Study on synergistic effect of ordered mesoporous carbon and carbon aerogel during electrochemical charge–discharge process

Study on synergistic effect of ordered mesoporous carbon and carbon aerogel during electrochemical charge–discharge process

Microporous and Mesoporous Materials 131 (2010) 261–264 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homep...

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Microporous and Mesoporous Materials 131 (2010) 261–264

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Study on synergistic effect of ordered mesoporous carbon and carbon aerogel during electrochemical charge–discharge process Dingcai Wu a,b,*, Xin Chen a, Sihong Lu a, Yeru Liang a, Fei Xu a, Ruowen Fu a,b,* a b

Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China DSAPM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China

a r t i c l e

i n f o

Article history: Received 19 September 2009 Received in revised form 2 December 2009 Accepted 30 December 2009 Available online 6 January 2010 Keywords: Ordered mesoporous carbon Carbon aerogel Synergistic effect Supercapacitor

a b s t r a c t Ordered mesoporous carbon/carbon aerogel (OMC/CA) composites have been obtained by a simple ultrasonic mixing technique. It is found that there exists a synergistic effect between OMC’s channel-like mesopores and CA’s network-like mesopores during electrochemical charge–discharge process; and the maximum synergistic coefficient arrives at 0.37 when CA content is 30 wt.%. As a result, the as-prepared OMC/CA composites can make full use of the two-dimensional order of mesoporous structure of OMC and the three-dimensional connectivity of mesoporous structure of CA, and thus, exhibit much better electrochemical properties as compared to OMC and CA alone. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction In recent years, supercapacitors have become one of hot topics in the field of energy storage due to their excellent performances, such as large capacitance, high power density, long cycle life, wide operating temperature range, short charging period and so on. They have many potential applications in electric vehicles, power supplies, memory protection of computer electronics, cellular devices, and others [1,2]. To date, the crucial technology of high performance supercapacitor lies in developing outstanding electrode materials. Generally, there are three types of fundamental electrode materials extensively studied: (i) porous carbons, (ii) metal oxides, and (iii) conducting polymers. Among them, porous carbons have stimulated significant scientific interest because of their good conductivity, stable physicochemical performance, low cost, and availability [3]. Ordered mesoporous carbon (OMC) and carbon aerogel (CA) are two novel types of mesoporous carbons developed in recent years, which are of two-dimensional (2D) hexagonal mesostructure and three-dimensional (3D) nano-network structure, respectively [4– 9]. Because of their respective unique mesopore structure, both OMC and CA have been proven to possess good electrochemical properties, especially power performance, which provides them

* Corresponding authors. Address: Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Tel.: +86 020 84112759; fax: +86 020 84115112. E-mail addresses: [email protected] (D. Wu), [email protected] (R. Fu). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.12.032

with a bright application prospect in supercapacitors [10–17]. On the other hand, the neighboring channel-like mesopores of OMC are not connected to each other; while the mesopores of CA are disordered. Obviously, such disadvantages are unfavorable for the transfer/diffusion of ions to some extent and in this way reduce the utilization rate of the surface for charge storage, especially during the large current charging–discharging process. Therefore, to further improve the electrochemical performance of both CA and OMC, these above structure disadvantages should be overcome. Herein, we report an interesting synergistic effect between OMC and CA during electrochemical charge–discharge process, which is schematically illustrated in Fig. 1. The nanostructure advantage of both OMC and CA can exactly offset the counterpart’s nanostructure disadvantage when these two carbon materials are simply mixed together. Consequently, the as-prepared OMC/CA composites can make full use of both the 2D order of mesoporous structure of OMC and the 3D connectivity of mesoporous structure of CA, leading to better electrochemical properties as compared to OMC and CA alone.

