Perovskite SOFC-related

Perovskites and Related Mixed Oxides for SOFC Applications... 

Perovskites and related mixed oxides have found a wide range of applications in catalysis, membrane separation, and solid oxide fuel cells (SOFCs). These applications involve the interactions of oxygen molecules with the oxide surface, as illustrated in Figure 38.1. 

Materials for Next Generation SOFCs relates two important strategies for improving fuel cell performance. The first section covers a nano-engineering solution, to produce thin layers to get surface (interfacial) ionic conductivity. The second section covers the use of new electrode materials based on Perovskite-related phases, Ruddleston-Popper phases, and double and triple perovskites. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

Perovskite-related oxides of the K2MF4 (A2BO4+J B-transition metal ion) structure (Figure 16) have been investigated by several groups as alternative mixed conductors for SOFC. 

The twin structure in small LSGMO ciystals tends to form chevronlike wall configurations that allows for a stress-lfee co-existence of four different orientation states. This pattern of domain walls is expected to be characteristic also for other perovskite-type compounds with a sequence of ferroelastic phase transitions related to those of LSGMO. Examples are mixed conductivity perovskites, which are used as electrodes and interconnectors in SOFC batteries. 

Perovskite related cathode materials have been shown to possess levels of ionic and electronic conductivity comparable to existing perovskite cathode materials. The development of these new cathodes is in the very early stages and significant research is required before these can compete with the established Lai.xSrxCoOs.g (LSC) and Lai.xSrxMnOg.g (LSM) based cathodes. However, one of the main advantages offered by these materials is the prospect of fast surface exchange and ionic diffusion at temperatures considerably lower than the established candidate materials thus enabling development of low temperature SOFC devices. 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

General While Ni/YSZ cermet is the most preferred anode material for SOFCs operating with hydrogen-containing fuels, there are several efforts to find alternative materials for the SOFC anode. Many studies on oxide anodes have been made for perovskite-related oxides. LaCr03 [230-237], LaFeOs [238], LaTiOs [239], and SrTiOs [240-243] systems are considered as possible alternative anode materials. 

A related study has been performed on perovskite-structured oxides for possible use as cheap hydrogen membranes and electrolytes for solid oxide fuel cells (SOFC). SOFCs have demonstrated excellent fuel efficiency and versatility, but the short operational lifetime of any fuel cell remains a major hindrance for commercial utilization [68]. Several factors contribute to... 

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