Electrochemical reduction of oxygen on quinone-modified carbon electrodes and on thin films of platinum and gold
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The electroreduction of oxygen on quinone-modified carbon electrodes and on thin gold and platinum films has been investigated, using the rotating disk electrode and rotating ring-disk electrode methods. The kinetic parameters for O2 reduction have been determined for all the electrodes studied. The covalent attachment of anthraquinone (AQ) and phenanthrenequinone (PQ) onto carbon electrodes was achieved by the electrochemical reduction of the corresponding quinone diazonium salt. The reduction of O2 on quinone-modified electrodes is catalysed by quinone radical anions and proceeds until the formation of peroxide. The kinetic parameters of oxygen reduction were determined for AQ-modified glassy carbon electrodes at various surface concentration of quinone and for boron-doped diamond and highly oriented pyrolytic graphite electrodes covalently modified with AQ and PQ. The electrocatalytic activity of PQ-modified electrodes was considerably higher than that of AQ-modified electrodes and this is primarily caused by more positive redox potential of PQ, which is the main factor that determines the electrocatalytic activity of quinone-modified electrodes towards O2 reduction. Thin Au and Pt films (nominal film thickness 0.25 to 20 nm) were prepared by vacuum-evaporation. The TEM measurements showed that the thinner films consist of separate metal clusters and the cluster size increases with metal loading. The rotating disk electrode studies revealed that O2 reduction mechanism on thin metal films is the same as on bulk metal electrodes. The specific activity of Au films only slightly decreased with decreasing film thickness in both 0.5 M H2SO4 and 0.1 M KOH solutions. The reduction of oxygen on thin Pt films on GC and Au substrates was studied in 0.1 M HClO4 and 0.05 M H2SO4 solutions. The specific activity of O2 reduction in HClO4 slightly decreased with decreasing film thickness; this was attributed to the stronger adsorption of surface oxygenated species that hinder the kinetics of O2 reduction. In H2SO4, the specific activity appeared to be independent of the Pt loading and was lower than in HClO4. This is due to the adsorbed sulfate ions, which block the sites for O2 adsorption and thereby reduce the O2 reduction activity. On thin Pt films, O2 is predominantly reduced to H2O.