Super-water-repellent surfaces can generate energy

Scientists knew that they could generate electricity by running salt water across an electrically charged surface. But they could never get the process to make enough energy to be useful. Now engineers have figured out a way to do that. Their trick: Make the water flow over that surface much more quickly. They achieved this by making the surface super water repellent.

Prab Bandaru is a mechanical engineer and materials scientist at the University of California San Diego. His team’s innovation grew out of frustration. None of the other things they tried had worked. One “spur of the moment thing … just happened to work,” he says with a laugh. It was hardly planned.

Scientists describe a surface that repels water as hydrophobic (HY-droh-FOH-bik). The term comes from the Greek words for water (hydro) and hating (phobic). The UCSD team describes the material it uses as super-hydrophobic.

Their new energy system starts with table salt, or sodium chloride. As its name suggests, this salt is made from bonded atoms of sodium and chlorine. When the atoms react to make salt, an electron from a sodium atom breaks off and attaches to a chlorine atom. This turns each neutral atom into a type of charged atom called an ion. The sodium atom now has a positive electrical charge. Opposite charges attract. So that sodium ion now is strongly attracted to the chlorine atom, which now has a negative charge.

When the salt is dissolved in water, the water molecules cause the association between the sodium and chlorine ions to loosen. As this salt water flows over a surface with a negative charge, its positively charged sodium ions will be attracted to it and slow down. Meanwhile, its negatively charged chlorine ions will keep flowing. This breaks the bond between the two atoms. And that releases the energy that had been stored within it.

The challenge was to get the water to move quickly enough. “When the chlorine flows away fast, then the relative velocity between the slow sodium and the fast chlorine is enhanced,” Bandaru explains. And that will increase the electric power it generates.

The team described its innovation on October 3 in Nature Communications. 

This use of a super-water-repellent surface to generate energy is “really, really exciting,” says Daniel Tartakovsky. He’s an engineer at Stanford University who was not involved in the research.

The innovation

Other researchers have tried to use water repellency to boost the energy production of a salt-water electric generator. They did it by adding tiny grooves to the surface. When the water flowed over the grooves, it encountered less friction as it traveled over the air. Yet even though the water flowed faster, the energy production didn’t increase very much. And that, Bandaru says, is because the air also cut the water’s exposure to the negatively charged surface.

His team tried different ways to get around this problem. They tried making the surface more porous. Their idea was to speed the water’s flow by providing even more air at the surface. “We were in the lab, thinking, ‘Why isn’t this working?’” he recalls. “Then we said, ‘Why don’t we put liquid inside [the surface]?’”

It was just a brainstorming idea. The researchers hadn’t done any calculations to figure out if it might work. They just tried replacing the air in the surface’s grooves with oil. And it worked! “We were very surprised,” Bandaru says. “We got a very, very high result for the [electrical] voltage.” To probe whether they’d made some mistake, Bandaru says, they quickly realized “‘We have to try this again!’”

They did several more times. And each time, the results came out the same. “It was reproducible,” Bandaru says. This reassured them that their initial success was no accident.

Later, they examined the physics of the liquid-filled surface. Recalls Bandaru, “It was one of those ‘Duh’ moments when we realized, ‘Of course it had to work.’”

Why it works

Like air, oil repels water. Some oils are far more hydrophobic than air — and can hold a negative charge. Bandaru’s team tested five oils to find which offered the best mix of water repellency and negative charge. Another advantage to using oil: It doesn’t wash away when the water flows over it because a physical force known as surface tension holds it to the grooves.

The team’s newly reported tests offer proof that the concept works. Other experiments will need to test how well it might work on a larger scale — one that might deliver a useful amount of electricity.

But the technique might find use even in small-scale applications. For instance, it might be used as a power source for “lab-on-a-chip” assays. Here, tiny devices perform tests on very small amounts of fluid, such a drop of water or blood. On a larger scale, it might be used to generate electricity from ocean waves, or even using the wastes moving through water-treatment plants. “It doesn’t have to be salt water,” Bandaru explains. “Maybe there is wastewater that contains ions. As long as there are ions in the liquid, one can use this scheme for generating voltage.”

Using a liquid like oil to speed up the flow of water while also conducting electricity might greatly improve the efficiency of such power systems. “If it works,” Tartakovsky says, it might even offer “a big breakthrough in battery technology.”

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.