2. Experimental CA was fabricated via a microemulsion-templated sol–gel polymerization method developed in our previous work [8]. OMC was synthesized according to the preparation procedure reported by Zhao and co-workers [5]. Briefly, hexadecane was dissolved in P123 aqueous solution and subsequently mixed with NaOH-catalyzed phenol–formaldehyde resol solution. After that, the mixture

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where I, v and m represent the current at the middle voltage of the potential window, the sweep rate and the mass of the carbon sample, respectively. 3. Results and discussion To evaluate the pore structure of the CA and OMC samples, N2 adsorption measurements have been carried out, and the corresponding N2 adsorption–desorption isotherms and Barrett–Johner–Halendar (BJH) mesopore size distributions are shown in Fig. 2. First of all, both these carbon samples show type IV isotherms with a distinct hysteresis loop, indicative of typical mesopore structure characteristic. Without doubt, for OMC, this loop at P/P0 = 0.4 is ascribed to its channel-like mesopores [5]; and for CA, the loop at P/P0 = 0.8 is attributed to the mesopores inside its three-dimensional nano-network [8]. The respective mesopore diameter for OMC and CA is calculated to be 3.6 and 18.9 nm by using BJH method (the inset in Fig. 2). In addition, Brunauer–Emmett–Teller (BET) calculation shows that the BET surface areas of OMC and CA are 576 and 586 m2 g1, respectively. Cyclic voltammograms at different sweep rates for the above OMC and CA samples are given in Fig. 3A and 3B, respectively. It is known that an ideal nanostructure should be capable of provid-

3

-1

dV/dlog(D) (cm g )

3

-1

50

4 3.6 nm 3 2 1 0 0

15

30

45

60

Pore diameter (nm)

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

700 600 500 400

4

-1

(B)

3

800

dV/dlog(D) (cm g )

ð1Þ

100

-1

I

v m

150

0.0

3

Cm ¼

(A)

200

0

Quantity adsorbed (cm g STP)

was reacted and then dried, followed by being carbonized for 3 h at 900 °C in N2 flow. The OMC/CA composites were obtained by ultrasonic mixing 320 mesh powders of OMC and CA for 1.5 h in ethanol solvent and then drying at 100 °C. The resulting composites are denoted as OMC/CA-CAxx, where xx represents the weight content (wt.%) of CA in the composite. The pore structure of the resulting CA and OMC samples was characterized by the ASAP 2010 surface area and porosity analyzer from Micromeritics. The carbon electrodes in the form of 1 cm  1 cm sheet are obtained by pressing a mixture film containing 92 wt.% carbon sample and 8 wt.% polytetrafluorethylene into a nickel foam current collector. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using an IM6e electrochemical workstation with a typical three-electrode configuration. Hg/HgO electrode was adopted as reference electrode, and the electrolyte was 6.0 mol/L KOH. The signal amplitude in EIS measurement was 5 mV. The specific capacitance (Cm) was calculated according to the following equation:

Quantity adsorbed (cm g STP)

Fig. 1. A scheme of synergistic effect of OMC and CA.

3 18.9 nm 2 1 0 0 20 40 60 80 100 Pore diameter (nm)

300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 2. N2 adsorption–desorption isotherms of (A) OMC, and (B) CA. The inset is BJH mesopore size distribution.

ing very fast ion transport pathways, and thus the electrical double layer can be re-organized quickly at the switching potentials, resulting in a rectangular-shaped CV curve [18]. That is, the rectangle degree of CV curve can reflect the ion diffusion rate within a

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0.12

(A)

-1

100 mV s

-1

0.06

50 mV s

Current (A)

-1

10 mV s 0.00 -0.06 -0.12 -0.18

-0.8 -0.6 -0.4 -0.2

0.0

0.2

0.4

Potential ( V vs.Hg/HgO)

0.08

(B)

-1

100 mV s

-1

50 mV s

0.04 Current (A)

-1

10 mV s 0.00 -0.04 -0.08 -0.12

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential ( V vs.Hg/HgO)

