Soft Electrodes

Highly Conductive, Capacitive, Flexible and Soft Electrodes Based on a 3D Graphene–Nanotube– Palladium Hybrid and Conducting Polymer Hyun-Jun Kim, Hyacinthe Randriamahazaka, and Il-Kwon Oh* Owing to the advent of next-generation electronic products such as wearable smart phones and flexible displays, and soft haptic devices, strong demand for flexible electrodes and soft energy storage devices with high rate capability and ultrahigh capacitance has stimulated a large number of academic and industrial studies. A two-dimensional (2D) graphene-based electrode can be one of the candidates to fulfill the requirements for such devices, because graphene nano-materials have prominent and inherent chemical and physical properties, such as good flexibility,[1] high electrical conductivity (103∼104 S/m),[2] large surface area (2,675 m2/g)[3] and ultrahigh mechanical strength (130GPa).[4] The intrinsic material properties of pure graphene without chemical or physical defects are theoretically exceptional to be used in energy storage devices and recently high-quality 2D graphene has been considered as a key material for next-generation electronic products. Meanwhile, unlike CVD-grown graphene[5] or mechanically exfoliated graphene,[6] mass-producible graphene flakes[2,7] for large scale energy storage applications are not good for constructing large scale flexible electrodes because of poor intersheet connections among isolated graphene flakes;[8] these flakes also have relatively poor material properties compared to the theoretical values because of uncontrolled defects and functionalized edges.[6,9,10] Also, the critical problems of 2D graphene flakes for energy storage electrodes include re-stacking phenomenon, agglomeration, low transverse electrical conductivity, and inefficient ion path ways, resulting in inaccessible surfaces and the capacitance

H.-J. Kim, Prof. I.-K. Oh Graphene Research Center KAIST Institute for the NanoCentury School of Mechanical Aerospace and Systems Engineering Korea Advanced Institute of Science and Technology (KAIST) 335 Gwahak-ro Yuseong-gu, Daejeon 305-701, Republic of Korea E-mail: [email protected] Prof. H. Randriamahazaka Univ. Paris Diderot Sorbonne Paris Cité, ITODYS UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf 75205 Paris Cedex 13, France DOI: 10.1002/smll.201401613 small 2014, DOI: 10.1002/smll.201401613

loss in comparison with the expected theoretical values.[6,8–10] Therefore, graphene-based three-dimensional (3D) or hierarchical carbon nanostructures[11–15,38] with much higher structural interconnectivity, very large accessible surface area and efficient pathways for ion/electron transport, have been investigated to overcome the problems of 2D graphene. Recently, Oh et al. developed a general strategy for the synthesis of 3D graphene-carbon nanotube-palladium (3D G-CNT-Pd)[15] or graphene-based hierarchical 3D carbon nanostructures[14] by using microwave irradiation methods; this structure showed ultrahigh specific capacity as an anode in lithium ion batteries.[14] However, in order to utilize electrode materials based on graphene flakes for flexible energy storage systems, some issues should be considered. First, when the electrode is bent, a binder that can improve the connectivity among graphene flakes[16] is needed in order to prevent crack formation in the electrode. The second issue is that the metal current collector and electrically conductive adhesives can be a major cause of the system failure due to layer delamination and contact resistance during large bending deformation because the interface layers between the current collector and the electrode are mainly linked by conductive adhesives via physical force.[16] Thus, a new electrode system that is mechanically stable under large bending deformation is necessary for the next-generation flexible electronic devices. Here, we report a highly conductive, capacitive, flexible and soft electrode based on three-dimensional graphene-nanotube-palladium nanostructures and Poly(3,4-ethylenedioxythiophene): Polystyrene sulfonate (PEDOT:PSS) conducting polymer. The PEDOT:PSS conducting polymer was used not only to intimately connect the graphene-based 3D flakes, but also to enhance the electronic transport of the original electrode without additional electrical connection parts such as a metallic current collector. In addition, with its high flexibility, the electrode can be used to support the applicability and to fulfill the requirements for a vast range of device applications, especially flexible energy storage systems and actuating devices. Prior to checking the flexibility of the electrodes, it is also important to improve the wettability between the PEDOT:PSS and graphene-based materials. The PEDOT:PSS solution can be rendered more hydrophobic by dissolving it in an organic solvent such as dimethyl formamide (DMF) and thus ensuring superior wetting of the surface of the graphene-based material.[17] The

