An adequate understanding of the conductivity of polyperfluorosulfonic acid (PFSA) membranes as a function of water content, or relative humidity, and temperature is necessary for an analysis of the functioning of proton-exchange membrane (PEM) fuel cells. Although much work has been done toward elucidating the microstructure and conduction mechanism in PFSA, a satisfactory theoretical model with a minimum of fitted parameters is not yet available. Such a model is developed here for the conduction of protons in hydrated Nafion ® or like membranes based on the dusty-fluid model for transport and the percolation model for structural aspects. Further, thermodynamics of dissociation of the acid groups in the presence of polar solvents such as water is included. The sorption of solvent from vapor is modeled using a finite-layer Brunauer-Emmett-Teller (BET) model. With the only fitted parameters employed being the BET constants, determined independently, and the ratio of diffusion coefficients representing the interaction of the protonated solvent molecules with solvent and that with the membrane, the model provides excellent correlation with a variety of experimental data.

A comprehensive pore transport model is proposed to describe proton diffusion within Nafion at various hydration levels by incorporating effects of water uptake and various proton transport mechanisms, namely, proton hopping along surface, Grotthuss diffusion, and ordinary mass diffusion of hydronium ions. The diffusion coefficients are predicted within a general random walk framework. The proton conductivity in contact with water vapor is accurately predicted as a function of relative humidity without any fitted parameters, considering the sorption isotherm proposed in the companion paper ͑Part I͒. A maximum conductivity in contact with liquid water is also predicted by the model for equivalent weight between 900 and 1000, in good agreement with the experimental measurements. The modeling framework could be extended to other proton conducting electrolytes for fuel cell applications.

Sustained potential oscillations are experimentally observed in a proton exchange membrane fuel cell with PtRu as anode catalyst and with H 2 /108 ppm CO as the anode feed when operating under a constant current density mode. These oscillations appear at fuel-cell temperatures below 70°C. A threshold value exists for both the current density and the anode flow rate at a given fuel-cell temperature for their onset. The temperature dependence of the oscillation period shows an apparent activation energy around 60 kJ/mol. The potential oscillations are believed to be due to the coupling of anode electro-oxidation of H 2 and CO on the PtRu catalyst surface, on which OH ad is formed more readily, i.e., at lower overpotentials. A simple kinetic model is provided that can reproduce the observed oscillatory phenomenon both qualitatively and quantitatively.

No abstract

A thermodynamic model is proposed to describe the sorption of water in Nafion based on the Flory-Huggins activity model and an appropriate osmotic pressure correction term for the chemical potential of water within the swollen membrane. The key variables for sorption are equivalent weight of ionomer, acid strength of the ionic groups, modulus of polymer elasticity, and interaction between water and polymer. The water uptake per unit mass of dry Nafion increases with the increasing acid strength of the functional groups, decreasing Young's modulus, and decreasing equivalent weight of Nafion. The model provides insights into the sorption and swelling behavior of ion-exchange membranes, and thus, may be useful in evaluating and designing alternate proton-exchange membranes for fuel cell applications. In a companion paper ͑Part II͒, a predictive model is presented for the transport of protons in Nafion.