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Electrochemical impedance spectroscopy measurements were performed with bare and with recast Nation filmcoated Pt electrodes in hydrogen saturated 85% phosphoric acid in order to determine the effects of the film on the transport properties of hydrogen. The impedance data were fit to a model equivalent circuit using nonlinear regression techniques. Two parallel gas diffusion paths in the Nation film can be inferred from the data. The diffusion coefficients were estimated for each diffusion path in the film. The concentration of hydrogen in the recast Nation film was found to be over fifteen times greater than in the surrounding solution.Electrochemical impedance spectroscopy and cyclic voltammetry measurements were performed with a Nation film-coated platinum electrode and a bare platinum electrode in nitrogen and hydrogen saturated 85% phosphoric acid in order to compare H2 transport to the Pt surface for these two cases. These studies are applicable to acid fuel cells where the gas fed electrodes are supported Pt particles.Nation is a cation exchange membrane that has hydrophilic clusters with sulfonate groups dispersed within a continuous hydrophobic fluorocarbon phase [e.g., (1-3)]. The microbiphasic nature of a Nation membrane is reflected in transport properties of species in the membrane. Anions have impeded permeability through Nation due to electrostatic interactions with the sulfonate groups in the hydrophilic clusters whereas cations readily permeate by hopping from cluster to cluster.Some previous studies on the gas transport properties in Nation membranes have been reported (4-7). Ogumi et al. reported the permeability of oxygen (4, 5) and hydrogen (5) in Nation membranes. The solubility of hydrogen in Nation was found to be comparable to the solubility of hydrogen in PTFE, that is, H2 solubility was reported to be about thirty time greater than in water. Ogumi et al. concluded that the higher concentration of gases in the Nation film was due to the hydrophobic perfluoro-carbon portion of the Nation film.Sakai et al. (6) studied gas diffusion in dried and hydrated membranes using time lag measurements. They found that hydrated membranes exhibited (i) a smaller gas solubility and (ii) a higher gas diffusivity as compared to the dry membranes. Their data suggested that the gas mainly diffuses through the hydrophilic clusters and channels of the wetted membranes.Gottsfeld et al. (7) performed a combination of quasi steady-state RDE measurements and linear sweep voltammetry measurements to measure oxygen reduction on both bare and Nation-coated Pt electrodes in 85% H3PO4 electrolyte. The oxygen concentration in the recast Nation coating was estimated to be 20 times larger than in the surrounding H3PO4. The greater oxygen concentration was attributed to 02 enrichment in the hydrophobic backbone region of the Nation film. Rubenstein et al. (8) investigated the Pt/Nafion/ Ru(bpy)3 ~ §247 system using ac impedance to determine the diffusion coefficients of the reactive ions and found two diffusion domains. ...

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