GDLs are typically designed as a bilayer structure consisting of a macro-porous backing material (carbon fiber paper support) and a micro-porous, carbon-based layer (MPL). The fibrous backing material governs the mechanical properties of the GDL (behavior upon compression, bending and shear strength, etc.) while the MPL ensures intimate contact to the catalyst layers, protects the delicate proton exchange membrane against perforation and plays an active role with respect to the water management in the cell during operation. There is consensus that this heterogeneous porosity brought about by this structure (hydrophilic/hydrophobic and various pore sizes) is advantageous for the performance.
Hydrophobic properties in the backing and the MPL are maintained by adding defined amounts of polytetrafluoroethylene (PTFE) to both sublayers. Various types of carbon particles (carbon blacks, graphite) can be used in the MPL in order to produce different levels of hydrophobicity. Furthermore, the MPL can be used as substrate to deposit catalyst particles for the manufacture of gas diffusion electrodes (GDEs).
SGL Carbon has been producing fully-treated SIGRACET® gas diffusion layers by reel-to-reel processes since 1999. Carbon paper-type (prepared by wet-laying of chopped PAN-based carbon fibers) gas diffusion layers are the preferred solution since they can be manufactured at high volumes (scalability) and low thickness. The following figures show the whole value chain of GDL manufacturing. All commercially available GDL materials to date are based on carbon fibers derived from polyacrylonitrile (PAN). PAN (co)polymers are processed into precursor fibers by wet-spinning. Subsequent stabilization and pyrolysis yields high tensile (HT) carbon fibers which are sized and chopped to enable suitable processing by means of papermaking technologies.
A primary carbon fiber web is laid by a papermaking technology and subsequent thermobonding. Thereafter, the obtained raw paper is impregnated with carbonisable thermoset resins (with optional addition of carbon fillers), cured and re-carbonized/graphitized. This serves to enhance the mechanical stability and conductivity as well as to adjust the desired porosity level.
Finishing of GDL comprises hydrophobic treatment of the substrate with PTFE and coating with a microporous layer (MPL).
A loading of the substrate with 5% (w/w) PTFE has proven to be sufficient for obtaining a pronounced hydrophobicity. MPLs typically contain 20 to 25% PTFE. This MPL composition has been identified as the optimum ratio for PEMFC performance across a broad range of operating conditions. Mean pore diameters of SIGRACET® GDLs are typically in a range from 0.1 to 0.3 µm (Hg-Porosimetry) or 1.5 to 3 µm (calculated from capillary flow porometry). The hydrophobic treatment produces water repellent properties for the substrate and for the MPL (water contact angles by sessile drop method > 130°).
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