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High-Performance PEDOT:PSS/Single-Walled Carbon Nanotube/Ionic liquid Actuators Combining Electrostatic Double-Layer and Faradaic Capacitors Naohiro Terasawa, and Kinji Asaka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01148 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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High-Performance PEDOT:PSS/Single-Walled Carbon Nanotube/Ionic liquid Actuators Combining Electrostatic Double-Layer and Faradaic Capacitors Naohiro Terasawa*and Kinji Asaka Inorganic Functional Material Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan. KEYWORDS

High

performance,

Poly(3,4-ethylenedioxythiophene)

(PEDOT),

actuator,

electrostatic double-layer, faradaic, hybrid type ABSTRACT: This paper presents the development of new hybrid-type poly(3,4ethylenedioxythiophene) (PEDOT) actuators produced by the film-casting method, in which both electrostatic double-layer (EDLC) and faradaic capacitors (FCs) occur simultaneously. The

electrochemical

and

electromechanical

properties

of

PEDOT:poly(4-

styrenesulfonate)(PSS), PEDOT:PSS/ionic liquid (IL), and PEDOT:PSS/single-walled carbon nanotubes (SWCNTs)/IL actuators are compared with those of a conventional poly(vinylidene fluoride)-co-hexafluoropropylene (PVdF(HFP))/SWCNT/IL actuator. It is found that the PEDOT:PSS/SWCNT/IL actuator provides a better actuation strain performance than a conventional (PVdF(HFP))/SWCNT/IL actuator, as its electrode is an electrochemical capacitor (EC) composed of an EDLC and FC. The PEDOT:PSS polymer

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helps produce a high specific capacitance, actuation strain and maximum generated stress that surpass the performance of a conventional PVdF(HFP) actuator. The flexible and robust films created by the synergistic combination of PEDOT and SWCNT may therefore have significant potential as actuator materials for wearable energy-conversion devices. A doublelayer charging kinetic model was successfully used to simulate the frequency dependence of the displacement responses of the PEDOT:PSS/IL and PEDOT:PSS/SWCNT/IL actuators. Introduction There are numerous advantages to conductive polymers (CPs), since these materials are relatively

environmentally-friendly

and

inexpensive,

exhibit

good

storage

capacity/porosity/reversibility, wide voltage windows and good conductivity when doped, and can be chemically modified to tune their redox activity.1–5 For all these reasons, CPs are thought to have promising applications in electrochemical capacitors (ECs). The redox reactions of CPs, during which the main polymer chain either takes on ions from the electrolyte (representing oxidation) or releases ions (reduction), can also allow these polymers to generate capacitance. Rather than occurring solely at the polymer surface, both these processes can also proceed in the interior of the CP.6 These reactions are almost completely reversible because the CP does not undergo any associated phase transitions or other structural changes.7 In terms of stability, conductivity and processability, the polythiophene-derived macromolecular species poly(3,4-ethylenedioxythiophene) (PEDOT) may be the best of the currently-available CPs.8 As a result, antistatic coatings, organic field-effect transistors, solid electrolyte capacitors, organic solar cells and light-emitting diodes have all been fabricated with this material, which can be obtained in large quantities from commercial sources.8 As well, aqueous colloidal dispersions of this polymer can be readily generated upon the addition


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of poly(4-styrenesulfonate) (PSS) to form the doped compound PEDOT:PSS. The conductivity of this doped version of the polymer can be adjusted between 0.1 and 3000 S/cm. This material is also thermally stable and displays good mechanical properties,9-10 and therefore has been applied to a variety of optical and electronic devices.11–14 Recently, PEDOT15,16 was explored as a candidate material for ECs because of its high stability, with electrochemical deposition succeeding in producing a highly porous amorphous polymer film on a gold plate. In addition, there have been articles by Oh et al. relating to PEDOT:PSS electrodes,17-19 and reports in which PEDOT/PSS electrodes were used to transform electrical energy to mechanical energy.20-21 Bergaud et al. used argon-plasma-induced surface polymerization of poly(ethylene glycol) monomethyl ether methacrylate (PEGMA) to develop PEDOT:PSS/PVDF-graft-PEGMA/ionic liquid (IL) ionic actuators, in which the PVDF-graft-PEGMA copolymers play an important role in binding the membrane to the electrode.22 Lubineau et al. also reported an improvement in the electrical conductivity of polycarbonate nanocomposites when using highly conductive PEDOT:PSS coated multiwalled carbon nanotubes (MWCNTs).23 Non-rigid substances capable of generating mechanical motion from electrical inputs have become of significant interest, especially with regard to uses in microelectromechanics, medicine, robotics, prosthetics, and tactile and optical displays.24 Artificial actuators that function like muscles and thus have various medical applications have also been proposed, based on rapid response electroactive polymer (EAP) actuators which function on low voltages.25,16 Our prior work27–29 successfully developed a gel-like, room temperature IL containing single-walled carbon nanotubes (SWCNTs), termed a “bucky-gel,” and used this material to fabricate the first-ever dry actuator.30 This device was composed of dual polymersupported bucky-gel electrode layers on either side of a polymer-supported IL electrolyte layer. When operated at low voltages in air, this device inherently has a long life time and is


