OATAO is an open access repository that collects the work of some Toulouse researchers and makes it freely available over the web where possible.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. Abstract This study builds upon previous work on single-layer invasion percolation in thin layers to incorporate a second layer with significantly different pore sizes and to study the impact of the resulting water configuration on gas-phase mass transport. We consider a situation where liquid water is injected at the assembly inlet through a series of independent injection points. The challenge is to ensure the transport of the liquid water while maintaining a good diffusive transport within the gas phase. The beneficial impact of the fine layer on the gas diffusion transport is shown. It is further shown that there exists a narrow range of fine layer thicknesses optimizing the gas transport. The results are discussed in relation with the water management issue in polymer electrolyte membrane fuel cells. Additional discussions, of more general interest in the context of thin porous system, are also offered.

Experimental results based on in-situ measurements at the interface between the catalyst layer and the gas diffusion layer (GDL) on the cathode side at the channel e rib scale show an interesting variation of the current density distribution as the mean current density is increased. It is found that the local current density below the rib median axis corresponds to a maximum at low to intermediate mean current densities and to a minimum when the mean current density is sufficiently high. Also, the higher is the current density, the more marked the minimum. From numerical simulations, it is shown that the current density distribution inversion phenomenon is strongly correlated to the liquid water zone development within the GDL.

PEMFC (Proton Exchange Membrane Fuel Cells) is considered as a promising solution for producing clean energy. To increase its industrial development for automotive application, performance and durability are still to be improved and cost needs to be reduced. Water management is one of the most important remaining bottlenecks to increase performance and durability of PEMFC. Nevertheless, despite numerous studies, many questions remain open and for instance it is not possible yet to propose reliable optimization of the porous layers (Gas Diffusion Layer, Active Layer…) to optimize transfers (and especially water management) in the MEA and thus increase performance and durability. One main reason is that water transfers in the MEA, and also the link between structural properties of these layers and performance, are not correctly described and understood. For instance, classical modeling used in PEMFC do not account for these structural properties despite the fact that they have a huge influence on performance. More descriptive modeling approaches have been proposed in the last years to better understand the link between structural properties of these layers and transfers in the MEA. Amongst those, Pore Network Modeling (PNM) is more and more applied to GDL. This approach, classically used for soil application, allows simulating two-phase flows in porous media taking into account local properties (Pore/Throat Size Distribution, local structure, local wettability…) which is an important progress compared to classical PEMFC modeling based on Darcy approach. This keynote lecture focuses on PNM and its application to GDL to better understand (gas and liquid) transfers in the MEA and to try and link local structural properties of GDL to the performance of PEMFC. Experimental results are presented all along the discussion either to validate the main hypothesis or to validate the PNM simulations. After a description of the PNM approach, two-phase transfers inside GDL are discussed: influence of wettability, thickness, local structure…; study of GDL degradation; role of the Micro Porous Layer (MPL); validity of classical Darçy “laws”… The two classical scenarii of water transfers in the GDL (liquid injection and/or condensation) are also discussed. The coupling between PNM and performance model, which is a step towards upscaling of PNM, is introduced by the analysis of performance loss of PEMFC as a function of hydrophobicity loss of GDL. Advanced results comparing two-phase PNM based on real 3D images of GDL and X-Ray tomography of liquid pattern inside the GDL are also presented as a step towards even more representative PNM modeling of MEA components. Finally it is shown how PNM can be extended to the Active Layers by coupling two-phase transfers to charge transfers by electrochemistry.