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Boron Nitride Nanotube Membrane Creates Energy by Controlling the Flow of Electrically Charged Ions in Water
Green energy advocates may soon turn blue. A new membrane could unleash the potential for “blue energy”, which uses the chemical differences between fresh and salt water to generate electricity. If researchers can enlarge the membrane to the size of a postage stamp in an affordable way, it could provide carbon-free energy to millions of people in coastal countries where freshwater rivers meet the sea.
“It’s impressive,” says Hyung Gyu Park, a mechanical engineer at Pohang University of Science and Technology in South Korea, who was involved in the work. “Our field has been anticipating this success for many years.”
The promise of blue energy lies in its magnitude: rivers discharge some 37,000 cubic kilometers of fresh water into the oceans each year. This intersection of fresh water and salt water creates the potential to produce a lot of electricity: 2.6 terawattsaccording to a recent estimate, roughly how much they can generate 2000 nuclear power plants.
There are several ways to produce energy from this mixture. And a few blue energy power stations have been built. But its high cost has prevented its widespread adoption. All blue energy approaches are based on the fact that salts are made up of ions or chemicals that carry a positive or negative charge. In solids, positive and negative charges attract each other, binding ions together. (Table salt, for example, is a compound made up of positively charged sodium ions bonded to negatively charged chloride ions.) In water, these ions are separated and can move independently.
By pumping positive ions, such as sodium or potassium, through a semi-permeable membrane, researchers can create two pools of water: one positively charged and one negatively charged. If they then dip electrodes into the pools and connect them with a wire, the electrons will flow from the negatively charged side to the positively charged side, generate electricity.
In 2013, French researchers produced such a membrane. They used a silicon nitride ceramic film, commonly used in the electronics industry, cutting tools and other uses, perforated with a single pore coated with a boron nitride nanotube (BNNT), a material which is being studied for its use in high resistance. compounds. among other things. Because BNNTs are strongly negatively charged, the French team suspected that they would prevent negatively charged ions in the water from crossing the membrane (because electrical charges repel each other). His intuition was correct. They found that when a membrane with a single BNNT was placed between fresh and salt water, the positive ions passed from the salty side to the cool side, but the negatively charged ions were mostly blocked.
The charge imbalance between the two sides was so strong that the researchers estimated that a single square meter of the membrane, filled with millions of pores per square centimeter, could generate around 30 megawatt hours per year. That’s enough to power three houses.
But creating even postage stamp-sized films proved impossible, because no one figured out how to line up all the long, thin BNNTs perpendicular to the membrane. Until now.
At the biannual meeting of the Materials Research Society, Semih Cetindag, Ph.D. lab student of mechanical engineer Jerry Wei-Jen Shan at Rutgers University in Piscataway, New Jersey, reported that his team has now deciphered the code. Nanotubes were easy. Cetindag says the lab simply buys them from a chemical supply company. The scientists then add them to a polymer precursor which is spread into a film 6.5 micrometers thick. To orient the randomly aligned tubes, the researchers wanted to use a magnetic field. The problem: BNNTs are not magnetic.
Cetindag therefore painted the negatively charged tubes with a positively charged coating; the molecules that made it up were too big to fit inside the BNNTs and thus left their channels open. Cetindag then added negatively charged magnetic iron oxide particles to the mix, which adhere to the positively charged coatings.
This gave the Rutgers team the leverage they were looking for. When the researchers applied a magnetic field, they were able to maneuver the tubes so that they were maximally aligned along the polymer film. They then applied ultraviolet light to harden the polymer, locking everything in place. Finally, the team used a plasma beam to etch some of the material onto the top and bottom surfaces of the membrane, making sure the tubes were open on both sides. The final membrane contained approximately 10 million BNNTs per cubic centimeter.
When the researchers placed their membrane in a small container separating fresh water from salt water, produced four times more energy per area than the previous French team’s BNNT experiment. This increase in power, Shan says, is likely due to the fact that the BNNTs they used are narrower and therefore better exclude negatively charged chloride ions.
And they suspect they can do even better. “We are not exploiting the full potential of membranes,” says Cetindag. This is because only 2% of BNNTS were actually open on both sides of the membrane after plasma treatment. Now, researchers are trying to increase the number of open pores in their films, which could one day give blue energy advocates a much-needed boost.