Oxygen reduction at polypyrrole electrodes - II. Experimental results

Abstract-Thecathodicreduction of molecular oxygen at a polypyrrole electrode in 0.5 M HaSO, is studied using a rrde. It appears that the polymeric layer is permeable and that the reduction of oxygen mainly occurs at the interface metal/polypyrrole. Due to the presence of the polymer layer, eventually formed peroxide is not immediately removed by convection, but is forced to diffuse through the polymer, back to the bulk of the electrolyte. Because of the accompanying increase of the mean residence time of peroxide near the metal/polypyrrole interface, the selectivity of oxygen reduction to water at the polypyrrole electrode is more than at the uncovered metal. However, the maximum current density at the polypyrrole electrode is less than at the uncovered metal. It is found, that for the so-called “oxidized polypyrrole electrode”, the polymer film probably catalyses the decomposition of hydrogen peroxide to water and molecular oxygen and thus contributes to a more favourable selectivity of the electrode. NOMENCLATURE disc surface area m2 concentration M, molm-3 diffusion coefficient m*s-’ potential V disc potential V potential V constant C mol- ’ (rotation) frequency thickness of the polymer film m collection efficiency number of electrons, in overall reaction equation-. ^-. p(H,U) water emctency Q charge passed during film per unit geometrical surface area Cm-’ T absolute temperature K “D disc potential scan rate Vs-’ b diffusion layer thickness m Y kinematic viscosity m2 s- ’ Superscripts ad adsorbed f polymer film S bulk 0 interphase metal/polypyrrole 1