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3-V Solid-State Flexible Supercapacitors with IonicLiquid-Based Polymer Gel Electrolyte for AC Line Filtering Yu Jin Kang, Yongju Yoo, and Woong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02690 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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ACS Applied Materials & Interfaces

3-V Solid-State Flexible Supercapacitors with IonicLiquid-Based Polymer Gel Electrolyte for AC Line Filtering Yu Jin Kang, Yongju Yoo, and Woong Kim* Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea

AUTHOR ADDRESS E-mail address: [email protected]

ABSTRACT State-of-the-art solid-state flexible supercapacitors with sufficiently fast response speed for AC line filtering application suffer from limited energy density. One of the main causes of the low energy density is the low cell voltage (1 V), which is limited by aqueous-solution-based gel electrolytes. In this work, we demonstrate for the first time a 3-V flexible supercapacitor for AC line filtering based on an ionic-liquid-based polymer gel electrolyte and carbon nanotube electrode material. The flexible supercapacitor exhibits an areal energy density that is more than

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20 times higher than that of the previously demonstrated 1-V flexible supercapacitor (0.66 vs. 0.03 µWh/cm2) while maintaining excellent capacitive behavior at 120 Hz. The supercapacitor shows maximum areal power density of 1.5 W/cm2 and time constant of 1 ms. The improvement of the cell voltage while maintaining the fast-response capability greatly improves the potential of supercapacitors for high-frequency applications in wearable and/or portable electronics. KEYWORDS: flexible supercapacitor; ion gel; carbon nanotube; AC line filter; power performance INTRODUCTION The demand for the development and improvement of next-generation electronics such as flexible, wearable, and portable electronics is ever increasing.1-4 Although various active components in electronic devices, such as transistors, light-emitting diodes, and sensors, have been intensively studied for these future applications,5-8 the passive components such as capacitors are relatively underdeveloped. Nevertheless, studies on the passive components are critical especially for the miniaturization of wearable and/or portable electronics. For example, the aluminum electrolytic capacitors (AECs) commonly used for AC line filtering in various power supplies are often one of the bulkiest components of electronic devices.9-12 The standard 60 Hz AC power line voltage is rectified to a 120 Hz pulsating DC signal, which is then smoothed to a sufficiently constant DC output voltage required by electronic circuits via AC line filtering.13 The replacement of the bulky AECs with supercapacitors may greatly contribute to the miniaturization of these devices; supercapacitors have higher volumetric energy density and are thus more compact than AECs.9-12,14-17 However, the response speed of current commercial


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supercapacitors with activated carbon based electrode material is not sufficiently fast and cannot be used for AC line filtering without significant improvement of the power characteristics.9-10,14 The implementation of relatively new electrode materials such as carbon nanotubes (CNTs), graphene, and silicon carbide (SiC) nanowires have significantly improved the response speed of flexible supercapacitors.10,14,18 The pore structure of CNT-, graphene-, and SiC-based electrode materials are relatively simple and open, which allows fast ion mobility in the pores. In contrast, the activated carbons based electrode material in commercial supercapacitors exhibits a narrow and tortuous pore structure, which greatly limits the response speed.10,14,19 However, despite the improvement in the rate capability using CNT, graphene, and SiC, the power performance has not been sufficiently high for AC line filtering applications, as indicated by the low negative phase angle (-φ = 15° at 120 Hz) or high relaxation time constants (τo = 10–170 ms) of most flexible supercapacitors with gel electrolytes.20-23 The ideal phase angle for AC line filtering applications is –90° at a frequency of 120 Hz, and the time constant corresponding to 120 Hz is 8.3 ms.11,12,15,17 Very recently, a sufficiently fast response flexible supercapacitor has been demonstrated (-φ ~75° at 120 Hz) for an ultrathin composite film of graphene and conductive polymers and H2SO4/poly(vinyl alcohol) (PVA) gel electrolytes; however, the cell voltage (V) was limited (~1 V).15 CNTs and graphene are considered promising electrode materials for high-voltage flexible supercapacitors for AC line filtering.11,15 For instance, the excellent performance of a CNT supercapacitor for AC line filtering was recently demonstrated with a liquid electrolyte.10,14 Advantages of the CNT film over other carbon based electrode materials such as vertically oriented graphene nanosheets, electrochemically reduced graphene oxides, and activated carbon have been described in details in these references.10,14 Briefly, the pore structure and the 3 ACS Paragon Plus Environment


