Inverse-Ordered Fabrication of Free-Standing CNT Sheets for

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Inverse-ordered fabrication of free-standing CNT sheets for supercapacitors Youngmi Koo, Vesselin Shanov, Sergey Yarmolenko, Mark J. Schulz, Jagannathan Sankar, and YH Yun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00891 • Publication Date (Web): 17 Jun 2015 Downloaded from http://pubs.acs.org on June 22, 2015

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Inverse-ordered fabrication of free-standing CNT sheets for supercapacitor

Youngmi Koo†, Vesselin N. Shanov‡, Sergey Yarmolenko†, Mark Schulz‡, Jagannathan Sankar†, and Yeoheung Yun*,†

†

Engineering Research Center, Department of Chemical, Biological, and Bio Engineering, North Carolina A&T State University, Greensboro, NC 27411, USA ‡

Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221, USA

*To whom correspondence should be addressed. Yeoheung Yun, Ph.D. North Carolina Agricultural & Technical State University NSF Engineering Research Center for Revolutionizing Metallic Biomaterials Fort Interdisciplinary Research Center Building, Room 119 Greensboro, NC 27411, USA Tel: (336) 285-3226 Fax: (336) 256-1153 E-mail: [email protected]

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Abstract Free-standing thin CNT sheets are challenged with handling and controlling for device fabrication and further transportation problem. In this paper, we report on the inverse-ordered fabrication method from thick carbon nanotube (CNT) sheets to thin free-standing CNT sheets. As proof of the concept, thin CNT sheets for supercapacitor were fabricated from thick 200 layers. The results show that thin CNT sheet was electrochemically stable and had enhanced capacitive performance. The smaller the number of layers is, the larger the specific capacitances we have (from 10.10 F g-1 to 51.37 F g-1). Energy and power density were increased from 0.50 to 2.57 Wh kg-1 and 0.33 to 2.31 kWkg-1 respectively. This simple and scalable inverse-ordered method is capable to fabricate CNT sheets in various forms, allowing fast trials on various applications.

Keywords: Carbon nanotubes (CNTs), CNT sheet, Supercapacitor, Nanostructure materials, Flexible device.


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1. Introduction Carbon-based materials (graphene, carbon nanotubes) have been extensively studied for application in energy storage [1, 2], energy conversion [3, 4], sensor [5, 6], self-powered wearable optoelectronics [7, 8], owing to their unique electrical, thermal, and mechanical properties. Carbon nanotubes (CNTs) and graphene sheets have been widely used as active materials in supercapacitors [9-13] or to help improving the performance of supercapacitor by incorporation with conducting polymers [14]. Recently, several results have proposed unique and various potential applications of the transparent and stretchable supercapacitors [15, 16]. Paperlike and film-using carbon-based materials incorporated with embedded materials have been developed using PDMS (Polydimethylsiloxane) [17, 18] to improve layer conductivity and flexibility. However, carbon nanotubes (CNTs) are still challenged with the controllability in alignment-dependent, interconnection-dependent, and layered thickness-dependent electrical conductivity. In order to address these problems, researchers suggested that super-aligned CNT arrays created via chemical vapor deposition (CVD) method were pulled [19] to fabricate highlyaligned CNT sheets. However, it is still challenging to fabricate low resistance and thin layer CNT sheets at large scale for commercialization. Challenges include; 1) handling of CNT sheets such as cutting, bonding, electrically connecting, and designing, and 2) transporting CNT sheets from one place to other since they are flimsy and easily damaged. In this work, we report on the inverse-ordered fabrication method from thick CNT sheets to thin free-standing CNT sheets. We fabricate various layers of CNT sheets by this method and explore electrochemical properties for supercapacitor. Especially, this paper studies the specific capacitance, energy and power density, coulomb efficiency, and alignment degree of CNT sheets.


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2. Experimental 2.1. Preparation of the multi-layered CNT sheet This free-standing CNT sheet was fabricated from 0.5 mm length of multi walled carbon nanotube (MWCNT) array, synthesized by a water-assisted chemical vapor deposition (CVD) [20, 21]. Long and multi-layer sheet was produced by pulling a bundle of nanotubes from one side of an aligned CNT array at a rate of ca. 17 mm/s. Dimensions and thickness of multi-layered sheet was controlled. Because of easy handling and stable delivery purpose, we fabricated 200 layers of CNT sheet on polytetrafluoroethylene (PTFE) substrate [22]. Then it was densified layer by layer using acetone to maintain the longitudinal nanotube orientation and the original dimensions (for more details see Supporting Information, Figure S1).

