Electrode polarization resistance

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... 

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]. 

Figure 10.5 Comparison of electrode polarization resistance of Ni anode, Ni-GDC cermet anode, and GDC-impregnated Ni anode in 97%CH4- 3%H20 at 800°C. Reprinted from Journal of Power Sources,...

The electrode processes in solid-electrolyte systems consist always of a number of serial and/or parallel steps. The characteristic steps of the gas electrode reactions include transport in the gas phase to (or from) the gas/electrode or gas/electrolyte interface, adsorption (or desorption) at these surfaces, diffusion to (or from) the reaction zone, and transfer reactions [14-24]. As a rule, the electrochemical reaction is believed to occur in the vicinity (within a few microns) of triple-phase boundary (TPB), the junction of the gas, electronic or mixed ionic-electronic conductor (electrode), and ionic conductor (electrolyte) the TPB length is mostly determined by the electrode microstructure formed during the cell fabrication. Actual location of the electrochemicaUy active sites depends generally on the bulk and surface transport properties of the electrode and solid-electrolyte materials. When the current I is passed or drawn through the cell, the working electrode potential vve deviates from the equilibrium value E. This deviation is characterized by the quantity of overpotential r] = we (see Chap. 1). The electrode polarization resistance defined as... 

A fairly large number of studies have demonstrated high power outputs up to 1100 and 1329 mW cm" at 600The OCV varies unsystematically in the range 0.72-0.92 V. No correlation can be found between the OCV of the SOFC and the power output. The theoretical voltage at 600 °C for Hg humidified with 3% water is 1.11 V. None of the cells reached the theoretical OCV value due to the internal electronic short circuit. The huge scatter in OCV is consistent with the fact that it is a complex function of the thickness of the electrolyte, electrode polarization resistances, and electrical contact to the cell as illustrated in Fig. 12.19. Some of the studies listed in Tables 12.3 and 12.4 are discussed in more detail below. 

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