Mathematical Modeling of Porous Structure and Operation of Cathode Catalyst Layers in PEM Fuel Cells
Highly efficient and environmentally clean energy conversion in Polymer Electrolyte Membrane (PEM) Fuel Cells is driven by electrochemical reactions that convert hydrogen and oxygen molecules into water. Water, the product of the overall reaction, is involved in all essential processes in the cell. Water management is, thus, a critical issue for fuel cell operation. It entails controlling water fluxes and maintaining appropriate levels of liquid water saturation in the different cell components. There are strong indications in experiment and modeling that the cathode catalyst layer (CCL) plays a major role in this context. Good operation of the CCL is closely linked to its composition (platinum/support phase, ionomer phase and pore space), porous structure, and wetting properties. The research team, in partnership with the NRC Institute for Fuel Cell Innovation, further analyzed a basic mathematical model of CCL function using the structural details of the layer, water transport from the membrane, liquid water formation by electrochemical reaction in the CCL, water transformation via evaporation and condensation and two-phase flow in liquid and vapor phases. Effects of composition of the CCL, porous structure, wetting properties of pores, operating conditions and boundary conditions at interfaces with membrane and gas diffusion layer were systematically studied. Suggestions for the optimization of water handling capabilities of CCLs and fuel cell power densities emerged, which are currently being tested in experiment.