Red-ox behaviour in the La0.6Sr0.4CoO3±δ-CeO2 system

Abstract

The compositions in the (100 À x)La 0.6 Sr 0.4 CoO 3AEd -xCeO 2 (LSCC) system with 5 # x # 76 are twophase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (R 3c) and modified ceria with fluorite structure (Fm 3m). The cross-dissolution of La, Sr, Co and Ce cations between the initial La 0.6 Sr 0.4 CoO 3AEd (LSC) and CeO 2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (R 3c) to cubic (Pm 3m) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grain's surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x ¼ 8-37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce 4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x ¼ 57-76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x ¼ 5-76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (d) above certain values: d > 0.08 in LSC, d > 0.13 in LSCC02, and LSCC with x ¼ 5-76. The LSCCx with x ¼ 5-37 and with x ¼ 57-76 show slightly different reduction behaviour under a(o 2 ) ¼ 7.4 Â 10 À5 . In the composites with a relatively low CeO 2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x ¼ 57-76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.

Evaluation of La0.3Sr0.7Ti1−xCoxO3 as a potential cathode material for solid oxide fuel cells

“…Two broad peaks, belonging to Co 2p 3/2 and Co 2p 1/2 electrons, were observed, both of which can be deconvoluted into two peaks, assignable to Co 3+ and Co 4+ , respectively. [39][40][41] With the integrated area of the peaks, the contents of each oxidation state of Co ions are calculated, which are listed in Fig. 8.…”

On the Existence of A‐Site Deficiency in Perovskites and Its Relation to the Electrochemical Performance

“…According to X‐ray photoelectron spectroscopy and Auger electron spectroscopy, the surface concentration of the transition metal cations in the cation stoichiometric perovskites is either the same as in the volume of crystallites or lower: [Mn]/([La]+[Sr]) = 0.1–0.96 for La 0.8 Sr 0.2 MnO 3 (depending on high‐temperature treatment and cooling conditions),22, 23 ([Co]+[Fe]/([La]+[Sr]) = 0.82–0.67 for La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 ,24 [Co]/([La]+[Sr]) = 0.31–0.52 and 0.29, respectively, for La 0.5 Sr 0.5 CoO 3 24 and La 0.6 Sr 0.4 CoO 3 25. However, Co surface concentration was found to raise in the two‐phase 92La 0.6 Sr 0.4 CoO 3 –8CeO 2 composition: [Co]/([La]+[Sr]) = 5.05 25. Although it was not possible to analyze the Ni surface concentration due to the interference of Ni2p and La3d 3/2 peaks, the [Fe]/[La] surface ratio amounts to 0.84 (the La concentration was defined from the La3d 5/2 peak) 19.…”

Oxygen storage capacity and thermal stability of the CuMnO2–CeO2 composite system

“…[33][34][35][36][37] CeO 2 has also been reported as an interlayer or electrolyte in the La 1Àx Sr x CoO 3Àd / CeO 2 composite system to improve the oxygen ionic mobility, in which some chemical reactions can take place at the interfaces between perovskite and CeO 2 . 38,39 Therefore, CeO 2 may function as an oxygen ionic mobility channel between oxides and their surrounding atmosphere. Here, we investigated the CuMnO 2 -CeO 2 composite system with the purpose of further optimizing the oxygen diffusion ability and overall oxygen storage capacity at low temperatures under alternating oxidative (i.e., air or O 2 ) and inert (i.e., Ar) atmospheres.…”