2.2. Alignment characterization X-Ray Diffraction (XRD) experiments were performed using Bruker AXS D8 Discover diffractometer with a CuKα X-ray radiation, parallel beam optics, LynxEye PSD detector, and centric Eulerian cradle. Orientation and alignment study of as-prepared 200 layered CNT sheet was performed through pole figure measurements by recording the intensity of graphitic diffraction peak at full rotation (phi) and 0 - 90° tilt (chi) of sample with step 2°. Sheet was placed on Si (001) wafer to avoid any background diffraction from substrate which can mask weak diffraction from nanotubes. Alignment of CNT sheet was evaluated by measuring orientation distribution of (002) peak intensity which strongly depended on nanotube orientation against incident X-ray beam. 2D analysis was performed by calculating orientation distribution function (ODF), which was fitted by Gaussian/Lorentzian ratio 45/55. Finally, quantitative alignment of CNT sheet was calculated by Herman’s orientation factor (HOF) [23];


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=

,

(1)

in which

< > =

!"#

,

(2)

where I(φ) is intensity of non-isotropic part of ODF at misalignment angle φ against preferred orientation at maximum of ODF [24]. Parameter HOF is equal 0 if no preferred orientation in the sample and parameter HOF is equal 1 for completely aligned sample.

2.3. Inverse-ordered fabrication Electrode template was drawn using Silhouette Studio® software and adhesive sheets were cut using CAMEO print (Silhouette, Utah, USA). Geometry of the template is following ; 1) inner size (opened window) is 10 mm width × 20 mm length and 2) outer size (adhesive sheet) is 20 mm width × 30 mm length (Figure 1). As-prepared 200-layered CNT sheet was cut into the size of 5 mm width × 30 mm length. Both adhesive templates in the manner of a window were sandwichly adhered to a CNT sheet. Each template sheet was adhered to the side of about 100 CNT sheet substrate which was not already affixed to an template sheet, kept under a 5 pound weight for at least about 5 minutes and then the two opposing template sheets on a single substrate comprising the 100-layered CNT sheet were pulled away from one another at a 30 degree angle to yield two 100-layered CNT sheet substrates, each attached to one template sheet. This process was repeated to reduce the substrates in a stepwise manner: from about 200-layered


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CNT sheet to about 100-layered CNT sheets; from about 100-layered CNT sheet to about 50layered CNT sheets; from about 50-layered CNT sheet to about 25-layered CNT sheets; and from about 25-layered CNT sheet to about 12.5-layered CNT sheets, based on equal mathematical division of CNT sheets to each template sheet. Generally, the resulting substrates of differing numbers of CNT sheets were 30 mm long by 5 mm wide and the thickness was determined by the number of CNT sheets. After reaching a substrate comprising approximately 12.5-layered CNT sheet, the substrate was transparent as well as free-standing, meaning that the substrate had three dimensional/structural robustness and no additional binder was necessary to hold the CNT sheets together. A free-standing substrate did not require binder, glass or silicon support to maintain its physical integrity.

Figure 1. Schematic illustration of the inverse-ordered fabrication of CNT sheets; mathematical statement (1/2n)(Li), where Li is the initial number of CNT sheet layers and n is the number of repeat.

2.4. Physical characterization


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Raman analysis of 200-layered CNT sheet using a LabRAM ARAMIS (HORIBA Scientific) with excitation laser beam wavelength of 633 nm was performed. The surface morphology was characterized by field-emission scanning electron microscope (FE-SEM, Hitachi 8000, 10 kV).

2.5. Electrochemical measurements The electrochemical measurements were done in a three-electrode setup: CNT sheets as the working electrode, a platinum wire electrode and an Ag/AgCl electrode served as counter and reference electrodes respectively. The cyclic voltammetry (CV), constant current chargedischarge, and electrochemical impedance spectroscopy (EIS) were performed using Reference 600TM potentiostat (Gamry Instrument, USA) in 6 M KOH aqueous electrolyte at room temperature. The specific capacitances of the free-standing CNT sheets, Cspe were calculated according to the following equation [25, 26]

$ %& =

× ∆) ∆* ×+

(3)

where, I (A) is the constant discharge current, ∆t (s) is the time for a full discharge, m (g) indicates the weight of the active materials in the electrode, and ∆V (V) represents the potential drop during discharge. The energy density (E) and power density (P) have been calculated from the charge/discharge data, using the following equation [27]

, = $-

. =

/ ∆)


(4)

(5)

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where, C is the calculated specific capacitance, V is the voltage window (0.6 V minus the IR drop), and ∆t is the discharge time. The columbic efficiency, η was estimated as [28, 29]

0 =

∆)1 ∆)2

× 100%

where, ∆td and ∆tc represent the discharge and charge time, respectively.


