Washington: A battery that takes advantage of the difference in salinity between freshwater and seawater to produce electricity has been developed by Stanford researchers.
Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery, said Yi Cui, associate professor of materials science and engineering, who led the research team.
The theoretical limiting factor, he said, is the amount of freshwater available.
"We actually have an infinite amount of ocean water; unfortunately we don``t have an infinite amount of freshwater," said Cui.
As an indicator of the battery`s potential for producing power, Cui`s team calculated that if all the world`s rivers were put to use, their batteries could supply about 2 terawatts of electricity annually – that`s roughly 13 percent of the world`s current energy consumption.
The battery itself is simple, consisting of 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.
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," said Cui.
"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," added Cui.
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," added Cui.
To enhance efficiency, the positive electrode of the battery is made from nanorods of manganese dioxide. That increases the surface area available for interaction with the sodium ions by roughly 100 times compared with other materials. The nanorods make it possible for the sodium ions to move in and out of the electrode with ease, speeding up the process.
The study has been published in the journal Nano Letters.