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A low-cost, high-performance battery chemistry developed by CU Boulder researchers could one day lead to scalable grid-level storage for wind and solar energy that could help electrical utilities reduce their dependency on fossil fuels.

The new innovation,听, outlines two aqueous flow batteries, also known as redox flow batteries, which use chromium and organic binding agents to achieve exceptional voltage and high efficiencies. The components are abundant in nature, offering future promise for cost-effective manufacturing.

鈥淲e鈥檙e excited to report some of highest performing battery chemistries ever, beyond previous limits,鈥� said Michael Marshak, senior author of the study and an assistant professor in CU Boulder鈥檚听Department of Chemistry. 鈥淭he materials are low-cost, non-toxic and readily available.鈥澨�

Renewable energy sources provide a growing share of U.S. electrical production, but currently lack a large-scale solution for storing harvested energy and re-deploying it to meet demand during periods when the sun isn鈥檛 shining and the wind isn鈥檛 blowing.

鈥淭here are mismatches between supply and demand on the energy grid during the day,鈥� said Marshak, who is also a fellow in the听Renewable and Sustainable Energy Institute (RASEI). 鈥淭he sun might meet the grid鈥檚 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鈥檇 take a car from zero to sixty.鈥�

Although lithium ion technology can provide power for smaller scale applications, you would need millions of batteries to backup even a small fossil fuel power plant for an hour,听Marshak says. But while the lithium ion chemistry is effective, it鈥檚 ill-suited to meet the capacity of an entire wind turbine field or solar panel array.

鈥淭he basic problem with lithium ion batteries is that they don鈥檛 scale very well,鈥� Marshak said. 鈥淭he 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鈥檚 just not cost-effective when you鈥檙e talking about this many megawatt hours.鈥�

Flow batteries have been identified as a more promising avenue. Aqueous batteries keep their active ingredients separated in liquid form in large tanks, allowing the system to distribute energy in a managed fashion, similar to the way a gas tank provides steady fuel combustion to a car鈥檚 engine when you push the pedal.

While there are some examples of flow batteries operating consistently for decades (such as in Japan), they have struggled to gain a broad foothold in commercial and municipal operations due in part to their unwieldy size, high operating costs and comparably low voltage.听

鈥淭he size is less of an issue for grid-scale systems, because it would just be attached to an already large structure,鈥� Marshak said. 鈥淲hat matters is cost, and that鈥檚 what we wanted to improve on.鈥�

The researchers went back to basics, re-examining flow battery chemistries that had been studied years ago, but abandoned. The key turned out to be combining organic binding agents, or chelates, with chromium ions in order to stabilize a potent electrolyte.

鈥淪ome people have taken this approach before, but hadn鈥檛 paid enough attention to the binding agents,鈥� said Brian Robb, lead author of the new study and a doctoral student in the听Department of Chemical and Biological Engineering (CHBE). 鈥淵ou 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.鈥�

Marshak, Robb and undergraduate researcher Jason Farrell customized a chelate known as PDTA, which creates a 鈥渟hield鈥� around the chromium electron, preventing water from hampering the reactant and allowing one of the battery cells to disperse 2.13 volts鈥攏early double the operational average for a flow battery.听

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

鈥淲e got this to work at the relatively neutral pH of 9, unlike other batteries which use highly corrosive acid that鈥檚 difficult to work with and difficult to dispose of responsibly,鈥� Robb said. 鈥淭his is more akin to laundry detergent.鈥�

鈥淵ou could order 15 tons of these materials tomorrow if you wanted, because there are existing factories already producing them,鈥� Marshak added.

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

鈥淲e鈥檝e solved the problem on a fundamental level,鈥� Marshak said. 鈥淣ow there are a lot of things we can try in order to keep pushing the performance limit.鈥澨