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Surface morphological studies of prepared carbon materials and composite electrodes were carried out using electron microscopy techniques as shown in Figures 1a∼d. The SEM image of the PEDOT:PSS conducting polymer pretreated with DMF shows a smooth and totally seamless surface of the flexible electrode. Figure 1b shows that the two-dimensional (2D) graphene sheets are well mixed with PEDOT:PSS (PEDOT:PSS + 2D rGO). Besides this, Figure 1c demonstrates the anchoring of palladium nanoparticles on the 2D graphene sheets mixed with PEDOT:PSS (PEDOT:PSS + 2D G-Pd). The three-dimensional (3D) Graphene-nanotube-palladium nanostructures that were introduced in our previous work[15] are well mixed with PEDOT:PSS (PEDOT:PSS + Scheme 1. Optical image of flexible electrode synthesized using 3D G-CNT-Pd) as can be seen in Figure 1d. And a pristine PEDOT:PSS, organic solvent, and 3D carbon nanostructures. PEDOT:PSS + 3D G-CNT-Pd electrode that is cut to dimentotal set of synthetic materials and the electrode is schemati- sions of 2 × 5 cm2 is shown as it comes in Figure 1e. To check cally illustrated in Scheme 1. the flexibility and the wettability of the electrode mentioned Figure 1 provides scanning electron microscope (SEM) above, two electrodes are compared: one is made of pristine micrographs and digital photographs of the flexible electrodes. PEDOT:PSS and 3D G-CNT-Pd only; the other is made of PEDOT:PSS pretreated with DMF and 3D G-CNT-Pd. As shown in Figure 1f, both electrodes can be easily bent, but no aggregation parts can be seen in the DMF-treated electrodes (see the right side image in Figure 1f). Such a flexible PEDOT:PSS electrode with well-dispersed 3D carbon nanoparticles can be considered an efficient potential candidate for various device applications such as energy storage devices and actuators. A bias voltage between −1.0 and 1.0 V was applied to compare the electrical conductivity of the electrodes. The electrical conductivity was calculated from the slope of the current-voltage (I–V) characteristics. All tracks exhibited ohmic behavior with linear I-V characteristics (Figure 2a). As can be observed in Figure 2a, the slope of I-V for the PEDOT:PSS + 3D G-CNT-Pd electrode is 12 times steeper than that of I-V for the pristine PEDOT:PSS electrode. This indicates that the electrical conductivity of PEDOT:PSS + 3D G-CNT-Pd is exceptionally higher than that of the pristine PEDOT:PSS electrode. In order to determine the effect of the electrode on charge transport in the electrolyte and to elucidate the interfacial electrochemical process, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed. The CV responses of all the electrode materials at a scan rate of 100 mV/s are presented in Figure 2b. The curve shape of the CV response for the PEDOT:PSS electrode is similar to those found in previously Figure 1. (a∼d) SEM images of electrodes and pristine materials(See inset images with scale [18,19] The PEDOT:PSS bar of 200 nm): (a) PEDOT:PSS, (b) PEDOT:PSS + 2D rGO, (c) PEDOT:PSS + 2D G-Pd, and (d) reported studies. PEDOT:PSS + 3D G-CNT-Pd; (e-f) Optical images of electrodes: (e) Pristine flexible electrode + 3D G-CNT-Pd electrode shows a and (f) Flexible electrode with well-dispersed graphene-based materials fabricated using DMF. maximum current density of 584.8 mA/cm3,

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Highly Conductive, Capacitive, Flexible and Soft Electrodes

100

(b)

PEDOT:PSS + 3D G-CNT-Pd PEDOT:PSS + 2D G-Pd PEDOT:PSS + 2D rGO PEDOT:PSS

50 0 -50 -100 -150 -200 -1.0

3

Specific capacitance (mF/cm )

(c) 7,000 6,000

-0.5

0.0

0.5

1.0

Potential (V) PEDOT:PSS+3D G-CNT-Pd PEDOT:PSS+2D rGO

600

PEDOT:PSS + 3D G-CNT-Pd PEDOT:PSS + 2D G-Pd PEDOT:PSS + 2D rGO PEDOT:PSS

3

Current density (mA/cm )

Current (mA)

150

PEDOT:PSS+2D G-Pd PEDOT:PSS

4,000 3,000 2,000

200 0 -200 -400 100 mV/s

-600

-1.0

-0.5

0.0

0.5

PEDOT:PSS + 3D G-CNT-Pd PEDOT:PSS + 2D G-Pd PEDOT:PSS + 2D rGO PEDOT:PSS Randles model

