Water management in proton-exchange membrane fuel cells (PEFCs) has a large impact on the performance of the device, as liquid water affects the transport properties of the gas diffusion layer (GDL). In this study, we develop an ensemble-based model of the liquid water distribution inside the GDL. Based on a water injection experiment, the wet structure of the porous medium is inspected via X-ray tomographic microscopy and, after an image segmentation process, a voxel-based meshing of the fiber, air, and water domains is obtained. Starting from the obtained dry fiber structure, a Metropolis-Hastings Monte Carlo algorithm is used to obtain the equilibrium distribution of liquid water that minimizes the surface free energy of the ensemble. The different water distributions from the Monte Carlo (MC) simulation and water injection experiment are identified as solution for different physical mechanisms both of which are present in a running fuel cell. The wet structure is then used to calculate saturation-dependent effective transport properties using the software geodict. Thereby, a strong influence of the saturation gradient on the macrohomogeneous transport properties is found.

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