When researchers at the Energy Department’s Pacific Northwest National Laboratory set out to make flow batteries better, they were not fooling around. They came up with solution based on sugar that can last all year, and more. The discovery could lead to new low-cost, long duration energy storage systems that can write fossil energy out of the picture by capturing wind and solar power for weeks, months, or even whole seasons.
The Flow Battery Solution For Long Duration Energy Storage
Flow batteries haven’t gone mainstream, yet, but they are slowly edging into the market.
Lithium-ion batteries are still the gold standard for energy storage systems that can be recharged over and over again. Lithium-ion technology is a good fit for all sorts of devices, from smart phones to electric vehicles. The problem kicks in at the grid scale, when energy storage is needed to smooth out kinks in the availability of wind or solar power, or both.
At that scale, banks of lithium-ion batteries can only last a few hours. Adding more banks to create a cascading effect could help stretch the timeline. However, that runs into prohibitive costs, including the cost of managing such a complex system.
To provide full day, utility scale energy storage as more wind and solar power enter the grid, a flow battery fits the bill.
Flow batteries leverage the ability of specialized liquids to create an electrical current when they flow adjacent to each other, separated by a thin membrane. The liquids are stored in tanks until needed, which means the flow battery can be cycled on or off rapidly, as needed. Scaling up does not appreciably add to the complexity of a flow battery system, because the scale depends primarily on the size of the tanks.
Today’s flow battery designs represent a 50-year process of improvement that began to take shape in the 1970s. Back then, corrosion, size, weight, toxicity, and low energy density were among the problems to be solved. Nevertheless, science loves a challenge.
NASA was on the forefront of the movement. Last year, the agency noted that its decades-old “liquid battery” — made with only iron, salt, and water — has been given new life by the long duration energy storage startup ESS. Legacy engineering firms like Cummins are also working to develop a new generation of flow batteries (see more CleanTechnica coverage here).
A Little Simple Sugar In Your Flow Battery
The Pacific Northwest National Laboratory’s contribution is the first ever use of β-cyclodextrin, a simple sugar derived from starch, in a flow battery formula.
Simple sugars are composed of just one or two molecules, in contrast to complex carbohydrates like starch. They can be synthesized in the lab, providing a more sustainable alternative to the materials mined for other battery formulas.
β-cyclodextrin is not just any simple sugar. It is a single, cone-shaped molecule commonly used in the pharmaceutical industry to fabricate drugs because it can house other molecules within the cone until its outer surface dissolves. That provides yet another advantage, in terms of having a supply chain knowledge base ready at hand.
Researchers have also been discovering additional uses for the “binding cavity” of β-cyclodextrin. For example, a team at Northwestern University is deploying β-cyclodextrin to develop an eco-friendly process for extracting gold from ore.
The PNNL team focused on β-cyclodextrin because they were looking for a straightforward way to introduce more fluorenol into their flow battery. Fluorenol is an alcohol derivative of fluorene that has begun to emerge in new flow battery technology, including research under the PNNL umbrella.
Apparently the idea behind the new research was to encapsulate fluorenol in β-cyclodextrin molecules, which would dissolve when introduced to a flow battery solution. That turned out to be not a very efficient way to deliver more fluorenol, but it did result in a fair amount of β-cyclodextrin making its way into the PNNL flow battery.
The β-cyclodextrin made an unexpected difference, acting as an agent that motivates more activity in other chemicals in the solution.
Better Flow Batteries, With Sugar
The team published their new findings last week in the journal Joule last week under the title, “Proton-regulated alcohol oxidation for high-capacity ketone-based flow battery anolyte,” in which they note the challenges involved in fluorenone-based flow batteries (fluorenone results from the oxidation of fluorene).
Ketones are the molecules produced by the body to break down fat when not enough sugar is available in the blood, of which fluorenone is one example. As the PNNL team points out, the activity level of fluorenone-based flow batteries is hampered by the slow rate at which alcohol oxidizes. In their solution, β-cyclodextrin acts as a proton regulator that kicks oxidation into high gear.
“Fluorenone-based flow batteries with the organic additive β-cyclodextrin demonstrate enhanced rate capability, high capacity, and long cycling,” they concluded.
“This study opens a new avenue to improve the kinetics of aqueous organic flow batteries by modulating the reaction pathway with a homogeneous catalyst,” they added, referring to the action of a catalyst dissolved in a solution, instead of a solid catalyst applied to a surface.
In a followup press release dated July 10, PNNL noted that the team tweaked their new formula until the flow battery achieved 60% more peak power than earlier versions. They ran it through continuous charge-discharge cycles for more than a year with only a minimal loss in capacity.
“This is the first laboratory-scale flow battery experiment to report more than a year of continuous use with minimal loss of capacity,” PNNL noted.
The principal investigator on the study, longtime PNNL researcher Wei Wang, emphasized that β-cyclodextrin is a totally new approach to flow battery formulation.
“We showed that you can use a totally different type of catalyst designed to accelerate the energy conversion. And further, because it is dissolved in the liquid electrolyte it eliminates the possibility of a solid dislodging and fouling the system,” he explained.
The year-long experiment ended only when the battery’s plastic tubing failed.
Success at the lab scale is another step towards scaling up to the football-field size envisioned for large scale, long duration energy storage systems. Next steps involve finding better plastic, and tweaking the formula to improve performance. Among other tasks, the team is searching for a molecule similar to β-cyclodextrin, but smaller.
PNNL is already planning to include the new flow battery in its forthcoming Grid Storage Launchpad initiative, which will formally kick off in 2024, so stay tuned for more on that.
Group Hug For US Taxpayers
PNNL is part of the sprawling network of national laboratories under the umbrella of the US Department of Energy representing an enormous investment in public funding for scientific research that benefits the common welfare, so group hug for the taxpayers.
In addition to Wang and first author Ruozhu Feng, other researchers working on the new flow battery are PNNL scientists Ying Chen, Xin Zhang, Peiyuan Gao, Ping Chen, Sebastian Mergelsberg, Lirong Zhong, Aaron Hollas, Yangang Lian, Vijayakumar Murugesan, Qian Huang, Eric Walter and Yuyan Shao, along with Benjamin J. G. Rousseau and Hammes-Schiffer of Yale.
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Image credit: “Researchers at PNNL developed a cheap and effective new flow battery that uses a simple sugar derivative called β-cyclodextrin (pink) to speed up the chemical reaction that converts energy stored in chemical bonds (purple to orange), releasing energy (electrons) to power an external circuit. A parallel reversible process (red-green) in the positive catholyte solution balances the positive and negative charges during charge and discharge” (animation by Sara Levine/PNNL).
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