Preparation of a Three-Dimensional Ordered Macroporous Carbon

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Preparation of Three-Dimensional Ordered Macroporous Carbon Nanotube/ Polypyrrole Composite for Supercapacitors and Diffusion Modeling Dan Zhang, Qi-Qi Dong, Xiang Wang, Wei Yan, Wei Deng, and Liyi Shi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405850w • Publication Date (Web): 16 Sep 2013 Downloaded from http://pubs.acs.org on September 19, 2013

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The Journal of Physical Chemistry

Preparation of Three-dimensional Ordered Macroporous Carbon Nanotube/Polypyrrole Composite for Supercapacitors and Diffusion Modeling Dan Zhang1,2, Qi-Qi Dong3, Xiang Wang3, Wei Yan3*, Wei Deng3 and Li-Yi Shi3* 1. Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200444, P. R. China 2. Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai 200072,China 3. Nano Science and Technology Research Center, Shanghai University, Shanghai，200444, P. R. China

Abstract Three-dimensional ordered macroporous (3DOM) carbon nanotube (CNT)/polypyrrole (PPy) composite electrodes for supercapacitor application were prepared through cyclic voltammetric co-polymerization from a solution containing both acid treated CNTs and pyrrole monomers. A self-assembled SiO2 colloidal crystal was used as the sacrificial template. After electrochemical co-polymerization, the template was removed, and a 3DOM CNT/PPy composite electrode was obtained. The specific capacitance of the composite reached 427 F g-1 at the scanning rate of 5 mV s-1, and it is calculated that ion diffusion contributed approximate 30 percent to the specific capacitance of the composite. A mathematical model of mass transport was proposed to evaluate the ion diffusion capability on the surfaces of 3DOM, nanoporous and planar films. The calculation results showed that the flux (i.e. ion flux per unit length) of 3DOM film was larger than that of planar film, while the flux of nanoporous film was close to planar film. The model indicates that 3DOM film is favorable for ion transportation, while nanoporous film does the opposite. The model partially explains the reason why the specific capacitance of the prepared 3DOM CNT/PPy composite is far above the specific capacitance values of other reported CNT/PPy composites, even nanoporous CNT/PPy composite. *

Corresponding author. Tel.: +86-21-66134852; fax: +86-21-66134852.

E-mail address: [email protected], [email protected] 1 / 32

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Keyword: 3DOM, CNT/PPY, composite, supercapacitor, diffusion modeling 1. Introduction An ideal electrical energy-storage device should store plenty of energy and provide powerful bursts when needed. The most common electrical energy-storage devices are capacitors and batteries. However, they can only do one or the other: capacitors provide high power, while batteries offer high storage. Supercapacitors fill in the gap between capacitors and batteries. Supercapacitors can store reversibly a higher energy than conventional capacitors, and they can also be operated at substantially greater specific power than most batteries 1. Generally, supercapacitors are classified into two types: pseudocapacitor and electric double-layer (EDL) capacitor. The former involves a Faradaic process while the latter is non-Faradaic

1, 2

. Metal oxide, electronic conducting polymer

(ECP) and carbon are often used as the electrode materials for supercapacitors3. Carbon nanotubes (CNTs) have been widely studied since their discovery in 19914. As a kind of high surface area carbon material, CNTs offer exceptional power and energy performance. Both Faradic and non-Faradic process are involved in CNTs-based supercapacitors1. Composites, which combine CNTs with either metal oxides or ECPs have been developed, and showed good performance as supercapacitor electrode materials

5, 6

. Due to the low cost as well as large specific

capacitance and good mechanic performance, the CNT/ECP composites are very promising and receiving much attention 7. Polypyrrole (PPy) is one of the most extensively studied ECPs on account of its aqueous solubility and high conductivity 8. PPy has a very large specific capacitance. However, same with other ECPs, PPy has poor mechanical stability because of typical shrinkage, breaking, cracks appearing in subsequent cycles, which is connected with volumetric changes of the polymer during

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intercalation/deintercalation of counter ions due to repeated intercalation and depletion of ions during charging and discharging9. CNT/PPy composites have been prepared10, 11. The entangled network of CNTs in composites can adapt to the volume change during intercalation/deintercalation of counter ions, which is contributive to improve the stability. Moreover, the open mesoporous network formed by the entanglement of nanotubes allows the ions to diffuse easily to the active surface of the composite components. In addition, CNTs greatly improve the conductivity of the composite1. High porosity is very important when PPy is used as the electrode material for supercapacitors because the porous structure can facilitate solvent uptake and dopant ion access12. However, polymerizing a continuous thin polymer film on a suitable porous, high surface area solid substrate, while preserving both the porous of the substrate and the functionality of the film, is challenging13. Kim et al. have tried to fabricate CNT/PPy electrode materials with controlled pore size for a pseudocapacitor using nanosized silica. The silica acted as a sacrificial filler to keep the porous structure of the CNT/PPy composites. The obtained CNT/PPy composites showed a specific capacitance of 250 F g-1 at 10 mV s-1 14. Recently, three-dimensional ordered macroporous (3DOM) films, with interconnected macropores (the so-called “inverse opals”), have attracted increasing attention due to their fascinating properties 15

. Due to the hierarchical porous structure, 3DOM films can provide high specific surface area and

facilitate ionic transport. Therefore, 3DOM films showed great performance as supercapacitor materials. Sawangphruk and Limtrakul developed a 3DOM MnO2 electrode. The specific capacitance of the 3DOM MnO2 electrode reached 765 F g−1 at the scan rate of 2 mV s−1 when the pore diameter was 200 nm16. Zhao et al. deposited a thin film of PANi on 3DOM carbon, and a specific capacitance of 1490 F g-1 was observed over the deposited PANi in the composite electrode 17.

