Scaleable ultra-thin and high power density graphene electrochemical capacitor electrodes manufactured by aqueous exfoliation and spray deposition Beatriz Mendoza-Sa ´ nchez * , Bertold Rasche, Valeria Nicolosi, Patrick S. Grant Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK ARTICLE INFO Article history: Received 27 June 2012 Accepted 15 September 2012 Available online 25 September 2012 ABSTRACT Graphene electrodes of high power density were manufactured by a surfactant-water based exfoliation method followed by a scaleable spray-deposition process. Cyclic voltammetry and galvanostatic charge–discharge experiments revealed a combination of electric double layer and pseudocapacitive behavior that, unlike the many graphene-oxide derived elec- trodes, was maintained to unusually high scan rates of 10,000 mV s 1 , reaching a maxi- mum capacitance of 543 lF cm 2 and with a capacitive retention of 57% at 10,000 mV s 1 . The performance of graphene electrodes was contrasted with carboxylated single walled carbon nanotubes that showed a sharp decrease in capacitance above 200 mV s 1 . Electro- chemical impedance spectroscopy analysis showed a fast capacitor response of 17.4 ms for as manufactured electrodes which was further improved to 2.3 ms for surfactant-free 40 nm thick electrodes. A maximum energy density of 75.4 nW h cm 2 gradually decreased as power density increased up to 2.6 mW cm 2 . Graphene electrodes showed 100% capac- itance retention for 5000 cycles at the high power scan rate of 10,000 mV s 1 . Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Electrochemical capacitors are high power density and long cycle life electrochemical energy storage devices for applica- tions where a high charge–discharge rate is required for short-term power management and delivery. For example, when combined with a battery, power pulses for short-term electrical demand in the millisecond range from electrochem- ical capacitors can extend the life and reduce the size of bat- teries in systems such as mobile phones where circuit RC times <1 s are needed [1,2]. A further recently suggested high power application is alternating current line filtering that de- mands a high frequency capacitive response with RC times <8.3 ms (120 Hz) [3]. The energy storage mechanism of a electrochemical capacitor consists of the formation of an electrical double layer at an electrode–electrolyte interface where the overall capacitance and energy density is proportional to the surface area of the electroactive material [4]. For this reason, nano- structured high surface area carbon materials have been exploited for electrochemical capacitor applications, firstly activated carbon which is fully commercialized, and more re- cently, carbon nanotubes and then graphene [5,6]. Graphene consists of one-atom thick sp 2 -bonded carbon sheet forming a honeycomb two dimensional (2D) nanostructure with un- ique electronic properties: ambipolar electric field effect with high charge carrier (massless Dirac fermions) mobilities inde- pendent of temperature, and a room temperature quantum Hall effect [7–11]. These electronic properties and a theoreti- cal surface area of 2600 m 2 g 1 suggest that graphene may be a promising candidate material for electrochemical capac- itor applications. However the realization of this potential in practice demands cost-effective and scaleable synthesis methods that preserve the key properties. Major challenges 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.09.035 * Corresponding author: Current address: Trinity College Dublin, Schools of Chemistry and Physics and CRANN, Dublin 2, Ireland. E-mail address: mendozab@tcd.ie (B. Mendoza-Sa ´ nchez). CARBON 52 (2013) 337 – 346 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon