If you have looked into flow batteries for any length of time, you will have found that the ferrocyanide/ferricyanide redox couple ( [Fe(CN)6] 4−, Fe(CN)6] 3−) is one of the most widely used in the field. This is because this redox couple has very high redox stability, great kinetics, significant solubility (0.7-1.2M depending on the exact salts used) and a redox potential that is lower to that of the Fe2+/Fe3+ redox couple (+0.22V and +0.5V respectively Vs saturated Ag/AgCl), with high stability under high pH conditions. However you might have also noticed that there are no published examples of flow battery systems where ferrocyanide salts are used in a common electrolyte, symmetric system. That is, a system where the battery starts with the same exact electrolyte on both the catholyte and anolyte and the redox reactions happen from this mixed state. Common examples are ZnBr2, ZnI2, Vanadium and Fe systems using simple FeCl2 or FeSO4 salts. Why is this the case? An initial reason is that ferrocyanide forms insoluble substances, Prussian blue or its analogues, with most heavy metal cations, so any battery that wants to do a metal reduction in the anolyte, such as the reduction of Zn2+ to Zn metal or the reduction of Fe2+ to Fe metal, would not work because you would precipitate these solids. While the above reason makes things more difficult, it is solvable. We have known from the late 1940s that solids of this type can be easily dissolved by using pyrophosphates (see here) and other strategies with strong chelating agents also work. This might tempt you to make a symmetric battery with something like potassium pyrophosphate, zinc chloride and ferrocyanide, where you reduce zinc pyrophosphate at the anode to zinc metal and oxidize ferrocyanide to ferricyanide at the cathode. However this is a bad idea. The reason is that anolytes using ferrocyanide, especially when the potential will subject the ferrocyanide side to low potential values, cause ferrocyanide to...