As renewable but intermittent sources like wind and solar continue to account for a larger part of our energy needs, flow batteries shape up as a very promising storage solution. These devices hold energy in fluids inside external tanks, but one thing holding them back is the expensive metals used as electrolytes, which to make things worse, require expensive parts to deal with the aggressive chemistry inside. Harvard scientists are claiming to have found a solution to both, in the form of a non-toxic flow battery that keeps its energy in organic molecules dissolved in neutral water.

The team at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have actually been chipping away at the flow battery problem for a while. Research in this area aims to negate the limitations of lithium-ion batteries, which are well-equipped to power our smartphones and electric cars, but aren't built to withstand the stresses that solar and wind power will place on the grid.

For that, we need something that is flexible, scalable and cost-effective, and in a lot of ways, flow batteries fit the bill nicely. With the energy stored in liquid form inside the external tanks, the batteries collect and release power by having these liquids exchange ions through a special membrane. This allows their capacity and power output to be tweaked by adjusting the size of the tank and area of the membrane, respectively.

But a key focus for researchers, including the Harvard team, lies in improving the materials used for the electrolytes, the energy-carrying liquids. Typical designs rely on vanadium and bromine dissolved in acid, but this is dangerous, expensive and causes energy storage to degrade after continuous charge and discharge cycles.

Back in 2014, the Harvard researchers managed to do away with vanadium in favor of a more environmentally-friendly molecule called quinone. Then in 2015 they improved on this yet again, replacing bromine with ferrocyanide, a compound commonly added to kitchen salt as an anti-caking agent. This left them with a flow battery based on an alkaline, rather an acidic solution, with the higher pH meaning less corrosion and tanks that could be made out of cheaper plastic. But in its latest step forward, the team is looking to split the difference.

"The batteries that our group designed previously used more corrosive solutions, either strong acids or strong bases," Roy Gordon, leader of the research, explains to New Atlas. "The new design uses neutral water solutions that are neither acidic or alkaline."

The new battery relies on the molecule viologen as the negative electrolyte, and the molecule ferrocene for the positive electrolyte. Putting these molecules to work in the battery first required the researchers to make them water-soluble and more resistant to degradation by modifying their molecular structure. This left them with molecules that could be cycled safely, so much so that the battery loses just one percent of its capacity every 1,000 cycles.

"Lithium-ion batteries don't even survive 1,000 complete charge-discharge cycles," said Michael Aziz, who co-authored the new research.

This incredibly long life span is a promising sign, but what has the researchers particularly excited is the battery's potential to drive down costs. Typically, flow batteries require the separator membrane to be made from expensive materials so they can endure the aggressive chemical reactions inside, something that can amount to one third of the total cost of the device. But with pH-neutral water inside, things could get a whole lot cheaper.

"It is hard to give an exact ratio of costs," Gordon tells us. "However, it is clear that the costs will be lower than those of currently used membranes, which are made of expensive fluorine-containing polymers. The hydrocarbon polymers that we use are made inexpensively from petroleum, without adding any expensive fluorine."

The US Department of Energy has set a goal of developing a flow battery that can store energy at less than US$100 per kilowatt-hour, something that could make renewables competitive with power coming from traditional plants. In 2015, researchers at the Pacific Northwest National Laboratory came up with flow state battery they claimed would cost $180 per kilowatt-hour. There's a bit to play out before the Harvard team can attach a price-tag to its creation, but the wheels are in motion.

"All the components of this technology already exist in the marketplace," Gordon continues. "We are working with several companies that are trying to bring together these components into practical systems. Prototypes should be available within the next few months. Large-scale production will depend on successful testing of these prototypes, and demonstration of favorable economics for their production."

The research is published in the journal ACS Energy Letters.