To create an economical redox flow battery, scientists from the South China University of Technology have developed a molecular compound that acts as a cost-effective electrolyte, facilitating a consistent flow battery that holds 99.98% capacity per cycle.
Photo of the aqueous redox flow battery. (Image Credit: Liwen Wang, South China University of Technology).
The researchers reported their approach in the August 14th issue in the Energy Material Advances.
Containing two tanks of opposing liquid electrolytes, the battery pumps the positive and negative liquids along a membrane separator squeezed in between electrodes, enabling ion exchanges to yield energy.
According to corresponding author Zhenxing Liang, professor in the Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, crucial work has been devoted to forming the negative electrolyte liquid, but not enough attention has been given to the positive electrolyte liquid.
Aqueous redox flow batteries can realize the stable electrical output for using unsteady solar and wind energy, and they have been recognized as a promising large-scale energy storage technology.
Zhenxing Liang, Study Corresponding Author and Professor, Key Laboratory of Fuel Cell Technology, School of Chemistry and Chemical Engineering, South China University of Technology
“Electroactive organic merit of element abundance, low cost and flexible molecular control over the electrochemical features for both positive and negative electrolytes are regarded as key to developing next-generation redox flow batteries,” Liang added.
Liang and his team concentrated on TEMPO, a chemical compound with effortlessly reversed oxidation states and high potential for energy, a preferred quality in positive electrolytes.
“However, TEMPO cannot be directly applied to aqueous redox flow batteries due to the high hydrophobicity of the molecular skeleton,” Liang said, clarifying that TEMPO, left unaltered, will not dissolve in the liquid necessary to enable the energy exchange in the flow batteries. “We developed a strategy to functionalize TEMPO with viologen, an organic compound that has highly reversible redox reactions, to improve TEMPO’s hydrophilicity.”
Liang says that viologen is extremely soluble in water, which boosts the ability of TEMPO to dissolve in water. Viologen also chemically removes electrons from atomic partners, which raises its potential to modify its oxidative state. Viologen is also a salt, which bestows TEMPO with what Liang describes “a decent conductivity” in an aqueous solution.
When the created viologen-altered TEMPO was verified in a flow battery, the team discovered that the battery could retain a capacity of 99.98% per cycle, meaning the battery could contain approximately all its stored energy when not actively used.
This work overcomes the disadvantages of TEMPO by viologen-functionalization and realizes its application in aqueous redox flow battery. The molecular design concept provides a strategy for novel organic electroactive materials and lays a foundation for the application of aqueous organic flow battery.
Zhenxing Liang, Study Corresponding Author and Professor, Key Laboratory of Fuel Cell Technology, School of Chemistry and Chemical Engineering, South China University of Technology
The other contributors are Shuzhi Hu, Liwen Wang, Xianzhi Yuan, Zhipeng Xiang, Mingbao Huange, Peng Luo, Yufeng Liu and Zhiyong Fu, all from the Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology.
Hu is also associated with the School of Materials Science and Engineering, Sun Yat-sen University.
This research was supported by the National Natural Science Foundation of China (21975081 and 21905114).
Journal Reference:
Hu, S., et al. (2021) Viologen-Decorated TEMPO for Neutral Aqueous Organic Redox Flow Batteries. Energy Material Advances. doi.org/10.34133/2021/9795237.