Effects of the molecular structure of fluorinated additives on the kinetics of cathodic oxygen reduction

The kinetics of oxygen reduction at a gold electrode was studied in 0.5 M sulfuric acid, in which different kinds of straight-chain [CF3(CF2)2CH2OH, CF3CF2CH2OH, and CF3CH2OH] and branched [(CF3)2CHOH] fluorinated alcohols were added. The adsorbed layers of the fluorinated alcohols were used as models of the fluorocarbon phase of the perfluorinated polymer electrolyte in gas-diffusion electrodes in proton-exchange membrane fuel cells. A rotating ring-disk electrode was used to determine kinetic parameters for O2 reduction and to detect intermediate H2O2 formation. The kinetics of oxygen reduction were strongly dependent on the molecular structure of fluorinated additives. The addition of the straight-chain fluorinated alcohols enhanced the kinetic current density while addition of the branched alcohol did not. The linear C3 fluorinated alcohol, CF3CF2CH2OH, gave the maximum enhancement effect. Oxygen is reduced predominantly via the two-electron series path in the range of 0.4 to 0.0 V at Au, on which no effect of fluorinated additives was observed. The rate constant for intermediate H2O2 reduction, k3, was negligible in the range 0.40–0.25 V, whereas it increased with decreasing ED in the range 0.25–0.0 V. In the lower potential range, k3 decreased with an increase in the concentration of fluorinated alcohol and this decreasing tendency was greatly dependent on the molecular structure of the fluorinated alcohol.