Synergetic design of enlarged surface area and pseudo-capacitance for fiber-shaped supercapacitor yarn

In various wearable energy storage devices, the shape of fiber or yarn has many advantages owing to their compatibility with the environment in which they are deployed. We present a systematic approach to maximizing the capacitance of a supercapacitor yarn by significantly increasing the yarn's surface area by growing a high density of nanorods around the yarn, followed by coating the surface with a pseudo-capacitive material. The two-step strategy is implemented using a dry-spun carbon nanotube yarn-based electrode, which is surrounded by a zinc oxide nanorod forest that is coated by a pseudo-capacitive nickel-cobalt layered double hydroxide material. The flexible as-prepared electrode exhibits a maximum capacitance of 1065 mF cm⁻² (1278 F g⁻¹) at a scan rate of 5 mV s⁻¹ and an excellent capacitance retention of 60.5% over 7000 cycles at a current density of 30 mA cm⁻². The outstanding performance of the composite yarn supercapacitor can be ascribed to the enhanced ion accessibility to the deep surface of the nickel-cobalt layered double hydroxide layer through the porous carbon nanotube yarn. Furthermore, the symmetric supercapacitor configuration demonstrated nearly 100% capacity retention at a bending angle of 150°.