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Herein, we report the preparation of 3DOM CNT/PPy composite electrode for supercapacitor application by cyclic voltammetric (CV) co-deposition. During the preparation, a self-assembled SiO2 colloidal crystal was used as a sacrificial template to obtain the ordered porous structure. The obtained 3DOM CNT/PPy composite showed great performance as the electrode material for supercapacitors. The specific capacitance of the 3DOM CNT/PPy composite reached 427 F g-1 at the scanning rate of 5 mV s-1 (413 F g-1 at the scanning rate of 10 mV s-1), which is much higher than 250 F g-1 given by the nanoporous CNT/PPy composite14. A mathematical model based on mass transport was proposed to describe the ionic diffusion capability on the surfaces of the 3DOM, nanoporous and planar films. Since CNTs is immobile, the charge balancing of CNT/PPy composite during electro-redox process could only be achieved by intercalation and expulsion of cations (K+)18. The simulation of K+ concentration distribution on the surface of nanoporous film showed overlap diffusion profiles over the space between individual nanopores due to the small size of nanopores. The overlap diffusion leads to the ion transport limited. Thus, the flux (i.e. ion flux per unit length) of nanoporous film is close to that of planar film. However, in the case of 3DOM, the overlap between the diffusion profiles decrease due to the large size of the mesopores, and the calculated ion flux of 3DOM film is larger than that of planar film. Although 3DOM film cannot provide as large specific area as nanoporous film, the total flux of 3DOM film is larger than that of nanoporous film. It is can be seen from the model that 3DOM structure is more favorable for ionic transportation than the other two films, which partially explain the reason why our prepared 3DOM CNT/PPy composite showed great performance as the supercapacitor electrode material.

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2. Experimental 2.1. Materials MWCNTs of 95% purity, with a diameter of ca. 25 nm and a length of approximately 10 µm, were commercially obtained from the Shenzhen Nanotech Port Co. Ltd (China). MWCNTs functionalized with carboxyl and hydroxyl groups were obtained by reflux with concentrated nitric acid for about 12h, and then washed with deionized water via centrifugation until the pH value reached above 6. The resulting functionalized MWCNTs were dispersed in deionized water under sonication to obtain a 0.3% MWCNT dispersion. Pyrrole monomer was distilled under reduced pressure before use. All other chemicals were of analytical grade and used as received.

2.2. Fabrication of macroporous CNT/PPy composite films The fabrication process of the macroporous CNT/PPy films is illustrated in Figure 1. Monodisperse silica spheres with the diameter of 500 nm (Alfa Aesar) were assembled on the gold substrates for the construction of SiO2 sacrificial templates. The gold substrates (the 55th institute of China electronic group) were prepared by sputtering a 200nm thick gold top layer onto the quartz wafers. Before use, the substrates were thoroughly rinsed with acetone, ethanol and water, and then vertically immersed into an ethanol suspension containing 0.09 g mL-1 monodispersed SiO2 spheres. This apparatus was covered with a 100 mL beaker, and kept at a temperature of 40 oC for 48 hours for the self-assembling of SiO2 spheres. Electrochemical preparation of the macroporous CNT/PPy composite films were carried out with a CV method in a three-electrode configuration on a CHI660C electrochemical workstation (Chenhua, China) from a solution containing 0.3% MWCNTs and 0.1 M pyrrole monomers. A platinum

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electrode and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, separately. A gold substrate covered by a SiO2 colloidal crystal template was used as the working electrode. The potential was swept between -0.5 to 0.70 V vs. SCE at a scanning rate of 0.1 V s-1. After hundreds of scanning cycles, a black film was observed to form on the gold substrate. The macroporous CNT/PPy composite electrode was obtained after the SiO2 colloidal crystal template was removed by diluted HF solution. The macroporous CNT/PPy composite films prepared by cyclic voltammetry for 500, 1000, 2000, 3000 and 6000 potential scanning cycles followed by the removal of SiO2 colloidal crystal templates are denoted as 500-CNT/PPy, 1000-CNT/PPy, 2000-CNT/PPy, 3000-CNT/PPy and 6000-CNT/PPy, respectively. As a comparison, planar CNT/PPy composite film was also prepared by cyclic voltammetry in the three-electrode configuration. A gold substrate with no SiO2 colloid crystal template was used as the working electrode. The potential was swept between -0.5 to 0.70 V vs. SCE at a scanning rate of 0.1 V s-1 for 3000 potential scanning cycles.

