There are several ways to generate electricity – batteries, solar panels, wind turbines, and hydroelectric dams – to name a few… and now, there’s rust.
New research by scientists at Caltech and Northwestern University in the U.S. outlines 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.
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 occur inside batteries.
In contrast, the phenomenon discovered by Tom Miller, Caltech Professor of Chemistry, and Franz Geiger, Dow Professor of Chemistry at Northwestern, does not involve chemical reactions, but instead converts the kinetic energy of flowing saltwater into electricity.
The phenomenon, known as the electrokinetic effect, has previously been observed in thin films of graphene (sheets of carbon atoms arranged in a hexagonal lattice) and is remarkably efficient.
The effect is around 30 per cent efficient at converting kinetic energy into electricity. For context, the best solar panels are only roughly 20 per cent 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,” Professor Miller explained. 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 scalable to larger sizes, Professor Miller detailed.
“It’s basically just rust on iron, so it’s pretty easy to make in large areas,” Miller says.
“This is a more robust implementation of the thing seen in graphene.”
Though rust will form on iron alloys on its own, the researchers needed to ensure it formed in a consistently thin layer. To do that, they used a process called physical vapour deposition (PVD), which turns usually solid materials, in this case iron oxide, into a vapour that condenses on a desired surface.
PVD allowed them to create an iron oxide layer 10 nanometres thick, about 10 thousand times thinner than a human hair.
When the research team 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 cm-2.
“For perspective, plates having an area of 10 square metres each would generate a few kilowatts per hour – enough for a standard U.S. home,” Professor Miller said.
“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, but it essentially works like this: The 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.
The Professor believes this effect could be extremely useful in specific scenarios where there are moving saline solutions, such as 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 said.
“You have saltwater flowing in your veins in periodic pulses. That could be used to generate electricity for powering implants.”
The paper describing their findings, Energy Conversion via Metal Nanolayers, appears in the July 29 issue of the Proceedings of the National Academy of Sciences.