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1 Carbon Letters Vol. 9, No. 2 June 2008 pp Electrochemical Properties of EDLC Electrodes Prepared by Acid and Heat Treatment of Commercial Activated Carbons Jingyu Wu 1, Ikpyo Hong 2, Sei-Min Park 2, Seong-Young Lee 2 and Myung-Soo Kim 1, 1 Department of Chemical Engineering, Myongji University, Yongin, Gyonggi-do , Korea 2 Carbon Materials Lab, RIST, Pohang, , Korea myungkim@mju.ac.kr (Received May 6, 2008; Accepted June 10, 2008) Abstract The commercial activated carbons are typically prepared by activation from coconut shell char or coal char containing lots of inorganic impurities. They also have pore structure and pore size distribution depending on nanostructure of precursor materials. In this study, two types of commercial activated carbons were applied for EDLC electrode by removing impurities with acid treatments, and controlling pore size distribution and contents of functional group with heat treatment. The effect of the surface functional groups on electrochemical performance of the activated carbon electrodes was investigated. The initial gravimetric and volumetric capacitance of coconut based activated carbon electrode which was acid treated by HNO 3 and then heat treated at 800 o C were 90 F/g and 42 F/cc respectively showing 94% of charge-discharge efficiency. Such a good electrochemical performance can be possibly applied to the medium capacitance of EDLC. Keywords : EDLC electrode, activated carbon, acid treatment, heat treatment, functional group, electrochemical properties r pt ƒže (EDLC, electric double layer capacitor) n p r l v rq o q sp rr ƒže, ƒže, r ƒ p e~ }l p Farad n p r l v e l r r p q p. r pt ƒže rl v p q e ƒže m r p p rvp rp m l rp rp l v q r p p m rp vp pl p rvp n ~ l v rq q p p [1-4]. r pt ƒže p r v ˆ q rp n p p r r, e, p lž} p p. p rp r pt ƒže r n ˆ q v, p ˆ l rp 1000 ~ 3000 m 2 /gp ˆp ˆ o n. p ˆ p k k p n r l p l rs rs rp q rso p ˆp 10 ~ 50 r n l Ž 10 ~ 100 r l p p [5-7]. trn ƒže p r q l p l rs ˆ n n r q o p rso d. n ˆ r ~ pn r l p r p r q eq pp p r q }n n ƒže p o 20% p r pp p [8]. r pt ƒže p r n ˆ q p r,, v p p p l q l p q l p. p r rn p v k p ˆ p k rs r, v r ~p p rl m } s p p p. l t n n r p l r v pmp d r p p, p rl t rp l r p lk [9-12]. ˆ p rp ˆ r q n r pt ƒže p p r tn n p. p r p ˆ p l k ˆp m sq, p p qnp ˆ p, p r. r pt ƒže p r vp p rp nk o n n ƒže p p eˆ o r r vp rp o pl k. p rp ml rs ˆ p n l p ˆ oq (-COOH, -COH, C=O ) k pv kk p ˆ. p n

