Pathway to commercialization of aqueous sulfur-based redox flow batteries

Polysulfide is one of the most promising materials for electrolytes used in large-scale aqueous redox flow batteries (RFBs) due to its inherent safety, high energy and low cost. However, potential polysulfide crossover results in a poor battery lifecycle, which prevents sulfur-based flow batteries from getting closer to commercialization.

With this in mind, a group of researchers in China has outlined a new pathway for the industrialization of this energy storage technology, which promises a competitive levelized cost of storage for long-duration energy storage. “In our work, we proposed an integrated strategy targeting sulfur-based flow battery commercialization, focusing on catalyst design, ion-selective membranes, and device integration,” the research’s corresponding author, Dongliang Chao, told pv magazine. 

RFBs devices work by storing energy in two separate tanks of fluids. The tanks need different redoxmers, which are redox-active molecules that can store energy in the batteries’ electrolytes. These molecules receive and send out electrons, or negatively charged particles. Their electrochemical properties can be significantly affected by the choice of supporting electrolyte. In order to enable the monitoring of redox flow battery performance in real time, carbon-based redoxmer molecules can be used as energy carriers and also to signal a problem known as “crossover.” This issue occurs when the redoxmers migrate to the wrong side of the battery.

The researchers stressed that polysulfide electrolytes could enable a low energy cost of $38.7 kWh thanks to the low cost of sulfur, which is approximately $0.15 kAh. By way of comparison, the U.S. Department of Energy set a cost target of $100 kWh for aqueous sulfur-based RFBs. “The majority of commercially available aqueous RFBs are vanadium-based RFBs with a lifespan of 10–20 years and an energy efficiency of over 80%,” they further explained. “However, the high cost of vanadium (US$12.7 kg) driving the high investment cost to $400–500 kWh, the low energy density (25 Wh l) and the low solubility of vanadium species hinder their widespread applications.

Sulphur-based RFBs have a limited lifespan of less than 200 cycles and potential solutions may come from membrane, catalyst and device engineering, the scientists said.

Membranes represent a significant portion of the final costs of redox flow batteries (RFB), with Nafion costs reaching around $500/m2. It is therefore important to identify the membrane technologies that could simultaneously ensure high performance and low-cost production. Charge reinforcement, hydrophobicity design and pore structure regulation were identified by the research team as an effective way to improve the stability of membranes. “Nonfluorinated-polymer membranes offer an economical advantage, but the balance between the mechanical strength and ionic conductivity still needs to be further explored and optimized,” it added.

As for the catalysts, the scientists said their improvement is required to reduce electrochemical polarization, improve energy efficiency and extend RFB operation. “We believe that the selection criteria of molecular catalysis having suitable redox potential, having no side reactions and enabling fast electrochemical kinetics are pivotal for regulating the reversibility and kinetics of the redox couples,” they emphasized, noting that systematic investigations will be necessary to identify alternative catalyst types and their corresponding catalytic mechanisms.

Moreover, the group said advanced counter electrodes should be developed to ensure RFBs’ stability and kinetics, while boosting energy density and reducing costs.

“Accelerating the transition of SRFBs from lab to fab requires interdisciplinary cooperation, including chemistry, materials science, electrical engineering and energy and environmental sciences. Academic research should keep pace with industry needs, to solve fundamental scientific and engineering problems for SRFBs to become a competitive energy storage technology,” the researchers concluded.

Their findings can be found in the paper “Aqueous sulfur-based redox flow battery,” published in nature reviews electrical engineering. The research team comprised academics from the Chinese University of Hong Kong (CUHK) and Fudan University.

In 2021, other researchers at CUHK developed a sulfur-based redox flow battery that can operate for 15 consecutive hours of runtime and for over 2,000 cycles without obvious capacity decay.

From pv magazine International.