2-dimensional hexagonal mesoporous carbon composite for supercapacitor

Synthetic Metals 161 (2011) 1623–1628

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Preparation of polyaniline/2-dimensional hexagonal mesoporous carbon composite for supercapacitor Shuangli Zhou, Shanshan Mo, Wujun Zou, Fengping Jiang, Tianxiang Zhou, Dingsheng Yuan ∗ Department of Chemistry, Jinan University, Guangzhou 510632, China

a r t i c l e

i n f o

Article history: Received 22 March 2011 Received in revised form 17 May 2011 Accepted 19 May 2011 Available online 12 July 2011 Keywords: Supercapacitors 2-D hexagonal mesoporous carbon Polyaniline Specific capacitance Specific energy

a b s t r a c t 2-D hexagonal mesoporous carbon is combined with polyaniline by a simple chemical oxidation polymerization. As-prepared samples are characterized via Fourier transform infrared spectrum, nitrogen sorption analysis and transmission electron microscopy. The experimental results demonstrate that polyaniline is linked with the functional groups on the surface of mesoporous carbon via the chemical bonds. The electrochemical tests reveal mesoporous carbon and polyaniline have a strong synergetic effect, which not only enhances the stability of polyaniline, but also increases the capacitance and energy density of the composite materials. A wide potential window of 1.1 V for the composite is obtained in aqueous electrolyte. The specific capacitance and specific energy density are as high as 470 F/g and 76.4 Wh/kg, respectively. © 2011 Elsevier B.V. All rights reserved.

1. Introduction As a novel type of energy storage device, the supercapacitor with quick charge/discharge and high power density has been attracted considerable attention due to its wide range of potential applications in hybrid electric vehicles, fuel cells, nano-electronics [1–5]. According to the mechanism of energy storage, two types of supercapacitors are under development: one is the electric double-layer capacitor (EDLC), in which stored energy is accumulated by the separation of electronic and ionic charges at the interface between a high surface area electrode and an electrolyte solution; another is the pseudocapacitor, in which the active species can be fast and reversibly oxidized and reduced in a certain range of potentials [6]. In recent years, mesoporous carbons (MCs) attracts broad interest because of its well-pore channels, high specific surface areas and narrow pore size distributions [7–10]. Therefore, MCs have been applied to adsorption [11,12], selective detection [13–15], catalyst support [16,17]. In addition, owing to the contribution of the surface groups being oxidized and reduced easily during the charge/discharge process, MCs have been studied as electrode materials [18], especially their composite materials have been widely investigated, for example, conducting electric polymers (CEPs)/MC and metal oxide/MC. Polyaniline (PANI), one of CEPs, is a promising electrode material with high specific capacitance [19], high electrical conductivity

∗ Corresponding author. Tel.: +86 20 85220597; fax: +86 20 85221697. E-mail address: [email protected] (D. Yuan). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.05.028

and simultaneously, has lots of advantages for the practical applications such as inexpensive monomer, ease of polymerization in aqueous and nonaqueous media, high stability in air and quick doping/dedoping [6,20–22]. However, compared to porous carbon, PANI has the main drawbacks of poor cycle stability derived from the big volumetric change and the unstable redox sites in the polymer backbone in the doping/dedoping process [23,24]. Consequently, the incorporation between porous carbon and PANI seems to be an effective method to fully utilize their respective advantages. The combination of PANI with various porous carbon materials has gradually attracted growing attention for electrochemical capacitors because of their interaction. Wang et al. [25] prepared whiskerlike polyaniline grown on the surface of mesoporous carbon, resulting in high specific capacitance and high-rate charge/discharge ability due to the loosely packed nanometer-scale PANI whiskers to create electrochemical accessibility for electrolyte ions and reduce the distance within the PANI bulk. Xing groups [26] studied a polyaniline-coated mesoporous carbon, the specific capacitance of polyaniline-coated mesoporous carbon was about three times higher than that of MC, due to the incorporation of PANI onto the pore surface of MC. However, the interactions between PANI and MCs are not mentioned and researched. Herein, we report the preparation of PANI/MC composite by a chemical oxidation polymerization. IR spectra reveal that MC and PANI are not a simple mechanical mixture, but take place chemical reaction, forming the long chain via the chemical bonds. The interactions between PANI and MC during the electrochemical tests are investigated in detail.

