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A characterization of the ionic conduction of the active layer of a polymer electrolyte membrane fuel cell (PEMFC) cathode by ac impedance measurement at open-circuit potential conditions was conducted. Porous electrode theory was used to derive a compact equation, ∂2normalΦ̑2/∂y2+∂lnffalse(yfalse)/∂y×∂normalΦ̑2/∂y−R/ffalse(yfalse)false(1+jnormalΩfalse)normalΦ̑2=0, to solve for the impedance response of a cathode at open-circuit potential conditions. This equation includes a parameter R, the ratio of an ionic resistance (evaluated at the active layer/membrane interface), to the total charge-transfer resistance of the active layer. The influence of an assumed ionic conductivity distribution profile ffalse(yfalse) on the error in the estimation of total double-layer capacitance of the active layer from the −1/false(ZnormalImωfalse) vs. ZnormalRe plot was also investigated in this work. The increase of ionic conductivity in the active layer of an air cathode with an increase in the ionomer loading was revealed from both impedance data and surface area measurements. A nonlinear parameter estimation method was used to extract the ionic resistance from the high-frequency region of the impedance data at open-circuit potential conditions. The assumed ionic conductivity distribution profile in the active layer was found to vary with ionomer loadings. © 2003 The Electrochemical Society. All rights reserved.

Oxygen reduction in a gas‐fed porous electrode attached to a proton exchange membrane is discussed. Experimental data and a mathematical model are presented for the test cell used. Various membrane and electrode assemblies were tested at different levels of platinum loading and Teflon® content. The model accounts for the diffusion and reaction of oxygen and the diffusion and reaction of hydrogen ions. Sulfuric acid was placed above the membrane in the test cell reservoir to provide a source of protons for the reduction of oxygen at the cathode. Based upon model predictions, it is shown that the transport of the protons in the active layer of the cathode is an important factor in the operation of the test cell.

Hydrogen gas diffusion coefficients and solubilities as well as water uptake values are reported for Dow's short‐side‐chain perfluoro‐sulfonic and ‐carboxylic membranes of different equivalent weight (EW). The diffusion coefficients and solubilities were determined with an electrochemical test cell. Hydrogen solubility decreases with increasing EW in the lower EW range and tends to level off at higher EWs for both types of membranes. Both hydrogen solubility and diffusion coefficients of a sulfonic membrane with EW higher than 800 are higher than the corresponding values of a carboxylic membrane of similar EW. An unusual maximum is observed in the diffusion coefficient‐EW plot of sulfonic membranes. Water uptake decreases with increasing EW for both types of membranes. Various trends in the hydrogen diffusion coefficients and solubilities are discussed in terms of a number of physical and morphological properties, such as the percent of crystallinity, intrusion of one phase into another phase, extent of ion‐pair formation, and pore sizes of membranes.

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The formal potentials of several redox couples incorporated in coatings of a perfluoropolycarboxylate on graphite electrodes were measured and compared with the formal potentials of the same couples in homogeneous solution. The differences observed agreed with those calculated from the Nernst equation with the independently measured incorporation coefficients for both halves of the redox couples. The dependences of the shifts in formal potentials on the nature of the incorporating complex ion, the ionic strength, and the temperature were determined and indicated that the incorporation equilibrium is governed by electrostatic and hydrophobic interactions that act in opposite directions. The incorporation of most cation s examined was driven by large increases in entropy which overcame the usually unfavorable enthalpy changes.Redox reactants incorporated in polyelectrolyte coatings on electrode surfaces will exhibit formal potentials that are shifted from their values in solution whenever the two oxidation states of the reactant have different equilibrium constants for incorporation by the polyelectrolyte. This situation has been recognized in earlier reports (1, 2) .and in a recent study this aspect of the electrochemistry of redox polymer coatings was examined in detail (3). In continuing studies of the electrochemical responses obtained from redox reactants at electrodes coated with Nation, I, and the re- coatings. Both the magnitude of the shift in formal potential and its sign were influenced strongly by the nature of the ligands coordinated to the metal center of the redox couples. It was the purpose of the experiments described here to document the behavioral differences observed with several redox couples in both polyelectrolytes and to attempt to understand their origins. The results indicate that both electrostatic and hydrophobic interactions of incorporated electroactive ions with the polyelectrolyte combine to determine the direction of shifts in formal potentials. The temperature dependences of the shifts in formal potential were also measured. They pointed to large entropic contributions to the differences in the equilibrium incorporation of the oxidized and reduced halves of the redox couples. ExperimentalMaterials.--Nafion I coatings were prepared as previously described (4) by evaporation of aliquots of a solution of the polymer supplied by E. I. du Pont de Nemours and Company several years ago. Polymer II was supplied in the form of a fine suspension of the methyl ester by Asahi Glass Company, Tokyo. To prepare coatings, aliquots of the suspension were evaporated on freshly cleaved electrodes. The resulting deposit was exposed to 1M NaOH for 5 min to convert the ester to the carboxylate, and the electrode was then washed thoroughly with water. The extent and uniformity of the de-esterification were not examined in detail, but it was established that increasing the time that the coatings were exposed to the 1M NaOH produced no changes in their behavior. Electrodes were prepared from pyrol...

Analysis of the data obtained by the electrochemical monitoring technique for diffusion of a gas through a membrane is considered. It is shown that combining a numerical method with a nonlinear parameter estimation technique provides a means to determine values for the diffusion coefficient and the solubility of the diffusing gas. It is shown that better accuracy can be obtained for the diffusion coefficient and solubility of this gas by using the method presented and all experimental data rather than only part of the data, as has often been done in the past.