Electrochimica Acta 46 (2000) 579 – 588 Electrocatalytic reduction of dioxygen at platinum particles dispersed in a polyaniline film C. Coutanceau, M.J. Croissant, T. Napporn, C. Lamy * Uniersite ´ de Poitiers, UMR CNRS 6503, 40, aenue du Recteur Pineau, 86022 Poitiers, France Received 19 April 2000; received in revised form 21 July 2000 Abstract Platinum based electrocatalysts for the oxygen reduction reaction were prepared by electrodeposition of Pt particles in a polyaniline film acting as a convenient matrix to achieve low Pt loadings (from 11 to 600 g cm -2 ). Using a rotating disc electrode or a rotating ring disc electrode allowed us to separate the different contributions in the overall reaction : diffusion of molecular oxygen in the electrolyte solution, diffusion inside the PAni film, adsorption process, and electron transfer. Koutecky – Levich analysis made possible to evaluate the kinetics parameters (total number of exchanged electrons, limiting current density, Tafel slope and exchange current density). These parameters vary with the Pt loading, displaying a steep increase at around 200 g.cm -2 , above which the behavior of bulk platinum is found. At lower Pt loadings, the kinetics is controlled by a Temkin isotherm, and a large amount of hydrogen peroxide is formed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrocatalysis; Platinum dispersed in polyaniline; Oxygen reduction reaction; Polyaniline www.elsevier.nl/locate/electacta 1. Introduction The dispersion at the molecular level of catalytic materials, such as platinum or platinum-based alloys, in an electron conducting matrix, is a very convenient way to decrease the particle size (and therefore the noble metal loading) and to enhance the electrocatalytic activ- ity [1]. Polyaniline (PAni) was chosen as the conducting polymer matrix since it is easy to synthesize in aqueous medium under reproducible conditions by electropoly- merization of aniline, and it is stable in a relatively wide potential window (from 0.0 to 1.1 vs RHE). Using noble metals dispersed in an electron conducting poly- mer, several electrocatalytic reactions were investigated, such as the reduction of protons [2–4] or of oxygen [5–10], and the oxidation of hydrogen [7,11–13] or of small organic molecules, such as formic acid [14 – 17], methanol [18–26] or ethylene glycol [27]. In most cases the reaction rate, i.e. the current densities, was in- creased and the overvoltage was greatly reduced. This is particularly observed for the oxidation of carbon monoxide [28], or of small organic fuels (CH 3 OH, HCHO, HCOOH) leading to carbon monoxide by dis- sociative chemisorption [19,24]. Moreover for practical applications, such as fuel cells, the amount of electro- catalyst has to be reduced to decrease the material cost. This is particularly true for the hydrogen/oxygen (air) fuel cell for which a goal of 1 g of platinum per kW is aimed. It is thus highly relevant to fuel cell development to investigate the catalytic activity of low loading plat- inum catalysts and to determine the lowest metal load- ing necessary to reach the practical performances in terms of current density and power density. In a preceding paper [13] we studied the electrooxida- tion of hydrogen at platinum particles dispersed in a polyaniline conducting matrix, with several platinum loadings ranging from 5 to 330 g cm -2 . Using a * Corresponding author. Fax: +33-59453580. E-mail address: claude.lamy@univ-poitiers.fr (C. Lamy). 0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(00)00641-1