Graphene supercapacitor breaks storage record

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Nov 26, 2010

Researchers in the US have made a graphene-based supercapacitor that can store as much energy per unit mass as nickel metal hydride batteries – but unlike batteries, it can be charged or discharged in just minutes or even seconds. The new device has a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C. These are the highest ever values for "electric double layer" supercapacitors based on carbon nanomaterials.

Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge than conventional capacitors. An important feature of supercapacitors is that there is an extremely narrow gap between the electrodes – which are ultrathin layers. This means that a large amount of electrical charge can be stored in a tiny volume.

The new device was made by Bor Jang of US-based Nanotek Instruments and colleagues. It has electrodes made of graphene mixed with 5wt% Super P (an acetylene black that acts as a conductive additive) and 10wt% PTFE binder. A sheet of carbon just one atom thick, graphene is a very good electrical conductor as well as being extremely strong and flexible.

The researchers coat the resulting slurry onto the surface of a current collector and assemble coin-sized capacitors in a glove box. The electrolyte-electrode interface is made of "Celguard-3501" and the electrolyte is a chemical called EMIMBF4.

We believe that this is truly a breakthrough in energy technology Bor Jang, Nanotek Instruments

The energy density values of the supercapacitor are comparable to that of nickel metal hydride batteries. "This new technology makes for an energy storage device that stores nearly as much energy as in a battery but which can be recharged in seconds or minutes," Jang explained. "We believe that this is truly a breakthrough in energy technology." The device might be used to recharge mobile phones, digital cameras and micro-EVs, he adds.

The team, which includes scientists from Angstron Materials in the US and Dalian University of Technology in China, are now working hard to further improve the energy density of the device. "Our goal is to make a supercapacitor that stores as much energy as the best lithium-ion batteries (for the same weight) but which can still be recharged in less than two minutes," said Jang.

His team first discovered that graphene could be used as a supercapacitor electrode material in 2006. Since then, scientists around the world have made great strides in improving the specific capacitance of these electrodes but the devices still fall short of the theoretical capacitance values of 550 F/g.

"Despite the theoretically high specific surface area of single-layer graphene (which can reach up to 2.675 m2/g), a supercapacitance of 550 F/g has not been reached in a real device because the graphene sheets tend to re-stack together," explained Jang. "We are trying to overcome this problem by developing a strategy that prevents the graphene sheets from sticking to each other face-to-face. This can be achieved if curved graphene sheets are used instead of flat ones."

The work was reported in Nano Letters.

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Giant Faraday rotation spotted in graphene

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Nov 15, 2010

The polarization of light can be rotated by almost 6° as it passes through a single sheet of graphene in a magnetic field, according to an international team of physicists. This latest property of graphene – a sheet of carbon just one atom thick – was unexpected because large rotations normally occur only in much thicker materials. The scientists believe that this newly discovered property of graphene could be exploited in new devices that switch light using electric and magnetic fields.

The fact that the polarization of light can rotate as it travels through a material exposed to a magnetic field is, of course, nothing new. Physicists have long known that it is to do with that fact that right- and left-circularly polarized light can propagate at different speeds. It means that when linearly polarized light passes through such a material, the right and left components of the light interfere such that the polarization is rotated by a certain angle when it emerges.

But because the size of this "Faraday angle" is proportional to the thickness of the material, graphene – being just one atomic layer thick – was not expected to generate a large rotation. However, Alexey Kuzmenko and colleagues at the University of Geneva have found that the material can twist the polarization of light by 0.1 radians, or about 6°. Researchers at the Fritz Haber Institute in Berlin and the University of Erlangen-Nueremberg – both in Germany – and the Lawrence Berkeley Laboratory in the US were also involved in the work.

According to Kuzmenko, the team made its discovery while using infrared light to study aspects of the quantum Hall effect in graphene. "We didn't expect to see a large [rotation] in graphene," he says "We expected to see a rotation of about 0.01 radians and instead we saw 0.1 radians." The result means that graphene has a bigger Faraday rotation per atomic layer than any other material – beating out its nearest semiconductor rivals in the infrared by a factor of 10.

The team measured the Faraday rotation by passing infrared light through a polarizing filter to create a linearly polarized beam. This beam was then sent through a graphene sample with a magnetic field perpendicular to its surface. After the light emerged, it was passed through a second polarizing filter and on to a detector. If the polarizations of the two filters are exactly 90° apart, no light should be detected. But if the polarization of the light is rotated as it passes through the graphene, the angle at which no light is detected will be shifted by the Faraday angle.

The physicists believe that the large rotation is a result of graphene's electrons behaving as if they have no mass. When subjected to a magnetic field, the electrons occupy a spectrum of circular "cyclotron" orbits that is very different to that found in other materials. Transitions between these orbits affect the circular polarization of the transmitted light and result in a much enhanced Faraday angle.

According to Kuzmenko, the effect could be used to create switches in which light can travel in one direction, but not in the opposite direction. These optical diodes, known as "Faraday isolators", are not currently available for infrared light.

One important benefit of making such magneto-optical devices from graphene is that the direction of the Faraday rotation can be reversed by simply applying an electric field to the graphene. In other materials, in contrast, this is only possible by reversing the applied magnetic field, which is a slower and more complicated process. The reason, according to Kuzmenko, is graphene's unique ability to change the sign of its charge carriers from negative to positive by simply applying an electric field.

Andrea Ferrari of the University of Cambridge in the UK believes that this newly discovered optical property of graphene is yet more evidence that the material's future lies in photonics and optoelectronics. "The Faraday effect and the associated magneto-optical Kerr effect are widely used in optical communications, data storage and computing," he told physicsworld.com. "These, combined with the [other known] properties of graphene, could lead to uniquely performing devices."

There are, however, several challenges involved in making practical devices. One is that about 10 independent layers of graphene would be needed to achieve a rotation of about 45° – which would be required in practical devices. Another problem is that graphene absorbs infrared light, which would lead to significant signal loss in devices.

The research is published in Nature Physics doi: 10.1038/NPHYS1816.

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Graphene Week 2011: Fundamental Science of Graphene and Applications of Graphene-Based Devices

Chaired by: Vladimir Falko, Lancaster University, UK; Andre Geim, University of Manchester, UK (Nobel Prize in Physics 2010); Karsten Horn, Fritz-Haber-Institut Berlin, DE & Sankar Das Sarma, University of Maryland, US.

The Graphene Week 2011 conference will be devoted to science and technology of graphene, advances in its growth and chemical processing, manufacturing graphene-based devices and studies of electronic transport, investigation of physical properties using ARPES, STM and AFM, emerging applications of this new material. It will also address studies of optical properties of graphene and their applications in optoelectronics, graphene manufacturing by mechanical and chemical exfoliation, synthesis on SiC, and growth on metals and semiconductors. Sessions will also cover the emerging studies of nanomechanical devices incorporating graphene flakes.

The conference programme will incorporate invited reviews, contributing talks, and two poster sessions. The organizers will welcome all researchers working on graphitic systems and scientists and postgraduate students from all countries.

Invited speakers will include:
? Igor Aleiner - Columbia U., US ? Antonio Castro Neto - Boston U., US/ U. of Singapore, SG ? Hongjie Dai * - U. of Stanford, US ? Yuriy Dedkov - FHI Berlin, DE ? Michael Fuhrer - Maryland U., US ? Roman Gorbachev - Manchester U., UK ? Francisco Guinea - Madrid U., ES ? Byung Hee Hong - Sungkyunkwan U., KR ? Euyheon Hwang - Maryland U., US ? James Hone - Columbia U., US ? Alexey Kuzmenko - Geneve U., CH ? Brian LeRoy - U. of Arizona, US ? Alan MacDonald - U. of Texas, US ? Charles Marcus - Harvard U., US ? Konstantin Novoselov - Manchester U., UK ? Tomás Palacios - MIT, US ? Marcos Pimenta - U. of Minas Gerais, BR ? Aron Pinczuk - Columbia U., US ? Eli Rotenberg - Berkeley U., NL ? Tomas Seyller - U. Erlangen-Nürnberg, DE ? Feng Wang - UC Berkeley, US ? Amir Yacoby - Harvard U., US ? Rozitsa Yakimova - Linköping U., SE ? Alex Zettl * - Berkeley U., US (* to be confirmed)

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Topical Research Meetings on Physics: Graphene and Related Two-Dimensonal Materials

The Institute of Physics is launching a new conference series entitled "Topical Research Meetings on Physics". The opening meeting of the series will focus on "Graphene and Related Two-Dimensional Materials". The meeting will focus on the physics and applications of graphene and related materials, and will consist of approximately ten invited talks from international research leaders, plus some shorter papers and posters on late-breaking news. There will be ample time for lively and interactive discussions. The following have already agreed to speak at the meeting:
Professor P Avouris (IBM Yorktown Heights)
A K Geim (University of Manchester)
Byung Hee Hong (Sungkyunkwan University, Korea)
P Kim (Columbia University, USA)

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