The vanadium redox flow battery systems have attracted attention because of their high energy storage capacity and good scalability. Moreover, the robustness and long lifetime make them highly promising. The cell membrane is a critical component because it defines the performance and energy efficiency. Exposure of the polymeric membrane to the highly oxidative and acidic environment of the vanadium electrolyte can result in membrane degradation. State-of-the-art for all-vanadium RFBs are ion exchange membranes, that are based on the perfluorinated polymer backbone, such as Nafion®. These membranes exhibit good stability in the electrolyte solution upon cycling, however there are also downsides: the poor membrane selectivity towards vanadium permeability leads to faster self-discharge of the battery. Moreover, the high costs of the currently used membranes reduce the competitiveness of the battery systems.

In addition, fluorinated polymers have very limited recyclability and substantial environmental concerns regarding the fluorinated precursors become an issue for manufacturers and society. Therefore, cost competitive and environmentally benign membranes, that possess a hydrocarbon-based backbone, are highly desirable. Para-polybenzimidazole (PBI) polymer is a promising candidate to meet these requirements. The conventional production process of PBI membranes uses organic solvents, leading thus to the accumulation of waste and rising the membrane price. BASF has introduced a production process without solvents for the Celtec®-P membrane, currently used for high temperature PEM fuel cells. A novel method for the transformation of PBI gel membranes (Celtec®-P) into dense PBI films has been recently developed.[1]

While the PBI gel membrane displays high vanadium permeability, the new dense PBI membranes show much lower vanadium crossover. The new generation of Celtec membranes also shows good ionic conductivity and promising performance in vanadium redox-flow batteries.[2] The results of ex-situ measurements and battery cycling tests will be presented in this poster, highlighting advantages of the new dense Celtec membranes for vanadium RFBs.

References

[1] L. A. Murdock, F. Huang, B. C. Benicewicz, Patent US 2021 0280883 A1

[2] L. A. Murdock, B. C. Benicewicz, L. Wang, F. Huang, Patent WO 2020 056268 A2.