Ultra-thin films of rust -- iron oxide -- can generate electricity when saltwater flows over them, scientists have found.
These films represent an entirely new way of generating electricity and could be used to develop new forms of sustainable power production, according to the study published in the journal 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, said Tom Miller, a professor at California Institute of Technology in the US.
In contrast, the new approach developed by researchers, including those from Northwestern University in the US, does not involve chemical reactions, but rather converts the kinetic energy of flowing saltwater into electricity.
The phenomenon, the electrokinetic effect, has been observed before in thin films of graphene -- sheets of carbon atoms arranged in a hexagonal lattice -- and it is remarkably efficient, researchers said.
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The effect is around 30 per cent efficient at converting kinetic energy into electricity. For reference, the best solar panels are only about 20 per cent efficient, they said.
"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 said.
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, Miller said.
"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," he said.
Though rust will form on iron alloys on its own, the team needed to ensure it formed in a consistently thin layer.
The researchers used a process called physical vapour deposition (PVD), which turns normally 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 researchers took that rust-coated iron and flowed saltwater solutions of varying concentrations over it, they found that it generated several tens of millivolts.
"For perspective, plates having an area of 10 square metres each would generate a few kilowatts per hour -- 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," he said.
The mechanism behind the electricity generation is complex, involving ion adsorption and desorption.
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.