Journal of Power Sources 173 (2007) 621–625 Short communication Structures and electrochemical performances of pyrolized carbons from graphite oxides for electric double-layer capacitor Ick-Jun Kim a,∗ , Sunhye Yang a , Min-Je Jeon a , Seong-In Moon a , Hyun-Soo Kim a , Yoon-Pyo Lee b , Kye-Hyeok An b , Young-Hee Lee b a Korea Electrotechnology Research Institute, 28-1, Seongju-Dong, Changwon 641-600, Republic of Korea b Sungkyunkwan University, Suwon 440-746, Republic of Korea Received 24 April 2007; received in revised form 11 June 2007; accepted 16 July 2007 Available online 3 August 2007 Abstract The structural features and the electrochemical performances of pyrolized needle cokes from oxidized cokes are examined and compared with those of KOH-activated needle coke. The structure of needle coke is changed to a single phase of graphite oxide after oxidation treatment with an acidic solution having an NaClO 3 /needle coke composition ratio of above 7.5, and the inter-layer distance of the oxidized needle coke is expanded to 6.9 ˚ A with increasing oxygen content. After heating at 200 ◦ C, the oxidized needle coke is reduced to a graphite structure with an inter-layer distance of 3.6 ˚ A. By contrast, a change in the inter-layer distance in KOH-activated needle coke is not observed. An intercalation of pyrolized needle coke, observed on first charge, occurs at 1.0V. This value is lower than that of KOH-activation needle coke. A capacitor using pyrolized needle coke exhibits a lower internal resistance of 0.57  in 1 kHz, and a larger capacitance per weight and volume of 30.3 F g -1 and 26.9 F ml -1 , in the two-electrode system over the potential range 0–2.5 V compared with those of a capacitor using KOH-activation of needle coke. This better electrochemical performance is attributed to a distorted graphene layer structure derived from the process of the inter-layer expansion and shrinkage. © 2007 Elsevier B.V. All rights reserved. Keywords: Electric double-layer capacitor; Activation procedure; Graphite oxide; Pyrolized carbon; Specific capacitance; Needle coke 1. Introduction Electric double-layer capacitors (EDLCs) have been widely used as energy storage devices for memory back-up systems, and are receiving considerable attention as a promising high-power energy source for electric devices and hybrid electric vehicles [1–3]. The energy storage mechanism of EDLCs is based on the fact that an electric double-layer is formed at the boundary between the electrode and an electrolyte. In electronic devices requiring higher energy density due to their smaller size, the higher specific volume capacitances of EDLCs is an important factor. However, conventional physically/chemically activated carbons possess a micro-porous structure with a moderately ∗ Corresponding author. Tel.: +82 55 280 1668; fax: +82 55 280 1590. E-mail address: ijkim@keri.re.kr (I.-J. Kim). high specific surface area, which has limited the specific vol- ume capacitance to less than 20 F ml -1 , due to low electrode density, in two-electrode systems [4]. In recent years, an alkali-activation procedure has been investigated to obtain larger specific volume capacitance from graphitizable carbons. These carbons, activated with alkaline solutions such as KOH, NaOH, and K 2 CO 3 at 700–900 ◦ C, exhibit larger specific volume capacitances of 20–30 F ml -1 [5–7]. It is reported that the first cycle, that is, electric field acti- vation, makes the electrolyte ions intercalate into the graphene layers of the carbons, which introduces the small pore struc- ture of the electrode. Such structural changes during the first cycle are irreversible and provide larger specific volume capac- itance. However, it has been also reported that this process has several problems, such as a high activation cost, corrosion of the activation vessel and the rapid degradation of capac- itance during charge–discharge cycling. This electrochemical 0378-7753/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2007.07.042