Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of local saturation and application to macroscopic continuum models

Abstract

Macroscopic continuum models are an essential tool to understand the complex transport phenomena that take place in gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs). Previous work has shown that macroscopic models require effective properties obtained under uniform saturation conditions to get a consistent physical formulation. This issue, mostly unappreciated in the open literature, is addressed in detail in this work. To this end, lattice Boltzmann simulations were performed on tomographic images of dry and water-invaded carbon-paper GDL subsamples with nearly uniform porosity and saturation distributions. The computed effective diffusivity shows an anisotropic dependence on local porosity similar to that reported for morphologically analogous GDLs. In contrast, the dependence on local saturation is rather isotropic, following a nearly quadratic power law. The capability of the local correlations to recover the layer-scale properties obtained from inhomogeneous GDLs is checked by global averaging. [...

Microstructure-property relationships in a gas diffusion layer (GDL) for Polymer Electrolyte Fuel Cells, Part I: effect of compression and anisotropy of dry GDL

“…In fact, our results for the SGL material are also close to the trend line described with e 2.5 . However, in contrast to the pronounced ip-tp anisotropy reported by Garcia-Salaberri for Toray [32,33], the anisotropy of our SGL material is much smaller.…”

“…But also with the T-S model the effective diffusivity of our GDL (SGL 25BA) is overestimated, which is compatible with the findings reported by Zamel and Li [5]. In recent studies by Garcia-Salaberri et al [32,33] it was shown that the gas diffusivity in GDL (Toray) follows the trends described by e 2.5 for ip and e 3.5 for tp directions. In fact, our results for the SGL material are also close to the trend line described with e 2.5 .…”

“…The changes of M-factors, diffusivities and microstructure components are well described by linear correlations with porosity, which is in contrast to exponential relationships that are usually given in literature for f(e) (see e.g. Bruggeman- [1] and TomadakisSotirchos-models [6], or more recent investigations [32,33]). Note: The difference between x-and y-directions is relatively small and therefore we consider the average of both values as being representative for the ip-direction.…”

“…D eff /D 0 ), which is a term that is often used in the PEFC community, e.g. [32,33]). A virtual materials testing (VMT) approach was applied in two previous studies in order to determine the M-factor empirically [23,24].…”

Versatile and efficient pore network extraction method using marker-based watershed segmentation

“…The SNOW algorithm was applied to an image of Toray 120 with no PTFE, used in previous studies [55,56]. This material has a porosity of 75% [57], a thickness of 360 microns, fiber diameter around 10 microns, and a permeability coefficient tensor [58] …”