake adversely affects the bending performance due to the loss of mechanical strength. In preparation, each IPMC strip was immersed in a mixture of methanol and an ionic liquid (50/50 wt.%) at 70 ◦ C for 2 h. Then, the IPMC was dried to remove volatile components. Fig. 5 shows that ionic liquids exhibited uptake levels similar to those of water (ca. 20 wt.% in this study) except [EMIm][AcO]. A relatively lower uptake of [EMIm][AcO] immediately after drying was due to the loss of absorbed water in the ionic liquid as observed from TGA experiments.
ters in a dry NafionTM membrane exhibited the strongest intensity at 1.10 nm, and those in the NafionTM membrane impregnated with water displayed at 1.21 nm, indicating swelling of the ionic clusters. In Fig. 6(a), the size of ionic clusters at the maximum distribution is indicated with an arrow. In the case of the NafionTM membranes containing the ionic liquids, the size of ionic clusters at the maximum distribution was slightly smaller than those in the NafionTM membranes with water, and decreased as the hydrophobicity of the ionic liquid increased except the extremely hygroscopic [EMIm][AcO]. It was expected that a part of the relatively hydrophobic ionic liquids could be distributed in the domain other than ionic clusters in the polymer. The NafionTM membrane impregnated with [EMIm][Br] ionic liquid registered the highest intensity of the ionic cluster peak with respect to the peak of the amorphous halo at 17.0◦ as shown in Fig. 6(b). This was an indicative of well-defined phase separation between the ionic clusters and the main chain region. The development of
3.2. Morphology of NafionTM membrane with ionic liquids Fig. 6 shows the X-ray diffraction (XRD) patterns of Li-NafionTM membranes containing ionic liquids. Inter-planar spacing (d value) was calculated with Bragg’s equation (2d sin = ). As reported in the literature [30], the XRD peaks in the range of 2 value of 12 ∼ 3◦ (corresponding to the d value range of 0.74 ∼ 2.94 nm) represented the size distribution of ionic clusters in the Li-NafionTM membranes containing ionic liquids, and the peaks around 17.0◦ were related to the amorphous halo of NafionTM . The ionic clus-
Fig. 5. Uptake of 1-ethyl-3-methylimidazolium-based ionic liquids in IPMC. The non-patterned area indicates the uptake level immediately after full drying of the samples. Consecutively, the IPMC samples were exposed to air for 2 h and the amount of water absorbed from the air was represented by the patterned area.
Fig. 6. X-ray diffraction (XRD) patterns of Li-NafionTM membranes with ionic liquids: (a) small- and (b) wide-ranged patterns.
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the ionic channel should contribute to actuator performance based on the ionic transport mechanism. 3.3. Electrical properties of IPMCs with ionic liquids Ion transport in ionic liquid-based IPMCs was expected to be more complicated than that in water-based IPMCs due to the nature of the ionic liquid. Fukushima et al. [17] proposed that when an electric potential was applied to the bucky-gel-based bimorph actuator, cations and anions of the built-in ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate; BMIm-BF4 ) were transported to the negatively and positively charged electrode layers, respectively, which form electric double layers on both electrode sides. The ionic transport resulted in an expansion of the negatively charged side as the BMIm+ ion is larger than the BF4 − ion, giving a bending deformation to the bimorph actuator. The ionic liquid-Li-IPMCs contained three mobile ionic species. Thus, possible ion transports in the ionic liquid-Li-IPMCs under an electrical potential included (i) the individual movement of lithium ions and dissociated ions of the ionic liquid as in the bucky-gelbased actuator and (ii) an electro-osmotic drag of lithium cations bound with associated ionic liquid molecules similar to conventional IPMCs. Ohno and co-workers claimed that the lithium ion, the smallest ionic source in the triple ion-type imidazolium salts, in a system similar to the ionic liquid-Li-IPMC, might be a major conducting ion in the salt [31,32]. The molecular migration of ionic liquids was also evidenced by the sweating phenomena of the ionic liquids on the negatively charged electrode of the ionic liquid-LiIPMC. Cyclic votammogram provided important information on the performance of IPMC. In the cyclic voltammetry, higher current density implies a larger ionic movement as well as a higher capacitance, which usually produced a larger bending deformation of the IPMC strip as reported in our previous report [33]. In Fig. 7, the current density of the IPMCs loaded with the different ionic liquids followed the order of [EMIm][NO3 ] > [EMIm][Br] > [EMIm][AcO] > [EMIm][TA]. Fig. 8(a) shows the frequency-dependent real impedance of the ionic liquid-based IPMCs. At a frequency of over 2 MHz, impedances of all the IPMCs were almost the same. However, the IPMCs with [EMIm][NO3 ] and [EMIm][Br] displayed lower impedances at lower frequencies compared to those with [EMIm][AcO] and [EMIm][TA], which was a tendency of materials possessing high
Fig. 8. (a) Frequency-dependent real impedance and (b) Nyquist plot of ionic liquidbased IPMCs measured between the two Pt surfaces of the IPMC.
capacitances of double layers [16]. In Fig. 8(b), the electrical resistances of IPMCs with [EMIm][Br], [EMIm][NO3 ], [EMIm][AcO], and [EMIm][TA], deduced from the intercept of the impedance plot to the abscissa real impedance axis (zero capacitance), were 684, 504, 1102, and 1129 [34]. The results from the cyclic voltammetry and electrochemical impedance spectroscopy predicted that the IPMCs with [EMIm][NO3 ] and [EMIm][Br] were superior to those with [EMIm][AcO] and [EMIm][TA] in actuation performance. 3.4. Actuation performance of IPMCs with ionic liquids
Fig. 7. Cyclic I–V curves of ionic liquid-based IPMCs. The test was performed by connecting the counter electrode (CE) and the reference electrode to one side of the IPMC and by connecting the working electrode (WE) to the other side. The curves were obtained after 10 cycles of a triangle voltage input of ±3 V with a scan rate of 100 mV s−1 .
Tip displacement of the ionic liquid-based IPMCs is exhibited in Fig. 9. The tip displacement may be the outcome of the combination of multiple factors. The size of anionic species could possibly be one of them. The IPMC containing [EMIm][NO3 ] measured the highest current density and lowest impedance. However, the [EMIm][Br]-based IPMC produced a slightly larger deformation than those with [EMIm][NO3 ]. This may be explained by the fact that Br− ion is smaller than NO3 − ion and thus the volume difference between anionic and cationic sides of the membrane becomes greater for [EMIm][Br]-based IPMC when imidazolium cations and pair anions migrate to opposite directions. Another factor contributing to the larger tip displacement of [EMIm][Br]-based IPMC may be the stronger association of Br− ions with water molecules [35], which allows a larger volume displacement under an electric potential. Our experimental results confirmed that [EMIm][Br] ionic liquid absorbed more water from the air than [EMIm][NO3 ]; the equilibrium water contents of [EMIm][Br] and [EMIm][NO3 ] in air were 14–17 and 10–14 wt.%, respectively. At 4 V, both IPMCs loaded with [EMIm][Br] and [EMIm][NO3 ] exhibited more extended gaps in tip displacement and faster response speed than those
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Fig. 9. Bending performance of ionic liquid-based IPMCs under (a) 3 V and (b) 4 V square waves at a frequency of 0.1 Hz.