2 138 Jingyu Wu et al. / Carbon Letters Vol. 9, No. 2 (2008) r vp o n p n p r p p nkp r v n nl r r vp r p ƒže p p n, p v q k e de } l p [13-15]. ˆ q p r nl r r sp r, v qp s, r p r p r s rq ƒže p r n p r tn n p. ˆ r p r r s rq ƒže p p r e n p l r l n qp p 5~10% p k [16]. l l coconut shell ˆ coal char ˆp r } l p l p r, l} l p l rl r pt ƒže p r q n l r r l l s m. v m l p l r p m, Boehm l p l r qp p s m kp r l r qp r p s m. x x rp 1170 m /gp 2 coconut shell ˆ 1077 m 2 /g p coal char ˆ(Samchully Carbotech Co.)p o k r } l r pt ƒže p r v n m. r p s p o p v CMC (carboxylmethycellulose sodium salt)m PTFE (polytetrafluroethylene, 60% dispersion in water. Aldrich), n v n mp p PP(poly propylene), r vp 1M TEABF4(tetraethylammonium tetrafluoroborate) mp n PC (polycarbonate)r kp n m. o p } o l v (Daejung Chem. & Metals Co., Ltd. 97%), m (Matsuneon Chem. Ltd., 35%), (Samchun pure chem. Co., Ltd. 97%), m (HCl : HNO 3 p p 3:1) n m. ˆp p kp r o l NaOH (Shinyo Pure Chem. Co., Ltd.), NaHCO 3 (Seoul Chem. Industry. Co., Ltd), Na 2 CO 3 (Wako Pure Chem. Industries Ltd.)p titrant n mp 0.1 N HCl tnk(daejung Chem. & Metals Co., Ltd) back-titrant n m. e. } m ˆp p mr r v v l } 150 o C m l 24 e k se. } ˆp rl o l v } m ˆp v o l dm m 10 o C/min 500 ~ 1000 o C v dm 1e k l} m. Micromeritics p ASAP 2020 modelp m e q n l } m l} l p ˆp r l m p, Boehm l p l l} m l r qp p k s r m.»yw p sƒ } m l} ˆp jet-millp pn l, rq carbon black, r CMC/PTFE l e planetary mill, aluminum foill Ž k v s l 150 o Cl 24e k s l r q n m. s r p n l glove box kl coin cellp rq mp rq cellp 23 o C ov cut-off rkp 0.1 ~ 2.5 V }p 5 p p C-Rate 2C 6 w p 0.5C l CC-CV r e p m. r p r pp o l IM6(Zahner, Germany) n l scan rate 50 mv/s l 0~2.5V p 10 p v r r m. x š y k p Coconut shell ˆ coal char ˆp jet-mill y k z o p p r o l v, m, m nkp okp pn l coconut shell coal char ˆp } m. 20 gp ˆl 200 mlp n kp 60 o Cl 2e k eˆ p Fig. 1. Particle size distribution of coconut shell based AC and coal char based AC.

3 Electrochemical Properties of EDLC Electrodes Prepared by Acid and Heat Treatment of Commercial Activated Carbons 139 l r v n l r p m. Jet-mill p Fig. 1l ˆ l. p pq 10 µm p l t m, pq coconut shell ˆp 7.2 µm, coal char ˆp 7.7 µm r ˆ lp r pt ƒže p r q n rr v p p pl. Fig. 2p p v l ˆ r p rn p ˆ p. Coconut shell ˆ coal char ˆp r v n mp, n p F/gpl, n p 39m 36 F/ccm. 100 p Ë rp, n p 54m 66 F/gp mp n p 32m 34 F/cc l n n t 82m 94%p r pp ˆ l. n p n n,, r l rp l p l r, ƒže rs pl q erp p ƒže p l vr rp m p t en o tn v n l rp m. Coconut shell ˆ o coal char shell ˆ o l l p kr p p coconut shell ˆp p coal char ˆl l p p. Table 1l o n coconut shell ˆ coal char ˆp ICP (Inductively Coupled Plasma)l p ˆ pnp p ˆ l. Coconut shell ˆl,, k p p p sq p l r rl pm p p qnl p l r v p e s r pp pl ln p v, coal char ˆp p rp r o pp r l n p r r p p Ž l. Fig. 3 coconut shell ˆ coal char ˆ o Fig. 2. Specific capacitance of coconut based AC and coal char based AC with different acid treatments. Fig. 3. Cyclic voltammograms of coconut shell based AC (A) and coal char based AC (B). Scan rate: 50 mv/s

