Rust nanolayers generate power from saltwater
These rust films represent an entirely new way of generating electricity and could be used to develop new forms of sustainable power production.
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. This electrokinetic effect has been observed before in thin films of graphene and can be remarkably efficient. The effect is around 30 percent efficient at converting kinetic energy into electricity, compared to 20 percent for solar panels.
“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,” said Miller. 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.
“It’s basically just rust on iron, so it’s pretty easy to make in large areas,” said Miller. “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 physical vapour deposition (PVD) to create an iron layer 10nm thick, After taking the metal film out of the PVD machine, rust formed spontaneously in air to a thickness of about 2 nanometers. 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 square cm.
“For perspective, plates having an area of 10 square meters each would generate a few kilowatts per hour, enough for a standard US home,” said Miller. “Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term.”
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.
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.”
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