Mechanism transition of mixed diffusion and charge transfer-controlled to diffusion-controlled oxygen reduction at Pt-dispersed carbon electrode by Pt loading, Nafion content and temperature

The mechanism transition in the oxygen reduction reaction at the Pt-dispersed carbon (Pt/C) electrode was investigated in an oxygen-saturated 0.5M H2SO4 solution. The reaction was monitored by acquiring data for Pt loading, Nafion content and temperature by analyses of the rotating disk voltammograms and potentiostatic current transients (PCTs). From the shape of the cathodic PCTs and the dependence of the initial current density on the potential drop, it is suggested that oxygen reduction at the Pt/C electrode is controlled by the charge transfer at the electrode surface mixed with the oxygen diffusion in the solution below the value of the potential drop, ΔEtr, needed for the occurrence of the mechanism transition, whereas oxygen reduction is purely governed by the oxygen diffusion in the solution above ΔEtr. In particular, it was noted that the value of ΔEtr remained nearly constant irrespective of the Pt loading and Nafion content. On the other hand, the value of ΔEtr decreased as temperature increased, which is ascribed to the fact that the contribution of the Cottrell current enhanced by temperature rise to the fall in ΔEtr is overwhelmed by that contribution of the Butler–Volmer current increased. Consequently, it is concluded that it strongly depends upon the extrinsic parameters such as Pt loading, Nafion content and temperature as well as the intrinsic parameters such as rate constant for interfacial reaction and oxygen diffusivity in the solution, which mechanism of the overall oxygen reduction reaction is operative.