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December 30th, 2010

Curved graphene sheets: Source of ultrahigh energy density supercapacitor?

Published on December 30th, 2010 | By: pwray@ceramics.org

SEM of curved graphene sheets (scale bar- 10 µm). Credit: Nano Letters, C. Liu, Zhenning Yu, David Neff, Aruna Zhamu and Bor Z. Jang.

A recent paper by a group of researchers working in the U.S. and China discusses the creation of a supercapacitor whose electrodes are prepared from curvy, single-layer graphene sheets, a method that yields remarkable energy densities. Their work appears in a recent issue of Nano Letters.

A major problem with current supercapacitors in electric vehicle applications is their low energy density compared to batteries. A good supercapacitor might have an energy density of 10 Wh/kg compared to 170 Wh/kg for good lithium-ion batteries or even 35 Wh/kg for a lead-acid battery. On the other hand, the batteries have long recharging times.

The researchers, who are connected with Nanotek Instruments, Angstrom Materials (a spin-off of Nanotek) and the Department of Materials Science and Engineering at the Dalian University of Technology, basically use an approach based on electrical double-layer capacitance that leverages the intrinsic high surface area and capacitance of graphene plus the higher voltages that are possible through the use of ionic liquid electrolytes. Heretofore, EDL capacitors typically have used activated carbon as a high surface area electrode material.

Ordinary single graphene sheets obviously have surfaces that are easy to put in contact with an electrolyte. The problem is that when one is dealing with a bunch of ordinary graphene sheets, they tend to restack themselves. When this occurs, the intergraphene pore sizes greatly limit the accessibility to the electroyte.

This group apparently has gotten around the restacking problem by finding a way – few details are provided – of giving the graphene sheets curves. In brief, they say they use a modified Hummers method to form graphene oxide. Then:

“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.”

This morphology causes the sheets to resist stacking during packing and compression into an electrode structure. They say their method maintains a pore size in the range of 2–25 nm.

So, by making coin-sized capacitor cells using the curved graphene and 1-ethyl-3-methlyimidazolium tetraflouroborate (EMIMBF4), they were able to achieve energy densitites of 85.6 Wh/kg at room temperature and 136 WH/kg at 80°C, measured at a current density of 1 A/g.

These energy densities are comparable to nickel metal hydride batteries — with an important difference: They can be charged or discharged rapidly.

 


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3 Responses to Curved graphene sheets: Source of ultrahigh energy density supercapacitor?

  1. Jun Yan says:

    It seems that the IR drop is high,compared to the other graphene-besed supercapacitor(see Fig.c and d ). Therefore, the rate compablity will not be as high as expected.

  2. Robert Radtke says:

    What is the ultimate temperature capability?

  3. This paper claims an ultrahigh energy density, which would be a revolution in the filed. Because of this, it quickly attracted wide attention. Unfortunately, such claims can hurt the entire field of capacitive energy storage by setting expectations unreasonably high. A few major problems with this paper are listed below:

    1. Cyclic voltammograms in Fig. 3b are distorted. They are very far from the rectangular shape of an ideal supercapacitor even at the lowest scan rate of 10 mV/s and they are asymmetric suggesting leakage and very poor Faradic efficiency. Thus, the capacitive behavior of this system is poor. It is expected to have a low power density and high losses.

    2. The electrode material that is claimed to have above 150 F/g capacitance without the pseudocapacitance contribution has only about 500 m^2/g of specific surface area (this number is hidden at the end of online Supporting Info, but a low SSA is obvious from nitrogen sorption isotherms in Fig, 2a) – one of the lowest for carbon electrodes used in supercapacitors. It is surprising how such a low SSA can lead to the gravimetric capacitance claimed by the authors. The claimed value of 154 F/g for a 500 m^2/g sample is about 50% higher than the ideal theoretical capacitance of graphene cited in the same article.

    3. Figure S2 in the Online Supporting Info shows a 10% decline in capacitance after only 500 cycles (and this is at 4 A/g – it will be much worse if cycled slowly). This means that only 50% of the initial capacitance will be left after 2500 cycles. Some Li-ion batteries have a longer cycle life! Supercaps are expected to serve from 500,000 to 1,000,000 cycles. The authors probably decompose the electrolyte by cycling cells outside the stability window of EMIMBF4 in contact with carbon (they went to 4 V to claim a high energy density). Thus, this device cannot be called a “supercapacitor” because it has neither high power, nor high Faradic efficiency or a long cycle life, which are 3 main advantages of supercapacitors over batteries.

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