Morphological reason for enhancement of electrochemical double layer capacitances of various acetylene blacks by electrochemical polarization

Morphological reason for enhancement of electrochemical double layer capacitances of various acetylene blacks by electrochemical polarization

Electrochimica Acta 53 (2008) 5789–5795 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

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Electrochimica Acta 53 (2008) 5789–5795

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Morphological reason for enhancement of electrochemical double layer capacitances of various acetylene blacks by electrochemical polarization Taegon Kim a,b , Chulho Ham c , Choong Kyun Rhee c,∗ , Seong-Ho Yoon a,∗∗ , Masaharu Tsuji a,b , Isao Mochida a a b c

Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka 816-8580, Japan Department of Chemistry, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea

a r t i c l e

i n f o

Article history: Received 26 January 2008 Received in revised form 14 March 2008 Accepted 15 March 2008 Available online 30 March 2008 Keywords: Electrochemical double layer capacitance Carbon materials Acetylene blacks Graphitization Pseudo-capacitance

a b s t r a c t Enhancement of electrochemical capacitance and morphological variations of various acetylene blacks caused by electrochemical polarization are presented. Acetylene blacks of different mean particle diameters were modified by air-oxidation and heat treatment to diversify the morphologies of the acetylene blacks before electrochemical polarization. The various acetylene blacks were electrochemically oxidized at 1.6 V (vs. Ag/AgCl) for 10 s and the polarization step was repeated until the capacitance values did not change any longer. These polarization steps enhanced the capacitances of the acetylene blacks and the specific enhancement factors range from 2 to 5.5. Such an enhancement is strongly related to morphological modification as revealed by transmission electron microscopic observations. The electrochemical polarization resulted in formation of tiny graphene sheets on the wide graphitic carbon surfaces, which were most responsible for the observed capacitive enhancement. Although the pseudo-capacitance increased after polarization by forming oxygenated species on the surfaces, its contribution to the total capacitance was less than 10%. The mechanism of the formation of the tiny graphene sheets during the electrochemical oxidation is described schematically. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction In industrial application of capacitor, the importance of carbon materials has been emphasized [1–10] because of their unique properties [11,12]. Carbon materials are diverse in structure, stable enough to resist chemically harsh conditions, easy to manipulate, and abundant enough for industrial applications. Surface area exposed to an electrochemical environment is known to be the most important factor in determining electrochemical double layer capacitance of carbonaceous materials. The key idea of the importance is that a high surface area is advantageous in forming electrochemical double layer. In reality, however, the correlations of surface areas to capacitances of carbon materials are very complicated. A specific example is that the capacitance of an activated carbon (1902 m2 /g) was 105 F/g [13], while the capacitance of a micro-porous carbon nanosphere (245 m2 /g) was 154 F/g [7]. This particular comparison definitely indicates that a simple

∗ Corresponding author. Tel.: +82 42 8215483; fax: +82 42 8218896. ∗∗ Corresponding author. Tel.: +81 92 5837801; fax: +81 92 5837798. E-mail addresses: [email protected] (C.K. Rhee), [email protected] (S.-H. Yoon). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.03.057

increase in the surface area of a carbon material, e.g. by making it porous, is not always a solution toward effective carbon electrodes for capacitor. To resolve the complications associated with capacitance applications, carbon materials have been approached from various aspects. Types of porosity in carbon materials, especially intra-particle pores, are crucial in understanding capacitive behavior of carbon [14–17]. Activation of carbon materials with steam or KOH, i.e. pore formation, results in free external surfaces and pore wall surfaces [18], which would lead to different adsorptive and capacitive behavior of electrolyte ions [19]. In addition, the accessibility of ions into pores, determined by their physical dimensions of entrances, ¨ and surely governs the capacitance [15,20–22]. Furthermore, Kotz co-workers suggested that the capacitive charge of a carbon black with extremely high surface area (>1200 m2 /g) could be saturated, because of limited capacity for pores to accommodate charge under a condition that the wall thickness of pores is compatible to the screening length of applied electric field [23]. Yoon and co-workers [24] reported a correlation between electrochemical double layer capacitance and mobility of protons in pores: as protons become more mobile in larger pores, the resultant capacitance becomes smaller. Since new aspects related to pores are emerging, it would

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Table 1 Surface areas of as-received and heat-treated acetylene blacks. Type

