Perovskite-type materials SOFCs cathodes

Perovskite-type materials have also been investigated as cathodes for SOFCs. Lanthanide-based perovskites showed a high conductivity and a high catalytic activity for oxygen reduction. Applying a thin porous layer of YSZ particles on LSM electrodes also increased the performance as the polarization resistance is reduced. Especially for operating at lower temperatures (650-700 °C), it is important to have an efficient cathode [36]. 

In order to examine the mechanism of the oxygen reduction in Lai xSrx. Coi yFey03 5 (LSCF) in this paper, complex studies over the stracture of the ionic and electronic defects, resulting from the deviation from stoichiometry or doping with different ions, were carried as a function of temperature and oxygen partial pressure. The results allowed for designing functional properties of perovskite-type oxides, perspective cathode materials for electrochemically effective IT-SOFC. 

An example of the use of nanoionic materials as the cathode in a SOFC is that of nanotubes of Lao.6Sro.4Co03 and Lao.6Sro.4Coo.2Feo.8O3 [136]. These perovskite-type mixed oxides (see Chapters 9 and 12) are widely used as cathode materials, and the nanotubes were prepared by denitration, microwave irradiation and calcination at 800 °C. The shape and size of the nanotubes was controlled by the porous template used, although their application in a fuel cell was not reported. 

The flexibility of the perovskite crystal structure and the opportunity to accommodate various dopants offer the possibihty to tailor material properties. The design of catalytic or electronic properties, such as ionic or electronic conductivity, is in the focus of solid oxide fuel cell (SOFC) research activities. Perovskite-type oxides are therefore a well-investigated class of materials and commonly applied as functional layers in SOFCs, as porous microstructured cathode layer on the air electrode side [32-36] and very recently as anode on the fuel side [35]. On a research level, perovskite-type oxides are also apphed as gas-tight electrolyte to separate anode and cathode compartments [37,38] or as an interconnector material [39,40]. Beside the stoichiometry and crystal structure, processing... 

Similarly to flame-made titanates, the precise control of the stoichiometry and material purity has an influence on the electronic properties of perovskite-type oxides such as LSC, LSCF, and BSCF. While conductivity is comparable with the highest reported in the literature (Figure 4.5), other unique properties are documented for these flame-made compositions. LSC, LSCF, and BSCF from a FSS process feature a pronounced shift of the temperature, at which the maximum conductivity is observed. The FSS process resulted in materials with an exceptional electronic conductivity, which may better match to a SOFC operated at intermediate or low temperatures. For example, LSCF cathodes based on flame-made nanopowders have shown polarization resistances in the range of 0.7 Q. cm at 592 °C [59], which are among the lowest reported for thick film layers of this material stoichiometry. Similar conclusions were drawn with respect to LSC-based cathodes, for which very low overpotentials were documented [60]. 

A large group of chemical compositions has been investigated as potential candidates for IT SOFC cathode materials. A recent article by Skinner has provided an overview of the progress of perovskite type oxides for the SOFC cathode, with an emphasis on the role of chemical compositions [41]. On the contrary, microstructure plays a major role in the cathode function as well. This is particularly true when the composite cathode, which shows a better performance compared to a single composition cathode, is used. Several authors have shown that electrode microstructure and transport properties have a profound effect on polarization. Tanner et al. [42] have shown that polarization resistance (Rp) depends upon the grain size, d, of the ionic conductor in the composite electrode and the volume fraction porosity, which was further derived as in (1) by considering the monolayer gas adsorption. A similar relation has been proposed as [43] ... 

Perovskite-type Sr-doped LaMnOs (Lanthanum Strontimn Manganites- LSM) has particularly attracted substantial interest as a promising material for cathode in SOFCs. This material has good properties such as chemical and thermal stability, and high catalytic activity for oxygen reduction. Additionally, it has a thermal expansion coefficient similar to that of a solid electrolyte (YSZ), and high electronic conductivity [2]. 

Perovskite-type oxides based on Mn, Co, Fe, or K2NiF4-type oxide with Ni are studied as cathode materials for SOFCs. For high-temperature SOFCs, LaMnOs-based materials are mainly used because of the high compatibility... 

In addition to noble metals, conducting perovskite family oxides are the preferred cathode materials. Lanthanum manganite (LaMnOs), which, when substituted with low valence elements, such as Ca or Sr, has a superior p-type electronic conduction due to the formation of large amount of Mn" ". Moreover, doped LaMnOa possesses adequate electrocatalytic activity for oxygen reduction, a reasonable thermal expansion match to YSZ, and stability in the SOFC cathode operating environment. Under the cathode operation conditions, doped LaMnOa has oxygen excess, or precisely speaking, has cation vacancies, and the oxide ion conductivity is relatively low in Sr doped LaMnOa (LSM), of the order of 10 S/ cm at 800 °C. 

Hence, by the combination of structural and transport properties, LSiF-LSNF nanoeomposite appears to be promising as cathode material for SOFC with apatite-type eleetrolyte provided the sintering temperature is below 1200 °C. The most promising approach consists in application of this composite as thin functional interlayer between the eleetrolyte and thin porous perovskite layer comprised of the same LSNF or other complex perovskite. This will help to mateh the thermal expansion coefficient of cathode and eleetrolyte as well as prevent too strong interaction between perovskite and electrolyte if sintering temperature will be high. 

Cathode materials for SOFCs based on any of the electrolytes described in Section 2.1.1 are of the perovskite structure type, generally La-based with transition metals located on the B site. Several authors " have summarised the range of perovskites investigated to date, concentrating on the conductivity, ion transport and compatibility of these materials. As such it is superfluous to continue the discussion in detail here. Instead we will refer to the main cathode types only, leaving the reader to consult the relevant literature for further details. 

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