A two-dimensional two-phase steady state model of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) cathode is developed to study the effects of various operating, design, and model parameters on the cell performance. The model domain consists of the cathode flow field, two layers of diffusion medium, catalyst layer, and the polymer membrane. In this work, the catalyst layer is modeled using flooded spherical agglomerate characterization. An expression is developed for the void fraction of the catalyst layer in terms of its design parameters. The developed model is validated with experimental data available in open literature. The effects of various operational parameters such as cell temperature, pressure, and the cathode air flow rate are studied in detail. The effects of the design parameters of the parallel flow field geometry and that of the diffusion medium are observed. In addition, the role of electric conductivity of the gas diffusion layer (GDL) on the cell current density is studied. Many limiting mechanisms take place in the cathode catalyst layer. Therefore, special emphasis is given on the parametric study of the catalyst layer. The effects of the catalyst layer design parameters such as the thickness, and the loadings of platinum and ionomer are studied. Finally, the issues related to water management are studied.
The effects of liquid water are considered in all the porous layers. Both electro-osmotic drag and back diffusion are considered for transport of liquid water in the membrane. The importance of modeling the membrane for capturing the cell performance is shown by simulating the effects of low concentration gradient of water in the membrane. The study on operating conditions showed that the optimum operating temperature of the cell is 80°C - 85°C and the performance of the cell is better at high pressures and flow rates. The study on design parameters suggests that the optimum porosity of the GDL for this cell is in the range of 0.7 to 0.8 and a thinner catalyst layer with high platinum and membrane content performs better than other combinations considered in this study. The detailed parametric study considering the effects of liquid water provides a pathway for different optimization studies in various layers.