2.3 Characterization The diameter and dispersion of the acid treated MWCNTs were observed by transmission electron microscopy (TEM, JEOL, JEM-200CX). The morphology of the composite films was examined using field-emission scanning electron microscopy (FE-SEM, JSM-6700F). The Raman spectra were measured with a Jobin-Yvon LabRam HR with a liquid nitrogen-cooled CCD multichannel detector at room temperature using conventional back scattering geometry. An argonion laser at a wavelength of 514.5 nm served as the laser light source. All the attenuated total reflection Fourier Transform Infrared Spectroscopic (ATR-FTIR) measurements were performed on a Bruker model VECTOR22

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instrument.

2.4 Electrochemical measurement Electrochemical measurements were conducted in a three-electrode electrochemical cell with a CHI660C electrochemical workstation. In the three-electrode cell, the prepared CNT/PPy composite electrodes were used as the working electrodes. A platinum plate and a SCE were used as the counter and the reference electrodes, respectively. The evaluations were performed in 1 M KCl aqueous solution in a potential window ranging from −0.4 to 0.2 V. The CV currents have been normalized to the mass of the composites. The specific gravimetric capacitance (Cp in F g-1) was obtained from the cyclic voltammetry results according to the following equation: Cp =

1 2v ∆V

(∫

Vc

Va

Vc

I (V ) dV − ∫ I ′(V )dV Va

)

(1)

where v is the potential scanning rate, Vc and Va represents the cathodic and anodic switching potential, respectively. ∆V means the potential scanning range; I(V) and I’(V) denote the normalized response current density (A g-1) on the anodic and the cathodic scans, respectively. Symmetric supercapacitors were assembled and performed in a two-electrode cell in 1 M KCl aqueous solution. The prepared CNT/PPy composite electrodes were separated with a paper separator. Galvanostatic charge–discharge tests of the assembled supercapacitors were conducted on a Land CT2001A (Wuhan, China) in the potential window ranged from −0.4 to 0.2 V. The cycling performances of the symmetric supercapacitors were also tested.

3. Results and Discussion 3.1 Characterization of WMCNTs and composite films

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Fig. 2a shows a typical SEM image of a SiO2 colloidal crystal template on a gold substrate. The template consists of well-ordered three dimensional arrays of SiO2 spheres and exhibits a face-centered-cubic structure with (111) planes parallel to the substrate. Fig. 2b displays a TEM image of the MWCNTs after acid treatment. It can be seen that the MWCNTs are well dispersed, and the average diameter of the MWCNTs is ca. 25 nm. Fig. 2c-2g shows the SEM images of 500-CNT/PPy, 1000-CNT/PPy, 2000-CNT/PPy, 3000-CNT/PPy and 6000-CNT/PPy composite films. In fig. 2c, MWCNTs are entangled and interconnected, forming a film with a three-dimensional porous structure. The average diameter of MWCNTs is approximately 40 nm, which is larger than that of the bare MWCNTs given in fig. 2b. The observation indicates that pyrrole monomers and MWCNTs could be electrochemical co-deposited on to the gold substrate, and PPy formed a uniform coating along the side walls of MWCNTs. As the CNT/PPy composite filled the voids in the SiO2 colloidal template during electropolymerization, holes resulting from the removal of SiO2 microspheres can be observed (red arrows). The composite film continued to grow by the repetition of potential cycling (fig. 2d). The holes resulting from the removal of SiO2 crystal template (red arrows) were larger and deeper than that in fig. 2c. The average diameter of MWCNTs increased up to 50 nm after 1000 scanning cycles, suggesting the pyrrole monomers keeping polymerization along the side walls of MWCNTs (fig.2d). The spaces between MWCNTs were gradually filled with PPy when the potential scanning cycles increased (fig. 2e). When the potential scanning cycles reached 3000 times, the space between MWCNTs was fully filled with PPy, and 3DOM CNT/PPy composite film could be obtained after the colloidal crystal SiO2 template was removed (fig. 2f). The 3000-CNT/PPy is the 3DOM CNT/PPy composite film in the following experiments. If the number of the potential scanning

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cycles kept increasing, PPy would pile up on the 3DOM CNT/PPy composite film and block the macropores (fig. 2g). Fig. 3 shows the FTIR spectra of acid treated MWCNTs (black line), 3DOM CNT/PPy composite (red line) and PPy (blue line). PPy was prepared with cyclic voltammetry in a three-electrode configuration from a solution containing 1 M KCl and 0.1 M pyrrole monomer. A gold substrate with no SiO2 colloidal crystal template was used as the working electrode. The potential was swept between -0.5 to 0.70 V vs. SCE at a scanning rate of 0.1 V s-1. In the spectrum of MWCNTs, the peak at 1215 cm-1 corresponds to C–N stretching vibration bond resulting from acid treatment. The carboxylic bond of C=O and –COO– stretching vibrate at 1720 and 1580 cm-1, respectively. The spectra of C=C stretching band are apparent at about 1636 cm-1 19. The presence of the C=O and-COO- vibrations reveals the existence of carboxylic groups in MWCNTs. In the IR spectrum of 3DOM CNT/PPy composite, all the three characteristic peaks of MWCNTs are clearly shown. The frequencies of C=O and –COO– stretching vibrations decrease, suggesting the strong interactions between PPy and the oxygen-containing functional groups on the surface of MWCNTs. The frequency of aromatic C=C stretching vibration, however, remains unchanged, indicating that PPy does not interact with the nonpolar side-walls of MWCNTs. The peak at c.a.980 cm−1 in pristine CNTs due to the C–H bend is not observed, indicating that the C-H defects on the pristine CNTs had been modified to oxygen containing groups during the acid treatment 20. In the spectrum of PPy, the peaks at 1658, 1531 and 1442 cm-1 can be attributed to fundamental vibrations of pyrrole ring 21. The broad bands located at 1351 and 1285 cm-1 are attributed to C–H or C–N in–plane deformation. Mode in the region from 1230 to 1100 cm-1 corresponds to the breathing vibration of the pyrrole ring, and the maximum is located at 1147 cm-1. The peak at 1086 cm-1

