Run Saltwater Over This Nanofilm to Make Electricity

New saltwater electrical generation method could power submarines or even implantable medical devices

3 min read

Margo Anderson is senior associate editor and telecommunications editor at IEEE Spectrum.

Image of water droplets on a slide with the Northwestern logo in the background.
Photo: Jana Butman/Geiger group

A new form of electricity generation involving saltwater (or other liquids with ions in them) running over ultrathin metal strips promises to add new life to the tidal or wave-energy power marketplace.

The new method centers around ultrathin iron or nickel surfaces that have been evaporated onto a plastic or glass slide. Ocean water droplets, for instance, running across the surface could be made to induce an electric current in the underlying metal film.

A number of other possible applications have been suggested, too, including a microsize power source powered by blood flow for implantable devices and even—running the process in reverse—a silent propulsion system for boats or submarines.

Franz Geiger, professor of chemistry at Northwestern University, says he first hatched this novel idea last summer when he was at a conference in Telluride, Colo. Seeing a presentation on inducing electrons to move through a sheet of graphene, he says he realized that he could achieve much the same effects with everyday, inexpensive ingredients.

“In my group we had developed these nano-metal films,” he says. “And I thought, ‘This has got to be the exact same thing.’ Except that now we go from [sheets that are] one carbon atom thick to 10 nanometers of iron. And as long as you insulate it, it’ll be great. It should totally work. And we can do hundreds of square meters of surface coverage with the metals in a single step, as opposed to trying to get a couple square inches of graphene using really complicated, multistep processes.”

The basic idea, Geiger says, is that the sodium ions in a drop of saltwater electrically induce a mirror charge in the 10- to 30-nanometer-thick metal film. A thin layer of oxidation on top of the film (rust in the case of iron), he says, should provide enough insulation to keep the positive sodium ions in the water drop separated from the freed electrons in the metal film.

Then as the water drop travels down the slide containing the film, it drags the mirror electrons in the metal with it. Which is an electrical current—though a very small one, in the case of a single saltwater drop.

Geiger says that even sitting in the room, he recognized this was a potentially significant discovery. “I sketched this on a sheet of paper and had my colleague sign it,” he says.

Geiger and six colleagues published their findings in a recent issue of Proceedings of the National Academy of SciencesThe process, they suggest, appears to be able to be scaled up.

Their initial calculations (though as yet untested) suggest that even generating kilowatt-hours’ worth of electricity might be achievable. By their estimates, a square 10-meter-by-10-meter stack of 100 thin plastic sheets (each sheet coated on both sides with the thin metal film) could potentially, using their method, generate as much as 2- to 5-kilowatt-hours of electrical power.

“That’s essentially a standard U.S. home, running an A/C and TV and those things,” he says. “And if you can do it with 100 of these plates, you can probably do it with a million.”

Of course, Geiger quickly adds, “We’ve got a device that works, and we’ve got a number of ways we can run it. But we have not, for example, yet lit up a lightbulb.”

Human blood also has ions in it, he says. So it’s possible—though realistically farther off still—to imagine using this power generation technology in an implanted device in a blood vessel, which could harness blood flow to generate small but possibly usable trickles of electricity.

On the other hand, plenty of other engineering problems would need to be solved first, including preventing the buildup of films on the implanted device—films that might break off into the bloodstream and potentially, if they built up in the heart or brain, trigger a heart attack or stroke.

Like any other power generation technology, Geiger’s group’s “metal nanolayer” technique could also be run in reverse. That is, instead of using moving drops of saltwater to generate electrical current in the film, use electrical currents in the film to move saltwater.

The device, he says, has no moving parts. So the saltwater would be moved silently. And ocean water is very salty, he adds. Which would only increase the effectiveness of this possible electric saltwater pump—or silent propulsion system.

Not surprising, then, that their paper acknowledges the Office of Naval Research, among other funders.

“Any time you put anything into water, your ultimate limit is going to be bio-buildup,” Geiger says (in other words, gunk). He adds, however, that it’s possible that problem could be solved by, he says, “zapping it.” That is, driving high currents through the film from time to time to cook the accumulated gunk off the surface, and then returning to normal electricity-generating or silent-propulsion operations.

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