Modelling of the flow in the Gas Diffusion Layer (GDL) of fuel cells
The focus of the established joint research between the IAM headed by JARA-ENERGY scientist Prof. Bernd Markert and IEK-3 headed by JARA-ENERGY scientist Prof. Detlef Stolten is the study, development and optimization of porous materials as central components of all electrochemical energy conversion devices. This would contribute to cost reduction and performance improvement of these devices. To this end, electrochemical devices are mainly constructed of a porous layers assembly, subjected to mechanical loadings, which rely on coupled electrochemical reactions and transport processes to convert or store energy.
Therefore, the operation in these devices can be described as a mechano-electro-chemical process in porous materials, which can be optimized by applying numerical, theoretical, and experimental studies on micro- and macro-scales. In this context, this study aims to enhance the design of polymer electrolyte fuel cells (PEFC) through deep understanding of the functionality and optimization of the design of the porous gas diffusion layer (GDL), the main gateway of reactants from the gas channels to the catalyst layer of the PEFC. The GDL is a porous medium constructed of randomly distributed micro fibers and saturated, as mentioned before, by one or more types of fluids. The compression of the GDL leads on the microscopic level to deformation and re-orientation of the fibers and reduction of the transport capability through the micro channels, which can be studied and quantified using micro-CT images. With this, parameters like the volume fractions can be quantified directly at different compression levels from the post-processing of the CT-images, and the possible anisotropy of the flow channels can be observed. As a multiphasic porous material, the GDL can properly be modeled on a continuum-mechanical basis using the Theory of Porous Media (TPM), accounting for the fiber deformation, the fluid flow, the fluid pressure and the thermal exchange. In the macroscopic treatment, the geometry of the individual fibers and the structure of the micro-channels are disregarded, and instead, they are assumed to be statistically distributed over a representative volume element (RVE). Applying a homogenization process to the RVE, an averaged continuum model is obtained, in which each spatial point is permanently occupied by all constituents in the sense of superimposed and interacting continua. Additionally, phenomena like the deformation-dependent permeability can be included in the TPM model, which are implicitly based on the microscopic channel geometry and fluid properties.
Dr.-Ing. Yousef Heider, M. Sc. Mohamad Chaaban and Univ.-Prof. Dr.-Ing. Bernd Markert
Institut für Allgemeine Mechanik (IAM), RWTH Aachen