with [EMIm][AcO] or [EMIm][TA], as depicted in Fig. 9(b). Fig. 10 also shows that the response currents diverged into two groups. The larger tip displacement and higher current of the IPMCs with [EMIm][Br] or [EMIm][NO3 ] all pointed to an abrupt increase in ionic mobility at a potential over 3 V. The charge density (mC cm−2 ) consumed in the IPMCs for 5 s under constant electric voltages of 3 and 4 V was calculated by integrating the area of the curve between 0 and 5 s in Fig. 10. The charge-specific displacement (mm/(mC cm−2 )), the concept of which was proposed by Onishi et al. [36] and which represents the energy efficiency of the displacement of actuators, was acquired by dividing the tip displacement for 5 s by the charge density consumed during that time. Table 2 also lists the charge-specific displacement at 3 and 4 V. By definition, the charge-specific displacement is the tip displacement of IPMC per unit charge density. The IPMCs loaded with [EMIm][Br] and [EMIm][NO3 ] yielded the values of 0.10–0.15 mm/(mC cm−2 ) at both 3 and 4 V, implying that tip displacement is linearly increasing with the charge density over the applied potentials. However, the IPMCs with [EMIm][AcO] and [EMIm][TA] recorded higher values of 0.21 and 0.24 mm/(mC cm−2 )
Fig. 10. Response current (for initial 5 s) under actuation of ionic liquid-based IPMCs: (a) 3 V and (b) 4 V square waves of 0.1 Hz.
at 3 V and 0.12–0.14 mm/(mC cm−2 ) at 4 V, respectively. The higher values of [EMIm][AcO]- and [EMIm][TA]-based IPMCs at 3 V are attributable to their lower charge densities compared to those of [EMIm][Br]- and [EMIm][NO3 ]-based IPMCs. It is because the migration of lithium cations is the only source of charge density in [EMIm][AcO]- and [EMIm][TA]-based IPMCs at 3 V. In the case of [EMIm][Br]- and [EMIm][NO3 ]-based IPMCs, migration of the dissociated ionic species of the ionic liquids also contributes to the charge density at 3 V. When applied electric potential was raised from 3 to 4 V, the charge density of the [EMIm][AcO]- and [EMIm][TA]-based IPMCs showed abrupt increases as these ionic liquids begin to dissociate at 3.6–4.0 V. The tip displacements of the [EMIm][AcO]- and [EMIm][TA]-IPMCs increased by mere 10–20%, while the charge densities were doubled over the electric potential of 3–4 V. A smaller size difference of cationic and anionic species of [EMIm][AcO] and [EMIm][TA] could be one of the reasons for less effective tip displacement improvement. Fig. 11 shows the linear sweep voltammogram (LSV) of the ionic liquid-based IPMCs. The electrical onset voltages measured on the
Table 2 Charge consumption and specific-charge displacement of ionic liquid-based IPMCs. Ionic Liquids
[EMIm][Br] [EMIm][NO3 ] [EMIm][AcO] [EMIm][TA]
Charge Density (mC cm−2 )
Tip Displacement for initial 5 s (mm)
Charge-Specific Displacement (mm/(mC cm−2 ))
3V
4V
3V
4V
3V
4V
14.97 18.48 8.43 6.64
24.49 27.97 15.51 14.72
2.27 1.83 1.75 1.59
3.27 3.24 1.92 2.02
0.15 0.10 0.21 0.24
0.13 0.12 0.12 0.14
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Fig. 11. Linear sweep voltammogram (LSV) of ionic liquid-based IPMCs. The test was performed by connecting the counter electrode (CE) and the reference electrode to one side of the IPMC and by connecting the working electrode (WE) to the other side. The curves were obtained scanning from 0 to 5 V with a scan rate of 100 mV s−1 .
IPMCs with [EMIm][Br] and [EMIm][NO3 ] were similar to those of the respective neat ionic liquids. And the IPMCs with [EMIm][AcO] and [EMIm][TA] showed no distinct onset voltages up to 4 V. The electrical onset voltages could be defined as the points of abrupt increase in current which reflect the increase of both electrolysis and ion transport in IPMC. Therefore, the high current beyond the electrical onset voltage resulted in a large deformation of IPMC at the expense of the inner solvent. A large difference in bending performance between the two groups of IPMCs shown in Fig. 9(b) corresponded well with their difference in current level at 4 V in Fig. 11. 3.5. Attenuation of displacement under dc potential Fig. 12(a) compares the normalized displacement vs. time for the ionic liquid-based IPMCs under a potential of 2.5 V. In addition to the back diffusion effect, a part of the performance decay was caused by the loss of inner solvent under a given electric field. It was found that the IPMCs containing [EMIm][Br] and [EMIm][NO3 ], especially in the case of that with [EMIm][Br], sustained its deformation more effectively than those with [EMIm][AcO] and [EMIm][TA]. Bennett and Leo reported that in the case of ionic liquid-based IPMCs, the attenuation of bending performance could be attributed to the decomposition of impurities in the ionic liquid and water absorbed from the air by means of the hygroscopic nature of NafionTM membrane and the ionic liquids [5]. In addition, the ‘sweating phenomenon’, a cross-over of the inner solvent, occurred most rapidly in the IPMC containing the most hydrophobic [EMIm][TA] in this study. For instance, the accumulation of ionic liquid at the negative-electrode surface was observed after less than 1 h for [EMIm][TA] under a constant bending deformation. However, [EMIm][Br] was observed from 3 h to a very limited extent. This should have caused a considerable loss of the inner solvent by squeezing out the IPMC (see Fig. 12(c)) [37]. Again, this is the reason why the IPMC containing [EMIm][TA] under 2.5 V has the actuation time shorter than that containing a more hydrophilic [EMIm][Br] ionic liquid. It was concluded that this sweating phenomenon was more distinct with the ionic liquids of hydrophobic nature. Fig. 12(b) plotted the attenuation of displacement of the ionic liquid-based IPMCs at 3.5 V, which exceeded the level of electrolytic onset voltage of [EMIm][Br] or [EMIm][NO3 ], but not for [EMIm][AcO] or [EMIm][TA]. IPMCs with [EMIm][Br] or [EMIm][NO3 ] displayed larger initial tip displacements but their tip displacements rapidly diminished from the significant loss of solvents by electrolysis at this voltage. The durability compared at 3.5 V corresponded with
Fig. 12. Durability test of ionic liquid-based IPMCs at (a) 2.5 V and (b) 3.5 V square waves of 0.1 Hz. (c) Negatively charged-electrode surfaces of the IPMCs with [EMIm][Br] and [EMIm][TA] ionic liquids after actuation for 1 h. D is the tip displacement and Do the initial tip displacement.