4 140 Jingyu Wu et al. / Carbon Letters Vol. 9, No. 2 (2008) Table 1. Ash contents of coconut shell based AC and coal char based AC Al Na Mg Si P K Ca Fe Total Coconut AC Coal char r p mp p CV (cyclic voltammogram) ˆ p. Coal char ˆp r p n mp, ~ p p 0.2 V l ˆ l p l p r vp qnl p coin cellp l ~ p p. 10 p coconut shell ˆ coal char ˆp CV l p ro yl p e v, p r p v rl r tp p r k p r kp l pp pl p p Ž. CV l p p rp p p n p ˆ 10 p v ˆl n p l, coconut shell ˆp n p coal char ˆl l ˆ lp Fig. 2l ˆ m p ˆ l. y k p ˆ o p p r l l p ƒže p pp e kr p eˆ o l Coconut shell ˆ coal char ˆp, m, v, m p } m. } ˆp ƒže p r v n mp } v kp o m l p v l rn rn p Fig. 4m Fig. 5l ˆ l. } ˆp n p o Fig. 4. Gravimetric capacitance of coconut based AC (A) and coal char based AC (B) with different acid treatments. Fig. 5. Volumetric capacitance of coconut based AC (A) and coal char based AC (B) with different acid treatments.

5 Electrochemical Properties of EDLC Electrodes Prepared by Acid and Heat Treatment of Commercial Activated Carbons 141 Table 2. Specific surface area of coconut based AC and coal char based AC with different acid treatment Sample SSA(m 2 /g) S mi (m 2 /g) S mi /SSA(%) S ext (m 2 /g) S ext /SSA(%) Coconut AC H 2 SO HCl+HNO HNO 3 HCl Coal char AC H 2 SO HCl+HNO HNO 3 HCl SSA: BET surface area; S mi : micropore area; S ext : external surface area l sp ˆ l, tl m } ˆp r v n p q p rn p ˆ. m } ˆp r v n p coconut shell ˆ coal char ˆp n p 75m 79 F/gp ˆ p 100 p n p F/gp ˆ l. v } ˆp r v n mp n p m } ˆ d v p kr p l. p v } ˆp l p rp r p p n kp r v n nl r r vp rp o tv o n r v tl r r vp rp l n p [25]. } ˆp n p v p } rp l p r l o p q l n p v l rp v l, ˆ rs rl ˆ v k p r q p p sq p p l p l l n p p p p Ž. Table 2 } l ˆ e p rp r p. } ˆp n p v mp n p o l p ˆ l. wy k p } ˆp ƒže p r q n mp n p o l l v lp r p l n p r ˆ l. } l r l p p r p } l p l ˆ l n rp l p p Ž. v } ˆp rn n p d p p p pl. v } l p l ˆp l p, Fig. 6. Contents of acidic surface functional group of coconut based AC (A) and coal char based AC (B) with different heat treatment temperatures after HNO 3 treatment.

6 142 Jingyu Wu et al. / Carbon Letters Vol. 9, No. 2 (2008) p l} l p r p f r qp ƒ Že p l m p q m. ˆp kp Boehm p pn l r mp e Fig. 6l ˆ l. Coconut shell ˆ coal char ˆ v } p l lactone group phenolic hydroxyl group v l. l } m p v l lactone groupp coconut shell ˆ coal char ˆl ˆ l l} l lactone groupp p k pl. phenolic hydroxyl groupp n, v ˆl p ˆ l, coal char p nl rp np, coconut shell p n l 1000 o C vp l} l np r v k ˆ l. p o p s l p r, sv s, ~rp p s m k l sn p, p l ƒ vp p n. p p l} m l p ˆ 800 o C p l kp r kv p p. v } ˆp l v m l l} l r q n mp p v l rn rn p Fig. 7 8l ˆ l. v } coconut shell ˆ coal char ˆp l} m d l rn p v lp p n m. p Fig. 9l l} m l n 100 p Ë r p n p ˆ l. l} m 800 o Cp coconut shell ˆ coal char ˆ p n p F/gp q p n p ˆ lp, 100 p n p 85m 76 F/g 94%m 93%p Ë r pp ˆ l. l} Fig. 7. Gravimetric capacitance of coconut based AC (A) and coal char based AC (B) with different heat treatment temperatures after HNO 3 treatment. Fig. 8. Volumetric capacitance of coconut based AC (A) and coal char based AC (B) with different heat treatment temperatures after HNO 3 treatment.

