Characterization of the activity of palladium-modified polythiophene electrodes for the hydrogen oxidation and oxygen reduction reactions

Pd particles were inserted into polythiophene (PT) films and studied for the activity for the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in acid media. In situ X-ray absorption spectroscopy (XAS) was employed for the characterization of the Pd particles and the results confirmed that they possess metallic character. At low potentials, hydrogen is incorporated into the Pd lattice resulting in an increase in both the average Pd-Pd bond distance as well as the structural disorder. XAS results also indicated that a considerable fraction of the total Pd atoms is located in the surface of the electrodeposit, as expected for such a high surface area material. No activity for the HOR is seen for the PT films in the absence of catalysts. For the catalyzed PT films, a chemical reaction involving atomic adsorbed hydrogen atoms on Pt or Pd and the carbon radical in the polaronic PT species may occur causing a progressive degradation of the film properties during HOR. The Pd-modified electrode exhibits a considerable catalytic activity for ORR. Above 0.3 V, only formation of hydrogen peroxide occurs, leading to a two electron ORR mechanism. For smaller potentials, further reduction of H2O2 to water occurs, resulting in a four electron global process. (C) 2003 The Electrochemical Society.