We present here an isothermal, one-dimensional, steady-state model for a complete polymer electrolyte fuel cell (PEFC) with a 117 Nation | membrane. In this model we employ water diffusion coefficients electro-osmotic drag coefficients, water sorption isotherms, and membrane conductivities, all measured in our laboratory as functions of membrane water content. The model pre.dicts a net-water-per-proton flux ratio of 0.2 H20/H § under typical operating conditions, which is much less than the measured electro-osmotic drag coefficient for a fully hydrated membrane. It also predicts an increase in membrane resistance with increased current density and demonstrates the great advantage of a thinner membrane in alleviating this resistance problem. Both of these predictions were verified experimentally under certain conditions.Fuel cells employing hydrated Nation or other hydrated perfluorinated ionomeric materials as the electrolyte are promising candidates for electric vehicle applications (1). The polymer electrolyte provides room temperature startup, elimination of many corrosion problems, and the potential for low resistance losses. Resistive losses within the fuel cell result, in part, from the decrease of membrane protonic conductivity following partial dehydration of the membrane. On the other hand, cathode flooding problems are caused when too much water is in the system. Clearly, water management within the fuel cell involves walking a tightrope between the two extremes.Spatial variations of water content within the polymeric electrolyte of a current~carrying fuel cell result from the electro-osmotic dragging of water with proton transport from anode to cathode, the production of water by the oxygen reduction reaction at the cathode, humidification conditions of the inlet gas streams, and "back-diffusion" of water from cathode to anode, which lessens the concentration gradient.The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and Hill (2-4) have carried out extensive modeling of transport properties in perfluorosulfonate ionomers based on dilute solution theory. Fales et al. (5) reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman (6) applied concentrated solution theory and employed literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set in these cases rather arbitrarily. A different approach was taken by Bernardi (7). She considered transport through the gas diffusion electrodes, assuming the membrane to be uniformly hydrated, corresponding to an "ultrathin membrane" case.We present an isothermal, one-dimensional, steady-state model for water transport through a co...

LIST OF SYMBOLS C~ ~the maximum adsorption sites on the electrode surface C~+ the concentration of adsorbed proton, H~+d in Eq. 1 CH, CHC, CHCm C~2 the concentration of adsorbed H~d in Eq. 4, HCOQd in Eq. 5, HCOOH in Eq. 6, and H2 in Eq. 20 Cp, Cco2 the concentration of H~ol and CO2 in solution kE, ko, kDES, kH2 the rate constant for reactions of Eq. 4, 5, 6, and 20, respectively %, ~DES the time constant defined by Eq. 9 and 12, respectively V~, V ~ the formal potential for Eq. 1 and 17

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Water uptake and transport parameters measured at 30~ for several available perfluorosulfonic acid membranes are compared. The water sorption characteristics, diffusion coefficient of water, electroosmotic drag, and protonic conductivity @ were determined for Nation 117, Membrane C, and Dow XUS 13204.10 developmental fuel cell membrane. The diffusion coefficient and conductivity of each of these membranes were determined as functions of membrane water content. Experimental determination of transport parameters, enables us to compare membranes without the skewing effects of extensive features such as membrane thickness which contributes in a nonlinear fashion to performance in polymer electrolyte fuel cells.

The ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed. The measurements were carried out in the presence of large dc currents. The impedance spectrum of the air cathode is shown to contain two features: a higher frequency loop or arc determined by interfacial charge-transfer resistance and catalyst layer properties and a lower frequency ioop determined by gas-phase transport limitations in the backing. The lower frequency ioop is absent from the spectrum of cathodes operating on pure oxygen. Properties of measured impedance spectra are analyzed by a PEFC model to probe the effect of ac perturbation. Comparison of model predictions to observed data is made by simultaneous least squares fitting of a set of spectra measured for several cathode potentials. The spectra reveal various charge and mass-transfer effects in the cathode catalyst layer and in the hydrophobic cathode backing. Three different types of losses caused by insufficient cell hydration, having to do with interfacial kinetics, catalyst layer proton conductivity, and membrane conductivity, are clearly resolved in these impedance spectra. The data reveal that the effective tortuous path length for gas diffusion in the cathode backing is about 2.6 times the backing thickness.

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