(345g) Preparation and Characterization of Microporous Layers for Gas Diffusion Media

The gas diffusion layers (GDLs) in PEMFC typically adopt a dual-layer structure which consists of a macroporous layer of carbon fiber paper and a microporous layer of carbon black powder and a hydrophobic agent. The latter is needed to provide proper pore sizes and to enhance an intimate electronic contact between the catalyst layer and the macroporous carbon layer [1]. In the present work, the GDL with a dual-layer structure was developed which enhances the catalyst utilization and the overall fuel cell performance. Critical parameters such as the electronic conductivity, the gas permeability and the pore size distribution (PSD) were optimized by optimizing the solvent composition, the PTFE content and the carbon loading in the microporous layer. The PTFE content and the carbon loading in the microporous layer were optimized to be 20 wt% and 0.5 mg cm^-2, respectively. It was shown that high PTFE contents result in a decrease in the porosity, which increases the oxygen transport resistance, while low levels of PTFE result in a flooding at the electrode interface, due to poor water removal from the catalyst layer. The mercury porosimetry and polarization studies indicated that the transport of gaseous oxygen and water is highly dependent on the micropore volume and pore size distribution in the GDL. The ac-impedance study suggested that the PEMFC performance is controlled by the oxygen reduction kinetics in the catalyst layer as well as the oxygen diffusion kinetics in the GDL, and consequently, the GDL with 0.5 mg cm^-2 carbon loading leads to the best performance due to better transport of oxygen and water through the GDL.

[1] M.V. Williams, E. Begg, L. Bonville, H. R. Kunz, J. M. Fenton, J. Electrochem Soc. 151 (2004) A1173.