Coatings redox, anodic oxidation mediation

The standard potential for the Ru(II/III) redox transformation is 712 mV in aqueous perchlorate media, whereas the standard potential for the ferrocyanide/ferricyanide couple is 375 mV. Hence the driving force for the mediation is some 337 mV, which corresponds to an equilibrium constant of 5 X 10 at 298 K. Thus we see that equilibrium lies very much on the rhs. Typical RDE voltammograms for the oxidation of Fe(CN)e in 0.1 M HCIO4 at uncoated glass carbon and metallopolymer-coated glassy carbon electrodes are shown in Fig. 2.24. Note that the reduction of Fe(CN) is quite sluggish. This is to be expected due to the unfavorable thermodynamics. Two anodic oxidation waves are observed at the metallopolymer-coated electrode. The first occurs at a potential where Fe(CN)6 is oxidized at the bare electrode, so it corresponds to the direct unmediated oxidation of substrate at the inner electrode/polymer interface. The second wave is due to the mediated oxidation via the Ru(II) redox sites, as just discussed. This mediated wave exhibits linear Koutecky-Levich behavior. It is clear that we are dealing with Case C here, since the direct unmediated oxidation of substrate occurs at a less positive potential than the mediated oxidation via the Ru(III) sites in the film. 

Still, these coatings are far from ideal and are not yet sufficiently stable for technical application. Their reoxidation is strongly hindered, necessitating a relatively high overpotential of several hundreds of millivolts and their stability is insufficient, with chromium bleeding out as chromate. Yet the results obtained by the method that allows fixing with an easily applicable method a soluble redox mediator to an oxidic anode is encouraging. The an-... 

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