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appropriate material density of the CNT film ensure both fast diffusion of even large ions in organic electrolytes such as tetraethylammonium and tetrafluoroborate and high energy density, respecitvely.10 This result suggests that CNT supercapacitors may also be compatible with ionic liquids with comparable ion sizes for high-frequency applications. Furthermore, ionic liquids have wider electrochemical stability windows than conventional aqueous and organic electrolytes, which will lead to a higher V.

24-26

It is also possible to fabricate gel-type

electrolytes by simply mixing the ionic liquid and silica nanoparticles or polymers, which enables the fabrication of flexible supercapacitors.27,28 However, it is also important to develop gel-type electrolytes for wearable and portable applications because conventional liquid electrolytes have intrinsic problems such as leakage and fast solvent evaporation.29,30 Moreover, the electrochemical stability window of gel-type electrolytes determines V. High V is an essential element for the realization of compact supercapacitors with high energy density (E ∝ V2).24,31 Most of the aforementioned flexible supercapacitors

accommodate

conventional

H2SO4/PVA-

or

H3PO4/PVA-based

gel

electrolytes.15,18,21-23,29,32,33 These gel electrolytes are based on aqueous solutions, and the cell voltage is limited to ~1 V because of the water decomposition. A 2-V flexible supercapacitor has been developed with an ionic-liquid-based gel electrolyte; however, the response speed was not sufficient (-φ = 15° at 120 Hz) for AC line filtering.20 In addition, 3-V flexible supercapacitors have been demonstrated with ionic-liquid-based silica or polymer gel electrolytes; however, the properties at high frequencies have not been characterized nor reported.27,28,34 In this work, we demonstrate 3-V flexible supercapacitors with AC line filtering capability (-φ = 80° at 120 Hz) based on ultrathin CNTs and ion gel electrolytes. The flexible supercapacitors with the CNTs and ion gels exhibit enhanced energy (Eareal) and power (Pareal) densities by 4 ACS Paragon Plus Environment

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orders of magnitude compared with those of previously demonstrated flexible supercapacitors. The response speed of the flexible supercapacitors is highly dependent on both the loading mass of the electrode materials and the thickness of the electrolytes. These two parameters are in a trade-off relationship and are optimized for both AC line filtering applications. Moreover, we successfully demonstrate the conversion of a 60-Hz AC signal to DC signal with the optimized supercapacitor incorporated in a filtering circuit. Finally, the performance of the CNT/ion gel supercapacitor is compared with those of representative fast-response flexible supercapacitors developed thus far.

EXPERIMENTAL SECTION Preparation and Characterization of Materials. Ultrathin CNT films (thickness = 50–300 nm) were prepared using a vacuum filtration technique and transferred onto Au-coated Al foil, as described elsewhere.10 Single-walled carbon nanotubes (SWNTs) were purchased from Sigma– Aldrich (average diameter ~1.3 nm, median aspect ratio ~850). The specific surface area of the SWNTs was 1070 m2/g, as analyzed using the Brunauer–Emmett–Teller method (BET, BELSORP-mini II).35 A poly(styrene–block-ethylene oxide–block-styrene) (PS–PEO–PS) triblock copolymer was purchased from Polymer Source with a number average molecular weight (Mn) of 83 kg/mol (Mn(PS) = 12 kg/mol and Mn(PEO) = 59 kg/mol). Gel electrolytes were prepared by mixing the PS–PEO–PS tri-block copolymer (50 mg) and 1-ethyl-3methylimidazolium bis(trifluoromethysulfonyl)imide ([EMIM][NTf2], C-TRI, 0.7 mL) in acetonitrile (1.4 mL). The mixture was stirred magnetically for 12 h in a glove box under an argon atmosphere. The morphology of the CNTs and poly(tetrafluoroethlyene) (PTFE) separator 5 ACS Paragon Plus Environment