(6)

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3. Results and discussions Figure 2 shows CNT sheet which was produced from MWCNT arrays. As shown Figure 2A, 200-layered CNT sheet is highly flexible and bendable. SEM image shows well-alignment structure of the CNT sheet. Electrical resistance of the sheet was less than 3 Ω/square. Raman analysis shows typically three characteristic peaks corresponding to the D, G, and G´ bands observed at 1328 cm-1, 1580 cm-1, and 2641 cm-1 in Figure 2C. The D band of the disordered carbon mainly corresponds to sp3 hybrid bonding and the G band of the graphitized carbon


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Figure 2. Physical properties of CNT sheet. (A) 200-layered CNT sheet which bent without specific binder, (B) SEM images, (C) Raman spectrum of the 200-layered CNT sheet. (D) Inverse-ordered fabricated CNT sheets from 200 layers to 12.5 layers.

contains sp2 hybrid bonding. The ratio of ID/IG was 0.77 for the 200-layered CNT sheet. The G´ band of the second-order harmonic of the D mode is caused by two-phonon scattering [30]. Figure 2D shows photographs of 200-layered CNT sheet and 12.5-layered CNT sheet fabricated by inverse-ordered process from 200-layered CNT sheet. After reaching 12.5 layers by inverseordered process, CNT sheet was transparent as well as free-standing structure. In addition, we measured resistances using end-to-end method (2 point method) with designed CNT sheets (10 mm width × 20 mm length). Resistances of each CNT sheets showed ; 1) less than 3 Ω for 200layered sheet, 2) 7.7 - 10.Ω range for 100-layered sheet, 3) 23 - 27 Ω for 50-layered sheet, 4) 46 50 Ω for 25-layered sheet, and 5) 78 - 90 Ω for 12.5-layered sheet.


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Figure 3. Alignment analysis for the 200-layered CNT sheet. (A) XRD patterns of MWCNT sheet at different orientations, (B) Pole figure for CNT sheet measured at 2Θ = 26°, (C) Rotational profiles (ψ-scans) of integral intensity of graphic peak (002) for MWCNT sheet at different tilt geometries χ, and (D) Orientation distribution functions (lines) for CNT sheet calculated by fitting of averaged azimuthal profiles (dots) obtained from ψ-scans in the range χ = 50 - 75°. Dots of corresponding color (Figure 3B) indicate positions of diffraction patterns presented on Figure 3A.


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Figure 3 shows XRD analysis for the alignment of MWCNT fibers in the sheet. Graphitic peak (002) was observed at 26.03° which corresponds to d = 3.42Å spacing between concentric graphitic shells (Figure 3A). Figure 3B shows pole figure for 002 orientation plane. Pole figure shows sharp fiber-type texture with very good nanotube alignment which supports our analysis of XRD patterns at different χ and ψ (Figure 3A). Maximum intensity of ODF at preferred orientation was observed at azimuthal angles ψ about 90° and 270° and background intensity at 0° and 180°. It was calculated to the full circle of ODF profile by averaging ψ-scans for range of χ = 50 - 75° with step 5° (Figure 3C) and shown by dotted lines on Figure 3D. Best fit of ODF showed average half-width of 30.7 ± 1.5° in Figure 3D. And calculated HOF was 0.78, which value shows good alignment fibers in the CNT sheet. Based on orientation measurement, ODF fitting, and HOF factor, we verified that fibers in the CNT sheet are highly aligned [31].


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Figure 4. CVs for inverse-ordered fabricated CNT sheet electrodes in 6 M KOH; (A) 200-layered CNT sheet, (B) 12.5-layered CNT sheet, (C) CVs of various layered-CNT sheets at constant scan rate, 1000 mV/s, (D) Current densities of CNT sheets with the scan rates of 25, 50, 100, 500, 1000, 2000, 3000 mV/s at 0.2 V potential.