LAB OLAB SAB OSAB

BET surface area (m2 /g) As-received

Heat-treated

72 109 443 1014

63 56 153 153

be too simplistic to explain all double layer capacitance effects by approximating pores with simple surface area factors without taking into account the morphology of the pore surfaces. Recently, our groups have investigated surface morphologies of various carbon materials to establish their correlations to electrochemical double layer capacitances [25,26]. A study on capacitive behavior of carbon nanofibers (CNFs) clearly confirmed that the edge surfaces of graphite were more effective by roughly 3–5 times in capacitive charging than the basal-plane surfaces of graphite [25]. On the other hand, a series of acetylene blacks was studied in terms of mean particle diameter, degree of oxidation and graphitization [26,27]. The origin of the capacitance of acetylene blacks was verified to be graphene sheets protruding from their surfaces, which are 2–3 times superior to the edge surfaces of graphite in capacitive charging. These observations indicate that morphologies of carbonaceous materials should be considered seriously in capacitor applications. On the other hand, electrochemical activation of carbon materials is known to enhance their capacitance performances [28–31]. A common interpretation for the capacitance enhancement is an increase in surface area by interconnecting the existing intraparticle pores [29] or oxidative removal of the tips of multi-walled nanotubes [31], including a contribution of pseudo-capacitance of O-containing surface functional groups and an increase in surface compatibility to adsorption of electrolyte ions. In this work, enhancement of electrochemical capacitance and morphological variations of various acetylene blacks caused by electrochemical polarization are presented. Because surface morphology plays a crucial role in defining capacitance as previously mentioned, we attempted to compare morphological changes of acetylene blacks before and after electrochemical oxidation to understand the origin of capacitance enhancement in a term of surface morphology. 2. Experimental In this work, four acetylene blacks were used and referred to large acetylene black (LAB), small acetylene black (SAB), oxidized large acetylene black (OLAB) and oxidized small acetylene black (OSAB). The mean particle diameters of LAB and SAB (supplied by Denki Kagaku Co., Japan) were 35 nm and 12 nm, respectively. OLAB and OSAB were obtained by oxidizing the plain LAB and SAB under air at 300 ◦ C for 1 h. The above four acetylene blacks were termed as as-received acetylene blacks, hereafter. On the other hands, the as-received acetylene blacks were graphitized under an argon atmosphere at 2800 ◦ C for 10 min with a heating rate of 20 ◦ C/min using a high temperature furnace (Kurata Giken, Japan). The surface areas of the as-received and heat-treated acetylene blacks are listed in Table 1 and their other physical properties are detailed in our previous reports [26,27]. Acetylene black electrodes were prepared by spreading and drying slurries of each acetylene black, 5% Nafion solution (Wako, Japan) and water on a Au disk. To evaluate the capacitances, cyclic voltammetry was performed in 0.5 M H2 SO4 solution (Wako, Japan) using a conventional three-electrode system. The reported poten-

Fig. 1. Cyclic voltammograms of the as-received acetylene blacks in 0.5 M H2 SO4 solution: (a) LAB, (b) OLAB, (c) SAB and (d) OSAB. The solid, dot and dash lines represent before polarization, after the 1st polarization step, and after the 20th polarization step, respectively. Scan rate: 10 mV/s.

tials in this work were measured against a home-made Ag/AgCl reference electrode. The morphologies of acetylene blacks in nanometer scale were observed with a transmission electron microscope (TEM, JEM2100F, JEOL, Japan). For TEM measurements, the electrochemically oxidized acetylene blacks were collected after polarization and washed with an alcohol to remove the Nafion binder. 3. Results 3.1. Polarization of as-received acetylene blacks Fig. 1 compares the cyclic voltammograms of the as-received acetylene blacks with those of the acetylene blacks polarized at 1.6 V in 0.5 M H2 SO4 solution. The cyclic voltammograms of the as-received acetylene blacks (solid lines) are rectangular, indicating that the capacitive behavior originates from electrochemical double layer capacitive charging. Polarization at 1.6 V induced significant changes in voltammograms (dashed lines): the increases in

Fig. 2. Comparison of the capacitance values of the as-received acetylene blacks before and after polarization.

