Electrochemical Capacitor Essay

Submitted By Nate-Dell
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Supporting Online Material for

Laser Scribing of High-Performance and Flexible Graphene-Based
Electrochemical Capacitors
Maher F. El-Kady, Veronica Strong, Sergey Dubin, Richard B. Kaner*

*To whom correspondence should be sent. E-mail: kaner@chem.ucla.edu

Published 16 March 2012, Science 335, 1326 (2012)
DOI: 10.1126/science.1216744
This PDF file includes:
Materials and Methods
Figs. S1 to S16
References (33–39)
Other Supporting Online Material for this manuscript includes the following:
(available at www.sciencemag.org/cgi/content/full/335/6074/1326/DC1)
Movie S1

Materials and Methods
1. Synthesis of Laser Scribed Graphene (LSG) electrodes: GO was synthesized from high purity graphite powder using a modified Hummer’s method as reported previously (33).
Dispersions of GO in water (3.7 mg/mL) were then used to make GO films on various substrates, including polyethylene terephthalate (PET), nitrocellulose membrane (with
0.4 µm pore size), aluminum foil, and regular Xerox paper, among others. GO films were made by either drop-casting or vacuum filtering GO dispersions onto substrates that were previously cut to the size of a CD/DVD media disc. The films were then allowed to dry for 24 hours under ambient conditions (Fig. S1). These films were affixed on top of a
LightScribe enabled DVD media disc and moved into the DVD optical drive for laser treatment. LightScribe is a direct labeling technology that patterns text and graphics onto the surface of a CD/DVD disc. LightScribe DVD drives are commercially available for ~
$20 and the LightScribing process is completely controlled by a standard desktop computer. The drive uses a laser (optimum power output = 5 mW, wavelength = 788 nm) to pattern a computer-generated image onto a light sensitive dye that changes color when hit with the laser; here, we use the GO film instead. The images are patterned in concentric circles, moving outward from the center of the disc as shown schematically in
Fig. 1. The laser irradiation process results in the removal of oxygen species and the reestablishment of the sp2 carbons. This causes a change in the conductivity of the film from the insulating graphite oxide, with a typical resistance of >20 MΩ/sq to highly conducting laser scribed graphene. We have shown that the number of times a film is laser-treated results in a significant and controllable change in conductivity (17). The electrodes used in the fabrication of the electrochemical capacitors are laser irradiated 6 times reaching an excellent conductivity of 1738 S/m. The laser irradiation process takes about 20 min per cycle. The produced laser scribed graphene (LSG) possesses very low oxygen content (only 3.5%) (17) that contributes to the very high cycling stability of the electrochemical capacitor. The thickness of the LSG layer, as measured from crosssectional SEM and profilometry, was found to be ~7.6 µm (Fig. S2). In the actual device, the area made accessible to the electrolyte was 1 cm2, which corresponds to a mass of
36.3 µg of the active material (LSG) per electrode or 72.8 µg for the device.
2. Electromechanical properties of laser scribed graphene electrodes: Flexible electronics have impacted our life in several ways, including flexible transistors, sensors, displays, etc. They are made possible by using flexible, mechanically robust and highly conducting electrodes. Nowadays, there is a great interest in using graphene films as flexible electrodes in these devices because of its excellent electrical, mechanical and optical properties (34). Here, we have investigated the electromechanical properties of the LSG electrodes to explore their potential for flexible devices. Fig. S3A shows the change in the resistance of an LSG electrode as a function of its bending radius. The electrical resistance shows a small decrease (2.8%) upon bending and this change is completely reversible by straightening the electrode back to its