A tiny device that uses salt to generate clean energy

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A new device that is smaller than a human hair can generate electricity from the difference in saltiness between seawater and freshwater. This could be a new source of clean energy along the coastlines of the world.

A team of researchers at the University of Illinois Urbana-Champaign has reported a design for a device that can convert the flow of salt ions into electric power in the journal Nano Energy. The device is made of nanoscale semiconductor materials and works by using a phenomenon called “Coulomb drag”. The team thinks that their device could be used to harvest energy from the natural salt gradients at the boundaries of seawater and freshwater.

The leader of the project, Jean-Pierre Leburton, a professor of electrical & computer engineering, said that their design was still a concept at this stage, but it was very versatile and showed great potential for energy applications. He said that they started with an academic question – ‘Can a nanoscale solid-state device extract energy from ionic flow?’ – but their design surprised them in many ways.

When seawater and freshwater meet, such as at the mouth of a river, salt molecules naturally move from higher concentration to lower concentration. This movement can be used to generate electricity because salt molecules are made of electrically charged particles called ions.

Credits: The Grainger College of Engineering at University of Illinois Urbana-Champaign

Leburton’s group designed a device that has a narrow channel where the ions flow through. The electric forces between the ions and the charges in the device cause the charges to move from one side to the other creating voltage and electric current.

The main author of the study, Mingye Xiong, a graduate student in Leburton’s group, said that they discovered two unexpected behaviors when they simulated their device. He said that they found that the device worked equally well whether the electric forces were attractive or repulsive. He also said that both positive and negative ions contributed to drag.

Xiong also said that there was an amplification effect. He explained that the ions were much heavier than the charges in the device, so they transferred a lot of momentum to the charges, boosting the underlying current.

The researchers also found that these effects did not depend on the specific shape of the channel or the choice of materials, as long as the channel was narrow enough to ensure closeness between the ions and the charges.

The researchers are in the process of patenting their findings, and they are studying how many devices can be connected to produce more power.

Leburton said that he believed that the power density of a device array could match or exceed that of solar cells. He also mentioned the potential applications in other fields like biomedical sensing and nanofluidics.

The study was published in the journal Nano Energy

The onset of electronic current in a doped silicon membrane induced by the long-range Coulomb interaction of ions flowing through a nanofluidic channel is established by a combined computational and analytical approach based on Green’s function technique and Boltzmann transport formalism. Characterized by an open circuit voltage and short circuit current, the electronic Coulomb drag provides a new paradigm for power harvesting. In addition, our model predicts a current amplification of the ionic drag current because of the large momentum transfer from heavy ions to charge carriers in silicon, which is achieved for both anions and cations flowing in the nanochannel irrespective of the dopant type in the semiconductor. The analysis indicates the versatility of this effect with respect to the nature of the electrolyte and the semiconducting materials, providing proper tuning of their structures and design configurations.