Energy Edge: Carbon Chicken WireVaughan Scully
To see what could be the future of the energy industry — and many other industries — simply draw a line with a pencil. Notice the smoothness with which the pencil glides over the paper. If you were to look really hard (or had an electron microscope handy) you could see that the markings made by pencil "lead," which is really graphite, a form of pure carbon, are made from millions of sheets of carbon just one atom thick joined together in a chicken wire-type pattern and stacked on top of each other.
These sheets are loosely bound together, separating easily, which gives the pencil tip its soft feel. While most of the graphite left on the paper will consist of flakes hundreds or thousands of layers thick, chances are some individual layers will detach as well. Those sheets are known as graphene, and some think they promise to create a revolution in the energy industry.
Graphene has many unique and interesting characteristics. It is the thinnest and possibly the strongest material known to man, yet it is also remarkably flexible. It conducts electricity at room temperature better than any other material, allowing electric current to flow at rates approaching the speed of light. It has an enormous surface area per unit of mass, and with current prices quoted near $1 per square micron, graphene is probably the most expensive substance on earth.
Research into graphene has exploded since 2004, when researchers at the University of Manchester developed a new process that isolated individual graphene sheets previously thought to be too unstable to exist at room temperature. In addition to potential applications in electronics and health care, graphene is being touted for its value as a key building block in new energy technologies including solar panels that can capture more sunlight, improved batteries, and possibly a solution to the vexing problem of storing hydrogen gas.
"It's taken off, really, in the past year or so," said Peter Blake, technical director of Graphene Industries, a Manchester, UK-based company that fabricates tiny slivers of graphene for sale to academic and corporate researchers. "We are struggling to keep up with demand."
One of the potential applications for graphene is in the fabrication of transparent conducting films, currently made of indium tin oxide, which is expensive and difficult to use. The world market for transparent conducting films made of indium tin oxide is currently about $1 billion annually (which does not include application) according to Unidym, a subsidiary of Pasadena-based Arrowhead Research (ARWR), which makes carbon-based films and other materials for the electronics industry.
Transparent conducting films are used to make touch screens, liquid crystal displays, flat panel televisions, and photovoltaic solar cells. With solar cells, the transparent conducting film is used to gather electricity produced by the photo-active layer of a "thin-film" cell and route it to the circuit. The high conductivity of graphene, together with the fact that carbon is one of the most abundant materials on Earth, and indium, one of the rarest, make graphene-based transparent conducting films a highly attractive market.
Another photovoltaic application for graphene is with dye-sensitized solar cells. These cells, which work on the same principal as photosynthesis, use titanium dioxide (TiO2) — the main ingredient in white paint — to capture sunlight. Titanium dioxide is used because of its massive surface area, which helps absorb photons from the sun instead of reflecting them. (Titanium dioxide does not trap light well, however, so it is covered in a dye.) Graphene has an even larger surface area for its weight than titanium dioxide, and thus could provide a more powerful cell by trapping more photons. Among the makers of dye-sensitized solar cells are: Australia’s Dyesol (DYSOF) and Japan's Sony (SNE).
One company, Photovoltaic Solar Cells (PVSO) of Fort Pierce, Florida, attempted to make graphene for such an application, but wasn’t able to secure enough funding to move beyond the research phase. "We used these sheets in the same way that TiO2 is used" in dye-sensitized solar cells, said Lawrence Curtin, the company’s founder. "It was used as the scaffolding for the dye." Curtin said he is trying to sell his interest in the company to move on to graphene-related projects.
Energy storage is also a major potential market for graphene. The massive surface area of graphene means it has a large surface area-to-weight ratio, which makes it interesting for developers of electrical components known as supercapacitors or ultracapacitors, because the charge-bearing particles (electrons) can be stored between layers. Capacitors are used throughout electronics to hold small amounts of electricity needed in quick bursts, rather than the slower but steady stream supplied by a battery. While they don’t hold as much charge as a battery, new capacitor designs are beginning to close that gap.
Rechargeable batteries hold electricity by means of a chemical reaction that is reversed when charge is being received.
Ultracapacitors, however, physically store charge-bearing particles in a porous material. Currently, activated carbon is used as the porous material since it has a relatively high surface area to weight ratio of 500 square meters per gram. Graphene, however, has a ratio of 2,630 square meters per gram, according to research published in August by a group at the University of Texas at Austin, which found that "ultracapacitors based on these materials could have the cost and performance that would dramatically accelerate their adoption in a wide range of energy storage applications." Maxwell Technologies (MXWL) makes ultracapacitors.
Other energy storage applications include using graphene instead of graphite as electrodes for the lithium-ion batteries many companies are developing for the new generation of plug-in hybrid electric vehicles due in showrooms in 2010. Graphene could also act as a type of sponge to hold hydrogen atoms if hydrogen fuels ever take off.