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Porosity and morphology of MPLs have a remarkable influence on the performance of PEFCs. However, until now, MPL porosity is largely undetermined for all commercial gas diffusion layers and the MPLs are commonly thought of as a homogeneous layer in thickness and porosity. Many model studies are based on this picture. However, these assumptions are not evidence-based, and can lead to erroneous interpretations of MPL functions. The challenge of having realistic models of MPLs lies in the lack of descriptions of MPL properties on the representative millimeter scale, while its pore structure is on the nanometer scale. Here, we use laboratory-based, X-ray tomographic microscopy (XTM) to determine the porosity, thickness and their spatial heterogeneity of microporous layers from 15 commercial GDL materials on the operation-relevant, millimeter scale. These properties are relevant to the diffusive transport through diffusion media (cf. Fick’s law). The porosity distribution is measured by quantifying the volume fraction of a saturating liquid, n-decane, in the fully saturated MPL with a large field of view (FoV) of >10 mm2 and a voxel edge length (determining the image resolution) of 3.6 μm. With optimized X-ray quantum noise control, regional average porosity differences >4.7% can be determined with a 95% confidence level (for every 50 μm 50 μm regions). This resolution in porosity determination allows to define and quantify the degree of spatial porosity heterogeneity for every MPL—a property of MPL that was previously unclear. Together with the MPL thickness heterogeneity (related to intrusion depth), the MPLs of the 15 commercially available gas diffusion layers from the three international manufactures Freudenberg, Sigracet® and CeTech, are characterized and classified into 5 groups. Contrary to the common homogeneous assumption, the majority of characterized MPLs (especially from Sigracet® and CeTech) actually come with various degrees of porosity and thickness heterogeneity. These differences among MPLs could result in very different water management properties of the respective GDL materials. The representative descriptions of MPL porosity, porosity heterogeneity and thickness heterogeneity also provides realistic input to modeling. Figure 1