Importance of Directed Water Removal: Intruding Microporous Layer Material into the Gas Diffusion Layer Substrate

Abstract

Proton exchange membrane fuel cells (PEMFC) are an essential component of net zero emission scenarios by the International Energy Agency (IEA), most prominent in the heavy-duty transportation sector.[1-2] During operation, the PEMFC is subject to different operating conditions, particularly wet conditions where liquid water removal is crucial. It was observed that a microporous layer (MPL), commonly consisting of a carbon component (e.g. carbon black, carbon fibers) and a hydrophobic binder (e.g. PTFE), placed at the interface of the catalyst layer (CL) and the gas diffusion layer substrate (GDL-S), has a large impact on the water removal properties. Among other advantages, the addition of an MPL can guide the localization of water clusters in the GDL.[3] However, if interfaces exhibit large interfacial gaps, as was demonstrated for the CL/MPL interface,[4] liquid water can accumulate in the large openings, thereby creating a mass transport barrier. Seeking to understand the importance of interfaces, which can either guide water formation or create obstacles, this study further investigates the MPL/GDL-S interface.We created an intruding MPL by pressing the MPL slurry into the GDL-S using mechanical force (further on referred to as “intruded-GDL”) and compared it to a GDL, where the MPL sits quasi on top (further on referred to as “sheet-GDL”). Figure 1a shows a cross-sectional scanning electron microscopy (SEM image) of the sheet-GDL, while Figure 1b shows an intruded-GDL. The boundaries between the MPL material and the GDL-S are marked in red. The MPL/GDL-S interface of the sheet-GDL is relatively flat, following only the surface contour of the GDL-S, while the MPL/GDL-S interface of the intruded-GDL is pressed into the GDL-S and locally penetrates deeper into the GDL-S. It is also visible that the penetration into the GDL-S is inhomogeneous. We characterized the altered morphology using SEM (see Figure 1), mercury intrusion porosimetry (MIP), and x-ray tomographic microscopy (XTM). We found that the MPL of the intruded-MPL intrudes significantly into the GDL-S and preferably fills the larger pores of the GDL-S.Both types of GDLs were subject to single-cell fuel cell testing under various operating conditions. We found that the intruding MPL poses an additional oxygen transport resistance at dry conditions compared to the sheet-GDL. Under conditions where liquid water formation can be expected, the intruded-GDL starts to have an advantage compared to the sheet-GDL. From the data obtained in this study, a delicate interplay between the additional dry transport resistance, the missing macropores that were filled with MPL material, and the guided water removal properties can be deduced. We can derive an improved water removal mechanism for the intruded-GDL as a cause of the structural changes, which can serve as a guideline to improve GDL design parameters.