Area specific impedance

Fig. 13 Cycling performance and area specific impedance (ASl) of MCMB/LiNio.gCoo.isAlo.osOa cells with a 0.8 M LiBOB/silane or 1.0 M LiPFe/ECiDEC (1 1) electrolyte. Reproduced with permission [7]. Cop5uight 2006 Elsevier...

Modification of the positive material, by replacing some Ni with Co and A1 ions in LiNi COyAljOg (x >0.75) dramatically improves its behavior. Improvement of the positive electrode material stability by substitution with aluminum was confirmed by Chen et al. [40]. Area-specific impedance of cells was greatly reduced during one month storage at 50°C, 80% SOC. 

Figure 17. Area specific impedance of a) aged cell, b) aged positive elecbode, c) fresh cell, d) fresh positive eiectr, e) aged negative eiectrode and 0 flesh negative etecbode. Derived from...

Figure 53. Idealized half-cell response of a thin solid electrolyte cell, (a) Cell geometry including working electrodes A and B and reference electrode (s). (b) Equivalent circuit model for the cell in a, where the electrolyte and two electrodes have area-specific resistances and capacitances as indicated, (c) Total cell and half-cell impedance responses, calculated assuming the reference electrode remains equipotential with a planar surface located somewhere in the middle of the active region, halfway between the two working electrodes, as shown in a.

The impedance is dependent on temperature, as can be seen in Figure 4, which shows the area specific resistance (ASR) of a cell as a function of cell temperature for different gas flow rates. For the same cell temperatures, lower ASR was observed for increasing gas flow rates due to the increased gas diffusion near the electrodes that effectively reduced the overpotential resistances [4], Because the anode and cathode are often conductive, the impedance of the cell is dependent largely on the thickness of the electrolyte. Using an anode supported cell structure, a YSZ electrolyte can be used as thin as 10-20 pm or even 1-2 pm [32, 33] as compared to 0.5 mm for a typical electrolyte supported cell [26],... 

The area specific resistances for the composite LSM-YSZ symmetrical cell, which was obtained from impedance spectroscopic measurements, are shown in Figure 3-24. The and were attributed to electrode overpotentials (the charge transfer and the gas diffusion process), while the... 

The capacitance is also determined from the impedance technique. In many coiroding systems, an interfacial capacitance associated with the electrified double layer scales linearly with the tme electrochemical surface area. An elec-trochemically based estimate of the surface area may be obtained if the area-specific capacitance is known or determined from the slope of a plot of C versus surface area [42]. 

A first difficulty results from the fact that the impedance data reflect the overall state of the tested surface integrating the contributions of non-rubbed and rubbed surfaces. Such data must thus be de-convoluted in order to obtain the specific impedances of these two types of surface states. A first approach to this problem might be to use models similar to those describing the impedance of a sample undergoing a localized corrosion (Oltra Keddam, 1990). In that specific case, the overall impedance can be considered as the result of two impedances in parallel, namely the impedance of the non-rubbed surface and the one of the rubbed surface. A strict interpretation requires further an evaluation of the areas of these surfaces, e.g. by using profilometry and surface observations by light optical or scanning electron microscopy. 

If the WE and CE possess different polarization resistances and time constants defined as t = / p x C (where C is the area-specific capacitance of the electrode/ electrolyte interface), this extension may also introduce artifacts in the impedance data [25, 31, 35]. In other words, when shifting the CE to RE, the equipotential surface probed by the RE approaches the CE/electrolyte interface at co —> 0 and the RE may no longer isolate the potential drop of the WE. In addition to the apparent increase in the electrolyte resistance, this causes a partial introduction of the CE impedance into the measured WE impedance [26, 31]. 

The term has the units of area specific resistance, Qcm, and is referred to as the charge transfer resistance, denoted by and is given by R, = An important point to note here is that a linear relationship between anS the current density, i, in the low current density limit does not imply ohmic relationship, since the response time for the process is long, and is determined by whatever is the underlying physical process. In the simplest case, the charge transfer process is describable by a parallel R — C circuit, in which case the time constant is given as RC. Thus, in DC measurements, the capacitive part is not reflected. At the same time, in the current interruption experiment, the voltage drop across the interface is usually not separable from the other time-dependent parts of the impedance. Measurement of frequency response, however, allows one to estimate both R and C. More about this is discussed later. 

The pulse-echo and resonance impedance techniques are capable of determining which layer contains a disbond, given an appropriate standard to compare against. Inspection standards are bond assemblies made to simulate specific areas of a part with disbonds purposefully placed in them. By comparing the signal response of a suspected void to that of a known disbond in the standard, the... 

Potential accident scenarios and flood locations were identified from plant drawings and tlic RHR system fault tree that identifies the equipment and support needed for RHR system operation. The equipment location was correlated with flood areas with consideration for plant features which may impede or divert the flow. The flood scenarios identify the effect on systems required to prevent core damage. Quantification accounts for the rate of rise of the flood relative to the critical level in each specific plant area. The time available for any recovery action is calculated from tiic volume and the flow rate. 

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