0.16

(C) -1

0.08 Current (A)

-1

100 mV s 50 mV s

10 mV s

-1

0.00 -0.08

are ordered but unconnected, whereas CA’s mesopores are connected but disordered), we decide to explore the possibility to improve their electrochemical properties by simply mixing these two samples (see Fig. 1). We found that when mixing them together, the as-obtained OMC/CA composites exhibit an enhanced rectangle degree in their CV curves at all the investigated sweep rates compared to OMC and CA alone, as shown in Fig. 3C. More importantly, the rectangle degree for the OMC/CA-CA30 is still satisfactory even at a high sweep rate of 100 mV s1. These results clearly demonstrate that the ion diffusion rates within the mixed mesopore systems of the OMC/CA composites are much faster than those within the pure mesopores of OMC and CA, especially during large current charging–discharging process. Such a faster ion diffusion rate inside the mixed mesopores of the composites can be further revealed by Nyquist plots in Fig. 4. Generally, the diameter of the semicircle in the high frequency range is referred to as the polarisation resistance (Rp). The Rp is believed to be associated with the porous structure of the carbon samples and reflect the ion diffusion into their pores [19,20]. We found that the Rp of OMC and CA is 0.151 and 0.794 X, respectively; whereas that of the OMC/CA-CA30 is as low as 0.060 X. This demonstrates effectively that the synergistic effect of pore structure between OMC’s 2D channel-like mesopores and CA’s 3D network-like mesopores remarkably improves the ion diffusion behaviors of the OMC/CA composites. On the other hand, the intrinsic resistance (Ri, i.e., bulk electronic resistance) of the carbon electrode material can be reflected by the high frequency intercept on the Z0 axis, considering the fact that both the electrolyte and cell-assembling technique are the same [19,20]. It can be seen that OMC/CA-CA30 has a slightly lower Ri (0.070 X) than OMC and CA samples (0.096 and 0.089 X, respectively), indicating that the composite exhibits a slightly higher conductivity as compared to OMC and CA samples, most likely due to the improvement of the packing of carbon particles after ultrasonic mixing treatment. However, it should be noted that such particle packing effect is insignificant in improving electrochemical properties compared with the synergistic effect of pore structure, because its contribution to conductivity improvement is far smaller than that of the above synergistic effect. As a result of the above improved ion diffusion performance, the OMC/CA composites exhibit higher utilization rate of the surface for charge storage and consequently lead to better capacitive

-0.16

30

-0.24 -0.8

-0.6

-0.4

-0.2

0.0

0.2

OMC OMC/CA-CA30 CA

0.4

25

Potential ( V vs.Hg/HgO) Fig. 3. Cyclic voltammograms of (A) OMC, (B) CA, and (C) OMC/CA-CA30.

-Z''(ohm)

carbon nanostructure. The higher the rectangle degree, the faster is the ion diffusion rate. It can be found that at 10 mV s1, both the samples give a good rectangular-shaped CV curve, indicating that both OMC’s 2D channel-like mesopores and CA’s 3D network-like mesopores are able to provide fast ion transport pathways at such a slow sweep rate. However, with increasing the sweep rate to 100 mV s1, the rectangle degree of CV curves for these two samples gradually decreases, demonstrating that neither of the above mesopores is perfect ion diffusion routes in the case of rapid charge–discharge operations. We believe that this should be related to their respective structure disadvantages: 2D channel-like mesopores of OMC are unconnected to each other, and 3D network-like mesopores of CA are disordered. Considering the fact that the structure characteristics of OMC and CA are very complementary to each other (OMC’s mesopores

20 1.0

15

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5

0.0 0.0

0 0

2

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6

0.6

0.9

8

1.2

1.5

10

Z'(ohm) Fig. 4. Nyquist plots for OMC, CA, and OMC/CA-CA30. The inset shows the expanded high-frequency region of the plots.

