University professor finds potential in batteries to remove salt from water

By Susan Szuch

The road to finding the right battery to take the salt out of water was a long one for Kyle Smith.

The mechanical engineering professor and graduate student Rylan Dmello recently published a study in the Journal of The Electrochemical Society that found a way to potentially provide fresh water using the material in batteries and the smallest amount of energy possible.

Smith had been doing research on batteries for energy storage for several years, but when he came to the University in fall 2014, something that happened in the lithium-ion batteries he studied piqued his curiosity.

Within lithium-ion batteries, a material called electrolyte, which allows electricity to flow from one electrode to the other, contains lithium ions — atoms that have a net positive or a net negative charge. These react with the electrolyte, which stores charge in either positive or negative charge of the battery. This reaction causes some salt to collect and be used inside of the battery.

While this is not the ideal thing to happen — accumulation and consumption of salt causes a battery to be less efficient — it reminded Smith of work done in the past using capacitors to take salt out of water.

“No one had ever thought about using the salt depletion process that occurs in lithium-ion batteries to get desalination out of a device that’s like a battery and not a capacitor,” Smith said.

The next step was to consider what kind of salt needed to be pulled out of the water. Instead of a lithium-based salt like what was found in the batteries he was used to working with, he instead had to focus on batteries that were able to pull sodium chloride out of the water.

As he and Dmello performed computational models to determine how a sodium-ion battery would be able to take the salt out of the electrodes before the opposing electrodes released salt back, they found that it wasn’t quite possible, and would take a high charging rate to make a conventional battery like that desalinate water.

This was a surprise to Smith, and it threw him off course for a bit, until he realized that adding a membrane to the battery might be the solution to his problem.

“My original hypothesis was that (the conventional battery) would work,” Smith said. “When (Dmello) showed me the results from the simulation, I was stuck for a while, thinking, and eventually contemplating about it when I realized, ‘You know what? There’s another way to do this, we could actually add another component into the cell.’”

Having a membrane in the battery solved the problem of the sodium diffusing from one electrode over to the opposite electrode. These membranes are used in dialysis, to take salt out of blood.

Smith was quick to note that this is not the end of research on this subject, but rather “the final conclusion of a preliminary study.”

“We are focusing on trying to realize this device, experimentally. Up until this point, we have predictions and we have a plan for the types of materials we’d like to use in this cell,” Smith said. “But where the rubber meets the road is really in actuality making a device.”

There is still a lot to study. It’s unclear how real sea water, which contains magnesium, calcium and sulfates, will react with the battery materials. Additionally, Smith is looking at how to tailor the battery to the size of the unit or the amount of desalinated water needed.

While Smith said there are currently no specific applications for which he intends to use the device, he acknowledged that there are many potential applications out there now: taking salt out of sea water, making briny water from estuaries drinkable, treating waste water or even using the desalinated water for manufacturing.

“We’re eager to find applications for it as these things develop. We’re excited about it.”

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