Oxygen reduction kinetics at platinum electrodes covered with perfluorinated ionomer in the presence of impurity cations Fe3+, Ni2+ and Cu2+

Abstract

The effect of impurity cations on the kinetics of oxygen reduction at a platinum surface covered with perfluoro-sulfonated ionomer film was investigated in 0.1 N H₂SO₄ in the presence of 0.001 N Fe³⁺, Ni²⁺ or Cu²⁺ ions, using rotating disk electrodes. The platinum disk of a rotating disk electrode was spin-coated with Nafion^® solution, and after drying the Nafion^® the film-covered platinum was tested for oxygen reduction kinetics. Koutecky–Levich plots and linear sweep voltammetry made possible the independent measurement of both charge transfer and diffusion kinetics of oxygen reduction at the same electrode. The rate of the charge transfer of the oxygen reduction reaction at the platinum/ionomer membrane interface was calculated along with the immersion time. It was discovered that the impurity ions hindered enormously the rate of the charge transfer step at platinum covered with perfluorosulfonated ionomer. No suppression effect for oxygen reduction was observed for bare platinum in a solution containing impurity ions, indicating that the hindrance effect is specific to the metal electrode/ionomer membrane interface. Also, both the diffusion coefficient of oxygen and oxygen concentration in the membrane were decreased by the presence of impurity cations. The relevance of the problem to the performance degradation of polymer electrolyte fuel cells is discussed in relation to the occurrence of contamination by these impurity ions during the operation of the cell.