Electrochimica Acta 56 (2011) 3543–3548 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Mass changes accompanying the pseudocapacitance of hydrous RuO 2 under different experimental conditions Suzana Sopˇ ci´ c a,1 , Marijana Kralji ´ c Rokovi ´ c a,1 , Zoran Mandi ´ c a,∗,1,2 , András Róka b , György Inzelt b,∗,1 a Faculty of Chemical Engineering and Technology, Department of Electrochemistry, University of Zagreb, Maruli´ cev trg 19, HR-10000 Zagreb, Croatia b Department of Physical Chemistry, Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary article info Article history: Received 2 June 2010 Received in revised form 12 October 2010 Accepted 12 October 2010 Available online 20 October 2010 Keywords: RuO2 Supercapacitors EQCN Pseudocapacitance Nafion abstract Pseudocapacitance reaction of hydrous ruthenium oxide was investigated by cyclic voltammetry com- bined with electrochemical quartz-crystal nanobalance (EQCN) in sulfuric acid as well as in neutral solutions of Na 2 SO 4 and K 2 SO 4 . The ruthenium oxide electrode was prepared by attaching the ruthenium oxide particles on gold covered quartz electrode. The results show that there are different types of charge taking place simultaneously during the redox reaction of ruthenium oxide electrode. Their contribution to the overall charge depends on the experimental conditions. Depending on the potential and electrolyte used the redox reaction of ruthenium oxide is accompanied either by mass loss or by mass gain. The aver- age molar masses of the species exchanged between the solid phase and the electrolyte solution depend on the potential and scan rate. The effect of Nafion TM top layer was also investigated. It has been found that it does not affect significantly the overall specific capacitance of ruthenium oxide electrode but the apparent molar masses of exchanged species decrease in comparison with the uncovered electrodes. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Since the discovery that ruthenium oxide shows capacitive responses over a wide potential range [1], a lot of research efforts have been made and a huge amount of data have been accumu- lated in order to reveal the exact mechanism and kinetics of the electrochemical reactions taking place during its redox transfor- mations [2–10]. It has been found that this material is almost ideal for supercapacitor applications due to its extremely reversible and reproducible electrochemical behaviour as well as the capability to store very high charge and to release it fast enough upon demand [3]. In order to find widespread use as active electrode material in supercapacitors, it is of paramount importance to maximize the uti- lization, i.e., to extract as much charge as possible per unit mass of ruthenium oxide. This would minimize ruthenium oxide load and keep manufacturing prices down. Thus, the capacitive responses of ruthenium oxides prepared by different methods including elec- trodeposition [11–13], sol–gel process [5,8–10,14–16], thermal preparation [2,5] and electrostatic spray deposition [6,7] have been investigated. The capacitance ranged from 100 F/g obtained for electrochemically prepared hydrous form of ruthenium oxide [13] ∗ Corresponding author. Tel.: +36 1 3722500; fax: +36 1 3722548. E-mail addresses: zmandic@fkit.hr (Z. Mandi ´ c), inzeltgy@chem.elte.hu (G. Inzelt). 1 ISE members. 2 Tel.: +385 1 4597164; fax: +385 1 3733640. to 720 F/g for amorphous hydrated form [16]. The ability to achieve such a high capacitance in the latter case is the consequence of the participation of deeper layers of ruthenium oxide in the redox reaction. It is generally accepted that the redox reaction involves the fine distribution of various mixed valence compounds and that the overall charge/discharge reaction can be represented by the following equation [3,6,7,9–11]: RuO x (OH) y + ze - + zH + ⇆ RuO x-z (OH) y+z (1) According to Eq. (1) transformation of ruthenium oxide goes between Ru(II) and RuO 2 states in the potential ranges between 0 and 1 V vs. Ag/AgCl (3 mol dm -3 KCl). Higher valence states of Ru(V) and Ru(VI) can be eventually be obtained at higher potentials. The surface electrochemical reaction given by Eq. (1) has been probed usually by electrochemical techniques. Another very useful technique for investigations of fine subtleties in the electrochemi- cal reactions is sensing small mass changes accompanying charge transfer by electrochemical quartz crystal nanobalance (EQCN). To the authors knowledge there have been only a few papers dealing with the EQCN investigation of ruthenium oxide redox reac- tions [3,6,12]. However, the results, especially the mass changes accompanying the redox reactions in these papers exhibit rel- atively high deviations. First attempt to study charge/discharge behaviour of ruthenium oxide by EQCN gave the mass loss/gain which corresponds exactly to the release of three water molecules and one proton per Ru-site during the electrochemical oxida- 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.10.035