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were characterized using scanning electron microscopy (SEM, Hitachi S4800). The thickness of the CNT film was estimated based on SEM and atomic force microscopy (AFM, Park Systems Xe-100) images. Fabrication and Characterization of Supercapacitors. Briefly, a CNT film was prepared via a vacuum filtration technique by pouring a CNT solution through a porous anodic aluminum oxide (AAO) membrane in a Buchner funnel. After chemically removing the AAO membrane, the remained CNT film was transferred onto a current collector. An electrode area was 1 cm2 and the CNT mass on each electrode was varied from 4 to 32 µg/cm2. Experimental conditions and fabrication procedure were described in details elsewhere.10 A small portion of the polymer/ionic liquid/acetonitrile solution was dropped on a CNT/Al foil electrode and dried in a vacuum oven at 80 °C for 8 h to evaporate the acetonitrile solvent. This step led to the formation of a gel on the surface of the CNT/Al foil. Two ion-gel-coated electrodes were assembled with a PTFE separator (Milipore, 0.1-µm pore size, 30-µm thickness) inserted between them. The assembly was pressed under a 10-kg steel disk for 10 min, which allows the ion gel to permeate into the pores of the separator. The distance between the two electrodes was controlled by the number of 30-µm thick separators (30, 60, and 90 µm). The electrochemical properties of the supercapacitors were measured in a two-electrode configuration using an electrochemical analyzer (Bio Logic, VSP300). The electrochemical impedance spectroscopy was performed by varying the frequency from 1 MHz to 0.1 Hz of an AC voltage (Vpeak = 10 mV) without DC bias. The input AC signal was generated using a function generator (Keysight Technologies, 33210A), and the output signal was recorded with a digital oscilloscope (Keysight Technologies, DSOX2004A). The AC input was rectified using a full-bridge diode (Multicomp, W02M), and a resistor (Sparkfun Electronics, 10 kΩ) was used as a load. 6 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION The flexible supercapacitor compatible with AC line filtering function has a layered structure consisting of Al foil current collectors, CNT thin-film electrodes, and a PTFE separator permeated with an ionic-liquid-based polymer gel electrolyte (Figure 1a). A uniform ultrathin film of CNTs can be fabricated via a vacuum filtration technique, as described in the experimental section. The CNT film was deposited on an etched Al foil. The etching process roughens and enlarges the Al surface, which typically reduces contact resistance between electrode materials and current collectors.36 For the examination of their morphology and thickness measurement, they were deposited on smooth SiO2/Si substrates (Figure 1b and c). Top- and cross-sectional- views of the randomly entangled CNTs are shown. The thickness of the CNT film was measured to be 130.0 ± 13.7 nm (mean ± one standard deviation) using AFM (Figure 1d). The surface morphology of the PTFE separator is shown in Figure 1e. The pore size, thickness, and porosity were approximately 100 nm, 30 µm, and 80%, respectively. Because of the porous nature of PTFE, gel-type electrolytes can easily permeate into the pores of the separator. Ion-gel electrolytes are excellent candidates for flexible and fast-responding supercapacitors because they are flexible and have a well-defined nanostructure, thereby allowing fast ion movement.28 In this work, an ionic-liquid-based tri-block copolymer was used as the gel-type electrolyte. Because of the hydrophobic PS and hydrophilic PEO segments, the polymers selfassembled and formed a nanostructure. According to the small-angle X-ray scattering (SAXS) analysis reported elsewhere,37 the electrolyte has a body-centered cubic (BCC) structure with the PS micelles (radius = 5–6 nm) located at the 8 corners and the center of the BCC unit cell 7 ACS Paragon Plus Environment


