Rust. The word brings images of decay and ruin to mind. Abandoned bridges. Old cars blistering beneath a hot sun.
But rust—iron oxide, in chemical terms—may be a valuable tool in the sustainable energy tool kit. New research shows that running saltwater over thin films of rust can generate electricity. This process could eventually be used to harness power from desalination plants, ocean buoys, bridges and more.
Franz Geiger, a professor of chemistry at Northwestern University, struck upon the idea after watching a demonstration of water droplets generating electricity as they rolled down graphene. As the water moved, it was attracting electrons in the graphene and dragging them along, which generated a current. This is an example of what’s known as the “electrokinetic effect.”
Graphene, a nanomaterial made of sheets of carbon atoms, has a number of remarkable qualities, but it’s difficult to prepare over large areas. Geiger wondered if the same thing would work if he used thin films of metal instead.
“Let’s try this out,’” he recalls thinking.
For the experiment, the researchers created a thin layer of metal using a process called physical vapor deposition, which involved turning iron into a vapor and condensing it on a glass surface. This produced an incredibly thin layer of metal, just 10 to 20 nanometers thick, or about 1,000 times thinner than a sheet of Saran wrap. When exposed to air, the iron film spontaneously developed an even thinner film of rust on top. The team then flowed saltwater across the material and watched what happened.
Electricity. The rust was generating power by the electrokinetic effect, just as the graphene had.
“We were just elated,” Geiger says. The chemists' findings were published last month in the Proceedings of the National Academy of Sciences.
The team estimates that flowing saltwater across one hundred 10-square-meter sheets of metal nanolayers could generate enough electricity per hour to power a standard U.S. home. They imagine it used in desalination plants or water treatment plants, where a pump would flow water over large areas of rust film to help recover a portion of the energy spent treating the water. Rust film could be used to cover buoys to generate power for scientific equipment at sea, or on bridges to create electricity for nearby structures. It could even be used to power medical devices. That’s because the process works with any ionic solution, not just saltwater.
“It can be normal saltwater, it could be blood, it could be brackish water,” says team member Tom Miller, a professor of chemistry at the California Institute of Technology. “The only thing you really need is that the [liquid] needs to be dripping or flowing or oscillating.”
The team has a grant from DARPA to develop the technology, and has filed a provisional patent application. They are scaling the process, and looking at how it works when the film is applied to 3-dimensional surfaces rather than sheets.
“I think this work is impressive,” says Donglei Fan, a professor of engineering at the University of Texas at Austin, who says she’s impressed by the energy conversion efficiency reported by the team. The rust films are about 30 percent efficient at converting kinetic energy into electricity, more efficient than the best solar panels.
There seems something almost magical about generating forward momentum from a material so associated with decay.
“For me it sort of confirms something I’ve always liked to think about the world,” says Emilie Lozier, a Northwestern PhD student who worked on the research. “Which is that there’s the extraordinary in the ordinary, and we don’t need to look for particularly exotic material to find things that are worth exploring.”