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 effect of contamination by Ca2 ions in proton conductive membranes f or polymer electrolyte fuel cells was investigated systematically. Ion and water transport characteristics of Nafion membranes, which were equilibrated with 0.02 to 0.03 kmol m3 of HC1/CaCl2 mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis, showed that Ca2 has much higher affinity than H to the ion exchange sites in Naf ion membranes. The water content in the membrane, as expressed by the amount of water per cationic site }120/SO;, decreased about 19% from 21 for H-form membrane to 17 for Ca-form membrane. The water transference coefficient was obtained from streaming potential measurements of Nafion 115 membranes of various H/Ca2 cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the Ca2 content in the membrane increased, especially when the equivalent fraction of H in the cationic exchange sites x became less than 0.5. Ionic transference numbers for H in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by H became less than 0.5. Membrane conductivity changed linearly with H composition in the membrane. In strong contrast to the interaction mode between H and Ca cations during ionic conduction, which appeared almost independent, a certain extent of interference was observed among water molecules as they were carried along by cations in the membrane. It was predicted that if Ca2 ions enter the fuel-cell membrane, they cause serious effects to membrane drying and result in deterioration of fuel-cell performance. The advantage of this methodology in the study of transport characteristics of fuel-cell membranes is stressed due to ease and accuracy of measurements.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.230.243.252 Downloaded on 2015-02-07 to IP
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.
Oxygen reduction characteristics of graphite electrodes modified with aggregated cobalt-porphyrins heat-treated at various temperatures and then impregnated in Nafion polymer were investigated systematically The aggregated cobaltporphyrin compound was adsorbed on graphite powder and then heat-treated at various temperatures ranging from 200 through 1200 'C. The catalysts were evaluated for electroreduction performances of oxygen on modified electrodes in sulfuric acid solutions. The electrocatalytic performances of catalysts as measured in rotating ring-disk electrodes showed that the most effective catalytic activity for oxygen reduction was attained for the aggregated cobalt-porphyrin compounds on graphite powder heat-treated at temperatures between 600 and 800 'C. The surface concentration of Co and N ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.226.37.5 Downloaded on 2015-03-08 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.226.37.5 Downloaded on 2015-03-08 to IP
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.