Component production for fuel and electrolyzer cells

The transition to a climate-neutral energy supply requires the expansion of renewable energies. The focus is also shifting to the use of hydrogen, which requires technological solutions from industry. Rehm Thermal Systems is making this contribution on several levels.

In the field of power electronics, vapor phase soldering under vacuum has established itself as a reliable process technology - for modules in inverters or charging stations, for example. By limiting the soldering temperature, temperature-sensitive components are protected. At the same time, the vacuum of the Condenso systems enables large-area solder joints with a low proportion of pores for optimum thermal bonding, which increases the service life of the assembly.

The expansion of Rehm's product portfolio will make a further contribution to the development of the hydrogen infrastructure for an independent European energy supply. With core competencies in the temperature range of -40 °C - 1200 °C, the company offers systems for material application and thermal production solutions for electrolyser and fuel cell components - scalable and highly automated. This field of application is closely linked to power electronics, in particular due to comparable requirements for electrical system integration and process technology.

Fuel and electrolyser cell types

There are various types of electrochemical cells, the most common being PEM cells (proton exchange membrane) and SO cells (solid oxide), which can be operated as fuel and electrolyser cells. In electrolysis, electrical energy is used to obtain hydrogen and oxygen from water, while the reverse process takes place in the fuel cell. The reaction of hydrogen and oxygen to water with the release of electrical energy takes place at the membrane of the fuel cell. This is where the cells differ, resulting in different operating temperatures for the two types: PEM cells operate at 60 - 80 °C, SO cells at 600 - 1000 °C - which is why the former are mobile and the latter are stationary.

Structure and components of the cells

In PEM fuel cells, the bipolar plate (BPP) performs central tasks: It conducts the reaction gases, conducts electricity and connects cell poles. In solid oxide fuel cells (SOFC), an interconnect plate (ICP) is used instead, which ensures the electrical connection of the cells and gas distribution. Both types of plate must meet high requirements in terms of electrical conductivity, gas tightness and thermal stability. To ensure these functions, they are given specific coatings. The respective membrane - polymer-based for PEM, ceramic for SOFC - is also given a functional coating. To ensure the functionality of the layers, precisely defined thermal production processes are followed in each case.

A BPP consists of two stamped metal halves that are welded together. The bead on the outer edge of the panel serves as a contoured mount for the seal and ensures a defined spring effect (see Fig. 1). This ensures that the sealing function is reliably maintained even under thermal and mechanical stress. The seals separate media, stabilize mechanically and prevent uncontrolled penetration of foreign particles into the cell as well as the escape of reaction products.

The sealing material for PEM cells consists of silicones or elastomers. As SO cells have a higher operating temperature, a glass paste is used for the seal, which softens at high temperatures and creates a tight connection between the components. In contrast to the BPP, the ICP does not consist of two stamped halves, but of a solid steel part that is specially designed for a higher temperature level.

The integrated flow field of the plates is an embossed channel pattern that guides the gases evenly over the cell surface and enables the removal of water and heat (see Fig. 1). This is coated on metallic BPP to protect against corrosion, create mechanical stability and guarantee electrical conductivity. In the case of graphite BPP, this is generally not done, as graphite naturally meets these requirements. As only ferritic steel can be used for ICP, a so-called Mixed Conducting Oxide (MCO) coating is applied here so that the chromium from the steel does not come into contact with the cell. In addition, both the ceramic membrane in SO cells and the MEA (Membrane Electrode Assembly) in PEM cells receive a coating for conductivity and reactivity.

Fig. 1: ZSW HyFaB generic stack metallic PEM bipolar plate - manufactured by EKPO

Production of the stack

After the BPP or ICP has been cut, embossed and cleaned, the flowfield is coated. In the case of PEM, this is done selectively by roller printing or dispensing or over the entire surface by PVD. In the area of SO cells, the MCO coating of the ICP is usually carried out by PVD or plasma spraying. The seal is also applied in each case - in the case of PEM cells by screen printing, dispensing or injection molding, whereas in the case of SO cells it is generally applied by screen printing or, in some cases, by dispensing. Dispensing offers a high degree of flexibility for different materials, precision and repeatability and prevents the formation of bubbles in the material. This process is mapped in the Rehm portfolio with the Protecto systems. Printing and dispensing processes require subsequent thermal treatment. After the material has been applied, the sheets are automatically handled in magazines or stacked product carriers and fed into the thermal system. PEM BPP is first dried at 80 - 100 °C to drive solvents out of the material and prevent the formation of bubbles in the seal. The gasket must then be vulcanized at 170 - 200 °C for complete chemical cross-linking. This thermal process is mapped as a step profile in a single pass. In injection moulding, a post-curing process of max. 180 °C may also be necessary to optimize quality. Magazine dryers with convection are suitable for processing BPP and ICP, which offer optimum zone separation through bulkheads and allow step profiles to be created (see Fig. 2). The magazines or stacked product carriers have space for hundreds of panels so that low cycle times of a few seconds per panel can be achieved in high-volume production.

In industrial series production, screen printing is also predominantly used for the coating of the ceramic membrane to ensure high precision. The functional layers are applied one after the other. After the printing process, each layer is dried at 80 - 180 °C, followed by sintering at 900 - 1600 °C to create a functional cell structure. Ceramic membranes require higher temperature gradients, which is why systems that combine infrared radiation with convection are used here. The sensitive 20 µm thin substrates are guided on stabilizing stainless steel or Teflon braided tapes.

After passing a quality test, the cells are assembled to achieve the desired overall performance. In addition, the mechanical connection with connecting plates is made by screwing or clamping in order to obtain a tight, stable stack.

Fig. 2: Magazine dryer with stepped profile for the production of BPP and ICP

Scalability of the processes

In order to design modern production lines efficiently, Rehm manufactures automated systems that enable repeatable production. The thermal systems are flexibly scalable and are customized according to the material requirements and production volume. They meet the requirements of DIN EN 1539 and are more energy-efficient than conventional batch ovens thanks to separate cooling zones.

While sufficient space is available in greenfield projects, existing production sites (brownfield environments) are increasingly facing the challenge of integrating new lines in a space-saving manner. The length of thermal systems can vary depending on the temperature profile. In order to do justice to limited space, the company also offers systems with vertical product guidance, whereby the footprint can be reduced by a factor of ten.

Individual line concepts with different degrees of automation and traceability variants are designed together with partners. The company's own Technology Center is already available during the development and prototyping phase to determine optimum process parameters.

Conclusion

In order to meet the growing requirements of hydrogen-based energy systems, efficient and adaptable production processes are crucial. Rehm Thermal Systems not only relies on proven technologies, but also develops customized solutions. With a view to increasing quantities, new cell concepts and higher levels of automation, a strong technological basis is offered - today and in the future.