Oxygen-ion conductors

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). 

CALCIA-STABILIZED ZIRCONIA AND RELATED FAST OXYGEN ION CONDUCTORS... 

Among the oxygen ion conductors, CaO or Y2C>3 stabilized Zr02 (CSZ and YSZ) [67, 68], and Y2C>3 or La203 stabilized Th02 [69] are frequently used. CSZ and YSZ are limited to oxygen partial pressures in the range from 10-13 to 1010 Pa at 1273 K [68], Lower partial pressures are allowed with the thoria-based... 

The maximum conductivity of fluorite-structured oxygen ion conductors is not only a function of dopant concentration, but also of dopant radius [9], For stabilized zirconia, the conductivity increases as the radius becomes close to that of Zr4 (rvm = 0.084 nm). The best value is reached for Sc3+ (rvlll = 0.087 nm) with a conductivity of 0.1 Scnr1 at 800°C for (ScjC o. ZrO o. ). For the sake of comparison,... 

Pure stoichiometric ceria is not a good oxygen ion conductor. The oxygen ion conductivity can be introduced dramatically by low valance doping. Thus the conductivity depends on the characteristics of the dopant elements and their concentrations. 

Kharton W, Yaremchenko AA, and Naumovich EN. Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. B. Perovskite-related oxides. J. Sol. St. Electrochem. 1999 3 303-326. 

Conductivity maximizes at relatively low concentrations of dopant, but not at the 50% one might expect this is because there is an elastic interaction between the substituted ion and the vacancy created. The best conductivity seems to be achieved when the crystal lattice is distorted as little as possible (i.e., when the dopant ion is similar in size to the cation it is replacing). Consequently, two of the best oxygen-ion conductors are zirconia... 

Much work with solid electrolyte electrochemical cells has involved the use of oxygen-ion conductors and it is therefore the use of these conductors which will be concentrated upon in this review article. 

Stability issues The many stability issues discussed for ceramic oxygen-ion conductors apply also to proton conductors. Reactions with acidic gas components... 

In the case of the high temperature SOFC discussed below the principles outlined above equally apply. The technical differences are that the cell runs typically on hydrocarbon fuels (e.g. natural or coal-gas) and that the electrolyte is an oxygen ion conductor rather than a proton conductor. The complex fuel molecules, in the presence of the water molecule and at the high operating... 

The fast-ion ceramic conductors of interest here are cubic stabilized zirconia (CSZ), an oxygen ion conductor, and a sodium aluminate (/f -alumina), a sodium ion conductor. Both are discussed in detail below. The overview by T.A. Ramanarayanan et al. [8] covering CSZ and its applications is recommended. 

Sensor. The control of the exhaust composition was essential to maintain the air-to-fuel ratio close to stoichiometric for simultaneous conversion of all three pollutants. This control came about with the invention of the 02 sensor.21,22 The sensor head of this device was installed in the exhaust immediately at the inlet to the catalyst and was able to measure the 02 content instantly and precisely. It generates a voltage consistent with the Nemst equation in which the partial pressure of 02 (P02)exhaust in the exhaust develops a voltage (E) relative to a reference. The exhaust electrode was Pt deposited on a solid oxygen ion conductor of yttrium-stabilized zirconia (Zr02). The reference electrode, also Pt, was deposited on the opposite side of the electrolyte but was physically mounted outside the exhaust and sensed the partial pressure (P02)ref in the atmosphere. E0 is the standard state or thermodynamic voltage. R is the universal gas constant, T the absolute temperature, n the number of electrons transferred in the process, and F the Faraday constant. 

R. I. Merino, N. Nicoloso, J. Maier, in Ceramic Oxygen Ion Conductors and Their Technological Applications, Ed. B. C. H. Steele, British Ceram. Proc., The Institute of Materials, Cambridge, 1996, pp. 43 K. Sasaki and J. Maier, Solid State Ionics 134 (2000) 303. 

It is clear from Figure 11 that, at present, proton conductors do not parallel the best oxygen ion conductors (i.e. a > 1 Scm ). 

The electrolysis reactions to produce hydrogen using oxygen ion conductors are ... 

C. The cell exhibited a steady current density of 700 mA/cm for 350 h. Ishihara et al. has also reported that doped PrGaOj is a fast oxygen ion conductor, but it does not seem to hold any advantage over LaGa03. 

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