Electrochemistry of Heterogeneous Graphitic Surfaces

Using a simple redox couple. Fig. 3.5 depicts the voltammetry obtained when using either a Basal Plane Pyrolytic Graphite (BPPG) (i) or (ii) an EPPG electrode of HOPG, and the responses are compared with numerical simulations (iii) assuming linear diffusion only, in that, all parts of the electrode surface are uniformly (incorrectly) electrochemically active. Two features of Fig. 3.5 are to be 

If the heterogeneous rate constants on the two electrode surfaces are similar, two peaks will be observed arising at similar potentials which will merge into one larger peak. If (i.e. Kasai) peaks will be observed if the 

Kubs measurable activity however it has been shown that this is not the case and only the is active, or sometimes reported as anomalously faster over that 

The rate of electron transfer for basal plane sites has been reported to correspond to 10 cm s for the oxidation of ferrocyanide and is considered to be possibly even zero [4-6]. How does one know that this is actually correct As shown in Fig. 3.9a, a strangely distorted voltammogram would be observed in the limit of very low defect density [23]. Due to the fact that two peaks have never been observed experimentally, it is generally accepted that edge plane electron transfer kinetics are anomalously faster over that of basal plane the latter is sometimes referred to as being inert [5,6,23]. Interested readers are directed to the elegant work of Davies et al. and Ward et al. to further appreciate this work [5,24]. 

Further evidence on the role of edge plane sites versus basal plane sites has been reported [4] by the selective blocking of the basal plane sites of HOPG with a polymer whilst the edge plane sites were left exposed. Identical voltammetric behaviour was observed with this modified surface when compared to that of the initial bare electrode and with numerical simulations, confirming the edge planes to be the sites of electrochemical activity Fig. 3.10 depicts how this was achieved. 

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