Flexible Solid-state Supercapacitors Based On PVA Aqueous Hydrogel Embedded Vertically Aligned Carbon Nanotube Arrays

Nowadays flexible all solid-state supercapacitors?FSCs?have raised more and more attentions due to the good operability and flexible fabrication process as well as high specific/volumetric energy density compared to conventional capacitors.Aligned carbon nanotube?ACNT?arrays are one of the ideal electrode candidates for energy storage due to its good capacity,highly efficient charge transfer rate,excellent rate performance and long cycle life.However the as-grown ACNT tend to collapse due to the weak interaction between the tubes during the transfer and fabrication process.This limitation makes ACNT an unsuitable candidate for FSC devices.In this paper we describe a simple and efficient method to fabricate a robust,flexible all-solid-state super capacitor device by embedding a conductive poly?vinyl alcohol?hydrogel into the ACNT arrays.The solid gel offers the hybrid device good mechanical stability and flexibility and the ACNTs remain its alignment and distribution during transfer,cutting,and bending process.The composite device shows a total electrochemical volumetric capacitance of 227 mF·cm^-3at a current density of 2.6mA·cm^-3.After 10000 charging-discharging cycles,a capacity loss is measured to be less than 3%?The maximum energy density of SACNT/gel composite device can reach 0.02mW·h·cm^-3with the average power density of 1m W·cm^-3.Such ACNT@PVA hybrid device exhibit tremendous electrochemical performance compared to the long entangled CNTs and short commercial CNTs powders based samples under same conditions.We also used in situ electrochemical oxidation to modify the outer wall of carbon nanotube arrays by simple electrochemical method,The specific capacitance and energy density can further increase to430 mF·cm^-3and 0.04 mW·h·cm^-3.Furthermore,we use ethyl alcohol to shrink the CNT array?ACNTs?to increase the density and mechanical strength.The density of the ACNT increases 8¹0 times while keeping the advantage of highly oriented.At the same time we embed the conductive PVA gel into the carbon nanotube array to fabricate flexible solid supercapacitor,further greatly enhanced the performance of the device.Then the conductive polymer polyaniline?PANI?was introduced into the device by in situ redox polymerization.The composite device ACNT@PVA@PANI shows a total electrochemical capacitance of793 mF·cm^-3at a current density of 16 mAcm^-3.The maximum energy density of composite device can reach 0.23 mW·h·cm^-3with the average power density of 17.43 mW·cm^-3.The high density densified array?DACNT?is used to replace the ACNT to assembly device of ACNT@PVA@PANI.It shows that the specific volumetric capacity of the device is as high as 4.53 Fcm^-3at the current density of 150 mAcm^-3,and the maximum energy density of the device is 0.91 mW·h·cm^-3,corresponding to the average power density of 116 mW·cm^-3.This easy-to-assemble hybrids thus potentially provide a new method for manufacturing wearable devices and implantable medical devices.