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(Figure 1f). The nearest center-to-center distance between the PS micelles is approximately 24– 28 nm. The spacings between the PS micelles are sparsely filled with PEO segments through which ions can travel. The structural analysis indicates that the diameter of the PEO channel is approximately 20 nm. Considering that the molecular weight of our polymer is almost twice as high as that of the polymer used in this SAXS analysis (83 kg/mol vs. 41 kg/mol), the channel size is expected to be even larger. The increase in the channel size according to the molecular weight of PEO segment of tri-block copolymer has been reported elsewhere.38 Because the channel diameter is much greater than the size of the electrolyte ions, the gel electrolyte exhibited a very high ion conductivity that is even comparable to that of neat liquid electrolyte (5–10 mS/cm).37,39,40 Because of the high ionic conductivity of the gel electrolyte originating from its nanostructure, the supercapacitors can exhibit fast frequency responses approaching those of supercapacitors with liquid electrolytes.


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CNT

(b)

(a) Al foil

CNT layers Ion gels/PTFE

500 nm (c)

CNT

1 µm (e)

PTFE

(d)

5 µm (f)

CNT

Thickness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 150 100 50 0 0

5

10

15

Distance (µm)

20

PS PEO

Ions

2 µm

Figure 1. (a) Schematic of all-solid-state supercapacitor. SEM (b) overview and (c) cross-section images of CNT film deposited on Si substrate. (d) AFM image of CNT film. (e) SEM images of PTFE. (f) Schematic of ion gels.



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The supercapacitor with CNT and ion-gel exhibited a fast frequency response that was highly dependent on the thickness of the CNT thin films. As the CNT film thickness decreases from approximately 300 nm (~32 µg/cm2) to 50 nm (~4 µg/cm2), the supercapacitors respond faster. -

φ at 120 Hz increases from 70.6° to 82.7°, respectively, with the decreasing thickness (Figure 2a and Table S1 in the Supporting Information). This increase occurs because the pore length along which the ions have to travel decreases as the CNT thickness decreases. However, the areal capacitance decreases from 233 to 43 µF/cm2, respectively, at 120 Hz as the thickness decreases (Figure 2b). This decrease occurs simply because the amount of the loading mass of the electrode material decreases. Therefore, there is a trade-off relationship between the response speed and capacitance. Nyquist plots reveal near vertical lines and similar equivalent series resistances (ESRs = 1.2–1.6 Ω) regardless of the electrode thickness (Figure 2c). As the thickness, and hence capacitance, decreases, the imaginary part of the impedance at the same frequencies increases. This finding indicates that the supercapacitors exhibit typical electrical double-layer capacitor (EDLC) behaviors. Consistently, τo decreases from 2.5 to 0.3 ms as the film thickness decreases (Figure 2d). The shorter τo corresponds to a faster response speed. The τo is the reciprocal of the frequency at the maximum of the normalized capacitance.41 We suggest that the optimum thickness is approximately 130 nm, where the supercapacitor exhibits -φ = 78.1° and Careal = 106 µF/cm2. Important properties with respect to the CNT loading mass are presented in the Supporting Information, Table S1.


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(b) 2

2

Careal (µF/cm )

80 60 40 2

4 µ g/cm 2 8 µ g/cm 2 12 µ g/cm 2 32 µ g/cm

20 0 -1

10

10

0

1

10

2

10

3

10

4

10

Frequency (Hz)

10

5

(c)

600 400

4 µg/cm 2 8 µg/cm 2 1 2 µg/cm 2 3 2 µg/cm

200 0 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10

6

10

Frequency (Hz)

(d) 20k

2

4 µ g/cm 2 8 µ g/cm 2 12 µ g/cm 2 32 µ g/cm

15k 1.0

10k 0.5

5k 0

0.0 1.0

0

5k

1.5

10k

Z' (Ω)

2.0

15k

20k

Normalized C" (%)

-Phase angle (°)

(a)

-Z" (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100 80

2

4 µg/cm 2 8 µg/cm 2 12 µg/cm 2 32 µg/cm

60 40 20 0 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10

Frequency (Hz)

Figure 2. CNT-mass dependent frequency response of all-solid-state supercapacitors with a gel electrolyte (~30 µm): (a) phase angle, (b) areal capacitance, (c) Nyquist plot, and (d) normalized capacitance.



