hurj spring 2015: 2014: issue 20 18 spotlight
science and engineering
Production of Supercapacitors from Liquid-Phase Exfoliated Graphene Alec Farid,§ Yinsheng Guo,‡ Elizabeth Thrall,‡ Giselle Elbaz,‡ Andrew Crowther,‡ and Louis E. Brus‡ § Johns Hopkins University, Baltimore, MD ‡ Department of Chemistry, Columbia University, New York, NY
Abstract Reported here is an effective, low-cost method for the preparation of graphene using a new liquid-phase exfoliation procedure, and the construction of a graphene supercapacitor. The graphene in these studies was produced using high concentrations of sodium cholate followed by treatment with the organic solvent acetonitrile. Treatment with acetonitrile reduced the surfactant concentration from 77% to 15%. The method yielded large amounts of few-layer graphene with high capacitance and conductivity. The supercapacitors were made by drop casting the concentrated graphene solutions onto aluminum substrates, and impedance and charge rate were measured. The capacitance varied between 0.5 mF/g to 0.1k F/g. Notably, all materials and equipment used in this method are easy to obtain and relatively safe, supporting the potential scalability and commercial production of graphenebased supercapacitors.
Introduction Graphene is one of the allotropes of carbon, arranged in a two dimensional lattice of hexagons that resembles a honeycomb pattern. It is very similar to graphite, which consists of multiple stacked layers of graphene. Graphene’s tensile strength is 130 GPa, which is about 3000 times stronger than structural steel, making it the strongest known material (1). This is attributed to the strength of the bonds between the carbon atoms (2). The hexagonal structure of graphene still allows elasticity while maintaining its structural integrity (3). In addition, graphene has remarkably high in-plane thermal conductivity of 5000 W·m-1·K-1, which is more than 10 times that of copper. Furthermore, graphene is being considered in the fabrication of supercapacitors. Supercapacitors, or double-layer capacitors (DLC), are known for having high power densities compared to batteries (4). Use of supercapacitors is appealing because under the correct conditions, they can charge very quickly while releasing stored charge slowly, acting like a quick charging battery. Supercapacitors are particularly appealing as replacements for batteries in electric cars or electronic devices such as phones. Instead of overnight charging for an electric car or phone, with supercapacitor energy storage, the devices could potentially be charged within minutes (5). Independent studies have shown that graphite can be exfoliated in specific solvents, generating few layers of
relatively pristine graphene (6,7) This method relies on solvents with low surface tension to facilitate graphene exfoliation.4 However, the drawback to this process is that the solvents currently used are generally expensive and toxic. A liquid phase process that uses water as the primary solvent and results in a significant yield of exfoliated graphene without oxidation or chemical treatments is highly desirable. Previous research has indicated a relatively straightforward method for producing graphene using surfactant-assisted aqueous phase exfoliation with ultrasonication (8). By modifying this method, we were able to develop a simple, safe, inexpensive, and high-yielding procedure for the exfoliation of graphene. The extracted graphene was of a higher quality and suitable for incorporation into functional capacitors.
Methods and Materials Materials Deionized, filtered water was used for all experiments. Graphite flakes (size 1-3mm), CAS Number 7782-42-5, were as needed. ≥99% pure, solid sodium cholate hydrate, CAS Number 206986-87-0, was used to make stock solutions in water that were then diluted. All materials were obtained from Sigma-Aldrich. Measurement and equipment methods All Raman spectroscopy measurements were performed with a 514 nm argon ion laser focused to a spot of about 1 µm in diameter using a 40x (0.6 NA) objective, with 500 µW power. The grating that was used had 600 grooves/mm. Raman scans were performed in high and low frequency ranges each for 60-300 sec. Thermogravimetric analysis (TGA) was performed using a GSC1 TGA in a nitrogen atmosphere at atmospheric pressure with sample sizes of greater than 2 mg and less than 20 mg, most being 10±2 mg for consistency. Measurements started with settling to 30°C for 2 min., then ramping to 150°C at 20°C/min. and settling for 30 min. This was followed by ramping to 1000°C at 10°C/min. Ultraviolet visible (UV-Vis) absorption spectroscopy was performed with a Hewlett Packard 8453 spectrophotometer at a wavelength range of 200-1100 nm Electrochemical measurements, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry (CP), were performed using a PARSTAT 2263 potentiometer with three electrode cell potentiometer with a working, ref-
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