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double layer current and the simultaneous appearance of pseudocapacitive current. In Fig. 2, the capacitance values measured at 0.55 V for the as-received acetylene blacks and at 0.65 V for the polarized ones, respectively, are displayed. The potentials for measurement of the capacitance values were the potentials where the pseudo-capacitance was absent. The as-received acetylene blacks clearly demonstrate the effects of mean particle diameter and chemical oxidation on the capacitive behavior as detailed in our previous report [26,27]. Briefly, the as-received acetylene blacks are more efficient in capacitive charging when their mean particle diameters are smaller and when their surfaces are oxidized. The origin of the capacitive currents of the asreceived acetylene blacks has been proved to be graphene sheets protruding from the surfaces of acetylene blacks: the more protruding graphene sheets, the more capacitive current. Formation of the protruding graphene sheets is favored by smaller mean parti-

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cle diameter (structural effect) and chemical oxidations to burst off graphitic layers (chemical effect). Electrochemical polarization brought significant increases in the capacitances of the acetylene blacks as shown in Figs. 1 and 2. The initial polarization step, i.e. holding at 1.6 V for 10 s in 0.5 M H2 SO4 solution, immediately doubled up the capacitance values, as displayed by the dotted lines in Fig. 1. Consecutive repeats of the polarization step up to 20 times, however, induced slow but continuous enhancements to certain values, depending on mean particle diameter. It is worthy to address that the capacitance of OSAB reached to a maximum value after the 5th polarization step and then decreased slightly. As shown in Fig. 2, the specific enhancement factors are 5.5 for LAB, 3.9 for OLAB, 5.2 for SAB, and 2.2 for OSAB, respectively. The enhancement factors of the un-oxidized asreceived acetylene blacks (LAB and SAB) were higher than those of the corresponding oxidized ones (OLAB and OSAB). After polariza-

Fig. 3. TEM images of the as-received acetylene blacks before polarization (left panel) and after polarization (right panel): (a) LAB, (b) polarized LAB, (c) OLAB, (d) polarized OLAB, (e) SAB, (f) polarized SAB, (g) OSAB and (h) polarized OSAB.

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tion, furthermore, the capacitance values of the acetylene blacks of identical mean particle diameter became comparable to each other within experimental uncertainty as demonstrated in Fig. 2. These observations imply that the polarization results in similar surface morphologies of the acetylene blacks, depending only on mean particle diameter. On the other hand, a pseudo-capacitive behavior appears, certainly due to surface functional groups of oxygen formed during electrochemical oxidation. While SAB and OSAB show a reversible redox couple around 0.30 V, LAB and OLAB display two redox couples at 0.0 V and 0.30 V. The redox peak at 0.30 V is a generally observed feature on oxidized carbon materials [25–27], while the one at 0.0 V is a newly observed characteristic. It is noteworthy that in contrast to the instant increase in the double layer charging current after the first polarization step, the pseudo-capacitance developed very slowly. Fig. 3 shows typical TEM images of the as-received and polarized acetylene blacks. Morphologies of various acetylene blacks were well documented in our previous work [26]: a high curvature (i.e. a small

mean diameter) of primary structural unit results in a high surface tension, which is relaxed somehow by bursting off the curved layers to flat graphene sheets protruding from primary structural unit. Because the curvature of the SAB primary structural unit is higher, more protruding graphene sheets result in as exemplified in Fig. 3(a) and (e). Furthermore, chemical oxidation facilitates formation of protruding graphene sheets probably by scissoring the upper graphitic layers as illustrated in Fig. 3(c) and (g). Polarization of the as-received acetylene blacks resulted in a serious morphological modification as shown in the right panel of Fig. 3. A close comparison of the TEM images before and after polarization indicates that the surfaces of protruding graphene sheets were covered with bumpy features to be rougher, and that the edges of graphitic layers became modulated like saw tooth. 3.2. Polarization of heat-treated acetylene blacks Morphological modification of the basal-plane surfaces of graphitic layers induced by electrochemical polarization was exam-

Fig. 4. TEM images of the heat-treated acetylene blacks before polarization (left panel) and after polarization (right panel): (a) GLAB, (b) polarized GLAB, (c) GOLAB, (d) polarized GOLAB, (e) GSAB, (f) polarized GSAB, (g) GOSAB and (h) polarized GOSAB. The heat treatment temperature: 2800 ◦ C.

