HIGHLIGHTS
• The volumetric power density of a redox flow battery was increased through the development of a submillimeter bundled microtubular membrane (SBMT).
• By using a zinc iodide redox, the volumetric power density for the SBMT was determined to be 10 times higher than a conventional planar flow battery cell. 
• The durability of the SBMT was found to be good with battery cycling for more than 220 hours, and the SBMT design can be scaled up for use commercially. 

The vulnerability in utilizing renewable energy sources such as solar and wind as the main power generation sources is their lack of reliability because the sun does not always shine, and the wind is not always present. As a result, attention is being paid to the development of energy storage devices that can be employed when renewable power generation is not possible. 

Redox flow batteries have been developed to act as energy reservoirs. In a previous TLT article,¹ new water soluble catholytes were developed based on sodium and potassium salts of iron (II) complexes of iron (II) complexes of bipyridine dicarboxylic acid and dicyanide. Catholytes are electrolytes situated in a tank by the cathode of the battery. The key to the performance of the iron complexes is their size, which minimizes crossover to the anolyte tank situated by the battery’s anode. By tuning the oxidation voltage of the iron (II) complexes, the researchers were able to improve the energy density of the catholyte. 

Current redox flow batteries consist of two large tanks containing the catholyte, anolyte and a membrane positioned between them. The planar ion-exchange membrane is part of the battery’s power module and also includes bipolar plates, gaskets and frames. 

Nian Liu, assistant professor in the School of Chemical and Bimolecular Engineering at Georgia Tech in Atlanta, Ga., says, “The planar configuration for redox flow batteries, which has been in use for many years, is hindered by an inferior volumetric power density. Bipolar plates and other inactive components are the source of this issue because they occupy much of the volume of the redox flow battery cell.” 

Attempts to optimize the planar configuration through preparation of individual battery cells at the centimeter and millimeter scale showed promise in improving the volumetric power density but proved to be unsuccessful. 

Liu found an alternative option based on the use of submillimeter bundled microtubular membranes (SBMT). He says, “SBMT have been used successfully in the chemical separation industry. The reason for SBMT’s potential is this configuration is much more efficient from a spatial standpoint. 

A SBMT exhibits a surface area of 10,000 square meters per a cubic module, which is an order of magnitude higher than conventional planar membranes used in flow batteries.” A SBMT contains a fiber-shaped membrane that is known as a hollow fiber. This design saves space and is a configuration that can be scaled up for use commercially. Liu and his colleagues have now developed a flow battery using SBMT that can demonstrate higher average charge and discharge power densities than conventional planar flow batteries. 

Zinc iodide redox system 
The researchers prepared a SMBT (see Figure 3) using a zinc iodide redox system. Liu says, “Initially, we considered working with vanadium, which is the main type of electrolyte used in redox flow batteries. But low-oxidation-state vanadium is not stable under ambient conditions. A zinc iodide redox was used instead where during charging, iodide is oxidized to triiodide at the cathode, and zinc ions are reduced to zinc metal at the anode.” 


Figure 3. A microtubular flow battery has been produced that exhibits the most space-efficient configuration with approximately 10,000 square meters of surface area per cubic meter of module. Figure courtesy of Georgia Tech.

Peak charge and discharge power densities of 1,322 and 306.1 watts per volume of battery cell, respectively, were realized. A conventional planar flow battery cell typically displays charge and discharge power densities of less than 60 and 45 watts per volume of battery cell, respectively. This means that the volumetric power density for the SBMT is 10 times higher. 

The researchers indicate that the SBMT membranes are the most space-efficient configuration that provide approximately 10,000 square meters of surface area per cubic meter of module. Transmembrane pressure is maintained without the need for supporting components, which saves space and improves efficiency. 

Another advantage of using zinc iodide chemistry is the high energy density due to the high concentration of electrolyte in the flow battery. Liu explains, “The energy density of zinc iodide is higher than vanadium. A combination of the SBMT cell stack with zinc iodide chemistry will make both the cell stack and the electrolyte tank compact.” 

Durability of the SBMT was found to be good with the battery cycling for more than 220 hours, which corresponds to more than 2,500 cycles at off-peak conditions. Electrochemical impedance spectroscopy was conducted to determine how the resistance between the two electrodes compares for the SBMT and conventional H-shaped electrolytic cells. Liu says, “Impedance is much lower for the SBMT because of the close packing of the cell and the smaller distance between the two electrodes.” 

The researchers intend to carry out more research using vanadium as the electrolyte, with practices to protect its oxidation by air. Initial experimentation shows that vanadium can be used and will cycle in a similar manner to the zinc iodide redox system. 

Liu says, “We also intend to demonstrate that the SBMT can be scaled up for use commercially. Currently we have linked 16 tubular membranes together, but our goal is to have systems with hundreds of tubular membranes. Our objective is to validate this idea on a larger scale.” 

Additional information can be found in a recent article² or by contacting Liu at nian.liu@chbe.gatech.edu. 

REFERENCES
1. Canter, N. (2021), “Water soluble organic catholyte for use in redox flow batteries,” TLT, 77 (12) pp. 16-17. Available here.
2. Wu, Y., Zhang, F., Wang, T., Huang, P., Filippas, A., Yang, H., Huang, Y., Wang, C., Liu, H., Xie, X., Lively, R. and Liu, N. (2023), “A submillimeter bundled microtubular flow battery cell with ultrahigh volumetric power density,” Processing of the National Academy of Science, 120 (2), e2213528120.