Researchers in South Korea have developed a highly soluble, stable organic redox-active molecule for use in aqueous redox flow batteries. The newly developed naphthalene diimide (NDI) molecule offered a higher storage capacity than existing vanadium devices.
Now, a research team from Korea Advanced Institute of Science and Technology (KAIST) and Pohang University of Science and Technology (POSTECH) in South Korea has developed a highly soluble and stable active molecule – naphthalene diimide (NDI). It was used instead of vanadates in water flow batteries.
Although NDI molecules are almost insoluble in water and therefore have been little studied, a South Korean research group managed to combine four ammonium functionalities and achieve a water solubility of up to 1.5 M. By controlling the π–π interactions of these organic molecules, the researchers have avoided severe side reactions and impaired cyclization that could have resulted from the formation of radicals during the electron transfer process.
“We have demonstrated the principles of molecular engineering by modifying an existing organic active molecule with low solubility and using it as an active molecule in redox flow batteries,” said Professor Hye Ryung Byon. “We have also shown that during a redox reaction we can use molecular interactions to suppress the chemical reactivity of radically formed molecules.”
Furthermore, the researchers showed that when a 1 M NDI solution was used in neutral redox flow batteries for 500 cycles, 98% of its capacity was retained. This means a 0.004% capacity loss per cycle, and only 2% capacity loss if the battery was used for 45 days.
The researchers also showed that the developed NDI molecule can contain two electrons per molecule, which means that 2M electrons can be stored for every 1M NDI solution used.
By comparison, vanadium, which is used in vanadium reduction flow batteries, which require a very concentrated solution of sulfuric acid, has a solubility of about 1.6 M and can only hold one electron per molecule, meaning it can store a total of 1.6 M electrons. Therefore, the newly developed NDI active molecule has a higher storage capacity compared to existing vanadium devices.
“If this was later used for aqueous redox flow batteries, in addition to its high energy density and high solubility, it would also have the advantage of being usable in neutral pH electrolytes,” Ryung Byon said. “Vanadium redox flow batteries currently use acidic solutions that cause corrosion, and we expect our molecule to solve this problem. As current lithium-ion-based ESS systems are flammable, we need to develop safer and cheaper next-generation ESS devices, and our research has shown promising solutions over here.”
The researchers discussed their findings in the paper “Controlling π-π interactions of highly soluble naphthalene imide derivatives for neutral pH aqueous redox flow batteries,” which was recently published in Advanced Materials.