Xuhai Wang

Modeling the Effects of Capillary Property of Porous Media on the Performance of the Cathode of a PEMFC

A two-phase model was developed to study the impact of the capillary property of the gas diffusion (GDL) and catalyst layers (CL) and the saturation-level jump condition at the interface between these two dissimilar materials on the liquid water transport rate and the liquid water saturation levels in these porous layers and the performance of the cathode of a proton exchange membrane fuel cell (PEMFC). The study involves adjusting various aspects of the capillary curves of these porous layers and studying how these changes affect the cathode electrochemical performance to identify the most sensitive aspects. The simulated results show that fuel cell performance and the liquid water saturation levels in the CL and GDL depend on both the individual two-phase properties of these components and their interactions at the GDL/CL interface. The boundary condition at the GDL/ channel interface for saturated air operation constrains the transport phenomenon in the GDL and CL to the hydrophobic regions of the capillary pressure curves. It was also found that capillary properties of the GDL had a more significant effect on the fuel cell performance than those of the CL, and better fuel cell performance was obtained with GDL and CL with high capillary diffusion capability and low hydrophilic porosity.

Modeling the Effects of the Microporous Layer on the Net Water Transport Rate Across the Membrane in a PEM Fuel Cell

In this study, a model was developed to evaluate the effects of the microporous layer (MPL) on the net liquid water transport rate across the membrane. The results support the hypothesis that the improvement in fuel cell performance observed when an MPL is used in the cathode side is related to its effect on the water transport process in the electrode and membrane. Due to its high hydrophobicity, the MPL increases the liquid water pressure in the cathode to levels much higher than that in the anode, resulting in an increased back-transport rate of liquid water from the cathode to the anode. This reduces the amount of water that is transported out of the cathode through the gas diffusion layer (GDL) to the cathode flow channels resulting in a lower saturation level in the GDL, and consequently, faster oxygen transport to the catalyst sites. This model showed that the state of zero-net-water-transport-across-the-membrane could be achieved with the appropriate capillary properties of the porous media. Two capillary properties of the MPL identified to have the greatest impact on the proton-exchange-membrane (PEM) fuel cell performance are the liquid water saturation level at p c = 0 (p g = p l ) and the slope of the capillary curve in the hydrophobic region.

An Experimental Study of Relative Permeability of Porous Media Used in Proton Exchange Membrane Fuel Cells

This study investigated gas and liquid relative permeability, one of the important two-phase transport properties of the porous media used in proton exchange membrane fuel cells. The experiment showed that the gas relative permeability obtained by the gravimetric analysis and neutron imaging agreed at low saturation levels, but differs at high saturation levels. The liquid relative permeability in the saturation level range of 0.3 to 0.8 was measured by neutron imaging. Because of dead air pockets in the porous materials and certain saturation levels are required to establish continuous flow of liquid water through gas diffusion media, liquid permeability at both high and low saturation levels could not be obtained. It was shown that the 3rd-order power correlation of gas permeability generally used in numerical simulation only fitted the porous media of A1 and A3 at low saturation levels.

Modeling the Effects of the Cathode Micro-Porous Layer on the Performance of a PEM Fuel Cell

In this study, a model was developed to evaluate the role of the micro-porous layer (MPL) and the effects of its properties on the liquid water saturation levels in the cathode porous media and the performance of a proton exchange membrane (PEM) fuel cell. The results validated that the fuel cell performance was improved by adding an MPL between the macro-porous gas diffusion layer (GDL) and the catalyst layer (CL) in the cathode side. The MPL, due to its high hydrophobicity, increases the liquid water pressure in the cathode to the levels much higher than those in the anode resulting in an increased back-transport rate of liquid water from the cathode to the anode. The more hydrophobic the MPL is, the higher liquid pressure increases, and, consequently, the higher driving force is created across the membrane to drive water from the cathode to the anode.

Experimental Study of Relative Permeability of Porous Media Used in PEM Fuel Cells

This study investigated gas and liquid relative permeability, one of the important two-phase transport properties of the porous media used in proton exchange membrane (PEM) fuel cells. The experiment showed that the gas relative permeability obtained by the gravimetric analysis and neutron imaging agreed at low saturation levels, but differs at high saturation levels. The liquid relative permeability in the saturation level range of 0.3 to 0.8 was measured by neutron imaging. Because of dead air pockets in the porous materials and certain saturation levels are required to establish continuous flow of liquid water through gas diffusion media, liquid permeability at both high and low saturation levels could not be obtained. It was shown that the 3rd-order power correlation of gas permeability generally used in numerical simulation only fitted the porous media of A1 and A3 at low saturation levels.

Root-Cause of Hysteresis in Capillary Pressure Curves of Porous Media Used in PEM Fuel Cells

Two-phase transport properties of porous media are crucial for water management strategies in PEM (proton exchange membrane) fuel cells. Due to the lack of twophase transport data in literature, Leverett function developed for sand was used in modeling liquid transport phenomena in porous media [1]. In recent years, experimentally measured capillary functions of the porous media used in PEM fuel cells have become available [29].

An Experimental Study of the Saturation Level in the Cathode Gas Diffusion Layer of a PEM Fuel Cell

A Proton Exchange Membrane (PEM) fuel cell with a flow field that can be switched between the serpentine flow mode and the interdigitated flow mode was used to measure the liquid water saturation level in the gas diffusion layer (GDL) of the cathode. The gas pressure drop across the GDL was used along with the correlations between the saturation level and gas relative permeability obtained by neutron imaging to determine the liquid water saturation level in the GDL during operation. The results showed that the saturation levels in the cathode GDL during the interdigitated mode was much lower than that during the serpentine mode leading to better oxygen gas access to the cathode catalyst layer and consequently better fuel cell performance, especially at high current densities and low oxygen stoichiometric flow rate.

A Modeling Study of the Effects of the Properties of Anode Porous Layer on the Performance of a PEM Fuel Cell

A few years ago we implemented a new water management approach called Water Management by Materials Engineering and Design [1]. The idea is to engineer the two-phase flow properties and the configuration of the gas diffusion materials used in the membrane-electrode-assembly (MEA) in a PEM fuel cell to achieve self-water management. Basically, the goal is modifying the capillary pressure property and permeability of the gas diffusion layers in the cathode and anode to force the water that is dragged from anode to the cathode by electro-osmotic drag to flow back to anode to achieve zero net water transport across the membrane. If this condition is achieved it will lead to a PEM fuel cell that does not require anode humidification and minimal cathode water removal requirement and a greatly simplified PEM fuel cell system.