RESEARCH PAPER CHINESE JOURNAL OF CATALYSIS Volume 31, Issue 5, 2010 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2010, 31: 541–546. Received date: 19 October 2009. *Corresponding author. Tel: +98-21-22853551; Fax: +98-21-22853650. E-mail: mjafarian@kntu.ac.ir Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved. DOI: 10.1016/S1872-2067(09)60069-3 Multistep Reduction of Oxygen on Polycrystalline Silver in Alkaline Solution M. JAFARIAN 1, *, F. GOBAL 2 , M. G. MAHJANI 1 , M. HOSSEINI ALIABADI 1 1 Department of Chemistry, K. N. Toosi University of Technology, 15875-4416, Tehran, Iran 2 Department of Chemistry, Sharif University of Technology, 11365-9516, Tehran, Iran Abstract: Oxygen reduction on a polycrystalline silver electrode was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The reaction occurred by a two-electron pathway. The steps in the mechanism were observed in the cyclic voltammograms recorded with different scan rates. The intermediates formed in the steps were detected by electrochemical impedance spectroscopy. Key words: silver; oxygen reduction; cyclic voltammetry; electrochemical impedance spectroscopy Electrochemical systems such as fuel cells and brine elec- trolyzers [1–3] need electrocatalysts with high activity, corro- sion resistance [4,5], and cost effectiveness [6,7]. The oxygen reduction reaction (ORR) determines the efficiency of these electrochemical systems because of its high over-voltage on different substrates, including Pt. Oxygen reduction on dif- ferent metals in alkaline solution has been investigated by a number of authors [8–15]. Oxygen is reduced by two different pathways: a single step four-electron pathway O 2 + 2H 2 O + 4e – ↔ 4OH – E 0 = 0.401 V vs NHE (1) and by consecutive steps that each involves two electrons O 2 + H 2 O + 2e – ↔ OH – + HO 2 – E 0 = –0.146 V vs NHE (2) HO 2 – + H 2 O + 2e – ↔ 3OH – (3) The mechanism and kinetics of oxygen reduction on dif- ferent metals have been investigated. Van Velzen et al. [9,10] showed that there was no pH dependence of the kinetics of oxygen reduction to hydrogen peroxide on mercury in neutral and alkaline media. Wroblowa et al. [11] showed that the mechanism of oxygen reduction on low carbon steel surfaces in alkaline solution is a four-electron reduction process. A study of zinc surfaces revealed that the mechanism of ORR can be affected by the potential dependence of the composition of the Zn/alkaline medium interface, and that the reaction occurred by a direct four-electron reduction to hydroxyl ions on clean zinc surfaces [12]. The rate and mechanism of ORR on corroded zinc depended on the thickness of the oxide or hydroxide film [13]. An investigation of an oxide-derived Pd electrode in alkaline medium showed that the ORR was catalyzed by Pd (I) [14]. The mechanism of ORR on Pd/C in alkaline medium followed the four-electron pathway because of the high cata- lytic activity of Pd for hydrogen peroxide reduction [15]. On silver electrodes, the activation energy for the surface disso- ciation of oxygen was high [16] (~15 kJ/mol) and this had a negative effect on ORR activity. In addition, silver is catalyti- cally active for the chemical decomposition of peroxide (Eq. 4), which has motivated researchers to study ORR on silver electrodes [17–26]. 2HO 2 – → O 2 + 2OH – (4) It has been shown that oxygen reduction to a hydroxide ion occurred through a four-electron reaction that is accompanied by the catalytic decomposition of peroxide that was formed. There is, however, still a debate over the mechanism of ORR on Ag. A detailed mechanism has been proposed by Adanuvor et al. [23]. This mechanism consisted of four main steps and a number of sub-steps. Experimental results obtained in rotating disk electrode (RDE) studies were in good agreement with this mechanism. However, these results were only for the main steps. Savinova et al. [24] obtained valuable results from a combination of cyclic voltammetry and Raman spectroscopy. According to their work, OH – was converted to adsorbed atomic oxygen that then diffused into the bulk of the electrode [24]: OH – ↔ OH ads + e – (5) OH – + OH ads ↔ O ads + H 2 O + e – (6)