the electrolytic stability of the ionic liquids, following the order of [EMIm][TA] > [EMIm][AcO] > [EMIm][NO3 ] > [EMIm][Br]. 4. Conclusions Imidazolium salts, [EMIm][Br], [EMIm][NO3 ], [EMIm][AcO], and [EMIm][TA], were successfully synthesized and tested to ascertain their thermal and electrical properties as inner solvents for IPMC. The performance of the ionic liquid-based IPMCs depended mainly on the size and ionic mobility of the anion in the ionic liquids, following the order of [EMIm][Br] > [EMIm] [NO3 ] > [EMIm][AcO] > [EMIm][TA]. [EMIm][Br] and [EMIm][NO3 ] exhibited enormously high thermal stability with initial weight loss
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above ca. 290 ◦ C. Solvent uptake of these ionic liquids was found to be comparable to that of water. From the LSV data, the electrical onset voltage may result from the abrupt increase of ionic transport and the electrolysis of ionic liquid, and the current at over the onset voltage may be the sum of the electrolysis and ionic transport in the IPMC. In addition, the attenuation of the performance of the ionic liquid-based IPMC proceeded either from the sweating phenomenon or from the electrolysis of the ionic liquid depending on the applied electric potentials and the type of ionic liquids. Acknowledgements This work was supported by a Korea Research Foundation grant (KRF-2006-005-J03302). The authors would also like to acknowledge the financial support of Konkuk University. References [1] S.N. Nasser, J.Y. Li, Electromechanical response of ionic polymer–metal composites, J. Appl. Phys. 87 (2000) 3321–3331. [2] M. Shahinpoor, Ionic polymer–conductor composites as biomimetic sensors, robotic actuators and artificial muscles—a review, Electrochim. Acta 48 (2003) 2343–2353. [3] Y.B. Cohen, Electroactive polymers as artificial muscles–reality and challenges, in: The 42nd AIAA Structures, Structural Dynamics, and Materials Conference, Gossamer Spacecraft Forum, Seattle, WA, USA, April 16–19, 2001. [4] M. Shahinpoor, K.J. Kim, Ionic polymer–metal composites: I. Fundamentals, Smart Mater. Struct. 10 (2001) 819–833. [5] M.D. Bennett, D.J. Leo, Ionic liquids as stable solvents for ionic polymer transducers, Sens. Actuators A: Phys. 115 (2004) 79–90. [6] N. Fujiwara, K. Asaka, Y. Nishimura, K. Oguro, E. Torikai, Preparation of gold–solid polymer electrolyte composites as electric stimuli-responsive materials, Chem. Mater. 12 (2000) 1750–1754. [7] J.W. Paquette, K.J. Kim, D.Y. Kim, Low temperature characteristics of ionic polymer–metal composite actuators, Sens. Actuators A: Phys. 118 (2005) 135–143. [8] Y. B. Cohen, X. Bao, S. Sherrit, S. S. Lih, Characterization of the electromechanical properties of ionomeric polymer-metal composite (IPMC), Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2002, San Diego, CA, USA, March 18–21, 2002. [9] Y.B. Cohen, S. Leary, A. Yavrouian, K. Oguro, S. Tadokoro, J. Harrison, J. Smith, J. Su, Challenges to the transition of IPMC artificial muscle actuators to practical application, in: The MRS Symposium: FF: Electroactive Polymers, Boston, MA, USA, November 29–December 1, 1999. [10] S. Leary, Y.B. Cohen, Electrical impedance of ionic polymeric metal composites, in: The SPIE’s 6th Annual International Symposium on Smart Structures and Materials, Newport Beach, CA, USA, March 1–5, 1999. [11] M. Shahinpoor, Y.B. Cohen, T. Xue, J.O. Simpson, J. Smith, Ionic polymer–metal composites (IPMC) as biomimetic sensors and actuators, in: The SPIE’s 5th Annual International Symposium on Smart Structures and Materials, San Diego, CA, USA, March 1–5, 1998. [12] S.N. Nasser, Micromechanics of actuation of ionic polymer–metal composites, J. Appl. Phys. 92 (2002) 2899–2915. [13] S.N. Nasser, S. Zamani, Experimental study of Nafion- and Flemion-based ionic polymer metal composites (IPMCs) with ethylene glycol as solvent, in: Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2003, San Diego, CA, USA, March 2–6, 2003. [14] J.W. Lee, K.V. Nguyen, S.Y. Park, Y.T. Yoo, Electrolytic stability of various inner solutions in an ionic polymer metal composite, J. Kor. Phys. Soc. 48 (2006) 1594–1600. [15] C.S. Kothera, D.J. Leo, Characterization of the solvent-induced nonlinear response of ionic polymer actuators, in: Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2005, San Diego, CA, USA, March 6–10, 2005. [16] B.J. Akle, M.D. Bennett, D.J. Leo, High-strain ionomeric-ionic liquid electroactive actuators, Sens. Actuators A: Phys. 126 (2006) 173–181. [17] T. Fukushima, K. Asaka, A. Kosaka, T. Aida, Fully plastic actuator through layerby-layer casting with ionic-liquid-based bucky gel, Angew. Chem. Int. Ed. 44 (2005) 2410–2413.
[18] F. Vidal, C. Plesse, D. Teyssié, C. Chevrot, Long-life air working conducting semiIPN/ionic liquid based actuator, Synth. Met. 142 (2004) 287–291. [19] H.T. Lim, J.W. Lee, Y.T. Yoo, Actuation behavior of a carbon nanotube/NafionTM IPMC actuator containing an ionic liquid, J. Kor. Phys. Soc. 49 (2006) 1101–1106. [20] K.J. Kim, M. Shahinpoor, Ionic polymer–metal composites: II. Manufacturing techniques, Smart Mater. Struct. 12 (2003) 65–79. [21] P. Bonhôte, A.P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Hydrophobic, highly conductive ambient-temperature molten salts, Inorg. Chem. 35 (1996) 1168–1178. [22] H.L. Ngo, K. LeCompte, L. Hargens, A.B. McEwen, Thermal properties of imidazolium ionic liquids, Thermochim. Acta 357–358 (2000) 97–102. [23] S.H. Yeon, K.S. Kim, S.J. Choi, H. Lee, H.S. Kim, H.G. Kim, Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids, Electrochim. Acta 50 (2005) 5399–5407. [24] J.S. Wilkes, Properties of ionic liquid solvents for catalysis, J. Mol. Catal. A: Chem. 214 (2004) 11–17. [25] C.P. Fredlake, J.M. Crosthwaite, D.G. Hert, S.N.V.K. Aki, J.F. Brennecke, Thermophysical properties of imidazolium-based ionic liquids, J. Chem. Eng. Data 49 (2004) 954–964. [26] S.I. Hsiu, J.F. Huang, I.W. Sun, C.H. Yuan, J. Shiea, Lewis acidity dependency of the electrochemical window of zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquids, Electrochim. Acta 47 (2002) 4367–4372. [27] J. Kielland, Individual activity coefficients of ions in aqueous solutions, J. Am. Chem. Soc. 59 (1937) 1675–1678. [28] S. Tezuka, R. Chitrakar, A. Sonoda, K. Ooi, T. Tomida, Studies on selective adsorbents for oxo-anions. Nitrate ion-exchange properties of layered double hydroxides with different metal atoms, Green Chem. 6 (2004) 104–109. [29] P. Atkins, J. Paula, Physical Chemistry, 4th ed. (Korean ver.), Oxford University Press, Seoul, 2004, p. 1221. [30] J.D. Nam, H.R. Choi, Y.S. Tak, K.J. Kim, Novel electroactive, silicate nanocomposites prepared to be used as actuators and artificial muscles, Sens. Actuators A: Phys. 105 (2003) 83–90. [31] M. Yoshizawa, H. Ohno, Triple ion-type imidazolium salts: a new class of singleion conductive matrix, Ionics 8 (2002) 267–271. [32] W. Ogihara, J. Sun, M. Forsyth, D.R. MacFarlane, M. Yoshizawa, H. Ohno, Ionic conductivity of polymer gels deriving from alkali metal ionic liquids and negatively charged polyelectrolytes, Electrochim. Acta 49 (2004) 1797–1801. [33] V.K. Nguyen, J.W. Lee, Y.T. Yoo, Characteristics and performance of ionic polymer–metal composite actuators based on Nafion/layered silicate and Nafion/silica nanocomposites, Sens. Actuators B: Chem. 120 (2007) 529–537. [34] Y.S. Park, Y. Yamazaki, Low methanol permeable and high proton-conducting Nafion/calcium phosphate composite membrane for DMFC, Solid State Ionics 176 (2005) 1079–1089. [35] H. Ohno, Electrochemical Aspects of Ionic Liquids, 1st ed., John Wiley & Sons Inc. Publication, New Jersey, 2005, pp. 146–147. [36] K. Onishi, S. Sewa, K. Asaka, N. Fujiwara, K. Oguro, The effects of counter ions on characterization and performance of a solid polymer electrolyte actuator, Electrochim. Acta 46 (2001) 1233–1241. [37] S.D. Pandita, H.T. Lim, Y.T. Yoo, H.C. Park, Degradation mechanism of ionic polymer actuators containing ionic liquids as a mixed solvent, in: Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2006, San Diego, CA, USA, February 26–March 2, 2006.
