Ferrocyanide exchange+ferricyanide

The simplest electrochemical reactions, which can be foxmd among the different kinds of electrode processes, are those where electrons are exchanged across the interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface (Bamford Compton, 1986), i.e. an ET (electron transfer) mechanism. The electrode acts as the source or sink of electrons for the redox reaction and is supposed to be inert. The reduction of ferricyanide to ferrocyanide (Angell Dickinson, 1972 Bamford Compton, 1986 Bruce et al., 1994 Iwasita et al., 1983) is an example of such a mechanism ... 

Fe(CN)5 /Fe(CN). Equimolal solutions of ferrocyanide and ferricyanide at pH 6.0 were found to impart Nemstian responses on platinum down to about 7x10 m in each valence. This corresponds to an exchange current of 8x10" amp cm". ... 

When the passive film present on a metal is sufficiently thin (< 2-3 nm), electrons exchanged between the metal and the electrolyte can cross the film by the tunneling. In this case, the reaction rate decreases as the film becomes thicker. On the other hand, the exchange rate does not depend, in principle, on the electronic properties of the film. Figure 6.29 shows polarization curves for the oxidation of ferrocyanide ions and for the reduction of ferricyanide ions measured on passive iron electrodes with different film thicknesses [31]. 

Zinc is electrodeposited from the sodium zincate electrolyte during charge. As in the zinc/bromine battery two separate electrolyte loops ( posilyte and negalyte ) 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 cationic exchange membrane in order to obtain acceptable coulombic efficiencies. The occasional transfer of soHd sodium ferrocyanide from the negative to the positive tank, to correct for the slow transport of complex cyanide through the membrane, is proposed [197]. 

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