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corresponds to the mode of in-plane deformation vibration of N+H2 which is formed on the PPy chains by protonation. The band of C–H and N-H in-plane deformation vibration is situated at 1028 cm-1 and the band of the C-C out of plane ring deformation vibration at 964 cm-1. The band of C–H out-of-plane deformation vibrations is located at 890 cm-1

22

. In the spectrum of 3DOM CNT/PPy

composite, all characteristic peaks from PPy are clearly shown with some shift for the interaction between MWCNT and PPy. Fig. 4 shows Raman spectra of acid-treated MWCNTs (red line) and 3DOM CNT/PPy composite (black line). In the spectrum of MWCNTs, the peak at 1330 cm-1 is called D-band, coming from the multiple phonons scattering of defects or amorphous carbon

23

. The peak at 1583 cm-1 is so-called

G-band, which is attributed to the E2g mode of the graphite wall, coming from the stretching of conjugated double bonds 24. The intensity ratio of D-band to G-band is associated with the extent of defects present in the MWCNTs and is sensitive to molecular interaction. The value of ID/IG for MWCNTs is 2.04. In the Raman spectrum of 3DOM CNT/PPy composite, the peak position of the G-band shifts from 1583 to 1580 cm-1, and the peak position of D-band shifted from 1330 to 1350 cm-1. The shifts of D and G bands are attributed to the presence of PPy coated on the MWCNTs. The value of ID/IG for the 3DOM CNT/PPy composite is 1.22, indicating that the interaction between PPy and the oxygen-containing functional group of MWCNTs. The spectrum of 3DOM CNT/PPy composite also shows the bands related to PPy. The double peaks at about 1050 and 1079 cm-1 are associated with the C-H in-plane deformation. The bands located at about 932 and 982 cm-1 are assigned to the ring deformation associated with dication (bipolaron) and radical cation (polaron) 25.

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3.2 Electrochemical Characterization of 3DOM CNT/PPy Film CV measurements were conducted to test the supercapacitor performance of the 3DOM CNT/PPy composite film. For comparison, the supercapacitor performances of PPy and planar CNT/PPy composite films were also evaluated with cyclic voltammetry in the three-electrode configuration. Fig. 5a-c gives the CV profiles of PPy (fig. 5a), planar CNT/PPy composite (fig.5b) and 3DOM CNT/PPy composite (fig.5c) films measured in 1 M KCl aqueous solution between -0.4 and 0.2 V at the scanning rate ranging from 5 to 200 mV/s. All of the currents have been normalized with respect to the mass of the films. In the fig. 5a, both the oxidation and reduction currents decreased towards the negative potential end, which is an indication of the polymer gradually becoming inactive and resistive

26

. As the

potential scanning rate increased, the CV curves of PPy electrode became more and more distorted in shape. In fig.5b, the CV curves of CNT/PPy composite film exhibited a rectangular shape when the scanning rate was smaller than 100 mV s-1. The almost straight and vertical current variations at the end potential suggested a fast charge/discharge switch resulting from high electronic and ionic conductivity of the planar CNT/PPy composite film

27

. In the oxidized CNT/PPy composites, the

highly conjugated CNTs can enhance the electron delocalization along the polymer chains. In addition, the incorporation of MWCNTs made the CNT/PPy composite show good conductivity even at negative potentials at which pure PPy would become non-conducting

27

. On the other hand, the

open mesoporous network formed by the entanglement of nanotubes allows the ions to diffuse easily to the active surface of the composite components. The specific capacitance of the planar CNT/PPy composite film was calculated to be 128 F g-1 at the scanning rate of 5 mV s-1 according to the equation 1. The rectangular-shape of planar CNT/PPy showed obvious distortion when the potential