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Notably, the performance of the supercapacitor with gel electrolyte was nearly comparable to that of the supercapacitor with liquid electrolyte (Figure 3 and Table 1). The supercapacitor with the electrode separation of ~ 30 µm only exhibited a slight reduction in -φ and areal capacitance and a slight increase in ESR and τo. Both the liquid- and gel-electrolyte based supercapacitors are suitable for 120-Hz AC line filtering applications. Although most AC line filtering functions have been based on supercapacitors with aqueous electrolytes,9,11,12,14,16,17 our results demonstrate that supercapacitors with ion-gel electrolytes can also be sufficiently fast for 120-Hz applications. Moreover, this result has been extended to gel electrolytes. This finding was observed because the ion gels can exhibit comparable ionic conductivity to that of neat liquid electrolytes; sufficiently large and organized nanochannels formed via self-assembly of the triblock copolymers allow fast ion diffusion in the ion gels. Importantly, the compatibility of the ionic liquid with fast-response supercapacitors enables high operation voltage (~ 3 V). The performance of the supercapacitor with gel electrolytes is naturally lower than that of the supercapacitor with organic electrolytes mainly due to lower ionic conductivity of electrolytes (Table 1).10 However, the higher voltage and the mechanical flexibility are noticeable advantages of the ion-gel based supercapacitors over the organic-liquid-electrolyte based supercapacitors. The frequency response of the supercapacitors is dependent on the thickness of the gel electrolytes. The thickness of the gel electrolytes can be controlled by the thickness or the number of PTFEs permeated with the gel. As the separation between two electrodes decreases, -

φ at 120 Hz increases from 69.4° to 78.1°, and the areal capacitance decreases from 106 to 83 µF/cm2, as observed in Figure 3a and 3b, respectively. The near-vertical Nyquist plots demonstrate the fast response of the supercapacitors (Figure 3c).10,42,43 The inset shows that the ESR decreases from 4.2 to 1.5 Ω. This change is mainly due to the reduction in ion resistance of 12 ACS Paragon Plus Environment

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the electrolytes, which is proportional to the thickness of the electrolytes. Finally, τo also decreases from 2.5 to 1.0 ms. All the results indicate that the frequency response becomes faster as the electrolyte thickness decreases.

(b) 80

400

2

Careal (µF/cm )

-Phase angle (° )

(a)

60 40 Liquid, 30 µm Gel, 30 µm Gel, 60 µ m Gel, 90 µ m

20 0 -1

10

10

0

1

10

2

10

3

10

4

10

10

5

200 100

Frequency (Hz)

(d) Liquid, 30 µ m Gel, 30 µm Gel, 60 µm Gel, 90 µm

12k 9k 5 4

6k

3 2

3k 0

1 0 0

0

3k

1

6k

Z' (Ω)

2

3

9k

4

5

12k

Normalized C" (%)

(c)

300

Liquid, 30 µ m Gel, 30 µm Gel, 60 µm Gel, 90 µm

0 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10

6

10

Frequency (Hz)

-Z" ( Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100 80

Liquid, 30 µm Gel, 30 µm Gel, 60 µm Gel, 90 µm

60 40 20 0 -1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10

Frequency (Hz)

Figure 3. Frequency response of liquid-electrolyte- or gel-electrolyte-based supercapacitors with various separator thicknesses (CNT mass on an electrode = 12 µg): (a) phase angle, (b) areal capacitance, (c) Nyquist plot, and (d) normalized capacitance.



