Effect of distribution, interface property and density of hydrogel-embedded vertically aligned carbon nanotube arrays...

In this paper we fabricate a robust flexible solid-state supercapacitor (FSC) device by embedding a conductive poly(vinyl alcohol) hydrogel into aligned carbon nanotube (CNT) arrays. We carefully investigate the effect of distribution, interface properties and densification of CNTs in the gel matrix on the electrochemical properties of an FSC. The total electrochemical capacitance of the device is measured to be 227 mF cm^-3 with a maximum energy density of 0.02 mWh cm^-3, which is dramatically enhanced compared with a similar device composed of non-parallel CNTs. Additionally, controllable in situ electrochemical oxidation greatly improved the compatibility between the hydrophobic CNTs and the hydrophilic hydrogel, which decreased the resistance of the device and introduced extra pseudocapacitance. After such oxidation treatment the energy storage ability further doubled to 430 mF cm^-3 with a maximum energy density of 0.04 mWh cm^-3 . The FSCs based on densified CNT arrays exhibited a much higher volumetric capacitance of 1140 mF cm^-3 and a larger energy density of 0.1 mWh cm^-3, with a large power density of 14 mW cm^-3. All devices show excellent stability of capacitance after at least 10 000 charge-discharge cycles with a loss of less than 2%. These easy-to-assemble hybrid arrays thus potentially provide a new method for manufacturing wearable devices and implantable medical devices.