Proton conductivity, perovskite oxides

Perovskites have two possible roles in these cells. The first of these, as a perovskite-derived proton conducting electrolytes, is described earlier (Section 5.3). In cells that use an oxide ion conducting electrolyte, perovskites are mainly employed as... 

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. 

As just stated, cerium or zirconium is available as a B-site cation for the protonconducting ABOi perovskite oxides. Figure 12.1 compares the temperature dependence of electrical conductivity of BaCeo 9Y0.1O3 and SrZro.9Yo.1O3 in moist hydrogen and in moist oxygen. Both specimens show the typical conductivity behavior of the proton-conducting perovskite oxides. The conductivity in... 

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. 

Figure 25. Proton conductivity of various oxides, as calculated from data on proton concentrations and mobilities, according to Norby and Larring (the type of dopant is not indicated see ref 187 for source data). The conductivity of oxides with a perovskite-type structure are shown by bold lines, and the conductivity of the oxide ion conductor YSZ (yttria-stabilized zirconia) is shown for comparison, (reproduced with the kind permission of Annual Reviews, http //www.AnnualReviews.org).

In the 1960s, all the tools needed to treat hydrogen defects in oxides [41-43] were fundamentally developed by Wagner and collaborators. However, up to the end of the 1970s, real developments in the field did not take place. During the last years of this decade, some significant studies were carried out [44-48], After that, it was realized that the introduction of defects in some perovskite structures determine the protonic conductivity of these materials. In this regard, Iwahara and... 

Development of compact fuel cells, created by combining proton conductive perovskite-type oxide ceramics with metal-hydride materials, has been already proposed [1], Our group expects that the compact fuel cells can be utilized under radiation environments such as fission and fusion reactors or cosmic [2], Therefore, it is very important to understand behaviors of electron and proton conductions under radiation environments. 

It is generally held that metal oxide perovskites with extrinsic oxygen vacancies react with atmospheric water. This entrains hydrogen into the lattice and leads to their significant proton conducting properties. The location of these hydrogen atoms was studied as a function of a series of dopants in a cerium based ceramic. The dopants were niobium, holmium... 

Consequently, we are in the startup phase of our program. Our first task was to identify candidate perovskite oxide materials with high protonic conductivities. We have identified ytterbium doped strontium cerate and yttrium doped strontium zirconate materials as possible electrolyte materials. Barium cerate perovskites exhibit higher protonic conductivity, but the reactivity with carbon dioxide would require pretreatment of the steam. 

One category of dense proton conducting membranes that has received considerable attention in the preceding decade is proton conducting perovskite type oxide ceramics [4-6]. The stoichiometric chemical composition of perovskites is represented as ABO3, where A is a divalent ion (A +) such as calcium, magnesium, barium or strontium and B is a tetravalent ion (B +) such as cerium or zirconium. Although simple perovskites such as barium cerate (BaCeOs) and strontium cerate... 

A series of perovskite compositions were synthesized using oxides and carbonates of the cations by conventional ceramic process. The synthesized powders were characterized using powder x-ray diffraction technique to ensure phase purity. Conductivity measurements were made in H2-H2O atmosphere to determine proton conductity. As the perovskite compositions are inherently mixed conducting, the transference numbers for proton and electron conduction were also determined by varying the partial pressures of hydrogen and steam across the membrane. 

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. 

For more simple systems, the predictive character of ab initio and quantum MD simulations has already made possible the directed identification of improved proton- conducting materials. A prominent example is yttria-doped BaZr03, an oxide with the perovskite structure forming highly mobile protonic defects in the presence of water vapor [22,23]. Quantum MD simulations [24,25] have revealed details of the proton-conduction mechanism, including the critical interactions, and electronic structure calculation have helped identify the best possible dopant [26]. 

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