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Printable Thin Film Supercapacitors UsingSingle-Walled Carbon Nanotubes

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Printable Thin Film SupercapacitorsUsing Single-Walled Carbon NanotubesMartti Kaempgen, Candace K. Chan, J. Ma, Yi Cui, and George Gruner

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Printable Thin Film SupercapacitorsUsing Single-Walled Carbon Nanotubes

Martti Kaempgen,†,‡ Candace K. Chan,†,§ J. Ma,‡ Yi Cui,*,| and George Gruner*,‡

Department of Physics and Astronomy, UniVersity of California, Los Angeles,

California 90095, and Department of Chemistry and Department of Materials Science

and Engineering, Stanford UniVersity, Stanford, California 94305

Received December 22, 2008; Revised Manuscript Received March 18, 2009

ABSTRACT

Thin film supercapacitors were fabricated using printable materials to make flexible devices on plastic. The active electrodes were made from

sprayed networks of single-walled carbon nanotubes (SWCNTs) serving as both electrodes and charge collectors. Using a printable aqueous

gel electrolyte as well as an organic liquid electrolyte, the performances of the devices show very high energy and power densities (6 W h/kg

for both electrolytes and 23 and 70 kW/kg for aqueous gel electrolyte and organic electrolyte, respectively) which is comparable to performancein other SWCNT-based supercapacitor devices fabricated using different methods. The results underline the potential of printable thin film

supercapacitors. The simplified architecture and the sole use of printable materials may lead to a new class of entirely printable charge

storage devices allowing for full integration with the emerging field of printed electronics.

Introduction. Portable electronic devices (e.g., mobile

phones, notebooks, cameras) are reaching a point where

increased functionality is limited by existing technologies

for energy management.1-3 Conventional charge storage

devices, such as batteries, cannot provide the peak power

needed without becoming too heavy or bulky. In addition,

future developments are moving toward thin, light, cheap,and often also flexible solutions, with wearable electronics

as one typical application. Here, the emerging field of

“printed electronics”, including printable transistors, solar

cells, and organic light-emitting diodes has the potential to

achieve these goals in future applications, but a comple-

mentary energy source is still missing. Although there have

been reports on flexible batteries4,5 and electrochemical

double-layer capacitors, also known as supercapacitors,6,7

these are rather thinned-down versions of conventional

devices. Truly printable charge storage devices that can be

easily fabricated using large-scale, solution-based, roll-to-

roll processing, while still displaying good electrochemical

performance, are still needed. Only fully printable charge

storage devices would allow for full integration into the

manufacturing process of printed electronics.

In this context, the idea of “printed power” has been

recently evaluated in our group.8 We have demonstrated that

both batteries and supercapacitors can be realized with

printable nanoscale materials9,10 Because of the higher power

densities possible compared to batteries, supercapacitors, in

addition to significant storage capacity, can provide current

boosts on high load demands to supplement battery perfor-

mance. Thus, supercapacitors are expected to help batteries

to meet current and future energy needs.

Here we discuss our progress toward fully printable high-performance supercapacitors based on thin films of single-

walled carbon nanotubes (SWCNTs). The architecture we

pioneered involves the use of such carbonaceous material

as both the electrode and charge collector and has several

advantages. Using the same material for this dual purpose

eliminates the metal-carbon charge collector-electrode

interface, leading to a simplified and lightweight architecture.

The high current carrying capability together with the

substantial mechanical strength of the SWCNT networks also

allows for the realization of flexible yet robust devices, which

are required for mobile applications. Because SWCNTs can

be solubilized in a variety of formulations and deposited fromsolution, SWCNT thin films can be easily deposited using

printing techniques.11-13 Thus, the thin film architecture based

on SWCNT networks has the potential for fully printable

devices with the ability of full integration with printable

electronics.

Compared to other carbonaceous materials, carbon nano-

tube networks provide a significantly higher conductivity due

to a decreased contact resistance from fewer interparticle

contacts. In the case of SWCNTs, the conductivity is high

enough to make a metallic current collector in many cases

unnecessary. In contrast, films of amorphous carbon, the state

* To whom correspondence should be addressed: ggruner@ucla.edu(G.G.) and yicui@stanford.edu (Y.C.).

† These authors contributed equally.‡ Department of Physics and Astronomy, University of California.§ Department of Chemistry, Stanford University.| Department of Materials Science and Engineering, Stanford University.

