Ultra-Thin Rust Layers Generate Electricity from Water

Researchers have devised a new method that uses a combination of saltwater and iron oxide, or rust, to generate a renewable source of electricity.

Researchers are finding many new creative ways to generate electricity to create renewable forms of energy. Now a cross-institutional team of researchers have come up with a new one that uses rather unique sources--a combination of rust and salt water.

Scientists from Caltech and Northwestern University have demonstrated how thin films of rust, or iron oxide, can generate electricity when saltwater flows over them.

The discovery is yet another option for sustainable power production that could help reduce carbon emissions around the world and help reduce the effects of climate change, Franz M. Geiger, a Dow professor of chemistry at Northwestern University, told Design News.

“If we want to achieve a reduction of carbon emissions by 50 percent over the next 10 years, any new sources of energy will help get us there,” Geiger told us, citing a goal some U.S. states have set for reduction in carbon footprint. “A broad energy source portfolio that can be subsequently down-selected will contribute to help achieve this goal.”

Researchers have developed a way to use a combination of rust, or iron oxide, and saltwater to generate electricity. Pictured are iron metal nanolayers on a glass support that the team from Caltech and Northwestern University used as part of their research. (Image source: Franz Geiger) 

Old Materials, New Reaction

Geiger and his colleagues want to add their new method for generating electricity to the list. While interactions between metal compounds and saltwater often generate electricity—this in fact is the basis for how typical batteries work—the technique of turning the interaction between rust and saltwater into electricity works a little differently, he told us.

Their method uses what’s called the electrokinetic effect, which converts kinetic energy into electricity. Geiger previously learned of a successful achievement of this effect at 30-percent energy efficiency using thin films of graphene, or sheets of carbon atoms arranged in a hexagonal lattice.  

However, that approach had its limitations, he told us, because graphene “is difficult to scale up to large areas and keep stable under aqueous flow conditions.”

“Having invented iron nanolayers some time earlier, I thought that replacing graphene with iron nanolayers should work as well,” Geiger told Design News.

It turns out that he was right. Inspired by Geiger’s idea, researchers developed thins films of oxide of a few nanometer thickness on top of a 10-nanometer-to-20-nanometer metal layer. As ions in the water approach the surface, they formed a current in the metal just a few nanometers below.

“When the oxide contains more than one oxidation state, the effect is boosted by 10 to 50 times,” Geiger told us.

Flow of Energy

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 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 is what allowed researchers to create the 10-nanometer-thick iron layer. Then, after taking the metal film out of the PVD machine, the rust formed spontaneously in air to a thickness of about 2 nanometers.

Flowing saltwater solutions of varying concentrations over the rust-coated iron generated electricity, researchers found. The reaction occurs because ions present in saltwater attract electrons in the iron beneath the layer of rust. As the saltwater flows, so do those ions, dragging the electrons in the iron along with them to generate an electrical current.

The team published a paper on its work in the journal Proceedings of the National Academy of Sciences.

Researchers envision numerous places they could use their method, including water-treatment or water-desalination plants or even in the human body, where “your heart beat modulating your blood flow could be used in vivo” to generate electricity, Geiger told Design News.


The team plans to continue its work to achieve a better understanding of how the process works, especially concerning energy production, he said. They also plan to work on scaling the process up as well as exploring a number of different substrates and metals to further optimize performance, Geiger told us.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.

 

 

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