Multiphysics modelling of PEM fuel cells - with reacting transport phenomena at micro and macroscales
Sammanfattning: This thesis presents numerical simulations of reacting transport phenomena in polymer electrolyte membrane (PEM) fuel cells. Broadly, the presented work is subdivided into macro and microscale simulations. In macroscale simulations, a unit PEM fuel cell with interdigitated flow field configurations is simulated keeping in mind all the essential transport mechanisms, i.e., transport of species, ions, electrons, heat and liquid water. Additionally, the impact of different material properties is also incorporated in the work such as anisotropy of species, electrons, temperature and liquid water diffusion in the gas diffusion layers. Furthermore, to increase the accuracy of predicted results, more stringent correlations have been applied for the correction of material phases in the catalyst layer for calculating the ion transport. For simulating the electrochemical reactions in the catalyst layers, an advanced agglomerate model has also been used that takes into account the morphological details of the materials present in the catalyst layers. Since, the liquid water transport in the gas diffusion layer represents one of the most critical phenomenons, a validated approach has been utilized instead of the conventional Leverett approach which has been the most common technique used so far in numerical simulations. The simulations with all the stated mechanism have revealed that the current densities predicted by earlier models were always overestimated and the limiting current density is found to be approximately 1.0 A/cm2, while in present work, the limiting current density is about 0.68 A/cm2. The second part of the thesis is dedicated to the generation and simulations of a section of the catalyst layer at microscales. The catalyst layer in PEM fuel cells consists of four different types of material each with a special function to serve. The main theme of work at this section is to segregate each material so that the behavior of each component can be explicitly studied and its response to various physical processes can be noted when subjected to operation. It is observed in simulations at such scale that the selected part of the catalyst layer is much descriptive than the macroscale simulations where it is much difficult to specify each material phase separately. Additionally, at microscales, the correction factors do not need to be defined explicitly because all the processes are confined to their respective phases. The work presented here is limited to single phase flow only and the solid phase is neglected, i.e., the electric current follows the same path as the ionic current due to the limitations in the computational resources.
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