7 Electrochemical Properties of EDLC Electrodes Prepared by Acid and Heat Treatment of Commercial Activated Carbons 143 m 800 o C p l n p p k p l. Coconut shell ˆ coal char ˆp n 800 o Cl 42m 37 F/cc ˆ lp, 100 p F/cc ˆ p p rn p q p ˆ l. n p l} m p v l v p l} m p v l ˆ p re p pmp kp v m p Ž, 800 o C p l n p p 800 o C p l ˆ p p kp l} m kvl s p p pmp kp p. Table 3p l} m l r r p. o l l }, l} ˆp rp v mp, l} m l 900 o C p l rp s j v } ˆl l l } ˆp micropore rp tl mesoporem macropore rp v ˆ l. Fig. 9. Specific capacitance with different heat treatment temperatures. (A): Gravimetric capacitance, (B): volumetric capacitance Coconut shell coal char n ˆp r } l p l p r, l} l p l rl l r q rs m. rs r qp r p t ƒže p r q p r r p s r q p r p l p p p ll. 1. Coconut shell coal char ˆp p } lp r pt ƒže p r q n mp r Table 3. Specific surface area and porosity parameters of samples Sample SSA(m 2 /g) S mi (m 2 /g) S ext (m 2 /g) V tot (cm 3 /g) V mi (cm 3 /g) V mi /V tot (%) D/nm Coconut HNO o C 700 o C 800 o C 900 o C 1000 o C Coal HNO o C 700 o C 800 o C 900 o C 1000 o C SSA: BET surface area; S mi : micropore area S ext : external surface area; V tot : total volume; V mi : micropore volume; D: average pore diameter

8 144 Jingyu Wu et al. / Carbon Letters Vol. 9, No. 2 (2008) l n p p CV pl p p r q p np r v k k. 2. ˆp } n rp v l n p v mp, l p l p kr p kr. l} l p l p k p micropore mesoporem macropore v l l m p 800 o C r l l} p v m. 3. v } l} l p l, rl ˆp r q n n n p p l coconut shell ˆp v 800 o C l} r q n 90 F/g, 42 F/cc 100 p 85 F/g, 40 F/ccp 94%p pp ˆ l o ˆ n p p ˆ l. š x [1] Kwon, O. J.; Jung, Y. H.; Oh, S. M. J. Power sources 2004, 125, 221. [2] Lee, J.; Kim, J.; Lee, Y.; Yoon, S.; Oh, S. M.; Hyeon, T. Chem. Mater. 2004, 16, [3] Bonnefoi, L.; Simon, P.; Fauvarque, J. F.; Sarrazin, C.; Dugast, A. J. Power Source 1999, 79, 37. [4] Tanahashi, I.; Yoshida, A.; Nishino, A. Denki Kagaku 1988, 56, 892. [5] Park, S. J.; Jung, W. Y. J. Colloid Interface Sci. 2002, 250, 93. [6] Inagaki, M.; Radovic, L.R. Carbon 2002, 40, [7] Burke, A. J. Power sources 2000, 91, 37. [8] Qiao, W. M.; Korai, Y.; Mochida, I.; Hori, Y.; Maeda, I. Carbon 2002, 40,351. [9] Qu, D. J. Power Source 2002, 109, 403. [10] Sarangapani, S.; Tilak, B. V.; Chen, C. P. J. Electrochem. Soc. 1996, 143, [11] An, K. H.; Jeon, K. K.; Heo, J. K.; Lim, S. C.; Bae, D. J.; Lee, Y. H. J. Electrochem. Soc. 2002, 149, [12] Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, [13] Park, S. J.; Jang, Y. S. J. Colloid Interface Sci. 2002, 249, 458. [14] Park, S. J.; Seo, M. K.; Rhee, K. Y. Carbon 2003, 41, 592. [15] Park, S. J.; Seo, M. K.; Rhee, K. Y. J. Phys. Chem. B. 2003, 107, [16] Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937.