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2. Experimental 2.1. Preparation of materials Aniline was purified by the distillation under reduced pressure and then stored in darkness until use. Other chemicals were of analytical reagent grade without further purification. The mesoporous carbon was synthesized via direct carbonization of silica/triblock copolymer/butanol copolymers which were obtained through one-step method in a sulfuric acid system using F127 and butanol as both structure-directing agent and carbon precursor, the detailed procedure as reported in the literature [27]. PANI was synthesized according to the following steps. Firstly, 0.9 mL of aniline, 20 mL of 1 mol/L sulfuric acid and 10 mL of ethanol were dispersed in 50 mL of deionized water with magnetic stirring in an ice-bath. After stirring for 10 min, 20 mL of 1 mol/L (NH4 )2 S2 O8 (APS) was added into the above mixture within 1 h drop by drop. The polymerization was carried out for 12 h in an ice-bath with the maintained magnetic stirring. Secondly, the suspension of PANI was filtered and rinsed several times with deionized water and ethanol to remove retained aniline monomer and oxidant until the filtrate became colorless. Finally, the precipitate was dried under vacuum at 60 ◦ C for 12 h to obtain the PANI. The preparation of PANI/MC composite was similar to that of PANI, the difference is the addition of 1.0 g C/Si complex into the mixture before adding APS. In addition, the as-prepared composite was transferred into 10% hydrofluoric acid solution for 24 h to remove the silica template. The product obtained by chemical oxidation polymerization was defined as MP1, and the mass ratio of PANI and MC was around 1:1. For comparison, the as-prepared PANI and MC with the mass ratio of 1:1 was transferred into a covered plastic tube via a direct mechanical mixing method for 30 min with 300-W ultrasonic to get the PANI/MC, denoted as MP2. 2.2. Characterization Transmission electron microscopy (TEM) analysis was performed on a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. The functional groups on the surface of samples were detected via Nicolet FT-IR 6700 spectrometer with KBr plate in the 800–4000 cm−1 region. Nitrogen adsorption–desorption isotherms were measured on Micromertics TriStar 3000. The pore size distribution (PSD) was calculated by the BJH method. All electrochemical measurements were conducted with CHI 660B electrochemical workstation (CHI, Shanghai, China) on a three-electrode cell, which consists of a working electrode, saturated calomel electrode (SCE), and a Pt foil auxiliary electrode. The working electrode was Pt foil (the area is 14 mm × 10 mm and thickness is about 0.1 mm) formed by well mixing 8.00 mg of active materials, 1.33 mg acetylene black and 0.67 mg binding reagent (Nafion, kynar flex, atochem, France). All the measurements were not deaerated by N2 bubbling at room temperature. 3. Results and discussion 3.1. Characterization of the morphologies and structures The microstructure of MC was characterized by TEM, as shown in Fig. 1a. It can be confirmed that MC are 2-D hexagonal mesoporous structure. To further verify the porous properties of MC, the nitrogen adsorption–desorption isotherm is shown in Fig. 1b. It can be seen that the isotherm is similar to the shape of type IV according to the IUPAC classification [28], suggesting that the MC is mesoporous structure. The pore-size distribution of MC is mainly located 2–12 nm, and the average pore size calculated by BJH algo-

Fig. 1. (a) TEM image of MC; (b) nitrogen adsorption–desorption isotherms and (c) pore size distribution of MC; (d) FT-IR spectra of MC, PANI, MP1 and MP2.

