The gas tightness of bipolar plates and also of sealings is a very important property for the construction and operation of fuel cell stacks. Permeating hydrogen through bipolar plates or sealings into the environment causes security risks and is not involved in the desired electrochemical reaction. In order to quantify the permeation rate, a special test rig has been developed, which is able to analyze unstructured solids in variable thicknesses with respect to their hydrogen- specific gas permeability. The apparatus is constructed and able to measure any samples size with a minimum diameter of 30 mm.
The following components are appropriate as test objects:
The construction of the test rig focusses on the lowest gas leakage rates , which are based on the mechanism of permeation and/or diffusion. One side of the sample is applied with hydrogen (H2) and after a defined period of time the hydrogen concentration is measured in the opposite chamber with a semiconductor sensor. The determined concentrations in ppm can be converted into common units of permeation / leakage.
The figure shows one exemplary study of untreated test specimens of a compound material for bipolar plates: Up to a measuring time of 25 min. and 40 ppm H2 a linear relation can be identified. Values with a large distance to the calibration point of 10 ppm causes impreciseness.
The consideration of the results near to the calibration point delivers permeation rates of 4-5 * 10E-7 mbar * l / s which is by definition in the field of gas leakage or virus tightness. That meets the requirements of fuel cells.
Parts of the work were realized during the project "development of lifetime prediction models of fuel cells in practible applications”. The project 03ET2007A (01 /2011 - 06/ 2013) was funded by the Federal Ministry of Economics (BMWi).