Scientists from Caltech and Northwestern University have found a way to generate electricity by combining saltwater with one of life's more undesirable compounds: rust.

The phenomenon discovered by Tom Miller, Caltech professor of chemistry, and Franz Geiger, Dow Professor of Chemistry at Northwestern, converts the kinetic energy of flowing droplets into current.

How their process works: The ions present in saltwater attract electrons in the iron beneath the layer of rust. As the saltwater flows, the ions attract, dragging along the electrons and generating an electrical current.

Such a "electrokinetic" effect has been discovered before in graphene. You can drag saltwater across the atom-thin material to produce electricity.

Scaling the single-layer graphene up to usable sizes, however, is a challenge. Miller and Geiger believe their iron oxide films are easier to produce.

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

To ensure that rust formed on the iron in a consistently thin layer, the researchers turned the solid iron into a gas and then condensed the vapor onto a surface. This kind of "physical vapor deposition" allowed Miller and Geiger to create an iron layer measuring 10 nanometers thick, about 10 thousand times thinner than a human hair.

After running saltwater of varying concentrations over their rust-coated iron, the duo generated several tens of millivolts and several microamps per cm2.

"For perspective, plates having an area of 10 square meters each would generate a few kilowatt-hours — enough for a standard US home," Miller said. "Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term."

Miller and Geiger spoke with Tech Briefs about the promising power of saltwater and rust.

Tech Briefs: Metal compounds and saltwater generate electricity. How is your electrokinetic effect different from this idea, and what led you to this discovery?

Tom Miller: The usual way in which metal compounds and saltwater generate electricity is via corrosion, which involves energy from chemical reactions. That is different than the mechanism we have found, in which the mechanical energy of saltwater moves over the iron film to generate electricity.

Franz Geiger: At a meeting in Telluride last year, I saw work on graphene, which produces electrical energy when drops of ionic liquids slide over it. Having invented oxide-terminated iron nanolayers a few years earlier, I thought that flowing ionic solutions over the metal films should work as well. The real surprise was how well the metal nanolayers responded, which our study indicates has to do with the chemical properties of the oxide overlayer that are lacking in the previously known approaches.

Tech Briefs: With the ability to generate electricity by flowing saltwater over rust, what kinds of applications do you envision?

Miller: There are many exciting applications that can be envisioned, associated with almost any situation in which you have dripping, flowing, or periodically oscillating saline solutions. This includes harvesting electricity from ocean waves, raindrops, and even the pulsing blood in our veins.

Tech Briefs: Blood in our veins?

Geiger: This approach would involve a stent coated with the metal nanolayers and then electrical readouts to produce a micro ampere or so per heart beat, given what we currently know about these devices. With 100 thousand heart beats per day, that’s about 0.1A that can be stored in a capacitor to drive an implanted device, say, an insulin pump, on demand.

Miller: I would add a note of caution. While in-vivo applications like this are very exciting, we would realistically have to contend with the formation of biofilms on the surfaces that could damp the electricity conversion effect. This is an important aspect of the ongoing research.

Tech Briefs: How easily can these oxide films be produced? Is it as simple as getting rust on iron? How do you make these metal films?

Miller: This is one of the major benefits of the nanofilms. They are extremely simple and scalable to produce.

Geiger: It’s about as easy as one, two, three: One is to form a metal nanolayer on a suitable substrate using physical vapor deposition, a common method used, for instance, for coating the inside of chip bags with metal thin films as a moisture barrier. Two is to take the sample out into air and letting its surface oxidize spontaneously, a process that stops after a few nanometers. Three is to flow the ionic solutions over it.

Tech Briefs: What is most exciting to you about this study, and the possibilities of electrokinetic power?

Geiger: The ease of scaling up the metal nanolayer to arbitrarily large areas and the ease with which plastics can be coated gets us to three-dimensional structures so that large volumes of liquids can be used. Foldable designs that fit, for instance, into a backpack are an option as well.

Given how transparent they are, it’s exciting to think about coupling the metal nanolayers to a solar cell, or to think about coating the outside of building windows with metal nanolayers so as to obtain energy during rain events. In vivo applications are also an option so as to generate power with each heart beat in a vein, for instance, to drive an implanted insulin pump.

Miller: I completely agree. The combination of scalability and robustness is exciting, and we’re looking forward to seeing what the future holds!

What do you think the future holds? Share your comments and questions for Miller and Geiger below.

The paper describing their findings, titled "Energy Conversion via Metal Nanolayers," is freely available and appears in the July 29 issue of the Proceedings of the National Academy of Sciences.

CORRECTION: An earlier version of this article misstated that plates having an area of 10 square meters each would generate a few kilowatts per hour. This has been changed to kilowatt-hours.