Article available under the terms of the CC-BY-NC-ND licence (https://creativecommons.org/licenses/by-nc-nd/4.0/) eprints@whiterose. AbstractKnowledge of the absolute permeability for the various porous layers is necessary to obtain accurate profiles for water saturation within the membrane electrode assembly (MEA) in a twophase model of a polymer electrolyte membrane fuel cell (PEMFC). In this paper, the gas permeability of gas diffusion layers (GDLS) coated with micro-porous layers (MPLs) of various carbon loadings for two different carbon blacks have been experimentally measured.The permeability of the GDL was found to decrease by at least one order of magnitude after the MPL-coating. Also, the permeability of the MPLs was shown to be lower than that of the carbon substrate by 2-3 orders of magnitude. Further, it was found that the gas permeability of the MPLs changes significantly from one carbon loading to another despite the use of a single weight composition for all the MPLs coated, namely 20% PTFE and 80% carbon black. This signifies the possible inaccuracy in estimating the MPL permeability through employing the cross-section SEM images as they do not resolve the MPL penetration into the carbon substrate.Finally, the MPL sintering was found to slightly decrease the permeability of the GDL.

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In an attempt to obtain accurate values of the gas permeability of the microporous layer (MPL), substrates with negligible MPL penetration and of known gas permeability, i.e. membrane filters, have been employed. The values of the MPL permeability obtained using the membrane filters were compared with those obtained conventionally using the carbon substrates. Due to MPL penetration, the MPL permeability obtained using the carbon substrate were found to decrease with carbon loading. On the other, due to negligible penetration, the MPL permeability obtained using the membrane filters were found to be almost invariant with the carbon loadings. Furthermore, the MPL permeability was found to be sensitive to the substrate used: more cracks (and subsequently substantially higher permeability) were shown by the MPLs coating the carbon substrates. This implies that the MPLs coating the carbon substrates and the MPLs coating the membrane filters are structure-wise different. It subsequently means that the MPL permeability obtained using the membrane filters cannot be used to estimate the MPL penetration into the carbon substrates.