Abstract
An understanding of the GDL properties is crucial for high-current-density operation of proton exchange membrane fuel cells (PEMFCs). The parameters porosity ( ε ) and tortuosity ( τ ) directly link the theoretical diffusivity in free space and the effective diffusivity in the structure. The τ / ε -ratio is therefore an important descriptor for the gas diffusion in a porous network. This study characterizes the τ / ε -ratio for gas diffusion layer substrate (GDL-S) materials from two suppliers (Toray, Freudenberg) and for one microporous layer (MPL) using limiting current measurements in an operating fuel cell. However, only the application of a new analysis method that considers the contribution from Knudsen diffusion allows for the τ / ε -determination of GDL-S materials which have carbon black added to the structure (e.g., Freudenberg GDL-S); this method can then also be used for the determination of the τ / ε -ratio of porous networks containing small pores like MPLs. Here, we assess the contribution of pressure-independent Knudsen diffusion to the overall diffusion and introduce a mixed diffusion coefficient, which includes both Knudsen and molecular diffusion. This analysis serves as a guideline to estimate the contribution of the respective diffusive processes for gas diffusion materials utilized in PEMFCs, where pore sizes resulting in different diffusion regimes are present.
Published Version
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