- What are the steps involved in the production of bipolar plates and what are the most common manufacturing methods?
- How are the bipolar plates assembled into a fuel cell stack?
- What are the different defects identified in the manufacturing of metal bipolar plates and what are common quality control measures taken to overcome this?
- How are the environmental impacts minimized for the manufacturing of metal bipolar plates?
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What are the steps involved in the production of bipolar plates and what are the most common manufacturing methods?
What makes bipolar plates for fuel cell and electrolyzer applications so special is firstly the large number of plates required to build a stack. Moreover, the geometries of bipolar plates are highly complex and place very high demands on the component. This special constellation means that a production line for series production of the bipolar plates must also meet special requirements.
A complete production line for series production of metallic bipolar plates includes the following sections/steps:
- Material: Depending on the application of the bipolar plate, the raw material must be defined. The mechanical and electrical as well as the chemical and thermal requirements of the planned applications must be taken into account. The definition of the raw material is the first necessary step and has a significant impact on all subsequent steps.
- Coating: Depending on the application of the bipolar plate, there are two different methods: pre- and post-coating. Pre-coating means that the material is coated in advance in a roll-to-roll process. In the case of post-coating, the finished product (the metallic bipolar plate) receives a coating only after all production steps have been completed.
- Geometry: The definition of the geometry of a metallic bipolar plate is usually an iterative process and is carried out depending on the planned application, the planned power density and the planned stoichiometric requirements for the complete electrochemical energy conversion process within the individual cells and the complete stack. In addition, the boundary conditions of all other components such as MEA, GDL, sealing and stack concept must be known and taken into account. These three requirements – material, coating and geometry – are the essential basis for the concrete design of a production line for the series production of metallic bipolar plates and at the same time already include the relevant input for all further economic considerations. They are, you could say, the “genetic fingerprint” of the production line design.
- Forming: A number of different processes can be used to form the metal, such as hydroforming, stamping, deep drawing or rotary forming. Depending on the plate and process requirements, one or the other method is used. If speed is required, for example, the roll-to-roll process can be a good option, provided the plate design does not have tight tolerances. If, on the other hand, the highest precision, repeatability and tightest tolerances in the process are required, hydroforming is the method of choice.
- Cutting: After forming, the plate gets both its contour and the manifolds for gas infeed. High precision and quality can be achieved by means of laser fusion cutting. These are important because the cut edges of the plates are used, among other things, as aligning elements in subsequent processes and any deviation between the cutting contour and the formed channel structures can lead to problems during operation of the fuel cell or electrolyzer. Likewise, the quality of the cut edges has a major impact on the quality of the final product, as only a completely burr-free cut enables the high demands on the overall service life of a stack to be met.
- Welding: Due to the conception of the stack design, it is possible that the single plates, namely anode and cathode, have to be welded together. This requires perfect positioning of the individual components as well as 100% zero-gap clamping and gas flushing in order to achieve an ideal welding result. Because this process step also requires maximum precision with short cycle times, welding with laser scanner technology is the most appropriate method.
- Straightening: Straightening of the bipolar plates is done to remove the warping from the plate. This is caused by residual stresses applied both during the production of the metal and during its further processing. Straightened, flat plates not only allow for easier handling in subsequent processes and stack assembly, but also ensure increased stack life and effectiveness.
- Cleaning: The single and bipolar plates are cleaned during the production process to create ideal conditions for the respective subsequent processes. In addition, there is a final cleaning of the finished metallic bipolar plate after all production steps have been passed through. It is then “ready for stacking”.
- Leak test: According to the design of the metallic bipolar plate, the leak test (= leakage rate) is performed on the single plate or the welded bipolar plate. Depending on the requirements defined by the customer, both for the leakage rate of the individual component and for the measurement method, the test takes place as a 100% final test using different methods. These are e.g. a simple pressure difference method with ambient air, a vacuum test or a helium leakage test. Taking into account the necessary traceability for each individual component, at the end of this process step the complete registration and correlation between the applied data matrix code and the previous history of the component takes place.
