Solid oxide fuel cells other materials

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

The difficulties in the development of HTSO fuel cells are in the area of stability of materials rather than in catalysis. Different materials, some of them ionic conductors with no electronic conductivity and others electronic conductors with no ionic conductivity, must be compatible with each other chemically at a high temperature and mechanically during temperature cycling. Improvements in materials are steadily made, but the more sophisticated materials developed for this purpose tend to increase the cost. Once the materials problems have been overcome, the inherent simplicity of the design and operation of high temperature solid oxide fuel cells may make them the most useful... 

Figure 3.19. Electrolyte conductivity cr for Lag gSr , jGaj gMgo njCoo 085O3 (open circles) and other materials (the unit AV is sometimes referred to as a siemens), as a function of temperature. (From Ishihara et al. (2004). Novel fast oxide ion conductor and application for the electrolyte of solid oxide fuel cell. /. European Ceramic Soc. 24, 1329-1335. Used by permission from Elsevier.)...

Another type of electrical conductivity observed in ceramics is ionic conductivity, which often occurs appreciably at elevated temperature a widely used material exhibiting this behavior is zirconia doped with other oxides such as calcia (CaO) or yttria (Y2O3). For this material, atomic oxygen is the mobile ionic species. Doped zirconia finds widespread use as oxygen sensors, especially as part of automobile emission control systems, where the oxygen content of the exhaust gas is monitored to control the air-to-fuel ratio. Other applications of ionic conducting ceramics are as the electrolyte phases in solid-oxide fuel cells and in sodium-sulfur batteries. 

Since these first reports, Iwahara and other investigators have studied the conductivities (both ionic and electronic), conduction mechanism, deuterium isotope effect, and thermodynamic stability of these materials. The motivation for most of this work derives from the desire to utilize these materials for high temperature, hydrogen-fiieled solid oxide fuel cells. In a reverse operation mode, if metal or metal oxide electrodes are deposited onto a dense pellet of this material and heated to temperature T, the application of an electric potential to the electrodes will cause a hydrogen partial pressure difference across the pellet according to the Nemst equation ... 

Development of planar solid oxide fuel cells (SOFCs) and other electrochemical devices, such as oxygen generators and sensors, makes it necessary to elaborate sealant materials for hermetization in high-temperature zone. The sealants should satisfy numerous requirements, including chemical stability, good adhesion and thermal expansion. 

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