Resource World Magazine

A LT ER NATI V E EN ER G Y R E VI E W Developments in Alternative Energy RESEARCHERS DEVELOP CARBON SOLAR CELL Stanford University scientists report that they have built the first solar cell made entirely of carbon, a promising alternative to the expensive materials used in photovoltaic devices. "Carbon has the potential to deliver high performance at a low cost," said Zhenan Bao, a professor of chemical engineering at Stanford. "To the best of our knowledge, this is the first demonstration of a working solar cell that has all of the components made of carbon. This study builds on previous work done in our lab." Unlike rigid silicon solar panels that adorn many rooftops, Stanford's thin film prototype is made of carbon materials that can be coated from solution. "Perhaps, in the future, we can look at alternative markets where flexible carbon solar cells are coated on the surface of buildings, on windows or on cars to generate electricity," Bao said. The coating technique also has the potential to reduce manufacturing costs. Researchers note that the device can be built using simple coating methods that don't require expensive tools and machines. The Bao group's experimental solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes. In a typical thin film solar 74 www.resourceworld.com cell, the electrodes are made of conductive metals and indium tin oxide (ITO). "Materials like indium are scarce and becoming more expensive as the demand for solar cells, touch screen panels, and other electronic devices grows," Bao said. "Carbon, on the other hand, is low cost and Earth-abundant." For the study, Bao and her colleagues replaced the silver and ITO used in conventional electrodes with graphene (sheets of carbon that are one atom thick) and singlewalled carbon nanotubes that are 10,000 times narrower than a human hair. "Carbon nanotubes have extraordinary electrical conductivity and light-absorption properties," Bao said. For the active layer, the scientists used material made of carbon nanotubes and "buckyballs" (soccer ball-shaped carbon molecules) just one nanometer in diameter. The research team recently filed a patent for the device. Other groups have reported making all-carbon solar cells, but they were referring to just the active layer in the middle, not the electrodes. One drawback of the all-carbon prototype is that it primarily absorbs nearinfrared wavelengths of light, contributing to a laboratory efficiency of less than 1% – much lower than commercially available solar cells. "We clearly have a long way to go on efficiency," Bao said. "But with better materials and better processing techniques, we expect that the efficiency will go up quite dramatically. "The Stanford team is looking at a variety of ways to improve efficiency. "Roughness can short-circuit the device and make it hard to collect the current," Bao said. "We have to figure out how to make each layer very smooth by stacking the nanomaterials really well. "The researchers are also experimenting with carbon nanomateri- by Jane Bratun als that can absorb more light in a broader range of wavelengths, including the visible spectrum. "Materials made of carbon are very robust," Bao said. They remain stable in air temperatures of nearly 1,100 degrees Fahrenheit. The ability of carbon solar cells to out-perform conventional devices under extreme conditions could overcome the need for greater efficiency. "Photovoltaics will definitely be a very important source of power that we will tap into in the future," Bao said. "We have a lot of available sunlight. We've got to figure out some way to use this natural resource that is given to us." INDUSTRY PARTNERS EXTRACT METHANE GAS FROM ALASKAN ICE Working in Alaska's Prudhoe Bay, the US Department of Energy and its industry partners, ConocoPhillips [COP-NYSE] and the University of Bergen in Norway, have drilled into a reservoir of methane hydrate that looks like ice but burns like a candle. With the boom in production from hydraulic fracturing, the United States is awash in natural gas for the near future and is considering exporting it, but the Department of Energy (DOE) wants to be ready with methane if there's a need. Methane, the main ingredient of natural gas, comes from buried organic matter after it's ingested by bacteria or heated and cooked. The gas migrates upward, under high pressure and low temperature, and can combine with water to form methane hydrate. Most deposits are below the sea floor, off the continental shelf or under permafrost. Shallow pockets of methane hydrate release potent greenhouse gas into the atmosphere. A Minerals Management Service study in 2008 estimated methane hydrate resources in the northern Gulf of Mexico at 21,000 DECEMBER 2012/JANUARY 2013

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