Relative Diffusivity of Partially Saturated GDLs

Gas diffusivity in GDLs of PEM fuel cells is significantly affected by water content, as water blocks pores and reduces gas flow. This effect increases at higher current densities due to more water production. However, few studies in the literature have experimentally measured gas diffusivity in partially saturated GDLs, primarily due to the challenges of controlling GDL saturation during measurement. Existing studies reported an exponential decay in diffusivity as saturation increases. However, the rate of diffusivity reduction differs between studies. One study suggests that the higher reduction in diffusivity observed in other papers is due to the GDL stacking to increase resistance to gas flow which causes the formation of a thin film of water at the interface between the stacked GDLs during the saturation process, obstructing the diffusion pathway and leading to inaccurate measurements.
In this study, we have developed a new ex-situ method using the symmetrical modified Loschmidt cell to measure gas diffusivity in partially saturated GDLs. This method ensures no water film between the layers, providing more accurate diffusivity measurements. The technique has been applied to two commercially available GDL samples (Toray and AvCarb) under different saturation levels (defined as water volume over the total pore volume of the GDL). Our results indicate a significant decline in relative gas diffusivity as the saturation level increases. Moreover, all GDLs show very low relative diffusivity at saturation levels above 0.3, explaining the deterioration in fuel cell performance under flooding conditions.