Acetylene black agglomeration in activated carbon based electrochemical double layer capacitor electrodes

Solid State Ionics 179 (2008) 1946–1950

Contents lists available at ScienceDirect

Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s s i

Acetylene black agglomeration in activated carbon based electrochemical double layer capacitor electrodes Hao Zhang, Wenfeng Zhang, Jie Cheng, Gaoping Cao ⁎, Yusheng Yang Research Institute of Chemical Defense, West Building, Number 35 Huayuanbei Road, Beijing 100083, People's Republic of China

A R T I C L E

I N F O

Article history: Received 27 December 2007 Received in revised form 29 May 2008 Accepted 2 June 2008 Keywords: Acetylene black Activated carbon Agglomeration Electrochemical double layer capacitors Electrochemical performance

A B S T R A C T The influence of acetylene black (AB) content (0–20 wt.%) on the electrochemical performance of activated carbon based electrochemical double layer capacitor (EDLC) electrodes has been studied systematically. The electrochemical performance was evaluated by galvanostatic charging/discharging, cyclic voltammetry, and alternating current impedance. Experimental results indicated that 5 wt.% AB was the best choice for electrodes presenting high energy density and good rate capability. AB ensures that the composite electrodes present high electronic conductivity; however, scanning electron microscope images show that excessive AB form large agglomerates, which stuff the voids among activated carbon particles and hinder the electrolyte ion transfer during the electrochemical double layer charging/discharging. Thus the composite electrodes with excessive AB lose their hierarchically porous structure and the subsequent ion-sieving effect deteriorates the electrochemical performance. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical double-layer capacitors (EDLCs) are power sources that store energy within the electrochemical double layer formed at a solid/solution interface [1]. EDLCs have attracted great interest because of their possible usage in power sources for electric vehicles [1]. Activated carbon (AC) is the electrode material used most frequently for EDLCs due to the low cost, high surface area, availability, and established production technologies [1]. Many efforts have been done to develop AC for use in EDLCs [2–5]. Carbon materials are available with specific surface areas (SSAs) of up to 500–3000 m2 g− 1 including AC, carbon blacks, glassy carbons, carbon microbeads, fibers, cloths, aerogels, and nanotubes [4–11], which possess a variety of microstructures, electrochemical properties, and a wide range of costs. Combination of these materials provides a flexible way to optimize the properties of electrodes for EDLC to balance the performance and costs. To go through with the optimization process, a good understanding of the interplay between overall properties of such composite electrodes and the characteristics of their carbon constituents is desired. The mechanism of energy storage in a carbon-based electrode is primarily based on the separation and accumulation of charge at the carbon/electrolyte interface [12]. In terms of kinetic considerations,

⁎ Corresponding author. Tel.: +86 10 66705840; fax: +86 10 66748574. E-mail address: [email protected] (G. Cao). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.06.002

high electronic conductivity is important and can be achieved by carbons with high degree of graphitization. However, the hightemperature graphitization process typically leads to a significant loss of SSA, resulting low specific capacitance and energy density. Although carbon nanotubes present both high conductivity and large surface area [8,9], they are much more expensive than ACs, which have large surface area but poor conductivity. Furthermore, the loading density of carbon nanotubes is very low. Aiming for EDLCs of low cost and good electrochemical performance, the present work focuses on composite electrodes that contain AC as the main source for providing high capacitance and acetylene black (AB) as the conductive additive. It is well-known that electrochemical performance is strongly affected by the property of conductive additives. Effects of conductive additive on electrochemical performance of battery have been fully studied [13–15]. Nevertheless, influences of the conductive additive on performance of EDLCs have not been fully assessed. The prime aim of this work is to understand the general electrochemical behavior of such composite electrodes and to determine the optimum content of AB for superior electrochemical performance. The influence of acetylene black (AB) content (0–20 wt.%) on the electrochemical performance of activated carbon based EDLC electrodes was systematically studied by galvanostatic charging/discharging and alternating current impedance. The microstructures of the composite electrodes were characterized by scanning electron microscope. Experimental results indicated that 5 wt.% AB was the best choice for electrodes presenting high energy density and good rate capability. The

H. Zhang et al. / Solid State Ionics 179 (2008) 1946–1950

1947

Fig. 1. Variations of (a) Cg and (b) Cv of AC with AB content. (c) Cyclic voltammetry curves of electrodes with various AB contents at 20 mV/s.

