Why are there no flow batteries with symmetric ferrocyanide electrolytes?

If you person looked into travel batteries for immoderate magnitude of time, you will person recovered that the ferrocyanide/ferricyanide redox mates ( [Fe(CN)6] 4−, Fe(CN)6] 3−) is 1 of the astir wide utilized successful the field. This is because this redox mates has very precocious redox stability, awesome kinetics, important solubility (0.7-1.2M depending connected the nonstop salts used) and a redox imaginable that is little to that of the Fe2+/Fe3+ redox mates (+0.22V and +0.5V respectively Vs saturated Ag/AgCl), pinch precocious stableness nether precocious pH conditions.

Potassium ferrocyanide, wikipedia image

However you mightiness person besides noticed that location are nary published examples of travel artillery systems wherever ferrocyanide salts are utilized successful a communal electrolyte, symmetric system. That is, a strategy wherever the artillery starts pinch the aforesaid nonstop electrolyte connected some the catholyte and anolyte and the redox reactions hap from this mixed state. Common examples are ZnBr2, ZnI2, Vanadium and Fe systems utilizing elemental FeCl2 aliases FeSO4 salts.

Why is this the case? An first logic is that ferrocyanide forms insoluble substances, Prussian bluish aliases its analogues, pinch astir dense metallic cations, truthful immoderate artillery that wants to do a metallic simplification successful the anolyte, specified arsenic the simplification of Zn2+ to Zn metallic aliases the simplification of Fe2+ to Fe metal, would not activity because you would precipitate these solids.

While the supra logic makes things much difficult, it is solvable. We person known from the precocious 1940s that solids of this type tin beryllium easy dissolved by utilizing pyrophosphates (see here) and different strategies pinch beardown chelating agents besides work. This mightiness tempt you to make a symmetric artillery pinch thing for illustration potassium pyrophosphate, zinc chloride and ferrocyanide, wherever you trim zinc pyrophosphate astatine the anode to zinc metallic and oxidize ferrocyanide to ferricyanide astatine the cathode. However this is simply a bad idea.

Cycling of 0.1M Hexacyanoferrate (taken from this paper). You tin spot that astatine 100 cycles it has intelligibly degraded a lot.

The logic is that anolytes utilizing ferrocyanide, particularly erstwhile the imaginable will taxable the ferrocyanide broadside to debased imaginable values, origin ferrocyanide to decompose astatine the anode to shape immoderate Fe metallic but besides free cyanide. The cyanide will past beryllium free arsenic Zn is already complexed by pyrophosphate but the Ferrocyanide inventory is difficult to recover. We cognize this happens from studies of ferrocyanide solutions nether reducing potentials (see here). There are immoderate evident cathodic peaks present that intelligibly show ferrocyanide is NOT unchangeable astatine reducing potentials beneath astir -0.5V. When cycled to antagonistic potentials for 100 times, it is clear that it is decomposing. Any free Fe will quickly shape Prussian bluish aliases hydroxides astatine basal pH which will past commencement destroying the battery. Add to that the truth that we are generating free cyanide and the inventory becomes overmuch harder to woody with.

The supra mightiness make you deliberation we tin past spell the different measurement and mates ferrocyanide pinch a higher redox imaginable mates instead, for illustration these Fe phenanthroline complexes, usage a ferricyanide anolyte alternatively of a ferrocyanide catholyte to create a travel artillery successful this way. The problem past is that we besides person important anodic existent generated astatine precocious potentials that correspond to cyanide oxidation and ferrocyanide demolition arsenic well.

The reality is, ferrocyanide is NOT good suited to activity successful a symmetric system. While it is very unchangeable erstwhile moving connected its ain successful an asymmetric system, it is not good behaved erstwhile exposed to potentials either +1V aliases -1V distant from its redox potential. This fundamentally precludes the creation of a viable travel battery, arsenic batteries pinch potentials beneath 1V are not going to beryllium economically viable, particularly fixed the further costs progressive successful creating a symmetric strategy utilizing ferrocyanide.

When utilizing ferrocyanide, besides see that while this brackish is comparatively safe successful its unaltered state, subjecting it to electrochemical maltreatment WILL make free cyanide and it’s apt to airs a important threat to you and others. For this reason, I would urge to enactment distant from testing ferrocyanides successful symmetric systems entirely, unless you are a trained master and professionally good equipped to grip some the imaginable operational hazards and wastes generated from its decomposition products.