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Nanotechnology battery uses salinity difference between fresh- and seawater to create electricity
Posted: Mar 28th, 2011
(Nanowerk News) Stanford researchers have developed a battery that takes advantage of the difference in salinity between freshwater and seawater to produce electricity.
Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery, said Yi Cui, associate professor of materials science and engineering, who led the research team.
The theoretical limiting factor, he said, is the amount of freshwater available. "We actually have an infinite amount of ocean water; unfortunately we don't have an infinite amount of freshwater," he said.
The cycle that generates electricity in Cui's new battery.
As an indicator of the battery's potential for producing power, Cui's team calculated that if all the world's rivers were put to use, their batteries could supply about 2 terawatts of electricity annually – that's roughly 13 percent of the world's current energy consumption.
The battery itself is simple, consisting of two electrodes – one positive, one negative – immersed in a liquid containing electrically charged particles, or ions. In water, the ions are sodium and chlorine, the components of ordinary table salt.
Initially, the battery is filled with freshwater and a small electric current is applied to charge it up. The freshwater is then drained and replaced with seawater. Because seawater is salty, containing 60 to 100 times more ions than freshwater, it increases the electrical potential, or voltage, between the two electrodes. That makes it possible to reap far more electricity than the amount used to charge the battery.
"The voltage really depends on the concentration of the sodium and chlorine ions you have," Cui said. "If you charge at low voltage in freshwater, then discharge at high voltage in sea water, that means you gain energy. You get more energy than you put in."
Once the discharge is complete, the seawater is drained and replaced with freshwater and the cycle can begin again. "The key thing here is that you need to exchange the electrolyte, the liquid in the battery," Cui said. He is lead author of a study published in the journal Nano Letters earlier this month ("Batteries for Efficient Energy Extraction from a Water Salinity Difference") [ http://pubs.acs.org/doi/abs/10.1021/nl200500s?journalCode=nalefd ].
In their lab experiments, Cui's team used seawater they collected from the Pacific Ocean off the California coast and freshwater from Donner Lake, high in the Sierra Nevada. They achieved 74 percent efficiency in converting the potential energy in the battery to electrical current, but Cui thinks with simple modifications, the battery could be 85 percent efficient.
To enhance efficiency, the positive electrode of the battery is made from nanorods of manganese dioxide. That increases the surface area available for interaction with the sodium ions by roughly 100 times compared with other materials. The nanorods make it possible for the sodium ions to move in and out of the electrode with ease, speeding up the process.
Other researchers have used the salinity contrast between freshwater and seawater to produce electricity, but those processes typically require ions to move through a membrane to generate current. Cui said those membranes tend to be fragile, which is a drawback. Those methods also typically make use of only one type of ion, while his battery uses both the sodium and chlorine ions to generate power.
Cui's team had the potential environmental impact of their battery in mind when they designed it. They chose manganese dioxide for the positive electrode in part because it is environmentally benign.
The group knows that river mouths and estuaries, while logical sites for their power plants, are environmentally sensitive areas.
"You would want to pick a site some distance away, miles away, from any critical habitat," Cui said. "We don't need to disturb the whole system, we just need to route some of the river water through our system before it reaches the ocean. We are just borrowing and returning it," he said.
The process itself should have little environmental impact. The discharge water would be a mixture of fresh and seawater, released into an area where the two waters are already mixing, at the natural temperature.
One of Cui's concerns is finding a good material for the negative electrode. He used silver for the experiments, but silver is too expensive to be practical.
His group did an estimate for various regions and countries and determined that South America, with the Amazon River draining a large part of the continent, has the most potential. Africa also has an abundance of rivers, as do Canada, the United States and India.
But river water doesn't necessarily have to be the source of the freshwater, Cui said.
"The water for this method does not have to be extremely clean," he said. Storm runoff and gray water could potentially be useable.
A power plant operating with 50 cubic meters of freshwater per second could produce up to 100 megawatts of power, according to the team's calculations. That would be enough to provide electricity for about 100,000 households.
Cui said it is possible that even treated sewage water might work.
"I think we need to study using sewage water," he said. "If we can use sewage water, this will sell really well."
Source: Stanford University (Louis Bergeron)
http://www.nanowerk.com/news/newsid=20754.php
Who needs batteries!!
Creating power from water
[With a little help from the sun!]
March 25, 2011 by Katie Gatto
Image credit: J. Am. Chem. Soc., 2009, 131 (11), pp 3838-3839. DOI: 10.1021/ja900023k [ http://pubs.acs.org/doi/abs/10.1021/ja900023k ]
VIDEOS:
Batteries charge very quickly and retain capacity, thanks to new structure
Paul Braun, professor of materials science and engineering, center, led the research group of graduate student Xindi Yu, left, and postdoctoral researcher Huigang Zhang that developed a three-dimensional nanostructure for battery cathodes that allows for dramatically faster charging and discharging without sacrificing energy storage capacity.
