Assembly and performance modeling of proton exchange membrane fuel cells
Proton exchange membrane (PEM) fuel cells are favored in many applications due to their simplicity and relatively high power density. However, there has been a lack of understandings of the fundamental mechanisms of assembly and manufacturing induced phenomena and their influence on performance and durability. This dissertation conducts a comprehensive analysis of assembly pressure induced phenomena in PEM fuel cells using multi-physics based modeling. Fundamental research has been conducted in assembly and performance modeling of PEM fuel cells and it is organized into three topics: (1) Development of an electrical contact model between bipolar plates (BPP) and gas diffusion layers (GDL): A micro-scale numerical model is developed to predict the electrical contact resistance between BPP and GDL by simulating BPP surface topology and GDL structure and determining the contact state. This micro-scale contact model is able to predict the contact resistance in PEM fuel cells effectively and accurately with good agreements with experiments. Such a model can be integrated with other fuel cell simulations to predict the overall fuel cell performance and is beneficial to understanding the basic mechanisms of contact behavior between the rough surface and a fibrous medium. (2) Development of a multi physics approach to study assembly pressure impact : A comprehensive multi-physics model is developed for analyzing the effects of assembly pressure by integrating gas mass transfer simulation based on the output from the finite element structure model with the contact resistance analysis. The PEM fuel cell power output first increases and then decreases as the assembly pressure increases. An optimal assembly pressure is observed. This study identifies the optimal assembly pressure theoretically for the first time and provides a fundamental understanding of assembly pressure effects in fuel cells. (3) Investigation of stack deformation, contact resistance and performance induced by assembly pressure and operating conditions : Assembly pressure along with membrane swelling induced by elevated temperature and humidity cause inhomogeneous compression and performance unevenness in the stack. A finite element structural model and flow analysis are developed to investigate the effects of assembly pressure, temperature and humidity. Simulation results show that elevated temperature and humidity will exaggerate inhomogeneous deformation in the GDL and membrane. Temperature and humidity also change the stress distribution in the fuel cell stack due to material property dependence and swelling strain, which lead to contact pressure increase and resistance reduction. Even though the overall performance is improved, significant variation of current distribution is observed due to the increased temperature and relative humidity.
This dissertation provides a comprehensive understanding of the electrical contact resistance, assembly pressure impact and performance in PEM fuel cell stack assembly and can be applied to other fuel cell types and configurations. Results from this dissertation can lead to improved manufacturing and assembly of PEM fuel cells as well as improved performance, which are two main technical barriers toward successful commercialization of PEM fuel cells.