Fuel cells produced from recycled carbon fibres
Carbon fibre reinforced plastics are gaining importance as components of aircraft — a trend which is increasing the need for sustainable recycling concepts. Now, researchers at Germany’s Fraunhofer Institute for Chemical Technology ICT have developed technology that converts recycled carbon fibres into materials for batteries and fuel cells.
Modern wide-body aircraft today consist of over 50% carbon fibre reinforced plastics (CFRP). The material is installed, for example, over a large area in the wings or fuselage. With carbon fibres embedded in a plastic matrix, the composite is lighter than previously used materials, while still being very stable. Due to their lower weight, the aeroplanes require less fuel.
So what’s the downside? As explained by Fraunhofer scientist Elisa Seiler, “The manufacturing and processing of the CFRP is currently very time-consuming. The demand for sustainable recycling concepts is therefore steadily increasing.”
The amounts of CFRP recycling material are tremendous: for the Airbus 350, for example, they add up to over 65 tons. “In addition to this, there are other relevant scrap quantities that already arise during production,” added Seiler.
The Fraunhofer ICT has many years of experience in the development of technologies for fibre-reinforced plastics. Now, its scientists are recovering materials for batteries and fuel cells out of recycled carbon fibres. With the help of partners, it has been possible to use recovered carbon fibres to produce a prototype of a bipolar plate — an electrode — on an industrial scale.
“Electric drives are now also a serious topic in the aviation industry,” said Seiler. “Manufacturers can directly perform value-preserving recycling by transferring materials from one application to the next.”
The carbon fibres are electrically conductive and are suitable as a substitute for natural graphite, which also consists of carbon: a resource-critical raw material for the German economy that currently has to be imported from China at great expense.
Another advantage is that recycled CFRP can be used for additive manufacturing processes such as 3D printing — a trend that makes production processes more efficient and saves costs, according to Seiler. Aircraft manufacturers also have to comply with the EU requirements that up to 85% of the average weight of a used vehicle must be recycled. Furthermore, in Germany the landfilling of CFRP is prohibited, and waste incineration plants can refuse to accept the material.
The CFRP experts have developed a special process with which carbon fibres can be recovered from the plastic matrix. To do so, they use microwave radiation to burn the plastic matrix that surrounds the fibres. So that the fibres do not burn up at temperatures of up to 900°C, the combustion has to be performed without oxygen.
“In technical jargon, this is called pyrolytic decomposition,” said Seiler. The use of microwave radiation is an energy-efficient process, as the whole oven no longer needs to be heated — just the component itself.
Next, the researchers embedded the recovered fibres in thermoplastic material. This composite material has similar properties to graphite and is suitable for the production of bipolar plates. As noted by Seiler, “Our prototype passed all the tests for conductivity, density and corrosion resistance perfectly.”
The researchers have thus proven that it is feasible to use recycled CFRP fibres to produce bipolar plates for batteries and fuel cells. The next steps include the characterisation of the bipolar plates in the battery cell network and studies concerning the life cycle assessment.
“Then, we want to tune the technology so that we can manufacture bipolar plates from recycled CFRP in series — for example, with an aviation partner,” said Seiler.
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