NANO

LETTERS

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of the art material in supercapacitor devices, have a rather

poor conductivity requiring an extra metallic foil as a current

collector, which adds extra weight. Carbon nanotube (CNT)

networks also have a superior robustness in terms of bending

and abrasion, and their functionalities do not suffer upon

mechanical stress14-16,18,19 Also, networks in general offer a

high fault tolerance since many different current pathwaysremain possible even with a few disconnected or missing

links within the network. CNT networks, in particular, also

have a high electrochemical stability due to slow kinetics of

carbon oxidation, which is at least comparable to amorphous

carbon.17 This allows, in combination with the increased

conductivity, the entire replacement of amorphous carbon

in many electrochemical devices without lowering the device

performance. This has already been demonstrated success-

fully for transparent electrodes,14,9 batteries,10 and fuel cells.18

The CNT electrodes, however, are often fabricated using

means that are difficult or too expensive to scale up for

widespread commercialization. These methods include vacuum

filtration to form thick, free-standing membranes also known

as “buckypapers”,19-21 chemical vapor deposition of verti-

cally aligned CNTs22-27 inside anodic alumina pores,28

electrophoretic deposition,29 and mixing with a binder30 and

subsequent pressing into a pellet.31,32 In many of these cases,

a metallic current collector is still needed. To date, devices

based on nanofiber networks without current collectors have

only been demonstrated for poly(3,4-ethylenedioxythiophene)(PEDOT).33 Regarding CNT networks, the double-layer

capacitance has been evaluated for thin films sprayed onto

metallic substrates.34 However, in both cases, the fundamental

behavior of single films was studied using two-electrode

measurements in liquid electrolyte, whereas in our case we

wish to assemble and evaluate the printable, symmetric two-

electrode device architecture.

In order to make our devices fully printable, we explored

the use of printable solid gel or polymer electrolytes. The

use of a liquid electrolyte in an electrochemical device

requires both robust encapsulation to prevent leakage and a

separator to avoid electrical contact between the electrodes.On the other hand, solid gel or polymer electrolytes35,36 offer

dual functionality as they combine the separator and the

electrolyte into a single layer. It also avoids potential leakage

since the electrolyte is bound within the polymer matrix. The

performance of gel electrolytes has already been demon-

strated to be comparable to its liquid counterparts.37,38 Also,

flexible devices based on PVA/H3PO4, the electrolyte of

choice within this work, have already been demonstrated.39

Device Fabrication. We have employed a simple device

architecture consisting of two thin SWCNT films that serve as

Figure 1. (a) Scanning electron microscopy image of as-depositedSWCNT networks. (b) Thin film supercapacitor using sprayedSWCNT films on PET as electrodes and a PVA/H3PO4 basedpolymer electrolyte as both electrolyte and separator.

Figure 2. Cyclic voltammetry of SWCNT supercapacitor in (a) PVA/H3PO4 polymer electrolyte and (b) LiPF6 /EC:DEC. Galvanostaticcharge/discharge curves measured with a 1 mA/cm2 (30 mA/mg) current density for a thin film SWCNT supercapacitor using (c) PVA/ H3PO4 polymer electrolyte and (d) LiPF6 /EC:DEC. The curves obtained using the other aqueous electrolytes investigated were similar and

are not displayed.B Nano Lett., Vol. xx, No. x, XXXX

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both electrodes and charge collectors enclosing a polymer or

liquid electrolyte. Purified SWCNT material (highly purified

P3 material, Carbon Solutions Inc.) was suspended as purchased

into water (1-2 mg/mL) with the aid of a tip sonicator

(Supporting Information). With an air brush pistol, the stable

suspension was sprayed onto polyethylene-therephthalate

(PET) substrates placed on a hot plate. During spraying, the

water evaporates and the SWCNTs form an entangled

random network on the PET (Figure 1a). Afterward, the

SWCNT-coated PET substrates were used as thin film

electrodes without any further treatment. The mass of the

SWCNTs coated on each substrate was determined by

weighing the PET before and after the deposition. All

SWCNT films used typically had a sheet resistance of

∼40-50 Ω, a transmittance of ∼12%, and a thickness of

∼0.6 µm. The gel electrolyte was prepared by mixingpoly(vinyl alcohol) (PVA) powder with water (1 g of PVA/

10 mL of H2O) and concentrated phosphoric acid (0.8 g).

Upon evaporation of excess water, the electrolyte solidifies.

