Analytical Modeling of PEM Fuel Cell Gas Diffusion Layers Deformation under Compression: Part 2 - Nonlinear 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. GDL mechanical behaviour can be divided into two 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. Using assumption of existing gap between the fibers in GDL micro-structure, the analytical model proposed in first part of this study for linear region, based on the unit cell approach, is extended to predict the nonlinear mechanical behaviour of GDL. The present unit cell model covers salient microstructural parameters and properties of the fibrous porous medium including: carbon fiber diameter, elastic modulus, pore size distribution, and gap size distribution between fibers. Bending of carbon fibers and closing the gaps between fibers are proved to be the main deformation mechanism of GDL deformation in nonlinear region. The statistical parameters of the gap distribution between carbon fibers in the GDL structure are calculated using experimental stress-strain data 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. In addition to the mechanical behaviour, the proposed model provides useful information about the microstructural properties of the GDL during compression such as gap distribution between fibers that can be used in the GDL transport properties prediction.

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