Large energy storage are achieving a high attention due to the wide application of the renewable energy such as solar, wind and hydroelectric power. Among all the technologies, a growing interest in flow battery has been observed due to their features of high efficiency, high safety, long cycle life as well as separation of the energy and power density. Although much work has focused on the development of FB technologies, low energy densities, high cost, and poor reliability have hindered their further commercialization. Therefore, it is vitally important to find a new flow battery system with low cost, high energy density, and excellent electrochemical kinetics. Zinc and iron are two common elements that have the abundant reserve in the earth crust with the relative high electrochemical activity. Furthermore, due to the high solubility of zinc and iron salts, the battery has the potential to achieve a high energy density. Most importantly, compared with some redox couples such as Br/Br- and Pb/Pb2+, zinc and iron salts are environment-friendly.
In this study, we use the FeCl2 and ZnBr2 as the redox reactants to establish neutral zinc-iron flow battery system. Using the glycine as the complex agent to overcome the intrinsic problem of the hydrolysis of Fe2+/Fe3+, the electrolyte stability can be greatly improved. Moreover, the complex effect of glycine enlarges the volume of the redox moiety which will alleviate the crossover problem. Furthermore, the application of the low-cost PBI porous membrane to replace the expensive ion exchange membrane ensure the battery with high ion conductivity. Besides, the ion exchange membrane suffers from the membrane fouling and may result in a high omhic resistance which can be confirmed by the area resistance test. The battery test indicate neutral zinc iron flow battery can be operated at 40 mA/cm2 with a high energy efficiency of 86%, even at 80 mA/cm2, an efficiency of nearly 78% can be obtained for more than 100 cycles. Above all, an energy density about 50 Wh/L can be achieved with the electrolyte of 2 M with the cost of less than 150 $/kW h. Based this study, we offer a good strategy to applies the FeCl2 electrolyte and provide a reference to develop low-cost battery system.