Perovskite protonic-electronic conductivity

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. 

State-of-the-art proton conductors comprise acceptor-substituted perovskites, such as the barium-based ones (BaCe03, BaZr03, etc.) which exhibit proton conductivities in excess of 0.01 S cm i [51-55] and strontium-based ones (SrCe03) with somewhat lower conductivities. Both BaCe03 and BaZr03 are almost pure ionic conductors, and the electronic conductivity would, as such, rate limit the H2 flux across membranes of these materials [56]. 

An ideal membrane must have high electronic and protonic conductivities it should be fairly thin (1-10 pm), and chemically stable for prolonged periods of time under the presence of various gases such as CO2, H2O and H2S etc. The low electronic conductivity limits the H2 flux of the protonceramic membranes. Under pure H2 atmosphere at 900 "C, the protonic conductivities of SCYb and BCNd are about 0.7 x 10 2 and 2.2 x 10 2 S cm , respectively. These values are sh tly lower than the oxygen ionic conductivity of yttria stabilized zirconia (YSZ) or lanthanum strontium cobaltite (LSC) perovskite-type ceramics in air at the same temperature, so that there is room for further improvement in protonic conductivity. 

Key words membrane reactor, perovskite, proton conducting membrane, mixed ionic-electronic conductor (MIEC), partial oxidation of methane... 

The conductivities due to each of these defects are determined by their concentrations multiplied by their mobilities. As a result of the electron-hole equilibrium, these defects cannot both be present in high concentration at the same time. Therefore, perovskite proton conductors normally display protonic, oxide ion and one type of electronic conductivity. For an HTPC in oxidizing conditions the main defects are Vo, OHo, and h/. The situation can be visualized in the diagram of Fig. 6, which indicates the domains favorable for applications in fuel cell electrolytes and hydrogen transport membranes. The domain for oxygen transport membranes (i.e., no proton conduction) is included for the sake of completeness. 

Even though perovskite proton conductors are selected for their protonic conductivity, possessing a minority electronic conductivity component is not necessarily a disadvantage. Figure 7a shows a plot of the total conductivity versus the oxygen partial pressure P02) a logarithmic scale. The curve follows an equation of the type ... 

In this equation, cr,- is the ionic conductivity, assumed to be independent of Pq2, while Op° and cr ° are parameters describing the other conductivity components at. Po2 of f hn [15]. In Fig. 7b, the ionic transport number cr,7(7,0, is plotted on the same logarithmic scale. In a fuel cell, for example, an H2/O2 ceU, the emf is dependent on the mean ionic transport number. In Fig. 7b, the mean icMiic transport number is the area ratio of the light colored block to the whole block. As a result, partial electronic conductivity limited to the high and low P02 regions is not a problem. This is also observed in practice, where H2/O2 or H2/air cells based on perovskite proton conductors give open circuit voltages within a few percent of the theoretical values [16]. 

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