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capable of rapid operation. Because actuators and related devices that have to respond rapidly must be very stable on an electrochemical basis, it is helpful to employ low volatility, highly conductive ILs having wide potential windows.31 We also previously reported the dependence of the electromechanical and electrochemical properties of such actuators on the type of IL, nanocarbon, and polymer used.29,32–35 See Figure 1. In recent years, ECs have also attracted significant interest due mainly to their high power densities and long lifespans, as well as their ability to bridge the power/energy gap between traditional dielectric capacitors and batteries/fuel cells.36,37 Indeed, much progress has been made in theoretical and practical research into the development of ECs, as evidenced by the large number of research articles and technical reports on the subject.38–43 Typically, ECs can be placed into one of two broad categories. The first is faradaic capacitors (FCs), which employ electrochemically active compounds such as metal oxides as electrodes and are capable of directly storing charge throughout the charging and discharging process.7,44,45 The other is electrostatic double-layer capacitors (EDLCs), which employ electrochemically inactive substances such as carbon particles as electrode materials. Throughout the EC charging and discharging of these devices, no electrochemical reaction occurs on the electrode

surface; there

is

only

the accumulation of

physical charge

at the

electrode/electrolyte interface. Regardless of the EC category, the device should possess a significant surface area along with a suitable pore-size distribution. It should also employ high-conductivity electrode materials to obtain the highest possible capacitance values. A third possible scenario is a hybrid capacitor,46 in which both EDLC and FC mechanisms occur simultaneously, with one being dominant. From the above description, it is clear that FCs or pseudocapacitors differ from EDLCs. The application of a potential to a FC generates rapid and reversible faradaic (or redox)


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reactions on the electrode surface. Conductive polymers and various metal oxides are known to allow these redox reactions.7,47–49 During this process, the charge moves across the double layer in a process that mimics the charging and discharging of a battery, such that a faradaic current passes through the capacitor cell. (a)

(b)


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Fig. 1 Configurations of (a) conventional polymer-supported PVdF(HFP)/SWCNT/IL gel actuator and (b) new-type of PEDOT:PSS, PEDOT:PSS/IL, and PEDOT:PSS/SWCNT/IL electrode actuators along with the molecular structures of the ILs and polymer used. The extraordinary mechanical50 and electrochemical51,52 properties of CNTs make them promising materials for electrochemical actuators based on the double-layer electrostatic mechanism.53 Bucky-gels are soft composite materials composed of CNTs dispersed in imidazolium ion-based ILs in such a way that the heavily entangled nanotube bundles are dismantled by cation–π interactions at the CNT surfaces to produce much finer bundles.54 As stated earlier, PEDOT doped with PSS (PEDOT:PSS) is currently one of the most commercially successful CPs for dispersion in water as colloidal particles. It is therefore also expected that CNTs can be dispersed very well by coating a thin layer of PEDOT:PSS onto it.23 Furthermore, a PEDOT:PSS/SWCNT/IL (type D) electrode layer prepared by ultrasonic dispersion possesses a large specific capacitance, which leads to a high degree of actuation. Liu et al.55 fabricated compact, flexible, and mechanically robust films based on interpenetrative nanocomposites composed of graphene/MnO2 and CNTs that exhibited superior electrochemical characteristics for use as supercapacitor electrodes. Liu et al.’s approach departs from earlier works because they took advantage of the synergistic effects arising from the combination of graphene and nanotubes. The CNTs in such films allow for electron conductance and provide mechanical reinforcement, while the graphene serves as a high-surface-area substrate for the direct growth of MnO2 nanoparticles. The uniformly interconnected CNT network in such a device will be highly conductive and porous, both of which are conducive to electronic and ionic transport. In a previous study,29 we investigated the voltage–current and voltage–displacement characteristics of a bucky-gel actuator by applying a triangle waveform voltage to an actuator


