Trung Van Nguyen

A Two-Dimensional, Two-Phase, Multicomponent, Transient Model for the Cathode of a Proton Exchange Membrane Fuel Cell Using Conventional Gas Distributors

A two-dimensional, two-phase, multicomponent, transient model was developed for the cathode of the proton exchange membrane fuel cell. Gas transport was addressed by multicomponent diffusion equations while Darcys law was adapted to account for the capillary flow of liquid water in the porous gas diffusion layer. The model was validated with experimental results and qualitative information on the effects of various operating conditions and design parameters and the transient phenomena upon imposing a cathodic overpotential were obtained. The performance of the cathode was found to be dominated by the dynamics of liquid water, especially in the high current density range. Conditions that promote faster liquid water removal such as temperature, dryness of the inlet gas stream, reduced diffusion layer thickness, and higher porosity improved the performance of the cathode. There seems to be an optimum in the diffusion layer thickness at the low current density range. The model results showed that for a fixed electrode width, a greater number of channels and shorter shoulder widths are preferred. The transient profiles clearly showed that liquid water transport is the slowest mass-transfer phenomenon in the cathode and is primarily responsible for mass-transfer restrictions especially over the shoulder.

An Along‐the‐Channel Model for Proton Exchange Membrane Fuel Cells

An along-the-channel model is developed for evaluating the effects of various design and operating parameters on the performance of a proton exchange membrane (PEM) fuel cell. The model, which is based on a previous one, has been extended to include the convective water transport across the membrane by a pressure gradient, temperature distribution in the solid phase along the flow channel, and heat removal by natural convection and coflow and counterflow heat exchangers. Results from the model show that the performance of a PEM fuel cell could be improved by anode humidification and positive differential pressure between the cathode and anode to increase the back transport rate of water across the membrane. Results also show that effective heat removal is necessary for preventing excessive temperature which could lead to local membrane dehydration. For heat removal and distribution, the counterflow heat exchanger is most effective.

Multicomponent Transport in Porous Electrodes of Proton Exchange Membrane Fuel Cells Using the Interdigitated Gas Distributors

Hydrodynamics of gases in the cathode of a proton exchange membrane fuel cell that is contacted to an interdigitated gas distributor are investigated using a steady‐state multicomponent transport model. The model describes the two‐dimensional flow patterns and the distributions of the gaseous species in the porous electrode and predicts the current density generated at the electrode and membrane interface as a function of various operating conditions and design parameters. Results from the model show that, with the forced flow‐through condition created by the interdigitated gas distributor design, the diffusion layer is greatly reduced. However, even with a much thinner diffusion layer, diffusion still plays a significant role in the transport of oxygen to the reaction surface. The results also show that the average current density generated at an air cathode increases with higher gas flow‐through rates, thinner electrodes, and narrower shoulder widths between the inlet and outlet channels of the interdigitated gas distributor.

Three-dimensional effects of liquid water flooding in the cathode of a PEM fuel cell

A two-dimensional model available in the literature for conventional gas distributors was expanded to account for the dimension along the length of the channel. The channel was discretized into control volumes in series that were treated as well mixed. An iterative solution procedure was incorporated in each control volume to determine the average current density and the corresponding oxygen consumption and water generation rates. Downstream channel concentrations were calculated based on stoichiometric flow rates and the solution obtained from the preceding control volumes. Comparison of the model results with experimental data and the existing two-dimensional model showed that accounting for the oxygen concentration variations along the channel and its effect on the current density is critical for accurately predicting the cathode performance. Variations in the current density along the channel were strongly influenced by the changes in oxygen concentration caused by consumption due to reaction and dilution caused by water evaporation. Operating parameters that facilitated better water removal by evaporation like higher temperature and stoichiometric flow rates and lower inlet stream humidity resulted in higher net current. Operating conditions that resulted in minimal loss in oxygen concentrations resulted in a more uniform current density distribution along the channel.

A Gas Distributor Design for Proton‐Exchange‐Membrane Fuel Cells

A nonconventional gas distribution design has been developed to improve the mass-transport rates of the reactants from the flow channels to the inner catalyst layers of the porous electrodes and to reduce the electrode water flooding problem in the cathode of proton-exchange-membrane fuel cells. Preliminary results validate the effectiveness of the design in achieving the above goals.

Effect of direct liquid water injection and interdigitated flow field on the performance of proton exchange membrane fuel cells

Proper water management is vital to ensuring successful performance of proton exchange membrane fuel cells. The effectiveness of the direct liquid water injection scheme and the interdigitated flow field design towards providing adequate gas humidification to maintain membrane optimal hydration and alleviating the mass transport limitations of the reactants and electrode flooding is investigated. It is found that the direct liquid water injection used in conjunction with the interdigitated flow fields as a humidification technique is an extremely effective method of water management. The forced flow-through-the-electrode characteristic of the interdigitated flow field (1) provides higher transport rates of reactant and products to and from the inner catalyst layers, (2) increases the hydration state and conductivity of the membrane by bringing its anode/membrane interface in direct contact with liquid water and (3) increases the cell tolerance limits for excess injected liquid water, which could be used to provide simultaneous evaporative cooling. Experimental results show substantial improvements in performance as a result of these improvements.

Effect of Thickness and Hydrophobic Polymer Content of the Gas Diffusion Layer on Electrode Flooding Level in a PEMFC

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 poly(tetralluoroethylene) (PTFE) 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.

Modeling Liquid Water Effects in the Gas Diffusion and Catalyst Layers of the Cathode of a PEM Fuel Cell

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 (PEM). 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.

A Two-Dimensional Two-Phase Model of a PEM Fuel Cell

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 (PEM). 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.

Three-Dimensional Simulation of Liquid Water Distribution in a PEMFC with Experimentally Measured Capillary Functions

Modeling of liquid water distribution in polymer electrolyte membrane fuel cells (PEMFCs) is challenged by uncertainties in characterizing the two-phase transport properties of the porous transport layer (PTL) and the catalyst layer (CL). Capillary pressure, representing both pore structures and wetting features of the porous media, is a key factor in controlling liquid water transport and distribution in the porous electrode of PEMFCs. By incorporating experimentally measured capillary pressure functions, a single-domain, three-dimensional, and two-phase transport model was developed to predict liquid water saturation in the PTL and CL of a PEMFC with long straight channels, where the two-phase flow complications were minimized at practical stoichiometry. In the cathode CL, the liquid water saturation was found to be higher under the channel than that under the ribs at high current densities. In the cathode PTL, however, the liquid water saturation level was observed to be lower under the channel than that under the ribs. At high current densities, the average water saturation levels are insensitive to current density and fall in the range of 0.4-0.5 in the CL and 0.2-0.3 in the PTL. Finally, the effects of liquid water on oxygen transport and cell performance were investigated.