Mekong River (Sông Cửu Long) could generate thousands of nuclear power plants worth of energy, thanks to a new ‘blue’ membrane

Green energy advocates may soon be turning blue. A new membrane could unlock the potential of “blue energy,” which uses chemical differences between fresh- and saltwater to generate electricity. If researchers can scale up the postage stamp–size membrane in an affordable fashion, it could provide carbon-free power to millions of people in coastal nations where freshwater rivers meet the sea.

Blue Energy’s promise stems from its scale: Rivers dump some 37,000 cubic kilometers of fresh water into the oceans every year. This intersection between fresh- and saltwater creates the potential to generate lots of electricity—2.6 terawatts, according to one recent estimate, roughly the amount that can be generated by 2000 nuclear power plants.

There are several ways to generate power from that mixing. And a couple of blue energy power plants have been built. But their high cost has prevented widespread adoption. All blue energy approaches rely on the fact that salts are composed of ions or chemicals that harbor a positive or negative charge. In solids, the positive and negative charges attract one another, binding the ions together. (Table salt, for example, is a compound made from positively charged sodium ions bound to negatively charged chloride ions.) In water, these ions detach and can move independently.

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, 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 BNNT , a material being investigated for use in high-strength composites, among other things. Because BNNTs are highly negatively charged, the 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 three 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 perpendiculars to the membrane. Until now.

The nanotubes were easy. The lab just buys them from a chemical supply company. The scientists then add these to a polymer precursor that’s spread into a 6.5-micrometer-thick film. To orient the randomly aligned tubes, the researchers wanted to use a magnetic field. The problem: BNNT are NOT magnetic.

Painted the negatively charged tubes with a positively charged coating; the molecules that made it up were too large to fit inside the BNNTs and thus left their channels open. Then added negatively charged magnetic iron oxide particles to the mix, which were affixed to the positively charged coatings.

That gave the team the lever it was looking for. When the researchers applied a magnetic field, they could maneuver the tubes so that most aligned across the polymer film. They then applied ultraviolet light to cure the polymer, locking everything in place. Finally, the team used a plasma beam to etch away some of the material on the top and bottom surfaces of the membrane, ensuring the tubes were open to either side. The final membrane contained some 10 million BNNTs per cubic centimeter.

When the researchers placed their membrane in a small vessel separating salt- and freshwater, it produced four times more power per area than the previous 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. “We’re not exploiting the full potential of the membranes,” the scientist says. That’s because only 2% of the BNNTs were actually open on both sides of the membrane after the plasma treatment. Now, the researchers are trying to increase the number of open pores in their films—which could one day give a long-sought boost to advocates of blue energy.