Zinc-ferricyanide system

Despite the fact that the zinc/ ferricyanide system employs an alkaline electrolyte, the electrode reactions are quite similar to those in zinc/halogen batteries and battery constructions are usually bipolar too. 

Zinc is electrodeposited from the sodium zincate electrolyte during charge. As in the zinc/bromine battery, two separate electrolytes loops ("posilyte" and "nega-lyte") are required. The only difference is the quality of the separator The zinc/ bromine system works with a microporous foil made from sintered polymer powder, but the zinc/ferricyanide battery needs a cation exchange membrane in order to obtain acceptable coulombic efficiencies. The occasional transfer of solid sodium ferrocya-nide from the negative to the positive tank, to correct for the slow transport of complex cyanide through the membrane, is proposed [54],... 

Besides the zinc-halogen systems, other zinc-based hybrid chemistries have been explored. One example is the alkaline zinc-ferricyanide redox flow cell that was demonstrated by Adams in 1979 [108]. The advantage of this zinc-ferricyanide redox flow cell includes high efficiency, high cell voltage, and low toxicity [109]. The electrode reactions are ... 

Coordinate initiation routes to principally amorphous polymer have been found that do offer certain advantages, although these routes have been slow to be adopted, probably due to the apparently complicated structures of the initiators. One of these processes employs zinc with a complex anion promoted with a low-molecular-weight polyether such as dioxane or diglyme (109). The initiator system is based on zinc cobalticyanide (hexacyanocobaltate), Zn3[Co(CN)6]2, or zinc ferricyanide (hexacyanoferrate), Zn3[Fe(CN)6]2, (110-113). 

II. The use of the zinc, ferrocyanide, ferricyanide redox system. Analy-... 

The use of zinc sulfate to catalyze the potassium ferricyanide oxidation procedure251 is worthy of comment. It is possible that other metals would also catalyze this oxidation, but their presence in the system would have a deleterious effect on thfe fluorescence of the final product, while Zn++ ions have relatively little effect. For instance, Cu++ ions would be expected to catalyze the oxidation stage, but they would also have a strong quenching effect on the fluorescence of the final products.144 Some of the Zn++ ions will also presumably be removed from the solution as insoluble zinc ferro-cyanide. Anton and Sayre have recently questioned the value of zinc sulfate as a catalyst at low pH.252... 

SDS micelles have also been used as a basis for light-induced charge separation processes. A hydrophobic, photooxidizable dye (e.g., a zinc porphyrinate) was, for example, dissolved in an anionic SDS micelle with copper(II) counterions. Upon excitation with visible light an electron was transferred first and very fast to the copper(II) coating, which then was reoxidized by anionic ferricyanide in the bulk water phase. The reduced ferrocyanide ion formed did not react with the oxidized porphyrin, because the anionic micelles and reductant repelled each other and the ferrocyanide was highly diluted by ferricyanide (Fig. 2.5.5). The energy of sunlight has thus initiated a simple vectorial reaction in a primitive membranous system. 

Other redox systems were also proposed in the past, such as the zinc/alkaline sodium ferricyanide [NajFeCCN) H2O] couple, and initial development work was performed. However, none of these efforts proved successful, mainly because of difficulties resulting from the efficacy and resistance of the ionic exchange membranes, until the development of the vanadium redox battery, by the University of New South Wales, Australia, in the late 1980s. Almost concurrently with this, development work started on VRBs at Sumitomo Electric Industries (SEI) of Osaka, Japan. Starting in the mid-1990s, VRB development has also been conducted at Mitsubishi Chemical s Kashima-Kita facility, although at a lower level of effort than at SEI. 

The system utilizes zinc and carbon as the negative and positive electrodes, respectively. On discharge, zinc is converted to a zincate ion ([Zn(OH)4] ) which is then converted and stored in the negative electrolyte tank as ZnO. Similarly, sodium ferro- and ferricyanides are stored as precipitates in the positive electrolyte tank. This unique design of solid storage in the electrolyte tanks results in a smaller footprint, but in a more complex system management and temperature control. 

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