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device at various frequencies. In order to quantitatively describe the frequency dependence of the actuation strain generated in the actuator, we proposed an electrochemical equivalent circuit model consisting of the lumped resistance and capacitance of the electrode layer, and the lumped resistance of the electrolyte layer. In the present work, we used the film-casting method to develop new hybrid-type (PEDOT:PSS/SWCNT/IL) actuators that exploit the synergistic effects of the EDLC and FC. The electrochemical and electromechanical properties of the PEDOT:PSS (type B), PEDOT:PSS/IL (type C), and PEDOT:PSS/SWCNT/IL (type D) actuators were compared with

those

of

a

conventional

(PVdF(HFP))/SWCNT/IL

actuator

poly(vinylidene (type

A).

In

fluoride-co-hexafluoropropylene addition,

it

is

expected

that

PEDOT:PSS/SWCNT/IL (both EDLC and FCs occur simultaneously) actuators will provide better actuation strain performance than a conventional (PVdF(HFP))/SWCNT/IL actuator in which only EDLC occurs. Figure 1 illustrates the configuration of the conventional actuator and new-type PEDOT:PSS, PEDOT:PSS/IL, and PEDOT:PSS/SWCNT/IL actuators.

Experimental Materials


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Fig. 2 Schematic showing a method for preparing PEDOT:PSS/SWCNT/IL actuators (Figure 1; type D) by hot-pressing an electrolyte film sandwiched by two electrode films.

PEDOT:PSS (Poly-ion complex; ratio of 1:2.5 by weight) was obtained from Aldrich (No. 768618). The SWCNTs (high-purity HiPcoTM SWCNTs, Unidym, Inc.) had an average diameter of 0.8–1.2 nm, average length of 0.1–1 µm and surface area 400-1000 m2/g. The ionic liquids (ILs) were 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI[BF4], Fluka) and 1-ethyl-3-methylimidazolium triflate (EMI[CF3SO3], Fluka); these were employed asreceived. Figure 1 shows the chemical structures of these compounds. The other reagents included poly(vinylidene fluoride-co-hexafluoropropylene (PVdF(HFP), Kynar Flex 2801, Arkema Chemicals, Inc.), methyl pentanone (MP, Aldrich), propylene carbonate (PC, Aldrich), and dimethylacetamide (DMAc, Kishida Chemical Co., Ltd.). All were used asreceived. Preparation of the actuator film32 Figure 2 shows a schematic drawing of the fabrication of PEDOT:PSS/SWCNT/IL actuators (Figure 1; type D) by hot-pressing an electrolyte film sandwiched between two


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electrode films. The PEDOT:PSS electrode layer was prepared by first mixing 100 mg PEDOT:PSS in 4 mL H2O, and then stirring for more than 1 h to produce a gelatinous mixture of these reagents. The electrode layer was fabricated by casting 1.6 mL of this electrode solution in a Teflon mold (2.5 × 2.5 cm2) and allowing the solvent to evaporate. This was followed by complete removal of the solvent in vacuo at 70 °C to produce a 70–80 µm-thick electrode film. The PEDOT:PSS/IL electrode layer was typically composed of 66.7 wt% PEDOT:PSS and 33.3 wt% IL, with each layer prepared by first mixing 200 mg PEDOT:PSS and 100 mg EMI[BF4] or EMI[CF3SO3] in 8 mL H2O. This was then stirred for more than 1 h to generate a gelatinous mixture, 2.4 ml of which was cast in a Teflon mold (2.5×2.5 cm2) using the same procedure as before to obtain electrode films 70–80 µm in thickness. The PEDOT:PSS/SWCNT/IL electrode layer was typically composed of 32 wt% PEDOT:PSS, 20 wt% SWCNT, and 48 wt% IL. Each layer was prepared by first mixing 80 mg PEDOT:PSS, 50 mg SWCNT, and 120 mg EMI[BF4] or EMI[CF3SO3] in 9 mL H2O, and then dispersing for more than 5 h. This was followed by casting 1.6 ml of the resulting gelatinous mixture in a Teflon mold as per the procedure outlined above. The conventional PVdF(HFP)/SWCNT/IL electrode layer was typically composed of 32 wt% PVdF(HFP), 20 wt% SWCNT, and 48 wt% IL, with each layer prepared by first mixing 80 mg PVdF(HFP), 50 mg SWCNT, and 120 mg EMI[BF4] or EMI[CF3SO3] in 9 mL DMAc and dispersing for more than 5 h, which generated a gelatinous mixture of these reagents. The electrode layer was fabricated by casting the above electrode solution (1.6 mL) in a Teflon mold (2.5 × 2.5 cm2) and allowing the solvent to evaporate. This was followed by the complete removal of the solvent in vacuo at 80 °C. The thickness of the obtained electrode films was 70–80 µm. Gel electrolyte layers were then fabricated by casting 0.3 mL of solutions composed of either one of the two ILs and PVdF(HFP) (0.5 mmol/100 mg) in a mixture of 1 mL MP and 250 mg PC in a Teflon mold (2.5 × 2.5 cm2). This was followed by solvent evaporation and the