200

100 20 15

50

10

1,000

(e)

0

5

0

50

100

150

0 10

0

50

-1

PEDOT:PSS + 3D G-CNT-P PEDOT:PSS + 2D rGO

PEDOT:PSS + 2D G-Pd PEDOT:PSS

Re(Z) (

(f)

15

20

25

30

100 150 200 250 300 350 400

Scan rate (mV/s) 10

1.0

Potential (V vs Ag/AgCl)

150

2

5,000

(d)

400

cm )

200

- Im(Z) (

(a)

2

cm )

-4

10

PEDOT:PSS + 3D G-CNT-P PEDOT:PSS + 2D rGO

PEDOT:PSS + 2D G-Pd PEDOT:PSS

-5

Co(F/cm )

10 -2

(s)

2

10

-6

ad

10

-3

10

-7

10

-8

-4

10

10

Figure 2. (a) I/V characteristics of electrodes, (b) Cyclic Voltammetry results for electrodes, (c) specific capacitances according to the scan rate at each electrode, (d) EIS spectra of electrodes and curves fitted to spectra using Randles-like model, (e) relaxation time for the adsorption, and (f) specific capacitance, CO.

whereas pristine PEDOT:PSS and PEDOT:PSS + 2D G-Pd electrodes show current density values of 48.8 mA/cm3 and 413.1 mA/cm3, respectively. Hence, the addition of graphene and graphene hybrid materials enhances the current density of the electrodes. And the high response of cyclic voltammetry implies synergetic utilization of both pseudocapacitance due to the presence of palladium nanoparticles, PEDOT:PSS and electric double-layer capacitance due to the unique arrangement of two different carbon allotropes, 1D carbon nanotubes vertically standing on 2D graphene sheets.[15] The current density was calculated from the CV response using the volume of the electrode dipped into the electrolyte. However, the entire electrode was not dipped into the electrolyte when tested because the current collector and the electrolyte were not allowed to react. The exceptionally high current density of the PEDOT:PSS + 3D G-CNT-Pd can be attributed to the unique three-dimensional graphenenanotube-palladium nanostructures, which provide low diffusion resistance to charge, easy electrolyte penetration and high electro-active areas.[15] The corresponding mean specific capacitances calculated from CVs at various scan rates are shown in Figure 2c. The mean specific capacitance of the PEDOT:PSS + 3D G-CNT-Pd small 2014, DOI: 10.1002/smll.201401613

electrode at various scan rates is found to be much higher than that of other electrodes within the potential window of [–1.0 V, 1.0 V]. Specific redox peaks can be noticed at voltages ranging from around –0.4 V to -0.6 V for the electrodes, containing palladium nanoparticles (PEDOT:PSS + 2D G-Pd and PEDOT:PSS + 3D G-CNT-Pd).[20] This indicates the participation of palladium nanoparticles in the electrochemical reactions, occurring on the electrode-electrolyte surfaces. The specific capacitances of the PEDOT:PSS, PEDOT:PSS + 2D rGO, PEDOT:PSS + 2D G-Pd, and PEDOT:PSS + 3D G-CNT-Pd electrodes were compared, and their ratios were found to be 1 : 3.74 : 6.61 : 8.16, at scan rate 50 mV/s. In addition, as shown in Figure 2c, the ratios at the scan rates of both 100 mV/s and 150 mV/s were 1 : 3.69 : 6.08 : 8.54 and 1 : 3.70 : 6.82 : 8.39, respectively. To obtain a deeper insight into the charge transport dynamics within the 3D graphene-nanotube-palladium nanostructures, we perform EIS measurements. Figure 2d shows the electrochemical impedance spectra of PEDOT:PSS, PEDOT:PSS + 2D rGO, PEDOT:PSS + 2D G-Pd, and PEDOT:PSS + 3D G-CNT-Pd electrodes in 0.5 M BMIMBF4/acetonitrile (AN). The interpretation of the impedance responses was carried out by fitting the experimental data

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using a generalized equivalent circuit model,[21] called the Randles-Frumkin-Melik-Gaikazyan model, which is shown in the inset of Figure 2d. This circuit contains the ionic resistance of the electrolyte (RS) in series with a parallel combination of double-layered capacitance (Cdl) and the adsorption resistance associated to the charge transfer (Rad), a Warburg −1/2 diffusion element ( Wdiff = σ diff ( jω ) ), and the adsorption capacitance (Cad), all of which were in series themselves. σdiff is the diffusion impedance. Here, the double-layered capacitance Cdl is replaced by a distributed electrical element, the constant phase element (CPE), with the following impedance[22] ZCPE =

