The kinetics of electroreduction of peroxodisulfate ions on single crystal cadmium and bismuth electrodes T. Thomberg, J. Nerut, R. Ja ¨ger, P. Mo ¨ller, K. Lust, E. Lust * Institute of Physical Chemistry, University of Tartu, 2 Jakobi Street, 51014 Tartu, Estonia Received 9 October 2004; received in revised form 21 December 2004; accepted 17 January 2005 Available online 16 February 2005 Abstract The electroreduction of the peroxodisulfate anions on the electrochemically polished (EP) Cd(0 0 0 1), Bi(1 1 1) and Bið01  1Þ planes has been studied by cyclic voltammetry, rotating disc electrode and impedance methods. The rate constant values of the heteroge- neous electroreduction reaction of the S 2 O 2 8 anion on the EP Cd(0 0 0 1) and Bi(hkl) planes dependent on electrode polarisation and base electrolyte concentration have been calculated. The values of apparent transfer coefficient a app corrected for the double layer effect (a app 6 0.15) only very weakly depend on the electrode potential and base electrolyte concentration. The very low values of the apparent charge transfer coefficient for S 2 O 2 8 electroreduction show that the activationless charge transfer mechanism is valid for EP Cd(0 0 0 1) and EP Bi(hkl) electrodes in various surface inactive base electrolyte solutions. This conclusion is in a good agreement with the theoretical models for the high hydrogen overvoltage metals based on the diabatic charge transfer mechanism from the metal to an ion [R.R. Nazmutdinov, D.V. Gluhov, O.A. Petrii, G.A. Tsirlina, G.N. Botikova, J. Electroanal. Chem. 552 (2003) 261]. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Electroreduction; Peroxodisulfate; Cadmium single crystal; Rate constant of heterogeneous reaction; Apparent transfer coefficient 1. Introduction Electroreduction of the peroxodisulfate anion has been suggested as a probe reaction for studying the influence of the electrical double layer (EDL) structure on the charge transfer mechanism from a metal to an an- ion [1–29]. Metal complex ions constitute interesting objects for systematic studies of so-called simple hetero- geneous electron transfer reaction. As summarised by Fawcett [20], the double layer effect observed may be varied significantly by varying the chemical nature and structure of the surrounding ligands as well as the chem- ical nature of the electrode material. On the basis of quantum chemical studies for the couples [Co(NH 3 ) 6 ] 3+/2+ , [Fe(H 2 O) 6 ] 3+/2+ and [Fe(CN) 6 ] 3/4 and a simple model for the double layer, it was demon- strated by Fawcett et al. [24] that the double layer effect for these reactions depends more on the charge distribu- tion on the ligands than on that for the central metal ion. Experimental data for Hg [1–4,13–17] and various polycrystalline sp-metals like Bi, Sb, Pb, Sn, Cd and Ag [5–9], as well as for single-crystal Ag(1 1 1) and Ag(1 0 0) planes [8,9] in the limited concentration region of the base electrolyte solution have implied that the reduction rate of the overall two-electron peroxodisul- fate anion is controlled by a single one electron rate- determining step. This rate-determining step can be characterised by an apparent rate constant k het of the heterogeneous reaction [4–6]. k het ¼ k cor expðz i F w 1 =RT Þ ¼ k 0 expðz i F w 1 =RT Þ exp½aF ðE  w 1 Þ=RT ; ð1Þ 0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.01.004 * Corresponding author. Tel.: +372 7 375 165; fax: +372 7 375 160. E-mail address: enn.lust@ut.ee (E. Lust). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 582 (2005) 130–143 Journal of Electroanalytical Chemistry