Triboelectric Capacitance Supplementation in Energy Storage Composites with Functionalized Carbon Fiber Electrodes

Structural supercapacitors integrating carbon fiber (CF) electrodes offer promising multifunctionality by combining load-bearing capacity with energy storage. Surface-functionalized CF electrodes paired with electrospun polymeric nanofiber separators can enhance triboelectric performance, while explaining one reason for increased capacitance of structural supercapacitors observed previously under mechanical deformation. Interlayer triboelectric charges from CF/separator interfaces are quantified in cyclic contact-separation mode, harnessing the triboelectric series and materials that are further apart to maximize the output. Triboelectric measurements reveal that desized CF produces the highest instantaneous current output, however, its inherently low dielectric storage and rapid self-discharge render it unsuitable for long-term energy retention. In contrast, CFs functionalized with tailored chemical groups can achieve both robust triboelectric charge generation and stable capacitive output. Furthermore, a hybrid piezoelectric-triboelectric layer within these CF composites enhances charge separation and leverages synergistic piezoelectric polarization to sustain higher voltage across the electrodes (e.g., charge density of 19.9 ± 0.2 µC m^-2 between silicon-functionalized CF and poly(vinylidene fluoride)-trifluoroethylene (PVDF-TrFE) separator, and 23.1 mF g^-1 specific capacitance in a structural supercapacitor device). This work underscores the potential of combining triboelectric and piezoelectric phenomena within structural supercapacitors on the way toward self-powered, load-bearing components in aerospace, automotive, and wearable electronics industries.