Acceleration of oxygen reduction rate by alkyl derivatives of uracil on Ptcatalysts used in fuel cells

Alkyl derivatives of uracil, namely, 5,6-dimethyluracil, 5-ethyluracil, and 5-methyluracil accelerate the rate of electrochemical reduction of oxygen on Pt catalysts in aqueous 0.5 M sulfuric acid. Two other uracil derivative s, namely, 1,3-dimethyluracil and 2,4-dimethoxypyrimidine, and uracil itsel f, also increase the rate of the oxygen reduction reaction, but to a much l esser degree. The organic molecules were adsorbed on the electrode surface from dilute solutions of 0.5 mM in the electrolytes. The organic adsorbates shift the open-circuit potential of the Pt electrode to more positive valu es and generate higher oxygen reduction currents at lower overpotentials. R eplacement of the adsorbed anions from the surface by the organic molecules facilitates oxygen reduction. Partial electron transfer between the electr ode and some of the adsorbed molecules also plays a role in their catalytic behavior. The degree of enhancement by the organic adsorbates reported her e for the rough Pt electrode is somewhat different from that found on a smo oth, polycrystalline electrode. These differences are explained based on th e effect of surface roughness and the time-dependent variation of the diffu sion layer thickness caused by the diffusion of oxygen to the electrode sur face. (C) 2001 The Electrochemical Society.