Sapphire-coloured methylene blue is a common ingredient in wastewater from textile mills, but scientists from the University at Buffalo in New York State said the pollutant could be reused for large-scale energy storage.
Energy firms and researchers are desperately searching for viable means of storing renewable energy, to manage supply when the Sun is not shining and the wind not blowing. Lithium-ion batteries have efficiency and cost issues, while hydrogen splitting is not yet widespread.
When dissolved in water, methylene blue is good at storing and releasing energy on cue, making it a promising candidate material for redox flow batteries – large, rechargeable liquid-based units.
Although the dye is also used as a medication, it can cause issues including hypertension, anaemia and abdominal pain.
“It can be harmful to health, so it's not something you want to dump into the environment without treating it,” said lead researcher Timothy Cook.
"What if, instead of just cleaning the water up, we could find a new way to use it? That's what really motivated this project," added first author Anjula Kosswattaarachchi.
As well as dyes, textile wastewater already contains salts – a vital ingredient for energy transfer in redox flow batteries – meaning it could potentially be used without treatment.
The scientists built two simple batteries using the dye dissolved in salt water to capture, store and release electrons. The first operated with near-perfect efficiency when it was charged and drained 50 times. Over time, however, the battery's capacity for storing energy fell as molecules of methylene blue became trapped on a membrane critical to the device's proper function.
Choosing a new membrane material solved this problem in the scientists' second battery. This device reportedly maintained the near-perfect efficiency of the first model, but had no notable drop in energy storage capacity over 12 cycles of charging and discharging.
The results mean that methylene blue is a viable material for liquid batteries, the researchers said. The team now hopes to take the research one step further by obtaining real wastewater from a textile mill that uses the dye.
The project has personal importance to Kosswattaarachchi, who previously worked in textiles developing new fabric technologies for the Sri Lanka Institute of Nanotechnology. The industry is one of the country's most important economic sectors, but causes pollution.
"We believe that this work could set the stage for an alternative route for wastewater management, paving a path to a green-energy storage technology," the researcher said.
The research was published in ChemElectroChem.
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