effect of AB content on the electrode microstructure and its influence on the capacitive performance of composite electrodes are presented. 2. Experimental The AC powder used in this work was prepared by activating apricot stone with potassium hydroxide (double weight of apricot stone) at 800 °C for 1 h. AB powder was a commercial product produced by Jinpu Corp. (Beijing, China). The SSAs of AC and AB were obtained from the analysis of the desorption branch of N2 isotherms (at 77 K, taken by Autosorb-6, Quantachrome) using density function theory (DFT, performed by Quantachrome Autosorb 1 software v. 1.51). Procedures for electrode preparation were as follows: AC powder, AB powder, and PTFE (as a binder, FR301B, 3F Corp., Shanghai, China) were mixed in different mass ratios. Electrode compositions were x wt.% AC, y wt.% AB and 3 wt.% PTFE, where x + y = 97 and y = 0, 5, 10, 15, and 20, respectively. The mixtures were dispersed in ethanol to allow a good homogenization of the binder suspension and then rolled to 0.40 ± 0.01 mm thick films. These films were dried in air at 120 °C for 4 h and electrodes of 11 mm in diameter were cut and used for the determination of the electrochemical performance. Nickel foams (400 gm− 2, 100 ppi) were used as current collectors. Sandwichtype capacitors were fabricated with two carbon composite electrodes separated by a polypropylene separator (0.12 mm in thickness, Scimat Corp.) and loaded in a testing cell. 7 M KOH was used as electrolyte. Measurement of the gravimetric specific capacitance (Cg) was performed by the galvanostatic charging/discharging using BT2000 (Arbin instruments) testing station. Current densities were in a range of 0.05–10 A g− 1, with the voltage ranged from 0 to 1.0 V. Cyclic voltammetry (CV) experiments, which were conducted by a Solartron 1280Z electrochemical test station controlled by CorrWare 2, were also used to characterize the capacitive performance of electrodes. The resistance (Re) and electronic conductivity of composite electrode was measured using a four-point method. The alternating current impedance measurements were conducted by Solartron 1280Z electrochemical testing station with frequency ranging from 20 k to 0.01 Hz on a dc voltage of 1.0 V and 5 mV amplitude. All the electrochemical performance characterization was conducted at 298 ± 1 K. Scanning electron microscopy (SEM, H-700H, Hitachi Corp.) was used to characterize the microstructure of AB, AC, and composite electrodes. Loading density of AC and the electrode density were calculated from the weight of AC and composites (including AC, AB, and PTFE) per apparent electrode volume, respectively.

contribution of AB to the capacitance can be neglected, considering the low specific capacitance and low content of AB. Fig. 1 (a) shows the variations of Cg (F g− 1) with AB contents. Six groups of dots (with different shapes) express the values of Cg at six current densities. The distinction between Cg is not remarkable at low current density (0.05 A g− 1). On the contrary, there is a general trend that electrode with 5 wt.% AB presents the highest Cg at high current densities (such as 10 A g− 1). Therefore, electrode with 5 wt.% AB presents best rate capability. Fig. 1 (b) shows the variations of volume specific capacitance (Cv, F cm− 3, Cg multiplied by the loading density of AC) with AB contents. Electrode with 5 wt.% AB presents the highest Cv at high current density. Compared with Cg, it is obvious that the decrease of Cv is more remarkable when AB content exceeds 5 wt.%. This is attributed to that the loading density of AC decreases as AB content increases (see Fig. 5). Fig. 1(c) shows the CV curves of electrodes with various AB contents at a high scan rate of 20 mV/s. All CV curves show rectangular shapes, indicating highly capacitive nature with rapid charge/discharge characteristics [16]. The CV curve of the electrode with 5 wt.% AB is wider than that of other electrodes, demonstrating that the electrode with 5 wt.% AB possesses higher capacitance. Fig. 2 shows the variations of equivalent series resistance (ESR) of EDLCs and Re with AB content. Both ESR and Re decrease as AB content increases and then tends to level off when AB content exceeds 5 wt.%, indicating that 5 wt.% is close to the percolation threshold for AB, the conductive phase [17–19]. The decrease of ESR enhances the maximum power of EDLC. Therefore, the maximum power of EDLC increases as AB content increases and then tends to level off when AB content exceeds 5 wt.%. Nyquist plots (20 k to 0.01 Hz) and their enlarged sections (20 k to 0.08 Hz) of EDLCs are shown in Fig. 3 (a) and (b), respectively. These plots show typical EDLC behavior, which start with a nearly 45° or a semi-circle impedance line and approaching an almost vertical slope at low frequency. It's noticeable that the shape and slope of these plots at frequency range from 0.1 to 0.01 Hz, which is a reflection of the

3. Results and discussion The SSA of AC is 1288 m2 g− 1, which is much larger than that of AB (55 m2m2 g− 1). Our experimental results show that the SSA of composite electrodes is close to that of AC, indicating that the adding of AB and PTFE does not decrease the effective surface area of AC. The

Fig. 2. Variations of ESR, Re, and electronic conductivity with AB content.

