Scientists have developed a new non-toxic flow battery that stores energy in organic molecules dissolved in water and could run for over a decade.
Flow batteries store energy in liquid solutions in external tanks - the bigger the tanks, the more energy they store. They are a promising storage solution for renewable, intermittent energy like wind and solar.
By modifying the structures of molecules used in the positive and negative electrolyte solutions, and making them water soluble, researchers at the Harvard University in the US were able to engineer a battery that loses only one per cent of its capacity per 1,000 cycles.
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"Lithium ion batteries do not even survive 1,000 complete charge/discharge cycles," said Michael Aziz from the Harvard.
"Because we were able to dissolve the electrolytes in neutral water, this is a long-lasting battery that you could put in your basement," said Roy Gordon from Harvard.
"If it spilt on the floor, it would not eat the concrete and since the medium is noncorrosive, you can use cheaper materials to build the components of the batteries, like the tanks and pumps," said Gordon.
"If you can get anywhere near this cost target then you change the world. It becomes cost effective to put batteries in so many places. This research puts us one step closer to reaching that target," said Aziz.
The key to designing the battery was to first figure out why previous molecules were degrading so quickly in neutral solutions, said Eugene Beh, postdoctoral fellow at Harvard.
By first identifying how the molecule viologen in the negative electrolyte was decomposing, Beh was able to modify its molecular structure to make it more resilient.
Next, researchers turned to ferrocene, a molecule well known for its electrochemical properties, for the positive electrolyte.
"Ferrocene is great for storing charge but is completely insoluble in water. It has been used in other batteries with organic solvents, which are flammable and expensive," said Beh.
However, by fictionalising ferrocene molecules in the same way as with the viologen, researchers were able to turn an insoluble molecule into a highly soluble one that could also be cycled stably.
"Aqueous soluble ferrocenes represent a whole new class of molecules for flow batteries," said Aziz.
The neutral pH should be especially helpful in lowering the cost of the ion-selective membrane that separates the two sides of the battery.
Most flow batteries today use expensive polymers that can withstand the aggressive chemistry inside the battery. They can account for up to one third of the total cost of the device. With essentially salt water on both sides of the membrane, expensive polymers can be replaced by cheap hydrocarbons.
The study was published in the journal ACS Energy Letters.
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