Anodic overpotentials

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

In order to relax 1 mol of compacted polymeric segments, the material has to be subjected to an anodic potential (E) higher than the oxidation potential (E0) of the conducting polymer (the starting oxidation potential of the nonstoichiometric compound in the absence of any conformational control). Since the relaxation-nucleation processes (Fig. 37) are faster the higher the anodic limit of a potential step from the same cathodic potential limit, we assume that the energy involved in this relaxation is proportional to the anodic overpotential (rj)... 

The anodic overpotential r controls both the rate and degree of oxidation, which means that the opening of the compacted structure is faster the greater the anodic potential, and oxidation is not completed until a steady state is attained at every anodic potential. This overpotential is also included in the constant a, with a subsequent influence on the two terms of the chronoamperometric equation. Both experimental and theoretical results in Fig. 43 show good agreement. 

Thus P is a structural parameter ranging between 0 and 1 that acts at the initial moments of the oxidation process of every segment the higher the degree of closure (v), the lower the probability (P) of any spontaneous conformational changes and the greater the anodic overpotential required to create a relaxation nucleus. Under these conditions both magnitudes are related by... 

Is the application of an anodic overpotential tj (= UWR - U R) equivalent to the application of a huge oxygen pressure P02,nernst computable from the Nemst equation ... 

Figure 5.32. Double layer capacitance as a function of overpotential of the system a) Pt/YSZ, b) Au/YSZ, c) Ni/YSZ and d) Au/YSZ before ( ) and after (O) prolonged anodic overpotential application.55 Reprinted with permission from the National Institute of Chemistry, Ljubljana, Slovenia.

Fig. 5.10 Relative band edge diagram for FeS2 and the energy position of some electron donor species. The thermodynamic reactions corresponding to corrosion processes at the anodic and cathodic sides are indicated as decomposition potentials due to holes, fip dec, and to electrons, n,dec> respectively. r]c and are the cathodic and anodic overpotentials, respectively, for the decomposition reaction of pyiite crystals in acid medium. (Reproduced from [159], Copyright 2009, with permission from Elsevier)...

Figure 7.6 Gerischer diagram for a redox reaction at an n-type semiconductor (a) at equilibrium the Fermi levels of the semiconductor and of the redox couple are equal (b) after application of an anodic overpotential.

Figure 8.7 Tunneling through the space-charge layer at equilibrium and for an anodic overpotential. Note that the band bending is stronger after the application of the overpotential. The arrows indicate electrons tunneling through the space-charge barrier.

FIGURE 2.14 (a) Influence of NiO particle size on the anode overpotential r (at a constant current density of 250 m A/cm2), anode ohmic resistance Rn, and anode interfacial resistance, RE, for cermets made from YSZ and three different types of NiO NiO-1, NiO-2, and NiO-3, of which the particle size is 1, 5, and 10 pm, respectively, (b) Influence of TZ3Y particle size on the anode overpotential rj (at a constant current density of 250 mA/cm2) for cermets made from 600°C-calcinated NiO-1 and six different TZ3Y powders with different particle sizes. (From Jiang, S.P. et al., Solid State Ionics, 132 1-14, 2000. Copyright by Elsevier, reproduced with permission.)... 

Jiang et al. [44] also found that the anode overpotential decreased as the YSZ particle size decreased in the range of -0.1 to 1.5 pm, as shown in Figure 2.14(b). 

For electrolyte-supported cells, many studies indicate that anode resistance decreases significantly as anodic current passes through the anode. For example, van Herle et al. [55] found that anode resistance decreased dramatically from 2.4 to 0.5 and to 0.1 fl when the cell current increased from 0 to 95 and then to 567 mA (Figure 2.20). Similarly, Primdahl and Mogensen [39] studied the effect of anode overpotential on the anode interfacial conductance and found that the anode interfacial resistance decreased significantly as the anode overpotential increased, which was also verified by Jiang and Badwal [43],... 

The influence of porosity on the electrochemical activity has not been studied much for electrolyte-supported cells because anode pastes for electrolyte-supported cells are made for screen printing, and thus contain significant amounts of organics, which almost guarantees sufficient porosity. In addition, since the anode thickness for electrolyte-supported cells is only on the order of 50 pm, the concentration polarization itself becomes much less of an issue. In fact, Jiang et al. [44] showed that anode overpotential for cermet anodes prepared with extra graphite pore formers... 

FIGURE 2.19 Anode overpotential versus current density in hydrogen with different concentration of H20. The plot at the bottom is the enlarged part for the polarization in region I. (From Jiang, S.P. and Badwal, S.P.S., J. Electrochem. Soc., 144 3777-3784, 1997. Reproduced by permission of ECS-The Electrochemical Society.)... 

FIGURE 2.21 Change of anode overpotential versus anode thickness for anodes made with and without 20 wt% graphite as pore former in electrolyte-supported cells. (From Jiang, S.P. et al., Solid State Ionics, 132 1-14, 2000. Copyright by Elsevier, reproduced with permission.)... 

The presence of a small amount of water vapor (up to pH20/pH2 = -0.03) in fuel reduces anode overpotential. For anode-supported cells, the use of pore formers is important to tailor the shrinkage during cofiring and to create adequate porosity for better performance. The difference in cell power output could differ by as much as 100% for cells as porosity changes from -30 to -50%. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

There is a high anode overpotential, which should be reduced by a better electrode-membrane assembling design. 

Regions characterized by large anodic overpotentials. Under such conditions, complete passivation and severe oxidation of most metal surfaces occurs. A breakdown of passive oxide layers and pitting corrosion is observed for transition-metal model systems. In this section are considered also the surfaces of electropositive metals such as aluminum. 

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