High-temperature solid electrolyte applications

Instead modem interest in mixed conduction theory is eaqpected to derive from high temperature solid electrolyte applications. 

Rare-earth elements are vital constituents of several prominent high-temperature solid electrolytes ranging from oxygen- or fluoride-ion conductors in the fluorite structures to protonic conductors in the doped perovskite phases and trivalent-ion conduction in Sc2(W04)3 and 3-alumina-type compounds. Solid electrolytes are considered as important for scientific studies and technological applications in vital areas such as fuel cells, batteries, sensors, process control and environmental protection. 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Another type of high temperature solid state O2 sensor that has been developed is based on the principle of electrochemical pumping of oxygen with Zr02 electrolytes. These sensors have higher sensitivity (generally, a first power dependence on Pq) than the Nernst cell and the resistive device and possess a number of other characteristics that make them very promising for many new applications. 

Ishihara, T., Sammes, N. M., Yamamoto, O., Electrolytes, in S., Singhal, C. Kendall, K. (2003). (eds). High temperature solid oxide fuel cell fundamentals, design and applications. Elsevier. Oxford., 2003, 96-97. 

Ishiahara, T., Sammes, N.M. and Yamamoto, O. (2003), Electrolytes, in High Temperature Solid Oxide Fuel Cells Fundamentals, design and Applications, Eds. S.C. Singhal and K. Kendall, Elsevier, UK, pp. 83-117. 

Alumina is one of the best electrical insulators, hence its dielectric applications. Electric conduction depends primarily on impurities which act as acceptors (for example, Mg, Fe, Co, V or Ni) or as donors (for example, H, Ti, Si, Zr or Y), the balance between electronic and ionic contributions depending on the temperature and the partial pressure of oxygen [KRO 84]. Among the impurities, sodium deserves a particular mention because the synthesis of alumina by the Bayer process brings into play sodic mediums sodium aluminate (Na2011Al203), also called beta alumina, is an ionic conductor with very high conductivity, which is why this compound is envisaged for solid electrolyte applications [WES 90]. 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

A sodium-sulfur cell is one of the more startling batteries (Fig. 12.23). It has liquid reactants (sodium and sulfur) and a solid electrolyte (a porous aluminum oxide ceramic) it must operate at a temperature of about 320°C and it is highly dangerous in case of breakage. Because sodium has a low density, these cells have a very high specific energy. Their most common application is to power electric... 

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