Gas Diffusion Layer

In polymer electrolyte membrane fuel cells (PEMFC) gas diffusion layers (GDL) – in combination with bipolar plates – are usually employed to distribute the reactants over the electrode surfaces. The GDL has not only the function to provide gas access to the catalyst layer, but also to allow the removal of the product water, to mechanically stabilize the membrane-electrode assembly and to provide electronic and finally also thermal conductivity between catalyst layer and bipolar plate. In PEMFC, usually porous carbon fiber papers or cloths infiltrated with polytetrafluoroethylene (PTFE, also denoted as Teflon) are used as gas diffusion layers to improve the GDL's ability to remove product water.

The image on the right shows an example of a carbon cloth based GDL. It represents a cross section of a 3D dataset acquired by X-ray computed tomography. One fiber bundle runs approximately from left to right, while two more bundles - that run perpendicular to the image plane - are cut approximately perpendicular to the fiber bundle axis.

 

Typical fiber diameters are between 5 and 10 um, typical porosities of GDLs are between 70 and 90 %. Instead of carbon cloths, also carbon papers are employed as gas diffusion layers (one example shown in the left image). Here, a 3D visualization of the 3D acquired again by X-ray computed tomography is shown. In a carbon paper based GDL, the fibers run approximately straight and are parallel to the plane defined by the carbon paper.

Quite frequently, gas diffusion layers are investigated by scanning electron microscopy which is a standard microscopy technique (not shown here). This technique allows not only the investigation of GDLs with much higher resolution (in the nm-regime), but also the discrimination of PTFE from the carbon fibers which is difficult in X-ray computed tomography.

Images published in: A. Pfrang, D. Veyret, G. Tsotridis, Investigation of gas diffusion layers of PEM fuel cells by x-ray computed tomography, Ulm ElectroChemical Talks 2010, Ulm, Germany, June 2010