KINETICS OF O-2 REDUCTION ON A PT ELECTRODE COVERED WITH A THIN-FILM OF SOLID POLYMER ELECTROLYTE

The kinetics of O-2 reduction on both oxidized and reduced Pt rotating disk surfaces covered with thin films (1 to 15 mu m) of recast polybe nzimidazole (PBI) or Nafion have been examined in 0.1 M aqueous soluti ons of H3PO4, H2SO4, and HClO4 at 25.0 degrees C. Polymer films were d eposited on a Pt disk electrode by a spin-coating technique. Experimen ts were performed formed at different rotation rates and potential sca n rates. This investigation was undertaken to establish to what extent the solid polymer electrolyte influences the mechanism and kinetics o f O-2 reduction. The number of electrons exchanged per O-2 molecule an d kinetic parameters for O-2 reduction (reaction order with respect to O-2, Tafel slope/transfer coefficient, and exchange current density) are not changed in the presence of the polymer films, indicating that the reaction path and kinetics are not altered in the presence of reca st PBI and Nafion films. The diffusion coefficient (D) and concentrati on (C) of O-2 in recast films were determined by means of linear sweep voltammetry measurements in quiescent solution. The D and C values in polymer films immersed in acidic solutions were found to have differe nt values than those in the solutions. D(O-2) is about 2.5 times lower in the recast films than in the solutions, whereas C(O-2) is slightly higher in the film. The largest concentration increase, by a factor o f two, was found in PBI film in 0.1 M HClO4. Cyclic voltammetric measu rements under argon atmosphere have shown that PBI and Nafion recast f ilms are electrochemically stable in the potential range between the o nsets of H-2 and O-2 evolution. A decrease in the effective electrocat alyst surface area due to deposition of solid polymer films was 15 to 20%.