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Table 1 Specific capacitances of the samples at different sweep rates. Sample

CA content (wt.%)

OMC OMC/CA-CA10 OMC/CA-CA20 OMC/CA-CA30 OMC/CA-CA40 OMC/CA-CA50 OMC/CA-CA60 OMC/CA-CA70 OMC/CA-CA80 OMC/CA-CA90 CA

0 10 20 30 40 50 60 70 80 90 100

Cm (F g1) 10 mV s1

50 mV s1

100 mV s1

132 134 150 165 160 136 133 134 127 122 119

122 133 145 158 154 131 130 126 122 118 112

118 133 142 156 153 130 127 123 119 116 104

4. Conclusions In summary, we have successfully fabricated OMC/CA composites by a simple ultrasonic mixing method. Based upon the experimental results, it can be found that in the as-prepared composites, OMC’s 2D channel-like mesopores and CA’s 3D network-like mesopores have an obvious complementary effect on ion diffusion properties, thereby improving the utilization rate of the surface for charge storage, especially in the case of rapid charge–discharge operations. As a result, the composites have both higher capacitance at various sweep rates and better capacitance retention ratio with increasing the sweep rate as compared to OMC and CA alone. The optimal CA content has been determined to be 30 wt.%. Considering that the various types of carbon nanomaterials have been widely used in many electrochemical fields and other applications needing rapid mass transport, we hope that the concept introduced in this paper can be further extended to the design and fabrication of other high-performance nanocarbon composites.

0.40

Synergistic effect coefficient

-1

10 mV s -1 50 mV s -1 100 mV s

0.32

OMC/CA-CA30 has a much slower decreasing rate in capacitance when the sweep rate is increased from 10 to 100 mV s1 (see Table 1), and thus, has an improved capacitance retention ratio (95%) as compared to OMC and CA alone (89% and 87%, respectively).

0.24

0.16

Acknowledgments

0.08

This research was supported by the Project of NNSFC (50802116, 50632040, 50972167), the Specialized Research Fund for the Doctoral Program of Higher Education (200805581014), the Natural Scientific Foundation of Guangdong Province (8451027501001421), and the Central Colleges Basic Scientific Research Fund for Youth Scholars of Sun Yat-sen University (09lgpy18).

0.00

0

20

40

60

80

100

CA content (wt. %) Fig. 5. Synergistic effect coefficients (SECs) of OMC/CA composites with various CA contents at different sweep rates.

performance than OMC and CA alone, as summarized in Table 1. It can be found that at various current densities, all OMC/CA composites have a higher capacitance than OMC and CA alone. Furthermore, when CA content equals to 30 wt.%, OMC’s 2D channel-like mesopores and CA’s 3D network-like mesopores have a best complementary effect, judging from the fact that the maximum capacitance always occurs in the case of such a CA content, no matter the sweep rate is high or low. For example, the capacitance of the optimum composite OMC/CA-CA30 is as high as 165 F g1 at 10 mV s1, whereas those at 10 mV s1 for OMC and CA are only 132 and 119 F g1, respectively. In order to clearly evaluate the synergistic effect of OMC and CA, we introduce a synergistic effect coefficient (SEC) to quantitatively express the complementary effect by using the following formula:

SEC ¼

C OMC=CA 1 C OMC  ð1  PCA Þ þ C CA  PCA

ð2Þ

where COMC/CA is the capacitance of the OMC/CA composites, COMC the capacitance of the OMC (e.g., 132 F g1 at 10 mV s1), CCA the capacitance of the CA (e.g., 119 F g1 at 10 mV s1), PCA the CA content. The results are given in Fig. 5. It can be found that the complementary effect between OMC’s 2D channel-like mesopores and CA’s 3D network-like mesopores obviously increases with an increment in the sweep rate. For example, for OMC/CA-CA30, the SECs at 10, 50 and 100 mV s1 are 0.28, 0.33 and 0.37, respectively. Therefore,

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