Analysis of Deformation of Gas Diffusion Layers and the Impact on Performance of PEM Fuel Cells
Sammanfattning: Proton exchange membrane (PEM) fuel cells have been promoted due to significant breakthroughs in various aspects and increasing public interests. The porous features of the gas diffusion layer (GDL) and the necessary assembly processes generate localized pressure forces on the channel/shoulder structure of the bi-polar plates (BPP). As a consequence, the assembly pressure acting on a single cell and a fuel cell stack has important influence on the geometric deformation of the GDL resulting in a change in porosity, permeability, and the resistance for heat and charge transfer in PEM fuel cells. It is expected that the cell performance is also affected by these physical parameters. To optimize the cell performance, it is necessary to consider the assembly effects, which is conducted by a numerical method in this work. The effect of the GDL porosity change caused by various compression ratios is investigated by a three-dimensional (3D) PEM fuel cell model based on the finite volume method (FVM). The model was validated and further applied to predict the transport phenomena including heat, mass and charges, as well as the effects on the cell performance. The simulation results show that a high compression ratio on the GDL leads to lower porosity, which is favorable for the heat removal from the cell. However, the compression has contradictory effects on the mass transfer and finally deteriorates the cell performance. To predict the GDL deformation and associated effects on the geometric parameters as well as porosity, mass transport properties and the cell performance, both the finite element method (FEM) and the FVM are applied, respectively. A non-homogeneous deformation, porosity, oxygen diffusion coefficient and the electric resistance of the GDL have been observed across the fuel cell in the in-plane direction. The obtained non-homogeneous physical parameters of the deformed GDL are applied for further computational fluid dynamics (CFD) analysis. The CFD results reveal that a higher assembly pressure decreases the porosity, GDL thickness, gas flow channel cross-sectional areas, oxygen diffusion coefficient, oxygen concentration and cell performance. It is found that, the reduction of the GDL porosity is a dominating factor that decreases the cell performance compared with the decreased gas channel flow area and GDL thickness in the assembly condition. A sufficient GDL thickness is required to ensure transfer of the fresh gas to the reaction sites far away from the channel. As the entire electric resistance is considered, the optimized cell performance is obtained if the cell is operating below 1 MPa assembly pressure. It is found from a newly developed electric resistance model that both through-plane resistance of a cell and the interfacial resistance between the GDL and BPP for electrons decrease with higher assembly pressures. Comparing with a zero-compressed cell, the cell operating at an assembly pressure above 2 MPa creates a new contact area between the GDL and BPP at the vertical interface. Therefore, the corner of a BPP close to the channel becomes the dominating zones for electron transfer. Finally, it is suggested that the assembly pressure should be considered properly in designing and manufacturing of PEM fuel cells.Popular science summaryProton exchange membrane (PEM) fuel cell is one of the promising fuel cells in conversion of chemical energy to electric energy with a relative high efficiency. It is widely known that the PEM fuel cell has nearly-zero pollutants if it is fueled by hydrogen. People can use the sustainable electric power without any noise in home usage, transportation and commutation facilities and so on. The current interest of this device is to replace combustion engines to release the environmental problems like CO2 emissions. A PEM fuel cell involves several technologies. Many achievements have been reached in the past decades. However, the cost and stability are two main limitations preventing wide use of PEM fuel cells. In various research and development fields, such as materials, design and manufacturing, some breakthroughs have been made in improving the cell performance. Even though large efforts have been paid in experiments, the closed-space and small-scale of the cell device make it hard to investigate. Therefore, numerical methods have become very popular and presented efficient ways to investigate the transport phenomena and optimizing the cell performance.The assembly process of a single cell or a cell stack is a necessary step to prevent gas leakage and decrease the contact resistance between the various layers. The porous carbon fibers in the gas diffusion layer (GDL) are touching the channel/shoulder structure of the bi-polar plates (BPP). As a consequence, the physical properties of the GDL, such as dimensions, porosity, mass transfer resistance, and interfacial resistances for heat and electrons will be changed. These factors may result in unexpected or decreased cell performance.In this work, the commercial software ANSYS and the newly developed open source code OpenFOAM (“Open Source Field Operation and Manipulation”) are applied to study the important assembly processes. The model in ANSYS predicts the GDL deformation behavior. Then the deformed GDL and the corresponding yield properties are implemented in the PEM fuel cell model to study the effects of the assembly pressure on the transport phenomena and cell performance. To optimize the cell performance, the electric resistance in the deformed bulk of a cell and the interfacial resistance between the GDL and BPP are considered. All the parameters are expressed as a function of the assembly pressure. To investigate the porosity effects independently, different porosities of the GDL caused by various assumed compression ratios are applied as initial conditions for the PEM fuel cell model. In the study of porosity effects, the GDL deformation and the electric resistance variations are neglected. Then the model is further extended to include real deformation of the GDL and the electron transfer effects, respectively. By evaluating several topics, the cell performance is optimized in terms of assembly pressures or compression ratios. Guidelines for design and manufacturing of PEM fuel cells can be set up based on this thesis.
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