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Table 1. Properties of supercapacitors as a function of electrolytes and separator thicknesses

CNT mass (µg)

Separator thickness (µm)

Careal at 120 Hz (µF/cm2)

-Phase angle at 120 Hz (°)

τ0 (ms)

ESR (Ω)

[EMIM][NTf2]

12

30

128

80.1

0.794

0.98

[EMIM][NTf2]/SOS

12

30

106

78.1

1.000

1.52

[EMIM][NTf2]/SOS

12

60

94.7

73.2

1.994

2.81

[EMIM][NTf2]/SOS

12

90

82.8

69.4

2.512

4.19

TEABF4/Acetonitrile

16

30

186

82.3

0.316

0.26

Electrolyte


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The outstanding power performances of the solid-state flexible supercapacitors are revealed using various characterization approaches. The cyclic voltammogram (CV) is almost rectangular at high scan rate (300 V/s), and the galvanostatic charge–discharge curves (GCD) are almost triangular at high current density (20 mA/cm2), which are ideal EDLC behaviors (Figure 4a,b and Figure S1 in the Supporting Information). The supercapacitors retain the areal capacitance well even at high current density. The capacitance decreases by approximately 20% from 342 to 273 µF/cm2 (or 57 to 46 F/g) while the current density increases from 1.5 to 24 mA/cm2 (Figure 4c). The cycle stability is also adequate; the capacitance decreases by approximately 15% over 10,000 charge–discharge cycles at a current density of 2 mA/cm2 (Figure 4d). This decrease can be attributed to the water absorption by ionic liquids over time. The significant degradation in capacitance over repeated charge-discharge cycles due to increased water content has been observed in various energy storage devices.44,45 We expect that the cycle stability can be improved via rigorous packaging. Csp and Careal were obtained using the following equations: ସ∙ூ

‫ܥ‬௦௣ = ெ∙ௗ௏/ௗ௧

(1)

ூ

‫ܥ‬௔௥௘௔௟ = ஺∙ௗ௏/ௗ௧

(2)

where I is the applied current, dV/dt is the slope of the discharging curve, M is the mass of CNTs on both electrodes, and A is the area of the cell. Csp is the specific capacitance of a single electrode and Careal is the areal capacitance of the supercapacitor cell. The CNT/ion-gel-based supercapacitor is superior to previous solid-state supercapacitors especially in terms of cell voltage and response speed. We have listed solid-state supercapacitors exhibiting fast-response capabilities in Table 2. Most of these supercapacitors rely on aqueous electrolytes and hence exhibit low operational voltages (< 1 V) and correspondingly low energy densities. One 15 ACS Paragon Plus Environment


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supercapacitor was fabricated with ionic-liquid-based gel electrolytes but exhibited an operation voltage of 2 V and a much lower response speed, as indicated by the low magnitude of the phase angle (φ ~ 15°).20


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(b) 300 V/s 100 V/s 50 V/s

400 200 0

-200 -400 1

2

Potential (V)

(c) 2

400 300 200 100 0

5

10

15

20

1

0.2

0.4

25 2

0.6

0.8

1.0

1.2

Time (s)

(d)

500

0

2

0 0.0

3

Capacitance retention (%)

0

2

20 mA/cm 2 10 mA/cm 2 5 mA/cm 2 2 mA/cm

3

Potential (V)

600

2

Careal (µF/cm )

(a)

Careal (µF/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 80 60 40 20 0

0

Current density (mA/cm )

2000

4000

6000

8000 10000

Cycle number

Figure 4. Electrochemical properties of an all-solid-state supercapacitor (separator thickness = 30 µm, mass of CNTs on an electrode = 12 µg): (a) CV curves, (b) GCD curves, (c) areal capacitance at various current densities, and (d) capacitance retention.



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Table 2. Performance comparison of all-solid-state flexible supercapacitors.

a

Gel electrolyte

b

Electrode

Voltage Window (V)

τ0 (ms)

−Phase angle at 120 Hz (°)

f at -45° (Hz)

Careal (µF/cm2)

Electrode Thickness (nm)

ESR (Ω)

Pmax (mW/cm2)

E (µWh/cm2)

reference

[EMIM][NTf2]/S OS

SWNT

3

1

78.1

1000

342

50-300

1.5

1500

0.66 (0.1 V/s)

This work

[EMIM][BF4]/ [PVdF-HFP]

f-RGO

2

2.4

3-V Solid-State Flexible Supercapacitors with Ionic ... - ACS Publications

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