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How a blue dye found in wastewater could power batteries

Methylene blue is dye, poison, medication

This liquid-based battery has methylene blue solution (left side), and a colorless solution of leuco methylene blue (right), which is methylene blue with added electrons. 
Photo: Meredith Forrest Kulwicki/University at Buffalo

When it comes to battery materials, there’s a lot of talk about lithium and cobalt, but some chemists think the batteries of the future could be powered by a commonly used dye.

The dye in question is methylene blue, and it creates an intense sapphire color. The dye looks good, but it can be harmful when it leaches into wastewater from textile mills. Anjula Kosswattaarachchi, now a PhD candidate in chemistry at the University at Buffalo, used to work with textiles at the Sri Lanka Institute of Nanotechnology, so she was familiar with the problem of methylene blue pollution.

At Buffalo, Kosswattaarachchi studies a type of liquid battery called flow batteries, and she also worked on a project that focused on removing hazardous materials from wastewater. She had the idea to combine the two concepts and has been working on a project to see whether wastewater with methylene blue might power these liquid batteries.

It’s early stages, and the team hasn’t worked with actual wastewater yet, but it could be an idea that kills two birds with one stone. The Verge spoke with Kosswattaarachchi about why methylene blue is a problem and how it could be a solution.

This interview has been lightly edited for clarity.

Tell me more about methylene blue. Is it common? Is it poisonous? And why is it used if it’s poisonous?

We use it because it’s so good at producing a particular color. It’s one of the most commonly used dyes. It can even be used as medicine when it’s at the perfect dose. But it’s hard to get that perfect dose, and when it’s not, it can be poisonous.

It can cause sicknesses, including cancer and genetic mutations. It’s also damaging to the environment because the dye is quite visible even at very low concentrations. So when it’s added to water, the sunlight penetration is hindered, and that harms photosynthesis. It’s probably the most-studied dye when it comes to research on how to remove it from the water.

So the idea is to use wastewater with methylene blue in flow batteries, right? Instead of extracting the dye from the water, you could just reuse the water to power the battery?

Right.

Can you explain what a flow battery is?

Basically, there are these two external tanks that hold these liquid electrolytes. The electrolytes are pumped through the center, and a chemical reaction happens to power the battery. It’s a relatively new field of study, but the advantage of a flow battery is that they’re safe. They’re liquid, so they won’t catch fire. The tanks can be really big, so you can power a lot of things at a large scale. They can store power for longer.

Where does methylene blue come in?

Methylene blue is part of the liquid that we’re going to be storing and pumping in the tanks. Usually, you have to add salt to the solution for the chemical reaction to work. Methylene blue in wastewater already has salt in it.

So what did you find in your experiment?

In our experiments, we built small batteries that had the methylene dye in salt water. One charged and drained 50 times. It’s quite stable, too.

We calculate that we should be able to generate up to 9.6 kilojoules per liter with methylene blue as the component in the battery. In practice, methylene will have to be combined with another material, so we’re still doing research on that material. And we still need to use samples of real wastewater to test. That’s the next step.