1 α Q ( jω )

(1)

where α is the CPE exponent, Q[F·s−(1-α)/cm2] is the CPE parameter, and j = −1 is the imaginary unit. As shown in Figure 2d, the experimental impedance data can be fitted using the Randles-Frumkin-Melik-Gaikazyan equivalent circuit; the values of each element are reported in Table 1 (using 0.5 M BMIM-BF4/AN as the electrolyte). The first Re(Z) intersection point at the left end of the semicircle represented the ionic resistance of the electrolyte (RS). As seen in the inset plotted in Figure 2d (showing data at high frequencies), we found that the experimental RS for PEDOT:PSS+3D G-CNT-Pd was smaller than those of other samples, unlike the results from fitting data in Table 1. The error between experimental data (symbols) and fitting data (solid line) generally appeared at high and low frequencies when the experimental impedance data are fitted by EIS analysis. Figure 2e shows that the presence of graphene decreases the adsorption relaxation time τad (τ ad = Rad ⋅ C ad ); the lowest τad is obtained with PEDOT:PSS + 3D G-CNT-Pd. However, with respect to the Warburg parameter σdiff, the electrodes that contain graphene-based materials are similar to each other. We have calculated the value of the specific diffusion impedance Wdiff at the frequency corresponding to the adsorption relaxation time τad; we show that this diffusion impedance is much lower that Rad for all electrodes. This behavior indicates that mass transport diffusion is not the rate limiting step. One can note that the Warburg parameter σdiff and Rad for PEDOT:PSS are higher than those for graphenebased electrodes. This indicates that the 3D graphene-nanotube-palladium nanostructures greatly improve the dynamics of the charge transfer and transport. Furthermore, the value of the parameter Cad of the PEDOT:PSS + 3D G-CNT-Pd

electrode was found to be markedly higher than that of the other electrodes. By taking into account the adsorption relaxation time τad, it can be seen that the PEDOT:PSS + 3D G-CNT-Pd electrode acted as a reservoir for electron storage in the ionic liquid electrolyte. Within the framework of this equivalent circuit, the impedance at infinite frequency depends on the CPE element. As can be seen in Table 1, the CPE exponent α is lower than unity. However, the CPE parameter Q cannot represent the capacitance when α < 1.[23] The physical origin of deviations from ideal behavior with α = 1 has been discussed and is a source of controversy. Two different viewpoints can be found in the literature. Some authors have explained that non-ideal behavior is related to the roughness of the electrode.[24,25] In such a theory, an increased surface roughness leads to a current density distribution along the surface and thus a distribution of the electrical doublelayer charging times. Other authors have reported that the non-ideal behavior is caused by the slow processes of the reorientation of ions, and the interaction of ions at the electrode surface.[26,27] Recent experiments have showed that the deviation from an ideal capacitor is mainly related to a slow process (reorientation and/or adsorption of ions) at the interface; the surface roughness seems to enhance the nonideality caused by the slow motions.[27] Brug et al. proposed a simple model to explain CPE behavior based on the idea of a double-layer capacity distribution along the interface caused by the surface inhomogeneity.[26] They obtained a characteristic capacitance Co which suggested to represent the doublelayer capacitance (at high frequency), which is related with the model parameters through ⎡ Q ⎤ C o = ⎢ (α −1) ⎥ ⎣ Rs ⎦

1/α

(2)

Figure 2f shows the characteristic capacitance Co for each electrode material, and indicates clearly the huge increase of Co in the presence of the graphene. Particularly, one can observe the high Co for the PEDOT:PSS + 3D G-CNT-Pd. This indicates that the graphene-based material increased the interface accessibility for the ions species. These three-dimensional graphene-nanotube-palladium nanostructures are very helpful for energy storage applications, such as flexible capacitors or supercapacitors.[28,29] To further examine the performance of flexible capacitive electrodes for practical application, a flexible capacitor without additional metallic current collectors was assembled

Table 1. Parameters of the generalized equivalent circuit model for each type of flexible electrodes with 0.5 M BMIM-BF4/AN as electrolyte. Rs [Ω·cm2]

Q [F·s−(1-α)/cm2]

α

Rad [Ω·cm2]

Wdiff [Ω−1·s5/cm2]

Cad [F/cm2]

τad [s]

CO [F/cm2]

