Ion and water transport characteristics of perfluorosulfonated ionomer membranes are investigated in the mixed cation form of H/Fe, H/Ni, and H/Cu systems. Nafion membranes, which were equilibrated with HCl/FeCl3, HCl/NiCl2, or HCl/CuCl2 mixed aqueous solutions of various mixing ratios, were prepared as test samples, and equilibrium and transport properties were measured systematically. Membrane cationic composition showed that trivalent cations had more affinity than divalent cations. Also larger valence cations caused less water content in the membrane. The membrane ionic conductivity was markedly influenced by counterions, and H+ mobility u H + was altered according to the nature of coexisting cations. In the presence of Cu2+, u H + increased from its inherent value, while in the presence of Fe3+, u H + decreased to a large extent, Ni2+ bringing about nearly no change in u H + . The ionic transference number of H+ was also influenced by coexisting cations in several ways. Despite the unique influence of impurity cations on the mobility of H+, the mobility of impurity cations was not affected by the presence of H+. The interaction between adjacent cationic species in the membrane ion exchange sites, although plausible in general for multivalence cations, appeared to be not specific due probably to the shielding of the cationic charge by water molecules or by sulfonic acid groups. The water transference coefficient t H 2 O as measured by streaming potential measurements showed unique changes with membrane ionic composition, and t H 2 O increased from 2.5 to over 13 by the presence of impurity ions. These impurity ions were found to result in more water molecules dragged than in the case of individual ions, when coexisting with the H+ ion. Overall, it was noted that the water molecules within the influence of impurity cations appeared to play a large role in the H+ movement in the membrane.

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.

Platinum electrodes covered with a perfluoro-sulfonated ionomer membrane show unique behaviors in comparison with bare platinum immersed in aqueous solutions. The electrochemical interface between the metal and the polymer can be different from the interface between the metal and the solution phase. In this context, platinum electrodes covered with such a polymer membrane are an interesting system, and deserve detailed study. Here the oxygen reduction reaction at the platinum surface covered with a perfluoro-sulfonated ionomer membrane is investigated kinetically in 0.1 N H2SO4 in the presence of 0.001 N Na+, K+, or Ca2+ ions, using rotating disk electrodes. It is discovered that the impurity ions, even in small amounts, hinder enormously the rate of the charge transfer step of oxygen reduction at the platinum covered with perfluoro-sulfonated ionomer. Especially the effect of Na+ in the membrane is very serious considering the fact that there exists only 2% Na+ of the exchange site in the membrane phase at this condition. Platinum covered with perfluoro-sulfonated ionomer membrane has historically attracted much research interest, based on the fact that oxygen concentration and H+ ion concentration in the membrane are both larger than those in normal acidic solutions, and could show larger catalytic activity than in bare platinum (Gottesfeld, S.; et al. J. Electrochem. Soc. 1987, 134, 1455. Lawson, D. R.; et al. J. Electrochem. Soc. 1988, 135, 2247). However, this expectation encountered disappointing failure (Zecevic, S. K.; et al. J. Electrochem. Soc. 1997, 144, 2973). Results here indicate that such a paradox could be accounted for by the effect of the metal−polymer interface that alters the reaction conditions of oxygen reduction.

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