The CNT networks and the gel electrolyte were sandwiched

together resulting in our device (Figure 1b). For comparison,

liquid electrolytes of 1 M solutions of H3PO4, H2SO4, and

NaCl in water as well as 1 M LiPF6 in 1:1 (by weight)

ethylene carbonate/diethylcarbonate (EC:DEC) (Novolyte)

were prepared as well.

Device Testing and Results. For the aqueous liquid

electrolytes, the measurements were made using a two-

electrode configuration inside a beaker (1.2 cm betweenelectrodes). For the organic electrolyte, the electrodes were

separated by a paper filter membrane (Whatmann 4) wetted

with the LiPF6 /EC:DEC electrolyte.

Cyclic voltammetry was performed to evaluate the stability

of the devices under the voltage ranges used (1 V for aqueous

electrolyte, 1 and 3 V for organic electrolyte). Galvanostatic

charge/discharge measurements were used to evaluate the

specific capacitance (C s) and the internal resistance of the

devices in a two-electrode configuration.

Cyclic voltammetry showed good electrochemical stability

and capacitance in the SWCNT networks (Figure 2a,b).

Significantly, the LiPF6 /EC:DEC electrolyte was stable up

to 3 V, allowing for higher energy densities to be possible.

Typical results of galvanostatic charge/discharge of our

devices are shown for the aqueous (using polymer electro-

lyte) and organic electrolyte in Figure 2c,d using a current

density of 1 mA/cm2 or 30 mA/mg. The charge/discharge

curves also showed good capacitive behavior.

Discussion. Internal Resistances. The internal resistances,

evaluated from the voltage drop at the beginning of each

discharge (indicated by the arrows in Figure 2c,d), are

summarized in Figure 3a for the PVA/H3PO4, the liquid

H3PO4, and LiPF6 /EC:DEC electrolytes using a discharge

current density of 1 mA/cm2 (30 mA/mg). In each case, the

SWCNT network had a sheet resistance of ∼40 Ω.

The difference between the internal resistance in aqueous

and organic electrolyte reflects the significantly lower

conductivity of LiPF6 /EC:DEC (10 mS/cm)40 compared toaqueous electrolytes (100-300 mS/cm).41 Note that there

were no significant differences between the internal resistance

of the PVA/H3PO4 and liquid H3PO4, indicating that the

polymer matrix itself has no significant influence on the

internal resistance when aqueous electrolytes are used.

However, compared to commercial supercapacitors, the

absolute device resistance is still higher for all electrolytes

used due to the rather thick electrolyte layer and the rather

high sheet resistance of the SWCNT film. Current work on

device optimization is addressing these issues.

Specific Capacitances. The specific capacitances C s were

calculated from the galvanostatic discharge curves using C s) i / -[∆V / ∆t ]m ) i / -slope × m, where i is the current

applied, the slope is the slope of the discharge curve after

the iR drop, and m is the mass of carbon nanotubes on one

electrode. In Figure 3b, the specific capacitances are pre-

sented for measurements obtained in various aqueous

electrolytes using a discharge current density of 40 µA/cm2

(1 mA/mg). The performance was comparable in all aqueous

electrolytes used, with C s ranging from ∼90 to 120 F/g. We

also note that the specific capacitance of our nanotube thin

films obtained using the polymer electrolyte was similar to

those obtained in liquid electrolytes, suggesting that the liquid

electrolyte can be substituted with a printable polymer

Figure 3. Electrochemical data as determined from galvanostatic charge/discharge measurements of supercapacitors using various liquidand gel electrolytes. (a) Internal resistance obtained using a 1 mA/cm2 (30 mA/mg) current density. (b) Specific capacitances obtained foraqueous electrolyte using a current density of 40 µA/cm2 (1 mA/mg). (c) Specific capacitances obtained using a 1 mA/cm2 (30 mA/mg)current density.

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electrolyte without losing performance. Figure 3c compares

the specific capacitance obtained when using the polymer

electrolyte with that obtained when using the organic

electrolyte under a 1 mA/cm2 (30 mA/mg) discharge current

density. With this higher current density, the specific

capacitance was ∼36 F/g when using the PVA/H3PO4

electrolyte. The specific capacitance in LiPF6 /EC:DEC was

lower at both 1 and 3 V charging. This can be explained by

ion association hindering the penetration into smaller pores.

