Weight and volume are often at a premium in engineering; any material that does not contribute to the load-carrying capacity is structurally parasitic. Current engineering design pursues optimisation of the individual components by utilising materials with improved specific properties. The alternative is to formulate multifunctional materials which can perform two or more functions simultaneously. Here, we develop multifunctional composite materials that can simultaneously carry mechanical loads whilst storing (and delivering) electrical energy. This work was initially motivated by the recognition that carbon-based materials are often used both for electrochemical devices, and high performance structural composites; in addition, both electrochemical devices and structural fibre-reinforced polymer composites are usually assembled in a laminated form. Thus, the goal was to produce a multifunctional structural supercapacitor built around laminated structural carbon fibre fabrics. Each cell of the proposed structural supercapacitor consists of two modified structural carbon fibre fabric electrodes, separated by a structural glass fibre fabric or polymer membrane, infused with a multifunctional polymeric electrolyte.
In order to simultaneously maximise the mechanical and electrical performance, the reinforcing fibres must be modified to increase electrochemical surface area whilst maintaining their intrinsic performance. Rather than using conventional activated carbon fibres, structural carbon fibres were treated to produce a mechanically robust, high surface area material, using a variety of methods, including direct etching, carbon nanotube sizing, and carbon nanotube in situ growth. One of the most promising approaches is integrate a porous bicontinuous monolithic carbon aerogel throughout the matrix. This nanostructured matrix both provides a dramatic increase in active surface area of the electrodes, and has the potential to address mechanical issues associated with matrix-dominated failures. The conflicting requirements for matrix stiffness and ionic conductivity can also be addressed using bicontinuous architectures.
Working structural supercapacitor composite prototypes have been produced and characterised electrochemically using impedance spectroscopy, cyclic voltammetry and charge-discharge measurements. The effect of introducing the necessary multifunctional resin on mechanical properties has also been assessed. Larger scale demonstrators have been produced, including a full size car boot/trunk lid as part of the STORAGE consortium.