There is no doubt that sustainable energy sources are the way of the future, but exactly how we generate that sustainable energy is anyone’s guess. While solar, wind and even hydroelectric energy generation is progressing forward every year, that has not stopped researchers from experimenting to find new ways to generate electricity.  In a recently released study, researchers at Northwestern University, and Caltech have discovered a new method for generating electricity from common rust, marking what could be a major milestone towards humans relying on 100% renewable energy.


The discovery came when a team of researchers at Northwestern flowed saltwater over a very thin film of metal that was itself coated in a thin layer of oxide. When seawater and rainwater flowed across the metal film’s surface it produced a small voltage. This is because the difference in salinity between the two fluids attracts electrons in the conductive metal layer and drags them back and forth. This conducting nanolayer is just 20-nanometers thick and is itself coated in a 2-nanometer thick layer of oxide, a feature that proved to be key to the whole process actually working.


Graphical representation of electrical energy conversion in metal nanolayers terminated by their thermal oxides. (Source: Northwestern University)

Graphical representation of electrical energy conversion in metal nanolayers terminated by their thermal oxides. (Source: Northwestern University)


“It’s the oxide layer over the nanometal that really makes this device go,” said Franz M. Geiger, the Dow Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences. “Instead of corrosion, the presence of the oxides on the right metals leads to a mechanism that shuttles electrons.”


Further sweetening the deal, the metal nanolayer film is so thin that its transparent to the eye, making it an excellent candidate for application to solar cells, vehicle windows, building, and home windows, and so much more. While there are plans to study this phenomenon with other ionic fluids such as blood, they stuck to seawater and rainwater in their experiments. Furthermore, a control experiment was conducted with pure iron oxide rust, and no voltage of any significance was reported.


The researchers found that they were not just limited to iron-based metallic nanolayers either, and discovered similar electrical generation from nickel, and vanadium as well, with a very thin oxide layer remaining key to the whole process working. With corrosion-resistant materials such as nickel producing the same electrical effect, this discovery could have a significant effect on medical devices such as stents and pacemakers, theoretically allowing those devices to generate their own voltage utilizing blood as the ionic fluid.


“The ease of scaling up the metal nanolayer to large areas and the ease with which plastics can be coated gets us to three-dimensional structures where large volumes of liquids can be used,” Geiger said. “Foldable designs that fit, for instance, into a backpack are a possibility as well. Given how transparent the films are, it’s exciting to think about coupling the metal nanolayers to a solar cell or coating the outside of building windows with metal nanolayers to obtain energy when it rains.”


While this new method produces voltage and currents close to similar graphene-based devices, this process is much simpler as it takes just one step to produce. Graphene and other methods utilize expensive multi-step processes to generate useable voltages. Being a one-step process allows for easy scalability, QA processing, and a much lower production cost. This savings in time and energy is due to a process called Physical Vapor Deposition which a solid such as nickel is vaporized, and then deposited onto a substrate such as glass, plastic, or other metals. The PVD process ensures a very thin layer of metal forms on the substrate’s surface, and then instantly oxidizes upon contact with the air. As I mentioned earlier, it’s this thin layer of oxide that helps produce the voltage.


“Thicker films of metal don’t succeed -- it’s a nano-confinement effect,” Geiger said. “We have discovered the sweet spot.”


In the lab, small samples that were tested were able to produce tens of millivolts and several microamps. This means that if scaled up to just 10 square meters, the panel would produce several kilowatts per hour. That’s enough electricity to power an average American home. If scaled up even larger, or coated onto a solar grid array, the amount of electricity produced could nearly double from just a single solar panel. “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,” Miller said. “Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term.”


Geiger is the study’s corresponding author; his Northwestern team conducted the experiments. Co-author Thomas Miller, professor of chemistry at Caltech, led a team that conducted atomistic simulations to study the device’s behavior at the atomic level. The study’s other co-authors are Mavis D. Boamah, Emilie H. Lozier, Paul E. Ohno and Catherine E. Walker of Northwestern and Jeongmin Kim of Caltech.