There are many ways to generate electricity ̵
A new study by Caltech and Northwestern University scientists shows that thin films of rust – iron oxide – can generate electricity when salt water flows over them. These films represent an entirely new way of producing electricity and could be used to develop new forms of sustainable energy production.
Interactions between metal compounds and salt water 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 works inside the batteries.
In contrast, the phenomenon discovered by Tom Miller, professor of chemistry at Caltech, and Franz Geiger, professor of chemistry in the Northwestern region, does not involve chemical reactions, but rather converts kinetic energy of the flow of salt water into electricity. .
The phenomenon, the electrokinetic effect, has been observed before in thin films of graphene – carbon atoms arranged in a hexagonal lattice – and is remarkably effective. The effect is about 30 percent effective in converting kinetic energy into electricity. For reference, the best solar panels are only about 20 percent efficient.
'A similar effect is observed in some other materials. You can take a drop of salt water and drag it through graphene and see how electricity is generated, "says Miller
However, it is difficult to produce graphene films and scale to usable sizes. Miller and Geiger iron oxide films are relatively easy to manufacture and are scalable to larger sizes, Miller says.
"It's basically just rust on iron, so it's pretty easy to make in large areas," Miller says. "This is a more stable implementation of the things seen in graphene."
Although rust would form on iron alloys by itself, the team had to ensure that it formed into a consistently thin layer. To do this, they used a process called physical vapor deposition (PVD), which converts normally solid materials, in this case iron oxide, into vapor that condenses on the desired surface. PVD allowed them to create an iron oxide layer with a thickness of 10 nanometers, about 10 thousand times thinner than human hair.
When they took this rust-covered rust and flowing brine with varying concentrations on it, they found that it generated several dozen millivolts and several microamperes per cm-2.
"In the long run, slabs of 10 square feet would each generate several kilos per hour – enough for a standard American home," Miller says. "Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near future."
The mechanism behind electricity generation is complex, involving ion adsorption and desorption, but it basically works like this: Ions present in salt water attract electrons to the iron beneath the rust layer. As salt water flows, so do these ions, and through this attractive force they drag the electrons into the iron along with them, generating electrical current. Miller says that this effect can be useful in specific scenarios where there is a saline solution, such as in the ocean or in the human body.
"For example, tidal energy or things that boil in the ocean, such as buoys, can be used to passively convert electricity," he says. "You have salt water that flows into your veins in periodic pulses. This can be used to generate electricity to power the implants."
A paper describing their discoveries, entitled "Conversion of energy through metallic nano-layers", appears in the July 29 issue of Proceedings of the National Academy of Sciences .
Bionic catalysts for the production of clean energy
Mavis D. Boamah et al. Energy Transformation through Metal Nanolayers, Proceedings of the National Academy of Sciences (2019). Doi: 10.1073 / pnas.1906601116
California Institute of Technology
Ultra-thin layers of rust generate electricity from running water (2019, July 30)
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