Anodic potential threshold

In the following cycles, the CV pattern does not show anymore the 4.15V anodic potential threshold, indicating a lowering of the anion intercalation overpotential. 

As observed in Table 2, some threshold potentials are recorded at very low anodic potentials indicating a conjugation between the porphyrins and the bifluorenyl cores. For example, the polymers derived from RuCO porphyrin, possess the lowest onset potential (measured at ca 0.1 V for poly(DMA)3(SBF) PRuCO) and those derived fiom the free-base porphyrins possess the more anodic potential limit of stability (measured at 1.55/1.6 V for poly(SBF)4PH2/poly(SBF) PH2). In the potential range of stability reported for each polymer, the p-doping process is reversible leading to the neutral polymer after reduction and the release of the anion from the polymer matrix. 

Figure 12. Dependence of the anodic peak current /peak on the potential scan rate v on a logarithmic scale obtained from the cyclic voltammograms for (a) Pt/polished AI2O3, (b) Pt/etched Ni, and (c) Pt/unpolished AI2O3 electrodes. The slope a means (d log /peak / d log v). Here, 14 and v0 means the upper and lower threshold scan rate, respectively. Reprinted from J. -Y. Go et al., A study on ionic diffusion towards self-affine fractal electrode by cyclic voltammetry and atomic force microscopy, J. Electroanal. Chem. 549, p. 49, Copyright 2003, with permission from Elsevier Science.

Well-established anode materials are Ni cermets such as Ni/YSZ composites. The presence of the second phase increases the contact area and prevents the catalytically active Ni particles from aggregating. The use of the composite becomes problematic if hydrocarbons are to be directly converted Ni catalyzes cracking, and the resulting carbon deposition deactivates the fuel cells. Therefore either pure H2 has to be used or the fuel has to be externally reformed. A third way is internal conversion of CHV with H20 to synthesis gas. The necessary steam addition, however, reduces the overall efficiency. Another problem of Ni cermets, if they are to be used at lower temperatures, is a potential oxidation of the Ni. Alternatives are Cu/Ce02 cermets in which Cu essentially provides the electronic conductivity and Ce02 the catalytic activity. Note that an efficient current collecting property of the electrode presupposes a metal concentration above the percolation threshold. 

The last example of this section deals with the optical function of hole-transport layers. In OLEDs some light is waveguided in the combined anode/organic layers due to total internal reflection [44], By increasing the thickness of the hole conductor the emitter can be optically decoupled from the contact. In a series of devices with increasing thickness of the hole conductor, deposited subsequently from solution, it was shown that a HTL thickness larger than 300 nm was necessary to accomplish this. As a very sensitive measure the threshold of amplified spontaneous emission (ASE) was used (Fig. 9.18, [45]). This is an important result with respect to a potential electrically driven organic laser diode. 

Several other factors, such as the temperature, the composition of the cement, the composition or surface roughness of the steel reinforcement, or the polarization with anodic or cathodic current may affect the chloride threshold. Some of these factors will be discussed later. Here it will be shown how the chloride threshold depends on properties of concrete and on environmental exposure (which, as discussed in Chapter 7, determines the potential of the steel). 

For passive reinforcement in non-carbonated and chloride-free concrete, current can flow only if there is a great enough increase in the potential of the anodic area to exceed the threshold for oxygen evolution (Figure 9.3). It was shown in Chapter 7 that at potentials below +600 mV SCE no iron dissolution or any other anodic process takes place, and thus it is impossible for the current to leave the metal. 

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