Ion association has been observed for 1 M EC:DEC

electrolytes.42 In addition, the higher viscosity of EC:DEC

(ηED:DEC ) 4.7 mPa·s, ηH2O ) 0.89 mPa·s) might lead to

incomplete wetting of the pores. Both phenomena potentially

decrease the effective surface area of the electrode.43

In general, our analysis revealed that the capacitance per

mass of our thin film devices was somewhat higher than that

of thick, free-standing SWCNT films known as Bucky papers(BP) when using the same current density of 1 mA/mg. For

supercapacitors made with BPs, we achieved a specific

capacitance of around 80 F/g in liquid aqueous electrolytes.

For supercapacitors made of thin sprayed films using the

same SWCNT material, however, the specific capacitance

was around 120 F/g. This indicates that wetting is facilitated

in thin CNT films, increasing the effective surface area. This

is true even when a solid gel electrolyte is used. However,

the amount of electrolyte is rather limited in gel electrolytes

and cannot fully penetrate into thicker films. This is

demonstrated in Figure 4 where the thickness dependence

of the capacitance per geometric area (F/cm2

) is shown.While the capacitance increases linearly for the liquid

electrolyte, saturation occurs for the gel electrolyte due to

limited penetration into the porous network. This limits the

performance that can be achieved, and leads to the conclusion

that gel electrolytes are more suitable for thin film electrodes

but less appropriate for applications when large storage

capacity is required. From Figure 4, we found the optimum

thickness of the CNT film to be around 2 µm for the gel

electrolyte, but it may vary for different PVA:H3PO4 ratios.

Energy and Power Densities. In Figure 5, the power (P)

and energy ( E ) densities of the sprayed SWCNT-based

supercapacitors were calculated using

P )V 2 ⁄ [4 RM ] (1)

and

E ) 0.5CV 2 ⁄ M

where V is the voltage applied, M is the total mass of theSWCNT coating on both electrodes, C the measured devicecapacitance, and R the internal resistance, respectively.44

These values are in good agreement with data of CNT-basedsupercapacitors reported by other groups including CNT-only based devices19,29,45,46 as well as chemical vapordeposition grown CNTs on metallic substrates25 or CNTmaterial mixed with binders.47 A commercially availableproduct is also included.48 However, because different currentdensities were used in the cited data, Figure 5 is used onlyto show the general range of our devices. It should also benoted that the devices of all other groups have used metalliccurrent collectors. Therefore, we wish to point out that ourdevice architecture can give comparable results even withthe simple printing deposition technique and lack of ad-ditional current collector. The RC constants for our deviceswere around 0.5 s for the aqueous electrolytes, and 0.1 and0.3 s for the organic electrolyte at 1 and 3 V, respectively.As seen in Figure 5, devices using the LiPF6 /EC:DECelectrolyte take advantage of the larger electrochemicalstability window allowing for a higher operating voltage.

Increasing the voltage up to 3 V increased both power densityand energy density roughly by 1 order of magnitude,confirming E ∼ V 2. This way, the higher operating voltagecompensates the disadvantage of a higher internal resistanceand a lower specific capacitance. The power density obtainedin the organic electrolyte (70 kW/kg) was also quite goodwhen compared to other devices. Higher power densitieshave been reported by Honda et al.25 but with a devicearchitecture using aligned MWNTs on Ta substrates whichcannot be commercially scaled. Even if the mass of theelectrolyte is taken into account assuming that all contributingmesopores have a volume of 40% of the entire CNTnetwork,49,50 the capacitance, power, and energy densities

of our devices decrease only by factors of 1.3 and 1.1 for

Figure 4. Thickness dependence of the capacitance per area forCNT films comparing a liquid (1 M H2SO4) and a gel electrolyte(PVA/H3PO4).

Figure 5. Energy density vs power density plot comparing theSWCNT thin film supercapacitors to values reported by other groups(numbers refer to references). The operating voltages of our devicesare also noted. Red and blue areas enclose the parameters we haveobserved, with the energy and power density normalized to theweight of the nanotubes (which make up the electrode + chargecollector) (solid lines) and to the weight of the nanotubes +electrolyte (electrode + charge collector + electrolyte) (dashedlines).

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PVA/H3PO4 and EC:DEC, respectively, when these poresare filled with electrolyte. This is indicated by the emptycircles in Figure 5.

