Flexible and wearable fiber microsupercapacitors based on carbon nanotube-agarose gel composite electrodes

Fiber electrodes provide interesting opportunities for energy storage by providing both mechanical flexibility and the opportunity to impart multifunctionality to fabrics. We show here carbon nanotube (CNT)-embedded agarose gel composite fiber electrodes, with a diameter of 120 μm, consisting of 60 wt % CNTs that can serve as the basis for flexible and wearable fiber microsupercapacitors (mSCs). Via an extrusion process, CNT bundles are induced to align in an agarose filament matrix. Due to the shear alignment of the CNT bundles, the dehydrated filaments have an electrical conductivity as high as 8.3 S cm^-1. The composite fiber electrodes are mechanically stable, enabling formation of twisted two-ply fiber mSCs integrated with a solid electrolyte. The fiber mSC shows a high capacitance (∼1.2 F cm^-3), good rate retention (∼90%) at discharge current densities ranging from 5.1 to 38 mA cm^-3, long cycle life under repeated charging/discharging (10% fade after 10 000 cycles) and good performance after at least 1000 cycles of deformation, with a radius of curvature of 12.3 mm (90° bend). After being coated with a thin layer of poly(dimethylsiloxane), the fiber mSCs could be cycled over 10 000 times under water. Impedance studies indicate that the superior performance is due to the high electrical conductivity along the aligned CNTs and the large electrode surface area that is accessible through the ion-conducting agarose.