The through-plane permeability has been experimentally measured for untreated and PTFE-treated gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells. Contrary to what the literature has previously shown, the results suggest that there exists an optimum value for the wet-proofing agent, PTFE, at which the though-plane permeability of the GDL is a maximum. The analysis that is undertaken to investigate the effect of the air compressibility shows that there is an underestimation in the through-plane permeability of the tested GDLs by of the order of 9% if the density of the air is assumed to be constant. Also, a non-dimensionlisation analysis shows that ignoring the non-Darcy effects at the maximum reported flow rates results in negligible errors for the untreated and treated GDLs.

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