Quinone-Bromine Redox Flow Battery (RFB) is a promising system for large-scale energy storage due to its high power and potential for low system cost [1]. As in a fuel cell, in a RFB the energy conversion device is separated from the energy storage containers, enabling independent scaling of energy and power.

To improve Quinone-Bromine RFB performances it is necessary to develop and study stable and efficient components, such as membranes. Membrane properties have a significant impact on the performance and efficiency of the system. In particular, there is a tradeoff between conductivity and crossover, where conductivity limits system efficiency at high current density and crossover limits efficiency at low current density.

Nafion has been the most widely used membrane for this application because of its high proton conductivity and chemical stability. However, various drawbacks, like high cost and low selectivity to Br^-/Br₂ species, limit battery service life. To overcome these limitations, in the last years different approaches have been adopted. In particular, to improve selectivity with respect to the pristine polymer, inorganic or inorganic-organic nanoparticles have been added to Nafion matrix.

In this work, sulfonated hypercrosslinked polystyrene (sHCP) nanoparticles have been employed for the fabrication of Nafion-based composite membranes with enhanced selectivity.

The use of fully organic fillers offer different advantages over inorganic ones, such as better compatibility with the polymer matrix and easier tunability of the degree of sulfonation, and thus of the ion exchange capacity (IEC). sHCP nanoparticles showing similar IEC with respect to Nafion (0.92 and 0.96 meq g^-1 respectively) have been prepared and different filler loadings have been considered. The performances of composite membranes have been tested both ex-situ as well as in situ, and compared to those of Nafion. More in detail, the in operando performances have been evaluated by polarization curves at different states of charge (SOC) and Electrochemical Impedance Spectroscopy (EIS). Capacity fading, and coulombic and energy efficiencies of the system have been studied by galvanostatic charge-discharge cycles. Battery self-discharge has been investigated, too. Moreover, permeability of membranes has been tested by Ion Chromatography on electrolytes, before and after battery cyclation.

Composite membranes displayed a significant lowering of the Br^-/Br₂ crossover, yet maintaining high proton conductivity.

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] 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)