For the stationary and mobile usage under consideration, the low-temperature polymer-electrolyte-membrane fuel cell (LT-PEM fuel cell) is a suitable choice. It runs at low operating temperatures of around 85° C and tolerates being frequently switched on and off.
A single fuel cell consists of a membrane-electrode unit, various seals and two bipolar plates for media distribution and/or temperature control and electrical contact. Due to the fact that a single fuel cell supplies only a low voltage, several cells are usually connected in series to form so-called stacks. Two endplates serve the media supply and discharge, and enable a uniform contact pressure for all cells in the stack. The overall objective of this research project is the development of a bio-based fuel-cell stack. It is divided into four subprojects:
Joining technology and function
In this subproject, the project partners determine the construction design, thereby taking into account the desired functionality, material-specific requirements, joining points and process-engineering possibilities. The focus is placed on the adhesive-technical design of the fuel-cell stacks by combining all the results from the subprojects and testing and validating them in real fuel-cell operation. In addition, a profitability analysis will be carried out and the recyclability of the bio-based fuel-cell stack will be evaluated.
Endplates made from wood-based material
Until now, the endplates have been made from aluminum or steel. To save manufacturing costs, they are mainly designed as solid block components and are therefore very heavy. In conventional PEM fuel-cell stacks, the endplates account for around 30 percent of the total weight. Furthermore, metallic endplates exhibit high thermal and electrical conductivity. For this reason, conventional fuel-cell stacks incorporate insulating plates, which lead to higher material and assembly costs. The project goal is the production of endplates made from high-performance, lightweight wood-based material. As dry wood is a very good thermal and electrical insulator, insulating plates can be dispensed with. A customized sandwich structure of the wood-based material should additionally save weight. The Fraunhofer WKI is responsible for the development of the endplates.
Bipolar plates made from bio-based compounds
For the production of bipolar plates, both metallic and graphitic materials are utilized. In order to ensure chemical resistance and a reduction in contact resistance, metallic bipolar plates require cost-intensive coating. As a result of their properties, graphite-polymer compounds have very long lifetimes even without a coating, whilst simultaneously achieving high electrical-conductivity values. In addition, compounds offer the opportunity of producing bipolar plates from renewable raw materials. The focus of the project lies on thermoplastic bipolar plates, as they can be produced more cost-effectively through injection molding/rolling than thermoset bipolar plates. Until now, thermoplastic bipolar plates have been manufactured using petroleum-based polypropylene and graphite/carbon black as a conductive filler. The aim of the project is to produce electrically and thermally highly conductive compounds from a bio-based biopolymer as a binder and natural graphite or carbonized wood as a filler. At the Fraunhofer WKI, we are developing suitable biopolymers for this purpose.
Bio-based adhesives and sealants
Conventional fuel cells are braced into stacks by means of clamping devices (e.g. screw connections, wires, tensioning straps). This joining technique is time-consuming and has technical disadvantages. For example, high contact pressures and, consequently, heavyweight clamping elements are required in order to achieve an adequate sealing effect. To simplify the joining and assembly processes and reduce the overall weight, adhesive bonding solutions are an option. Their fundamental suitability has already been successfully investigated in preceding projects. The aim of the project is the development of bio-based adhesives and sealants for the joints within the fuel cell.
Fraunhofer Institute for Wood Research 
