Electrolytes 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... 

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.

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 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. 

The data presented in Table 1 permit several conclusion to be drawn on the potential use of these materials as electrolytes in electrochemical devices with practical values for the area-specific resistance. Only the j0"-alumina and NASICON electrolytes possess sufficient conductivities for use as membranes with thicknesses of 1 mm or greater. The sodium ion conductivities of polycrystalline NASICON and /l"-alumina are comparable. The glassy electrolytes must be used in the form of thin films ( < 50 pm and possibly under 10-15 pm) or as capillary tubes with very thin walls (10-50 pm). 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

Fuel cell performance of the composite LSM-YSZ/YSZ/Ni-YSZ cell was investigated using forming gas (10 vol% H2 in N2) as the fuel (Figure 3-25). The results showed that a maximum power density of about 0.26 W cm2 as obtained at a temperature of 850°C. The temperature dependence of the area specific resistances of the asymmetrical cell is shown in Figure 3-26. The electrode overpotential was estimated to 0.3 Q cm2 at 800°C, which is the total of anode and cathode overpotential. It appeared that about half of the overpotential originated from the anode, because the cathode overpotential determined from the symmetrical cell test was found to be about 0.14 Q cm2 at 800°C. The performance of the cell was mainly limited by the electrolyte resistance. The decrease in the electrolyte thickness would decrease electrolyte resistance. It can be concluded that the net shape technology can be successfully applied for the fabrication of cathode and anode electrodes. 

Despite the number of reports on the structural and transport properties of many apatite electrolytes, their potential use in SOFC devices has not been widely studied (Brisse et al., 2006 Yoshioka et al., 2008 Bonhomme et al., 2009). In this sense,Tsipis et al. (2007) and Yaremchenko et al. (2009) reported the electrochemical behaviour of different cathode materials in contact with silicate apatite electrolytes and they found that silicon migration towards the surface layer blocks the electrochemical reaction zones increasing the electrode polarisation. In addition, high area-specific resistances were found (Brisse et al, 2006) with Ni0-La9SrSi< 026.5 cermets. However, a complete single fuel cell with apatite-type electrolyte, using different electrode materials, has not been reported before. Hence, further studies are still needed for a better characterisation of these potential electrolyte materials for SOFCs on several issues, such as the chemical reactivity and electrochemical performance between apatite electrolytes and different... 

The current state-of-the-art SOFC anode-supported cells based on doped zircona ceramic electrolytes, ceramic LSM cathodes, and Ni/YSZ cermet anodes are operated in the temperature range 700-800°C with a cell area specific resistance (ASR) of about 0.5 O/cm at 750°C. Using the more active ceramic lanthanum strontium cobalt ferrite (LSFC)-based cathodes, the ASR is decreased to about 0.25 Q/cm at this temperature, which is a more favorable value regarding overall stack power density and cost-effectiveness. 

Here a is the conductivity or the reciprocal of the resistivity, having a unit of ohm cm or more commonly S cm In electrochemical devices, the area-specific resistance (ASR, ohm cm ) of a flat sheet membrane electrolyte is of engineering importance and can be expressed as the product of the resistance and the surface area, or as the ratio between the thickness and the conductivity. The ASR is directly proportional to the voltage loss (V) of the electrolyte at the current density i (A cm ), because V = ASR x i. The expression of the thickness to conductivity ratio indicates that high conductivity (electrolyte thickness (L) lead to a low cell resistance. 

Li H., Nogami M. Ordered mesoporous phosphosilicate glass electrolyte film with low area specific resistivity. Chem. Commun. 2003 236-237... 

We have already discussed some materials that could be used as low-temperature SOFC electrolytes. Referring again to Figme 7.22, if we assume that the electrolyte should not contribute more than 0.15 Vcm to the total cell area specific resistance, then for a thickness (L) of 15tim, the associated specific ionic conductivity (a) of the electrolyte should exceed 10 Scm (since a = L/ASR = 0.0015/0.15). 

The electro-catalyst layers, which are porous, conduct both ions and electrons to facilitate oxidation and reduction reactions. For example, the electrocatalyst layers in most PEMFCs are 5-30 pm thick. The ion conductivity of these layers varies from 1 to 5 S/m and hence electro-catalyst layer area specific resistance values vary from 0.01 Q cm to 0.03 Q cm (or 10-300 mQ cm ). The Nafion electrolyte in PEMFC has a conductivity of 10 S/m when hydrated. Hence, for electrolyte thickness of 50-200 pm, the area specific resistance varies from 50 mil cm to 200 mil cm. Thus, it can be seen that the electro-catalyst layer contribution to ohmic resistance is significant. In Table 5.3, typical thickness and area-specific resistance values for selected fuel components are listed. 

In order to improve the performance of SOFC, a thirmer yttria-stabilized zirconia (YSZ) electrolyte is considered for lower ohmic resistance and for operation in the intermediate temperature range of 500°C-800°C. Ionic conductivity decreases with decrease in temperature and hence the area-specific resistance (ASR) of an electrolyte increases with lower operating temperature. Fabricating the electrolyte in a dense and thinner film reduces the ASR or the resistance to ionic transport, allowing a lower operating temperature. For this purpose, efforts are being made in fabricating SOFC cell on the basis of either a thicker anode-supported or a thicker cathode-supported SOFC... 

Figure 12.19 Maximum power and cell voltage (at OCV) as a function of the relative anode and cathode area specific resistances./= r /r jis the base cathode area specific resistance r = 0.21V fi cm ), and similarly for the anode (r = 0.525 f cm ). The solid and dotted lines show the power output and cell voltage, respectively. The black and gray lines represent anode and cathode, respectively. The calculation was for a 30-pm-thick Ceo9Gdo,0,95 x (CGOlO) electrolyte at 600 °C for a hydrogen to steam ratio of one.

The conduction properties of electrolytes are the most important factors in determining the operational temperature [11]. Here, the conversion efficiency is described in terms of conduction properties. The oxide ion conductivity determines the area-specific resistance contributed by the electrolyte. The contribution increases with increasing thickness of the electrolyte plate (film). 

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