Atmospheric plasma coating of polymer bipolar plates

In times of global sensitization to economic and especially ecological issues, awareness of energy-efficient whole solution strategies across the entire value chain and the sustainable use of available resources is also growing. Before profitable mass production of bipolar plates can begin, a whole series of developments and preliminary investigations are necessary in the product creation process, to determine the optimum efficiency depending on the design and execution. As this cannot be done with the help of simulations alone, experimental investigations are indispensable.

With the coating processes currently available on the market, prototype, pre-series and small series production is very time-consuming and cost-intensive. This is where the approach investigated by the research institute ITW Chemnitz comes in, to provide an easily moldable and cost-effective base material with a suitable coating, to carry out preliminary and small series tests in an energy-, time-, cost- and material-efficient way.

With the help of the combination of low-cost additive base material production and universally applicable coating technology implemented in this project, it is possible to manufacture different bipolar plate designs flexibly and cost-effectively, without neglecting the required industrial parameters. As a result, the envisaged versatile production technology should make it possible to produce prototypes as well as pre-series and small series in an energy-, time-, cost- and material-efficient manner and thus even out the path for industrial practice.

Coating technology plays a major role
In the course of the investigations, a low-energy plasma was used, into which the coating material used was definably fed in the form of microparticles. This enables a firm bond between the coating material and the substrate (see Fig. 1). Due to the technologically dependent low thermal load on the substrate to be coated, it is possible to create material combinations that seem at first glance unrealistic (in the context presented, a polymer as substrate and copper powder as coating material). Another advantage of the process used is the application under atmospheric conditions. In contrast to comparative processes such as physical or chemical vapor deposition, a prior evacuation and working in a vacuum are not necessary. Furthermore, the high degree of flexibility and the possibility of partial coating should be positively highlighted.

Search for suitable substrate material
When searching for and selecting a suitable substrate material, various challenges had to be accounted for:

• the required temperature resistance (should be oriented toward the operating temperatures of PEM fuel cells of about 110 °C),
• easy and variable processing (base structure should be producible using selective laser sintering, to ensure high design flexibility),
• the good as well as cost-effective availability of the raw material.

Several potential substrate materials were examined in more detail and tested for their suitability for coating. For this, variants available on the market were also modified so that they were optimized for the planned application. After a comprehensive series of tests, consisting of coating trials, optical analyses, surface measurements, simulative studies and thermal post-treatment tests with regard to temperature resistance, the choice fell on a glass fiber-modified polybutylene terephthalate (PBT). This material was modified by the targeted addition of glass fibers such that all the required technical parameters were achieved. In addition, the modified PBT has the best coating properties.

From the idea to industry-oriented flow field design
One of the major challenges within the investigations was the development of an industry-oriented flow field design taking into account the material-specific and technological limits of the processes used. This involved working out and defining the manufacturing limits of selective laser sintering, taking into account the material and the target application, as well as the technological limits of the subsequent coating process. To this end, various parameter and geometry studies were carried out on industrially used flow field designs. In the end, a meandering flow field structure with the following dimensions was realized:

Tab. 1: Realized flow field structure

The four wing-shaped hold-down devices (see Fig. 2) are required for fixation during the coating process and can be easily removed afterwards.

Developed polymer base body (left) and coating result (right)

To counteract any distortion, a metallic sample holder was used. This test setup ensures a targeted removal of the applied temperature and thus an optimum coating result. Both optical surface analyses and adhesion tests based on the cross-cut test in accordance with DIN EN ISO 2409 yielded satisfactory results and reveal a high potential for the abovementioned prototype, pre-series and small series production.

The studies were supported with funding from the German ministry for economic affairs and climate protection.