Scientists at Caltech (Pasadena, California, USA) and Northwestern University (Evanston, Illinois, USA) collaborated on a research project that places thin films of rust—also known as iron oxide—in saltwater to generate electricity.
A conventional method of conducting electricity is to have metal compounds interact with saltwater in order to product a chemical reaction. This method, which is used inside batteries, works because metal compounds can be converted to new compounds.
By contrast, the process pioneered by Tom Miller of Caltech and Franz Geiger of Northwestern coverts the kinetic energy of flowing saltwater into electricity. Known as the electrokinetic effect, this process has been observed in thin films of graphene and converts kinetic energy into electricity at about 30% efficiency. “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,” says Miller.
The problem with graphene is that it’s too costly a material to scale up to useable sizes. On the other hand, iron oxide is both easy to acquire, affordable, and scalable. “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.”
Because naturally spreading rust was too thick for their purposes, Miller and Geiger would need to produce thin layers. To achieve this, they turned to a process called physical vapor deposition (PVD), which turns solid materials into a vapor that condenses into a 10-nm-thick iron layer, which is thousands of times thinner than a human hair. A 2-nm-thick layer of rust then forms once the iron is exposed to air, and the rust-coated iron is introduced to flowing seawater to generate several tens of mV and several microamps per cm-2.
“For perspective, plates having an area of 10 square meters each would generate a few kilowatts per hour—enough for a standard U.S. home,” says Miller. "Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term.”
According to Miller, one of those promising potential applications includes objects that already operate in saltwater environments. “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.”
Miller, Geiger, and their co-authors recently published their findings in a paper entitled “Energy Conversion via Metal Nanolayers” for Proceedings of the National Academy of Sciences.
Source: Caltech, www.caltech.edu.