J. Phys. Chem. C 2010, 114, 19867–19874
19867
Growth of Polyaniline on Hollow Carbon Spheres for Enhancing Electrocapacitance Zhibin Lei, Zhongwei Chen, and X. S. Zhao* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, 117576, Singapore ReceiVed: September 2, 2010; ReVised Manuscript ReceiVed: October 12, 2010
Hollow carbon spheres (HCS) with specific surface areas as high as 2239 m2/g were prepared by chemical vapor deposition with ferrocene as the carbon precursor and colloidal silica spheres as the template. Chemical oxidative polymerization of aniline in the presence of the HCS yielded composite materials with a layer of polyaniline (PANI) deposited on the external surface of the HCS. The electrocapacitive properties of the composite materials (HCS-PANI) with different PANI contents were evaluated using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques. Results showed that the specific capacitances of the HCS before and after PANI coating were, respectively, 268 and 525 F/g in an aqueous H2SO4 electrolyte, which is almost doubly enhanced. A maximum energy density of 17.2 Wh/kg was achieved for the HCS-PANI electrode at a discharge current density of 0.1 A/g. However, the energy density of the HCS-PANI electrodes with higher PANI contents (>65 wt %) declined quickly as the power density increased. An asymmetric supercapacitor using the composite material as the positive electrode and HCS as the negative electrode showed good electrochemical stability, with 73% of the capacitance, 75% of the energy density, and almost 100% of the power density being retained after 1000 cycles at a current density of 1.0 A/g. 1. Introduction Electrochemical capacitors, which are also known as supercapacitors, are electrochemical energy storage devices with unique properties, such as high power, long cyclic life, and fast charge-discharge rates.1,2 Supercapacitors have been used as power back-up systems for emergency use and uninterruptible power supplies for electric or hybrid electric vehicles.1-3 Carbon materials have been used as supercapacitor electrodes, because of their high surface area, good electronic conductivity, and excellent stability.4-6 However, in comparison with batteries, the present supercapacitors with high-surface-area carbon materials as electrodes exhibit a lower energy density (
