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Graphene-Based Supercapacitor with an Ultrahigh Energy Density

Nanotek Instruments, Inc. and ‡Angstron Materials, Inc., Dayton, Ohio 45404, United States
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China
§These authors contributed equally to this work
*Corresponding Author:
Summarized in this manuscript are research results that are of great scientific and technological significance.
Technologically, the nano graphene-based, ionic liquid-enabled supercapacitor provides a specific energy density of 85.6 Wh/kg (based on the total electrode weight) at room temperature and 136 Wh/kg at 80°C, both measured at a current density of 1 A/g (corresponding to a high charge/discharge rate).
These are the highest energy density values ever reported for nano carbon material-based supercapaitors dominated by the electric double layer (EDL) mechanism, with minimal contribution from any redox-type pseudocapacitance mechanism.
These energy density values are comparable to that of the Ni metal hydride battery. This new technology provides an energy storage device that stores nearly as much energy as in a battery, but can be recharged in seconds or minutes. We believe that this is truly a breakthrough in energy technology.
Since our research group’s 2006 discovery that graphene can be used as a supercapacitor electrode material, scientists around the globe have been making great strides in improving the specific capacitance of graphene-based electrodes. However, the results still fall sort of the theoretical capacitance of 550 F/g. Despite the theoretically high specific surface area of single-layer graphene (up to 2,675 m2/g), this value has not been achieved in a real supercapacitor electrode due to the high tendency for graphene sheets to re-stack together. To this end, our research has made the following scientific
In this report, we point out that the best strategy to achieve a high capacitance in graphene-based electrodes is to find a way to effectively prevent graphene sheets from sticking to one another face-to-face.
This face-to-face aggregation of graphene sheets can be effectively prevented if curved graphene sheets (instead of flat-shaped) are prepared.
We further confirm that this curved graphene morphology enables the formation of mesopores (> 2 nm) that are readily accessible and wettable by ionic liquids, which are significantly larger in molecular size than, for instance, KOH or H2O in the conventionally used aqueous electrolyte. In order to take advantage of the high operating voltage of ionic liquids (> 4 volts), the pore sizes must be sufficiently large to let the ionic liquid enter the pores and form double layers of charge therein.
Some Experimental Details
Synthesis of Graphene Oxide
Graphene oxide was synthesized according to a modified Hummers method. As an example to illustrate the process, 65 mL 98% sulfuric acid was slowly added to a mixture of 5 grams of natural graphite and 2.5 grams of sodium nitrate in an ice water bath. Then, 15 grams of potassium permanganate was added, one small amount at a time. The oxidation of the mixture was allowed to proceed overnight. The mixture was diluted to 1 L, and 30% hydrogen peroxide was added slowly to quench excess potassium permanganate to give a greenish yellow suspension. The suspension was injected into a forced convention oven in which a stream of compressed air was introduced to produce a fluidized-bed situation. Upon removal of the solvent or liquid, we obtained the desired curved graphene sheets.
Fabrication of Electrode
The pH value of 1 L graphene oxide suspension (1 g/L) was tuned to 10 by 2M sodium hydroxide solution. Subsequently, 0.2 mL hydrazine monohydrate was added and the mixture was heated to 95°C and held for 2 hours. Then it was filtered and dried at vacuum oven to form graphene powder. The electrode material was coated on Al foil. The compressed electrode was
dried in a vacuum oven at 120°C for 12 hours before use. Coin-size capacitor cells were assembled in a glove box using EMIMBF4 ionic liquid electrolyte.
Electrochemical Testing
The specific capacitance was measured with galvanostatic charge/discharge tests using an Arbin SCTS system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on a CH Instruments electrochemical workstation. The capacitance was calculated according to vtICΔΔ×=/)(, where I is the constant discharging current, is the voltage difference from 60% to 40% of the discharge voltage range, and vΔtΔ is the time required to go from 60% to 40% of the discharge voltage range.
Characterization of Mesoporous Graphene Nanostructure
To analyze the surface area of the mesoporous graphene nanostructure, low-temperature nitrogen sorption experiments were performed by using a volumetric adsorption apparatus (Quantachrome Instruments). The volume of mesopores was derived from the data of BJH pore size distribution.
2. Results and Discussion
Supercapacitor performance
05010015020025030001234 Voltage (V)Discharge time (s)
Figure S1. (a) Discharge curve at 0.5, 1 and 2 A/g current density, and (b) discharge curve at 1 A/g and 2, 2.5, 3, 3.5 and 4 V, using EMIMBF4 ionic liquid electrolyte. Each electrode is 6.6 mg.
From the charge/discharge curve shown in Figure S1a at various current densities we obtained an energy density of 76.3, 85.6 and 90 Wh/kg at 2, 1 and 0.5 A/g current density, respectively. The value changes little with the variation in current density. From the discharge curve comparison of different charging voltages from 2 to 4 volts in Figure S1b, the straight discharge lines are nearly parallel to each other, indicating the good double layer performance.
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