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scanning rate was 200 mV s-1, indicating the poor high-rate capability of the planar CNT/PPy composite. Fig. 5c shows the CV curves of the 3DOM CNT/PPy composite film in 1M KCl at different potential scanning rates. It can be seen that the CV curves of the 3DOM CNT/PPy composite film always maintain a good rectangular-shape even when the potential scanning rate was 200 mV s-1. An ideal electrode for supercapacitor application should have good electronic conductivity, high specific surface area and interconnected porous structure. 3DOM structure can provide a large surface area. Moreover, the large ordered pore size and the continuous connected windows are favorable for the penetration of electrolyte and ion transportation even at high scan rates. The specific capacitance of the 3DOM CNT/PPy composite was calculated to be 427 F g-1 at the scanning rate of 5 mV s-1, which is much larger than that of planar CNT/PPy composite. Fig. 6a shows the CV curves of the 500-CNT/PPy, 1000-CNT/PPy, 2000-CNT/PPy, 3000CNT/PPy and 6000-CNT/PPy composite films in 1 M KCl aqueous solution at the scanning rate of 5 mV s-1. All of the CV curves exhibit rectangular-like shapes without obvious redox peaks. Fig. 6b shows the potential scanning cycle dependence of the specific capacitance. The specific capacitance increased with the potential scanning cycles from 500 to 3000. However, the specific capacitance decreased when the potential scanning cycles reached 6000 because some of the macrospores of the 3DOM structure were blocked, which also blocked the ionic transportation. It is surprising to find that the specific capacitance of our prepared 3DOM CNT/PPy composite film is not only larger than the planar CNT/PPy composite film, but also larger than the nanoporous CNT/PPy composite film given by the literature14. The charge-discharge curves of the assembled symmetric supercapacitors at a constant current of

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0.2 mA g−1 are shown in fig7a. The capacitance can be calculated according to the following equation:

C

∗

∗

(2)

where I is charge–discharge current, t is the discharge time, V is the electrochemical window. The capacitances of the symmetric supercapacitors assembled with planar and 3DOM CNT/PPy composite electrodes were calculated to be 46.51 F g-1 and 195.7 F g-1 respectively. The cycling life test over 1000 cycles for the supercapacitors were carried out by repeating the charge-discharge test between -0.4 and 0.2 V. Fig. 7b shows the continuous galvanostatic charge-discharge curves after 500 cycles for the supercapacitor assembled with the planar CNT/PPy composite electrodes. The insert in fig.7b shows the capacitance retention ratio of the capacitor as a function of the cycle number. The cell exhibits good electrochemical stability with a retention of 89.9% after 1000 cycles. Fig.7c shows the continuous galvanostatic charge-discharge curves after 500 cycles for the symmetric supercapacitor assembled with the 3DOM CNT/PPy composite electrodes. The insert in fig.7c shows the capacitance retention ratio of the capacitor as a function of the cycle number. The capacitance dropped by 12.6% after 250 cycles and only 72.5% of the capacitance remained after 1000 cycles. The poorer electrochemical stability of the supercapacitor assembled with 3DOM CNT/PPy composite electrodes may result from the removal of the SiO2 template by diluted HF solution. 3.3 Limitation of Mass Transport Trasatti et al. used an analysis modeling to characterize the charge storage of RuO2, a well-known pseudocapacitive material28. In his analysis, the overall capacity (Qtotal) of a pseudocapacitive material was assumed to contain two contributions, Qout and Oinn:

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total Q out inn

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(3)

Where, Qout came from the double-layer and pseudocapacitance, and the Qinn depended on ion diffusion 29. The relationship between Qtotal and the minus or positive square root of scanning potential rate (v-1/2 or v1/2) for the three CNT/PPy composite films are shown in fig.8. In fig.8a, the plot of Qtotal vs. the minus square root of scanning potential rate ( / ) yielded a straight line whose y-intercept ( ∞) was the Qout . When ∞, Qinn was zero because the potential scanning rate was so large that ions in bulk solution could not diffuse onto the surface of film in time. The Qout of planar, 3DOM CNT/PPy composite films were given in table I. Since the specific surface area of 3DOM film is larger than that of planar film, the Qout of the 3DOM film is naturally much larger than that of planar CNT/PPy composite film. When 0, Qinn reaches maximum value. As shown in fig. 8b, the plot of Qtotal vs. / also yielded a straight line whose y-intercept ( 0) was a combination of Qout and maximum Qinn. The maximum Qinn for planar, 3DOM CNT/PPy composite films were also listed in table I. The Qinn of 3DOM CNT/PPy composite film is 154.72, which is larger than that of planar film, proving that ion diffusion has also contributed a lot to the charge storage of 3DOM CNT/PPy composite film. The Qout and the maximum Qinn of the nanoporous CNT/PPy composite film reported by Kim et al. were also calculated according to their data and were listed in table I 14. Compared with the planar composite film, the Qout of the nanopoprous composite film increases due to large specific area. The maximum Qinn, however, decreases, indicating that nanoporous structure is not favorable for ion transportation. The finding is surprising because porous film (no matter macroporous film or nanoporous film ) was believed to be favorable for ion transportation. 14 / 32


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Table I The Qout and Qinn of planar, 3DOM and nanoporous CNT/PPy compositie films Qout（F g-1） -1

Maximum Qinn（F g ）

Planar

3DOM

Nanoporous 14

65.99

368.48

427.0

91.79

154.72

52.85

Since CNTs is immobile, the charge balancing of CNT/PPy composite during electro-redox process could only be achieved by intercalation and expulsion of cations (K+) 18. A two-dimensional, steady state, isothermal numerical model was applied to evaluate the total diffusion flux of cation K+ from the bulk solution to the surface of films and estimate the spatial variations in concentration. Ions transport is dominated by the diffusion through a stagnant layer in the electrolyte. According to the transport layer approach, the proportionality between surface and bulk concentration and current can be described as 30:

