Carbon fiber electrodes for structural supercapacitors were modified to achieve increases in specific capacitance and energy density. In cyclic voltammetry tests with a liquid electrolyte, the specific capacitance of a commercial carbon fiber fabric weave was 0.84 F g −1 or lower, depending on the sweep rate. Impregnation of the carbon fiber with multiwall carbon nanotubes (MWCNT) or electrochemical functionalization of the carbon fiber increased the capacitance to 3 F g −1 . Electrochemical functionalization of the MWCNT-impregnated carbon fiber further increased the capacitance to 7.2 F g −1 . Electrochemical synthesis and deposition of polyaniline on carbon fiber electrodes increased the electrode capacitance to 26 F g −1 . MWCNT and polyaniline were incorporated into structural supercapacitors made of carbon fiber electrodes, glass fiber separator, and poly(ethylene glycol)-based solid polymer electrolyte. Incorporation of MWCNT increased the specific capacitance and energy density 28-fold, to 125 mF g −1 and 17.4 mWh kg −1 , respectively. Mechanical properties were comparable to previously demonstrated structural energy storage devices. Although the specific capacitance of these solid-state supercapacitors was significantly higher than what was previously demonstrated, the energy density, rate capability, and mechanical performance still require significant improvements for multifunctional structural energy storage devices to be competitive with conventional supercapacitors and carbon fiber composites. In multifunctional material systems, two or more functions are performed by the same material, component, or structural unit.1 The goal in the design of such systems is a smaller volume or mass compared to a combination of corresponding monofunctional elements. One of the most common types of multifunctional material systems is structural energy storage, in which a battery, capacitor, or supercapacitor acts as a structural element for the overall system. Various forms of structural energy storage have been described, 2 and these systems have been proposed for application in communications satellites, 3 spacecraft, 4 ground vehicles, 5 and unmanned aerial vehicles (UAV).
6-9Small UAVs are typically powered by rechargeable batteries and are prime candidates for incorporation of a structural battery, for example as part of a wing or fuselage. Incorporation of structural energy storage can theoretically increase the flight endurance time in small UAVs because of the interdependence of the subsystem weights, amount of available energy, and flight endurance. 7,9 Theoretical analysis by our own group predicted as much as 200% increase in flight endurance when structural batteries are fully incorporated into a small UAV.
9Even modest increases in flight time, such as 10% or 20%, would provide significant benefit because missions are often limited to less than an hour endurance.Much of the previous work on structural energy storage has focused on lithium-ion batteries. Some common approaches are to incorporate commerc...