Energy & Power
“This is [battery] chemistry I’d be happy to put in my basement”
By Anthony Capkun
September 24, 2015 – A team of Harvard scientists and engineers say they have demonstrated a rechargeable battery that could make the storage of electricity from intermittent energy sources like solar and wind safe and cost-effective for both residential and commercial use.
“This is chemistry I’d be happy to put in my basement,” said prof. Michael J. Aziz, principal investigator of the project. “The non-toxicity and cheap, abundant materials placed in water solution mean that it’s safe—it can’t catch on fire—and that’s huge when you’re storing large amounts of electrical energy anywhere near people.”
This new research builds on earlier work by members of the same team that, they feel, could enable cheaper and more reliable electricity storage at the grid level.
In the operation of the battery, electrons are picked up and released by compounds composed of inexpensive, earth-abundant elements (carbon, oxygen, nitrogen, hydrogen, iron and potassium) dissolved in water. The compounds are non-toxic, non-flammable and widely available.
The new battery chemistry was discovered by post-doctoral fellow Michael Marshak and graduate student Kaixiang Lin working together with co-lead author prof. Roy Gordon.
“We combined a common organic dye with an inexpensive food additive to increase our battery voltage by about 50% over our previous materials,” said Gordon. The findings “deliver the first high-performance, non-flammable, non-toxic, non-corrosive and low-cost chemicals for flow batteries.”
Unlike solid-electrode batteries, flow batteries store energy in liquids contained in external tanks, similar to fuel cells. The tanks (which set the energy capacity), as well as the electrochemical conversion hardware through which the fluids are pumped (which sets peak power capacity), can be sized independently, the researchers explain. Since the amount of energy that can be stored can be arbitrarily increased by scaling up only the size of the tanks, larger amounts of energy can be stored at lower cost than traditional battery systems.
The active components of electrolytes in most flow battery designs have been metal ions such as vanadium dissolved in acid. The metals can be expensive, corrosive, tricky to handle, and kinetically sluggish, leading to inefficiencies, the researchers added.
Last year, Aziz and his Harvard colleagues demonstrated a flow battery that replaced metals with organic (carbon-based) molecules called quinones, which are naturally occurring chemicals that are integral to biological processes like photosynthesis and cellular respiration. While quinones in aqueous solution formed the negative electrolyte side of the battery, the positive side relied on a conventional bromine-bearing electrolyte that is used in several other batteries.
But bromine’s toxicity and volatility make it somewhat unsuitable for regular consumer-type settings.
So the team started the search for a new recipe that would provide comparable storage advantages using chemicals that could be safely deployed in homes and businesses. Their new battery replaces bromine with a non-toxic and non-corrosive ion called ferrocyanide.
“It sounds bad because it has the word ‘cyanide’ in it,” explained co-lead author Marshak. “Cyanide kills you because it binds very tightly to iron in your body. In ferrocyanide, it’s already bound to iron, so it’s safe. In fact, ferrocyanide is commonly used as a food additive, and also as a fertilizer.”
Because ferrocyanide is highly soluble and stable in alkaline rather than acidic solutions, the Harvard team paired it with a quinone compound that is soluble and stable under alkaline conditions, in contrast to the acidic environment of their original battery developed last year.
Marshak compares exposure to the concentrated alkaline solution to coming into contact with a damaged disposable AA battery. “It’s not something you want to eat or splash around in, but outside of that it’s really not a problem.”
There are other advantages to using alkaline solution. Because it is non-corrosive, the flow battery system components can be constructed of simpler and much less expensive materials, such as plastics.