| Fuel cells are electrochemical devices to directly convert the chemical energy of hydrogen and oxygen into the electrical energy. Since the final reaction product is only water, fuel cells become one of the most favorable green power sources. Especially the proton exchange membrane (PEM) fuel cell, owing to the compact configuration, rapid start-up, high efficiency, low noise and low operation temperature, it is widely regarded as not only the optimal power source of a modern automobile, but also one of the perfect power sources for some special military weapons and equipment, for example a submarine requiring a high concealment ability.Porous electrode consisting of the gas diffusion layer (GDL) and the catalyst layer is one of the essential components of PEM fuel cells. GDL is the important functional structure material with high electronic conductivity and excellent performance of permeability. GDL, with a thickness about 100-300μm, is generally made from stochastic distributed carbon fibers or orthogonal woven of carbon fiber bundles. The main roles of GDL in a PEM fuel cell stack are: (1) to allow the gaseous reactants (fuels: oxygen or air in cathode and hydrogen in anode) to move towards the catalyst layer region, requiring a high permeability; (2) to provide a large number of the micro-paths for the reaction product (liquid water) and non-reaction gaseous reactants to flow towards the flow channel, requiring a good transport ability for the two phase mixed-flow; (3) to give a low interfacial contact resistance and to reduce the Omhic overpotential working together with the bipolar plates; (4) as an important media to transport the electron and thermal; (5)as an important structure to support catalyst, requiring a high specific surface; (6) to connect and support the structure and system, requiring a certain mechanical strength and stiffness. The first five functions are affected directly by the assembly load (pressure) of the structure. The final function is more closely related to the assembly load. In the assembly (packaging) process of a fuel cell stack, the GDL gives a large and inhomogeneous deformation. The mechanical and physical properties of GDL depend strongly on this inhomogeneous compression pressure. When the assembly pressure is unreasonably high, on one hand, the reaction efficiency of the fuel cell will decrease due to the decrease in the porosity of the GDL. On the other hand, the related components of the electrode (i.e., the exchange membrane) may reach the yield state and even is destroyed. However, an unreasonable low assembly pressure will give a high interfacial contact resistance and therefore reduces the system efficiency. It may also cause the failure of either the stack structure or the mechanical seal. However, most of the previous studies have neglected those effects. This not only affects the efficiency of the fuel cell stack but also reduces the reliability of the fuel cell stack. During the past few years, assembly mechanics of fuel cell stack is thus received more and more attention. Most of the studies are focused on the effect of the assembly pressure on the mechanical properties, physical properties and micro fluid transport ability of the GDL. Taking the PEM fuel cells as the studied objective and the numerical simulation as the main studying method, this dissertation investigates several mechanics problems of the PEM fuel cells under multiphysical fields, especially the dependence regulation of the structural and physical properties of the fuel cells on the clamping pressure. The effects of assembly pressure on the performance of a single fuel cell and fuel cell stacks are studied in macroscale. On the other hand, the formation, growth and transport process of the micro-droplets is studied in microscale to reveal the fundamental mechanism of liquid water development in the GDL. Therefore, effects of the assembly pressure and the transport mechanisms of liquid water in GDL on the PEM fuel cell performance are studied in three different structure scales, including fuel cell stack (decimeter), electrode (millimeter) and fiber structure in GDL (micrometer).This dissertation not only proposes a model of the contact resistance between rib and GDL, but also develops a numerical method to study the effect of the compression deformation of the GDL on the performance of PEM fuel cells. First, finite element method (FEM) is used to analyze the contact resistance between the rib and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and reaction products. It is found that the GDL compression deformation induced by the clamping pressure strongly affects the contact resistance, the GDL porosity distribution, and the cross section area of the gas channel, which, in turn, influence the over-potential and finally influence the system efficiency. The numerical results show that the fuel cell performance decreases with increasing the compression deformation if the contact resistance is negligible (ideal condition), but there exists an optimal compression deformation if the contact resistance is taken into account (actual condition). This suggests that the compression of GDL has a significant effect on the fuel cell performance, especially in the high current density region. In order to research the effect of the clamping pressure on the performance of fuel cell stacks, a simplified assembly model is proposed.No matter whether the inhomogeneous compression of GDL is considered, the numerical simulation of the PEM fuel cells always involves the two-phase flow transport problem in porous media. A two-phase flow model has to be used to analyze the problem of water balance in the electrode, i.e. the water management, one of the key technologies affecting the performance and the lifetime of the PEM fuel cells. The traditional models use the macroscopic approach based on the unsaturated flow theory to investigate liquid water transport in the PEM fuel cells. These methods are easily mastered, but cannot be incorporated the GDL morphology. Therefore they can neither reveal the mechanisms of liquid water transport nor give an enlightening idea for the optimal design of the GDL microstructure. The flooding phenomenon, one of the most important technology problems in the PEM fuel cells, has not been well solved for a long period. Based on the micro-flow principle, this dissertation adopts equivalent capillary model and the Lattice Boltzmann method to study the formation, growth and transport process of the liquid water in the porous media. The simulation result has a guiding significance for high performance porous electrode design.
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