Redox flow batteries (RFBs) systems, as one of the most promising electrical energy storage systems, provide an alternative solution to the problems of balancing power generation and consumption. These batteries are not only able to provide energy where and when it is required but also reduce the high capital cost of managing peak demands on the grid as well as large investments in grid infrastructures. Cost, reliability and safety are the most important factors to be considered to develop these systems. Because of good safety characteristics and high power densities (e.g., VRBs), aqueous systems have attracted widespread interest. The most widely studied redox species, vanadium, is expensive and toxic in its oxidized form. Therefore, organic or organometallic redox species could be used since they can offer more flexibility to tune the redox activity, redox potential, solubility, and stability.

Here, we will introduce a new low-cost aqueous redox flow battery system based on organometallic redox species. The theoretical energy density of this system can reach 90 Wh/L. Fig. 1(a) and (b) show the typical charge-discharge curves of this system (before and after electrolytes optimization) and cell efficiency performances over cycling, respectively (after optimization). It can be seen the open circuit voltage (OCV) increases from 1.52 V to 1.83 V and the system shows a very stable cell performance. The underlying mechanisms will be discussed.