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. (C) The Author(s) 2017. Published by ECS.