Using the PVA/H3PO4 aqueous electrolyte and the LiPF6 /

EC:DEC organic electrolyte as model, unoptimized systems,

we have shown that the sprayed SWCNT films can demon-

strate good supercapacitor performance without the additional

need of a metallic current collector.

Conclusion. In conclusion, we have successfully demon-

strated that thin film supercapacitors based on SWCNTnetworks in combination with a printable gel electrolyte have

great potential as printed charge storage devices. Even though

our devices are not yet optimized in terms of electrical

conductivity of the nanotube films and the amount of the

various active components, their performance already spans

the typical range of conventional supercapacitor devices for

all electrolytes used. This can be explained by the increased

effective surface area in the thin films maximizing the

efficiency of thin film CNT supercapacitors compared to the

thick electrodes in regular devices. The simplified architec-

ture with the sole use of printable materials has the potential

to lead to a new class of light, flexible, and printable charge

storage devices. In combination with the established field

of printed electronics, the concept of printed power offers a

new platform of all kinds of lightweight devices.

Acknowledgment. Y.C. acknowledges support from the

Global Climate and Energy Project at Stanford, US Office

of Naval Research, and King Abdullah University of Science

and Technology. C.K.C. acknowledges support from a

National Science Foundation Graduate Fellowship and Stan-

ford Graduate Fellowship.

Supporting Information Available: Description of ma-

terials and methods. This material is available free of charge

via the Internet at http://pubs.acs.org.

References(1) Freiman, J. F. Aerosp. Electron. Syst. Mag., IEEE 1994, 9, 3–6.(2) McCallum, J. C. Price-Performance of Computer Technology; CRC

Press: Boca Raton, FL, 2002.(3) Cross-layer modeling and optimization for wireless multimedia systems,

Scientific Report; Interuniversity Microelectronics Centre (IMEC),Leuven, Belgium, 2006.

(4) Flexible primary thin film battery, www.powerpaper.com.(5) Flexible Li ion thin film battery, www.powerstream.com.(6) CapXX G-series, www.cap-xx.com.(7) PowerStor Aerogel Supercapacitors, F-Series, www.cooperbuss-

mann.com.(8) Kiebele, A; Kaempgen, M; Gruner, G. Nanotechnol. Law Bus. 2008,

5, 7–16.(9) Kaempgen, M.; Ma, J.; Gruner, G.; Wee, G.; Mhaisalkar, S. G. Appl.

Phys. Lett. 2007, 90, 264104–264106.(10) Kiebele, A.; Gruner, G. Appl. Phys. Lett. 2007, 91, 144104–144107.(11) Shiraishi, M.; Takenobu, T.; Iwai, T.; Iwasa, Y.; Kataura, H.; Ata,

M. Chem. Phys. Lett. 2004, 394 (1-3), 110–113.(12) Wei, T.; Ruan, J.; Fan, Z.; Luo, G.; Wei, F. Carbon 2007, 45 (13),

2712–2716.(13) Zhou, Y.; Liangbing, H.; Gruner, G. Appl. Phys. Lett. 2006, 88, 12–

123109.(14) Kaempgen, M.; Duesberg, G. S.; Roth, S. Appl. Surf. Sci. 2005, 252

(2), 425–429.

(15) Cao, Q.; Hur, S.-H.; Zhu, Z.-T.; Sun, Y. G.; Wang, C.-J.; Meitl, M. A.;

Shim, M.; Rogers, J. A. Ad V. Mater. 2006, 18, 304–309.(16) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gruner, G. Nano

Lett. 2005, 5, 757–760.(17) Liang, L.i.; Xing, Y. J. Electrochem. Soc. 2006, 153, A1823–A1828.(18) Kaempgen, M.; Lebert, M.; Nicoloso, N.; Roth, S. Appl. Phys. Lett.

2008, 92, 0941031–0941033.(19) Zhou, C.; Kumar, S.; Doyle, C. D.; Tour, J. M. Chem. Mater. 2005,

17 , 1992–2002.(20) Kim, J. H.; Nam, K.-W.; Ma, S. B.; Kim, K. B. Carbon 2006, 44

(10), 1963–1968.(21) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. J. Electroanal. Chem.

