Perovskite-type mixed-conducting

Our research is focused on the control of both the microstructure and the architecture of perovskite-type mixed-conducting reactors for high temperature applications, especially for oxygen separation from air and syngas production. 

Since the membrane conducts oxygen ions, in the great majority of the cases through a mechanism involving anion vacancy diffusion, an equivalent counterflux of electrons should take place for charge neutrality membrane materials should be mked conductors. Perovskite-type mixed-conducting materials are considered suitable candidates for use in dense membrane reactor chemical looping processes, since they fulfill most of the required characteristics. 

Some of the many different types of catalysts which have good catalytic properties for the OCM reaction qualify as membrane materials. Membrane reactors for OCM were designed and tested by Nozaki et al. (1992). Three kinds of reactors were developed the first one consisted of a porous membrane covered with a thin film of catalyst (type I) the second one, a dense ionic-conducting membrane (non porous) with catalytic layer (type II) and the third one was a membrane made of perovskite-type mixed oxides which was active for OCM (type III). Figure 11 presents the diagram for the membrane reactor system and table 13 shows the different materials used for supports and coated catalysts. 

Some perovskite-type oxides show protonic conduction and are useful for hydrogen-related electrochemical devices, including application to solid oxide fuel cells (SOFCs). Iwahara et al. reported protonic conductivity of strontium-cerate-based perovskite-type oxides in 1981 [1], Since that time, various perovskite-type proton-conducting oxides have been found. For use of the proton-conducting perovskite oxides, we should understand not only their merits but also their weak points. This chapter concerns the protonconducting properties of typical cerium- and zirconium-containing perovskite oxides from the points of view of conductivity, stability, electrode affinity, and dopant effect. Mixed conduction occurring in a special composition of the perovskite oxide is also introduced. 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

The limits of integration are the oxygen partial pressures maintained at the gas phase boundaries. Equation (10.10) has general validity for mixed conductors. To carry the derivation further, one needs to consider the defect chemistry of a specific material system. When electronic conductivity prevails, Eqs. (10.9) and (10.10) can be recast through the use of the Nemst-Einstein equation in a form that includes the oxygen self-diffusion coefficient Dg, which is accessible from ionic conductivity measurements. This is further exemplified for perovskite-type oxides in Section 10.6.4, assuming a vacancy diffusion mechcinism to hold in these materials. 

Additional attempts have been presented to render hosts with the fluorite and the related pyrochlore structure electronically conductive by doping with mixed-valence and/or shallow dopants. The list of dopant materials examined includes oxides of elements of, for example, Ti, Cr, Mn, Fe, Zn, Fe, Sn, Ce, Pr, Gd, Tb and U. In general, however, the extent of mixed conductivity that can be obtained in fluorite-type ceramics is rather limited, by comparison with the corresponding values found in some of the perovskite and perovskite-related oxides considered in the next section. 

The considerations in this chapter were mainly prompted by the potential application of mixed-conducting perovskite-type oxides to be used as dense ceramic membranes for oxygen delivery applications, and lead to the following general criteria for the selection of materials... 

Y. Teraoka, H. M. Zhang, K. Okamffoto and N. Yamazoe, Mixed ionic electronic conductivity of Lai-xSrxC0i-yFey03.8 perovskite-type oxides. Mater. Res. Bull, 33 (1988) 51. 

III. ACTIVATION OF MIXED-CONDUCTING PEROVSKITE-TYPE OXIDE CATHODES... 

Figure 9.5 Comparison of the oxygen ionic (a) and electronic (p- and n-type) (b) conductivity of selected solid electrolytes and mixed conductors under oxidizing conditions. The partial ionic conductivity of perovskite-type...

The perovskite-type catalysts (ref.l), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref.4 and 5), the adsorption and thermal desorption of CO, C2H5 and O2 (ref.6 and 7), the reactivity of lattice oxygen (ref.8), the electric conductance of catalysts (ref.9), the pattern of poisoning by SO2 (ref. 10 and 11), the improvement of crushing strength of support (ref. 12) and determination of the activated surface of complex metal oxides (ref. 13) have also been reported. 

Teraoka Y, Zhang HM, Okamoto K, Yamazoe N. Mixed ionic-electronic conductivity of Lai-xSrxCoi-yFeyOs-s perovskite type oxides. Mat Res BuU. 1988 23 51-8. 

Sata N, Yugami H, Akiyama Y, Sone H, Kitamura N, Hattori T, Ishigame M. Proton conduction in mixed perovskite type oxides. Solid State Ionics. 1999 125 383-387. 

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. 

The perovskite-type ceramic membranes have attracted much attention from major chemical and petrochemical companies in the USA. Companies currently involved in the development of the mixed-conducting ceramic membranes include Air Products, Praxair, BP and Amoco. The largest currently existing consortium developing this technology is headed by Air Products and sponsored by... 

Teraoka, Y., Zhang, H. M., Okrunoto, K. Yattrazoe, N. Mixed ionic-electronic conductivity of Lal-xSrxCol-yFey03-8 perovskite-type oxides. Materials Research Bulletin 23, 51-58, doi Doi 10.1016/0025-5408(88)90224-3 (1988). 

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