Vanadium Redox Flow Batteries (VRFBs) are considered as a key technology in the field of large-scale energy storage systems [1, 2]. Unfortunately, their capabilities are compromised by the crossover of active species through the membrane which reduces the overall efficiency during operation and hinders the long-term capacity storage [3]. In this work, an extremely selective proton exchange material is proposed. “Zip-like” Ion Exchange Membranes (ZIEMs) are prepared by means of an acid-base reaction between the cationic and anionic functionalities of two interacting ionomers. The strong electrostatic crosslinking interactions between the two components generate an overall improvement of thermal and mechanical properties, and the permeation of vanadium ions is strongly hindered (the best performing membrane has a selectivity of 3.1⸱10⁵ S⸱min⸱cm^-3, which is almost 50 times higher than Nafion). The best performing membranes show outstanding VRFB performance with a stable coulombic efficiency (>98% at 50 mA⸱cm^-2), and a retained capacity of 82% after 100 cycles compared to the 21% of the same device with a Nafion 212 membrane. The results presented here show how ZIEM family of membranes can be regarded among the best performing ion exchange membranes up-to-date.

Acknowledgements

The authors wish to thank the fruitful collaboration within the project “Development of high-selectivity ion-exchange membranes for application in next-generation redox flow batteries” funded by ENI S.p.A., contract 2500026228. In addition, the authors wish to thank the SID2020Project of the Department of Industrial Engineering, University of Padova “A New frontier in Hybrid Inorganic-Organic Membranes for Energy Con-version and Storage Devices” (Prot. BIRD201244) for funding.

References

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[2] C. Sun, E. Negro, A. Nale, G. Pagot, K. Vezzù, T.A. Zawodzinski, L. Meda, C. Gambaro, V. Di Noto, An efficient barrier toward vanadium crossover in redox flow batteries: The bilayer [Nafion/(WO₃)_x] hybrid inorganic-organic membrane, Electrochimica Acta, 378 (2021) 138133.

[3] V. Di Noto, K. Vezzù, G. Crivellaro, G. Pagot, C. Sun, L. Meda, I.A. Rutkowska, P.J. Kulesza, T.A. Zawodzinski, A general electrochemical formalism for vanadium redox flow batteries, Electrochimica Acta, 408 (2022).