Biographies Jang Woo Lee completed his MS in materials chemistry and engineering at Konkuk University, Korea in 2006, and his BS in industrial chemistry at Konkuk University in 2000. He graduated with a bachelor’s degree in industrial chemistry from Konkuk University, joined Pharmacia & Upjohn and worked from 2000 to 2002, and joined Hanmi Fine Chemical as an antibiotic synthesizer from 2002 to 2004. He entered the graduate school at Konkuk University in the fall of 2004 and is currently in a doctor’s course in the Department of Materials Chemistry and Engineering at Konkuk University. He is involved with the Artificial Muscle Research Center at the university as an assistant researcher. Young Tai Yoo graduated from the Virginia Polytechnic Institute and State University in 1988, receiving his PhD in chemical engineering. He joined Eastman Kodak and worked as a research scientist from 1988 to 1991. He then joined the Department of Materials Chemistry and Science at Konkuk University. His research interests include bioactive polymers, drug delivery systems, and actuators.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Anion effects in imidazolium ionic liquids on the performance of IPMCs Jang-Woo Lee a,b , Young-Tai Yoo a,b,∗ a b
Artificial Muscle Research Center, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea Department of Materials Chemistry and Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 August 2008 Received in revised form 16 January 2009 Accepted 22 January 2009 Available online 31 January 2009 Keywords: Ionic polymer–metal composite Actuators Ionic liquids Nafion
a b s t r a c t Four ionic liquids were explored for as inner solvents of IPMC to overcome the shortcomings of water, especially its high volatility and low electrolysis potential. The imidazolium salts were composed of 1-ethyl-3-methylimidazolium [EMIm] cation and anions including bromide [Br], nitrate [NO3 ], acetate [AcO], and trifluoroacetate [TA]. The 1 H NMR studies confirmed the structures of the four ionic liquids and indicated that [EMIm][AcO] and [EMIm][TA] had stronger interactions between anion and C(2) hydrogen in imidazolium cation compared with [EMIm][NO3 ] and [EMIm][Br]. [EMIm][Br] and [EMIm][NO3 ] exhibited enormously low volatility with initial weight loss above ca. 290 ◦ C. Two-probe linear sweep voltammetry (LSV) revealed that the electrolytic stability decreased in the order of [EMIm][TA] > [EMIm][AcO] > [EMIm][NO3 ] ∼ [EMIm][Br], whereas current density in IPMCs was in the reverse order. Thus, [EMIm][Br]- and [EMIm][NO3 ]-based IPMCs exhibited a superior performance in terms of tip displacement and response speed compared to [EMIm][AcO]- and [EMIm][TA]-based ones. These results corresponded with the size and ionic mobility of anions in imidazolium salts. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A typical IPMC is constructed with an ionic polymer membrane and two platinum electrode layers. A polymeric membrane contains mobile metal cations and water as an inner solvent. Under an electric potential of 1–3 V, hydrated metal cations in the ionic membrane are attracted toward the negatively charged electrode, which subsequently triggers the electro-osmotic flow of the inner solvent. As a result of the mass transport from one side to another, volume difference takes place between the two sides of the IPMC strip, resulting in a bending deformation of the composite film [1,2]. The IPMCs can be considered as viable actuators, producing a large bending deformation, a fast response, and a low operational voltage; however, their practical application is limited due to major drawbacks. Some of these include a back relaxation under dc potentials, a relatively low actuation force, a low reproducibility, and, most importantly, a rapid loss of water through natural evaporation and electrolysis at over 1.23 V [3]. In particular, the loss of water causes a reduced flux of metal cations and inner solvent and damage to the electrode. Abundant studies in the literature have discussed these aspects of IPMC [4–14] and many attempts have been made to replace water, the conventional inner solvent in IPMC, with less
∗ Corresponding author at: Artificial Muscle Research Center, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea. Tel.: +82 2 450 3207 fax: +82 2 444 0711. E-mail address: [email protected] (Y.-T. Yoo). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.01.041
volatile and stable organic compounds. Nasser used mixtures of ethylene glycol and glycerol as inner solvents for IPMCs [12,13]. However, the use of organic solvents alone resulted in a limited and sluggish actuation performance. Lee et al. reported that mixtures of polar organic solvents and deuterated water could improve the electrolytic stability [14]. Recently, ionic liquids have been recognized as alternatives to some conventional organic solvents as they exhibited low vapor pressure, a wide electrochemical window, high electrical conductivity, and low viscosity [5,15–19]. Bennett and Leo chose 1-ethyl-3methylimidazolium trifluoromethanesulfonate ([EMIm][TfO]) and 1-ethyl-3-methylimidazolium bromide ([EMIm][Br]) ionic liquids as inner solvents of IPMC actuators. They observed enormously improved stabilities but slow responses with the IPMCs containing the ionic liquids compared to water-based IPMCs [5]. Kothera and Leo reported the linearity of the voltage-to-current relationship of IPMCs with [EMIm][TfO] ionic liquid [15]. Akle et al. found that the charge accumulation at the polymer–electrode interface of IPMCs with ionic liquids is the key to producing high strain in ionic polymer transducers [16]. Fukushima et al. reported the bucky-gel-based bimorph actuator, having an electrolyte consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF(HFP)) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4 ]), with two electrodes consisting of single-walled carbon nanotubes (SWNTs), [BMIm][BF4 ], and PVdF(HFP) [17]. Vidal et al. presented a conducting semi-IPN/ionic liquid-based actuator, and Lim et al. a multi-walled carbon nanotube (MWNT)/ionic liquid-based IPMC [18,19].
