KIER Creates Viologen Redox Flow Battery to Replace Vanadium

A technology has been developed to replace the active material in high-capacity ESS “redox flow batteries” with a more affordable substance.

^{*Redox Flow Battery: A term derived from the synthesis of the terms Reduction, Oxidation and Flow. It is a battery that stores electrical energy in the form of chemical energy through oxidation and reduction reactions of active materials in the electrolyte on the surface of the electrode and converts it back into electrical energy when needed. It is capable of large-scale storage, can be used in the long term thanks to periodic replacement of the electrolyte and its main advantage is the absence of fire hazard.}

The research team of Dr. Seunghae Hwang from the Energy Storage Research Department of the Korea Energy Research Institute has successfully improved the performance and lifetime of redox flow batteries, an important high-capacity energy storage device, by introducing functional groups* that replace active materials and improve solubility and stability.

^{*Functional group: A group of atoms within an organic compound that determines the properties of the compound and plays a role in defining its characteristics.}

To expand the use of renewable energy such as solar and wind power, a long-term energy storage system is needed that can store electricity generated under favorable weather conditions for more than 8 hours and reuse it when needed. Among these systems, redox flow batteries, which have a lower fire hazard and a longer service life of more than 20 years compared to commonly used lithium-ion batteries, are being actively researched globally. The Republic of Korea is also focusing on developing* low-cost, high-efficiency technologies that will be widely adopted around 2030.

^{*Energy Storage Industry Development Strategy (October 2023), Ministry of Trade, Industry and Energy}

Although vanadium is currently commercialized as an active material in redox flow batteries, its limited supply has spurred recent research into alternatives. Organic compounds such as viologens, made from naturally occurring elements such as carbon and oxygen, are particularly notable for their affordability and potential to replace vanadium. However, viologens have the disadvantage of low solubility, which reduces overall energy density, and instability upon repeated charging and discharging, requiring the development of technologies to overcome these issues.

To address these issues, researchers introduced functional groups into viologens. These functional groups fit into viologens like building blocks, improving their solubility and stability.

To increase the solubility of viologens, the researchers introduced sulfonate and ester functional groups, which have hydrophilic properties. These two functional groups generate attractive forces between molecules through interactions with water molecules (electrolyte) on the surface of viologens, thus facilitating the dispersion of viologens in water.

Viologens are structured like a sandwich, consisting of two molecular layers. When charged, these layers frequently combine, transforming into a structure that can no longer store energy. To address this problem, the researchers introduced alpha-methyl functional groups that act as obstacles. These functional groups introduce a twist into the layered structure and generate repulsion between the molecules, suppressing side reactions and thus improving the efficiency and stability of energy storage.

The application of the active material developed by the researchers to redox flow batteries confirmed that the energy density was more than twice that of vanadium redox flow batteries. In addition, after 200 charge and discharge cycles, the batteries demonstrated a Coulomb efficiency of 99.4% (discharge capacity relative to charge capacity) and a capacity retention of 92.4%, indicating improved performance and stability.

Dr. Seunghae Hwang, lead author of the paper containing the research results, said, “In response to climate change and to expand the use of renewable energy, it is necessary to facilitate energy storage through the development of redox flow batteries that have both a competitive price and a long cycle life.” She added, “This research enables the design of active materials that offer both affordability and longevity, thereby contributing to the early commercialization of redox flow batteries.”

The research results were published in the prestigious materials science journal ACS Applied Materials and Interfaces (IF 9.5) and the study was conducted with the support of KIER.