A rechargeable battery developed by researchers from Stanford University employs the difference in salinity between freshwater and saltwater to generate a current. The technology could make it possible to harness power from anywhere freshwater enters the sea, such as river mouths or estuaries, Yi Cui, associate professor of materials science and engineering, who led the research team, said.

As the researchers explained in the March issue of the journal Nano Letters, the battery essentially uses two electrodes—one positive, one negative—immersed in a liquid containing electrically charged particles or ions. In water, the ions are sodium and chlorine, the components of ordinary table salt. The positive electrode is made from nanorods of manganese dioxide, which increases the surface area available for interaction with the sodium ions by roughly 100 times compared with other materials, Cui said. The researchers continue to search for a better material for the negative electrode than the silver used for the experiments, which is too expensive to be practical.

5. Worth one’s salt. A rechargeable battery developed by Stanford University researchers employs the difference in salinity between freshwater and saltwater to generate power. In the first step, a small electric current is applied to charge the battery, pulling ions out of the electrodes and into the water. In the second step, the freshwater is purged and replaced with seawater. In the third step, electricity is drawn from the battery for use, draining the battery of its stored energy, and in the final step, seawater is discharged and replaced with river water, for the cycle to begin anew. Courtesy: Yi Cui

Initially, the battery is filled with freshwater and a small electric current is applied to charge it up. The freshwater is then drained and replaced with seawater. Because seawater is salty, containing 60 to 100 times more ions than freshwater, it increases the electrical potential, or voltage, between the two electrodes. That makes it possible to reap far more electricity than the amount used to charge the battery. “The voltage really depends on the concentration of the sodium and chlorine ions you have,” Cui said. “If you charge at low voltage in freshwater, then discharge at high voltage in sea water, that means you gain energy. You get more energy than you put in.”

Once the discharge is complete, the seawater is drained and replaced with freshwater and the cycle can begin again. “The key thing here is that you need to exchange the electrolyte, the liquid in the battery,” Cui said.

In their lab experiments, Cui’s team used seawater they collected from the Pacific Ocean off the California coast and freshwater from Donner Lake, high in the Sierra Nevada. They achieved 74% efficiency in converting the potential energy in the battery to electrical current, but Cui thinks with simple modifications, the battery could be 85% efficient.

Other researchers have used the salinity contrast between freshwater and seawater to produce electricity, but those processes typically require ions to move through a membrane to generate current. Cui said those membranes tend to be fragile, which is a drawback. Those methods also typically make use of only one type of ion, while his battery uses both the sodium and chlorine ions to generate power.

Cui admitted that one significant theoretical limiting factor is the amount of freshwater available. However, the researchers claim that their batteries could supply 2 TWh of power annually if all the world’s rivers were put to use. According to the team’s calculations, a power plant operating with 50 cubic meters of freshwater per second could have a capacity of up to 100 MW.

The battery would be best suited for the Amazon River, which drains a large part of South America, but other continents, such as Africa and North America could also benefit. Cui even suggested that treated sewage water might work. “If we can use sewage water, this will sell really well,” he said.

Sonal Patel is POWER’s senior writer.