- Sealing: Depending on the stack concept, the sealing is then applied. There are also different methods available for this process step. From single-sided to double-sided application of the seal with a dispenser to the application of the seal using screen printing, up to the tool-based process using injection molding, the necessary seal is applied to the single plate or the bipolar plate.
How are the bipolar plates assembled into a fuel cell stack?
The stack concept is already defined in the design phase – together with the design of the metallic bipolar plates and all required components. It forms another component of the “genetic fingerprint” since the stack concept defines all functional as well as all basic economic conditions.
Usually, stacking is carried out according to the pick-and-place principle, since continuous and discontinuous processes have to be combined. The metallic bipolar plates are aligned using index features and stacked together with the other components, such as membranes, gas diffusion layers and, if necessary, seals, to form a complete stack, pressed and then conditioned in a further process step.
Example: Bipolar plate production line
What are the different defects identified in the manufacturing of metal bipolar plates and what are common quality control measures taken to overcome this?
- Material: The base material, supplied as a precision strip in coil form, may vary in material thickness depending on the strip width. These must be corrected in the production process. In addition, the raw material may contain mechanical defects in the form of impressions caused by dirt in the rolling process. When using pre-coated material, coating defects and scratches may also be present.
- Leakage: The tightness of the component is the most important prerequisite for the function and is 100% tested by means of a leakage test.
- Warping: For the use of fuel cells and electrolyzers, metallic bipolar plates are stacked and clamped in stacks of sometimes several hundred plates. In addition to the bipolar plates, extremely flexible and sensitive components such as membranes, gas diffusion layers and seals are also installed. If the bipolar plates used are not flat but warped, this leads to an inhomogeneous distribution of pressure between the components when a stack is pressed, which has a negative impact on the effectiveness and service life of the stack. If the generated transverse forces are too strong, this even leads to microcracks in the sensitive components, and this can result in a total breakdown of the stack.
- Contamination: Any kind of contamination of the individual components can lead to degradation of MEA (membrane electrode assembly) and complete breakdown of the fuel cell or the electrolyzer during operation, or to a significant reduction in service life. Therefore, each process step must be precisely examined and designed both in terms of cleanliness, but also in terms of the choice of fixture materials.
How are the environmental impacts minimized for the manufacturing of metal bipolar plates?
Minimizing the environmental impacts of bipolar plate production involves adopting sustainable practices and technologies throughout the manufacturing process. Here are several strategies to reduce the environmental footprint of bipolar plate production:
- Material selection plays a crucial role in reducing the environmental impact therefore choosing the right materials with a lower environmental impact is important. For example, consider using recycled metals in the production of bipolar plates. Recycled stainless steel can significantly reduce the demand for new raw materials. Alternative materials with lower embodied energy and environmental impact, such as lightweight alloys.
- Implementing energy-efficient manufacturing processes. This can include optimizing machine efficiency, using energy-efficient equipment, and incorporating renewable energy sources into the production facility. Cogeneration or combined heat and power (CHP) systems could be one of the best options to maximize energy utilization and minimize waste. Minimize waste generation through efficient material usage and recycling programs within the manufacturing facility. Establish systems for recycling scrap and unused materials, reducing the overall demand for raw materials and minimizing the environmental impact associated with extraction and processing.
- Implementation of water conservation measures to reduce water usage in the manufacturing process. Consider water recycling systems to treat and reuse water within the facility, reducing the environmental impact associated with water extraction and discharge.
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Graebener® Bipolar Plate Technologies is part of Graebener® Maschinentechnik, a medium-sized, family owned machine building company with locations in Netphen (Germany) and Houston (USA) as well as various international representatives. For 20 years, Graebener® has been one of the first companies to focus on the research and development of manufacturing processes and machines for fuel cell and electrolyzer components. The company has dedicated itself to setting the quality standard for manufacturing technologies by developing innovative processes and machines that help to manufacture fuel cell and electrolyzer components, such as the metallic bipolar plate, that are tailormade, integrable, scalable and thus economically efficient.
Last update: 15.11.2023