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The increase in the capacitances of the heat-treated acetylene blacks after polarization was accompanied with morphological changes as shown in the right panel of Fig. 4. The top layers of the graphitic surfaces, smooth before polarization, became rough (the right panel of Fig. 4). The graphitic surfaces parallel to the electron beam of TEM were imaged jagged, while the perpendicular ones were imaged bumpy as exemplified in the right panel of Fig. 4. The absolute capacitance values of the heat-treated acetylene blacks are, however, much lower than those of the as-received ones, because polarization of the heat-treated acetylene blacks did not lead to formation of large protruding graphene sheets like those on the as-received ones. Such a small morphological change may be due to a greater stability of thick graphitic layers of the heat-treated acetylene blacks. 4. Discussion

Fig. 5. Cyclic voltammograms of the heat-treated acetylene blacks in 0.5 M H2 SO4 solution: (a) GLAB, (b) GOLAB, (c) GSAB and (d) GOSAB. The solid, dot and dash lines represent before polarization, after the first polarization step, and after the twentieth polarization step, respectively. The heat treatment temperature: 2800 ◦ C. Scan rate: 10 mV/s.

ined also. A heat treatment of the acetylene blacks at 2800 ◦ C turned the entire surfaces of the as-received acetylene blacks to surfaces covered with smooth and wide basal planes of graphite as shown in the left panel of Fig. 4. The capacitances of the heat-treated acetylene blacks (2 F/g in average) were extremely small (solid lines in Fig. 5). A single step of electrochemical polarization of the heattreated acetylene blacks at 1.6 V for 10 s increased immediately their capacitance values (dot lines in Fig. 5). Further polarization resulted in a gradual increase in capacitance values as in the case of the as-received acetylene blacks, concomitantly with pseudocapacitive peaks (dashed lines in Fig. 5). As demonstrated in Fig. 6, the capacitance values after polarization of the heat-treated acetylene blacks remarkably depend only on mean particle diameter: they are approximately 7 F/g for the heat-treated LAB (GLAB) and OLAB (GOLAB), and 23 F/g for the heat-treated SAB (GSAB) and OSAB (GOSAB), respectively. The specific enhancement factors are 3.4 and 4.5 for GLAB and GOLAB, 10.2 and 10.5 for GSAB and GOSAB, respectively.

Fig. 6. Comparison of the capacitance values of the heat-treated acetylene blacks before polarization and after polarization. The heat treatment temperature: 2800 ◦ C.

The morphological variations after electrochemical polarization would be condensed to roughening the surfaces of the acetylene blacks. A schematic diagram for the roughening process of acetylene black is illustrated in Fig. 7. During electrochemical oxidation of highly oriented pyrolytic graphite (HOPG), hollow blisters are well known to form on the top surface of HOPG by intercalation of intact electrolyte through surface defects [32–35]. The inner graphitic layers of the hollow blisters become graphitic oxides, and electrolytic gases such as CO2 and O2 evolve inside the blisters. At a higher potential like 1.9 V (vs. SCE), the blisters broke up, so that the basal-plane surface turns out to be an amorphous surface rich in edges of graphite. During electrochemical polarization at 1.6 V, the surfaces of the acetylene blacks may undergo a roughening process similar to that on HOPG. Fig. 7(a) depicts a part of the studied acetylene blacks before polarization: there are protruding graphene sheets on a curved graphitic layer. When a high potential (1.6 V in this case) is applied, the intercalation of the electrolyte may take place to form blisters (Fig. 7(b)). As the oxidation continues, the blisters would break up to tiny graphene sheets (Fig. 7(c)). The bumpy features observed on the heat-treated acetylene blacks as in Fig. 4 may be the blisters or the tiny graphene sheets produced by intercalation. Furthermore, the roughened surfaces of the protruding graphene sheets after polarization as displayed in Fig. 3 may be surfaces filled with such tiny graphene sheets. Here, these particular tiny graphene sheets should be distinguished, in terms of size, from the inherent large protruding graphene sheets (e.g. Fig. 3(e) and (g)): the former comes form electrochemical polarization, while the latter originates from small mean particle diameter and chemical oxidation. A comparison of the blisters formed on the basal-plane surfaces of HOPG and acetylene blacks would be instructive. The blisters formed on HOPG are as large as 10 ␮m [32], while those on the acetylene blacks are less than 10 nm, judging from the roughening features in Figs. 3 and 4. Also, it takes several minutes to form blisters on HOPG, while a single polarization for 10 s is enough to produce the tiny graphene sheets via blisters as signaled abrupt increase in capacitance in Figs. 1 and 5. The definite differences in size and formation time may be ascribable to the differences in the number of defects on the surfaces of acetylene blacks and HOPG. In other words, the number of defects on the acetylene blacks would be higher than that on HOPG, because of extremely high degree of graphitization of HOPG. Therefore, the formation of blisters in nanometer scale on the protruding graphene sheets is triggered on the numerous defects in a short time. Morphological modifications other than the formation of tiny graphene sheets are worthy to discuss further. As shown in Fig. 3, the edges of the protruding graphene sheets became jagged, indi-

