Flow batteries are a promising technology for storing energy produced by intermittent renewable sources such as wind and solar. To make these sources competitive to energy produced from traditional power plants, the U.S. Department of Energy (DOE) set a cost goal for a storage battery at less than $100 per kilowatt-hour.

Flow batteries store energy in liquid solutions in external tanks – the bigger the tanks the more energy they store. However, the energy storage capacity in existing versions often degrades after many charge-discharge cycles, requiring periodic maintenance of the electrolyte.

Michael J. Aziz and others at Harvard University developed a metal-free flow battery.A team of Harvard researchers modified the structures of molecules used in both the positive and negative electrolytes mentioned to make them water-soluble. Using the approach, they constructed a battery that loses 1% of its capacity per 1000 cycles – better performance, they say, than even lithium ion batteries.

"This work on aqueous soluble organic electrolytes is of high significance in pointing the way towards future batteries with vastly improved cycle life and considerably lower cost," says Imre Gyuk, Director of Energy Storage Research at the Office of Electricity of the DOE.

Eugene Beh, first author of the study in ACS Energy Letters, found that the molecule viologen in the negative electrolyte was decomposing. He was able to modify its molecular structure to make it more resilient.

The team wanted to use ferrocene, as the positive electrolyte, but while it is well-suited for storing charge it is completely insoluble in water. Ferrocene has been used in other batteries with organic solvents, which are flammable and expensive, says Beh.

The researchers were successful in modifying the ferrocene molecules so that they were highly soluble and could be cycled stably.

"Aqueous soluble ferrocenes represent a whole new class of molecules for flow batteries," says Michael Aziz, leader of the research team.

The molecules are dissolved in neutral pH water, which should be helpful in lowering the cost of the membrane that separates the two sides of the battery. Existing flow batteries typically use expensive polymers for the membrane in order to withstand the aggressive chemistry inside the battery.

These can account for up to one third of the total cost of the device. With essentially salt water on both sides of the membrane, inexpensive hydrocarbons can replace the expensive polymers.