Harvard University
A team of Harvard scientists and engineers has demonstrated a
rechargeable battery that could make storage of electricity from intermittent
energy sources like solar and wind safe and cost-effective for both residential
and commercial use. The new research builds on earlier work by members of the same team that could enable cheaper and more reliable electricity storage at the grid level.
The mismatch between the availability of intermittent wind or
sunshine and the variability of demand is a great obstacle to getting a large
fraction of our electricity from renewable sources. This problem could be
solved by a cost-effective means of storing large amounts of electrical energy
for delivery over the long periods when the wind isn't blowing and the sun
isn't shining.
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, making them safer and cheaper than other battery systems.
"This is chemistry I'd be happy to put in my basement,"
says Michael J. Aziz, Gene and Tracy Sykes Professor of Materials and Energy
Technologies at Harvard Paulson School of Engineering and Applied Sciences
(SEAS), and project Principal Investigator.
"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."
The research appears in a paper published in the journal Science.
This new battery chemistry was discovered by post-doctoral
fellow Michael Marshak and graduate student Kaixiang Lin working together with
co-lead author Roy Gordon, Thomas Dudley Cabot Professor of Chemistry and
Professor of Materials Science at Harvard.
"We combined a common organic dye with an inexpensive food
additive to increase our battery voltage by about 50 percent over our previous
materials," says 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.
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.
Last year, Aziz and his Harvard colleagues demonstrated a flow
battery that replaced metals with organic (carbon-based) molecules called
quinones, which are abundant, 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. The high performance and low cost of
the technology, which Harvard has licensed to a company in Europe, hold the
potential to provide scalable grid-level storage solutions to utilities.
But bromine's toxicity and volatility make it most suitable for
settings where trained professionals can deal with it safely behind secure
fences.
So the team began searching for a new recipe that would provide
comparable storage advantages -- inexpensive, long lasting, efficient -- using
chemicals that could be safely deployed in homes and businesses. Their new
battery, described in a paper published today in the journal Science,
replaces bromine with a non-toxic and non-corrosive ion called ferrocyanide.
"It sounds bad because it has the word 'cyanide' in
it," explains co-lead author Marshak, who is now assistant professor of
chemistry at the University of Colorado Boulder. "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.
"First generation flow batteries were single-element
couples -- transition metals like vanadium or iron or chrome," says
Michael Perry, Project Leader for Electrochemical Systems at United
Technologies Research Center, who was not involved in the work.
"Now we're
seeing the possibility of engineered molecules giving us the properties and
attributes that we want in one complete system. More work is required and
justified but the Harvard team is really demonstrating the promise of
next-generation chemistries."
Robert F. Savinell, Distinguished University Professor and
George S. Dively Professor of Engineering at Case Western Reserve University,
another battery expert who was not part of the Harvard research, agrees that
the new technology offers significant advantages over other flow batteries
concepts, including "potential very low costs with sustainable materials,
high efficiencies at practical power densities, and safe and simple
operation."
He adds: "It should be expected that this flow battery
approach will have a short development and scale-up path for fast commercial
introduction."
Harvard's Office of Technology Development has been working
closely with the research team to navigate the shifting complexities of the
energy storage market and build relationships with companies well positioned to
commercialize the new chemistries.
The demand for battery storage is driven by regulatory factors
as much as economic ones. In some states, as well as many parts of the world,
if it can't be instantaneously used by meeting electricity demand, solar energy
incident on solar panels goes to waste unless the electricity is stored.
However, in many states, customers have the right to sell electricity produced
by rooftop solar panels at high consumer rates under a regulatory scheme called
net metering. Under those circumstances, consumers have little incentive to
install batteries.
But market experts like William W. Hogan, Raymond Plank
Professor of Global Energy Policy at Harvard Kennedy School, believe that such
policies are ultimately "uneconomic and unsustainable." And as more
and more homeowners install solar panels, utilities are opposing requirements
to buy electricity from their customers.
Hogan says net metering is one of a series of "regulatory
gimmicks designed to make solar more attractive" and predicts that
eventually consumers with rooftop photovoltaic panels will lose the option of
exchanging electricity for discounts on their utility bills. When that happens,
these homeowners have an incentive to invest in battery storage.
That's the emerging market opportunity that Tesla Motors
entrepreneur Elon Musk hopes to leverage with his company's recently-announced
Powerwall system. But the flow battery design engineered by Aziz and his
Harvard colleagues offers potential advantages in cost and the length of time
it can maintain peak discharge power compared to lithium batteries.
"This has potential because photovoltaics are growing so
fast," Aziz says. "A cloud comes over your solar installation and BAM
-- the production goes crashing down. Then the cloud goes away and the
production goes shooting up. The best way of dealing with that is with
batteries."
Watch video: How a flow batter works-https://youtu.be/4ob3_8QjmR0.