"#$ !"#$ %

(4)

where km is the mass transport coefficient. Supposing 3D porous structure is homogeneous, the mass transport of K+ nearby the film surface can be treated as a diffusion transport of diluted species in a 2D domain shown in Fig. 9, and the surface concentrations of K+ can be described as a function of the bulk concentrations and modeled by Fick’s second law as: &'k &

D)

&* 'k & *

&* 'k &+ *

,0

where ck is the concentration of K+; D is the diffusion coefficient of K+, 1.87E-9

(5) 31

; t is time. Here,

the finite element method is employed to solve this partial differential equation subjected to the following initial and boundary conditions: At the top of the resolve domain: - at . /

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(6)


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By assuming a constant ions concentration at the upper boundary was defined for the boundary layer, where C0 is the bulk concentration of free K+, L is the height of the domain. At the bottom of the resolve domain, a flux was calculated: &'k

012 ∙ 4 5 )

&

&'k &+

,7 89$ :

(7)

where rads is similar to an intercalate or repulsion rate of free K+ to occupy a free site or leave an occupied site near the location of the CNTs, here rads is treated as a big number, in order to eliminate the influence of the intercalation or repulsion kinetics. The symmetry condition was applied to the side of the resolve domain:

1n2 ∙ 4 5 )

&'k &

&'k &+

,7 0

(8)

where the initial K+ flux at the boundary is set to 0. In our case, the governing equations were discretized along all spatial coordinates and solved iteratively using COMSOL. Mesh independence is ensured throughout the simulations. The CFD simulation can predict the concentration distribution of the cation K+ nearby the interface of electrode. As shown in the fig. 9, the concentration profile of cations K+ driven by the concentration gradient reveals their diffusion patterns on the surfaces of the films. The simulated spatial distribution of K+ concentration shows that the isoline of 0.00833M maintains only the entrance to the pore for the nanoporous structure (fig.9b), while the isoline of 0.09M appears in the below part of the pore for the 3DOM case (fig.9c). It indicates that the cations K+ could reach inside walls of the pore more easily, which implies that the transport limited current remains for nanoporous structure. As shown in fig. 9b, the concertration diffusion profiles overlap over the space between the individual micro-pore due to the small size of pore. This results in approximately planar diffusion geometry at larger distances

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to the film surface, which was comparable to that of a non-structured planar film surface (fig.9). As the increasing of the pore size, the overlap between the diffusion profiles decreases, the ions can approach the inside wall of the pore, and then transfer much deeply. Fig. 9 also shows a comparison between nanoporous and 3DOM in the form of flux and F phy/geo which is the ratio between a physically measureable length (the length of the planar surface) and a real geometry length (the length of the curved surface). The simulated results reveal that the flux of the 3DOM is 35.6 percent more than that of the nanoporous, while the F phy/geo decreases by 5.7 percent compared with the latter. Therefore the total flux estimated of the 3DOM film is approximately 1.3 times that of the nanoporous film. This conclusion was found to be in good agreement with the experimental data. The

total flux has to be deliberately considered because the redox kinetics of PPy is likely to be coupled with the ions mass transport in the actual conditions. In the present work, the removal of monodisperse silica spheres leaved a three dimensional porous structure of entangled CNTs, therefore the pore size of the final CNT/PPy composite film could be controlled by the size and amount of monodisperse silica spheres. By means of combining experiments with simulation, a critical diameter of the pore might be found to achieve a compromise between a higher accessible specific surface area and a better mass transport, which will be carried out in details in the next paper.

4. Conclusion

In this paper, 3DOM CNT/PPy composite was prepared by electrochemical co-polymerization for supercapacitor applications. By modeling the mass transport of the ions, the diffusion patterns of K+ on the surfaces of 3DOM, nanoporous and planar films were numerical simulated, and the results

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showed that the 3DOM film could provide the highest flux among the three kinds of films, which indicates that 3DOM film is favorable for ion transportation. This model partially accounts for the large specific capacitance values of our 3DOM CNT/PPy composite as well as other 3DOM materials.

Acknowledgement

The authors acknowledge the supports of National Natural Science Foundation of China (11202124, 20805030, 21001072, 21102088 and 21174081); The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning; Key subject of Shanghai Municipal Education Commission (J50102) and Shanghai Leading Academic Discipline Project (S30107).

