Given the discontinuity of renewable energies availability, such as wind and solar, the development of proper storage systems is a crucial point for their economic dispatchability. Redox flow batteries (RFB) are among the most promising technologies as it is possible to scale power (electrode area) and energy (the volume of the electrolyte solutions is proportional to the capacity of the battery) independently.

In this work a RFB systembased on a metal-free redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS), is studied and reported [1].

Combining the quinone/hydroquinone couple with the Br₂/Br^-redox couple, the system presents a suitable design, allowing an easy scale up, but it is necessary to develop and study stable, efficient and low cost components, such as membranes and electrodes to improve battery performances [2].

The membrane is a critical component that determines the completion of RFB systems for practical applications. Membranes are designed to avoid electrical contacts between the anode and cathode, allowing the passage of protons to close the circuit, and must exhibit good ionic conductivity, low permeability of the active species, and high chemical stability.

Perfluorosulfonic polymers, such as Nafion, are commonly used in these applications. However, when used in AQDS/Br based RFBs, Nafion membranes suffer from the crossover of Br^-ions which results in decreased energy efficiency. In this work, several ionic exchange membranes (CEM) have been investigated and compared to Nafion, according to their performances. The permeability of membranes has been tested by Ion Chromatography on electrolytes, before and after their use inside RFBs systems. Ionic conductivities have been evaluated by electrochemical impedance spectroscopy.

Moreover, to enhance system performances, different types of electrodes have been studied, such as carbon papers and sintered titanium fibers/powders with specific bromine/bromide coatings. The behavior of each electrode has been analyzed by cyclic voltammetry (CV) and tested inside a lab-scale AQDS /Bromine RFB cell.

The overall performances have been then evaluated by polarization curves at different state of charge (SOC) and Electrochemical Impedance Spectroscopy (EIS). Coulombic and energy efficiencies of the system have been studied by galvanostatic charge-discharge cycles. By cyclic voltammetry and UV-vis spectroscopy changes in the electrolytes composition have been also investigated and kinetics mechanisms have been examined and compared.

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).

[2] Aishwarya Parasuramana, Tuti Mariana Lima, Chris Menictasc, Maria Skyllas-Kazacos, Electrochimica Acta, 101, 2013, 27-40