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Scheme 1. The reaction mechanism of MC-PANI.

rithm is centered at 5.2 nm based on the adsorption branch (Fig. 1c). BET specific surface area of MC is 1050 m2 /g. Fig. 1d gives IR spectra of the samples. A broad absorption in the IR spectrum of the four samples from 3400 to 3500 cm−1 is attributed to the stretching –OH and –NH modes arising from MC and PANI. In the IR spectrum of MC, the small peaks at 2921 and 2850 cm−1 are originated from the stretching vibration of C–H bond, the peaks at 1743 and 1632 cm−1 corresponds to the stretching vibration of C O in COOH and C C, respectively. The key characteristic peaks at 1402 and 1069 cm−1 correspond to the stretching vibration of C–O in C–OH functional groups and C–C–O, respectively. In PANI, the small peaks at 2922 and 2851 cm−1 are originated from the benzenoid ring stretching vibration of C–H bond, and the characteristic bands at 1575 and 1498 cm−1 attribute to C C a stretching deformation mode of the quinoid and benzenoid rings. Bands at 1302, 1242 and 806 cm−1 arise from the C–N stretching of the secondary aromatic amine, C–N stretching vibration in the polaron structure and an aromatic C–H out-of-plane bending of 1,4-disubstituted benzene ring, respectively. Band at 1139 cm−1 is the results of a vibration mode of the N Q N stretching mode [7,29,30]. All characteristic peaks for MP1 (1575, 1496, 1301, 1142 and 802 cm−1 ) are in good agreement with MC and PANI, but show a red-shift to lower wavenumbers, because of the carbonyl groups from MC are linked with the nitrogen of PANI backbone via the chemical oxidation polymerization into the PANI backbone (see Scheme 1), and this is confirmed by the disappeared peak at 1743 cm−1 for C O. However, the spectra of MP2 (1581, 1499, 1300, 1242, 1139 and 825 cm−1 ) are similar to those of MC and PANI. 3.2. Elecrochemical properties of as-prepared materials and probable reaction mechanism It is well known that there are three kinds of oxidation states of PANI [19,31–33], where m (0 ≤ m ≤ 1) corresponds to the ratio of reduced unit and 1 − m corresponds to the ratio of oxidized unit

in PANI. Three separate oxidation states of PANI are involved, in which the fully reduced state (m = 1) is designated as leucoemeraldine base (LB), the half-oxidized form (m = 0.5) is designated as emeraldine base (EB) and the fully oxidized form (m = 0) is designated as pernigraniline base (PNB). LB, EB and PNB are protonated, corresponding to salt forms designated as LB, emeraldine salt (ES) and pernigraniline salt (PNS) (the anions in the structures are denoted as A− ), respectively [19,31]. Brett and co-workers [33] had investigated that the oxidation of PANI-LB to PANI-ES occurred in E1 = +0.15 V (vs. SCE) and PANI-ES to PANI-PNS occurred in E2 = +0.76 V (vs. SCE). However, in this study, we find that PANI-LB is oxidized to PANI-ES to take place at E1 = +0.34 V (vs. SCE). Fig. 2a presents the cyclic voltammogram curves of MP1 between −0.1 and 1.0 V at a scan rate of 50 mV/s in 1 mol/L−1 H2 SO4 solution. It can be observed that two obvious oxidation peaks are around +0.34 V and +0.76 V at the first cycle, respectively. Moreover, a new peak appears and gradually increases the maximum peak value at the 150th cycle (see Fig. 2a “b” point). This is because the main structure of polyaniline is LB and EB at the beginning, which are oxidized to form ES and PNS, corresponding to the point of “a” and “c”, respectively. With the continuous oxidation in the cyclic process, LB is gradually oxidized to become ES and PNS. As a result, “c” peak gradually disappears and “b” peak strongly appears at E = +0.6 V, which is determined by the experimental details of polyaniline synthesis, electrolyte and pH as Harsha S. Kolla and Duke Orata described in their papers [31,32]. When the potential scans negatively, there are two corresponding reduction peaks. In order to evaluate the electrochemical characteristics of the materials, cyclic voltammetry is employed to study the electrochemical capacitance performance. Fig. 2b shows the CVs of MC at different scan rates in 1 mol/L H2 SO4 solution. Compared with the curves at different scan rates, all the curves are given a pair of symmetrical redox peaks, which indicates that there are a large number of functional groups in MC, mainly for C O and –OH, testified by the IR spectra of MC. The C O is combined with the H+ to become –OH in the reduced state, which would be re-oxidized into C O in

