By Simon Brugger, CEO, CellForm
The bipolar plate
Green hydrogen technology is centered around two processes: electrolytic splitting of water into hydrogen and oxygen (electrolysis) with the external input of energy, and the use of energy from the reverse reaction (fuel cell). Both run in a so-called stack. This stack represents the heart of the technology, in which the electrochemical reactions take place. The main component of this stack is the bipolar plate, which consists of two very thin metallic plates welded together and is used several hundred times per single stack.
As the term itself suggests, the bipolar plate serves as a carrier for the two poles of the fuel cell: the negative anode plate and the positive cathode plate. The plates are responsible for the distribution and reaction process of the gases in the fuel cell as they control the chemical reaction of hydrogen and oxygen to electrical energy and water. They also ensure the electrical contact between the individual cells and separate the membrane electrode assemblies (MEA). The membrane electrode unit serves as a semi-permeable membrane, which is only permeable to protons and forces the electrons emitted by the hydrogen from the anode via an electrical device to the cathode. The remaining protons migrate through the membrane and finally react with the oxygen atoms to form water. A cooling medium for temperature control of the individual cells is passed between the tightly welded individual plates. The channel structure of the so-called flow field serves as the contact surface between hydrogen and oxygen and can be seen in the opening image.
As already mentioned, this chemical reaction takes place in reverse direction when occurring in an electrolyser. Each stack consists of up to 500 bipolar plates, which are installed in the corresponding application.
Hydrogen: advantages and disadvantages
Hydrogen has many advantages as an energy carrier compared to alternative technologies such as batteries. For example, the resources required are significantly more sustainable in terms of materials extraction and recycling, the refuelling time in the mobile sector is significantly shorter, and hydrogen-based energy chains have a potentially lower total cost of ownership. However, the main advantage of hydrogen is closely associated with the energy transition: in a future with energy from fully renewable sources, our society faces a major challenge, namely the geographical and temporal differences between energy production and consumption. Renewable energy generated from wind and sun – in contrast to nuclear and fossil energy – does not always match our usage patterns. This presents us with the challenge of storing energy on a large scale in order to transport and use it time independently. Under the given conditions, the only solution to this challenge is hydrogen.
At the same time, hydrogen has a major disadvantage compared to batteries when it comes to overall efficiency. Green hydrogen is produced sustainably by means of electrolysis and returned to its original energy state in the fuel cell. Although this conversion results in the advantages mentioned above, however, a large part of the original primary energy is lost. This disadvantage is the central problem that must be solved in the best possible way.
Importance of bipolar plates
The bipolar plate has enormous potential to improve the efficiency of the stack. More specifically, the embossed channels with their specific structure and shape have the greatest potential to improve performance and make the fuel cell and electrolyser more efficient. In order to fully exploit this potential, new manufacturing methods with the greatest possible design freedom are required. One such method is the CellForm® technology.
The challenge for manufacturers is to form and weld the thin substrate material for the plates into fine structures with demanding starting geometries, free of cracks and thus free of leakage while being capable of reproducing this process in large quantities.
Bipolar plates are mainly made of metallic and graphite materials. Different materials are associated with different properties and advantages for the functionality of the bipolar plates. Due to slight efficiency advantages and a lack of manufacturing methods for competitive metallic bipolar plates, the graphite alternative dominated in the past. However, especially in sophisticated applications, graphite bipolar plates have significant volumetric and gravimetric deficits compared to metallic variants. Graphite is also very brittle and can break easily. Nevertheless, graphite plates are often used in stationary applications where installation space is not a limiting factor. The flow field is usually produced by milling, hot pressing or injection moulding.
Regarding the cost perspective, metallic plates are in a leading position. With the right manufacturing method, sheet thicknesses can be further reduced to as little as 0.05 mm. In this range, metal is at a completely different price level than graphite. Considering that several hundred bipolar plates are used for a single stack, the cost implications for the final application are enormous. Another advantage of the metallic variant is that the material has a positive influence on the cold-start capability of the fuel cell.
Due to the corrosive behaviour of the metallic material, metallic bipolar plates are coated with chromium nitride, for example. This is usually done with a thermochemical treatment. The CellForm® technology was developed for metallic bipolar plates and can form and weld both pre-coated and post-coated plates.
Production of bipolar plates
There are several production technologies for bipolar plates, depending on the materials and the applications. They vary in terms of cycle time, filigree of the flow field, and costs. A finished bipolar plate consists of an anode and a cathode, which are formed separately. The two individual pieces are then tightly welded together. The difficulty in producing bipolar plates lies in the extremely thin sheet thickness. Forming with such a thin starting sheet and such a precise and demanding geometry of the channel structure quickly leads to cracks due to physical restrictions, making the bipolar plate unusable. The image below clearly shows the problem of cracking caused by inappropriate forming technologies.
As mentioned earlier, the challenge for manufacturers of bipolar plates are the high-quality requirements with low error tolerances, which must be guaranteed in series production with high quantities. Only those who meet these requirements will be able to succeed in this growing and competitive market.
There are several production processes for bipolar plates, which are still in development. Many manufacturing processes will be limited in their output quantity for future large-scale production due to physical restrictions (e.g., heat generation). This problem is not yet apparent with small quantities but will become more and more apparent in the coming years with higher demand. CellForm relies on a proprietary multi-stage forming process that can be easily scaled up. The scalability of the manufacturing process determines the production costs and price of the individual bipolar plate and thus significantly affects the attractiveness of the climate-friendly hydrogen technology as a whole.
The potential of fuel cell technology in general and of bipolar plates as a key element of the fuel cell in particular is outstanding. With fuel cell electric vehicles expected to have a global market share of 3% by 2030 and an average of 450 bipolar plates per vehicle, nearly 1.5 billion bipolar plates per year could be needed in this area alone. Further demand for bipolar plates for other application areas such as trucks, ships, trains, etc. is not included in this calculation. Given these enormous quantities, only manufacturing processes that can significantly reduce the price per plate through short cycle times while meeting the quality requirements discussed above will manage to prevail.
CellForm covers the entire manufacturing process of metallic bipolar plates with a multi-stage forming process and downstream laser welding. With the developed CellForm technology, bipolar plates with industry-leading features can be manufactured with process reliability. These are 5% more efficient, 50% thinner/lighter and prospectively 90% cheaper than competing products. CellForm is tackling the problems of hydrogen technology disruptively and consistently with high R&D activities, thus advancing the global energy transition. The fuel cell stack is raised to a new level of efficiency and the possibilities for sustainable energy production are expanded. For more information about the CellForm technology, visit www.cellform.eu.