为什么没有对称亚铁氰化物电解质的液流电池？

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 Fe 2+ /Fe 3+ redox couple (+0.22V and +0.5V respectively Vs saturated Ag/AgCl), with high stability under high pH conditions. Potassium ferrocyanide, wikipedia image 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.常见的例子是使用简单的 FeCl 2 或 FeSO 4 盐的 ZnBr 2 、ZnI 2 、钒和 Fe 系统。为什么会这样呢？ 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 Zn 2+ to Zn metal or the reduction of Fe 2+ to Fe metal, would not work because you would precipitate these solids.虽然上述原因让事情变得更加困难，但这是可以解决的。 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.然而这是一个坏主意。 0.1M 六氰基铁酸盐的循环（摘自本文）。您可以看到，在 100 个周期时，它的性能明显下降了很多。 The reason is that anolytes using ferrocyanide, especially when the potential will subject the ferrocyanide side to low potential values, cause ferrocyanide to decompose at the anode to form some Fe metal but also free cyanide.然后氰化物将游离，因为锌已经与焦磷酸盐络合，但亚铁氰化物库存很难恢复。我们通过对还原电位下的亚铁氰化物溶液的研究知道这种情况会发生（参见此处）。这里有一些明显的阴极峰，清楚地表明亚铁氰化物在还原电位低于约 -0.5V 时不稳定。当循环到负电势100次时，很明显它正在分解。任何游离铁在碱性 pH 值下都会迅速形成普鲁士蓝或氢氧化物，然后开始损坏电池。再加上我们正在产生游离氰化物，库存变得更加难以处理。 The above might make you think we can then go the other way and couple ferrocyanide with a higher redox potential couple instead, like these Fe phenanthroline complexes , use a ferricyanide anolyte instead of a ferrocyanide catholyte to create a flow battery in this way. The problem then is that we also have significant anodic current generated at high potentials that correspond to cyanide oxidation and ferrocyanide destruction as well.现实情况是，亚铁氰化物不太适合在对称系统中工作。 While it is very stable when working on its own in an asymmetric system, it is not well behaved when exposed to potentials either +1V or -1V away from its redox potential. This basically precludes the creation of a viable flow battery, as batteries with potentials below 1V are no