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Carbon fibers are thin (diameter = 7 μm), light (real density = 1.8 g/cm³), strong (strength up to >6 GPa), and stiff (Young’s modulus up to 900 GPa) (Figure 1.19). These fibers exceed any other fiber material in its specific properties and come close to the theoretically predicted properties of pure graphite. The properties depend on the temperature weakly only. Only the presence of oxygen limits the application at elevated temperatures. Carbon fibers can be made from different fiber precursors. These fiber precursors can be polyacrylonitrile (PAN), pitch, or rayon. During the history of the carbon fiber development, PAN‐based carbon fiber won the race. Reasons had been the relatively easy processing and the wideness of achievable mechanical properties. Mesophase pitch‐based carbon fibers are only competitive in applications with extreme stiffness. Embedded in a polymer matrix (carbon fiber reinforced polymer [CFRP]) or a carbon matrix (carbon fiber reinforced carbon [CFRC]), superior material properties are the outcome. These spectacular properties were soon recognized for military application, an area where functionality overrules cost (Figure 1.20). As the carbon fiber price dropped, application in sport articles followed. Today carbon fibers are common in the civil aviation industry. Weight and thus the reduction of operational cost made the use of CFRPs’ attractive.

Figure 1.19 Mechanical properties of carbon fibers.

Figure 1.20 Carbon fiber fields of application.

Modern planes will contain more than 50% of their constructional parts made from CFRPs. In the area of wind power, the blade length is going to exceed 70 m. To provide the necessary stiffness to these blades, the application of CFRPs is unavoidable. CFRPs are indispensable for the requested energy saving in transportation and the start into e‐mobility. These new markets will cause a heavily increasing demand for carbon fibers (Figure 1.21). Forecast expects a growth in demand from 20 000 t in 2010 to 270 000 t in 2030. The production know‐how (precursor) and production capacities are concentrated in companies based in Japan or the United States (Figure 1.22). Europe started to establish its own independent position in this market.

Figure 1.21 Carbon fiber demand and capacity.

Figure 1.22 Carbon fiber producers and their estimated capacities. Sources: The Global CF and CC Market 2018, Sauer & Kühnel; Annual Report Composites United 2019).

The production cost for CFRPs has to be reduced to become competitive versus the traditional construction materials steel and aluminum. The cost for carbon fibers production is linked to the oil price and energy pricing; the biggest potential today is in the manufacturing process for CFRPs itself. Automation and reasonable lot sizes are the keys to success. The development of matrix systems that will accelerate the manufacturing processes and enable the recycling into new components is necessary. Thermoplastic polymers will partially replace the currently used thermosetting resin systems. The fiber surface has to be modified to provide the required interaction with the respective polymer system. On a long‐term perspective, precursor fibers based on renewable materials and “green” matrices will be the answer to the current CO₂ footprint discussions. All this needs big efforts in research and development.

Figure 1.23 Energy and power density for different storage systems. Source: R. Kötz, M. Hahn, R. Gallay, 9th UECT‐Ulm, Electrochemical Talks, 2004, Neu‐Ulm, Germany.

Lightweight construction is one precondition for the breakthrough of e‐mobility. Another challenge is the storage of electrical energy. Different storage systems are available or under development. Due to their differences in power density and loading and unloading characteristics, the intelligent combination of all of them is necessary. This could provide the desired acceleration and cruising range (Figure 1.23). Capacitive systems with fast unloading enable the powerful acceleration: Li‐ion batteries will cover the midsize cruising range, and fuel cell system will provide the energy for longer distances. In all these systems that are available or under development, carbon materials play an essential role.

The anode in Li‐ion batteries is made from graphite. The electrical characteristics are determined by a bunch of parameters, from the raw material source toward processing temperatures, grain shaping, coating, and many others. Natural graphite‐based anode material provides good charging and discharging characteristics. An advantage of natural graphite versus synthetic graphite is the unneeded graphitization treatment. A forecast for the expected demand for Li‐ion batteries storage capacity is shown in Figure 1.24.

As in many other cases the know‐how and production capacity are located in Japan (Figure 1.25). Europe with its high end car industry did hard to invest into this development. Currently, Europe is struggling to catch up.

Figure 1.24 Expected Li‐ion battery demand. Sources: H. Takeshita, IIT, 25th Int. Batt. Sem and Exhibit. Fort Lauderdale, FL, USA, 2008; Next‐mobility news 09/2017; Roland Berger Study on LiB /2018.

Figure 1.25 Li‐ion anode material producer and their capacity.

Electrical discharge layer capacitors (EDLCs) are fast loading and unloading systems. In contrary to the Li‐ion batteries, in which the intercalation in between the graphite layers is the storage process, EDLCs require an easy accessible high surface area with a preferred porosity in the nano‐range for the adsorption/desorption of charge carriers. Suitable carbon materials can be produced from a wide variety of sources. One source is from renewables like nutshells and others that are known from the production of activated carbons. Also synthetic sources can be used. Essential is the activation of the carbon surface. One advantage of these EDLCs is their high cycle life with more than one million cycles.

In a fuel cell the reactive components hydrogen and oxygen are separated from each other by a gas diffusion layer (GDL). The components diffuse through a gas penetrable layer formed by carbon materials until they reach the catalyst (Figure 1.26). The application of fuel cells is expected to concentrate on automotives and less on portable and stationary systems (Figure 1.27). Main industrial players in the field of GDL are located in Germany and Japan (Figure 1.28).

Figure 1.26 Fuel cell schematics.

Figure 1.27 Fuel cell demand distribution by application.

Redox flow battery systems are suitable for stationary energy storage systems. As carbon components they contain graphite felt and a bipolar plate out of graphite. Although this storage system is not yet widely installed, the forecast is promising (Figure 1.29). Yet the production capacities are small (Figure 1.30).

Graphite is an interesting candidate for systems for the storage of thermal energy. The thermal conductivity of fine‐dispersed graphite can be used in cooling and heating systems, for example, for the room conditioning of buildings or the storage of thermal energy. These systems are developed and tested currently. Latent heat storage systems have been commercially installed in air‐conditioning system for trucks.

Figure 1.28 Gas diffusion layer production capacity.

Figure 1.29 Redox flow battery production. Source: EscoVale Study – FlowBatteries, Dec. 2006.