Conventional battery packs usually consist of several individual cells that are connected with each other via wires. Not only is this costly and time-consuming, it also entails the danger of hot spots – areas in which the wires get too hot. In addition, every single one of these cells has to be packaged, meaning that a large portion of the battery consists of inactive material that does not contribute to battery performance. Bipolar batteries are designed to solve this problem by connecting the individual cells with each other using flat bipolar plates. However, this gives rise to other challenges: either the bipolar plates are made of metal and prone to corrosion, or they are made of a carbon-polymer composite, in which case they have to be at least several millimeters thick as a result of the manufacturing process.
Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen have now developed an alternative with flexible electrodes that can be made on a roll-to-roll process.
“We manufacture bipolar plates from polymers that have been made electrically conductive,” said Dr Anna Grevé, department head at Fraunhofer UMSICHT. “In this way, we can produce very thin plates and – compared with conventional cells connected by wires – save over 80 percent of the material used.”
The graphite/polymer material does not corrode and can be reshaped, making it possible to create embossed structures for fuel cells. “We can make plates that are so flexible that you can wrap them around your finger, as well as ones that are completely stiff,” she said. The bipolar plates can also be welded together, where conventional bipolar plates are unsuitable for welding due to the thermal and mechanical stressing of the material during manufacture. Joining them in such a way that neither gases nor liquids can pass through the joints requires seals, but these quickly porous, and they also take up space. Welding the plates avoides the need for the seals.
The primary challenge consisted in developing the material and the manufacturing process. “We use commercially available polymers and graphites, but the secret is in the recipe,” said Grevé. As the material is made up of about 80 percent graphites and only about 20 percent polymer, the processing methods have little in common with ordinary polymer processing and the plates can be manufactured in any size.. “We were able to fulfill all requirements in one process. Consequently, the plates can be used just as they are when they come out of the machine,” she added.
The researchers are already able to produce relevant sample quantities of the new bipolar plates, working with Saueressig to translate a previous method of producing bipolar plates for fuel cells into a continuous process. This is currently being shown in a a 3.2 square meter bipolar plate that enables the construction of large-scale redox flow batteries.