The mechanism of how impurity cations suppress the kinetics of oxygen reduction reaction on platinum surface covered with perfluoro-sulfonated ionomer film was studied in 0.1 N H2SO4 in the presence of various kinds of impurity ions of several concentrations. Impurity cations tested were Li+, Na+, K+, Ca2+, Fe3+, Ni2+, and Cu2+ with the amount of 0.1%, 1%, and 10% as compared with H+ in the solution. Platinum disk of a rotating disk electrode was spin-coated with Nafion solution, and after drying the Nafion film-covered platinum was obtained. The electrochemical measurements were performed to evaluate both charge transfer and diffusion kinetics of oxygen reduction at the Nafion film-covered electrode. It was discovered that the impurity ions hindered enormously the rate of charge-transfer step at platinum covered with perfluoro-sulfonated ionomer. The suppression started already at 0.1% level of impurity concentration, but did not increase much at over 1% level. No suppression effect for oxygen reduction was observed for a bare platinum in the solution containing impurity ions, indicating that the effect is specific to the metal electrode−ionomer membrane interface. Also both the diffusion coefficient of oxygen and oxygen concentration in the membrane decreased by the presence of impurity cations. It was implied that all the process is related to the reorientation of polymer networks in the membrane, which might bring about the modification of electric double layer at the platinum−ionomer interface.

The oxygen reduction reaction (ORR) at platinum electrodes covered with perfluorosulfonated ionomer film was studied in 0.05 mol dm-3 H2SO4 in the presence of various kinds of ammonium ions. Ammonium ion derivatives (R4N)2SO4 (R = H, CH3) and (R4N)HSO4 (R = C2H5, C3H7, and C4H9) were added with various amounts in the solution. The electrochemical measurements of ORR were performed to evaluate both charge transfer and diffusion kinetics of oxygen reduction at the Nafion film covered platinum rotating disk electrode. ORR was affected differently for different molecular weights of ammonium derivative ions. The suppression of ORR started in a short time for ammonium derivative ions of high molecular weight, reflecting the strong adsorption at the platinum surface. It was inferred that these ions strongly distorted the electric field at the platinum|ionomer interface. From the knowledge, a new method to inhibit the ORR degradation at platinum|ionomer interface was proposed, and several kinds of carboxylic acids or amino acids were tested as additives in the ionomer film. It was discovered that these additives successfully inhibit the ORR degradation brought about by impurity cations. After investigating the ORR kinetics, it was found that the additives prevented the degradation of the charge transfer step effectively, but the diffusion process in the film was degraded more by these additives.

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