Redox Flow Battery Made of Chromium with Organic Binding Generates 2.13V

July 28, 2019 by Scott McMahan

University of Colorado researchers have developed a low-cost, high-performance battery chemistry that could one day lead to scalable grid-level storage for solar and wind energy. The innovation, reported in the journal Joule, describes two aqueous flow batteries, also called redox flow batteries, which use chromium and organic binding agents to attain outstanding voltage and high efficiencies.

These components are abundant in nature, offering the potential for cost-effective manufacturing.

"We're excited to report some of highest performing battery chemistries ever, beyond previous limits," said Michael Marshak, senior author of the study and an assistant professor in CU Boulder's Department of Chemistry. "The materials are low-cost, non-toxic and readily available."

Renewable energy sources deliver an increasing share of U.S. electrical production, but they need a large-scale solution for storing harvested energy and re-deploying it to meet the demand during periods when the wind isnt' blowing, and the sun isn't shining.

"There are mismatches between supply and demand on the energy grid during the day," said Marshak, who is also a fellow in the Renewable and Sustainable Energy Institute (RASEI). "The sun might meet the grid's needs in the morning, but demand tends to peak in the late afternoon and continue into the evening after the sun has set. Right now, utility companies have to fill that gap by quickly revving up their coal and natural gas production, just like you'd take a car from zero to sixty."

University of Colorado Boulder (CU) PhD student Brian Robb (left) and Assistant Professor Michael Marshak. (Photo: Patrick Campbell/CU Boulder)

Although currently available lithium-ion technology can provide power for smaller-scale applications, you would need millions of batteries to back up even a small fossil fuel power plant for an hour, Marshak said. But while the lithium-ion chemistry is effective, it is still not well suited to meet the needs of an entire wind turbine field or solar panel array, Marshak contends.

"The basic problem with lithium-ion batteries is that they don't scale very well," Marshak said. "The more solid material you add, the more resistance you add and then all of the other components need to increase in tandem. So in essence, if you want twice the energy, you need to build twice the batteries and that's just not cost-effective when you're talking about this many megawatt-hours."

Flow batteries have been identified as a promising alternative. Aqueous batteries use large tanks to hold their active ingredients separated in liquid form. The liquid form lets the system distribute energy in a managed fashion, similar to the way a gas tank delivers steady fuel combustion to a car's engine when you push the pedal.

Marshak noted that while there are some examples of flow batteries that have operated consistently for decades (such as in Japan), they have struggled to gain wider adoption in commercial and municipal operations due in part to their high operating costs, unwieldy size, and comparably low voltages.

"The size is less of an issue for grid-scale systems because it would just be attached to an already large structure," Marshak said. "What matters is cost, and that's what we wanted to improve on."

The researchers re-examining flow battery chemistries that had been studied years ago, but abandoned. The key they discovered was combining organic binding agents, or chelates, with chromium ions to stabilize a potent electrolyte.

"Some people have taken this approach before, but hadn't paid enough attention to the binding agents," said Brian Robb, lead author of the new study and a doctoral student in the Department of Chemical and Biological Engineering (CHBE). "You need to tailor the chelate for the metal ion and we did a lot of work finding the right one that would bind them tightly."

Marshak, Robb, and undergraduate researcher Jason Farrell specially made a chelate known as PDTA, which forms a "shield" around the chromium electron, preventing water from inhibiting the reactant and letting one of the battery cells deliver 2.13 volts, nearly double the operational average for a flow battery.

PDTA is a chemical spinoff of EDTA, an agent already used in some food preservatives, hand soap, and municipal water treatments due to its bacteria-thwarting properties. EDTA is considered non-toxic. The chemistry also employs the benign form of chromium, the same kind in stainless steel surgical instruments.

"We got this to work at the relatively neutral pH of 9, unlike other batteries which use highly corrosive acid that's difficult to work with and difficult to dispose of responsibly," Robb said. "This is more akin to laundry detergent."

"You could order 15 tons of these materials tomorrow if you wanted because there are existing factories already producing them," Marshak added.

Marshak and Robb filed a patent on the innovation with help from Venture Partners at CU Boulder. They intend to continue optimizing their system, including scaling it up in the lab to cycle the battery for even longer periods of time.

"We've solved the problem on a fundamental level," Marshak said. "Now there are a lot of things we can try in order to keep pushing the performance limit."