References

(1) Pan, H.; Li, J.; Feng, Y. P. Carbon Nanotubes for Supercapacitor. Nanoscale. Res. Lett. 2010, 5, 654-668. (2) Zhang, L. L.; Zhao, X. S. Carbon-based Materials as Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38, 2520-2531. (3) Zhao, X.; Sanchez, B.M.; Dobson, P. J.; Grant, P.S. The Role of Nanomaterials in Redox-based Supercapacitors for Next Generation Energy Storage Devices. Nanoscale 2011, 3, 839–855. (4) Lijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56-58. (5) Reddy, A. L. M.; Shaijumon, M. M. S.; Gowda, R.; Ajayan, P. M. Multisegmented Au-MnO2/Carbon Nanotube Hybrid Coaxial Arrays for High-Power Supercapacitor Applications. J. Phys. Chem. C. 2010, 114, 658–663. 18 / 32


Page 18 of 32

Page 19 of 32

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


(6) Liu, J.; Sun, J.; Gao, L. A Promising Way To Enhance the Electrochemical Behavior of Flexible Single-Walled Carbon Nanotube/Polyaniline Composite Films. J. Phys. Chem. C. 2010, 114, 19614– 19620. (7) Yin, Y.; Liu, C.; Fan, S. Well-Constructed CNT Mesh/PANI Nanoporous Electrode and Its Thickness Effect on the Supercapacitor Properties. J. Phys. Chem. C. 2012, 116, 26185–26189. (8) Wang, L.; Li, X.G.; Yang, Y. L. Preparation, Properties and Applications of Polypyrroles. React. Func. Polym. 2001, 47, 125–139. (9)

Lota,

K.;

Khomenko,

V.;

Frackowiak,

E.

Capacitance

Properties

of

Poly(3,4-ethylenedioxythiophene)/Carbon Nanotubes Composites J. Phys. Chem. Solids. 2004, 65, 295-301. (10) Zhou, C.; Kumar, S. Functionalized Single Wall Carbon Nanotubes Treated with Pyrrole for Electrochemical Supercapacitor Membranes. Chem. Mater. 2005, 17, 1997-2002. (11) Lee, H.; Kim, H.; Cho, M. S.; Choi, J.; Lee, Y. Fabrication of Polypyrrole (PPy)/Carbon nanotube (CNT) Composite Electrode on Ceramic Fabric for Supercapacitor Applications. Electrochim. Act. 2011, 56, 7460-7466. (12) Nyholm, L.; Nystrom, G.; Mihranyan, A.; Stromme, M. Toward Flexible Polymer and Paper-Based Energy Storage Devices. Adv. Mater. 2011, 23, 3751-3769. (13) Miharanyan, A.; Nyholm, L.; Bennett, A.E.G.; Stromme, M. A Novel High Specific Surface Area Conducting Paper Material Composed of Polypyrrole and Cladophora Cellulose. J. Phys. Chem. B. 2008, 112, 12249-12255. (14) Kim, J. Y.; Kim, K. H.; Kim, K. B. Fabrication and Electrochemical Properties of Carbon Nanotube/Polypyrrole Composite Film Electrodes with Controlled Pore Size. J. Power. Source. 2008,

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176, 396-402. (15) Teng, Y.; Fu, Y.; Xu, L.; Lin, B.; Wang, Z.; Xu, Z.; Jin, L.; Zhang, W. Three-Dimensional Ordered Macroporous (3DOM) Composite for Electrochemical Study on Acetylcholinesterase Inhibition Induced by Endogenous Neurotoxin. J. Phys. Chem. B. 2012, 116, 11180–11186. (16) Sawangphruk, M.; Limtrakul, J. Effects of Pore Diameters on the Pseudocapacitive Property of Three-dimensionally Ordered Macroporous Manganese Oxide Electrodes. Mater. Lett. 2012, 68, 230-233. (17) Zhang, L. L.; Li, S.; Zhang, J.; Guo, P.; Zheng, J.; Zhao, X. S. Enhancement of Electrochemical Performance of Macroporous Carbon by Surface Coating of Polyaniline. Chem. Mater., 2010, 22, 1195-1202. (18) Chen, G.Z.; Shaffer, M. S. P.; Coleby, D.; Dixon, G.; Zhou, W.; Fray, D. J.; Windle, A. H. Carbon Nanotube and Polypyrrole Composites: Coating and Doping. Adv. Mater. 2000, 12, 522-525. (19) Kim, S. D.; Kim, J. W.; Im, J. S.; Kim, Y. H.; Lee, Y. S. A Comparative Study on Properties of Multi-walled Carbon Nanotubes (MWCNTs) Modified with Acids and Oxyfluorination. J. Fluor. Chem. 2007, 128, 60-64. (20) Peng, C.; Jin, J.; Chen, G. Z. A Comparative Study on Electrochemical Co-deposition and Capacitance of Composite Films of Conducting Polymers and Carbon Nanotubes. Electrochim. Act. 2007, 53, 525-537. (21) Liu, J.; Wan, M. Composites of Polypyrrole with Conducting and Ferromagnetic Behaviors. J. Polym. Sci. 2008, 38, 2734-2739. (22) Omastova, M.; Trchova, M.; Kovarova, J.; Stejskal, J. Synthesis and Structural Study of Polypyrroles Prepared in the Presence of Surfactants. Synth. Metal. 2003, 138, 447-455.