1948

H. Zhang et al. / Solid State Ionics 179 (2008) 1946–1950

Fig. 3. (a) Nyquist plots (20 k to 0.01 Hz) and (b) their enlarged sections (20 k to 0.08 Hz) of composite electrodes with different AB contents.

essential pore characteristics of AC, are almost the same. Nyquist plot of electrode with 5 wt.% AB shows near vertical slope at 0.4 Hz, indicating good power output performance. On the contrary, Nyquist plots of electrodes with more than 5 wt.% AB show non-vertical slopes even at 0.1 Hz, indicating inferior power output performance. Taberna and co-workers [20] reported that a Bode plot of the real (C' (ω)) and imaginary parts (C'' (ω)) of the complex capacitance (C (ω)) as a function of frequency was useful for gaining insight into the electrochemical properties of EDLCs. These plots allow an overview of the whole frequency behavior of EDLCs. Fig. 4 (a) presents the real part of capacitance (C' (ω)) changes versus frequency. C' (ω) decreases sharply as frequency increases from 0.01 to 1 Hz. The huge dependence of capacitance on frequency indicates that a great deal of the surface area of AC has little effect on the forming of electrochemical double layer at high frequency. EDLC electrodes with more than 5 wt.% AB show more remarkable frequency-reliant characteristics, which are reflections of inferior rate capability. Fig. 4 (b) presents the imaginary part of capacitance (C'' (ω)) changes versus frequency. For EDLC, a maximum of C'' (ω) is observed at a characteristic frequency (f0). The reciprocal of f0 represents the minimum discharging time τ0, which can be used for evaluating the rate capability of EDLCs [20]. Electrode with 5 wt.% AB presents the least τ0 (6.3 s), indicating that superior discharging rate can be delivered. This is consistent with the conclusion obtained from the galvanostatic charging/discharging. Based on the results above, we claim that 5 wt.% AB ensures electrode presenting higher specific capacitance and better rate capability and power output performance. In addition, we evaluated the electrochemical performance of composite electrodes in 6 M NaOH electrolyte. Experimental results also exhibited that 5 wt.% AB ensured composite electrodes presenting higher specific capacitance and better power output performance. Fig. 5 shows the variations of AC loading density and electrode density with AB content. AC loading density decreases as AB content increases, which is attributed to the low packing density of AB.

However, electrode density increases as AB content increases from 0 to 15 wt.%, indicating that AB particles fill in the voids among AC particles. Electrode density tends to level off and decreases as AB content exceeds 10 wt.%, revealing that the voids among AC particles has been stuffed by AB particles and then AB particles begins to form large agglomerates (see Fig. 6 (d) and (e)). These agglomerates can separate AC particles and the heterogeneity of the electrode may lead to the deterioration of the electrochemical performance. This hypothesis is proved by SEM results. SEM image of AB clusters, AC particles, composite electrodes with 5 and 20 wt.% AB are shown in Fig. 6. AB clusters are formed by submicrometer primary particles and the diameter of AB clusters ranges from 2 to 10 µm (see Fig. 6 (a)). The diameter of AC particles ranges from 2 to 20 µm (see Fig. 6 (b)). The morphology and microstructure of the composite electrodes varies with the AB contents, which has been shown as schematic diagrams in Fig. 6 (e). Fig. 6 (c) is a SEM image of electrode with moderate AB (5 wt.%). It's obvious that AB particles fill the voids among AC particles. The conductive network is formed by both AC and AB particles. AB has nice electronic conductivity thus the composite electrode's electronic conductivity is good (see Fig. 2, Re and conductivity of electrodes with 5–20 wt.% AB are lower than 0.05 Ω and higher than 1.15 S/cm). Furthermore, the electrode still possesses a great deal of large pores (larger than 1 µm, see the middle image in Fig. 6 (e)) among AC particles and AB. These macropores (N50 nm) among AC and AB particles and the meso/micropores in the AC particles compose a hierarchically porous structure [21], which enhances the ionic conductivity of the composite electrode greatly. Therefore, electrode with moderate AB (5 wt.%) obtains both good electronic conductivity and ionic conductivity. EDLCs assembled with these electrodes have low ESR, high power density, and good rate capability. Fig. 6 (d) is a SEM image of electrode with excessive AB (20 wt.%). It is noticeable that AB clusters stuff the voids among AC particles and there are many large AB agglomerates. This electrode presents good electronic conductivity (see Fig. 2), yet the big pores among AC particles are narrowed or even

Fig. 4. Bode plots of the (a) real and (b) imaginary capacitance of EDLCs as a function of frequency.