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Although many have reported on ionic liquid-based actuators, very few have discussed the effects of structural factors of ionic liquids, particularly the anions, affecting the properties of ionic liquids and the performance of IPMCs containing them. This paper will focus on the thermal and electrochemical properties of imidazolium-based ionic liquids with various anions of Br− , NO3 − , AcO− , and TA− , and the IPMCs impregnated with these ionic liquids. The effect of the anions in the ionic liquids on the performance of IPMCs will also be investigated. 2. Experimental
2.5. Actuation test An IPMC strip for the bending performance was cut to the size of 0.5 cm × 2.5 cm. The strip was supported vertically in air and fixed to 0.5 cm length on both sides; thus, the actual effective length of the samples was 2 cm. Electric power was applied with an ac source/analyzer (Model 6811B, Agilent). The displacement measurement was performed using a laser beam device (LB-11, Keyence) and a video camera which were connected to a computer by a data acquisition (DAQ) system (SCB-68, National Instrument). The deformation response was measured under a square wave input.
2.1. Preparation of IPMC IPMCs containing lithium ions (Li+ ) were produced from NafionTM -117 membrane, and prepared via a well-known electroless plating method of platinum [20]. 2.2. Synthesis and characterization of ionic liquids Four imidazolium salts including [EMIm][Br], [EMIm][NO3 ], [EMIm][AcO], and [EMIm][TA] were prepared via metathesis reactions [21–23] and dried before use. Scheme 1 illustrates the metathesis reaction procedure for the synthesis of ionic liquids. 1 H NMR (Spectrospin400 UltraShieldTM , Bruker) and FT-IR (Genesis II FTIR Spectrometer, Mattson) spectroscopy were used to identify the chemical structure of the ionic liquids prepared in this study. Thermal analysis was performed using a differential scanning calorimeter (DSC 2010, TA Instruments) and thermogravimetric analysis (TGA 2050, TA Instruments) with a heating rate of 10 ◦ C min−1 under nitrogen atmosphere. The electrolytic stability of each ionic liquid was estimated using a source meter (Model 2400, Keithley). X-ray diffraction (D/Max-2200, Rigaku Denki) patterns were obtained under 40 kV and 30 mA of Cu K␣ radiation with a characteristic wavelength of 0.154 nm, in a 2 range of 1–80◦ with a scanning rate of 4◦ min−1 in order to identify the size distribution of ionic clusters in the Li-NafionTM matrices containing the ionic liquids. 2.3. Preparation of ionic liquid-based IPMCs In order to impregnate the IPMCs with the ionic liquids, each IPMC strip was immersed in a mixture of methanol (50 wt.%) and a dry ionic liquid at 70 ◦ C for 2 h. Then, the swollen IPMC samples were dried to remove methanol. 2.4. Characterization of ionic liquid-based IPMCs Electrical properties of the IPMC samples were measured with a complex impedance analyzer (IM6ex, Zahner) and a multichannel potentiostat/galvanostat (WMPG1000, Wonatech).
3. Results and discussion 3.1. Properties of 1,3-dialkylimidazolium salt ionic liquids Ionic liquids exhibit a relatively high conductivity, a wide window of electrochemical stability, and a low viscosity as well as an extremely low volatility [22,24]. Especially, the high electrochemical stability and low volatility of the inner solvent are essential to the stable operation of the IPMC in a dry environment. 1,3-Dialkylimidazolium salts were chosen since they met many important criteria of inner solvents in IPMC. In Fig. 1, proton NMR spectra are shown with the assignments of the protons in the 1-ethyl-3-methylimidazolium salts. The structure of the four imidazolium salts was confirmed by matching the integrated area of each peak with the number of the corresponding hydrogen. The four 1-ethyl-3-methylimidazolium salts have different anions with varying degrees of basicity. In imidazolium salts, the basic anions strongly associated with the electron deficient C(2) hydrogens via hydrogen bonds. Bonhôte et al. reported that degree of hydrogen bonding could be compared by the chemical shift of the C(2) hydrogen in the 1 H NMR spectrum of ionic liquid as an indicator of the electron donating capability of the anions [21]. According to the chemical shift of the hydrogen shown in Fig. 1(b) using [EMIm][Br] as a reference, the degree of hydrogen bonding, i.e., the basicity of the anions, decreases in the order of [EMIm][AcO] > [EMIm][TA] ∼ [EMIm][NO3 ] > [EMIm][Br]. Rapid loss of water in IPMC occurs primarily from its high volatility. Fig. 2 shows the TGA measurements of weight loss vs. temperature for the 1-ethyl-3-methylimidazolium-based ionic liquids. In particular, [EMIm][Br] and [EMIm][NO3 ] exhibited higher thermal stability with the initial weight loss above ca. 290 ◦ C compared to [EMIm][AcO] and [EMIm][TA]. Ngo et al. reported that the substitution of the abstractable acidic C(2) hydrogen of imidazolium cations with alkyl groups increases the thermal stability of imidazolium salts [22]. Fredlake et al. [25] postulated that imidazolium ionic liquids with anions of lower basicity possess higher thermal stabilities. These studies correspond with the observation that [EMIm][Br] and [EMIm][NO3 ] with anions less basic
Scheme 1. Metathesis reaction of imidazolium salt type ionic liquid.
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Fig. 3. DSC curves of [EMIm][Br] ionic liquid in bulk and IPMC. An IPMC sample containing the ionic liquid of 21 wt.% was used.
Fig. 1. (a) 1 H NMR peaks of ionic liquids and (b) chemical shift of C(2) protons (peak e) of ionic liquids on zero basis of [EMIm][Br]. Deuterated chloroform (CDCl3 ) was used as a 1 H NMR solvent.
and thus less capable of abstracting C(2) hydrogen from imidazolium cation exhibited the stability higher than the [EMIm][AcO] and [EMIm][TA] ionic liquids. Initial weight loss over 100–140 ◦ C was attributed to the evaporation of absorbed water. A relatively larger weight loss of [EMIm][AcO] was attributed to the extremely hygroscopic nature of this compound. It was found that dried
Fig. 2. Thermal stability of 1-ethyl-3-methylimidazolium-based ionic liquids. The ionic liquids were dried before the test.
[EMIm][AcO] absorbed moisture of up to 46% of its weight within an hour. Among the four 1,3-dialkylimidazolium salts, [EMIm][Br] shows a melting phase transition above room temperature, m.p. of 75 ◦ C. Thermograms of [EMIm][Br] run in neat and in IPMC (21 wt.% in IPMC) were compared in Fig. 3. Crystallization and melting phase transition of [EMIm][Br] were not observed when it was impregnated in IPMC. The structure of the ionic liquid is viewed as a loose ionic interaction between anions and cations with delocalized charge. It is expected that the anions and cations of ionic liquid are easily associated with the respective counter ions in NafionTM , retarding the crystallization. Another important feature of ionic liquids was characterized by their electrolytic stability and wider electrochemical window. Under an electrical potential, the reduction of EMIm+ ion occurs at the cathode side and the oxidation of anion at the anode side [23,26]. Two-probe linear sweep voltammetry (LSV) was used to assess the electrolytic stability of the imidazolium type ionic liquids with various anions during the operation. As shown in Fig. 4, the electrolytic onset voltage of the ionic liquids, which is the starting voltage of the oxidation of each anion at the anode, inter-correlated to the reduction of EMIm+ at the cathode, decreasing in the order of [EMIm][TA] > [EMIm][AcO] > [EMIm][NO3 ] ∼ [EMIm][Br]. These results indicated that the electrolytic stability of the imidazolium ionic liquids varied with the stability of the anion; the electrolytic
Fig. 4. Electrolytic stability of 1-ethyl-3-methylimidazolium-based ionic liquids. The ionic liquids were dried before the test, and the data were recorded between the tips of a pair of gold probes (diameter = 1 mm) immersed in the ionic liquid, with a scan rate of 100 mV s−1 .
