Flexible carbon fiber based structural supercapacitor composites with solvate ionic liquid-epoxy solid electrolyte

The "mass-less" energy storage technology based on the concept of multifunctional structural energy storage composites has the potential to revolutionize the design and performance of electric vehicles in the future. This work presents the fabrication process and properties of a structural supercapacitor laminate made of surface functionalized carbon fiber (CF), glass fiber separator and an epoxy-solvate ionic liquid bi-continuous electrolyte. CF surface has been electrochemically functionalized with a covalently grafted layer of redox-active poly(o-phenylenediamine) significantly improving the pseudocapcitance. The bi-continuous electrolyte contains 70% (w/w) lithium-triethylene glycol dimethyl ether bis(trifluoromethanesulfonyl)imide [Li-G3]TFSI in RIM935/RIMH936 epoxy resin. This fabricated supercapacitor device demonstrated a specific capacitance of 909 mF/g at 0.5 mA g−1, a 45-fold improvement compared to a device made with unfunctionalized CF. The device showed a maximum energy density of 181.79 mW h kg−1 at 0.5 mA g−1 and power density of 6.18 W kg−1 at 2.5 mA g−1, while maintaining excellent thermal stability, and possesses a multifunctional index of 1.01. They also possess a capacitance retention of 77.4% and coulombic efficiency of 90.9% after 10 000 charge–discharge cycles. Most notably, we show that the capacitance of the composite can be improved simply by bending the device at different angles, effecting a 3-fold increase when bent at 90° and 135°. This is the first time that the geometry of a composite has been shown to influence the electrochemical performance of a capacitor device and opens new avenues of investigation into structural energy optimization.