LSCF

A mathematical model was developed to predict the interfacial polarization resistance, Rp. According to the coarsening theory, the characteristic length L evolves with time t in the form of 

the time-dependent polarization resistance with the micro-structure could be written in the form by combining Equations (7.1), (7.2) and (7.7) as  

Novel nano-structured cathodes with high performance and stability are fabricated with SSC. For example, the polarization resistance at 700°C was reduced from 0.103 2cm for a blank porous LSCF electrode to 0.071 2cm with the impregnation of a thin LSCF layer, due to an 

Financial support from the Ministry of Science and Technology of China (2012CB215403) is greatly appreciated. 

Nanostructured (Ba,Sr)(Co,Fe)03 5 impregnated (La,Sr)Mn03 cathode for intermediate-temperature solid oxide fuel-cells. J. Electroch. Soc. 157 (2010), PP.B1033-B1039. 

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To circumvent problems associated with the link atoms different approaches have been developed in which localized orbitals are added to model the bond between the QM and MM regions. Warshel and Levitt [17] were the first to suggest the use of localized orbitals in QM/MM studies. In the local self-consistent field (LSCF) method the QM/MM frontier bond is described with a strictly localized orbital, also called a frozen orbital [43]. These frozen orbitals are parameterized by use of small model molecules and are kept constant in the SCF calculation. The frozen orbitals, and the localized orbital methods in general, must be parameterized for each quantum mechanical model (i.e. energy-calculation method and basis set) to achieve reliable treatment of the boundary [34]. This restriction is partly circumvented in the generalized hybrid orbital (GHO) method [44], In this method, which is an extension of the LSCF method, the boundary MM atom is described by four hybrid orbitals. The three hybrid orbitals that would be attached to other MM atoms are fixed. The remaining hybrid orbital, which represents the bond to a QM atom, participates in the SCF calculation of the QM part. In contrast with LSCF approach the added flexibility of the optimized hybrid orbital means that no specific parameterization of this orbital is needed for each new system. 

LaCo03 is rhombohedral from room temperature to 1000°C. For LSCF perovskite, the phase structure depends on the dopant level and temperature. In the La0 8Sr0 2CO, l ev03 system, the room-temperature phases are rhombohedral and orthorhombic for 0 < y < 0.7 and 0.8 < y < 1, respectively [79], For y = 0.8, i.e., I. a, xSrxCo0 2Fe0 sO3 the orthorhombic/rhombohedral transition at room temperature occurs at x = 0.3. When Sr content is increased to x > 0.6, the second phase was observed, while the main phases were rhombohedral for 0.6 < x < 0.8 and cubic for x > 0.8 [80],... 

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 addition of an ionic conductive phase, such as GDC, also promotes the elec-trocatalytic activity of an MIEC cathode. Hwang et al. [108] studied the electrochemical activity of LSCF6428/GDC composites for the 02 reduction and found that the activation energy decreased from 142 kJmol-1 for the pure LSCF electrode to 122 kJmol1 for the LSCF/GDC composite electrodes. Thus, the promotion effect of the GDC is most effective at low-operation temperatures (Figure 3.12). This is due to the high ionic conductivity of the GDC phase at reduced temperatures. 

FIGURE 3.12 Impedance curves of LSCF ad LSCF/GDC composite cathodes in air measured at different temperatures. (From Hwang, H.J. et al., J. Power Sources, 145 243-248, 2005. With permission.)... 

FIGURE 4.9 Electrochemical performance of the LSCF cells tested with various raw chro-mia-forming metal meshes. Performance of the LSCF cell tested with no mesh was also plotted for comparison [203],... 

Single-phase perovskite MIECs such as Sr-doped lanthanum cobaltite (LSC), lanthanum ferrite (LSF), lanthanum cobalt ferrite (LSCF) and samarium cobaltite (SSC), and Ca-doped lanthanum ferrite (LCF) [13] are sometimes used alone in SOFC cathodes, as depicted in the lower right-hand comer of Figure 6.1, but combining an... 

MIEC with an additional ionically conductive phase, such as GDC or SDC, typically extends the electrochemically active region still further due to the higher ionic conductivity of GDC and SDC compared to that of the perovskites. The optimal composition of a two-phase composite depends in part on the operation temperature, due to the larger dependence of ionic conductivity on temperature compared to electronic conductivity. A two-phase composite of LSCF-GDC therefore has an increasingly large optimal GDC content as the operating temperature is reduced [14], A minimum cathode Rp for temperatures above approximately 650°C has been found for 70-30 wt% LSCF-GDC composite cathodes, while at lower temperatures, a 50-50 wt% LSCF-SDC composite cathode was found to have a lower Rp [15]. 

Fu C, Sun K, Zhang N, Chen X, and Zhou D. Electrochemical characteristics of LSCF-SDC composite cathode for intermediate temperature SOFC. Electrochim. Acta 2007 52 4589-4594. 

The intermediate- and low-temperature materials exhibit both significant oxygen ion and electron conductivity. This means that the oxygen reduction can take place everywhere in the cathode and is not restricted to the TPBs as in the LSM-type cathodes. An option for compensating the relatively low oxygen ion conductivity of LSC and LSCF at operating temperatures around 600 °C is by mixing the material with (doped) ceria [79, 80],... 

All materials in the Lai- r ,Coi- Fe/)3-(5 (LSCF) family of materials have electronic transference numbers approaching unity. The electronic structure LSC and LSF has often been described in terms of partially delocalized O p—Co band states based on the tg and e levels of crystal-field theory. In... 

A notable exception may be Sr-doped ceria. As reported by Liu and Wu, LSCF electrodes on 10 mol % Sr-doped ceria exhibit a significant high-frequency impedance arc in air at 650—750 °C, which is comparable in frequency (but somewhat smaller) to that observed for identically processed and tested LSCF electrodes on 8 mol % YSZ. One explanation may be found in the thermodynamic data for the Sr— Ce—O system, which shows that the solubility of SrO in Ce02 is less than 10 mol % at these temperatures.Thus, precipitation of SrCeOs at the LSCF/ceria interface is favored, depending on the exact firing conditions and A/B ratio of the perovskite. Oddly, the authors did not consider this possibility. 

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