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.

The anode flow rate of a proton exchange membrane ͑PEM͒ fuel cell involving Pt anode electrocatalyst is found to strongly influence the single cell performance when H 2 containing trace amounts of CO is used as the feed. The performance drops dramatically due to CO poisoning as the anode flow rate increases until a large overpotential is reached when it levels off. This effect of the flow rate on the extent of poisoning is found to be reversible and is explained as depending on the actual concentration of CO in the anode chamber which in turn depends on the feed content, the flow rate, and CO oxidation kinetics on Pt. Further, it is found that oxygen permeating across the PEM from the cathode side also appreciably affects the anode overpotential by providing another route for CO oxidation. A CO inventory model is provided that explains the observed phenomena in a PEM fuel cell operating with H 2 /CO as anode feed and a cathode feed with different oxygen pressures.Proton exchange membrane ͑PEM͒ fuel cells power plants for stationary and mobile applications are planned to be operated on reformate gas feed that would inevitably contain trace amounts of CO. 1,2 Extensive investigations have been performed to evaluate the CO tolerance of the PEM fuel cells with different electrocatalysts and under different conditions. It has been reported that the fuel cell temperature and CO content of the gas mixture are the key parameters that determine the performance of a given catalyst. 3-8 The poisoning effect of CO has also been investigated in electrochemical cells incorporating liquid electrolytes such as H 3 PO 4 or H 2 SO 4 . 9-12 These investigations provide much insight into the effect of CO poisoning on the performance of PEM fuel cells. The present work is motivated by our desire to systematically study the mechanisms and kinetics of standard fuel cell catalyst, such as Pt and PtRu, at real PEM fuel cell conditions. During the study of CO poisoning on PEM fuel cell performance with Pt anode catalyst, we found the strong influence of certain operating parameters, namely the anode flow rate and cathode oxygen partial pressure, which have not so far been documented.The performance of a single fuel cell is normally studied in the laboratory under a constant flow rate or stoichiometry. Though PEM fuel cell anodes fed with pure H 2 are often operated near a stoichiometry of one, reformate gas mixtures from catalytic reformers require higher flow rates, because considerable amounts of CO 2 and N 2 are also present in the gas mixtures, with H 2 concentrations ranging from 40 to 75% depending upon the reforming system and specific process. 13 It is shown in this work that the anode flow rate and cathode oxygen partial pressure are very important operating parameters influencing the extent of CO poisoning when operating with a feed containing essentially pure H 2 along with trace amounts of CO. We report here for the first time, experimental results on the effect of anode flow rate and cathode oxygen partial pressure on the per...

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A phenomenological theory is provided for water sorption and proton transport in polymer electrolyte membranes (PEMs) as well as in polymer-inorganic nanocomposite membranes (NCPEMs) that not only serves to rationalize the sorption and conductivity behavior of conventional PEMs such as Nafion but also provides a framework for rational design of improved PEMs and NCPEMs. The thermodynamic model, which considers the effect of osmotic pressure on the activity of free water within the membrane pores, predicts the entire sorption isotherm and provides a plausible explanation for the so-called Schroeder's paradox. The transport model incorporates the various mechanisms of proton transport, namely, surface hopping, Grotthuss diffusion, and en masse diffusion. As the design of alternate PEMs suitable for effective proton transport under hot and dry conditions is a key current technological goal, the rational design of NCPEMs for this purpose is considered here in detail on the basis of an extension of the transport model to account for the influence of the inclusion of functional additives in NCPEMs. The results also point to the reason that Nafion is an excellent PEM, because the hydrophobic nature of its backbone induces water away from surface into pore bulk where efficient proton diffusion occurs.

The design of higher temperature composite proton-exchange membranes ͑PEMs͒ with adequate performance under low relative humidity ͑RH͒ is discussed here based on experimental and theoretical considerations. The approach is based on enhancing the acidity and water sorption of a conventional polymer electrolyte membrane by incorporating in it a solid acidic inorganic material. A systematic investigation of the composite Nafion/inorganic additive PEMs based on characterization of water uptake, ionexchange capacity ͑IEC͒, conductivity, and fuel cell polarization is presented. The effects of particle size, chemical treatment, additive loading, and alternate processing methodologies are investigated. The most promising candidate investigated thus far is the nanostructured ZrO 2 /Nafion PEM exhibiting an increase of ϳ10% in IEC, ϳ40% increase in water sorbed, and ϳ5% enhancement in conductivity vs. unmodified Nafion 112 at 120°C and 40% RH. This appears to be an attractive candidate for incorporation into a membrane-electrode assembly for improved performance under these hot and dry conditions.