Modeling Capillary Transport and Gas Diffusion in Carbon-Fiber Papers Using a Hybrid CFD Formulation

Abstract

Improved modeling of the membrane electrode assembly (MEA) and operation is essential to optimize proton exchange fuel cells (PEFCs). In this work, a hybrid model, which includes a pore network formulation to describe water capillary transport and a continuum formulation to describe gas diffusion, is presented. The model is validated with previous data of carbon-paper gas diffusion layers (GDL), including capillary pressure curve, relative effective diffusivity, g(s), and saturation profile. The model adequately captures the increase of capillary pressure with compression, the nearly cubic dependency of g(s) on average saturation, s^{\rm avg}, and the shape of the saturation profile in conditions dominated by capillary fingering (e.g., running PEFC at low temperature). Subsequently, an analysis is presented in terms of the area fraction of water at the inlet and the outlet of the GDL, A_w^{in} and A_w^{out}, respectively. The results show that gas diffusion is severely hindered when A_w^{in} is exceedingly high (>80%), a situation that can arise due to the bottleneck created by flooded interfacial gaps. Furthermore, it is found that s^{avg} increases with A_w^{out}, reducing the GDL effective diffusivity. Overall, the work shows the importance of an appropriate design of MEA porous media and interfaces in PEFCs.