The electrode polarization

The electrode polarization is resulted from the accumulation of charge on electrode surfaces and the formation of the electrical double layers. When the applied electric field is a dc field, the ions are distributed in a 

02 M KCl aqueous solution, the electrode polarization becomes more serious. The dielectric constant reaches as high as 10 at around 10 Hz frequency. For monodispersed anionic polystyrene latex particle of a mean 

K is the reciprocal of the Debye length, D is the diffusion coefficient of ions, 2d is the spacer thickness between two electrodes, and ca is the frequency. Reproduced with permission from A. D. Hollingsworth, and D.A. Saville, J. Colloid Intcrf. Sci., 272(2004)235. 

A strong electrode polarization was observed in low frequencies, and it became much stronger when the water droplet concentration increases from 1 vol% to 9.5 vol%. The electrode polarization disappeared at high frequency for all water-in-oil microemulsions of different water content. Since there is always such a strong electrode polarization in any system of electrolytes, dielectric measurement at low frequency becomes difficult. Correction methods for the electrode polarization in low frequency area have been developed [15-17]. Furtlier description on the electrode polarization will be addressed in tlie next section. 

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We and others have been involved in the study of such systems including Cu/Au(lll),85 86 Ag/Au(lll),87 Pb/Ag(lll),88 and Cu/Pt(lll).89 The first three systems involved the use of epitaxially deposited metal films on mica as electrodes.90 92 Such deposition gives rise to electrodes with well-defined single-crystalline structures. In the last case a bulk platinum single crystal was employed. Because of the single-crystalline nature of the electrodes, polarization dependence studies could be used to ascertain surface structure. 

During the process of charge-discharge of the electrode, polarization varies with time due to modification in the crystal size. 

Figure 3.5 [36], For the 02 reduction reaction on freshly prepared LSM electrodes, the initial polarization losses are very high and decrease significantly with the cathodic polarization/current passage (see Figure 3.5b). Consistent with the polarization potential, the impedance responses at open circuit decrease rapidly with the application of the cathodic current passage. For example, the initial electrode polarization resistance, RE, is 6.2 Qcm2 and after cathodic current treatment for 15 min RK is reduced to 0.7 Qcm2 see Figure 3.5 (a). The reduction in the electrode polarization resistance is substantial. The analysis of the impedance responses as a function of the cathodic current passage indicates that the effect of the cathodic polarization is primarily on the reduction in the low-frequency impedance [10]. Such activation effect of cathodic polarization/current on the electrochemical activity of the cathodes was also reported on LSM/YSZ composite electrodes [56-58], Nevertheless, the magnitude of the activation effect on the composite electrodes is relatively small.

Various strategies have been developed to improve the electrocatalytic activities of the LSM-based cathodes. Murray and Barnett [68, 69] showed that the addition of YSZ and gadolinia-doped ceria (GDC) phase to LSM significantly reduced the electrode polarization resistance. Figure 3.7 shows the electrode polarization resistance of the LSM/GDC tested in the air [69], The electrode polarization resistance... 

LSC/LSGM interface with different Sr content in LSC are shown in Figure 3.11 [97], The difference of the polarization current density comes from the difference of the activity at the LSC/LSGM interface. Thus, the higher Sr content in LSC promotes the 02 reduction and reduces the electrode polarization resistance. LSC/LSGM and LSC/Ce02-YSZ systems were also studied by others [98-101],... 

The electrochemical performance of La,.6Sr0 4Coo.2Feo.803 (LSCF6428) composition was characterized by Esquirol et al. [102], At 600°C, the conductivity of a porous LSCF coating with thickness of 10 pm was 52.8 and 29.2 Scm 1 when sintered at 1000 and 850°C, respectively. This conductivity is considerably lower than the conductivity values for dense LSCF (300 to 400 Scm-1)- The electrode polarization resistance of LSCF sintered at 850°C was 7.5, 0.23, and 0.03 ohm cm2 at 502, 650, and 801°C, respectively, lower than the electrode polarization resistance values at the same temperatures for the LSCF cathode sintered at 1000°C. The results show that the electrochemical activity of the LSCF electrode for the 02 reduction at... 

The electrocatalytic activity of MIEC cathodes also depends strongly on the properties of the electrolyte, as shown by Liu and Wu [109], The electrode polarization resistances, RE, or area specific resistance (ASR) measured by the electrochemical... 