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complete removal of the solvent in vacuo at 80 °C. The thickness of the gel electrolyte films obtained was 20–30 µm. The actuator film was fabricated by hot-pressing the electrode and electrolyte layers containing the same IL. The typical thickness of the resulting actuator films was 150–175 µm. This is less than the sum of the two electrode layers and the one electrolyte layer; that is because the thickness of each layer was reduced as a result of hot-pressing. Displacement measurement56 The actuator experiments were conducted using a triangular voltage applied to a 10×1 mm2 actuator strip clipped by two gold disk electrodes. The displacement at a point 5 mm away (free length) from the fixed point was continuously monitored from one side of the actuator strip using a laser displacement meter (Keyence, LC2100/2220). A potentio/galvanostat (Hokuto Denko, HA-501G) and a waveform generator (Yokogawa Electric, FC 200) were used to activate the bucky-gel actuator while the electrical parameters were simultaneously measured. The measured displacement, δ, was transformed into the actuation strain difference between the two bucky electrode layers, ε, using the following equation, on the assumption that the cross sections are plane planar at any position along the actuator; i.e., there is no distortion of the cross section: ε=2dδ/(L2+δ2)

(1)

where L is the free length and d is the thickness of the actuator strip.57 Characterization of the electrode and electrolyte The double-layer capacitance of the polymer-supported bucky-gel electrode (φ7 mm) was estimated by cyclic voltammetry (CV), which was measured using a two-electrode configuration with a potentiostat (Hokuto Denko, HSV-100). The electrical conductivities of the electrodes were evaluated using the four-probe DC current method, wherein a linear


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sweep wave of current is applied from outer probe electrodes and the voltage is measured by inner probe electrodes. Current-voltage curves were obtained using a potentio/galvanostat (Hokuto Denko, HA-151) with a waveform generator (Yokogawa Electric, FC 200). The conductivity of the gel electrolyte layer was determined by impedance measurement using a Solatron 1250 Impedance Analyser. Young’s moduli for the electrodes were estimated from their respective stress-strain curves, which were measured using a thermal stress-strain instrument (Seiko, TMA/SS 6000). Results and discussion Mechanical and electrochemical properties of the electrode Table 1 compares the Young’s moduli of the PEDOT:PSS, PEDOT:PSS/IL PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrode layers. The modulus of the PEDOT:PSS electrode (328 MPa) was much greater than that of the PEDOT:PSS/IL electrode (102–133 MPa). Similarly, the moduli of the PEDOT:PSS/SWCNT/IL electrodes (214–255 MPa) were higher than those of the PVdF(HFP)/SWCNT/IL electrodes (138–175 MPa). Given that the Young’s modulus of the PVdF(HFP)/IL electrode was about 30 MPa, this data provides evidence that highly entangled SWCNTs form a network of open mesopores35,53-55. Table 1 Young’s moduli (MPa) of PEDOT:PSS, PEDOT:PSS/IL, PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrodes.


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Table 2. Specific capacitance (F g-1) (C, equal to C1/weight of the PEDOT:PSS or SWCNT) of PEDOT:PSS, PEDOT:PSS/IL, PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrodes (applied triangular voltage: ±0.5 V, sweep rate: 1 mV/s).