2000, 488 (2), 92–98.(22) Pushparaj, V. L.; et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 13574–

13577.(23) Emmenegger, Ch.; Mauron, Ph.; Sudan, P.; Wenger, P.; Hermann,

V.; Gallay, R.; Zuttel, A. J. Power Sources 2003, 124 (1), 321–329.(24) Chen, J. H.; Li, W. Z.; Wang, D. Z.; Yang, S. X.; Wen, J. G.; Ren,

Z. F. Carbon 2002, 40 (8), 1193–1197.(25) Honda, Y.; Haramoto, T.; Takeshige, M.; Shiozaki, H.; Kitamura, T.;

Ishikawa, M. Electrochem. Solid-State Lett. 2007, 10 (4), A106–A110.(26) Kimizuka, O; Tanaikea, O.; Yamashitaa, J.; Hiraokaa, T.; Futabaa,

D. N.; Hataa, K.; Machidac, K.; Suematsuc, S.; Tamamitsuc, K.;Saekib, S.; Yamadab, Y.; Hatoria, H. Carbon 2008, 46 , 1999–2001.

(27) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.;Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat.

Mater. 2006, 5 (12), 987–994.(28) Ahn, H.-J.; Sohn, J. I.; Kim, Y. S.; Shim, H.-S.; Kim, W. B.; Seong,

T.-Y. Electrochem. Commun. 2006, 8, 513–516.(29) Du, C.; Pan, N. J. Power Sources 2006, 160 (2), 1487–1494.(30) Zhang, H.; Cao, G.; Yang, Y.; Gub, Z. J. Electrochem. Soc. 2008,

15, K19–K22.(31) Liu, C. G.; Fang, H. T.; Li, F.; Liu, M.; Cheng, H. M. J. Power Sources

2006, 160 (1), 758–761.(32) Pico, F.; Rojo, J. M.; Sanjuan, M. L.; Anson, A.; Benito, A. M.;

Callejas, M. A.; Maser, W. K.; Martinez, M. T. J. Electrochem. Soc.

2004, 1516 , A831–A837.(33) Nguyen, H. D.; Ko1, J. M.; Kim, H. J.; Kim, S. K.; Cho, S. H.; Nam,

J. D.; Lee, J. Y. J. Nanosci. Nanotechnol. 2008, 8 , 4718–4721.(34) Zhao, X.; Chul, B. T. T.; Ballesteros, B.; Wang, W.; Johnston, C.;

Sykes, J. M.; Grant, P. S. Nanotechnology 2009, 20, 065605-1-9.(35) Manuel Stephan, A. Eur. Polym. J. 2006, 41 (1), 21–42.(36) Vincent, C. A. Prog. Solid-State Chem. 1987, 17 , 145–261.(37) Nohara, S.; Wada, H.; Furukawa, N.; Inoue, H.; Morita, M.; Iwakura,

C. Electrochim. Acta 2003, 48, 749–753.(38) Morita, M.; Qiao, J. L.; Yoshimoto, N.; Ishikawa, M. Electrochim.

Acta 2004, 50, 837–841.(39) Sunga, J-H.; Kima, S-J.; Jeong, S-H.; Kima, E-H.; Lee, K-H. J. Power

Sources 2006, 162 (2), 1467–1470.(40) Smart, M. C.; Ratnakumar, B. V.; Huang, C. -K.; Surampudi, S., Proc.

ECS Meeting, 193rd 1998.(41) Hamann, C. H., Vielstich, W., Elektrochemie, 4th ed.; Wiley-VCH:

New York, 2005.(42) Stewart, G. S.; Newman, J. J. Electrochem. Soc. 2008, 155 (1), F13–

F16.(43) Conway, B. E., Electrochemical Capacitors; Kluwer Academic/Plenum

Publishers: New York, 1999.(44) Frackowiak, E. Phys. Chem. Chem. Phys. 2007, 9 (15), 177417785).(45) Niu, C. M.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys.

Lett. 1997, 70 (11), 1480–1482.(46) Ma, R.; Liang, J.; Wei, B.; Zhang, B.; Xu, C.; Wu, D. Bull. Chem.

Soc. Jpn. 1999, 72 (11), 2563–2566.(47) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung,

D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H. Ad V. Mater. 2001, 13 (7),497–500.

(48) Maxwell Technologies (www.maxwell.com), data based on theirmedium cell with highest power density available (model BCAP0310).

(49) Lafi, L.; Cossement, D.; Chahine, R. Carbon 2005, 43 (7), 1347–1357.

(50) Hemraj-Benny, T.; Bandosz, T. J.; Wong, S. S. J. Colloid Interface

Sci. 2008, 317 (2), 375–382.

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