In current research, electrochemical energy storage systems have gaining interest because they constitute an essential element in the development of sustainable energy technologies [1,2]. Among them, rechargeable flow batteries (RFBs) are one of the most promising technologies for the integration
in grid-connected electricity, especially if combined with unpredictable and intermittent renewable energy sources, due to their high efficiency, power/energy independent sizing and room temperature
operation [3]. At the moment, among all RFBs systems, the most investigated and advanced technology is the vanadium based RFB, characterized by an energy efficiency equal to 80% and energy density ranging 15-45 Wh/l [4]. However, nowadays the main bulk of research is focused on finding an economically convenient and technically competitive flow battery chemistry, able to ensure long lifetime and high energy efficiency [5,6]. In this study, a zinc-iron RFBs based on sulfate and sulfamate electrolytes will be presented, discussing the achievement of a charge density in the range 30-70 Wh/l. The combination of high energy efficiency of the Zn-Fe RFB with its ability to withstand a large number of charge/discharge cycles and the low cost, makes this battery system suitable for energy storage applications.

References
[1] Alotto, M. Guarnieri, and F. Moro. Renewable and Sustainable Energy Reviews 29 (2014): 325-335.
[2] Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick and Q. Liu. Journal of Applied Electrochemistry (2011), 41(10), 1137.
[3] Kear, A.A Shah, and F.C. Walsh. International journal of energy research, (2012), 36(11), 1105-1120.
[4] Ding, H. Zhang, X. Li, T. Liu, and F. Xing. The Journal of Physical Chemistry Letters, (2013), 4(8), 1281-1294.
[5] C- Xie, Y. Duan, W. Xu, H. Zhang, and X Li. Angewandte Chemie International Edition, (2017) 56 (47), 14953-14957.
[6] Selverston, R. F. Savinell, and J. S. Wainright. Journal of The Electrochemical Society 164, (2017): A1069-A1075.