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ACS Meeting News: Predictive approach to better batteries

by Stu Borman
April 3, 2017


Redox flow batteries can be quite large, exemplified by these shipping-container-sized units. Credit: UniEnergy Technologies

Redox flow batteries (RFBs) aid solar and wind energy facilities by storing energy when it’s generated and then giving it back to the electrical grid when the sun isn’t shining or the wind isn’t blowing. But scientists and engineers would like to improve the size, cost, and longevity of RFB systems, and a new predictive chemistry-based strategy could help achieve those goals.

The technique identifies nonaqueous electrolytes for RFBs that will enable the batteries to have high energy density and high stability. It was developed by Melanie S. Sanford of the University of Michigan, Shelley D. Minteer and Matthew S. Sigman of the University of Utah, and coworkers (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b00147). Sanford discussed the approach on Sunday during a Presidential Events session at the ACS national meeting in San Francisco.

To charge an RFB, generated energy drives electrons in a circuit toward an electrode where a redox-active electrolyte, the anolyte, is reduced. Another redox electrolyte, the catholyte, is simultaneously oxidized at a separate electrode, releasing electrons back to the circuit. Counterions pass through an ion-exchange membrane between the electrodes to maintain charge neutrality. During discharge, the current direction reverses, so the catholyte is reduced and the anolyte is oxidized, restoring each electrolyte’s original redox state. RFB anolyte and catholyte solutions are stored in separate reservoirs, and the amount of electrical energy the batteries can handle increases with the size of the reservoirs.

RFBs that work with water-based electrolyte solutions are already used at solar and wind energy plants, but they have low energy densities, storing only modest amounts of energy per unit volume. They therefore tend to be large and expensive, often housed in multiple units the size of shipping containers. Nonaqueous-electrolyte RFBs have much greater energy densities, making them smaller and cheaper, but the electrolytes tend to be unstable and break down quickly.

Scientists can increase an RFB’s energy density by boosting the difference between the electrochemical potentials of the anolyte and catholyte redox reactions. Ideally, the anolyte reaction’s redox potential will be as low as possible, and the catholyte’s as high as possible. Both electrolytes also need to be stable enough to be recycled—repeatedly oxidized and reduced.

In the new technique, Sanford and coworkers synthesize nonaqueous electrolyte candidates, measure their redox potentials and decomposition rates, and optimize them by performing multidimensional analysis calculations that have traditionally been used in asymmetric catalysis and drug discovery. The calculations use parameters that relate chemical structure to redox potential and charged-state lifetime to predict electronic and structural modifications likely to optimize an electrolyte’s properties. The scientists then synthesize and test the predicted compounds.

In one experiment, predicted modifications to a pyridinium-based anolyte yielded a revised compound with about 10% lower redox potential, 10% higher energy density, and greatly improved stability. Over 200 redox cycles, the modified compound had zero loss in charge-storage capacity, compared with about 50% loss for the original anolyte.

Mitch R. Anstey of Davidson College, an expert on flow battery anolytes and catholytes, commented that the technique “could be broadly useful for generating new types of battery electrolytes. It’s a beautiful adaptation of existing predictive calculations for use in a cutting-edge field.”

In the future, Sanford told C&EN, “we plan to assess other classes of molecules for energy storage applications and to use the approach to develop more highly soluble analogs.” Improving the solubility of compounds in nonaqueous electrolyte solutions is yet another property that could be optimized to build better redox flow batteries.

Related stories:

How redox flow batteries could stabilize our electric grids

A new and improved flow battery


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