// Simulation of mass transfer in the microstructures

The transport of reactants to the catalyst by the gas diffusion layer (GDL) is the limiting factor for the performance of a PEMFC. Inhomogeneities of the GDL microstructure, due to uneven compression or blockage of pores with liquid water, produce a nonuniform current density distribution. This lowers the cell’s performance and accelerates component ageing. Regarding to the GDL, conventional CFD models make simplifying assumptions and use integral parameters such as porosity and permeability to account for the porous structure. This may make mass transport appear more uniform than it actually is.

For the project, convective mass transport was simulated in a microscopically accurate GDL substrate using the CFD programme, AVL FIRE® (see fig. below). The structures of the various GDL substrates were digitised by means of computer tomography (μCT) with a nominal resolution of 0.5 μm (see fig. above). In a following step, the 3D surface geometry was processed and the surface mesh was imported into AVL and used to generate the 3D calculation mesh for the GDL substrate pores. The GDL fibres have an average diameter of approximately 7-10 μm, the considered section size is 500 μm at a GDL thickness of 200 μm. The resulting computational mesh has about 10 million elements.

It was demonstrated that mass transfer on the microscopic level of real GDL pores does not take place uniformly and that there are preferred transport routes that depend on the GDL material. Using these results, transport parameters for standard CFD programs can be determined, thereby increasing the accuracy of models.