Surface modification of bipolar plates for fuel cells by plasma treatment

Abstract

Fuel cells are regarded as modern and promising alternative energy sources, therefore they have earned scientific interest. Proton exchange membrane fuel cells, which are of main interest for research and development in ZBT (Duisburg), typically consist of two electrodes – bipolar plates, separated by a proton exchange membrane. During the process of fabrication of graphite/polypropylene composite bipolar plates the surface of bipolar plates is enriched by polypropylene significantly reducing the electrical conductivity and thus the efficiency of the fuel cells. Surface conductivity can be improved by plasma treatment. In this work a plasma source by IPLAS was used in a remote mode. This device allows to vary a lot of parameters to find the treatment conditions suitable for carbon bipolar plates. It is shown how plasma treatment removes polymer from the surface. The most suitable experimental parameters for this task are found. The removal of polypropylene from the surface of bipolar plates is corroborated by the measurements using FTIR spectroscopy method. Furthermore, it is shown by Raman spectroscopy that graphite on the surfaces of the samples exposed in plasma changed its structure from nanocrystalline to polycrystalline. Fewer interfaces between the crystals lead to the reduction of electron scattering and hence the conductivity of the plasma treated samples is higher than the conductivity of the untreated ones. The etching effect on the surface is correlated with the plasma chemistry, what can be observed by optical emission spectroscopy. Various changes of surface chemistry and also morphology and topography of the surface after the plasma treatment are described and explained. The measurements of area specific resistance of bipolar plates showed that the electrical properties of bipolar plates for fuel cells were uccessfully improved by plasma treatment. It is demonstrated that plasma treatment by air at atmospheric pressure is an efficient method to improve the electrical conductivity of bipolar plates.