Activated carbon fiber yarns with birnessite-type MnO₂ and oxygen-functional groups for high-performance flexible asymmetric supercapacitors

Carbon fiber yarn (CFY) is deemed to be an ideal fiber flexible substrate because of its excellent mechanical flexibility, conductivity, and low-cost. In this study, positive electrode was prepared by hydrothermal process to form a birnessite-type MnO2 interconnected 3D nanostructure loading on the carbon fiber yarn (MnO2@CFY). Using an acid-etching process, the oxygen-containing functional groups successfully introduce to the surface of carbon fiber yarn (OCFY) as the negative electrode. The asymmetric all-solid-state flexible fiber-shaped supercapacitor (AFFSC) based on as-prepared MnO2@CFY and OCFY delivers an excellent volumetric energy density of 11.05 mWh cm(-3) at a volumetric power density of 100.32 mW cm(-3), maintaining 95.33% capacitance retention after 5000 bending cycles.