It is a challenge to monitor the health of flow batteries in real time. Argonne researchers have designed a fluorescent molecule for this.
The research continues to charge on redox flow batteries, a possible solution for storing energy on the electricity grid. But such batteries remain somewhat of a black box when it comes to diagnosing performance. At the US Department of Energy's (DOE) Argonne National Laboratory, scientists have discovered fluorescence as a way to shed light on what happens to power batteries while they work.
Redox flow batteries work by storing energy in two separate tanks of liquid, which is similar to the positive and negative terminals of an AA battery. But flow batteries are designed for much larger scales than what you could hold in your hand. The capacity of a power battery, or stored energy, can be easily selected with larger tanks. This flexibility makes them attractive for use on the grid, where they can store fluctuating output from wind turbines and solar panels.
To make these fluids, battery scientists need molecules called redoxmers, named after the reversible "redox" reactions they undergo. During these reactions, the molecules receive and release electrons, or negatively charged particles, that can be used to store energy in the batteries. Each tank needs a different redoxmer: one that is positively charged and one that is negatively charged.
The battery fluids also contain supporting electrolytes, which are saline solutions that conduct electricity between the battery's positive and negative terminals. The salts move across a membrane between the tanks to balance the charge or discharge of the redoxmers.
Developing and deploying these batteries requires the ability to track their performance in real time and in ways that go far beyond the tiny 'battery status' icons that we are used to on our phones and computers. In a study funded by the Joint Center for Energy Storage Research (JCESR), a DOE Energy Innovation Hub led by Argonne, researchers set out to design carbon-based redoxmer molecules that can both transport energy in the battery and signal a problem becoming crossover mentioned. , when the redoxmers migrate to the wrong side of the battery.
"Crossover is a major problem for power batteries," said Lu Zhang, a chemist at Argonne. "In this case it is particularly challenging because we are dealing with very small molecules dissolved in an electrolyte and with membranes that are porous."
In an ideal world, redoxmers stay in their respective compartments. But when crossover occurs and the redoxmers penetrate the membrane between the two tanks, it can degrade battery performance.
The research team came up with two chemical variations of a common anode (negative) redoxmer, 2,1,3-benzothiadiazole (BzNSN). BzNSN is very stable during charging, dissolves easily in a solvent-based electrolyte, and can be designed to fluoresce under ultraviolet light. These properties make it an excellent candidate for a health self-reporting agent within some types of power batteries.
To measure the molecules' fluorescence in common power battery fluids, the researchers relied on fluorimetry measurements from the Center for Nanoscale Materials, a DOE Office of Science User Facility, which showed different behavior depending on the electrolyte salts used. Electrochemical stability measurements also indicated that one molecular design in particular retained its electrochemical function and stability for days under a charged state. In a separate experiment, the telltale fluorescent glow under ultraviolet light was used to detect redoxmer crossover in real time, showing how the motions of the redoxmer molecule change depending on the composition of the electrolyte.
The study marks the first time that fluorescence has been used to monitor a health condition in power battery design. "The fluorescence detection offers a great advantage over other methods because it is very sensitive," said Lily Robertson, a postdoctoral fellow at Argonne. "We see the molecule the moment it passes." Other detection methods such as cyclic voltammetry or absorption spectroscopy, which measure current or absorption of light, may interfere with a working battery or have sensitivity limits. The fluorescence also provides a unique visual handle that reveals nuanced interactions with the other components of the battery fluid, such as the supporting electrolyte.
Demonstrating the ability to use fluorescence as a beacon for power battery crossover problems paves the way for similar self-reporting opportunities for other health metrics.
"We envision that we could apply this to most battery cycling parameters, such as capacity decay," said Zhang.
The article "Fluorescence-Enabled Self-Reporting for Redox Flow Batteries" was published in the journal ACS Energy Letters in August.
Lily A. Robertson et al .; Fluorescence Activated Self Reporting for Redox Flow Batteries, ACS Energy Letters (2020). DOI: 10.1021 / acsenergylett.0c01447
A Bright New Outlook for Self-Reporting Batteries (2021, Feb. 3)
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