Mechanical Interactions between Gas Diffusion Layers and Bipolar Plates in low Temperature Fuel Cells

Kurzfassung

In contrast to stiff backing materials (e.g. carbon paper) softer ones like carbon cloth are compressed over the ribs of the gas distributors or impressed into the channels when the PEFC is assembled. During fuel cell optimisation the interactions between the gas diffusion layer and the flow field are frequently neglected; hence flow fields as well as gas diffusion layers are commonly optimized independently. The DLR has investigated these interactions with a two-stage approach: At first the basic influence of the mechanical compression was analysed by measuring the generated current and the local current density distribution in the potentiostatic mode as a function of the contact pressure. These investigations were carried out with a segmented laboratory cell with an active area of 25 cm2. The overall performance as well as the local current density distribution shows a strong dependency on the compression state of the cell. Thereby, an optimal contact pressure exists, where the influences of the contact resistance and the mass transfer are minimized. The impression depth of the gas diffusion layer into the flow field channels was determined as a function of contact pressure by two different methods: optically and electronically. For this purpose an appropriate device was developed that consists of a copper plate with spark-eroded channels, ridges of acrylic glass therein and the gas diffusion layer on the ribs. While the outer appearance of this device is equal to the gas distributor of the fuel cell, this set-up acts as a parallel connection of different capacities. Accordingly, the depth of impression in the gas diffusion layer can be determined by measuring the change of the electrical capacity if the material is pressed in the channel structures.