Table 2 presents the specific capacitance (F g-1) (C, equal to C1/weight of the PEDOT:PSS or

SWCNT)

of

PEDOT:PSS,

PEDOT:PSS/IL,

PEDOT:PSS/SWCNT/IL

and

PVdF(HFP)/SWCNT/IL electrodes. Table 2 shows that the specific capacitance of the PEDOT:PSS and PEDOT:PSS/IL electrodes were similar at 22–26 F g-1 at a slow sweep rate of 1 mV s-1. The specific capacitances of these electrodes are believed to result from both the EDLC (SWCNT) and FC (PEDOT:PSS) mechanisms, with the latter providing the greatest contribution. This differs from a conventional PVdF(HFP)/SWCNT/IL gel electrode, in which EDLC is the only mechanism. The capacitance of the PEDOT:PSS/SWCNT/IL electrodes at a slow sweep rate of 1 mV s1

(128–149 and 227–244 F g-1 equal to C1/weight of PEDOT:PSS and SWCNT, respectively)


12

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was much higher than that of a conventional PVdF(HFP)/SWCNT/IL electrode (51–54 F g-1 equal to C1/weight of SWCNT). This is explained by the specific capacitance of the PEDOT:PSS/SWCNT/IL electrode resulting from both the EDLC and FC mechanisms, whereas the conventional polymer-supported PVdF(HFP)/SWCNT/IL gel electrode relies solely on the EDLC mechanism. That is, the electrodes in the PEDOT:PSS actuator system create an FC, while the PEDOT:PSS in the base polymer replaces PVdF(HFP) as the actuator. Meanwhile, the SWCNT skeleton increases the electroconductivity and provides an EDLC for the actuator. In addition, the evidence from the Young’s moduli data that highly entangled SWCNTs form a network of open mesopores27,58-60 suggests that they can be more easily reached by the ions. This behavior can be explained by a synergistic effect between PEDOT:PSS and the SWCNTs.55 The PEDOT energy storage mechanism mainly results from the pseudocapacitance that is produced by the fast faradaic redox reactions between the PEDOT and electrolyte.41, 61 This process allows such materials to store greater amounts of energy than the SWCNT found in typical double-layer capacitors,41 and so the high capacitance obtained with PEDOT may be ascribed to the pseudocapacitance effect. The reversible oxidation and reduction of PEDOT is described by the reaction below.62 PEDOT:PSS + δEMI+ + δe = (PEDOT:PSS)1-δ(EMI)δ, (0 < δ < 1)

(2)

Table S1 summarizes the electrical conductivities of the PEDOT:PSS, PEDOT:PSS /IL, PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrode layers, revealing the conductivity of the PEDOT:PSS/IL electrode (3.6–4.4 S cm-1) to be much higher than that of the PEDOT:PSS electrode (0.5 S cm-1). Note that PEDOT:PSS has a tunable conductivity of 0.1 to 3000 S/cm,9-10 and that it has been reported that the addition of certain organic solvents such as ethylene glycol can produce a significant improvement in the conductivity of


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PEDOT:PSS thin films.63 The electrical conductivities of the PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrodes were similar at 12.6–15.7 and 13.9–17.5 S cm-1, respectively, suggesting that PEDOT has little effect on the conductivity. Electromechanical properties and performance of the actuator Figure 3 shows the actuation strain values obtained for the new-type PEDOT:PSS, PEDOT:PSS/IL, and PEDOT:PSS/SWCNT/IL electrode actuators as a function of the applied triangular voltage (±2 V) frequency (IL: EMI[BF4]). As actuation strain is dependent on the measurement frequency, all of the EMI[CF3SO3]-based PEDOT:PSS, PEDOT:PSS/IL, and PEDOT:PSS/SWCNT/IL actuators exhibit the same tendency. Note that the actuation strain of the PEDOT:PSS/SWCNT/IL actuator was larger across a wide frequency range of 5–0.005 Hz than those of the PEDOT:PSS and PEDOT:PSS/IL actuators, but that at low frequencies (0.01–0.005 Hz) the PEDOT:PSS/SWCNT/IL electrode had a higher specific capacitance than the PEDOT:PSS and PEDOT:PSS/IL electrodes. Furthermore, the actuation strain of the PEDOT:PSS/SWCNT/IL actuator in this low frequency range was greater than that of the conventional PVdF(HFP)/SWCNT/IL actuator. This can be explained by the specific capacitance of the PEDOT:PSS/SWCNT/IL electrode originating from both the EDLC and FC mechanisms, with the greater contribution from the latter, whereas the FC mechanism

does

not

contribute

to

the

actuation

strain

in

a

conventional

PVdF(HFP)/SWCNT/IL electrode.