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Page 20 of 32

Page 21 of 32

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


(23) Yang, C.; Lin, Y.; Nan, C. W. Modified Carbon Nanotube Composites with High Dielectric Constant, Low Dielectric Loss and Large Energy Density. Carbon 2009, 47, 1096-1101. (24) Han, G.; Shi, G. Novel Route to Pure and Composite Fibers of Polypyrrole. J. Appl. Polym. Sci. 2007, 103, 1490-1494. (25) Han, G.; Yuan, J.; Shi, G.; Wei, F. Electrodeposition of Polypyrrole/Multiwalled Carbon

Nanotube Composite Films. Thin. Solid. Films. 2005, 474, 64– 69. (26) Peng, C.; Jin, J.; Chen, G. Z. A Comparative Study on Electrochemical Co-deposition and Capacitance of Composite Films of Conducting Polymers and Carbon Nanotubes. Electrochim. Acta. 2007, 53, 525-537. (27) Peng, C.; Zhang, S.; Jewell, D.; Chen, G. Z. Carbon Nanotube and Conducting Polymer Composites for Supercapacitors. Prog. Nat. Sci. 2008, 18, 777-778. (28) Baronetto, D.; Krstajic, N.; Trasatti, S. Reply to “note on a method to interrelate inner and outer electrode areas” by H. Vogt. Electrochim. Acta. 1994, 39, 2359-2362. (29) Conway, B. E.; Pell, W. G. Double-layer and Pseudocapacitance Types of Electrochemical Capacitors and Their Applications to the Development of Hybrid Devices. J. Solid. State .Electrochem. 2003, 7, 637–644. (30) Lyons, M. E. G. Reaction/Diffusion at Electrode/Solution Interfaces: The EC2 Reaction. Int. J. Electrochem. Sci. 2009, 4, 1116– 1127. (31) Samson, E.; Marchand, J.; Snyder, K. A. Calculation of Ionic Diffusion Coefficients on the Basis of Migration Test Results. Mater. Struct. 2003, 36, 156-165.

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Figure 1. Schematic illustration of the fabrication process of macroporous CNT/PPy composite films: (A) self-assembling of a SiO2 colloidal template; (B) immersing the SiO2 colloidal template into the electrolytic solution; (C) electrochemical co-polymerization and (D) removal of the SiO2 colloidal template.

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Figure. 2. (a) A SEM image of a SiO2 colloidal crystal template; (b) a TEM image of the acid treated MWCNTs; the SEM images of (c)500-CNT/PPy, (d)1000-CNT/PPy, (e)2000-CNT/PPy, (f) 3000-CNT/PPy and (g) 6000-CNT/PPy.

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Absorbance (a.u.)


Figure. 3. FTIR spectra of acid treated MWCNT (black line), 3DOM CNT/PPy composite (red line) and PPy (blue line)

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D band 1350

12000

G band 1580

10000

8000

6000

4000

932 982 1050

intensity (a.u.)

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2000

0 500

1000

1500

2000

-1

Raman shift(cm )

Figure. 4. Raman spectra of acid treated MWCNT (black line) and 3DOM CNT/PPy composite (red line).

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Figure. 5. CV curves of (a) PPy film, (b) planar CNT/PPy composite film and (c) 3DOM CNT/PPy composite film in 1 M KCl aqueous solution at different potential scanning rate.

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Figure. 6 (a) CV curves of the 500-CNT/PPy, 1000-CNT/PPy, 2000-CNT/PPy, 3000-CNT/PPy and 6000-CNT/PPy composite films in 1 M KCl aqueous solution at the scanning rate of 5 mVs-1; (b) Relationship between the specific capacitance values of macroporous CNT/PPy composites films and potential scanning cycles at the scanning rate of 5 mVs-1.

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(a)

0.3

3DOM Planar

0.2 0.1

Potential ( V )

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

0.0 -0.1 -0.2 -0.3 -0.4 0

200

400

600

800

1000

Time ( s )

Figure. 7 (a) Galvanostatic charge/discharge curves of symmetric supercapacitors assembled with planar CNT/PPy composite film (curve a) and 3DOM CNT/PPy composite film (curve b) at current density of 0.2 A g-1. (b) Continuous galvanostatic charge/discharge curves of the symmetric

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supercapacitor assembled with the planar CNT/PPy composite films after 500 cycles. The insert is the cycle performance of the supercapacitor. (c) Continuous galvanostatic charge/discharge curves of the symmetric supercapacitor assembled with the 3DOM CNT/PPy composite films after 500 cycles. Insert is the cycle performance of the supercapacitor.

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(a)a

(b)

Figure. 8 Determination of the infinite and near zero sweep-rate capacitance of PPy/CNT: (a) the capacity as a function of sweep rate v-1/2, the y-intercept represents the infinite sweep rate capacity; (b) the capacity as a function of sweep rate v1/2, the y-intercept represents the extremely low sweep rate capacity, the largest contribution of the diffusion-related processes

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Figure. 9 Schematic description of the spatial distribution of the cations K+ concertration of during a mass transport dominated by the diffusion for the electrolyte over the surface of electrodes made of the PPy/CNTs composite material: (a) with a planar surface (b) with nano-size micro-pores (Diameter of 30nm) (c) with well-ordered 3D macro-pores (Diameter of 300nm)

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Graphic for TOC

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Preparation of a Three-Dimensional Ordered Macroporous Carbon

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