rXXXX American Chemical Society A dx.doi.org/10.1021/jp205568v | J. Phys. Chem. C XXXX, XXX, 000–000 ARTICLE pubs.acs.org/JPCC Graphene-Based Flexible Supercapacitors: Pulse-Electropolymerization of Polypyrrole on Free-Standing Graphene Films Aaron Davies, Philippe Audette, Blake Farrow, Fathy Hassan, Zhongwei Chen, Ja-Yeon Choi, and Aiping Yu* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute for Sustainable Energy, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 b S Supporting Information ’ INTRODUCTION There has recently been a growing demand for energy storage systems with high power for use in such diverse applications as hybrid electric vehicles, personal electronics, and industrial power backup. 1À6 Recent attention has focused on supercapaci- tors (also known as ultracapacitors or electrochemical capacitors) to address these demands. 2,6À14 Supercapacitors are a promising new energy storage system on account of the high chargeÀ discharge rates, simple mechanism, long cycle-life, and high power density that they possess. 5,6,13À17 Focus has presently turned to the creation of flexible supercapacitors for use in various personal soft portable electronics such as cell phones and mp3 players, where flexibility is becoming an increasingly desirable quality. 18,19 Figure 1 shows a schematic representation of a flexible supercapacitor made from graphene/polypyrrole. Supercapacitors gain their capacitive properties from two separate mechanisms; electric double-layer capacitance (EDLC) and pseudocapacitance. 5,6,9,13,15À17 EDLC is a result of the accu- mulation of electrostatically charged layers at the interface between the electrode and electrolyte and is therefore greatly influenced by the surface area of the electrode material. 1,6,8À10,13,15,20 To max- imize EDLC, various forms of high surface area carbon have been investigated such as carbon nanotubes, activated carbon black, and graphene. 1,6,8,12,15,17,21À25 As a prominent material, graphene (G) sheets are 2D, single-atom thick layers of sp 2 -bonded carbon. 26À29 These are of particular interest because it has been recently shown that optically transparent, flexible G films can be created while preserving G’s good electrochemical properties. 15,19,28À35 Pseudocapacitance is a result of reversible, fast faradic reac- tions occurring between an electroactive electrode material and the electrolyte. 1,6,9,10,13,15,17,20 Some pseudocapacitive materials that have been widely studied are metal oxides and electrically conducting polymers (ECPs). 1,2,4,5,8,9,13,15,17,20,21,36,37 ECPs, such as polyaniline and polypyrrole (PPy), have received much attention because of their fast electrochemical switching, low cost, and high specific capacitance values. 2,11,38À41 PPy is parti- cularly appropriate for this application because of the water solubility of the pyrrole monomer as well as the much lower carcinogenic risks associated with its degradation products compared with polyaniline. 3,39,41 A variety of electrode synthesis methods have been employed to create conductive polymerÀcarbon nanostructure hetero- structures, including one-pot copolymerization, and electrode- position on prefabricated CNT membranes. It is anticipated that the new heterostructure can bring the EDLC and pseudocapa- citive behavior together, leading to a significantly enhanced performance and stability. 6,8,23,36 However, copolymerization with graphene or CNT suspensions suffers from polymeric aggregation and high electrode resistances because of poor interconnection between conducting structures, whereas post- fabrication electrodeposition often blocks electrolyte channels at the outer surface and does not form a conformal coating of polymer. 39,41,42 Much attention has been given to the pulse Received: June 14, 2011 Revised: July 19, 2011 ABSTRACT: A simple method has been implemented to create flexible, uniform grapheneÀpolypyrrole composite films using a pulsed electro- polymerization technique for supercapacitor electrodes. Applying the pseudocapacitive contribution of conformal redox-active polypyrrole to graphene supercapacitor electrodes results in high performance while still maintaining the inherent flexibility of graphene films. Specific capacitances as high as 237 F/g were obtained for a moderate total deposition time of only 120 s, which is approximately four times higher than the blank scaffold, graphene films. This flexible supercapacitor film exhibited very high energy and power densities with values of ∼33 Wh/kg and ∼1184 W/kg, respectively, at a scan rate of 0.01 V/s. This increase was attributed to the favorable nucleation of new polymer chains at defects on the graphene surface, which become less favorable as defect sites are occupied by existing polymer nanoparticles.