Gas Diffusion Media with Optimized Water Transport
Authors: Manzi-Orezzoli Victoria , Siegwart Muriel , Schmidt Thomas J. , Boillat Pierre 
Published: February 2020

The water management in polymer electrolyte fuel cells (PEFC's) needs to be properly balanced to maximize the power output of the fuel cell. The polymer electrolyte membrane requires to be hydrated in order to be proton conductive. However, since the water transport and gas reactant transport occurs through the same media, the so called gas diffusion media (GDM), accumulation of water in the GDM can block the access of reactant gases to the catalyst limiting the performance of the cell.

The GDM is usually composed of two layers: the gas diffusion layer (GDL) and the microporous layer (MPL). In the majority of cases the GDL consist of a highly porous structure made of carbon fibers coated with a fluoropolymer to enhance its hydrophobicity. The GDL has a porosity of around 80% and a mean pore size diameter of 20 µm. The MPL, on the other hand, is formed by carbon particles and fluoropolymer. The porosity is around 50% and the mean pore size is for most MPL types around 50 nm [1].

Most of the optimization strategies for water management comprise modification on the GDM. Several groups have developed techniques aiming at increasing the water permeability of the GDM locally in order to improve water transport while keeping free pathways for the reactant gases [2]. Our group in particular has developed such a material based on a chemical modification of the GDL coating. We use radiation grafting to selectively alter regions on the GDL, generating a GDL with a hydrophilic and hydrophobic pattern [3].

Our previous studies show that the incorporation of the hydrophilic regions to the GDL improves the cell performance in comparison with the base material. We have also studied the incorporation of a self-standing MPL to the patterned GDL, see Figure 1 a). In this case the overall performance of both the base and the patterned GDL increase and the patterned GDL still shows better performance than the base GDL. Nevertheless, when using an MPL the effectiveness of the pattern in the GDL is reduced due to the difference in water injection caused by the presence of the MPL [4]. The MPL requires a much higher capillary pressure for the water to permeate through, therefore any special feature such as cracks, holes or regions with a lower thickness define the injection points into the GDL. Once the water reaches the GDL it will preferentially be transported through the hydrophilic areas, but since it had previously gone through the MPL which imposes a higher capillary pressure barrier the effectiveness of the pattern is limited..

To resolve this issue, we created GDM's with MPL's with chemically modified MPL fluoropolymer binder in the hydrophilic regions of the GDL (see Figure 1 b) and locally increased porosity in regions of the hydrophilic pattern in the GDL (see Figure 1 c).

Here, we will present our synthetic methods and the material characterization, as well as the influence of the modifications on water distribution and cell performance.

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Figure 1