An electrode-kinetic investigation of CO and CO/H 2 oxidation in phosphotungstic acid (H 3 PW 12 O 40 ) electrolyte A. S. Aricò a,* , E. Modica a , P. Cretì a , P. L. Antonucci b and V. Antonucci a a C.N.R Institute for Transformation and Storage of Energy, Salita S. Lucia sopra Contesse 39, 98126 S. Lucia, Messina, Italy b University of Reggio Calabria, Faculty of Engineering, C.da Feo di Vito, 89100 Reggio Calabria, Italy. (Received June 17, 1999; received in revised form December 2, 1999) Abstract: An electrode-kinetic investigation of the oxidation of CO and CO/H 2 mixtures on a Pt-Ru/C catalyst was carried out in phosphotungstic acid (PWA) electrolyte. The chemical, structure and surface properties of the Pt-Ru/C catalyst were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of temperature, CO partial pressure and proton concentration on the electrochemical oxidation rate was studied by steady-state galvanostatic polarization measurements. An apparent activation energy of about 50 kJ·mol -1 at 0.6 V vs. NHE was observed for CO oxidation. Fractional reaction orders close to 0.5 and -0.4 with respect to carbon monoxide and proton concentration, respectively, were determined. The PWA electrolyte appeared to promote the water discharging process. Tafel slopes of about 138 mV·dec -1 and 127 mV·dec -1 were observed for CO and CO/H 2 oxidation, respectively, indicating that a single electron transfer was involved in the r.d.s. These results appear to be consistent with the bifunctional theory. Both the surface migration of adsorbed species and the oxidative removal of CO seem to contribute to the slow step of the electro-oxidation process. Key words: CO electrooxidation, Pt-Ru catalyst, phosphotungstic acid, reaction orders, activation energy, Tafel slopes. * To whom correspondence should be addressed. Fax: +39-090 624247, e-mail: arico@itae.me.cnr.it 1. INTRODUCTION In the last years, heteropolyacids have became the subject of remarkable and widespreading technological interest. The application of these compounds in catalysis and electrochemistry is now an important and growing area of technology [1-3]. The source of this interest mainly lies upon their unique structural and chemical characteristics, as well as on their capacity to exert various catalytic functions. Moreover, phosphotungstic acid (PWA) has recently aroused a considerable interest as proton conducting electrolyte in low temperature H 2 -O 2 fuel cells [4-6]. It was shown that the electro-reduction of oxygen is strongly promoted in presence of phosphotungstic acid [4] and an electrochemical performance of about 700 mW cm -2 has been obtained with a PWA-based fuel cell at room temperature and atmospheric pressure [5]. These results were attributed to the presence of Keggin units in the phosphotungstic acid structure which determine a strong Bronsted acidity and a suitable oxygen affinity [7], allowing to obtain high exchange current densities for oxygen reduction on Pt [4]. Furthermore, PWA is characterized by high protonic conductivity both in its 29-water molecules hydrate form [8] and in concentrated aqueous solutions [9]. One of the main drawbacks that still limits the practical application of low temperature fuel cells, employing hydrogen derived from reforming of methanol or methane, is the anodic Pt catalyst poisoning by carbon monoxide, even when it is present at low levels in the feed stream [10-11]. The strong chemisorption of Journal of New Materials for Electrochemical Systems, 3, 207-220 (2000) © J. New Mat. Electrochem. Systems 207