| Proton exchange membrane fuel cells have emerged as one of the leaders for the replacement of fossil fuel powered internal combustion engines. Water removal from the cell is one of the top concerns regarding fuel cell performance for transportation applications. During lower power output or high temperature operation, water removal in the vapor phase can dominate. The rate of water vapor diffusion through the porous cathode gas diffusion layer (GDL) of the fuel cell is limited by the porosity and tortuosity formed by the solid fiber matrix. In this work an experimental apparatus is designed to measure the rate of water vapor diffusion across the GDL to determine an effective diffusion coefficient. The effects of microporous layer (MPL) coating, GDL thickness, and polytetrafluoroethylene (PTFE) loading on the diffusion coefficient is demonstrated. Commercially available diffusion media are tested and include Mitsubishi Rayon Corp. Grafil U-105 series, SGL SigracetRTM 25, 35, and 10 series, and Toray TGP-H-120 series.;Standard corrections, such as the Bruggeman correction, used in fuel cell literature are found to overpredict the effective diffusion coefficient for the GDL. The MPL was found to produce a significant resistance to water vapor diffusion due to its smaller pore diameters, lower porosity, and an increase in tortuosity. The GDL Grafil U-105 A produced a higher effective diffusion coefficient of 0.070 cm2/s compared to the SGL 25BC value of 0.063 cm2/s. Confocal scanning laser microscope images indicated that the MPL for the Grafil U-105 A sample is possibly thinner, thus explaining some of the reduction in diffusion resistance. Thickness was found to have no influence on the effective diffusion coefficient for samples without MPL. PTFE causes a rapid decrease in effective diffusion coefficient from 0.095 cm2/s for TGP-H-120 0% PTFE to 0.024 cm2/s for TGP-H-120 40% PTFE. Comparison to other studies from the literature show good agreement with the present work thus validating the dynamic method for use in diffusion coefficient measurements in fuel cell diffusion media. |
