Battery tech can give fresh water from sea

The technology that charges batteries for electronic devices could provide fresh water from salty seas, a new study suggests. Electricity running through a salt water-filled battery draws the salt ions out of the water.

“We are developing a device that will use the materials in batteries to take salt out of water with the smallest amount of energy that we can,” said Kyle Smith, professor at the University of Illinois at Urbana-Champaign.

The most-used method, reverse osmosis, pushes water through a membrane that keeps out the salt, a costly and energy-intensive process. By contrast, the battery method uses electricity to draw charged salt ions out of the water.

The researchers were inspired by sodium ion batteries, which contain salt water. Batteries have two chambers, a positive electrode and a negative electrode, with a separator in between that the ions can flow across.

“When the battery discharges, the sodium and chloride ions — the two elements of salt — are drawn to one chamber, leaving desalinated water in the other. In a normal battery, the ions diffuse back when the current flows the other direction. The researchers had to find a way to keep the salt out of the now-pure water. In a conventional battery, the separator allows salt to diffuse from the positive electrode into the negative electrode,” Smith said.

“That limits how much salt depletion can occur. We put a membrane that blocks sodium between the two electrodes, so we could keep it out of the side that is desalinated,” Smith said.

The battery approach holds several advantages over reverse osmosis. The battery device can be small or large, adapting to different applications, while reverse osmosis plants must be very large to be efficient and cost effective, Smith said. The pressure required to pump the water through is much less, since it’s simply flowing the water over the electrodes instead of forcing it through a membrane. This translates to much smaller energy needs, close to the very minimum required by nature, which in turn translates to lower costs, researchers said.