As shown in Figure 4, CVs were performed to study the effect of varying the number of layers in the CNT sheet electrodes on the electrochemical behavior of those electrodes. Figure 4A-B shows the CVs of a representative CNT sheets: about 200 layers and about 12.5 layers. The CVs were rectangular-shaped within the applied potential. The peak current response of 200-layered CNT sheet is roughly 80 times larger than that of 12.5 layers. Figure 4C shows CVs


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of different number of CNT sheets in CNT substrate at constant scan rate, 1000 mV/s. With an increase in the number of CNT sheets, an increase in current density was observed. Also it shows typical capacitance behavior (rectangular shape) [25, 32]. Figure 4D describes the current response of different number of layers in CNT sheets at various scan rates, from 25 to 3000 mV/s. As the scan rate and number of layers increased, the current linearly increased at constant potential, 0.2 V.


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Figure 5. Impedance plots of CNT sheet electrodes fabricated by inverse-order process in 6 M KOH. (A) The Nyquist diagram of the 200, 100, 50, 25, and 12.5 layers with frequency ranging from 100 kHz to 0.1 Hz, (B) Bode plots of the 200, 100, 50, 25, and 12.5 layers between 100 kHz and 0.1 Hz frequency, (C) Equivalent circuit for the impedance spectra of different layered CNT sheets.


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Furthermore, electrochemical impedance measurements were carried out to understand electrochemical performance as a supercapacitor with different layers of CNT sheets. Figure 5AB shows the impedance spectra of the different layered CNT sheets as Nyquist plots and the bode plots; the dots represent experimental data, and lines represent a model of an equivalent circuit. The equivalent circuit which was used to explain the impedance plots is shown in Figure 5C. Rs is the electrolyte resistance (the equivalent series resistance, ESR), CPEL and RL are the capacitance and resistance of CNT sheet electrode, and CPEDL and RCT are the double-layer capacitance and charge-transfer resistance, respectively. ZW is the Warburg impedance related to the ionic diffusion into the CNT sheet. The fitted data for all circuit parameters is shown in Table 1. In Figure 5A, all CNT sheets behave like a supercapacitor at low frequency. As shown in inset of Figure 5A, the 12.5-layered CNT sheet also has a supercapacitor property. In high-to-mid frequency range, layers of CNT sheets are thinner, shapes with a slop of about 45° are clearer. These CNT sheets exhibit a “Warburg like” shape for double layer charging of porous electrode. In case of thinner layered sheets, the electrolyte could permeate into the chink of the individual CNT sheet. In 12.5 layers case, it shows more remarkable slope. Also, no distinct semi-circle was observed in high frequency region. It is well known that the semi-circle is related to the presence of an interface between the electrode and the current collector and the electrical charge transfer in the electrode material due to the Faradic process. It can be explained that all the CNT sheets were directly used as electrode and current collector simultaneously. It suggests that CNT sheets inversely fabricated from thick CNT sheets can directly be a binding free electrode without current collector.


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Table 1. Electrochemical parameters of equivalent circuits obtained from best fit to impedance data for the different layered CNT sheets (Figure 5A).

Layers

Rs

CPEL

n1

n

(Ω)

(S·s )

12.5

11.16

1.58 × 10-4

25

3.543

50

RL

CPEDL

n2

n

RCT

ZW

(Ω)

(S·s1/2)

(Ω)

(S·s )

6.77 × 10-1

11.55

4.41 × 10-5

9.69 × 10-1

2.56 × 105

-2.30 × 109

1.10 × 10-3

7.24 × 10-1

2.67

9.53 × 10-4

9.01 × 10-1

1.83 × 104

-4.60 × 105

1.924

7.49 × 10-4

8.07 × 10-1

1.242

1.53 × 10-3

9.11 × 10-1

1.45 × 105

-32.61

100

2.731

2.36 × 10-3

6.83 × 10-1

2.317

2.31 × 10-3

8.90 × 10-1

5.75 × 104

-1.431

200

1.85

1.38 × 10-2

7.11 × 10-1

1.57

7.04 × 10-3

9.76 × 10-1

2.66 × 103

-3.15


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Figure 6. Electrochemical behavior of the CNT sheet electrodes for supercapacitors fabricated by inverse-ordered process. (A) Galvanostatic charge/discharge curves at 1.5 mA, (B) Current per mass of the different 200, 100, 50, 25 layers CNT sheets at 1.5 mA, gram capacitance as a function of the 200, 100, 50, 25 layers, (C) Energy and power density of the supercapacitor (in the 6 M KOH electrolyte) as a function of the different layers, and (D) Cyclic stability of the CNT sheets of the 200 and 50 layers at 1.5 mA, 12.5 layers at 0.1 mA .

