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South Korean scientists have developed a highly soluble and stable redox-active organic molecule for use in aqueous redox flow batteries. The newly developed naphthalene diimide (NDI) molecule offers superior storage capacity over existing vanadium devices.
Performance results of an aqueous redox flow battery using 1 M of the developed NDI molecule as the cathodic electrolyte and 3.1 M of ammonium iodine as the anodic electrolyte. Using a 1.5 M KCl solution.
(a) Schematic diagram of a redox flow stack.
(b) Voltage-capacity graph according to cycle in a redox flow cell.
(c) Graphs of capacity and coulombs, voltage and energy efficiency held at 500 cycles.
Redox flow batteries are one of the most promising technologies for large-scale stationary storage applications due to their low capital cost, low flammability, and long service life of more than 20 years. However, as the price of vanadium, the most commonly used active material in redox flow batteries, has risen in recent years, scientists have actively sought replacement redox materials.
Now, a research team from the 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 in place of vanadates in the slurry stacks.
Although NDI molecules are nearly insoluble in water and thus little explored, the South Korean research team managed to bind four ammonium functionalities and achieved a solubility of up to 1.5M in water. By regulating the p-p interactions of these organic molecules, the researchers have avoided the severe side reactions and reduced cyclability that could have caused radical formation during the electron transfer process.
“We have demonstrated the principles of molecular design by modifying an existing low-solubility organic active molecule and using it as the active molecule for 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."
In addition, the researchers demonstrated that when a 1M NDI solution was used in redox-neutral flow batteries for 500 cycles, 98% capacity was maintained. This means a capacity deterioration of 0.004% per cycle, and only 2% of its capacity would be lost if the battery operated for 45 days.
The researchers also showed that the developed NDI molecule can contain two electrons per molecule, meaning that 2M electrons could be stored in every 1M NDI solution used.
For reference, the vanadium used in vanadium redox flow batteries, which require a highly concentrated sulfuric acid solution, has a solubility of approximately 1.6M and can only hold one electron per molecule, meaning it can store a total of 1.6M electrons. Therefore, the newly developed NDI active molecule shows a higher storage capacity compared to existing vanadium devices.
“If this were to be used later for aqueous redox flow batteries, along with its high energy density and high solubility, it would also have the advantage of being able to be used in pH-neutral electrolytes,” Ryung Byon said. "Vanadium redox flow batteries currently use acidic solutions, which cause corrosion, and we hope our molecule will solve this problem." As current lithium-ion batteries are flammable, we need to develop safer and cheaper new generation batteries, and our research is very promising.”
The researchers report their findings in the paper Controlling p-p interactions of highly soluble naphthalene diimide derivatives for neutral pH aqueous redox flow batteries. neutral pH), recently published in Advanced Materials. |
