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The effective diffusivity of gaseous species in partially-saturated finite-size porous media is a valuable parameter for mathematical modeling of many processes, but it is difficult to measure experimentally. In this work, the effective diffusivity of carbon-fiber gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs) was determined by performing lattice Boltzmann (LB) simulations on X-ray tomographic reconstructions of invading water configurations. Calculations on dry GDLs were in close agreement with previous experimental data; the effective diffusivity was reduced by the addition of PTFE due to the loss of pore volume and the higher tortuosity of transport paths. The effect of water saturation was significantly larger. It was found that the resistance of water to gas transport was extremely dependent on the saturation distribution through the porous medium, particularly the peak saturation, and not just the average saturation as is typically considered in the literature. [...

In proton-exchange-membrane fuel cells, flooding of the cathode gas-diffusion layer (GDL) hinders the gaseous reactant transport and thereby limits cell performance. The understanding of the effective diffusivity of the reactants through the GDL is essential for performance optimization and material design. In this paper, the effective diffusivities of unsaturated and partially-saturated GDLs are experimentally examined using an ex-situ electrochemical limiting-current method for various, uncompressed GDLs including different PTFE loadings. For unsaturated (including PTFE loadings) and partially-saturated (no PTFE) GDLs, the experimental results follow a power law with respect to porosity and saturation, respectively. PTFE treatment favorably changes the liquid distribution for improved gas-transport pathways, and a new correlation is proposed using a cumulative log-normal distribution function; however, the impact of PTFE on the overall effective diffusivity depends on the specific GDL structure. This work provides insights for fuel-cell models and transport phenomena, which can lead to the optimal GDL material design and cell operation.

X-ray computed tomography was used to investigate geometrical land and channel effects on spatial liquidwater distribution in gas-diffusion layers (GDLs) of polymer-electrolyte fuel cells under different levels of compression. At low compression, a uniform liquid-water front was observed due to water redistribution and uniform porosity; however, at high compression, the water predominantly advanced at locations under the channel for higher liquid pressures. At low compression, no apparent correlation between the spatial liquid water and porosity distributions was observed, whereas at high compression, a strong correlation was shown, indicating a potential for smart GDL architecture design with modulated porosity.

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. [...