Gas diffusion layer (GDL), which consists of a macro porous substrate (GDBL, gas diffusion backing layer) and micro porous layer (MPL), is the crucial component which is closely related with water management in PEMFC due to its basic function that is to drain out liquid water to catalyst layer from flow channels. The MPL, which is composed of carbon powder and a polymer binder, such as polytetrafluoroethylene (PTFE), is a hydrophobic layer that contains several micro pores, and these micro pores serve to improve the water removal ability. Thus, MPL has been recently been used to solve the mass transport problems in PEMFCs, which are caused by water flooding. In this study, a MPL was modified to improve the proton exchange membrane fuel cell (PEMFC) performance under low humidification condition.
We developed a novel hydrophobic/hydrophilic double micro porous layer (MPL) that was coated on a gas diffusion backing layer (GDBL). The carbon slurries for MPL were prepared using the following procedures. The carbon black (Vulcan XC-72, Cabot) was dispersed in a mixture of isopropyl alcohol and deionized water (80:20 volume ratio) using ultrasonication and mechanical stirring and then the polymer binder was added. The hydrophobicity of the MPL was controlled by changing the polymer binder. PTFE was used to fabricate the hydrophobic MPL, while another fluorinated polymer, which had a similar structure to PTFE but contained less fluorine, was used to fabricate the hydrophilic MPL. The hydrophobic/hydrophilic double MPLs (PA2ADL2 and PA2ADL3) were prepared in two steps. First, the hydrophobic (or hydrophilic) MPL slurry was applied onto the wet-proofed carbon fiber paper using screen printing technique. This was followed by a second coating with the hydrophilic (or hydrophobic) MPL slurry. The hydrophilic MPL was located on the surface of PA2ADL2 and in the middle of PA2ADL3. For comparison purposes, a single hydrophobic MPL (PA2AML1) was prepared.
The contact angle of the MPL surface was measured using the Sessile drop method (Phoenix 300, SEO Co.). The water and vapor permeability of GDL was characterized using a home built system. A single cell test was carried out using a 25 cm2 active area test cell in humidified H2 / air gases under low humidification conditions, R.H. 50%.
The best cell performance was obtained from the PA2ADL3, and PA2ADL1 resulted in the worst cell performance. In other words, both PA2ADL2 and PA2ADL3 containing hydrophilic MPL showed the better cell performance than PA2AML1, which contained a single hydrophobic MPL. We identified that PA2ADL2 and PA2ADL3 containing a hydrophilic MPL, retained a much larger amount of water than PA2AML1 through measuring water and vapor permeability of GDLs. This result demonstrated that the hydrophilic MPL had the ability to absorb significant amounts of water, like a sponge. PA2ADL3 had a hydrophobic MPL on its surface and a hydrophilic MPL in the middle of the GDL. The hydrophobic MPL surface prevented water flooding and the hydrophilic MPL, which was located inside of the GDL, absorbed water to prevent dehydration of the polymer electrolyte membrane. The polymer electrolyte membrane has to be hydrated for protons to be conducted from the anode to cathode. Under the low humidification condition, the polymer electrolyte membrane can be easily dehydrated, resulting in a decrease in cell performance. As the amount of water generated at the cathode catalyst layer decreases due to low cell performance, dehydration of the polymer electrolyte membrane can be accelerated. In the cathode GDL, air must diffuse from the flow channel to the catalyst layer through the MPL. If the MPL retains water, the air will be the more humidified while diffusing through the MPL. The hydrophilic MPL that was located in the middle of the GDL acted as an internal humidifier. Therefore, PA2ADL3 containing a hydrophilic MPL between the hydrophobic MPL and GDBL showed the best cell performance. In contrast, PA2ADL2 had a hydrophilic MPL on the surface; thus, water produced at the cathode catalyst layer can be easily stagnant between the cathode catalyst layer and surface of the PA2ADL2 when the cell was operated under the low humidification condition. Consequently, the stagnant water could cover up the active site in the catalyst layer, which was the reason why the performance of PA2ADL2 was lower than PA2ADL3