As carbon fiber-reinforced plastics continue to become major components of aircraft, it’s increasing the need for sustainable recycling concepts. Engineers at the Fraunhofer Institute for Chemical Technology (ICT) recently developed a process that converts recycled carbon fiber into materials for batteries and fuel cells.
“Today, wide-body aircraft consist of more than 50 percent carbon fiber reinforced plastics (CFRP),” says Elisa Seiler, an ICT engineer. “The amounts of CFRP recycling material are tremendous. For the Airbus 350, for example, they add up to more than 65 tons. In addition to this, there are other relevant scrap quantities that already arise during production.”
Seiler and her colleagues involved in the Graphit 2.0 project developed a process that can be used to recover materials for batteries and fuel cells out of recycled carbon fibers. They recently produced a prototype of a bipolar plate.
The aim of the Graphit 2.0 project is to develop a process enabling recycled carbon fibers to be used as secondary graphite for high-value application in energy storage devices, such as redox-flow batteries.
In the first part of the project, the engineers developed a process to obtain a secondary raw material from carbon fibers through a mechanical and thermal treatment. This secondary raw material serves as a substitute for graphite. In the second part of the project, the secondary graphite was tested in bipolar plates for redox-flow batteries.
“Electric drives are now also a serious topic in the aviation industry,” says Seiler. “Manufacturers can directly perform value-preserving recycling by transferring materials from one application to the next.
“The carbon fibers are electrically conductive and are suitable as a substitute for natural graphite, which also consists of carbon,” Seiler points out. “[This] resource-critical raw material currently has to be imported from China at great expense. Recycled CFRP can also be used for additive manufacturing applications.”
Aircraft manufacturers have to comply with the European Union requirements that have been in force since 2015—they demand that up to 85 percent of the average weight of a used vehicle has to be recycled. In some countries, such as Germany, the landfilling of CFRP is prohibited and waste incineration plants can refuse to accept the material.
Seiler and her colleagues developed a process that enables carbon fibers to be recovered from the plastic matrix. They use microwave radiation to burn the plastic matrix that surrounds the fibers. The combustion has to be performed without oxygen so that the fibers don’t burn up at temperatures of up to 900 degrees C.
“This is called pyrolytic decomposition,” says Seiler. “The advantage of microwave radiation is energy efficiency. A whole oven no longer needs to be heated—just the component itself.
“Our polymer engineers embed the recovered fibers in thermoplastic material,” explains Seiler. “This composite material has similar properties to graphite and is suitable for the production of bipolar plates.
“Our prototype passed all the tests for conductivity, density and corrosion resistance perfectly,” claims Seiler. “We’ve proven that it’s generally feasible to use recycled CFRP fibers to produce bipolar plates for batteries and fuel cells. This shows that recycling works in a holistic approach.
“The next steps are the characterization of the bipolar plates in the battery cell network and studies concerning the life cycle assessment,” says Seiler. “Then, we want to tune the technology so that we can manufacture bipolar plates from recycled CFRP in series.”