Voltammetry at Heterogeneous Surfaces

Carbon comes in a variety of allotropes which exhibit a wide range of different properties graphite is one of the most common of these forms. A recent review by R.E. McCreery [Chem. Rev. 108 (2008) 2646] provides a good overview on a number of carbon-based materials and their use in electrochemistry. 

Electronically, graphite maybe described as a semi-metal. This is as a result of its substantially lower density of states, as compared to that of a metal. Graphite does not exhibit a band gap as there is overlap between the conduction and valence bands, and hence there is a non-zero density of states at the Fermi level. 

As mentioned above, the electronic structure of graphite is anisotropic, and this anisotropy is reflected in the differing electrical resistance of the material perpendicular and parallel to the carbon layers. Further, as a result of this 

Since their rediscovery in 1991 by S. lijima [Nature 354 (1991) 56], carbon nanotubes have been the focus of a huge amount of research. 

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The experimental assessment of the ET theory [1] has been the central theme in the SECM study of tip reactions. Kinetic parameters for a heterogeneous ET reaction at the UME tip are determined by tip voltammetry. The tip reaction of a redox mediator that is initially present in the bulk solution (i.e., the oxidized form of a redox couple, O, in Figure 6.2) is monitored as tip current at various tip potentials to obtain a steady-state voltammogram. In tip voltammetry based on the total positive feedback current (Figure 6.2a), the tip-generated species, R, is electrolyzed at the surface of an electroactive substrate so that the original mediator is regenerated at a diffusion-limited rate. When the tip is positioned within a tip diameter away from the substrate, the redox molecules efficiently diffuse between the tip-substrate gap to enhance the tip current in comparison to its... 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

From the voltammograms of Fig. 5.12, the evolution of the response from a reversible behavior for values of K hme > 10 to a totally irreversible one (for Kplane < 0.05) can be observed. The limits of the different reversibility zones of the charge transfer process depend on the electrochemical technique considered. For Normal or Single Pulse Voltammetry, this question was analyzed in Sect. 3.2.1.4, and the relation between the heterogeneous rate constant and the mass transport coefficient, m°, defined as the ratio between the surface flux and the difference of bulk and surface concentrations evaluated at the formal potential of the charge transfer process was considered [36, 37]. The expression of m° depends on the electrochemical technique considered (see for example Sect. 1.8.4). For CV or SCV it takes the form... 

Fleischmann et al. [549] studied the electro-oxidation of a series of amines and alcohols at Cu, Co, and Ag anodes in conjunction with the previously described work for Ni anodes in base. In cyclic voltammetry experiments, conducted at low to moderate sweep rates, organic oxidation waves were observed superimposed on the peaks associated with the surface transitions, Ni(II) - Ni(III), Co(II) -> Co(III), Ag(I) - Ag(II), and Cu(II) - Cu(III). These observations are in accord with an electrogenerated higher oxide species chemically oxidizing the organic compound in a manner similar to eqns. (112) (114). For alcohol oxidation, the rate constants decreased in the order kCn > km > kAg > kCo. Fleischmann et al. [549] observed that the rate of anodic oxidations increases across the first row of the transition metals series. These authors observed that the products of their electrolysis experiments were essentially identical to those obtained in heterogeneous reactions with the corresponding bulk oxides. 

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