Posted on 10/24/2013 1:12:03 PM PDT by Red Badger
Using porous silicon coated with graphene, material scientists at Vanderbilt University have developed the first supercapacitor that is made out of silicon.
Solar cells that produce electricity 24/7, not just when the sun is shining. Mobile phones with built-in power cells that recharge in seconds and work for weeks between charges.
These are just two of the possibilities raised by a novel supercapacitor design invented by material scientists at Vanderbilt University that is described in a paper published in the October 22 issue of the journal Scientific Reports.
It is the first supercapacitor that is made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers. In fact, it should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings.
“If you ask experts about making a supercapacitor out of silicon, they will tell you it is a crazy idea,” said Cary Pint, the assistant professor of mechanical engineering who headed the development. “But we’ve found an easy way to do it.”
Instead of storing energy in chemical reactions the way batteries do, “supercaps” store electricity by assembling ions on the surface of a porous material. As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries.
These properties have allowed commercial supercapacitors, which are made out of activated carbon, to capture a few niche markets, such as storing energy captured by regenerative braking systems on buses and electric vehicles and to provide the bursts of power required to adjust of the blades of giant wind turbines to changing wind conditions. Supercapacitors still lag behind the electrical energy storage capability of lithium-ion batteries, so they are too bulky to power most consumer devices. However, they have been catching up rapidly.
Research to improve the energy density of supercapacitors has focused on carbon-based nanomaterials like graphene and nanotubes. Because these devices store electrical charge on the surface of their electrodes, the way to increase their energy density is to increase the electrodes’ surface area, which means making surfaces filled with nanoscale ridges and pores.
“The big challenge for this approach is assembling the materials,” said Pint. “Constructing high-performance, functional devices out of nanoscale building blocks with any level of control has proven to be quite challenging, and when it is achieved it is difficult to repeat.”
So Pint and his research team – graduate students Landon Oakes, Andrew Westover and post-doctoral fellow Shahana Chatterjee – decided to take a radically different approach: using porous silicon, a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer.
This allowed them to create surfaces with optimal nanostructures for supercapacitor electrodes, but it left them with a major problem. Silicon is generally considered unsuitable for use in supercapacitors because it reacts readily with some of chemicals in the electrolytes that provide the ions that store the electrical charge.
With experience in growing carbon nanostructures, Pint’s group decided to try to coat the porous silicon surface with carbon. “We had no idea what would happen,” said Pint. “Typically, researchers grow graphene from silicon-carbide materials at temperatures in excess of 1400 degrees Celsius. But at lower temperatures – 600 to 700 degrees Celsius – we certainly didn’t expect graphene-like material growth.”
When the researchers pulled the porous silicon out of the furnace, they found that it had turned from orange to purple or black. When they inspected it under a powerful scanning electron microscope they found that it looked nearly identical to the original material but it was coated by a layer of graphene a few nanometers thick.
When the researchers tested the coated material they found that it had chemically stabilized the silicon surface. When they used it to make supercapacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated porous silicon and significantly better than commercial supercapacitors.
The graphene layer acts as an atomically thin protective coating. Pint and his group argue that this approach isn’t limited to graphene. “The ability to engineer surfaces with atomically thin layers of materials combined with the control achieved in designing porous materials opens opportunities for a number of different applications beyond energy storage,” he said.
“Despite the excellent device performance we achieved, our goal wasn’t to create devices with record performance,” said Pint. “It was to develop a road map for integrated energy storage. Silicon is an ideal material to focus on because it is the basis of so much of our modern technology and applications. In addition, most of the silicon in existing devices remains unused since it is very expensive and wasteful to produce thin silicon wafers.”
Pint’s group is currently using this approach to develop energy storage that can be formed in the excess materials or on the unused back sides of solar cells and sensors. The supercapacitors would store excess the electricity that the cells generate at midday and release it when the demand peaks in the afternoon.
“All the things that define us in a modern environment require electricity,” said Pint. “The more that we can integrate power storage into existing materials and devices, the more compact and efficient they will become.”
Research associate Jeremy Mares, graduate student William Erwin, Assistant Professor of Chemical and Biomolecular Engineering Rizia Bardhan and Associate Professor of Electrical Engineering and Computer Science Sharon Weiss also contributed to the research, which was funded by National Science Foundation grants CMMI 1334269 and EPS 1004083 and Army Research Office grant W911BF-09-1-0101.
Publication: Landon Oakes, et al., “Surface engineered porous silicon for stable, high performance electrochemical supercapacitors,” Scientific Reports 3, Article number: 3020; doi:10.1038/srep03020
Material Scientist As distinquished from Material girl, who collects material, but learns nothing.
They move through the graphene as a wave. It’s a wave! The moment to applaud would be now.
It may open up space travel in ways we haven’t dreamed of.
I could see these caps revolutionizing cyclotron research...smaller but much more powerful storage caps needed to store energy for the power bursts necessary to smash atoms...the size of the equipment needed being reduced by orders of magnitude.(reminds me of the condensor used to build the “Interocitor” in “This Island Earth”).
It is serendipitous irony that Carbon, whose Atomic number is 6, which is the basic building block of all life, may also provide the breakthru’s needed for unlimited clean energy production and/or highly efficient storage of electrical power.
I agree! The possibilities a mind-boggling! As an aside,“This Island Earth” is one of my favorite movies. Saw it at The Willow Theater in 1955, when I was 9!
This movie needs a serious remake, though I don’t criticize anything about the original. Another I like is “Forbidden Planet” which was groundbreaking as it influenced a host of directors and sci fi writers. One can see its influence on the Star Trek series, for example.
“Earth vs The Flying Saucers” is another good one!
https://en.wikipedia.org/wiki/Earth_vs._the_Flying_Saucers
Just for perspective, the best lithium polymer batteries deliver about 250 Wh/Kg.
The power density for supercaps is pretty good though, as is their longevity.
Will Dr Pint name the consortium to exploit this nanotech “Pint-sized materials”?
Thanks Red Badger.
That graph shows grapheme caps topping out at about 4.5 Wh/Kg energy density.
Just for perspective, the best lithium polymer batteries deliver about 250 Wh/Kg.
The power density for supercaps is pretty good though, as is their longevity.
..............
I’m not getting this. If grapheme caps top out at about 4.5 Wh/Kg energy density....why are they so much more interesting/better/important/powerful than lithium polymer batteries which deliver about 250 Wh/Kg? Isn’t 250 Wh/Kg many orders of magnitude better/important/powerful than 4.5 Wh/Kg?
It’s not the energy storage that’s attractive, it’s the power density. Lithium batteries take hours to recharge. Supercaps can be recharged in seconds (or deliver their whole charge in seconds).
Batteries generally have fairly short lives; this is certainly true with lithium batteries, whose high performance characteristics are offset by the fact that they're really only just barely stable from a chemistry standpoint. There have been a number of spectacular fires resulting from damaged or overcharged lithium batteries; as they age they become less stable; most lithium battery packs include little special-purpose "smart power" chips to keep them from exploding. These chips monitor their temperature, charge state, and other parameters to keep them stable.
For that reason, I imagine that supercaps might find uses in satellites and other applications where maintenance is impossible.
Another application for supercaps would be in hybrid cars, where you have a regular engine supplying power, and an electrical power source supplying additional power for acceleration, and storing power from regenerative braking.
Graphene - like transistors were in the 50’s....
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.