In a stack assembly cells are clamped together and therefore mechanical interactions are induced into the stack. The mechanical behaviour has a strong influence on the performance. For example the gas diffusion layer (GDL) is a porous layer which will lose its porosity if the pressure level is too high, whereas the contact area between GDL and flow field needs high pressure for a low contact resistance [1].

In this study experiments are performed to investigate the mechanical behavior of different GDL materials in a Polymer Electrolyte Membrane (PEM) Fuel Cell. The relationship between pressure on the active area and the performance of a single cell is determined. For this purpose, a test cell from Baltic Fuel Cells GmbH, Germany, is used, which allows to change the pressure level during fuel cell operation and to record performance curves. Thus, the optimum operating point of the cell can be determined. To distinguish which mechanical effects have an influence, ohmic resistance [2] and permeability measurements depending on the mechanical load on the GDLs were performed. To analyze the mechanical structure scanning electron microscope (SEM) pictures were taken from the different GDLs.

The test results regarding the optimum pressure level show that the performance decreases strongly at pressures below 0.6 N/mm². This is explained by the increasing contact resistance between the individual components. At higher pressure levels the expected performance optimum is achieved for some GDL materials. The SEM pictures reveal that the fibres took mechanical damages starting from dents up to GDL areas with a high amount of broken fibres. But some materials show a different behaviour from that. For example a pressure level above 1 N/mm² results in a constant performance plateau, an increase of the pressure up to 3 N/mm² does not increase or decrease the performance anymore. The contact resistance is no longer noticeably reduced at these pressures, while even at the maximum pressure of 3 N/mm², enough gas passes through the porous GDL to the membrane, so the porosity is still sufficient.

These results show that the mechanical behavior influences the characteristics of the GDL and the performance of the fuel cell. A minimum pressure of 1 N/mm² on the entire active area should be ensured for the used GDL types for an optimum performance of a PEM fuel cell, whereas the maximum clamping pressure depends strongly on the type of GDL which is used.

1. D. Qiu, P. Yi, L. Peng, X. Lai; Assembly design of proton exchange membrane fuel cell stack with stamped metallic bipolar plates; International Journal of hydrogen energy 40 (2015) 11559-11568
2. D. Qiu, H. Janßen, L. Peng, P. Irmscher, X. Lai, W. Lehnert; Electrical resistance and microstructure of typical gas diffusion layers for the proton exchange membrane fuel cell under compression; (2018) submitted