Figure 6 shows some capacitive performances of the four different layered CNT sheets. Figure 6A shows the comparison of e typical charge/discharge curves of CNT sheet electrodes between 0 and 0.6 V at constant current of 1.5 mA. The charge curves of the CNT sheets having between about 100-layered and about 25-layered CNT sheets were nearly symmetric with fast charging and discharging capabilities compared to their corresponding discharge curves in the potential range [33]. It is not shown here; these electrodes indicated a high reversibility between charge and discharge processes at cycle performance. On the other hand, 200-layered CNT sheet showed an asymmetric behavior during the charge/discharge process. The energy storage mechanism in this supercapacitor is based on the ion accumulation at the electrode-electrolyte interface. Possibly reason for asymmetric charing/discharging time is that electrolyte is difficult to penetrate quickly into the dense and thick CNT sheets at the electrode-electrolyte interface. Also, poor interfacial contact of the thicker layered CNT sheets might affect for the high internal resistance of the supercapacitor. This can be a factor to indicate lower coulombic efficiency. Figure 6B-C describes the specific capacitance, coulombic efficiency, energy density, and power density of the four different layers, respectively. With the increase of layer in CNT sheets, specific capacitances were decreased. In particular, the specific capacitance of the 25-layered


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CNT sheet dramatically increased more than others. Also, the coulombic efficiency of the 50layered CNT sheet reached almost 100 %, implying good charge/discharge reversibility for the supercapacitor (Figure 6B). Energy and power densities of the four different CNT sheets showed an inverse relationship with the number of CNT sheets. This behavior could be ascribed to the depletion of the immobilization of the charge carriers during charge and discharge processes. As a result, we suggest that CNT sheets which have an adequate stack and large surface area exhibit better electrochemical properties than the excessively dense CNT sheets as shown by the higher specific capacitance, higher coulombic efficiency, higher energy density, and higher power density in Figure 6. Electrochemical cyclic stability of CNT sheets was collected up to 1000 cycles. Figure 6D shows specific capacitance vs. cyclic number of the three different layers, 12.5-, 50-, and 200-layered CNT sheets. All CNT sheets were stable up to 500 cycles at least. CNT sheets thicker than 12.5-layered sheet were showed higher rate capability.

Table 2. Capacitance performance summary of different layers of CNT sheets fabricated by inverse-ordered process (at constant current 1.5 mA).

Parameters

12.5 Layers*

25 Layers

50 Layers

100 Layers

200 Layers

Charge time (s)

4

5

3

14

50

Discharge time (s)

3

4

3

4

6

Energy density (Wh/kg)

0.50

2.57

1.09

0.78

0.54

Power density (W/kg)

606

2312

1312

702

325


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Charge/discharge efficiency (%) *

75

80

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100

29

12

Result for 12.5 layered CNT sheet was measured at constant current 0.15 mA.

A comparison of the results of the different layered CNT sheets which are fabricated by inverse-ordered process from highly aligned and 200-layered CNT sheet was summarized in Table 2. From results, 25-layered and 50-layered CNT sheets have better performance ability than others as a supercapacitor based on [34] at constant current of 1.5 mA. Specific capacitance, 10.10 F g-1 for 12.5-layered CNT sheet was also measured at narrowly low current such as 0.15 mA. It was showed that electrochemical property of 12.5-layered CNT sheet has in whole, decreased, as shown in Table 2. Poor interfacial contact of the 12.5-layered CNT sheet might be the main reason for the high internal resistance of the supercapacitor due to increasing resistance by loose connection as shown in Figure 5. That is, capacitance efficiency of the dense and thick CNT sheets are not fully decreased, the reason may be that the surface area for accessing part of CNT sheets strands of electrolyte is smaller than that of thinner CNT sheets.

4. Conclusions We firstly introduced inverse-order process method to fabricate thin CNT sheets from thick CNT sheets. The different layered CNT sheet electrodes fabricated by inverse-order process showed superior capacitive performances as a supercapacitor. This method and result showed a potential capability such as free-standing substrate and replacement of transparent conductive electrode for energy, environment, and sensor applications.

Acknowledgements


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This research was supported by the Office of Naval Research (N0001411103151), ARO Grants (W911NF-14-1-0143) at North Carolina A & T State University. The NSF funding at the University of Cincinnati through grant SNM-1120382 is gratefully acknowledged.