PEDOT:PSS

10.76

1.742 × 10−5

0.607

311.1

0.09129

1.49 × 10−4

4.64 × 10−2

6.76 × 10−8

PEDOT:PSS + 2D rGO

10.37

1.591 × 10−5

0.646

152.0

0.00107

1.20 × 10−4

1.83 × 10−2

1.33 × 10−7

PEDOT:PSS + 2D G-Pd

10.67

1.732 × 10−4

0.639

87.9

0.00130

1.07 × 10−4

9.43 × 10−3

4.91 × 10−6

PEDOT:PSS + 3D G-CNT-Pd

12.71

2.168 × 10−4

0.706

1.9

0.00117

1.87 × 10−4

3.54 × 10−4

1.85 × 10−5

Electrode

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Highly Conductive, Capacitive, Flexible and Soft Electrodes

Figure 3. (a) Optical images of electrode, capacitor and the performance for practical application using electrode with no additional current collector, (b) Conductivity change of electrodes under bending cycle test; the inset photo shows the bending process, (c) Stress-strain curves for electrodes under uniaxial tension.

using two electrodes that were separated by a PVdF-HFP film containing BMIM-BF4 as electrolyte,[30,31] as shown in Figure 3a and Figure S1. There were no current collectors or conducting paste between the electrode and the lead wire in the capacitor system, a red light emitting diode was turned on by connecting the charged capacitor for only a short charging time. Optical images of the circuits, which show the charging capacitor and the performance of the flexible capacitor based on our electrodes, are provided in Figure S1 (See also, Movie S1). Moreover, the mechanical properties of the electrodes were assessed by bending cycle and tensile tests. With the good dispersion of 3D G-CNT-Pd into the PEDOT:PSS conducting polymer and strong interaction between 3D G-CNTPd and the polymer, mechanical stiffness and strength were greatly increased, as shown in Figure 3c. The electrical conductivity of the PEDOT:PSS + 3D G-CNT-Pd electrode is found to be higher than that of the pristine PEDOT:PSS electrode for each bending cycle, showing the great advantages of using 3D G-CNT-Pd as an electrode for electrical applications (See Figure 3b.). The inset photograph in Figure 3b shows the electrical conductivities of electrodes attached to double-sided tape as a function of the bending times with a radius of 10 mm. The electrical conductivities of the electrodes were measured using a four-probe station. The stressstrain curves of the electrodes under uniaxial tensile stress are presented in Figure 3c. It has been observed that the ultimate strength and strain of the PEDOT:PSS + 3D G-CNT-Pd small 2014, DOI: 10.1002/smll.201401613

electrode are higher than those of the pristine PEDOT:PSS electrode. Ultimate strength and strain at breaking point for the PEDOT:PSS + 3D G-CNT-Pd electrode are found to be 33.2 MPa and 3.0%, respectively. However, the pristine PEDOT:PSS electrode has lower values of ultimate strength (13.6 MPa) and strain (1.2%) at break, as compared to the PEDOT:PSS + 3D G-CNT-Pd electrode. It is obvious that the addition of 3D G-CNT-Pd to the PEDOT:PSS polymer has a synergistic influence on the mechanical as well as the electrical properties. In other words, the stronger interfacial interaction between G-CNT-Pd and the PEDOT:PSS polymer matrix has significant effects on the mechanical properties, which are considered as the most important factors in flexible energy storage systems.[32–35] In summary, a highly conductive, capacitive, flexible and soft electrode was fabricated by employing three-dimensional graphene-nanotube-palladium nanostructures and conducting polymer. Electrochemical analyses of the electrodes indicate that the flexible electrodes have potential for use in energy storage devices and other flexible electronic devices. Flexible and soft electrodes fabricated by mixing 3D graphene-nanotube-palladium nanostructures with DMFpretreated PEDOT:PSS conducting polymer were also analyzed using the current-voltage (I-V) characteristics, cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS), bending cycle, and tensile tests. Furthermore, a capacitor based on our electrode

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demonstrated LED turning on/off performance after short charging time. The above results suggest that the fabricated flexible electrodes without any additional metallic current collectors exhibited increased charge mobility and good mechanical properties; these electrodes also allowed greater access to the electrolyte ions and hence are suitable for use in energy storage applications, sensors and actuators, for all of which electrodes need to be flexible.