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Table 1 Properties and characteristics of 1,3-dialkylimidazolium salts. Species [EMIm][Br] [EMIm][NO3 ] [EMIm][AcO] [EMIm][TA]
Radius of Hydrated Anion (nm) a
0.30 0.30a (0.33b ) 0.45a nad , >0.45
Anionic Mobility (in water) (10−8 m2 s−1 V−1 ) c
8.09 7.40c 4.24c nad , <4.24
Electrolytic Onset Voltage (V)
Decomposition Temperature (◦ C)
3.0 3.1 3.6 4.1
299.3 287.8 216.1 171.8
a
From Ref. [26]. From Ref. [27]. c From Ref. [28]. d No data are reported but it is expected that with a larger radius of hydrated anion of CF3 COO− than that of CH3 COO− , the ionic mobility of CF3 COO− in water is lower than that of CH3 COO− due to the fact that the electro-negativity of fluorine is higher than hydrogen. b
onset voltage decreases with the increasing oxidation tendency of the anion at the anode, combining the reduction capability of EMIm+ ion at the cathode. The highest electrochemical stability of [EMIm][TA] was attributed to the high thermodynamic formationstability of the CF3 COO− ion due to the strong electron-withdrawing influence of fluorine. The electrochemical decomposition occurs at the interface between electrolytes and contact electrodes, where an electrical double layer is formed. Thus, high mobility of Br− and NO3 − ions could have contributed to their rapid electrochemical decomposition. Table 1 summarizes the properties and characteristics of each ionic liquid [27–29]. A proper level of solvent uptake is required to exhibit a high ionic conductivity in IPMC, although an excessive uptake adversely affects the bending performance due to the loss of mechanical strength. In preparation, each IPMC strip was immersed in a mixture of methanol and an ionic liquid (50/50 wt.%) at 70 ◦ C for 2 h. Then, the IPMC was dried to remove volatile components. Fig. 5 shows that ionic liquids exhibited uptake levels similar to those of water (ca. 20 wt.% in this study) except [EMIm][AcO]. A relatively lower uptake of [EMIm][AcO] immediately after drying was due to the loss of absorbed water in the ionic liquid as observed from TGA experiments.
ters in a dry NafionTM membrane exhibited the strongest intensity at 1.10 nm, and those in the NafionTM membrane impregnated with water displayed at 1.21 nm, indicating swelling of the ionic clusters. In Fig. 6(a), the size of ionic clusters at the maximum distribution is indicated with an arrow. In the case of the NafionTM membranes containing the ionic liquids, the size of ionic clusters at the maximum distribution was slightly smaller than those in the NafionTM membranes with water, and decreased as the hydrophobicity of the ionic liquid increased except the extremely hygroscopic [EMIm][AcO]. It was expected that a part of the relatively hydrophobic ionic liquids could be distributed in the domain other than ionic clusters in the polymer. The NafionTM membrane impregnated with [EMIm][Br] ionic liquid registered the highest intensity of the ionic cluster peak with respect to the peak of the amorphous halo at 17.0◦ as shown in Fig. 6(b). This was an indicative of well-defined phase separation between the ionic clusters and the main chain region. The development of
3.2. Morphology of NafionTM membrane with ionic liquids Fig. 6 shows the X-ray diffraction (XRD) patterns of Li-NafionTM membranes containing ionic liquids. Inter-planar spacing (d value) was calculated with Bragg’s equation (2d sin = ). As reported in the literature [30], the XRD peaks in the range of 2 value of 12 ∼ 3◦ (corresponding to the d value range of 0.74 ∼ 2.94 nm) represented the size distribution of ionic clusters in the Li-NafionTM membranes containing ionic liquids, and the peaks around 17.0◦ were related to the amorphous halo of NafionTM . The ionic clus-
Fig. 5. Uptake of 1-ethyl-3-methylimidazolium-based ionic liquids in IPMC. The non-patterned area indicates the uptake level immediately after full drying of the samples. Consecutively, the IPMC samples were exposed to air for 2 h and the amount of water absorbed from the air was represented by the patterned area.
Fig. 6. X-ray diffraction (XRD) patterns of Li-NafionTM membranes with ionic liquids: (a) small- and (b) wide-ranged patterns.
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the ionic channel should contribute to actuator performance based on the ionic transport mechanism. 3.3. Electrical properties of IPMCs with ionic liquids Ion transport in ionic liquid-based IPMCs was expected to be more complicated than that in water-based IPMCs due to the nature of the ionic liquid. Fukushima et al. [17] proposed that when an electric potential was applied to the bucky-gel-based bimorph actuator, cations and anions of the built-in ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate; BMIm-BF4 ) were transported to the negatively and positively charged electrode layers, respectively, which form electric double layers on both electrode sides. The ionic transport resulted in an expansion of the negatively charged side as the BMIm+ ion is larger than the BF4 − ion, giving a bending deformation to the bimorph actuator. The ionic liquid-Li-IPMCs contained three mobile ionic species. Thus, possible ion transports in the ionic liquid-Li-IPMCs under an electrical potential included (i) the individual movement of lithium ions and dissociated ions of the ionic liquid as in the bucky-gelbased actuator and (ii) an electro-osmotic drag of lithium cations bound with associated ionic liquid molecules similar to conventional IPMCs. Ohno and co-workers claimed that the lithium ion, the smallest ionic source in the triple ion-type imidazolium salts, in a system similar to the ionic liquid-Li-IPMC, might be a major conducting ion in the salt [31,32]. The molecular migration of ionic liquids was also evidenced by the sweating phenomena of the ionic liquids on the negatively charged electrode of the ionic liquid-LiIPMC. Cyclic votammogram provided important information on the performance of IPMC. In the cyclic voltammetry, higher current density implies a larger ionic movement as well as a higher capacitance, which usually produced a larger bending deformation of the IPMC strip as reported in our previous report [33]. In Fig. 7, the current density of the IPMCs loaded with the different ionic liquids followed the order of [EMIm][NO3 ] > [EMIm][Br] > [EMIm][AcO] > [EMIm][TA]. Fig. 8(a) shows the frequency-dependent real impedance of the ionic liquid-based IPMCs. At a frequency of over 2 MHz, impedances of all the IPMCs were almost the same. However, the IPMCs with [EMIm][NO3 ] and [EMIm][Br] displayed lower impedances at lower frequencies compared to those with [EMIm][AcO] and [EMIm][TA], which was a tendency of materials possessing high
Fig. 8. (a) Frequency-dependent real impedance and (b) Nyquist plot of ionic liquidbased IPMCs measured between the two Pt surfaces of the IPMC.
capacitances of double layers [16]. In Fig. 8(b), the electrical resistances of IPMCs with [EMIm][Br], [EMIm][NO3 ], [EMIm][AcO], and [EMIm][TA], deduced from the intercept of the impedance plot to the abscissa real impedance axis (zero capacitance), were 684, 504, 1102, and 1129 [34]. The results from the cyclic voltammetry and electrochemical impedance spectroscopy predicted that the IPMCs with [EMIm][NO3 ] and [EMIm][Br] were superior to those with [EMIm][AcO] and [EMIm][TA] in actuation performance. 3.4. Actuation performance of IPMCs with ionic liquids
Fig. 7. Cyclic I–V curves of ionic liquid-based IPMCs. The test was performed by connecting the counter electrode (CE) and the reference electrode to one side of the IPMC and by connecting the working electrode (WE) to the other side. The curves were obtained after 10 cycles of a triangle voltage input of ±3 V with a scan rate of 100 mV s−1 .
