Kinetics of mixed-controlled oxygen reduction at nafion-impregnated Pt-alloy-dispersed carbon electrode by analysis of cathodic current transients

The effects of Co alloying to Pt catalyst and Nafion pretreatment by NaClO₄ solution on the rate-determining step (RDS) of oxygen reduction at Nafion-impregnated Pt-dispersed carbon (Pt/C) electrode were investigated as a function of the potential step ΔE employing potentiostatic current transient (PCT) technique. For this purpose, the cathodic PCTs were measured on the pure Nafion-impregnated and partially Na⁺-doped Nafion-impregnated Pt/C and PtCo/C electrodes in an oxygen-saturated 1 M H₂SO₄ solution and analyzed. From the shape of the cathodic PCTs and the dependence of the instantaneous current on the value of ΔE, it was confirmed that oxygen reduction at the pure Nafion-impregnated electrodes is controlled by charge transfer at the electrode surface mixed with oxygen diffusion in the solution below the transition potential step |ΔE _tr| in absolute value, whereas oxygen reduction is purely governed by oxygen diffusion above |ΔE _tr|. On the other hand, the RDS of oxygen reduction at the partially Na⁺-doped Nafion-impregnated electrodes below |ΔE _tr| is charge transfer coupled with proton migration, whereas above |ΔE _tr|, it becomes proton migration in the Nafion electrolyte instead of oxygen diffusion. Consequently, it is expected in real fuel cell system that the cell performance is improved by Co alloying since the electrode reaches the maximum diffusion (migration) current even at small value of |ΔE|, whereas the cell performance is aggravated by Nafion pretreatment due to the decrease in the maximum diffusion (migration) current.