Perovskite evolution

Our recent our works show that even higher activity and stability can be demonstrated by the three-layer electrodes with nickel layer, active in the oxygen evolution, middle layer with catalyst, active in the oxygen reduction (Mn02, pyropolymer or a perovskite), and a diffusion (waterproof) layer,... 

Evolution of secondary phases. Another concern has been continued formation of LZ and SZ secondary phases at the perovskite/YSZ interface as a function of time or current density. - Accelerated testing, achieved by sustained heat treatments of the electrode, suggests that degradation can occur by this mechanism.However, whether such thermal treatments can be meaningfully extrapolated to predict natural degradation processes is unclear. 

So far, the bonding and surface structure aspects of electrocatalysis have been presented in a somewhat abstract sort of way. In order to make electrocatalysis a little more real, it is helpful to go through an example—that of the catalysis of the evolution of oxygen from alkaline solutions onto substances called perovskites. Such materials are given by the general formula RT03, where R is a rare earth element such as lanthanum, and T is a transition metal such as nickel. In the electron catalysis studied, the lattice of the perovskite crystal was replicated with various transition metals, i.e., Ni, Co, Fe, Mn, and Cr, the R remaining always La. 

Fig. 7.111. Current density (based on real surface area) for oxygen evolution on perovskites at an overpotential of 0.3 V vs. M—OH bond strength. The transition-metal ions (M) in perovskites are indicated with different symbols. (Reprinted from J. O M. Bock-ris and T. Ottagawa, J. Electrochem. Soc. 131 2965,1984. Reproduced by permission of The Electrochemical Society, Inc.)...

The considerations of this reaction of Oz evolution on an oxide catalyst again show the importance of electronic factors and bonding. However, the discussion covers only the essentials the reality of the catalysis of perovskites in oxygen evolution involves several other factors that can be referred to here only briefly. 

Fig. 7. 112. Schematic representation of the first and second (rate-determining) steps of the mechanism of the evolution of 02 on perovskites, involving a series of proton transfers. (Reprinted from J. O M. Bockris and T. Ottagawa, J. Phys. Chem. 87 2964,1983.)...

An increasing amount of attention is being given to oxides as possible anodes for oxygen evolution because of the importance of this reaction in water electrolysis. In this connection, numerous studies have been carried out on noble metal oxides, spinel and perovskite type oxides, and other oxides such as lead and manganese dioxide. Kinetic parameters for the oxygen evolution reaction at a variety of single oxides and mixed oxides are shown in Table 3. 

The kinetics of oxygen evolution have been investigated at a variety of perovskite oxides, mainly in alkaline solution. Notwithstanding the work of Bockris and co-workers [269] on the electrocatalytic activity of the perovskite analog oxide Nax W03 for oxygen reduction, the first report of a study of the electrocatalytic activity of perovskite oxides was by Meadowcroft [270] for oxygen reduction on La(1 l)SrICo03. 

Bockris et al. [87, 290, 291] have recently reported results of a comprehensive program of surface characterization of a large number of perovskite oxide electrodes in oxygen evolution investigations. Anodic and cathodic oxygen reactions were studied in detail as a function of the solid-state surface properties of these materials. Capacity-potential curves were analysed in terms of the Mott-Schottky treatment and indicated that the potential distribution in the oxide corresponds to a depletion of electrons at the oxide electrode surface in the potential region where oxygen reduction... 

Several oxides with perovskite related stmctures can also be intercalated with oxygen ions by an electrochemical method. The oxide Sr2Fe20s with the brownmillerite stmcture has been electrochemically oxidized to SrFeOs. The reaction was carried out by controlled potential electrolysis at a potential below that for oxygen evolution in 1 M aqueous KOH at room temperature. Bulk oxidation was confirmed by Mossbauer spectroscopy and X-ray difflaction. Similar results have been obtained for electrochemical oxidation... 

Studying the temperature evolution of UV Raman spectra was demonstrated to be an effective approach to determine the ferroelectric phase transition temperature in ferroelectric ultrathin films and superlattices, which is a critical but challenging step for understanding ferroelectricity in nanoscale systems. The T. determination from Raman data is based on the above mentioned fact that perovskite-type crystals have no first order Raman active modes in paraelectric phase. Therefore, Raman intensities of the ferroelectric superlattice or thin film phonons decrease as the temperature approaches Tc from below and disappear upon ti ansition into paraelectric phase. Above Tc, the spectra contain only the second-order features, as expected from the symmetry selection rules. This method was applied to study phase transitions in BaTiOs/SrTiOs superlattices. Figure 21.3 shows the temperature evolution of Raman spectra for two BaTiOs/SrTiOa superlattices. From the shapes and positions of the BaTiOs lines it follows that the BaTiOs layers remain in ferroelectric tetragonal... 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

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