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20

a

10

b

scan rate 50 mV/s

15

b

c

0 st

1 cycle

-5

4

Current / A/g

a

5

2 0 -2 -4

-10

-6

-15

th

10 cycle

MP1

-8

-20

MC

-10

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.2

0.0

0.2

Potential / V (vs SCE)

5

10

0 -5

-15

5

1.0

0 -5

-15

PANI

MP1

-20

-20 -0.2

0.0

0.2

0.4

0.6

0.8

-0.2

1.0

0.0

0.2

20

f

2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s

10 5

4

0.6

0.8

1.0

0.8

1.0

MC PANI MP1 MP2

3

Current / A g-1

15

0.4

Potential / V (vs SCE)

Potential / V (vs SCE)

Current / A/g

0.8

-10

-10

e

0.6

2 mV/s 5 mV s 10 mV/s 20 mV/s 50 mV/s

15

Current / A/g

Current / A/g

10

20

d

2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s

15

0.4

Potential / V (vs SCE)

20

c

5 mV/s 20 mV/s 50 mV/s

6

10

Current / A/g

2 mV/s 10 mV/s

8

0 -5 -10

2 1 0 -1 -2 -3

-15

-4

MP2 -20 -0.2

0.0

0.2

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0.6

0.8

-0.2

1.0

0.0

0.2

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0.6

Potential / V (vs SCE)

Potential / V (vs SCE)

Fig. 2. (a) CVs of MP1 at the scan rate of 50 mV/s for 10 cycles; CVs of MC (b) and PANI (c), MP1 (d) and MP2 (e) at various scan rates; comparison for the CVs of four samples at the scan rate of 5 mV/s (f) in the electrolyte of 1 mol/L H2 SO4 .

the discharge, therefore, presenting a strong reversible redox peaks. These functional groups improve the hydrophilicity and wettability of MC and it is advantageous for the aqueous supercapacitor. The functional groups will also contribute to the pseudocapacitance via the redox reactions during the charge/discharge process [34,35]. A probable mechanism is expressed as follows: C–OH ↔

C O + H + + e−

C O + e− ↔

C–O−

The slopes of current variation near the vertex potentials are vertical at higher scan rate, indicating quick electrolyte diffusion within the mesopores and low mass transfer resistance (Rmt ) of MC, therefore, the 2-D MC as electrode materials of electrochem-

ical capacitors is a good candidate. As can be seen in Table 1, the capacitance of MC has reached up to 153 F/g at the scan rate of 2 mV/s. Obviously, pure MC, due to relatively low specific capacitance, is far from the practical application. Thus, we have designed to com-

Table 1 The specific capacitance (F/g) of MC, PANI, MP1 and MP2 calculated from CVs in 1 mol/L H2 SO4 electrolyte at the different scan rates. Scan rate (mV/s)

2

5

10

20

50

MC PANI MP1 MP2

153 318 470 296

138 281 422 258

125 251 342 218

114 216 298 190

94 171 238 142

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Scheme 2. The electrons’ channels of MC-PANI.

E=

1 2 CV 2

E P= t

(1)

a

MP1 1 Cycle MP2 1 Cycle MP1 4000 Cycle MP2 4000 Cycle

15

Current / A/g

10 5 0 -5 -10 -15 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential / V (vs SCE)

