There is an untapped energy source on the world’s coasts: the difference in salinity between seawater and freshwater. A new nanodevice could use this difference to produce energy.

A group of researchers from the University of Illinois at Urbana-Champaign reported in the journal Nano Energy about the design of a nanofluidic device that can convert ion flow into useful electricity. The team believes their device could be used to harvest energy from natural ion flows at the interface between seawater and freshwater.

“Although our project is still a concept at this stage, it is quite versatile and already shows great potential for energy applications,” said project leader Jean-Pierre Leberton, professor of electrical and computer engineering at the University of Illinois. “It all started with an academic question: ‘Can a nanoscale solid-state device extract energy from ion flow?’ — but our design exceeded our expectations and surprised us in many ways.”

When two bodies of water with different salinities meet, such as where a river flows into the ocean, salt molecules naturally flow from higher concentration to lower concentration. The energy of these currents can be harvested because they consist of electrically charged particles called ions formed from dissolved salt.

Leberton’s group has developed a nanoscale semiconductor device that exploits a phenomenon called “Coulomb resistance” between the ions and electric charges present in the device. As ions pass through the narrow channel of the device, electrical forces cause the charges of the device to move from side to side, creating a voltage and electric current.

While simulating their device, the researchers discovered two surprising behaviors. First, we expected Coulomb resistance to arise primarily from the attraction between opposite electric charges, while simulations showed that the device worked equally well when the electric forces repel each other. Both positively and negatively charged ions contribute to resistance.

“It is equally remarkable that our study shows that there is an amplification effect,” said Minghe Xiong, a graduate student in Leburton’s group and lead author of the study. “Since the moving ions are very large compared to the charges of the device, the ions impart large momentum to the charges, increasing the underlying current.”

The researchers also found that these effects were independent of the specific configuration of the channel and material choice, provided the diameter of the channel was narrow enough to allow proximity between ions and charges.

The researchers are in the process of patenting their results and are investigating how arrays of these devices could be scaled up for practical electricity generation.

“We believe the power density of a range of devices could match or exceed the power density of solar cells,” Leberton said. “And that’s not to mention potential applications in other areas. Source