No abstract

A technique to measure an average mass-transport resistance of a polymer electrolyte fuel cell in situ is described. Experimental polarization curve data are extrapolated to the limiting current, at which the driving force for diffusion can be accurately estimated. The technique gives very reproducible results and is sensitive to changes in the mass-transport characteristics of the cell. Using a resistance-in-series approach, the mass-transport characteristics of the electrode and gas diffusion media ͑GDM͒ can be separated. For the particular system used in this study, it is estimated that the GDM accounts for about 25% of the total mass-transport resistance of the fuel cell assembly. Bulk gas diffusion accounts for less than half of the measured mass-transport resistance. The measured mass-transport parameter may also be utilized for comparison of fuel cell systems, analysis of the effect of operating conditions, or for tracking changes during durability testing.The performance of a polymer electrolyte fuel cell ͑PEFC͒ is limited by the electrochemical kinetics, proton transport through the ionomer, electronic resistances, and mass-transport limitations. Transport of the reactants from the gas flow channels to the active sites in the electrode are influenced by the gas diffusion media ͑GDM͒ structure, the flow-field design, the degree of flooding in the cell, and the electrode architecture. It is desirable to use a universal measure of the mass-transport characteristics of a fuel cell assembly to compare different GDM, electrodes, and flow-field configurations. Such a parameter may also be utilized for comparison of fuel cell systems, analysis of the effect of operating conditions, or for tracking changes during durability testing. Previous studies have focused on estimating the transport characteristics of PEFCs by comparison of experimental data to model calculations, 1-4 by inspecting the high current density region of the polarization curve, 5,6 or by analyzing the entire polarization curve with respect to all losses in the cell. 7 Attempts have also been made to measure the limiting current directly and through extrapolation by running the fuel cell at constant flow rate and with dilute oxygen feed streams. 8 The goal of this study was to develop the in situ measurement of a parameter, which can be performed on any fuel cell test station or fuel cell system and which also provides a direct quantitative measure of the mass-transport characteristics.This paper summarizes the development of the method. First, the necessary equations to calculate the mass-transport coefficient from a measured limiting current are derived and experimental difficulties in obtaining a limiting current are described. The resulting technique to measure the mass-transport coefficient is then tested for reproducibility, station-to-station variability, and sensitivity to changes in mass-transport resistance and accuracy. Initial results are used to estimate the contribution of fuel cell components to the total masstransport resistance....

No abstract

For reduced system complexity and compactness, it is desirable to operate polymer electrolyte fuel cells ͑PEFC͒ with low humidification. In order to more fully understand and optimize performance, there is a need for combined distributed current, species, and impedance data. This paper presents results of a series of experiments at various anode and cathode humidity levels with distributed current, species, and high frequency resistance ͑HFR͒ data. This provides much insight into the characteristic operating performance of PEFCs under low-humidity operation. Results show that the degree of water saturation in the anode greatly influences local performance through local anode dryout, even for the thin 18 m electrolytes used in this study. A characteristic curve has been developed to predict the qualitative shape of the current profile for a coflow arrangement under fully dry to fully humidified inlet combinations. These results should also be of great interest to those seeking experimental data for model validation.

SUMMARYProton Exchange Membrane (PEM) fuel cells have been developed extensively since their introduction over thirty years ago. A key component, the polymer electrolyte membrane, acts as both a separator and an electrolyte in the operating fuel cell. Composite membranes offer the capability of using a wide variety of ionomeric polymers that may be mechanically too weak to use as freestanding films. These thinner membranes can replace thicker non-reinforced membranes, thereby increasing performance while simultaneously increasing durability. However, additional advancements will be necessary to meet aggressive operating conditions of higher temperatures and/or lower humidities, as well as longer operating lifetimes demanded in both stationary and automotive applications. In this paper, these challenges for fuel cell membranes are considered.PEM membrane requirements are discussed in terms of two different parameters: temperature and relative humidity. The effect of these two operating parameters on the proton conductivity of PEM fuel cell membranes and the resulting effect on fuel cell performance are examined using experimental observations. Numerical simulations are used to assess the influence of water transport properties on the local hydration state of the membrane inside the running fuel cell. Finally, the challenge of longer membrane life is explored by examination of recent studies on reinforced and non-reinforced membranes. These results illustrate the benefit of reinforced membranes in terms of membrane durability and therefore cell lifetime.