Figure 29.5, depicts the diagram of an electron capture detector. The metal block of the detector housing itself serves as a cathode, whereas an electrode polarizing lead suitably positioned in the centre of the detector housing caters for a collector electrode (anode). The radioactive source from a beta-emitter is introduced from either sides of the detector housing below the electrode polarizing lead. 

From the potential energy curves shown in Fig. 7-7, it appears that the reduction in the activation energy, Ag] - Ag, caused by the electrode polarization is not equal to but smaller than the reaction afSnity, Zier, imposed by the polarization. Hence, we obtain Eqn. 7-27 ... 

Consequently, by measuring the polarization curves for the transfer reaction of anodic holes both at a photoexdted n-type electrode and at a dark p-type electrode of the same semiconductor, we obtain the relationship between the Fermi level of the electrode (polarization potential E) and the quasi-Fermi level of interfadal holes in the photoexcited n-type dectrode as a function of... 

The opposite conclusion was reported by Lin et al., who used a three-electrode configuration to study the electrode polarization of the MCMB anode and LiCo02 cathode under galvanostatic conditions. They found that in all cases the polarization at the MCMB anode surface far outweighs that at the cathode to such an extent that the potential profile... 

A1 electrode, the polarization of the Co top electrode dropped to 6% and the barrier height increased to 1.8 eV. Santos et al. rationalized the increase in the barrier height in the absence of the AI2O3 layer to the surface dipole layer that is known to exist at Al/Alqs interfaces [49]. One of the key findings of the Santos study was the observation of a positive TMR value both with and without the AI2O3 layer, as expected based on the known sign of the electrode polarization [17]. 

For the less reactive, longer n-alcanols the activation of the nickel hydroxide electrode is necessary (Table 3). For that purpose several layers of black nickel oxide hydroxide are deposited on the electrode surface from a buffered nickel sulfate solution by changing the electrode polarity every 5-10 sec n.isb,20,21)... 

The thermodynamically possible conversion efficiency, however, is only partly realized in a practical fuel cell. Two basic losses are encountered (I) the ohmic loss and (2) the electrode polarization, that is. Ihe deviation of... 

Figure 16 shows a comparison of the electrode polarization behavior with that obtained with an assumption of a constant concentration, and shows the influence of the transfer coefficient... 

The electrode polarization curve characteristics exhibited above are typical of those seen in many fuel cell electrodes. Thus a potential application of the ADM approximate solutions is determining the key electrochemical and mass transport parameters. 

Figure 5.9 UV-vis-spectroelectrochemical determination of the oxidative doping onset directly related to the HOMO level case of poly[(4,4"-dioctyl-2,2 5, 2"-terthiophene-3 -yl)ethyl acetate]. Note very profound spectral changes upon increasing the electrode polarization potential from 0.400 V to 0.425 V versus Ag/Ag-.149 (Reprinted with permission from K. Bugaet al., J. Mater. Chem. 2006,16, 2150-2164. Copyright the Royal Society of Chemistry.)...

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Bias-dependent measurements were performed in order to check to what extent the mechanism depends on the electrical operation conditions. Fig. 43 shows double-logarithmic plots of the electrode polarization resistance (determined from the arc in the impedance spectrum) versus the microelectrode diameter observed at a cathodic bias of —300 mV and at an anodic bias of +300 mV respectively. In the cathodic case the electrode polarization resistance again scales with the inverse of the electrode area, whereas in the anodic case it scales with the inverse of the microelectrode diameter. These findings are supported by I-V measurements on LSM microelectrodes with diameters ranging from 30-80 pm the differential resistance is proportional to the inverse microelectrode area in the cathodic regime and comes close to an inverse linear relationship with the three-phase boundary (3PB) length in the anodic regime [161]. 

Ionic conductivity has another important implication. The resin system acts like an electrolyte thus, all of the electrode polarization effects that can be observed in conventional electrolytes can also be observed in resins. The effect of electrode polarization is discussed in Section 3.2. 

Both alternating and direct current techniques can be used (see also impedance spectroscopy), but the electrode polarization effects should be minimized or taken into account in all cases. For this goal, a four-electrode method where the potential probes are placed between current probes, is often used. 

The - current efficiency methods are based on the analysis of amount of electrochemical reaction products during or after passing an electrical current through a material [ii—v]. Taking into account the electrode polarization, one can then determine the ion -> trans-... 

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