Flexible solid-state supercapacitors based on shrunk high-density aligned carbon nanotube arrays

Abstract

Nowadays flexible solid-state supercapacitors (FSCs) have received more and more attention than conventional capacitors due to the good operability and flexible fabrication process as well as high specific/volumetric energy density. In general, carbon based materials including amorphous carbon, carbon nanotube, grapheme, etc. can be used to fabricate electrolytic double-layer capacitance (EDLC)-type FSCs due to its extraordinary cyclic stability at high current density. Aligned carbon nanotube (ACNT) arrays are one of the ideal electrode candidates for energy storage due to their good capacity, highly efficient charge transfer rate, excellent rate performance and long cycle life compared with those of other carbon-based materials carbon nanotubes. However, the low density and the weak interaction between the carbon tubes cause the CNT arrays to tend to easily collapse during processing and transferring. Thus pure carbon nanotube arrays are unable to be directly used to assemble flexible electronic devices. In this paper, we use ethyl alcohol to shrink the CNT array to increase the density and mechanical strength. At the same time we embed the conductive polyvingle alcohol (PVA) gel into the carbon nanotube array to fabricate a flexible solid supercapacitor. Hydrogel-based solid electrolytes have been long considered to be used to prepare FSCs, because this method possesses obvious advantages including low cost, good environmental compatibility and simple manufacturing process. The ACNT/PVA complex can maintain good mechanical stability and flexibility during its folding and bending, and can also keep the high orientation of carbon nanotubes. The maximum capacitance of the hybrid flexible device can reach 458 mFcm-3 at a current density of 10 mAcm-3, which is much higher than the capacitance reported in the literature. After 5000 charging-discharging cycles, a capacity still keeps nearly 100%. The maximum energy density of CNTs/gel composite device can reach 0.04 mWhcm-3 with an average power density of 3.7 mWcm-3. The capacitance can be further increased to 618 mFcm-3 by a simple in-situ electrochemical oxidation treatment. The energy density can be further increased to 0.07 mWhcm-3 by the electro-oxidation treatment. The electrochemical performance of the device is far superior to that of EDLC-typed FSC reported in the literature. Additionally the equivalent series resistance (RESR) of the devices decreases from 120 to 30 and also the charge transfer resistance declines from 90 to 10 . This is mainly due to the effect of pseudo capacitance and electro-wetting effect caused by electro-oxidation. This easy-to-assemble hybrid devices thus potentially pave the way for manufacturing wearable devices and implantable medical devices.