Double layers, directed metal oxidation

In an earlier discussion pertaining to activation polarization it was assumed that dissolving metal ions move directly into solution and there is a plentiful supply of ions to be deposited during cathodic reduction. The above assumption is often not correct especially for oxygen reduction because O2 takes time to diffuse in solution to the corroding interface, and metal ions take a definite time to cross the double layer, hence, in oxidation and reduction reaction there exists a metal ion concentration across the double layer. In a polyelectrode system, two separate electrode processes occur ... 

In this method the creation of defects is achieved by the application of ultrashort (10 ns) voltage pulses to the tip of an electrochemical STM arrangement. The electrochemical cell composed of the tip and the sample within a nanometer distance is small enough that the double layers may be polarized within nanoseconds. On applying positive pulses to the tip, the electrochemical oxidation reaction of the surface is driven far from equilibrium. This leads to local confinement of the reactions and to the formation of nanostructures. For every pufse applied, just one hole is created directly under the tip. This overcomes the restrictions of conventional electrochemistry (without the ultrashort pulses), where the formation of nanostructures is not possible. The holes generated in this way can then be filled with a metal such as Cu by... 

The direct observation of waves on electrode surfaces with video cameras is often possible in metal dissolution reactions where, in general, different double-layer potentials are coimected with distinct thicknesses of salt or oxide layers. The latter are so pronounced that they possess visible contrast in the reflectivity. Obviously, this direct imaging is restricted to reactions that are accompanied by drastic changes of the electrode morphology. Hence, the reactions that are mechanistically the most difficult ones to understand are the easiest ones to study from an experimental point of view. 

The TER-XSW method opens the possibility of using XSW to profile nanoscale metal structures and ion distributions above solid surfaces and at fluid-solid interfaces. Applications of TER-XSW have included direct observation of the diffuse electrical double layer at the charged membrane and electrolyte interfaces (Bedzyk et al. 1990 Wang et al. 2001), structural characterization of self-assembled organic monolayers (Lin et al. 1997), and determining metal ion partitioning at oxide-biofilm interfaces (Templeton et al. 2001). These applications are discussed later in this chapter. 

Features in Figure 3.16 that correspond to Pt oxide formation and reduction are labeled and explained in the pictorial legend. It should be noted for the illustrated CV that the surface processes do not involve any supplied reactants. They correspond to the transformation of interfacial water molecules into surface species and vice versa. A CV shows the amount of electronic charge withdrawn from the metal surface in anodic scan direction (oxidative current, j > 0) or transported to it in cathodic scan direction (reductive current, < 0). The electrical charge flux generated or consumed at the interface is the result of double layer charging and faradaic processes. An interesting aspect of cyclic voltammetry is that the surface is never in a steady state. The potential is continuously ramped up and down, at a constant scan rate, Vs, and between precisely controlled upper and lower potential bounds. 

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