Tip displacement of the ionic liquid-based IPMCs is exhibited in Fig. 9. The tip displacement may be the outcome of the combination of multiple factors. The size of anionic species could possibly be one of them. The IPMC containing [EMIm][NO3 ] measured the highest current density and lowest impedance. However, the [EMIm][Br]-based IPMC produced a slightly larger deformation than those with [EMIm][NO3 ]. This may be explained by the fact that Br− ion is smaller than NO3 − ion and thus the volume difference between anionic and cationic sides of the membrane becomes greater for [EMIm][Br]-based IPMC when imidazolium cations and pair anions migrate to opposite directions. Another factor contributing to the larger tip displacement of [EMIm][Br]-based IPMC may be the stronger association of Br− ions with water molecules [35], which allows a larger volume displacement under an electric potential. Our experimental results confirmed that [EMIm][Br] ionic liquid absorbed more water from the air than [EMIm][NO3 ]; the equilibrium water contents of [EMIm][Br] and [EMIm][NO3 ] in air were 14–17 and 10–14 wt.%, respectively. At 4 V, both IPMCs loaded with [EMIm][Br] and [EMIm][NO3 ] exhibited more extended gaps in tip displacement and faster response speed than those
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Fig. 9. Bending performance of ionic liquid-based IPMCs under (a) 3 V and (b) 4 V square waves at a frequency of 0.1 Hz.
with [EMIm][AcO] or [EMIm][TA], as depicted in Fig. 9(b). Fig. 10 also shows that the response currents diverged into two groups. The larger tip displacement and higher current of the IPMCs with [EMIm][Br] or [EMIm][NO3 ] all pointed to an abrupt increase in ionic mobility at a potential over 3 V. The charge density (mC cm−2 ) consumed in the IPMCs for 5 s under constant electric voltages of 3 and 4 V was calculated by integrating the area of the curve between 0 and 5 s in Fig. 10. The charge-specific displacement (mm/(mC cm−2 )), the concept of which was proposed by Onishi et al. [36] and which represents the energy efficiency of the displacement of actuators, was acquired by dividing the tip displacement for 5 s by the charge density consumed during that time. Table 2 also lists the charge-specific displacement at 3 and 4 V. By definition, the charge-specific displacement is the tip displacement of IPMC per unit charge density. The IPMCs loaded with [EMIm][Br] and [EMIm][NO3 ] yielded the values of 0.10–0.15 mm/(mC cm−2 ) at both 3 and 4 V, implying that tip displacement is linearly increasing with the charge density over the applied potentials. However, the IPMCs with [EMIm][AcO] and [EMIm][TA] recorded higher values of 0.21 and 0.24 mm/(mC cm−2 )
Fig. 10. Response current (for initial 5 s) under actuation of ionic liquid-based IPMCs: (a) 3 V and (b) 4 V square waves of 0.1 Hz.
at 3 V and 0.12–0.14 mm/(mC cm−2 ) at 4 V, respectively. The higher values of [EMIm][AcO]- and [EMIm][TA]-based IPMCs at 3 V are attributable to their lower charge densities compared to those of [EMIm][Br]- and [EMIm][NO3 ]-based IPMCs. It is because the migration of lithium cations is the only source of charge density in [EMIm][AcO]- and [EMIm][TA]-based IPMCs at 3 V. In the case of [EMIm][Br]- and [EMIm][NO3 ]-based IPMCs, migration of the dissociated ionic species of the ionic liquids also contributes to the charge density at 3 V. When applied electric potential was raised from 3 to 4 V, the charge density of the [EMIm][AcO]- and [EMIm][TA]-based IPMCs showed abrupt increases as these ionic liquids begin to dissociate at 3.6–4.0 V. The tip displacements of the [EMIm][AcO]- and [EMIm][TA]-IPMCs increased by mere 10–20%, while the charge densities were doubled over the electric potential of 3–4 V. A smaller size difference of cationic and anionic species of [EMIm][AcO] and [EMIm][TA] could be one of the reasons for less effective tip displacement improvement. Fig. 11 shows the linear sweep voltammogram (LSV) of the ionic liquid-based IPMCs. The electrical onset voltages measured on the
Table 2 Charge consumption and specific-charge displacement of ionic liquid-based IPMCs. Ionic Liquids
[EMIm][Br] [EMIm][NO3 ] [EMIm][AcO] [EMIm][TA]
Charge Density (mC cm−2 )
Tip Displacement for initial 5 s (mm)
Charge-Specific Displacement (mm/(mC cm−2 ))
3V
4V
3V
4V
3V
4V
14.97 18.48 8.43 6.64
24.49 27.97 15.51 14.72
2.27 1.83 1.75 1.59
3.27 3.24 1.92 2.02
0.15 0.10 0.21 0.24
0.13 0.12 0.12 0.14
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Fig. 11. Linear sweep voltammogram (LSV) of ionic liquid-based IPMCs. The test was performed by connecting the counter electrode (CE) and the reference electrode to one side of the IPMC and by connecting the working electrode (WE) to the other side. The curves were obtained scanning from 0 to 5 V with a scan rate of 100 mV s−1 .
IPMCs with [EMIm][Br] and [EMIm][NO3 ] were similar to those of the respective neat ionic liquids. And the IPMCs with [EMIm][AcO] and [EMIm][TA] showed no distinct onset voltages up to 4 V. The electrical onset voltages could be defined as the points of abrupt increase in current which reflect the increase of both electrolysis and ion transport in IPMC. Therefore, the high current beyond the electrical onset voltage resulted in a large deformation of IPMC at the expense of the inner solvent. A large difference in bending performance between the two groups of IPMCs shown in Fig. 9(b) corresponded well with their difference in current level at 4 V in Fig. 11. 3.5. Attenuation of displacement under dc potential Fig. 12(a) compares the normalized displacement vs. time for the ionic liquid-based IPMCs under a potential of 2.5 V. In addition to the back diffusion effect, a part of the performance decay was caused by the loss of inner solvent under a given electric field. It was found that the IPMCs containing [EMIm][Br] and [EMIm][NO3 ], especially in the case of that with [EMIm][Br], sustained its deformation more effectively than those with [EMIm][AcO] and [EMIm][TA]. Bennett and Leo reported that in the case of ionic liquid-based IPMCs, the attenuation of bending performance could be attributed to the decomposition of impurities in the ionic liquid and water absorbed from the air by means of the hygroscopic nature of NafionTM membrane and the ionic liquids [5]. In addition, the ‘sweating phenomenon’, a cross-over of the inner solvent, occurred most rapidly in the IPMC containing the most hydrophobic [EMIm][TA] in this study. For instance, the accumulation of ionic liquid at the negative-electrode surface was observed after less than 1 h for [EMIm][TA] under a constant bending deformation. However, [EMIm][Br] was observed from 3 h to a very limited extent. This should have caused a considerable loss of the inner solvent by squeezing out the IPMC (see Fig. 12(c)) [37]. Again, this is the reason why the IPMC containing [EMIm][TA] under 2.5 V has the actuation time shorter than that containing a more hydrophilic [EMIm][Br] ionic liquid. It was concluded that this sweating phenomenon was more distinct with the ionic liquids of hydrophobic nature. Fig. 12(b) plotted the attenuation of displacement of the ionic liquid-based IPMCs at 3.5 V, which exceeded the level of electrolytic onset voltage of [EMIm][Br] or [EMIm][NO3 ], but not for [EMIm][AcO] or [EMIm][TA]. IPMCs with [EMIm][Br] or [EMIm][NO3 ] displayed larger initial tip displacements but their tip displacements rapidly diminished from the significant loss of solvents by electrolysis at this voltage. The durability compared at 3.5 V corresponded with
Fig. 12. Durability test of ionic liquid-based IPMCs at (a) 2.5 V and (b) 3.5 V square waves of 0.1 Hz. (c) Negatively charged-electrode surfaces of the IPMCs with [EMIm][Br] and [EMIm][TA] ionic liquids after actuation for 1 h. D is the tip displacement and Do the initial tip displacement.
