(435h) Modeling of the Evaporation of the Polymer Slurry in the Porous Media

The polymer electrolyte fuel cell (PEFC) is a potential power resource for the new-generation energy society. However, it takes too high costs and has too low output power density to apply the common use at the present. In order to breakthrough these technical issues, it is not only to develop effective materials, but also important to optimize mechanical characteristics of component parts. PEFC has the electrode with the pours structure made of the chained particles of carbon and the platinum (Pt) catalyst, which is called the catalyst layer (CL). CL is a key component to improve the PEFC. The reason the electrode is porous structure is to increase the surface area to obtain the high power density. In addition, to enhance the proton conductivity on the CL, the chained particles of the carbon are coated with the ionomer which is used as a proton conductive polymer. In the fabrication process of CL, it is expected that ionomer is mixed well with carbon in CL slurry and that ionomer is covered on the carbon by evaporation of solvent. However, from the direct observation of actual CL, it is reported that ionomer makes isolated large pores and that it causes to induce the decrement of the effective surface area in CL. Thus, the formation of these heterogeneous porous structure should be avoided to create PEFC with the high output power density.
Although present experiments suggest that the migration of the polymer produces isolated pores, its dominant factor has not been obvious yet. Since it can be difficult to make factors of the migration obvious experimentally, the numerical calculation is appropriate to use.
As for the present numerical models for the transport phenomena in the sub-micro-scale porous media, almost of them are only considered of the mass and momentum transport without phase transition. However, the model including phase transition is required in this study because the coating process has the evaporation of the slurry. Therefore, the numerical model is proposed to estimate the permeation and evaporation process of the polymer slurry in porous media. The proposed model considers of the mass, momentum and energy transport. The distribution of the slurry is determined by the mass transport equation and it is treated as the time advance of the local volumetric fraction. The flow field is determined by the momentum transport, which is based on the Darcy’s law in the porous media. The phase transition is determined by the energy equation and the fixed polymer is treated as the high viscosity fluid.
Finally, the calculation result is compared with the experiment and the validation of the proposed model is conducted.