Richard S. Fu

Consideration of the Role of Micro-Porous Layer on Liquid Water Distribution in Polymer Electrolyte Fuel Cells

Evidence of a region of gradual property change between the micro-porous layer and the macroporous layer of bilayer diffusion media is presented, and a mathematical model describing the effects of this gradual interfacial region is developed. The model results in a continuous liquid water saturation distribution across the diffusion media compared to the sharp discontinuity in the liquid phase saturation predicted by the earlier sudden interface models. High-resolution neutron radiography is used to measure the water content profile across the diffusion media, and the results are compared with the model predictions. The effect of the geometric blur, and uncertainty in-neutron radiography, is accounted for by applying the effects of geometric blur to model results. When the blur is considered, the neutron radiography results are found to be very similar to model predictions.

Water Transport Across a Polymer Electrolyte Membrane under Thermal Gradients

A fundamental experimental and numerical study of the water transport across a perfluorosulfonic acid membrane under a temperature gradient is presented. The water transport phenomenon was experimentally investigated through water flux measurement and neutron radiography. The experimental observations found that water is transported in the direction from the high temperature side to the low temperature side, when both sides of the membrane are sufficiently humidified, and suggest that the transport mechanism is concentration gradient driven. The neutron radiography measurements detected the presence of water content gradient across the membrane and higher water content is seen at a larger thermal gradient. A numerical model was developed to investigate the experimental results. Water transport predictions agreed qualitatively but more accurate material and transport property characterizations are needed for further improvement.

Heat and Mass Transfer in Polymer Electrolyte Fuel Cells in Ultra-Low Humidity Operation

Polymer electrolyte fuel cells (PEFCs) typically utilize a PFSA (perfluorosulfonic acid) based polymer electrolyte, which requires to be well hydrated for proton conductivity. However, system constraints on weight and volume, efficiency considerations and flooding potential in the cells and stack forces a decreased humidity of the reactant streams. Coupled with a desire in increased power density through larger current densities, heat and mass (particularly water) transport in low relative humidity operation become significantly important. We have performed an experimental and computational study on the operation and performance of PEFCs. Standard 25 cm fuel cell hardware (Fuel Cell Technologies, Albuquerque, NM) is used with commercially available membrane-electrode assemblies with Nafion 212 membranes, and 0.4 mg/cm Pt loading on each side (Ion Power Inc, Newark, DE) to obtain steady-state polarization curves under various conditions. Figure 1 shows the polarization curves obtained with various cathode reactants in order to identify the effect of gas composition on water and oxygen transport. The stoichiometry of the reactants are kept very high (>15) in order to minimize the spatial variations. It is clear that the reactant gas has a significant impact on the performance of the fuel cell, even at very low current densities. Variation of cathode reactant is typically utilized to identify the limiting current density due to oxygen transport limitation, but its effect is usually dominant in high current density (mass transfer limited) region. It is clear that the composition of the cathode mixture has an impact on the water content of the membrane through water transport across the membrane-electrode assembly (MEA) and the gas diffusion media (GDM). This finding is supported by the high frequency resistance (HFR) measurements performed in conjunction with the polarization curve measurements. Not only the cathode reactant has an impact on membrane resistance and consequently on the water content, but the resistance also found to have a significant dependence on cell voltage (and current density). Even though the cell produces more water as the cell voltage decreases, the temperature in the cell rises more rapidly than the water production therefore the overall water activity in the cell starts to decrease after a certain a cell voltage. The cell voltage at this point is dependent on the cathode composition, since effective diffusivity of water vapor is a function of cathode gas mixture. The model results also support this finding, such as shown in Figure 3. Figure 3 shows the predicted temperature contours across a PEFC, operating at 80oC (i.e. the flow plate temperatures are kept constant at 80oC), with 21%O2 (balance N2) as the cathode reactant. At 0.3 V, even though the predicted current density is less than 0.6 A/cm, the temperature may exceed 84oC at portions of the cell. This 4oC difference causes 20% increase in saturation pressure, which decreases the water activity by 20%.

An Internal Water Management Scheme for Portable Polymer Electrolyte Fuel Cells

Performance of polymer electrolyte fuel cells (PEFCs) is highly dependent on water content of the membrane and a humidification scheme becomes a necessity to operate PEFCs at a high efficiency. However, conventional humidification schemes require external humidifiers, which are usually bulky and impractical for portable PEFCs. In this paper we propose an innovative approach for humidification of the polymer electrolyte membrane, using an internally built-in mass (water) exchanger (MX) embedded in the bipolar plates. We present the validation of the concept using a multi-dimensional, isothermal computational fluid dynamics (CFD) solution of the water transport in the proposed MX. An optimal range of operation of the MX is investigated and effects on PEFC performance are studied.Copyright

Validation of a Computational Polymer Electrolyte Fuel Cell Model

Computational models of polymer electrolyte fuel cells (PEFCs) of various degrees of complexities have been reported in the recent years and are capable of simulating detail transport phenomena within the PEFCs where experimental methods cannot. A thorough model validation is necessary for the model results to be used in analysis and design. Water transport in PEFCs has a strong effect on the performance regardless of the operating conditions. In low humidity cases, especially, the amount of water exchange from anode to cathode has a strong role in governing the water content in the anode side of the polymer electrolyte membrane (PEM), consequently on the ionic conductivity of the anode side of the membrane and catalyst layer. In this work, we present results on validation of the previously developed CFD based PEFC model for low humidity conditions using current density and species distribution data along the flow direction provided in the open literature. An excellent current density profile was obtained and species profiles were found to capture the data trend well, especially for the first half of the flow distance. Our findings suggest that, an accurate water transport modeling is paramount in capturing the overall PEFC behavior.Copyright