Researchers led by Korea’s UNIST developed a new redox flow battery concept that uses iron and chromium ore in redox chemistry. The proposed battery assembly is reported to achieve a stable 500-cycle lifetime and a high energy density of 38.6 Wh L−1.
“We are in the early stages of investigating the economic aspect of these batteries,” said lead study author Hyun-Wook Lee. pv magazine. “Although we have not yet performed a thorough levelized cost of storage (LCOS) assessment, our early findings suggest potential cost competitiveness, especially given the environmentally friendly nature of our batteries and promising performance metrics.”
The battery is based on a chromium-based negative electrolyte, or negolyte, and strong-field cyanide ligands, which researchers claim can mitigate the Jahn-Teller effect. The latter is a geometric distortion of a nonlinear molecular system that reduces its symmetry and energy and in redox flow batteries reduces redox potential, stability and fast kinetics.
Preventing the unwanted decomposition of the chromium(II) chloride-Cr(II) complex used in the battery is a crucial step to avoid these problems during the electrochemical cycling of redox batteries, which facilitates a stable and fast redox reaction.
The battery also uses negolyte based on hexacyanometalate, an inorganic compound with the formula Cr(CN)6. “Reliable Electrochemical Properties of Cr(CN).6 are unique due to their low redox potential of –1.15 V,” the team explained. The electrolyte was based on sodium cyanide (NaCN), which is claimed to offer excellent electrochemical stability, especially against highly reducing potentials, combined with high water solubility.
Using thermodynamic density functional theory (DFT) simulations, the researchers found that the negolyte coordinates well with the strong-field ligands used in the battery and is able to reduce the Jahn–Teller effect.
The proposed battery assembly can achieve a stable 500-cycle life and a high energy density of 38.6 Wh L−1according to the research team.
“This hexacyanometalate-based redox flow battery is capable of reversible redox reactions at a relatively high potential as an aqueous system, exploiting the low potential of Cr(CN).6 and improved cycling performance at high current densities,” it stated. “Compared to conventional and other flow batteries, this cell assembly has superior characteristics in terms of whole cell potential and cycling performance.”
The new battery concept is presented in the study “Full-Hexacyanometallate Aqueous Redox Flow Batteries Exceeding 1.5 V in an Aqueous Solution”, published Advanced energy materials. The research group includes researchers from e.g Nanyang Technological University in Singapore and Ulsan National Institute of Science and Technology (UNIST) in South Korea.