Boron nitride nanotube membrane creates power by controlling the flow of electrically charged ions in water //

By pumping the positive ions—like sodium or potassium—to the other side of a semipermeable membrane, researchers can create two pools of water: one with a positive charge, and one with a negative charge. If they then dunk electrodes in the pools and connect them with a wire, electrons will flow from the negatively charged to the positively charged side, generating electricity. //

In 2013, French researchers made just such a membrane. They used a ceramic film of silicon nitride—commonly used in industry for electronics, cutting tools, and other uses—pierced by a single pore lined with a boron nitride nanotube (BNNT), //

Because BNNTs are highly negatively charged, the French team suspected they would prevent negatively charged ions in water from passing through the membrane (because similar electric charges repel one another). Their hunch was right. They found that when a membrane with a single BNNT was placed between fresh- and saltwater, the positive ions zipped from the salty side to the fresh side, but the negatively charged ions were mostly blocked.

The charge imbalance between the two sides was so strong that the researchers estimated a single square meter of the membrane—packed with millions of pores per square centimeter—could generate about 30 megawatt hours per year. That’s enough to power more than 400 homes. //

But creating even postage stamp–size films has proved impossible, because no one has figured out how to make all of the long, thin BNNTs line up perpendicular to the membrane. Until now. //

When the researchers placed their membrane in a small vessel separating salt- and freshwater, it produced 8000 times more power per area than the previous French team’s BNNT experiment. That power boost, Shan says, is likely because the BNNTs they used are narrower, and thus do a better job of excluding negatively charged chloride ions.

And they suspect they can do even better.