References (1) Chabi, S.; Peng, C.; Hu, D.; Zhu, Y. Ideal Three-Dimensional Electrode Structures for Electrochemical Energy Storage. Advanced Materials 2010, 26, 2440-2445. (2) Pushparaj, V.L.; Shaijumon, M. M.; Kumar, A.; Murugesan, S.; Ci, L.; Vajtai, R.; Linhardt, R. J.; Nalamasu, O.; Ajayan, P. M.Flexible Energy Storage Devices Based on Nanocomposite Paper. PNAS 2007, 104, 13574–13577. (3) Koo, Y.; Malik, R.; Alvarez, N.; White, L.; Shanov, V. N.; Schulz, M.; Collins, B.; Sankar, J.; Yun, Y. Aligned Carbon Nanotube/copper Sheets: a New Electrocatalyst for CO2 Reduction to Hydrocarbons. RSC Advances 2014, 4, 16362-16367. (4) Yang, Z.; Chen, T.; He,R.; Guan, G.; Li, H.; Qiu, L.; Peng, H. Aligned Carbon Nanotube Sheets for the Electrodes of Organic Solar Cells. Advanced Materials 2011, 23, 54365439. (5) Tortorich, R. P.; Song, E.; Choi, J. Inkjet-Printed Carbon Nanotube Electrodes with Low Sheet Resistance for Electrochemical Sensor Applications. Journal of The Electrochemical Society 2014, 161, B3044-B3048. (6) Srivastava, R. K.; Srivastava, S.; Narayanan, T. N.; Mahlotra, B. D.; Vajtai, R.; Ajayan, P. M.; Srivastava, A. Functionalized Multilayered Graphene Platform for Urea Sensor. ACS Nano 2011, 6, 168-175. (7) Su, F.; Miao, M. Asymmetric Carbon Nanotube-MnO2 two-ply yarn supercapacitors for wearable electronics. Nanotechnology 2014, 25, 135401. (8) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603. (9) Noked, M.; Okashy, S.; Zimrin, T.; Aurbach, D.; Composite Carbon Nanotube/Carbon Electrodes for Electrical Double-Layer Super Capacitors. Angewandte Chemie International Edition 2012, 51, 1568-1571. (10) Hu, S.; Rajamani, R.; Yu, X. Flexible Solid-State Paper Carbon Nanotube Supercapacitor. Applied Physics Letters 2012, 100 104103-1-4. (11) Yu, D.; Dai, L. Self-Assembled Graphene/Carbon Nanotube Hybrid Films for Supercapacitors. The Journal of Physical Chemistry Letters 2009, 1, 467-470. (12) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326. (13) Jha, N.; Ramesh, P.; Bekycarova, E.; Itkis, M. E.; Haddon, R. C. High Energy Density Supercapacitor Based on a Hybrid Carbon Nanotube-Reduced Graphite Oxide


