(A) study on mechanism transition of mixed diffusion and butler-volmer-controlled oxygen reduction at Pt-dispersed carbon electrode by analyses of potentiostatic current transients and rotating disk voltammograms

Abstract

In the present work, the mechanism transition for oxygen reduction reaction at the Pt-dispersed carbon (Pt/C) electrode by the Pt loading and temperature was investigated in an oxygen-saturated 0.5 M $H_2SO_4$ solution using rotating disk electrode technique and potentiostatic current transient technique. Especially, the rotating disk voltammograms were measured on the Pt/C electrode in order to determine the oxygen reduction pathway, and the cathodic current transients were analysed to establish the boundary condition at the electrode surface during oxygen reduction reaction. From the analysis of rotating disk voltammograms, it is suggested that during oxygen reduction reaction the direct four-electron reduction predominantly proceeds rather than two-electron reduction. In addition, from the analysis of the cathodic current transients measured on the Pt/C electrode, it is concluded that oxygen reduction reaction at the Pt/C electrode is controlled by charge transfer at the electrode/electrolyte interface mixed with oxygen diffusion in the solution below the value of potential drop $ΔE_{tr}$ needed for mechanism transition, whereas oxygen reduction reaction is governed by oxygen diffusion in the solution above the value of $ΔE_{tr}$. For the Pt/C electrodes with various Pt loadings, it is confirmed that the value of $ΔE_{tr}$ remains nearly constant irrespective of Pt loading. This implies that the Butler-Volmer current measured under the ‘constraint by Butler-Volmer behaviour’ and the Cottrell current specified under the ‘constraint of constant concentration’ increase as well with increasing Pt loading due to the increase in the electrochemical active area. On the other hand, it is also recognised that the value of $ΔE_{tr}$ decreases as temperature increases, which is ascribed to the fact that the enhanced Cottrell current is overwhelmed by the increased Butler-Volmer current. From those results determined from the Pt/C electrodes, it is concluded that it...