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Fig. 3 Actuation strain (ε) calculated from the peak-to-peak displacements of new-type PEDOT:PSS,

PEDOT:PSS/IL, 32

PVdF(HFP)/SWCNT/IL

PEDOT:PSS/SWCNT/IL

and

conventional

electrode actuators as a function of the applied triangular voltage

(±2 V) frequency (IL: EMI[BF4]). Table 3 presents the maximum actuation strain values for the PEDOT:PSS, PEDOT:PSS/IL, PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrode actuators. The maximum actuation strains of the PEDOT:PSS and PEDOT:PSS/IL electrode devices (0.18%–0.22% and 0.26%–0.48%, respectively) indicate that both actuators have the potential to be used in real-world applications (e.g., transparent actuators when the more thin film is without SWCNTs).

Meanwhile,

we

see

that

the

maximum

actuation

strain

of

the

PEDOT:PSS/SWCNT/IL electrode device (0.56%–0.57%) was 1.3–1.5 times greater than that of the PVdF(HFP)/SWCNT/IL electrode device (0.38%–0.43%). These results indicate that PEDOT can act as both a base polymer and FC electrode, whereas PVdF(HFP) can only be base polymer. Thus, the specific capacitance of the PEDOT:PSS/SWCNT/IL electrode is most likely the result of both EDLC and FC, effects with the largest contribution coming from the latter. This indicates that a PEDOT:PSS/SWCNT/IL actuator with two roles (i.e.,


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base polymer and FC electrode) can generate sufficient actuation strain for real-world applications. Table 3. Comparison of maximum actuation strain (%) for PEDOT:PSS, PEDOT:PSS/IL, PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrodes.

Table

4

Maximum

generated

stresses

(MPa)

of

PEDOT:PSS,

PEDOT:PSS/IL,

PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrodes.

Table 4 compares the maximum generated stresses of the PEDOT:PSS, PEDOT:PSS/IL, PEDOT:PSS/SWCNT/IL and PVdF(HFP)/SWCNT/IL electrode layers. The maximum stress (σ) generated during actuation was calculated using the maximum actuation strain (εmax) and Young’s modulus (Y) according to Hooke’s law (σ = Y × εmax). The maximum generated


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stresses (σ) of the PEDOT:PSS and PEDOT:PSS/IL electrodes of 0.59–0.72 and 0.35–0.49 MPa, respectively, demonstrate that both actuators could be used in for real-world applications (e.g., transparent actuators when the more thin film is without SWCNTs). Meanwhile, the maximum generated stress (σ) of the PEDOT:PSS/SWCNT/IL actuators (1.2–1.45 MPa) was approximately 1.9–2.3 times larger than that of the conventional PVdF(HFP)/SWCNT/IL actuator. Thus, the PEDOT actuator generates sufficient maximum stress for real-world applications such as tactile displays. Actuation mechanism A possible process explaining the bending of conventional PVdF(HFP)/SWCNT/IL actuators was suggested in a prior publication by our group.28 In this mechanism, cations contained in the gel electrolyte layer move to the cathode layer while anions migrate to the anode layer upon applying a voltage between the two electrode layers. The resulting ion transport within positive and negative nanotubes of the electric double layer shrinks the anode layer while swelling the cathode layer. As a result, the actuator bends toward the anode side. It has also been reported that in the case of a polymer/IL composite electrochemical artificial muscle, bending occurs due to ion transfer in the vicinity of the electrode, while the bending direction depends strongly on the ionic transport number.64 In the present work with PEDOT actuators, PEDOT was used as both the base polymer and FC electrode and so the anode layer swells and expands with ion migration (FC). We therefore propose that swelling and expanding of the PEDOT anode layer by the FC mechanism provides a major contribution at low frequencies, whereas the EDLC mechanism provides a minor contribution (i.e., ion transport causes swelling of the cathode layer and shrinkage of the anode layer). As a result, the actuator bends toward the anode side. However, as FC is not the only contributor


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to the actuation strain, some other factor would also affect it (e.g., Young’s modulus and the presence of PSS and/or bulky poly-anions). See Figure 4.

