Redox flow batteries (RFB) are one of the most promising energy storage systems used to overcome the intermittence limitations of renewable energies. In RFB, electrical energy production is supplied by two electrolytes that undergo redox reactions in two half cells. These latter are separated by a membrane, whose primary function is to allow the proton flow, while preventing the transport of the active species to avoid battery self discharge.

Traditional aqueous RFBs, such as all-vanadium (redox battery, VRB) [1], or more innovative anthraquinone-disulphonic acid (AQDS) combined with the Br₂/Br^- redox couple [2], used Nafion membranes as separator because of the high ion conductivity and chemical stability of this polymer. However, its low selectivity causes a significant drop of battery performances over time.

In this work, several ionic exchange membranes (CEM) have been investigated and their performances have been compared to Nafion. Hydrocarbon-based membranes have been tested because highly rigid aromatic macromolecular chains and tinier ionic clusters are expected to decrease electrolyte crossover. In this framework, the use of sulfonated poly(phenylene sulfide sulfone) polymers (sPSS) as chemically stable ion-conducting membranes for aqueous RFB is proposed [3].

To further increase membrane selectivity, sPSS has been loaded with sufonated hypercrosslinked polystyrene nanoparticles (sHCP). sHCP have been synthesized through Friedel-Craft alkylation to obtain HCP, followed by post-functionalization to attach sulfonic groups. sHCP structure and morphology has been assessed by TGA, FT-IR, SEM and BET.

Composite (sPSS-sHCP) and pristine (sPSS) membranes have been characterized ex situ and in operando for all vanadium Redox Flow Battery applications, through galvanostatic cyclation at a current density of 150 mA/cm².

The results evidenced the higher performances of the composite membranes with respect to sPSS and Nafion, resulting in a self-discharging time over 3.5x higher and capacity fading about 33% lower.

Acknowledgements

The present work was carried out with the support of the “European Union's Horizon 2020 research and innovation programme”, under H2020-FTIPilot-2015-1 (Grant Agreement n. 720367-GREENERNET) and GREENERSYS project supported by Provincia Autonoma di Trento ITALY

References

[1] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green, New all-vanadium redox flow cell, J. Electrochem. Soc. 133, 1057-1058 (1986)

[2] B. Huskinson, M. P. Marshak, C. Suh, S. Er, M. R. Gerhardt, C. J. Galvin, X. Chen, A. Aspuru-Guzik, R. G. Gordon, and M. J. Aziz, Nature, 505, 195 (2014).

[3] M.Branchi, M. Gigli, B. Mecheri, D. de Porcellinis, S. Licoccia and A. D'Epifanio

J. Mater. Chem. A, 5, 18845 (2017)