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Architecture. Advanced Energy Materials 2012, 2, 438-444. (14) Dong, X.; Wang, J.; Wang, J.; Chan-Park, M. B.; Li, X.; Wang, L.; Huang, W.; Chen, P. Supercapacitor Electode Based on Three-Dimensional Graphene-Polyaniline Hybrid. Materials. Chemistry and Physics 2012, 134, 576-580. (15) Chen, P.; Shen, G.; Sukcharoenchoke, S.; Zhou, C. Flexible and Transparent Supercapacitor Based on In2O3 Nanowire/Carbon Nanotube Heterogeneous Films. Applied Physics Letters 2009, 94, 043113. (16) Niu, Z.; Dong, H.; Zhu, B.; Li, J.; Hng, H. H.; Zhou, W.; Chen, X.; Xie, S. Highly Stretchable, Integrated Supercapacitors Based on Single-Walled Carbon Nanotube Film. Advanced Materials 2013, 25, 1058-1064. (17) Chen, T.; Peng, H.; Durstock, M.; Dai, L.High-Performance Transparent and Strechable all-Solid Supercapacitors Based on Highly Aligned Carbon Nanotube Sheets. SCIENTIFIC REPORTS 2014, 4, 3612. (18) Wang, X.; Zhang, Y.; Zhi, C.; Wang, X.; Tang, D.; Xu, Y.; Weng, Q.; Jiang, X.; Mitome, M.; Golberg, D.; Bando, Y. Three-Dimensional Strutted Graphene Grown by Substate-Free Sugar Blowing for High-Powe-Density Supercapacitors. Nat Commun 2013, 4, 2905. (19) Jakubinek, M. B.; Johnson, M. B.; White, M. A.; Jayasinghe, C.; Li, G.; Cho, W.; Schulz, M. J.; Shanov, V. Thermal and Electrical Conductivity of Array-Spun MultiWalled Carbon Nanotube Yarns. Carbon 2012, 50, 244-248. (20) Jayasinghe, C.; Li, Y. S.; Abot, J. L.; Shanov, V. N.; Fialkova, S.; Yarmolenko, S.; Sundaramurthy, S.; Chen, Y.; Cho, W.; Chakrabarti, S.; Li, G.; Yun, Y.; Schulz, M. J. Nanotube Responsive Materials. MRS Bullentin 2010, 35, 682-692. (21) Li, W.; Jayasinghe, C.; Shanov, V.; Schulz, M. Spinning Carbon Nanotube Nanothread under a Scanning Electron Microscope. Materials 2011, 4, 1519-1527. (22) Pöhls, J.; Johnson, M. B.; White, M. A.; Malik, R.; Ruff, B.; Jayasinghe, C.; Schulz, M. J.;Shanov, V. Physical Properties of Carbon Nanotube Sheets Drawn from Nanotube Arrays. Carbon 2012, 50, 4175-4183. (23) White, J.L.; Spruiell, J.E. The Specification of Orientation and its Development in Polymer Processing. Polymer Engineering and Science 1983, 23, 247-256. (24) Wang, B.N.; Bennett, R.D.; Verploegen, E.; Hart, A.J.; Cohen, F.E. Quantitative Characterization of the Morphology of Multiwall Carbon Nanotube Films by SmallAngle X-ray Scattering. The Journal of Physical Chemistry C 2007, 111, 5859-5865. (25) Shah, R.; Zhang, X.; Talapatra, S. Electrochemical Double Layer Capacitor Electrodes Using Aligned Carbon Nanotubes Grown Directly on Metals. Nanotechnology 2009, 20, 395202-395206. (26) Niu, Z.; Zhou, W.; Chen, J.; Feng, G.; Li, H.; Ma, W.; Li, J.; Dong, H.; Ren, Y.; Zhao, D.; Xie, S. Compact-Designed Supercapacitors Using Free-Standing Single-Walled Carbon Nanotubes Films. Energy & Environmental Science 2011, 4, 1440-1446. (27) Lin, J.; Zhang, C.; Yan, Z.; Zhu, Y.; Peng, Z.; Hauge, R. H.; Natelson, D.; Tour, J. M. 3Dimensional Graphene Carbon Nanotube Carpet-Based Microsupercapacitors with High Electrochemical Performance. Nano Letters 2012, 13, 72-78. (28) Mandal, M.; Ghosh, D.; Giri, S.; Shakir, I.; Das, C. K. Polyanilne-Wrapped 1D CoMoO4·0.75H2O Nanorods as Electrode Materials for Supercapacitor Energy Storage Application. RSC Advances 2014, 4, 30832-30839. (29) Hsieh, C.; Chou, Y.; Chen, W. Synthesis and Electrochemical Characterization of


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Carbon Nanotubes Decorated with Nickel Nanoparticles for Use as an Electrochemical Capacitor. J. Solid State Electrochem 2008, 12, 663-669. (30) Lehman, J. H.; Terrones, M.; Mansﬁeld, E.; Hurst, K. E.; Meunier, V. Evaluating the Characteristics of Multiwall Carbon Nanotubes. Carbon 2011, 49, 2581-2602. (31) White, J. L.; Spruiell, J. E. The Specification of Orientation and its Development in Polymer Processing. Polymer Engineering and Science 1983, 23, 247-256. (32) Burke, A. Ultracapacitors: Why, How, and Where is the technology. Journal of Power Sources 2000, 91, 37-50. (33) Wang, H.; Liang, Y.; Mirfakhrai, T.; Chen, Z.; Casalongue, H. S.; Dai, H. Advanced Asymmetrical Supercapacitors Based on Graphene Hybrid Materials, Nano Research 2011, 4, 729-736. (34) Mutlu, A. T. Characterization of CNTs and CNTs/MnO2 Composite for Supercapacitor Application. diposit.ub.edu. 2010, Chapter 5, 1-12.


