A research team from Northwestern University and California Institute of Technology (Caltech) has successfully generated electricity by flowing water over very thin layers of low-cost, oxidized metals, such as iron.
These films provide a whole new way of producing electricity and can possibly be used for developing new kinds of sustainable power production. In addition, the films include a 10 to 20 nm-thick conducting metal nanolayer that is insulated with a 2 nm-thick oxide layers.
When pulses of ocean water and rainwater alternate and move across the nanolayers, a current is produced. Due to the variation in salinity, the electrons are dragged along in the metal below the oxide layers.
It’s the oxide layer over the nanometal that really makes this device go. Instead of corrosion, the presence of the oxides on the right metals leads to a mechanism that shuttles electrons.
Franz M. Geiger, Dow Professor of Chemistry, Weinberg College of Arts and Sciences, Northwestern University
The thin films are transparent, a trait that could be exploited in solar cells. The scientists are planning to examine the technique, utilizing other ionic liquids, such as blood. Advances in this field could enable its applications in various implantable devices, including stents.
“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 stated. “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.”
The study, titled “Energy Conversion via Metal Nanolayers,” was recently reported in the journal, Proceedings of the National Academy of Sciences (PNAS).
Geiger is the corresponding author of the study; the experiments were performed by his Northwestern team. Thomas Miller, the study co-author and professor of chemistry at Caltech, headed a research team that performed atomistic simulations to examine the behavior of the device at the atomic level.
The latest technique creates currents and voltages that are similar to graphene-based devices reported to have around 30% efficiencies—analogous to other methods being analyzed (graphene and carbon nanotubes). However, the new method involves a one-step fabrication process from earth-abundant elements, rather than a multistep fabrication one.
This simplicity enables scalability, low cost, and quick implementation. A provisional patent has been filed by Northwestern University.
Among the various metals examined, the team discovered that vanadium, nickel, and iron worked best. A pure rust sample was tested as a control experiment, but it did not generate a current.
The mechanism behind the production of electricity is rather complicated, in which adsorption and desorption of ions are involved. However, it basically functions like this—the ions in the saltwater/rainwater lure electrons in the metal below the oxide layer. As the saltwater/rainwater flows, the ions also flow. Through that attractive force, the ions pull the electrons in the metal along with them, producing an electrical current.
“There are interesting prospects for a variety of energy and sustainability applications, but the real value is the new mechanism of oxide-metal electron transfer,” Geiger stated. “The underlying mechanism appears to involve various oxidation states.”
The researchers utilized a process known as physical vapor deposition, or PVD, which converts typically solid materials into a vapor that condenses on a required surface. This PVD process helped in depositing these materials onto glass metal layers that had a thickness of just 10 to 20 nm. Subsequently, an oxide layer forms suddenly in the air and grows to a 2 nm-thickness and then stops growing.
“Thicker films of metal don’t succeed—it’s a nano-confinement effect,” Geiger stated. “We have discovered the sweet spot.”
When the devices were tested, they produced several tens of millivolts as well as several microamps/cm2.
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. Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term.
Thomas Miller, Study Co-Author and Professor, Division of Chemistry and Chemical Engineering, Caltech
Other co-authors of the study are Mavis D. Boamah, Emilie H. Lozier, Paul E. Ohno, and Catherine E. Walker of Northwestern and Jeongmin Kim of Caltech.
The study was supported by the National Science Foundation (award number CHE-1464916), the Office of Naval Research (award number N00014-10-1-0884) and the Defense Advanced Research Projects Agency (DARPA) through the Army Research Chemical Sciences Division (award number W911NF1910361).