The effect of thickness and wetproof level of the gas diffusion layer on electrode flooding and cell performance was investigated. Three types of gas diffusion media were tested: SGL SIGRACET carbon papers, with and without a microporous layer, and Toray TGPH carbon paper without a microporous layer. Overall, it was found that SGL carbon paper with the microporous layer gave the best fuel cell performance even at low air stoichiometries. It was also found that adding poly͑tetrafluoroethylene͒ ͑PTFE͒ to the gas diffusion layer could enhance gas transport and water transport when a cell operates under flooding condition, but excessive PTFE loading could lead to a high flooding level in the catalyst layer. It is our opinion that a combination of hydrophobic pores for gas transport and hydrophilic pores for liquid water transport within the macroporous layer is needed. It is also our opinion that the optimal ratio of hydrophobic and hydrophilic pores depends on the pore size and its distribution. Finally, it was observed that without the microporous layer, thinner gas diffusion materials were more sensitive to liquid water accumulation than the thicker ones.

This paper describes a two-phase, one-dimensional steady-state, isothermal model of a fuel cell region consisting of the catalyst and gas diffusion layers bonded to a proton exchange membrane ͑PEM͒. A thin film-agglomerate approach is used to model the catalyst layer. The effect of water flooding in the gas diffusion layer and catalyst layer of the cathode on the overall cell performance was investigated. The simulation results confirmed that the water-flooding situation in the catalyst layer is more severe than that in the backing layer since water is first produced in the catalyst layer. The catalyst layer should be considered as an individual domain. The effect of operating parameters that affect the water generation and removal process, such as the inlet relative humidity of the cathode and anode streams and operating temperature was studied. The results show good agreement with the experimental observations.

No abstract

A two-dimensional, two-phase, steady-state, isothermal model was developed for a fuel cell region consisting of the catalyst and gas diffusion layers bonded to a proton exchange membrane ͑PEM͒. This model extends the previously published one-dimensional model of the gas diffusion and catalyst layers to two dimensions in order to account for the effects of the shoulder of the gas distributor and the electronic conductivity of the solid phase. The new model was validated with experimental results and then used to investigate the effect of the relative dimensions of the shoulders and channels on the cell performance. The effects of the in-plane liquid water permeability and electronic conductivity of the gas diffusion layer on cell performance were also examined. It was found that more channels, smaller shoulder widths on the gas distributor, and higher in-plane water permeability of the gas diffusion layer can enhance the transport of liquid water and oxygen, leading to better cell performance. The in-plane electronic conductivity of the gas diffusion layer was found to have minimal effect on the cell performance. However, a highly nonuniform distribution of electronic current was formed within the gas diffusion and catalyst layers when the in-plane electronic conductivity was low.

The conductivity of a proton-conducting membrane ͑PEM͒ depends on the characteristics of the ionic clusters both within the polymeric structure and on its outer surfaces. This work explores the use of conductive atomic force microcopy to characterize the surface ionic activity of these membranes and investigates the effect of the surface ionic activity on the performance of PEM fuel cells. Results obtained show that only a fraction of the membrane surfaces is active and that this fraction increases with the relative humidity in the gas phase. Also, no correlation exists between the membrane surface ionic activity and its topography as expected.In a proton exchange membrane ͑PEM͒ fuel cell, a proton conducting membrane like Nafion is used as the electrolyte. The proton conductivity of this membrane, which strongly affects the performance of a PEM fuel cell, depends on the characteristics of the ionic clusters both within the polymeric structure and on its outer surfaces. The bulk conductivity of these membranes has been extensively studied. 1-12 However, no study has been conducted to determine the effect of the morphology and activity of the ionic clusters on the surface of these membranes on the performance of PEM fuel cells. Furthermore, the effects of membrane preparation and treatment processes ͑sulfonation, protonating, boiling, drying, hot pressing, etc.͒ on the topography and activity of the ionic clusters on the membrane surface are not known. It is known, however, from water absorption, contact angle, and MRI experiments that the bulk membrane and its outer surface behave differently. This difference in behavior is also known to depend on the membrane pretreatment processes. 1,4-6,13 It has been postulated that the ionic clusters that are normally found outside of the Teflon structure when the membrane is hydrated reorganize and migrate into the hydrophobic Teflon structure when the membrane becomes dehydrated, resulting in a membrane with greatly reduced surface ionic conductivity. Existence of a Fluorine-Rich SkinThe first study of the surface ionic activity of proton conducting membranes and its effect on the performance of a fuel cell using these membranes was conducted in 1999. 14,15 In this study X-ray photoelectron spectroscopy ͑XPS͒ was used to determine the surface elemental composition, specifically the carbon-to-sulfur ratio, of Nafion 1100 membranes subjected to various treatment and surface modification processes. 14-17 Next, these membranes were incorporated into membrane-electrode-assemblies ͑MEAs͒ and tested in a PEM fuel cell to determine whether there was a correlation between the surface elemental composition and the fuel cell performance. The results of the XPS study are summarized in Table I, and those from the fuel cell test are shown in Fig. 1.Note that the surface composition of a dry, as-received Nafion 1100 membrane has lower sulfur-to-carbon ratio than that calculated from the chemical formula for Nafion 1100, 0.024 vs 0.053. The results also show that pretreatment with sulfuri...