Published: June 21, 2011 r2011 American Chemical Society 14006 dx.doi.org/10.1021/jp201673e | J. Phys. Chem. C 2011, 115, 14006–14013 ARTICLE pubs.acs.org/JPCC Functionalized Graphene-Based Nanocomposites for Supercapacitor Application Ashish Kumar Mishra and Sundara Ramaprabhu* Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India 1. INTRODUCTION The increasing popularity of various portable electronic devices and motor vehicles has increased the demand of energy storage devices. In this regard, different batteries and high performance capacitors are the focus of the scientific commu- nity. Supercapacitors, also called electrochemical capacitors or ultracapacitors, are able to provide a huge amount of energy in a short period of time, making them indispensable for surge- power delivery. 1,2 Supercapacitors are mostly used as comple- mentary devices to batteries and fuel cells which have high energy densities but cannot supply a high power during short time scale. Electrochemical capacitors have been considered as promising high-power sources for digital communication de- vices and electric vehicles. Depending on the charge-storage mechanism, they are basically classified into two types: electric double-layer capacitors (EDLCs) based on carbon electrodes 3,4 and pseudocapacitors with certain metal oxides 5À8 or conduct- ing polymers as electrode materials. 9 While the storage me- chanism in carbon-based EDLCs is through electrostatic forces, fast Faradaic redox reactions are responsible for the charge- storage mechanism in pseudocapacitors. 10 Transition metal oxides have been explored as potential electrode materials for use in supercapacitors; their charge storage mechanisms are based predominantly on pseudocapa- citance. RuO 2 has been found to have high capacitance due to redox transitions that even penetrates into the bulk of the material; however, the cost of Ru is one of the concerns for commercial acceptance. 11À14 The present trend in the ongoing research on supercapacitors is to develop economical electrode materials with a high capacity of charge storage and energy density. Cheap metal oxides with comparable characteristics are being investigated, for example, oxides of Ni, Co, In, Sn, Fe, Mn, and so forth, and conducting polymers are another class of material under investigation due to their excellent electroche- mical properties and low cost. Among the various conducting polymers, polyaniline (PANI) has been studied extensively because of its cost-effective and easy synthesis procedure and has good environmental stability, redox reversibility, and elec- trical conductivity. 9,10 However, chemically prepared nano- structured conducting polymers are usually powdery and insulating in their dedoped states. 15 Carbon-based nanomaterials having a high surface area and good electrical conductivity has been attracted the attention of scientific community for different applications. These carbon- based nanomaterials (activated carbon, carbon nanotubes, and graphene) have been used as substrate for metal oxide nano- particles for supercapacitor applications. 16À25 These conduct- ing carbon materials provide a fast electron transfer rate during Faradaic charge transfer reactions and hence enhance the capacitance. Additionally, these carbon nanomaterials provide Received: February 20, 2011 Revised: June 10, 2011 ABSTRACT: A modern technological society demands the use and storage of energy on a large scale. In this regard, the development of high performance supercapacitors is the focus of current scientific research. Graphene, due to its excellent properties, has attracted attention for supercapacitor applications. In the present work, graphene is synthesized via hydrogen-induced exfoliation and is further functionalized to decorate with metal oxide (RuO 2 , TiO 2 , and Fe 3 O 4 ) nanoparticles and polyaniline using the chemical route. Materials are characterized by electron micro- scopy, X-ray diffraction, Fourier transform infrared, and Raman spectros- copy techniques. Electrochemical performance of as-prepared graphene (HEG), functionalized graphene (f-HEG), RuO 2 -f-HEG, TiO 2 -f-HEG, Fe 3 O 4 -f-HEG, and PANI-f-HEG (PANI = polyaniline) nanocomposites is examined using cyclic voltammetry and galvanostatic chargeÀdischarge techniques for supercapacitor applications. A maximum specific capacitance of 80, 125, 265, 60, 180, and 375 F/g for HEG, f-HEG, RuO 2 -f-HEG, TiO 2 -f- HEG, Fe 3 O 4 -f-HEG, and PANI-f-HEG nanocomposites, respectively, is obtained with 1 M H 2 SO 4 as the electrolyte at the voltage sweep rate of 10 mV/s. The specific capacitance for each nanocomposites sustains up to 85% even at higher voltage sweep rate of 100 mV/s. A simple and cost-effective preparation technique of graphene and its nanocomposites with good capacitive behavior encourages its commercial use.