H. Zhang et al. / Solid State Ionics 179 (2008) 1946–1950

1949

sieving effect becomes more serious as the discharge rates become higher. Therefore, electrodes with excessive AB present inferior rate capability and power output performance. Note that the suitable content of AB will depend on the particle size and particle distribution of AC. Theoretically, smaller AC particles are more susceptible to AB agglomerates because of smaller voids among AC particles. 4. Conclusions

Fig. 5. Variations of AC loading density and electrode density with AB content.

stuffed, thus the composite electrode lose the hierarchically porous structure and the ion transfer is hindered during charging/discharging (see the right image in Fig. 6 (e)), resulting in that a large fraction of surface area of micropores in AC particles has little effect on the forming of electrochemical double layer at high current densities. This ion-

Through the analysis of the variations of Cg, Cv, ESR, and rate capability of electrodes with different AB content, it was found that 5 wt.% AB was the best choice for EDLCs presenting high energy, high power density, and superior rate capability. When moderate AB is used, AB particles fill the voids among AC particles and then enhance the electronic conductivity of the electrode, moreover, the electrode possesses a hierarchically porous structure, which ensures the electrode possessing good ionic conductivity and having superior power output performance and rate capability. On the contrary, excessive AB tends to form large agglomerates, which stuff the voids among AC particles, hinder the electrolyte ion transfer during charging/discharging, and then deteriorate the electrochemical

Fig. 6. SEM images of (a) AB, (b) AC, composite electrodes with (c) 5 wt.% and (d) 20 wt.% AB. (e) Schematic representation of the microstructure of composite electrodes with various AB contents.

1950

H. Zhang et al. / Solid State Ionics 179 (2008) 1946–1950

performance of electrodes. The study of the AB agglomeration in EDLC electrodes can help us get a good understanding of the effects of conducting additives on electrochemical performance of EDLCs, which is also helpful for the optimization of the EDLCs' electrochemical performance.

[4] [5] [6] [7] [8] [9] [10]

Acknowledgements

[11] [12]

This work was financed by the National Science Foundation of China (No. 20633040) and National 863 project (No. 2006AA03Z342 and No. 2006AA11A163).

[13] [14] [15]

References [1] A. Nishino, J. Power Sources 60 (1996) 137. [2] D. Lozano-Castelló, D. Carzorla-Amorós, A. Linares-Solano, S. Shiraishi, H. Kurihara, A. Oya, Carbon 41 (2003) 1765. [3] J. Gamby, P.L. Taberna, P. Simon, J.F. Fauvarque, M. Chesneau, J. Power Sources 101 (2001) 109.

[16] [17] [18] [19] [20] [21]

H. Shi, Electrochim. Acta 41 (1996) 1633. I. Tanahashi, A. Yoshida, A. Nishino, J. Appl. Electrochem. 21 (1991) 28. C. Lin, B.N. Popov, H.J. Ploehn, J. Electrochem. Soc. 149 (2002) A167. I. Tanahashi, J. Appl. Electrochem. 35 (2005) 1067. H. Zhang, G.P. Cao, Y.S. Yang, J. Power Sources 172 (2007) 476. H. Zhang, G.P. Cao, Y.S. Yang, Zhennan Gu, J. Electrochem. Soc. 155 (2008) K19. L. Bonnefoi, P. Simon, J.F. Fauvarque, C. Sarrazin, J.F. Sarrau, A. Dugast, J. Power Sources 80 (1999) 149. H.K. Song, H.Y. Hwang, K.H. Lee, L.H. Dao, Electrochim. Acta. 45 (2000) 2241. B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Press, New York, 1999. J.K. Hong, J.H. Lee, S.M. Oh, J. Power Sources 111 (2002) 90. S. Kuroda, N. Tobori, M. Sakuraba, Y. Sato, J. Power Sources 119-121 (2003) 924. K. Tatsumi, K. Zaghib, H. Abe, S. Higuchi, T. Ohsaki, Y. Sawada, J. Power Sources 54 (1995) 425. B. Fang, L. Binder, J. Phys. Chem. B. 110 (2006) 7877. N.L. Wu, S.Y. Yang, J. Power Sources 110 (2002) 233. M. Liu, Z. Wu, Solid State Ionics 107 (1998) 105. H. Hu, M. Liu, Solid State Ionics 109 (1998) 259. P.L. Taberna, P. Simon, J.F. Fauvarque, J. Electrochem. Soc. 150 (2003) A292. E.S. Toberer, T.D. Schladt, R. Seshadri, J. Am. Chem. Soc. 128 (2006) 1462.