A new prototype “flow battery” uses safe, abundant compounds to store electricity.

A new prototype “flow battery” uses safe, abundant compounds to store electricity.

Harvard School of Engineering and Applied Sciences

A cleaner, greener way to store solar and wind energy

With the continuing rise of solar and wind power, the hunt is on for cheap batteries that are able to store large amounts of energy and deliver it when it’s dark and the wind is still. Last year researchers reported an advance on one potentially cheap, energy-packing battery. But it required toxic and caustic materials. Now, the same team has revised its chemistry, doing away with the noxious constituents—an advance that could make future such batteries far cheaper and simpler to build.

The new design is what’s known as a “flow battery,” which is usually far larger than your sleek lithium-ion cell or lead-acid car battery. Unlike conventional batteries, flow batteries don’t package all their battery components together. Rather, they separate the chemical power supply—a pair of liquids known as electrolytes—from the electrodes needed to tap that power. This design makes it easy to increase the battery’s energy storage capacity simply by increasing the amount of electrolytes stored in external tanks. That has many engineers eyeing these batteries as a way to store the overabundance of solar and wind power at periods of peak production for use at times when their production is off.

At the heart of flow batteries is a sandwich of electrodes, known as a stack, separated by an ion-conducting membrane. The electrolytes are pumped through the stack during charging and discharging. In most designs, when the battery is discharged to provide power, a positively charged electrode strips electrons from molecules in one electrolyte and sends them through a circuit to charge-accepting molecules in the second electrolyte. This process produces positively charged ions in the first electrolyte that travel through the membrane into the second electrolyte, where they balance the charges coming in from the electrons. When the battery is charged, the flow of electrolytes, electrons, and ions is reversed and electrons are dumped into the first electrolyte.

Last year, researchers led by Harvard University materials scientist Michael Aziz created a high-energy storage flow battery that used organic compounds called quinones as the electron-storing components of one of its two electrolytes. The other, however, contained bromine, a toxic compound that also readily corrodes steel and other materials commonly used to contain the liquid electrolyte and pipe it around. That forced the Harvard team to contain it with expensive, corrosion-resistant materials, a solution too costly for most applications.

Aziz and his colleagues wondered whether there was a way to change the chemistry of their battery so that it wouldn’t need such caustic materials. Most batteries use electrolyte solutions that are either acidic or alkaline, as these are adept at ferrying electric charges from one side of the battery to the other, which is essential for a battery to both charge and discharge. In the team’s previous battery, both the quinone and bromine were dissolved in acidic solutions.

The Harvard team realized that a possible bromine replacement was a charge-carrying molecule called ferrocyanide, which sounds dangerous but is actually used as a food additive. Ferrocyanide, however, dissolves in alkaline solutions, not acidic ones. So Aziz and his colleagues tweaked the chemical structure of their quinone—ripping off a couple of sulfur groups and replacing them with pairs of hydrogen and oxygen atoms—in the end converting the compound into one that readily dissolves in an alkaline solution.

The scheme worked, and as the researchers report today in Science, the battery readily stores power with only components that are cheap, abundant, and nontoxic.

For now, Aziz notes the alkaline quinone battery stores only about two-thirds of the energy per volume as the previous acid-based version. But because it doesn’t require expensive materials to deal with bromine, it’s likely to be far cheaper to produce and friendlier to use. “This is chemistry I’d be happy to put in my basement,” Aziz says. And that may not be far off. A flow battery using the new quinones and ferrocyanide would likely only have to be the size of a couple of hot water tanks to store the energy produced by a conventional home rooftop solar array.

“The chemistry sounds great,” says David Keith, an energy expert at Harvard, who was not part of the current study. He notes that most homeowners and small businesses don’t yet need to use batteries as a backup for their renewable power supplies, because they are connected to the grid that can supply power when needed.

Aziz agrees, but adds that there are already niche applications where installing batteries makes financial sense. Many businesses, for example, are charged not only for the number of kilowatt hours of energy they use, but also the peak rate at which it is delivered, Aziz explains. That’s because businesses that use most of their power all at once have to have extra electrical power lines to deliver it all, and utilities charge them for it. By installing batteries, those businesses can use stored power to reduce the need for extra electrical connections, and thus reduce their costs. Such applications, Aziz predicts, are only going to grow. If so, cheap, nontoxic flow batteries could become a key part of our energy future.