b

MC PANI MP1 MP2

300

Capacitance / F/g

250 200 150 100 50 0 0

1000

2000

3000

4000

Cycle number

c -1

O

and C O doped ions. Secondly, there are a large number of bonds on the surface of MC [7,29,30,33,36], which have some kinds of resonance behavior (Scheme 1a). These groups can bond with amino-produced in the polymerization process of aniline, forming carbon-based-aniline complex (Scheme 1b), which can continue reacting with aniline to produce carbon-based-polyaniline compound (Scheme 1c and d). The peak at 1743 cm−1 originated from stretching vibration of C O bond in MC is disappeared in MP1, where this is verified by IR analysis in Fig. 1d. Finally, when MC is used as the carrier, the volume contraction of PANI caused by the ions reject-into/out becomes weaker. PANI coated on the surface of MC to produce long-chain carbon-based polyaniline compound (Scheme 1c, d and Scheme 2), even the molecular chains of PANI are broken down into smaller fragments in the charge/discharge process, the MC matrix still links with them together to retain the electronic channel (see Scheme 2) and consequently, the capacitance of MP1 is significantly increased. The capacitance of MP1 is up to 470 F/g at the scan rate of 2 mV/s. This synergetic effect is also presented by cycle-life testing, as shown in Fig. 3. Fig. 3a shows the comparison of CV curves for MP1 and MP2 between 1st cycle and 4000th cycle at a scan rate 20 mV/s. Compared to MP2, it can be seen that a visible difference of MP1 at 1st cycle is shown in Fig. 3a, and the specific capacitance of MP1 is much larger than that of MP2, further confirming between PANI and MC in MP1 to be combined via chemical bonds. However, CV of MP2 is similar to the simple superposition of CVs of PANI and MC at 1st cycle. After 4000 cycles, the electrochemical behaviors of MP2 are like pure MC, but then MP1 still maintains the original CV shape and only the capacitance is dropped off. As shown in Fig. 3b, there is only a slight decrease (8.5% loss) after 4000 cycles, revealing the excellent cycle stability of MC. In contrast, the specific capacitance for PANI is decreased 91% for the duration 4000 consecutive cycles in Fig. 3b and the stability is also very poor. The 4000-cycle-life testing indicates the stability of MP1 (48% loss in specific capacitance) is visibly better than that of MP2 (the loss of 62%). For measuring the practical supercapacitance of as-prepared materials, the procedure of recording charge/discharge relations at constant-current is also employed at range from 200 mA/g to 5 A/g. The obtained capacitance value is used to calculate the energy density (E) and the power density (P). Based on the following equations [37], the curves of E and P for the four samples are given in Fig. 3c.

It can be observed that the energy density of MP1 reaches 76.4 Wh/kg at a low power density of 0.112 kW/kg. Although the energy density of PANI, MP1 and MP2 quickly drops off with the increase of power density, the energy density of MP1 still remains at 47.3 Wh/kg at a power density of 2.56 kW/kg, implying that MP1 shows excellent electrochemical performance as supercapacitors’ material.

Energy density (Wh kg )

bine the high capacitive PANI with 2-D MC. Fig. 2c, d and e show the cyclic voltammograms of PANI, MP1 and MP2 electrodes at the scan rates ranged from 2 mV/s to 50 mV/s, respectively. The comparative curves for four samples at 5 mV/s are shown in Fig. 2f. It can be seen that the peak area of MP1 is larger than that of MP2 and PANI, and is much larger than that of MC, revealing the existence of the synergetic effect between the MC and the PANI. The probable mechanism for the synergetic effect involves: first, H+ ions of H2 SO4 can directly be doped on PANI, resulting in the short distance for the

80 75 70 65 60 55 50 45 40 35 30 25 20 15

MC PANI MP1 MP2

0

500

1000

1500

2000

2500

3000

-1

Power density (W kg ) (2)

where C, V and t represent the capacitance, potential window and the discharge time, respectively.

Fig. 3. CVs of MP1 and MP2 at a scan rate of 20 mV/s for the 1st and 4000th cycles (a); the dependence of the specific capacitance vs. cycle numbers for four electrode materials (b); and plot of specific energy density vs. specific power density for four electrode materials (c).

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4. Conclusion The polyaniline has successfully been incorporated with 2-D mesoporous carbon via the chemical oxidation polymerization, which has been confirmed by FT-IR spectra. The electrochemical tests illustrate mesoporous carbon and polyaniline have a strong synergetic effect. Due to the formation of chemical bonds, the stability of PANI on MC is visibly improved, and significantly increasing the specific capacitance and energy density of the composite materials. Thus, this composite could be a potential candidate as the electrode material for the supercapacitors.

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Acknowledgements

[22]

This work was supported by National Natural Science Foundation of China (20876067 and 21031001) and the Fundamental Research Funds for the Central Universities (21609203).

[23]

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