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.