Variothermal process with up to 330°C mould temperature
High-temperature PEM fuel cells are propagated for applications in energy supply for decentralized and mobile applications. Due to the necessary temperature stability of materials to be used, the known materials and production methods from classic PEM fuel cells are not suitable. The process optimization for the production of bipolar plates is a central task of the working group stack components of the department fuel cells and systems.
Viable bipolar plates for HT-PEM fuel cells are producible by injection moulding with PPS compound. That showed the ZBT several years ago. However, the very high viscosity and heat conductivity of the compound prevented an increase in the active area of more than 50 cm². In a successor project, the production of bipolar plates with an active area of 163 cm² now been examined with the contribution of an oil variothermal process.
Together with the project partner IPE of the University of Duisburg-Essen new compounds have been developed, characterized and processed to test specimen and bipolar plates. The determination of material properties formed the basis of the material understanding of the variothermal injection moulding and allowed to carry out extensive simulations of the injection moulding process of highly filled PPS compounds. Here they were used for tool design and the matching between simulation and experimental measurement data.
For the new stack design 2 different bipolar designs have been developed in one-piece and two-piece design. The corresponding mould inserts were designed and the master tool adapted.
For the test series 3 compounds with different property profiles were produced. A very positive influence of variothermal processes on the electrical resistivity and their homogenization over the entire active area was determined. Regardless of injection moulding parameters have been set. Essential precondition is a variothermal process with a mould insert temperature between 280 and 330° C during injection and lower than 220°C during ejection. Isothermally at about 200°C the resistivity level increases and the fluctuation width is considerably greater. Obviously the filler tolerance increases with the variothermal process. Only on this way the complete filling of the one-piece bipolar plate was enabled.
Nevertheless the high viscosity of the compounds requires high injection pressures (>2500 bar).
The recognition that port areas are very sensitive areas also at variothermal injection moulding processes and must be designed accordingly is also new. The tendency to flow line induced cracks increased variothermal.
Despite the challenging production of bipolar plates two multi-cell fuel cell stacks could be produced and extensively characterized with injection moulded and machined bipolar plates with 163 cm² active area from two different PPS compounds.
In addition, both stacks showed their functionality over several hundred hours operating time and generate important knowledge for material and fuel cell development.
The research project “Process development of a variothermal injection moulding process for extensive bipolar plates for use in reformat gas provided high temperature PEM fuel cells with a nominal capacity of at least 2 kWel. for CHP applications” was performed together with the project partner University of Duisburg-Essen, Institute for Product Engineering , department of Engineering Design and Plastics Machinery and with the member association IUTA e.V.