the electrolytic stability of the ionic liquids, following the order of [EMIm][TA] > [EMIm][AcO] > [EMIm][NO3 ] > [EMIm][Br]. 4. Conclusions Imidazolium salts, [EMIm][Br], [EMIm][NO3 ], [EMIm][AcO], and [EMIm][TA], were successfully synthesized and tested to ascertain their thermal and electrical properties as inner solvents for IPMC. The performance of the ionic liquid-based IPMCs depended mainly on the size and ionic mobility of the anion in the ionic liquids, following the order of [EMIm][Br] > [EMIm] [NO3 ] > [EMIm][AcO] > [EMIm][TA]. [EMIm][Br] and [EMIm][NO3 ] exhibited enormously high thermal stability with initial weight loss
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above ca. 290 ◦ C. Solvent uptake of these ionic liquids was found to be comparable to that of water. From the LSV data, the electrical onset voltage may result from the abrupt increase of ionic transport and the electrolysis of ionic liquid, and the current at over the onset voltage may be the sum of the electrolysis and ionic transport in the IPMC. In addition, the attenuation of the performance of the ionic liquid-based IPMC proceeded either from the sweating phenomenon or from the electrolysis of the ionic liquid depending on the applied electric potentials and the type of ionic liquids. Acknowledgements This work was supported by a Korea Research Foundation grant (KRF-2006-005-J03302). The authors would also like to acknowledge the financial support of Konkuk University. References [1] S.N. Nasser, J.Y. Li, Electromechanical response of ionic polymer–metal composites, J. Appl. Phys. 87 (2000) 3321–3331. [2] M. Shahinpoor, Ionic polymer–conductor composites as biomimetic sensors, robotic actuators and artificial muscles—a review, Electrochim. Acta 48 (2003) 2343–2353. [3] Y.B. Cohen, Electroactive polymers as artificial muscles–reality and challenges, in: The 42nd AIAA Structures, Structural Dynamics, and Materials Conference, Gossamer Spacecraft Forum, Seattle, WA, USA, April 16–19, 2001. [4] M. Shahinpoor, K.J. Kim, Ionic polymer–metal composites: I. Fundamentals, Smart Mater. Struct. 10 (2001) 819–833. [5] M.D. Bennett, D.J. Leo, Ionic liquids as stable solvents for ionic polymer transducers, Sens. Actuators A: Phys. 115 (2004) 79–90. [6] N. Fujiwara, K. Asaka, Y. Nishimura, K. Oguro, E. Torikai, Preparation of gold–solid polymer electrolyte composites as electric stimuli-responsive materials, Chem. Mater. 12 (2000) 1750–1754. [7] J.W. Paquette, K.J. Kim, D.Y. Kim, Low temperature characteristics of ionic polymer–metal composite actuators, Sens. Actuators A: Phys. 118 (2005) 135–143. [8] Y. B. Cohen, X. Bao, S. Sherrit, S. S. Lih, Characterization of the electromechanical properties of ionomeric polymer-metal composite (IPMC), Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2002, San Diego, CA, USA, March 18–21, 2002. [9] Y.B. Cohen, S. Leary, A. Yavrouian, K. Oguro, S. Tadokoro, J. Harrison, J. Smith, J. Su, Challenges to the transition of IPMC artificial muscle actuators to practical application, in: The MRS Symposium: FF: Electroactive Polymers, Boston, MA, USA, November 29–December 1, 1999. [10] S. Leary, Y.B. Cohen, Electrical impedance of ionic polymeric metal composites, in: The SPIE’s 6th Annual International Symposium on Smart Structures and Materials, Newport Beach, CA, USA, March 1–5, 1999. [11] M. Shahinpoor, Y.B. Cohen, T. Xue, J.O. Simpson, J. Smith, Ionic polymer–metal composites (IPMC) as biomimetic sensors and actuators, in: The SPIE’s 5th Annual International Symposium on Smart Structures and Materials, San Diego, CA, USA, March 1–5, 1998. [12] S.N. Nasser, Micromechanics of actuation of ionic polymer–metal composites, J. Appl. Phys. 92 (2002) 2899–2915. [13] S.N. Nasser, S. Zamani, Experimental study of Nafion- and Flemion-based ionic polymer metal composites (IPMCs) with ethylene glycol as solvent, in: Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2003, San Diego, CA, USA, March 2–6, 2003. [14] J.W. Lee, K.V. Nguyen, S.Y. Park, Y.T. Yoo, Electrolytic stability of various inner solutions in an ionic polymer metal composite, J. Kor. Phys. Soc. 48 (2006) 1594–1600. [15] C.S. Kothera, D.J. Leo, Characterization of the solvent-induced nonlinear response of ionic polymer actuators, in: Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials 2005, San Diego, CA, USA, March 6–10, 2005. [16] B.J. Akle, M.D. Bennett, D.J. Leo, High-strain ionomeric-ionic liquid electroactive actuators, Sens. Actuators A: Phys. 126 (2006) 173–181. [17] T. Fukushima, K. Asaka, A. Kosaka, T. Aida, Fully plastic actuator through layerby-layer casting with ionic-liquid-based bucky gel, Angew. Chem. Int. Ed. 44 (2005) 2410–2413.
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Biographies Jang Woo Lee completed his MS in materials chemistry and engineering at Konkuk University, Korea in 2006, and his BS in industrial chemistry at Konkuk University in 2000. He graduated with a bachelor’s degree in industrial chemistry from Konkuk University, joined Pharmacia & Upjohn and worked from 2000 to 2002, and joined Hanmi Fine Chemical as an antibiotic synthesizer from 2002 to 2004. He entered the graduate school at Konkuk University in the fall of 2004 and is currently in a doctor’s course in the Department of Materials Chemistry and Engineering at Konkuk University. He is involved with the Artificial Muscle Research Center at the university as an assistant researcher. Young Tai Yoo graduated from the Virginia Polytechnic Institute and State University in 1988, receiving his PhD in chemical engineering. He joined Eastman Kodak and worked as a research scientist from 1988 to 1991. He then joined the Department of Materials Chemistry and Science at Konkuk University. His research interests include bioactive polymers, drug delivery systems, and actuators.