Rusting: Undesirable Effect or Scientific Opportunity?
Rust or iron oxide is a reddish-brown coating observed on the surface of metals which is caused by a reduction-oxidation reaction between iron and oxygen in the presence of moisture/water.
Typically, rust forms on pieces of metal that were not protected by galvanisation (or otherwise of course metals that have been long-exposed to moisture in the atmosphere). Naturally, it is generally an undesirable effect, and manufacturers go to great lengths to protect their products against it during metal fabrication, especially as it reduces the tensile strength and thickness of metals over time.
A team of researchers—led by Franz M. Geiger, dow professor of chemistry at Northwestern University with support from Tom Miller, professor of chemistry at the California Institute of Technology (Caltech), have generated a measurable flow of electrons by applying a salt-based (i.e. saline) solution over a thin coating of rust. A similar method has been used by scientists to transfer electrons across graphene/solution interfaces.
Principle of Operation
The principle of generating electric power from rust is kinetic-electric current transduction via nanolayers formed on the surface of suitable metals. As saltwater flows over the rust coating, the droplets create kinetic energy, and the ions in the solution attract electrons from the metal underneath the rust, ultimately generating an electric current.
For the experiments, the researchers used a variety of metals including iron, nickel, aluminium, vanadium, and chromium. The best conversion efficiencies were obtained in iron, nickel, and vanadium.
Although rust forms in layers on most metals, a uniformly thin layer is required to produce a tangible current. The researchers used a technique known as physical vapour deposition (PVD) to achieve this. The process involves vaporising solid iron, then condensing it onto a glass surface to produce a consistently thin layer. To find the optimal design for highest conversion efficiency, the scientists experimented with linear flow and oscillatory motion (using varying and constant salinities respectively).
At aqueous flow velocities of a few centimetres per second, the iron, nickel, and vanadium (between 10 to 30 nm layers of them) generated tens of millivolts of open-circuit potential—with current densities in the order of several mA cm−2.
Left to right: section A of the above image displays iron and aluminium nanolayers, while section B shows a Teflon cell and flow channel, for which the annotating dashes and arrows indicate substrate and aqueous flow direction respectively. Image credit (for each): Northwestern University.
Is the Technology Scalable?
According to Tom Miller, the technology can be scaled to small—or arbitrarily large—areas, as long as ideal conditions in the working model are maintained. On a large scale, 3D structures could be used to both support large volumes of saline solution and increase power output.
The researchers say a 10 m2 surface (perhaps positioned on a roof), combined with a suitable flow (such as rainfall) can generate several kilowatt-hours (aka kWh) of electricity—enough to power a standard U.S. home. Similarly, it can also be implemented in miniature applications, such as medical devices.
Geiger suggested that a nanofilm-coated stent could receive microamperes—aka one-millionth (10-6) of an ampere—of electric current provided by blood flow from 100,000 heartbeats a day. Miller, however, cautioned that such applications could be impacted by the formation of biofilms on the surfaces which can lower the power conversion efficiency.
The work, published in Proceedings of the National Academy of Sciences is covered in a paper titled, ‘Energy Conversion via Metal Nanolayers’. It is supported by the Office of Naval Research, the National Science Foundation, and the Defense Advanced Research Projects Agency (aka DARPA) via the Army Research Chemical Sciences division.
A sheet of rusted iron metal. Image credit: Pixabay.
Generating electricity from rust is yet another scientific breakthrough that shows great promise for cost-effective power generation. While the technology involved is still in its infancy, the metals it uses can be sourced cheaply, and rust is easy to generate, of course.
That said, rust is a bit trickier to produce in the uniformly thin layers that are required by the research. Fortunately, the scientists found a way around this by using physical vapour deposition, which allows rust to be scaled over miniature, or large, areas on flexible or rigid substrates. The main challenge of this method, however, will be to find ways to optimise the conversion efficiencies to provide tangible amounts of sustainable power that is suitable for most applications.
Suffice to say, though, if and when a full solution is reached, such applications would benefit tremendously.