Photo by
L. Brian Stauffer
Paul Braun, professor of materials science and engineering, center, led the research group of graduate student Xindi Yu, left, and postdoctoral researcher Huigang Zhang that developed a three-dimensional nanostructure for battery cathodes that allows for dramatically faster charging and discharging without sacrificing energy storage capacity.
3/21/11 | Liz Ahlberg, Physical Sciences Editor | 217-244-1073; eahlberg@illinois.edu
CHAMPAIGN, Ill. — The batteries in Illinois professor Paul Braun’s lab look like any others, but they pack a surprise inside.
Braun’s group developed a three-dimensional nanostructure for battery cathodes that allows for dramatically faster charging and discharging without sacrificing energy storage capacity. The researchers’ findings will be published in the March 20 advance online edition of the journal Nature Nanotechnology.
Aside from quick-charge consumer electronics, batteries that can store a lot of energy, release it fast and recharge quickly are desirable for electric vehicles, medical devices, lasers and military applications.
“This system that we have gives you capacitor-like power with battery-like energy,” said Braun, a professor of materials science and engineering. “Most capacitors store very little energy. They can release it very fast, but they can’t hold much. Most batteries store a reasonably large amount of energy, but they can’t provide or receive energy rapidly. This does both.”
The performance of typical lithium-ion (Li-ion) or nickel metal hydride (NiMH) rechargeable batteries degrades significantly when they are rapidly charged or discharged.
Making the active material in the battery a thin film allows for very fast charging and discharging, but reduces the capacity to nearly zero because the active material lacks volume to store energy.
Braun’s group wraps a thin film into three-dimensional structure, achieving both high active volume (high capacity) and large current. They have demonstrated battery electrodes that can charge or discharge in a few seconds, 10 to 100 times faster than equivalent bulk electrodes, yet can perform normally in existing devices.
This kind of performance could lead to phones that charge in seconds or laptops that charge in minutes, as well as high-power lasers and defibrillators that don’t need time to power up before or between pulses.
Braun is particularly optimistic for the batteries’ potential in electric vehicles. Battery life and recharging time are major limitations of electric vehicles. Long-distance road trips can be their own form of start-and-stop driving if the battery only lasts for 100 miles and then requires an hour to recharge.
“If you had the ability to charge rapidly, instead of taking hours to charge the vehicle you could potentially have vehicles that would charge in similar times as needed to refuel a car with gasoline,” Braun said. “If you had five-minute charge capability, you would think of this the same way you do an internal combustion engine. You would just pull up to a charging station and fill up.”
All of the processes the group used are also used at large scales in industry so the technique could be scaled up for manufacturing.
The key to the group’s novel 3-D structure is self-assembly. They begin by coating a surface with tiny spheres, packing them tightly together to form a lattice. Trying to create such a uniform lattice by other means is time-consuming and impractical, but the inexpensive spheres settle into place automatically.
Then the researchers fill the space between and around the spheres with metal. The spheres are melted or dissolved, leaving a porous 3-D metal scaffolding, like a sponge. Then a process called electropolishing uniformly etches away the surface of the scaffold to enlarge the pores and make an open framework. Finally, the researchers coat the frame with a thin film of the active material.
The result is a bicontinuous electrode structure with small interconnects, so the lithium ions can move rapidly; a thin-film active material, so the diffusion kinetics are rapid; and a metal framework with good electrical conductivity.
The group demonstrated both NiMH and Li-ion batteries, but the structure is general, so any battery material that can be deposited on the metal frame could be used.
“We like that it’s very universal, so if someone comes up with a better battery chemistry, this concept applies,” said Braun, who is also affiliated with the Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at Illinois. “This is not linked to one very specific kind of battery, but rather it’s a new paradigm in thinking about a battery in three dimensions for enhancing properties.”
The U.S. Army Research Laboratory and the Department of Energy supported this work. Visiting scholar Huigang Zhang and former graduate student Xindi Yu were co-authors of the paper.
Editor's note: To contact Paul V. Braun, call 217-244-7293; e-mail pbraun@illinois.edu.
The paper, "Three-Dimensional Bicontinuous Ultrafast-Charge and -Discharge Bulk Battery Electrodes." is available online.
[ http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.38.htm ]
http://news.illinois.edu/news/11/0321batteries_PaulBraun.html
See also:
http://www.engadget.com/2011/03/25/styrofoam-touches-electrodes-to-create-incredibly-fast-charging/
Energy production and storage developments using nanotechnology.
