Area-specific resistance

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... 

Generally, the absolute magnitude of Q is not as important as the ratio leak rate to the total flow rate Qieaf/QtotJ. The leakage rate given by Equation (5.4) is the volume flow rate at the temperature and pressure of the leakage flow, and must be corrected to standard conditions for comparison with reactant feed rates. The total required flow rate of fuel or air to the stack is proportional to the stack current, which increases with the electrochemically active area and is inversely proportional to the cell area specific resistance (R"). 

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.

As a result of the transferred species, loss mechanisms occur. In terms of the first law of thermodynamics these losses are well known as polarisation losses. Polarisation losses are sensitively influenced by numerous mechanisms, which are strongly non-linear with respect to a change of the operational parameters like the current density, electrical potentials, temperature, pressure, gas compositions and material properties. These parameters are assumed to be constant in case of a differential cell area. Thus, the loss mechanisms are summarised in a constant area specific resistance ASR [ 2cm2]. A change of the local overpotential (EN(Uf) — Vceii) at constant ASR complies with a proportional change in the local current density. 

A fixed cell area of 1 cm2, a temperature of 800°C, a total pressure of 1 bar, an oxygen partial pressure of 0.21 bar and an area specific resistance of 1 2cm2 are chosen to compare the cell performances at uniform conditions. The total current of... 

With this choice of geometiy and conductivities the area specific resistance is ASRo 0.35 ohm cm2 at T = 900°C. The ASRo is quite sensitive to temperature, mainly due to the change of the electrolyte conductivity with temperature [3], Once the ASRo is known, the ohmic losses can be evaluated using Ohm s law ... 

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],... 

Figure 9 (a) Area-specific resistance of a button cell as a function of time for 1 100-hour test ... 

Table 1 Area specific resistances (ASR) measured on the curves presented in Figure 3 for the two stacks of three cells...

FIGURE 10.2 Area-specific resistance vs. electrolyte thickness for YSZ, CGO, and LSMG at 500 and 600°C. 

For an ionic conducting solid electrolyte to be seriously considered for use in a practical electrochemical device, which operates at a given temperature T, the maximum value for the area-specific resistance f as should be about 0.5 Q-cm. J as is the product of the electrolyte resistivity p at T in ohm-centimeters and the membrane thickness t (cm) in the direction of current flow. Table 2 lists maximum limits on electrolyte resistivity for various electrolyte membrane thicknesses. 

Table 1. Alternative Sodium Ion Conductors/Sodium Ion Resistivity Maximum Thickness For Area-Specific Resistance < 0.5 Q-cm (300°C)...

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