Conductive graphene fibers for wire-shaped supercapacitors strengthened by unfunctionalized few-walled carbon nanotubes

Graphene fibers are a promising electrode material for wire-shaped supercapacitors (WSSs) that can be woven into textiles for future wearable electronics. However, the main concern is their high linear resistance, which could be effectively decreased by the addition of highly conductive carbon nanotubes (CNTs). During the incorporation process, CNTs are typically preoxidized by acids or dispersed by surfactants, which deteriorates their electrical and mechanical properties. Herein, unfunctionalized few-walled carbon nanotubes (FWNTs) were directly dispersed in graphene oxide (GO) without preoxidation or surfactants, allowing them to maintain their high conductivity and perfect structure, and then used to prepare CNT-reduced GO (RGO) composite fibers by wet-spinning followed by reduction. The pristine FWNTs increased the stress strength of the parent RGO fibers from 193.3 to 385.7 MPa and conductivity from 53.3 to 210.7 S cm^-1. The wire-shaped supercapacitors (WSSs) assembled based on these CNT-RGO fibers presented a high volumetric capacitance of 38.8 F cm^-3 and energy density of 3.4 mWh cm^-3. More importantly, the performance of WSSs was revealed to decrease with increasing length due to increased resistance, revealing a key issue for graphene-based electrodes in WSSs.