Novel method for the preparation of carbon supported nano-sized amorphous ruthenium oxides for supercapacitors Yong-Hee Lee, Jong-Gil Oh, Hyung-Suk Oh, Hansung Kim * Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-gu, 120-749 Seoul, Republic of Korea article info Article history: Received 14 April 2008 Received in revised form 29 April 2008 Accepted 6 May 2008 Available online 13 May 2008 Keywords: Supercapacitors Glycolic acid Zeta potential Ruthenium oxide Pseudocapacitance abstract Nanostructured amorphous RuO 2 Á xH 2 O/C composite materials are prepared via a modified sol–gel pro- cess using glycolic acid. The glycolate anion, which dissociates from glycolic acid at pH 7, behaves as a stabilizer by adsorbing onto the RuO 2 Á xH 2 O surface, thus resulting in particles with a size of about 2 nm. As evidenced by zeta potential measurements, the surface charge of RuO 2 Á xH 2 O becomes more electronegative as the amount of glycolic acid increases. After heat treatment at 160 o C to remove the sta- bilizer, RuO 2 Á xH 2 O/C is found to exhibit an amorphous structure. The specific capacitance of RuO 2 Á xH 2 O/C particles (40 wt% Ru) prepared in the presence of glycolic acid (0.3 g L À1 ) is 462 F g À1 , which is 30% higher than that of the material prepared in the absence of glycolic acid. Both the nanosized particles and the amorphous structure mainly contribute to this increase in the specific capacitance. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Hydrous ruthenium oxide (RuO 2 Á xH 2 O) has received extensive attention for applications in supercapacitors because it undergoes fast faradaic redox reactions over a wide voltage range, with excel- lent reversibility [1]. It is well known that the rutile part of hydrous ruthenium oxide (RuO 2 Á xH 2 O) provides the path for electron transportation, while the structural water at the boundaries helps in proton transport [2,3]. The both pathways are optimized by con- trolling the amount of water content in the RuO 2 Á xH 2 O. When the RuO 2 Á xH 2 O is treated at high temperature (>200 o C), the phase transition occurs to crystalline RuO 2 [4]. The resulted rutile struc- ture restricts the faradaic reaction to occur near to surface and ac- counts for a low specific capacitance. On the other hand, the hydrous ruthenium oxide with amorphous structure prepared by the sol–gel method allows the protons to diffuse into the oxide particle. Consequently, the bulk of the oxide is utilized for pseud- ocapacitance and a relatively higher specific capacitance is ob- served [5]. The capacitance of RuO 2 Á xH 2 O varies considerably depending on the preparation technique used. Hu et al. prepared the RuO 2 Á x- H 2 O by a hydrothermal synthesis route [6] and the anodic deposi- tion technique [7]. The sol–gel process was modified using methanol and NH 3 solution [8,9]. The uniform size distribution of RuO 2 Á xH 2 O ranged between 1.5 and 3 nm exhibited up to 1589 F g À1 . The particle size of the RuO 2 Á xH 2 O also plays a key role in increasing the utilization of the material. Since proton diffusion through the amorphous ruthenium oxide is slow, a decrease in the particle size of the oxide increases the utilization of RuO 2 [10]. Miller et al. [11] deposited nanosized RuO 2 particles with a size of about 3 nm on a carbon aerogel by chemical vapor impregna- tion. However, the high annealing temperatures they used approx- imately 300 o C initiated a crystalline transition of RuO 2 and resulted in a specific capacitance of 330 F g À1 , which is much lower than that obtained from the sol–gel method. Popov et al. applied the colloidal method to prepare amorphous RuO 2 Á xH 2 O/carbon nanocomposites [12]. This low-temperature process ensures the growth of nanodimensional amorphous-ruthenium-oxide parti- cles. The specific capacitance was estimated to be approximately 863 F g À1 . In the present work, we propose a new methodology for prepar- ing nanostructured amorphous ruthenium oxides using glycolic acid. The glycolate anion which dissociates from glycolic acid in alkaline solution acts as a stabilizer by adsorbing onto the RuO 2 Á xH 2 O particles, thus preventing aggregation. Furthermore, re- moval of this stabilizer can be achieved by means of heat treatment at 160 o C [13]. This temperature is low enough to maintain the amorphous structure of the oxide. The mechanism involved in this process is supported by the changes in the surface charge of RuO 2 Á xH 2 O determined using the zeta potentials. 2. Experimental Measured amounts of RuCl 3 Á xH 2 O and carbon black (Ketjen black 300 J) were added to 600 ml of an aqueous solution to 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.05.005 * Corresponding author. Tel.: +82 2 2123 5753; fax: +82 2 312 6401. E-mail address: elchem@yonsei.ac.kr (H. Kim). Electrochemistry Communications 10 (2008) 1035–1037 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom