(A) Study on mechanism of oxygen reduction at Pt-dispersed carbon electrode and in gas diffusion electrode for polymer electrolyte membrane fuel cell

Abstract

The present work deals with oxygen reduction at Pt-dispersed carbon electrode and in gas diffusion electrode for PEMFC using rotating disc voltammetry, ac-impedance spectroscopy and potentiostatic current transient technique. Chapter III is concerned with the mechanism transition in oxygen reduction at the Pt-dispersed carbon electrode by analyses of the rotating disk voltammograms and the potentiostatic current transients. From the comparison of the instantaneous current experimentally measured from the cathodic current transients with those theoretically calculated from the Butler-Volmer equation and from the Cottrell equation, it is confirmed that oxygen reduction is controlled by charge transfer at the electrode surface mixed with oxygen diffusion in the solution below the transition potential drop. In contrast, oxygen reduction is purely governed by oxygen diffusion in the solution above the transition potential drop. In addition, the transition potential drop remained nearly constant irrespective of the Nafion content, but the transition potential drop decreased with increasing temperature due to the substantial increase of the Butler-Volmer current as compared to that increase of the Cottrell current. In chapter IV, the effects of Co alloying to Pt catalyst and Nafion pre-treatment by NaCl solution on the mechanism transition in oxygen reduction at Pt-dispersed carbon electrode were investigated as a function of the cathodic potential step employing potentiostatic current transient technique. From the shape of the cathodic PCTs and the dependence of the instantaneous current on the potential step, it was confirmed that oxygen reduction at pure Nafion-impregnated electrodes is controlled by charge transfer at the electrode surface mixed with oxygen diffusion in the solution below the transition potential drop, whereas oxygen reduction is purely governed by oxygen diffusion above the transition potential drop. On the other hand, the rate-d...