Research 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.
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 new phenomenon does not involve chemical reactions but rather converts the kinetic energy of flowing saltwater into electricity.
The phenomenon, called the electrokinetic effect, has been observed before in thin films of graphene — sheets of carbon atoms arranged in a hexagonal lattice — and is remarkably efficient. The effect is about 30 percent efficient at converting kinetic energy into electricity. For reference, the best solar panels are only about 20 percent efficient.
It is difficult to fabricate graphene films and scale them up to usable sizes. The new iron oxide films are relatively easy to produce and scalable to larger sizes.
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 vapor deposition (PVD), which turns normally solid materials — in this case iron — into a vapor that condenses on a desired surface. PVD allowed them to create an iron layer 10 nanometers thick, about 10,000 times thinner than a human hair. 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 cm2. For reference, plates having an area of 10 square meters each would generate a few kilowatts per hour — enough for a standard U.S. home. Less demanding applications, including low-power devices in remote locations, are more promising in the near term.
The mechanism behind the electricity generation involves 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.
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 buoys could be used for passive electrical energy conversion. Also, since human beings have saltwater flowing in their veins in periodic pulses, it could be used to generate electricity for powering implants.