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demonstrated in Figs. 2 and 6. Specifically, the capacitance values of the acetylene blacks of SAB and OSAB converge to 150 F/g, while those of LAB and OLAB do to 20 F/g. Furthermore, the capacitance values of the heat-treated acetylene blacks are 7 F/g for GLAB and GOLAB and 23 F/g for GSAB and GOSAB, respectively. These particular dependences are obviously due to that the acetylene blacks of smaller mean particle diameter have higher surface area exposed to electrochemical environment during polarization (Table 1). In addition, the smaller capacitance values of the heat-treated acetylene blacks certainly result from the more graphitic surfaces, thus resistive to formation of blisters. Accordingly, the capacitances of the heat-treated and polarized acetylene blacks support that presence of tiny graphene sheets and surface area play role in the capacitances of electrochemically polarized acetylene blacks. The number of graphene sheets on an acetylene black, formed by a chemical oxidation or electrochemical polarization, may have a saturation values as a function of mean particle diameter. Providing that the capacitance is proportional to the number of graphene sheets, an oxidation by electrochemical and/or chemical means would lead to a certain number of graphene sheets. During a polarization, for an example, OSAB reached to a maximum value (163 F/g) and then decreased to the value of polarized SAB (140 F/g). Because OSAB has more protruding graphene sheets than SAB, the number of tiny graphene sheets formed during electrochemical oxidation on OSAB would be higher than that on SAB. Thus, the capacitance value of OSAB reaches to a maximum, higher than that of SAB and further polarization may lead unstable graphene sheets on OSAB to be electrochemically oxidized off from the surface. Thus, the total number of graphene sheets on OSAB becomes compatible to that on SAB. 5. Conclusions

Fig. 7. Schematics of formation of tiny graphene sheets on the wide surfaces of graphitic layers and protruding graphene sheets: (a) before polarization, (b) during polarization and (c) after polarization.

cating local oxidative removal of carbon atoms to CO2 in the perpendicular direction. However, the increasing portion of the edge induced in this way would not be high as much as that coming from formation of tiny graphene sheets, so that the effect of the electrochemical polarization on the edges of the protruding graphene sheets would be negligible. On the other hand, formation of various oxygenated carbon species [36] is notable as indicated by the redox couples at 0.0 V and 0.3 V in Figs. 1 and 5. In spite of appearance of the pseudo-capacitance, its contribution to the total capacitance increase after electrochemical oxidation is less than 10%. Thus, the tiny graphene sheets are most responsible for the capacitance enhancement of the acetylene blacks by polarization. The maximum capacitance values of the acetylene blacks after polarization strongly depends on mean particle diameter as

The results presented in this work demonstrate that an electrochemical polarization of acetylene blacks induces enhancement of their capacitive behavior and that the specific enhancement are strongly related to morphological modification. During electrochemical oxidation, tiny graphene sheets form on the surfaces of graphitic layers and protruding graphene sheets. The particular tiny graphene sheets are responsible for the observed increases in capacitances of the acetylene blacks. As well as electrochemical double layer capacitive behavior, the pseudo-capacitance increases after polarization by forming oxygenated species on the surfaces. However, the contribution of the pseudo-capacitance to the total capacitance is much less significant than the electrochemical double layer one. Therefore, it is concluded that presence of tiny graphene sheets and surface area play role in the capacitances of electrochemically polarized acetylene blacks. Acknowledgements The work performed in Japan was carried out partially within the framework of Seed-Hakkutsu program in JST. The authors in Japan acknowledge the financial support of Japan Science and Technology Agency of Japan. On the other hand, the work performed in Korea was supported by the Next Generation Growth Engine Program funded by the Korean Government (MOCIE) (100283558). The authors thank Denki Kagaku Co., Japan for the kind supply of the acetylene blacks used in this work. References [1] A. Nishino, J. Power Sources 60 (1996) 137. [2] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [3] J.H. Jang, S. Han, T. Hyeon, S.M. Oh, J. Power Sources 123 (2003) 79.

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