Water droplets on a metal nanolayer. Photo: Franz Geiger, Northwestern University.
Water droplets on a metal nanolayer. Photo: Franz Geiger, Northwestern University.

There are many ways to generate electricity – batteries, solar panels, wind turbines and hydroelectric dams, to name a few examples. And now there's also rust.

New research conducted by scientists at California Institute of Technology (Caltech) and Northwestern University shows that thin films of rust – iron oxide – can generate electricity when saltwater flows over them. These films represent an entirely new way of generating electricity and could be used to develop new forms of sustainable power production. The scientists report their findings in a paper in the Proceedings of the National Academy of Sciences.

Interactions between metal compounds and saltwater often generate electricity, but this is usually the result of a chemical reaction in which one or more compounds are converted to new compounds. Reactions like these are what is at work inside batteries.

In contrast, the phenomenon discovered by Tom Miller, a professor of chemistry at Caltech, and Franz Geiger, a professor of chemistry at Northwestern, does not involve chemical reactions. Rather, it works by converting the kinetic energy of flowing saltwater into electricity.

This phenomenon, the electrokinetic effect, has been observed before in thin films of graphene – sheets of carbon atoms arranged in a hexagonal lattice – and is around 30% efficient at converting kinetic energy into electricity. For reference, the best solar panels are only about 20% efficient.

"A similar effect has been seen in some other materials. You can take a drop of saltwater and drag it across graphene and see some electricity generated," Miller says.

However, it is difficult to fabricate graphene films and scale them up to usable sizes. The iron oxide films discovered by Miller and Geiger are relatively easy to produce and are also scalable to larger sizes.

"It's basically just rust on iron, so it's pretty easy to make in large areas," Miller explains. "This is a more robust implementation of the thing seen in graphene."

Though rust will form on iron alloys on its own, the team needed to ensure it formed in a consistently thin layer. To do that, they used a process called physical vapor deposition (PVD), which turns normally solid materials, in this case iron oxide, into a vapor that condenses on a desired surface. PVD allowed them to create an iron oxide layer just 10nm thick.

When they took that rust-coated iron and flowed saltwater solutions of varying concentrations over it, they found that it generated several tens of millivolts and several microamps per cm2.

"For perspective, plates having an area of 10m2 each would generate a few kilowatts per hour – enough for a standard US home," Miller says. "Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term."

The mechanism behind the electricity generation is complex, involving ion adsorption and desorption. Essentially, though, ions present in saltwater attract electrons in the iron beneath the layer of rust. As the saltwater flows, so do those ions, and through that attractive force, they drag the electrons in the iron along with them, generating an electrical current.

Miller says this effect could be useful in specific scenarios where there are moving saline solutions, like in the ocean or the human body.

"For example, tidal energy, or things bobbing in the ocean, like buoys, could be used for passive electrical energy conversion," he says. "You have saltwater flowing in your veins in periodic pulses. That could be used to generate electricity for powering implants."

This story is adapted from material from Caltech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.