Fig. 4 Schematic of the response model used for new-type PEDOT:PSS/SWCNT/IL actuators.

Prior research29 has investigated the voltage–current and voltage–displacement relations for a bucky-gel actuator when a voltage with a triangular waveform is applied at various frequencies. In this way, both the time constant of the response and the low frequency limit actuation strain can be predicted. In the present work, variations in the displacement with frequency for PEDOT:PSS/SWCNT/IL and PEDOT:PSS/IL electrode actuators were successfully predicted by employing a double-layer charging kinetic model. This process involves both EDLC and FC mechanisms, and so is much like the model used for SWCNTbased actuator. That is, it allows for calculation of the theoretical values for the response time constant and the actuation strain at the low-frequency limit. However, the variations in the displacement with frequency were not successfully predicted for the PEDOT:PSS electrode (electrolyte; PVdF(HFP)/IL) actuators with the double-layer charging kinetic model. This


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suggests that the process may only involve the FC mechanism, and so is different from the model used for the SWCNT-based actuator. See the Supporting Information. Conclusions New hybrid-type (PEDOT:PSS/SWCNT/IL) actuators combining the synergistic effects of an electrostatic double-layer and faradaic capacitor were developed using the film-casting method. Through comparison of the electrochemical and electromechanical properties of these actuators to a conventional PVdF(HFP)/SWCNT/IL actuator, it is believed that the specific capacitances of the PEDOT:PSS, PEDOT:PSS/IL and PEDOT:PSS/SWCNT/IL electrodes result from both the EDLC and FC mechanisms; the latter providing the greater contribution. This differs from the conventional polymer-supported PVdF(HFP)/SWCNT/IL gel electrode, which only uses the EDLC mechanism. PEDOT and SWCNT are also considered to have a synergistic effect. In terms of actuation strain, PEDOT acts as both a base polymer and FC electrode, while PVdF(HFP) is only a base polymer. The specific capacitance of the PEDOT:PSS/SWCNT/IL electrode results from both EDLC and FC, and since FC is only a small contributor to the actuation strain, some other factor such as Young’s modulus and the presence of PSS and/or bulky poly-anions would also affect it. Nevertheless, the maximum actuation strain and maximum generated stress (σ) of the PEDOT:PSS/SWCNT/IL actuator were approximately 1.3–1.5 and 1.9–2.3 times greater than those of the conventional PVdF(HFP)/SWCNT/IL actuator, respectively. The

variation

in

displacement

with

frequency

for

the

PEDOT:PSS/IL

and

PEDOT:PSS/SWCNT/IL actuators was determined based on simulations employing a double-layer charging kinetic model that incorporates the oxidization and reduction reactions of the PEDOT: this model is also applicable to SWCNT-based actuators. This was used to


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predict low-frequency actuation strains resulting from the various electromechanical mechanisms and was able to provide response time constants that mimicked those for an equivalent circuit composed of an ionic resistance, double-layer and faradaic capacitances, and an electrode resistance in series. Based on the data acquired in this study, flexible and robust films operating via the synergistic effects of PEDOT combined with SWCNT show promise as electrode materials for wearable and energy conversion devices. This same concept can also be used in conjunction with other electrochemical materials containing PEDOT for energy conversion systems to increase the range of applications.

ASSOCIATED CONTENT Supporting Information. Equivalent circuit models and simulation parameters are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partly supported by a KAKENHI Grant-in Aid for Scientific Research C (No. 24550264) from JSPS.


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ABBREVIATIONS EC, electrochemical capacitor; EDLC, electrostatic double-layer capacitor, FC, faradaic capacitor; PEDOT, poly(3,4-ethylenedioxythiophene); PSS,

poly(4-styrenesulfonate);

(PEDOT/PSS); CPs, Conductive polymers; EAP, electroactive polymer; PVdF(HFP), poly(vinylidene

fluoride-co-hexafluoropropylene;

PEGMA,

poly(ethylene

glycol)

monomethyl ether methacrylate; SWCNT, single walled carbon nanotube. REFERENCES (1)

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Table of contents entry


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