Analytical Modeling of PEM Fuel Cell Gas Diffusion Layers Deformation under Compression: Part 1 - Linear Behaviour Region

In the Proton exchange membrane PEM fuel cell stack, transport characteristics of the porous gas diffusion layer (GDL) change due to the GDL microstructural parameters variation in a result of compressive loading during the fuel cell assembly and operation. The GDL is a carbon fiber based highly porous media in the form of paper or cloth. In addition to the reactants and reactions products transportation, GDL provides mechanical support for the membrane assembly against the compressive loads imposed by bipolar plates. According to the results of compression tests, GDL mechanical behaviour can be divided into regions; nonlinear and linear. In the compressive loads smaller than a critical pressure, GDL has a nonlinear mechanical behaviour and its compressive modulus is a function of deformation level. After reaching the critical pressure, the compressive strain increases linearly with the applied stress. Using unit cell approach and assuming a simplified geometry for the complex and random microstructure of the GDL, a new mechanistic model is developed that can accurately predict the mechanical behaviour of GDL. The unit cell model covers salient microstructural parameters and properties of the fibrous porous medium including: carbon fiber diameter, elastic modulus, and pore size distribution. Bending of carbon fibers is proved to be the main deformation mechanism of GDL deformation in linear region. A comprehensive optical measurement study with statistical analysis was performed to determine the effective geometrical parameters of the model for a number of commercially available GDL samples. A comparison between the present model and the GDL stress-strain data shows that the model can predict accurately the mechanical behaviour of the GDL material. The nonlinear behaviour of GDL will be addressed in the second part of this study.

Keywords: Gas Diffusion Layer, Porous media, Fuel cell, analytical modeling, mechanical behaviour, compression