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Table of Contents/Abstract Graphic

Figure 1. Schematic illustration of the inverse-ordered fabrication of CNT sheets; mathematical statement (1/2n)(Li), where Li is the initial number of CNT sheet layers and n is the number of repeat.


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Figure 2. Physical properties of CNT sheet. (A) 200-layered CNT sheet which bent without specific binder, (B) SEM images, (C) Raman spectrum of the 200-layered CNT sheet. (D) Inverse-ordered fabricated CNT sheets from 200 layers to 12.5 layers.


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Figure 3. Alignment analysis for the 200-layered CNT sheet. (A) XRD patterns of MWCNT sheet at different orientations, (B) Pole figure for CNT sheet measured at 2Θ = 26°, (C) Rotational profiles (ψ-scans) of integral intensity of graphic peak (002) for MWCNT sheet at different tilt geometries χ, and (D) Orientation distribution functions (lines) for CNT sheet calculated by fitting of averaged azimuthal profiles (dots) obtained from ψ-scans in the range χ = 50 - 75°. Dots of corresponding color (Figure 3B) indicate positions of diffraction patterns presented on Figure 3A.


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Figure 4. CVs for inverse-ordered fabricated CNT sheet electrodes in 6 M KOH; (A) 200layered CNT sheet, (B) 12.5-layered CNT sheet, (C) CVs of various layered-CNT sheets at constant scan rate, 1000 mV/s, (D) Current densities of CNT sheets with the scan rates of 25, 50, 100, 500, 1000, 2000, 3000 mV/s at 0.2 V potential.


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Figure 5. Impedance plots of CNT sheet electrodes fabricated by inverse-order process in 6 M KOH. (A) The Nyquist diagram of the 200, 100, 50, 25, and 12.5 layers with frequency ranging from 100 kHz to 0.1 Hz, (B) Bode plots of the 200, 100, 50, 25, and 12.5 layers between 100 kHz and 0.1 Hz frequency, (C) Equivalent circuit for the impedance spectra of different layered CNT sheets.


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Figure 6. Electrochemical behavior of the CNT sheet electrodes for supercapacitors fabricated by inverse-ordered process. (A) Galvanostatic charge/discharge curves at 1.5 mA, (B) Current per mass of the different 200, 100, 50, 25 layers CNT sheets at 1.5 mA, gram capacitance as a function of the 200, 100, 50, 25 layers, (C) Energy and power density of the supercapacitor (in the 6 M KOH electrolyte) as a function of the different layers, and (D) Cyclic stability of the CNT sheets of the 200 and 50 layers at 1.5 mA, 12.5 layers at 0.1 mA .


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Table 1. Electrochemical parameters of equivalent circuits obtained from best fit to impedance data for the different layered CNT sheets (Figure 5A).

Layers

Rs

CPEL

n1

n

(Ω)

(S·s )

12.5

11.16

1.58 × 10-4

25

3.543

50

RL

n2

CPEDL n

RCT

ZW

(Ω)

(S·s1/2)

(Ω)

(S·s )

6.77 × 10-1

11.55

4.41 × 10-5

9.69 × 10-1

2.56 × 105

-2.30 × 109

1.10 × 10-3

7.24 × 10-1

2.67

9.53 × 10-4

9.01 × 10-1

1.83 × 104

-4.60 × 105

1.924

7.49 × 10-4

8.07 × 10-1

1.242

1.53 × 10-3

9.11 × 10-1

1.45 × 105

-32.61

100

2.731

2.36 × 10-3

6.83 × 10-1

2.317

2.31 × 10-3

8.90 × 10-1

5.75 × 104

-1.431

200

1.85

1.38 × 10-2

7.11 × 10-1

1.57

7.04 × 10-3

9.76 × 10-1

2.66 × 103

-3.15


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Table 2. Capacitance performance summary of different layers of CNT sheets fabricated by inverse-ordered process (at constant current 1.5 mA).

Parameters

12.5 Layers*

25 Layers

50 Layers

100 Layers

200 Layers

Charge time (s)

4

5

3

14

50

Discharge time (s)

3

4

3

4

6

Energy density (Wh/kg)

0.50

2.57

1.09

0.78

0.54

Power density (W/kg)

606

2312

1312

702

325

Charge/discharge efficiency (%)

75

80

100

29

12

*

Result for 12.5 layered CNT sheet was measured at constant current 0.15 mA.


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Abstract Graphic


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Inverse-Ordered Fabrication of Free-Standing CNT Sheets for

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