U.S. Department of Energy’s Brookhaven National Laboratory scientists Dong Su and Eric Stach have helped to uncover the nanoscale structure of a novel form of carbon, contributing to an explanation of why this new material acts like a super-absorbent sponge when it comes to soaking up electric charge.
The excitement is about a new material recently created at The University of Texas at Austin. Called Activated Graphene, it can be incorporated into “supercapacitor” type energy storage devices. Activated graphene properties offer remarkably high storage capacity while retaining other attractive attributes such as super fast energy release, quick recharge time, and an astonishing lifetime of at least 10,000 charge/discharge cycles.
This is major news on the electron storage front.
Eric Stach, a Brookhaven materials scientist and co-author on the paper describing the material published in Science yesterday May 12, 2011 effuses a bit saying, “Those properties make this new form of carbon particularly attractive for meeting electrical energy storage needs that also require a quick release of energy – for instance, in electric vehicles or to smooth out power availability from intermittent energy sources, such as wind and solar power.”
A quick recap on super or ultra capacitors: these devices store electrical charges packing up charged ions on the surfaces within the device. It can be a metaphor to your own body storing up static electricity and fast dumping it when you touch a lower charged person or ground. The surfaces in a capacitor are electrodes, a positive and negative. Between them is an electrolyte. The electric charge coming in is stored in the ions at the interface of the electrodes and the electrolyte. The more surface area of the electrodes the more total capacity is available for a given volume. Today’s common capacitor will take up a comparatively small amount and discharge really quickly – not your ideal storage arrangement like a battery where the total charge would seem to be huge.
But batteries are chemical reaction chambers, where the charging and discharging effect chemical reactions within the battery. You can get a lot in, but getting it in and out is a slow process compared to a capacitor. Big total charge – battery. Fast charge and discharge – capacitor. Long life and big cycling – capacitors. The electron storage predicament described in a few words, but get them both together and change history.
That’s what the EEStor interest is about.
The new material developed by the UT-Austin research group may change that. The experimental supercapacitors made from activated graphene have an energy-storage capacity, or energy density, that’s approaching the energy density of common lead-acid batteries. At the same time the experimental units retain the high power density, or rapid energy release that is a characteristic of capacitors. The claimed 10,000-cycle life equates into more than 27 years of daily charge and discharge cycles.
Look out EEStor.
University of Texas team leader Rodney Ruoff sums that up in an understating way saying, “This new material combines the attributes of both electrical storage systems. We were rather stunned by its exceptional performance.”
This end of the story is even more interesting. The Texans had set out meaning to create a more porous form of carbon (lots more surface area for ions) by using potassium hydroxide to restructure chemically modified graphene platelets, a form of carbon where the atoms are arrayed in tile-like rings laying flat to form single-atom-thick sheets. Such “chemical activation” has been previously used to create various forms of “activated carbon,” which have pores that increase surface area and are used in filters and other applications, including other supercapacitors.
But the Texan’s new form of carbon was testing so much more superior to others used in supercapacitors, the UT-Austin researchers knew they’d need to characterize its structure at the nanoscale.
To exploit the new material much deeper understanding is required. Ruoff theorized that the material consisted of a continuous three-dimensional porous network with single-atom-thick walls, with a significant fraction being “negative curvature carbon,” similar to inside-out buckyballs. There’s the amazing idea.
So Ruoff turned to Stach and his colleague Dong Su at Brookhaven for help with further structural characterization to verify or refute the hypothesis. The Brookhaven team conducted a wide range of studies at the Lab’s Center for Functional Nanomaterials (CFN), the National Synchrotron Light Source (NSLS), and at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory.
The three facilities are joined by support from the US Department of Energy’s Office of Science. Stach said, “At the DOE laboratories, we have the highest resolution microscopes in the world, so we really went full bore into characterizing the atomic structure. Our studies revealed that Ruoff’s hypothesis was in fact correct, and that the material’s three-dimensional nanoscale structure consists of a network of highly curved, single-atom-thick walls forming tiny pores with widths ranging from 1 to 5 nanometers, or billionths of a meter.” Lots of places for those ions to be charged.
The study includes detailed images of the fine pore structure and the carbon walls themselves, as well as images that show how these details fit into the big picture. “The data from NSLS were crucial to showing that our highly local characterization was representative of the overall material,” Stach said.
“We’re still working with Ruoff and his team to pull together a complete description of the material structure. We’re also adding computational studies to help us understand how this three-dimensional network forms, so that we can potentially tailor the pore sizes to be optimal for specific applications, including capacitive storage, catalysis, and fuel cells,” Stach said.
Meanwhile, the scientists say the processing techniques used to create the new form of carbon are readily scalable to industrial production. “This material – being so easily manufactured from one of the most abundant elements in the universe – will have a broad range impacts on research and technology in both energy storage and energy conversion,” Ruoff said.
But what would it cost to build a 1000 amp capacitor?
This is major news, if still a little hard to grasp, this material has good numbers, no mystery, no secrecy and is in the open. Maybe the activated graphene solution isn’t so energy dense as the EEStor capacitor might be, but its competitive and it seems much more likely to get to market and much sooner as well.
For those in the field, the press releases at Brookhaven, The University of Texas at Austin and the paper published in Science are must reads. The Science page of Supporting Online Material even includes a video. (It’s a very large file.)