Hierarchically-structured Co₃O₄ nanowire arrays grown on carbon nanotube fibers as novel cathodes for high-performance wearable fiber-shaped asymmetric supercapacitors

Fiber-shaped supercapacitors (FSCs) are lightweight and flexible energy storage devices that have potential applications in portable and wearable electronics. However, FSCs have flawed energy density stemming from a small specific capacitance and low operating voltage, which limits their practical application. This study puts forth a simple and effective approach to grow well-aligned three-dimensional cobalt oxide nanowire arrays (Co₃O₄ NWAs) directly on carbon nanotube fibers (CNTFs). The hybrid fibers obtained an ultrahigh specific capacitance of 734.25 F cm⁻³ (2210 mF cm⁻²) in a three-electrode system. Benefiting from their intriguing features, we successfully fabricated an all-solid-state fiber-shaped asymmetric supercapacitor (FASC) prototype with a stable potential window of 1.6 V. Where the vanadium nitride nanowires/carbon nanotube fibers (VN NWAs/CNTFs) acted as negative electrode and the KOH poly(vinyl alcohol) (PVA) worked as the gel electrolyte. The electrochemical results suggested that the device possessed a high energy density of 13.2 mWh cm⁻³ at a current density of 1.0 A cm⁻³. Besides, the FASC exhibited excellent mechanical flexibility and structural stability. Therefore, this device has great potential for next-generation wearable energy-storage devices. © 2018 Elsevier B.V.