CUHK Progress in Biomimetic Molecular Catalyst Highly Promising for Future Energy-Efficiency of Redox Flow Batteries

Professor Lu Yi-chun (R) and PhD student Lei Jiafeng (L) from the Department of Mechanical and Automation Engineering (MAE) at CUHK demonstrate a prototype of an iron-sulfur redox flow battery using biomimetic molecular catalysts. Courtesy of CUHK

The Chinese University of Hong Kong (CUHK) has developed a molecular catalyst that can increase the reaction rate of redox flow batteries, significantly reducing energy consumption by three times, and boosting charging power by nearly six times. It is believed that this approach can be widely applied in other redox flow battery systems. This groundbreaking research was recently selected as the cover story in the December issue of the world-leading scientific journal “Nature Energy.”

Lithium-ion batteries are commonly employed for energy storage, but when thousands of them are stacked together, there is a risk of overheating, causing combustion and explosion, making them unsuitable for large-scale energy storage. Aqueous redox flow batteries, on the other hand, are lower in cost, much safer, and offer design flexibility in terms of power and energy. However, the high price of traditional commercial all-vanadium redox flow batteries hinders their further development.

In contrast, sulfur is highly abundant, and the storage cost is 1,000 times lower than that of vanadium. However, sulfur-based redox flow batteries suffer from two bottlenecks, poor cycle life and low energy efficiency, which hinder their further development. In 2021, Professor Lu Yi-chun of the Department of Mechanical and Automation Engineering (MAE) of the School of Engineering at CUHK proposed a charge-reinforced ion-selective (CRIS) membrane that significantly extended the cycle life by reducing the crossover rate of polysulfide. However, the commercialization process still faced challenges with low energy efficiency.

Ms. Lu’s team then proposed a molecular catalyst, riboflavin sodium phosphate (FMN-Na), to accelerate the conversion of polysulfides with high energy efficiency. Riboflavin is actually vitamin B2, and riboflavin derivatives function as energy carriers in the human body. Inspired by the natural electron transport chain, Prof. Lu and her team adopted FMN-Na to accelerate the reaction rate of the flow battery.

The team found that this biomimetic strategy can drastically reduce the battery’s overpotential by more than three times, and increase energy efficiency from 53 to 76 percent, thereby increasing charging power by nearly sixfold. The iron-sulfur flow batteries can operate stably for more than 2,000 cycles (with an expected life span of more than 20 years), and this same strategy is also found to work with sulfur-iodide flow batteries, achieving high stability on more than 1,300 cycles.

Collaborating with the company they founded, Ms. Lu’s team demonstrated a 100 sq. cm (about 16 sq. in.) battery stack with an operating current density of up to 100 milliamperes per sq. cm, showing enormous potential for practical applications.

Ms. Lu said that their work shows that homogeneous catalysis is an effective approach to addressing the sluggish kinetics of polysulfides. She also believed that this approach could be widely applied to other flow battery systems.