Experimental Section Preparation of 3D Graphene-Nanotube-Palladium Nanostructures: For the three-dimensional carbon nanostructures, three materials were used: graphite powder (99.98%, SAMJUNG C&G), palladium (II) acetate catalytic precursor (98%; Sigma-Aldrich), and an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4), MERCK). Graphite oxide was prepared using the modified Staudenmaier method.[36] Similar quantities (0.5 g each) of these ingredients were mixed and ultra-sonicated for 30 min. Subsequently, the resulting mixture was subjected to microwave irradiation at 700 W for about 10 min to yield a fluffy powder comprising the three-dimensional carbon nanostructures. Preparation of Flexible Electrode: A number of different flexible electrodes were fabricated by mixing various graphene-based materials (described in the scheme), poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), and N,N-dimethyl formamide (DMF). The PEDOT:PSS solution, which is usually used as a conducting polymer, was purchased from H.C. Starck (CLEVIOS PH 1000). The weight ratio of PEDOT to PSS was 1:2.5. The organic solvent, DMF, was purchased from SigmaAldrich. Reduced graphene oxide (rGO), palladium nanoparticles decorated on graphene (G-Pd), and graphene-nanotube-palladium nanostructures (G-CNT-Pd) were the graphene-based materials used. The PEDOT:PSS (2 ml/mg) solution was mixed with DMF (0.1 ml/mg); the solution was stirred for 30 min at room temperature. Then, the graphene-based materials (10 mg) were dispersed into the above solution, and the solution was stirred again for 1 h to create a homogeneous suspension. Next, the well-mixed solution was poured into a glass Petri dish to make the electrode and was cured at 45 °C in a slight vacuum (–0.09 MPa) for 15 h to remove any residual solvent. Electrical Analysis: The current-voltage (I-V) characteristics were measured using a multichannel potentiostat/galvanostat/ impedance analyzer (VersaStat3, Princeton Applied Research). A bias between -1 and 1 V was applied in order to compare the electrical conductivity of the electrodes whose size was 2 × 1 cm2. Electrochemical Analysis: Electrochemical analysis of the materials was performed via cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS), using a multichannel potentiostat/galvanostat/impedance analyzer (VersaStat3, Princeton Applied Research). In the case of the flexible electrodes, the electrochemical properties of the electrodes were studied via cyclic voltammetry and galvanostatic charge/discharge using a three-electrode half-cell system; the properties were also studied through EIS, using a two-electrode half-cell system with 0.5 M 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4)/acetonitrile (AN) as the electrolyte. The cyclic voltammetry measurements were taken over a voltage window that

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extended from –1.0 V to 1.0 V (vs. Ag/AgCl), at varying scan rates of 50, 100, and 150 mV/s. Galvanostatic charge/discharge curves were measured within the potential window from 0 V to 0.6 V (vs. Ag/AgCl) at a current density of 10 A/cm3. The EIS-based measurements were performed over frequencies ranging from 100 kHz to 10 Hz with the amplitude of the alternating current (AC) voltage of 50 mV (rms) for an open-circuit potential of 0.3 V. A 0.5 M solution of BMIM-BF4 in acetonitrile (AN) was used for the electrolyte. The obtained data were fitted to the proper equivalent circuits using ZSimpWin 3.21 software. To calculate the specific capacitance, we used the general equation; C = Q/(Vol* ΔE) = ∫ IdE /(Vol* ΔE*ν). The volumetric specific capacitance C[F/cm3] of the electrode is determined using the charge Q, integrated from cyclic voltammetry curves ∫ IdE and divided by scan rate v[V/s]. Then, the total charge Q is divided by the volume Vol[cm3] of the electrode and the potential window ΔE [V].[37]

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by a National Research Foundation of Korea Grant funded by the Korean Government (2012R1A2A2A01047543). It was also supported by the International Cooperation of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government ministry of Knowledge Economy (No. 20128510010050). The authors gratefully acknowledge the financial support provided by Defense Acquisition Program Administration and Agency for Defense Development under the contract UD130049GD. H.R. thanks the CNRS and the Université Paris Diderot for their support.

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small 2014, DOI: 10.1002/smll.201401613

Highly Conductive, Capacitive, Flexible and Soft Electrodes

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small 2014, DOI: 10.1002/smll.201401613

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Received: June 4, 2014 Revised: July 21, 2014 Published online:

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Highly conductive, capacitive, flexible and soft electrodes based on a 3D graphene-nanotube-palladium hybrid and conducting polymer.

Highly conductive, capacitive and flexible electrodes are fabricated by employing 3D graphene-nanotube-palladium nanostructures and a PEDOT:PSS conduc...
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