Fuel cell materials and fabrication

Hydrogen fuel cells are regarded as highly efficient power generators that produce/generate electrical energy by reversing water electrolysis. Due to their low emissions and good efficiency, there are numerous international efforts to further develop fuel cell technology. The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is often the focus of these efforts.

This is due to the fact that it is characterized by a high power density in combination with a high energy conversion. At the same time PEMFC also has low operating temperatures, in addition to short start-up times (fast cold start) and uses pure hydrogen and ambient air as an oxidant. In particular, stationary power generation systems and battery replacement devices as well as fuel cell powered transportation systems are seen as important areas of application.

The further development of fuel cells is mostly concerned with optimizing the systems and components. This mainly includes materials for fuel cells, but also takes into account Important aspects such as operating conditions and the potential of cost reduction with regard to fuel cell materials and manufacturing processes.  Engineering thermoplastics are the preferred materials for fuel cells because they are easy to process and contribute to significant savings due to their relative low weight. They also have a very long service life. In addition, their inherent electrical insulation makes them ideal for fuel cell systems.

As a leading manufacturer and processor of engineering and high performance thermoplastics, Ensinger specializes in development of hydrogen fuel cell materials and processing techniques for fuel cell components such as bipolar plates and end plates.

Certain characteristics are necessary for the bipolar plate which results from the application itself. In mobile applications, the primary concern is volume and weight, especially since bipolar plates can account for about 80 % of the total weight of a fuel cell and take up almost the entire volume.

In practice, coated, thin metal plates are still frequently used. However, in the selection of bipolar plate materials, compounds made of plastics are increasingly coming to the fore in terms of TCO (Total Cost of Ownership). Production scalability and production costs continue to play a decisive role for commercialisation, as the cost share of bipolar plates currently amounts to up to 40 %. In this respect, plastic materials also show great potential for achieving economies of scale, especially in production techniques. Another advantage of PEMFC bipolar plates made of thermoplastic materials is the significant increase in service life, which at over 20,000 hours, is more than double that of metallic alternatives. 

Furthermore, bipolar plates made of thermoplastic materials have the advantage of recyclability in comparison to thermosets. Thermoplastics are also characterised by a significantly longer lifetime and easier processing due to the fact that thermosets require a time-consuming hardening processes during which gases are released that increase the porosity of the material.

Ensinger offers solutions from the field of high performance and engineering plastics that are tailored to individual applications and have been tested in field trials. Together with our partner, the Center for Fuel Cell Technology (ZBT) in Duisburg, Ensinger has developed highly conductive materials that are particularly suitable for fuel cell bipolar plates. The special, electrically and thermally optimised formulations bring their advantages to the stationary as well as the mobile sector.

The granules for bipolar plates are available as compounds based on thermoplastic PP (polypropylene) and PPS (polyphenylene sulfide), and are known under the brand name TECACOMP HTE. They have been specially optimised for PEMFC production by hot compression or injection molding. They are suitable for operating temperatures from 60 °C up to 200 °C for use in direct methanol fuel cells (DMFC), LT-PEM fuel cells and alkaline fuel cells (AFC) as PP alternatives. The compounds can also be used as alternatives to PPS in HT-PEM fuel cells and phosphoric acid fuel cells (PAFC). 

The compounds from the TECACOMP HTE series, which have already been tested, prove technically under demanding conditions to be very suitable for a wide range of relevant applications. Economies of scale in injection molding and hot pressing processes can significantly reduce component production costs. 

With the granules of the TECACOMP HTE series, bipolar plates with sizes of approximately 350 - 500 mm in length, a width of up to 160 mm and a thickness ≥0.4 mm can be made. Since Ensinger offers several plastic processing methods, we are able to manufacture the fuel cell bipolar plates themselves as well. Please feel free to contact us for this purpose if necessary.

End plates in fuel cells have the task of clamping and fixing the fuel cell stack in place, as well as sealing the membrane electrode units and protecting them from chemical reactions. 

The stack clamping pressure is of central importance here, ensuring on the one hand cohesion, but also leak-proof operation of the stack. If the contact pressure between the electrode and the bipolar plate is insufficient, performance is severely impaired due to excessive contact resistances and losses, and component failure is even possible. As stack dimensions increase, so does the corresponding requirement for contact pressure. In addition, the two compression resistant end plates form the anode and cathode side terminations of the stack or electrolytic block. They can be used to connect cables and connections (usually pipes), for the gas, electrolyte or cooling lines that lead to and from the stack. Mechanical clamping of the stack is usually done by tie rods or spring components to guarantee mechanical safety and tightness.

Since the fuel cell end plate is exposed to a variety of loads, the performance of the fuel cell stack is significantly influenced by its geometry, the connection method, but also the fuel cell materials used in the end plate. 

In the past, metals were primarily used for end plates due to their high mechanical strength and stiffness as well as heat resistance, despite these designs being associated with complex manufacturing processes and high costs. There were also significant disadvantages due to heat loss and corrosion of the material which results in the possibility of metal ions diffusing into the electrolyte, which in turn affects the conductivity and output voltage. The high thermal conductivity of metallic materials leads to a loss of heat capacity, which requires more time for the unit to reach operating temperature, and the use of additional thermal insulators.

Working with our customers, we have successfully replaced metallic materials and thermoset composites with thermoplastic materials in fuel cell end plates. Our fuel cell materials, in particular, our glass fiber reinforced PPS TECATRON GF40 black (shape) offers ideal properties for use in end plates. PPS material is characterised by excellent chemical resistance, very low water absorption and a low risk of ion leaching (due to the low foreign ion content). 

Thanks to the high mechanical strength and stiffness of PPS GF40, the required contact pressure can be built up in the fuel cell stack. The high dimensional stability and creep resistance of TECATRON GF40 natural and black results in very low deformation - even over longer periods under load and at higher temperatures. Furthermore, TECATRON GF40 meets further construction-specific requirements for flame resistance with UL94 V-0.

The processing and precise machining of PPS GF40 is particularly demanding, as the material tends to suffer from severe warpage and can lead to chipping during milling if it is not manufactured correctly. 
Complicating matters further are the complex geometric features typical of fuel cell end plates in the 5-axis range. These are characterized by many breakthroughs in component geometry and pockets as well as holes near the edges. It is also not uncommon to see larger components of over half a meter in length, as well as major discrepancies in the width/length to thickness ratio. 

Our many years of expertise in processing reinforced plastics such as TECATRON GF40 black enable us to manufacture high-precision fuel cell end plates for our partners and customers with low distortion. Especially in the prototyping phase, with the prospect of scaling production towards injection molding, precision in manufacturing is essential. 

The prototypes machined by us from TECATRON GF40 have proven to be extremely suitable in various external test runs, even with an operating time of over 10,000 hours.

You can take a closer look at Ensinger's end plate made of TECATRON GF40 black by zooming and rotating here: