Oxygen anion-conducting electrolytes

Dense membranes have been used to feed hydrogen for hydrogenation reactions.Improved yields have been observed, due not to the above kinetics reasons, but attributed to the better availability of the active available on the membrane surface. Most dense membrane use has been to feed oxygen. Early studies considered silver membranes, but cost and low permeation rates did not favor these. More recent work has used solid oxide electrolytes as membranes. Initially, investigators used yittria- and calcia-stabilized zirconias (YSZ or CSZ), which had reasonable oxygen anion conductivity. Their low electron conductivity dictated the use of an external circuit, as shown in Figure 4(a). 

SOFCs, SOECs, and SOERs all consist of three parts an electrolyte, an anode, and a cathode. The electrolyte is an oxygen anion conducting but electrically insulating solid oxide. This dense ceramic layer also acts as a barrier to prevent gas mixing between the two gas chambers. If we take the SOFC as an example. Fig. 1, the cathode is in contact with air, while the anode is in contact with fuel. The difference in oxygen partial pressure (pO-2)... 

All fuel cells have three components, anode, electrolyte, and cathode, as shown in Figure 38.3b. The anode in the SOFC is responsible for electrical chemical oxidation of fuels, the cathode is responsible for electrochemical reduction of O2 to 0 , and the electrolyte is responsible for the transport of 0 from cathode to anode. Both electrodes, that is, anode and cathode, must possess electrical and oxygen anion conductivities as well as catalytic activities. The most widely used and extensively studied fuel cells use Ni/... 

As shown on Figure 9.1 when the circuit is opened (I = 0) the catalyst potential starts increasing but the reaction rate stays constant. This is different from the behaviour observed with O2 conducting solid electrolytes and is due to the fact that the spillover oxygen anions can react with the fuel (e.g. C2H4, CO), albeit at a slow rate, whereas Na(Pt) can be scavenged from the surface only by electrochemical means.1 Thus, as shown on Fig. 9.1, when the potentiostat is used to impose the initial catalyst potential, U r =-430 mV, then the catalytic rate is restored within 100-150 s to its initial value, since Na(Pt) is now pumped electrochemically as Na+ back into the P"-A1203 lattice. 

Oxygen anion conductors solid electrolyte Oxygen, chemisorbed - chemisorption of oxygen Oxygen conducting solid electrolyte - solid electrolyte... 

The electrical conductivity of rare-earth oxide fluorides was first investigated for the development of a binary anion conductive solid electrolyte. As a result, it was found that the binary rare-earth oxide fluorides exhibited oxide ion conductivity. Among them, the conductivity of neodymium europium oxide fluoride, Nd2Eu203F6, was reported to be much higher than that of yttria-stabilized zirconia, YSZ, practically used as an oxygen sensor [34]. The electrical conductivities of the binary rare-earth oxide fluorides vary not only with the combination of Ln203 with Ln F3... 

The fluorite oxides are the classical oxygen ion conducting oxide materials the study of these materials as electrolytes derives from the early investigations of Walther Nemst 1900. The fluorite structure illustrated in Figure 13 is best described for the purposes of the present discussion as a primitive cubic array of anions (O ) with half the cube centers occupied by cations the latter form a face-centered-cubic (fee) arrangement of the cation sublattice. The unoccupied cube centers play a central role in the defect physics and ionic conductivity of fluorites, because they... 

The PEVD sample utilized in this investigation is a solid electrochemical cell with a ytterbia and yttria stabilized zirconia pellet (8%Yb303-6%Y303-Zr03) as the solid electrolyte to conduct oxygen anions from the source to the sink side. A commercially available Pt thick film paste was screen printed on the center of both surfaces of the solid electrolyte disk. Two Pt meshes, with spot welded Pt leads,... 

As it is impossible to analyze all types of electrolyte tvithin the limitations of this chapter, the reader is directed towards many comprehensive reviews where the known solid electrolytes are classified according to their technological functions [38], the nature of their transition to a highly conducting state [13], the constituent chemical species [39], or their crystal structures [40]. Other recent surveys have been devoted to systems with 3-D ionic migration [41] and to the electrolytes with a certain type ofmobile species (e.g., oxygen anions [42]). Information on these groups of ionic conductors can be found in Chapters 7-9. Irrespective of classifications and microscopic mechanisms, the partial ionic conductivity (G,) of a solid can be expressed as... 

Based on the Wagner method [16], the required character of conductivity can be achieved by the introduction of admixture into the basis oxide, which has a common anion with the basis oxide, and the cation has less valence. Type and quantity of the defects are stipulated by the admixture and its concentration. As a rule, the majority of well-known solid electrolytes with pure oxygen-ionic conductivity have a fluorite CaFj crystalUne structure [17]. 

Oxygen Anion Transport in Soiid Oxides, Fig. 2 Conductivity versus temperature for the three most common SOFC electrolyte materials. The required... 

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