Oxygen nonstoichiometry of La1−xSrxCoO3−δ (0 < x ≤ 0.6)

doi:10.1016/0022-4596(90)90066-7

Journal of Solid State Chemistry

Volume 87, Issue 1, July 1990, Pages 69-76

https://doi.org/10.1016/0022-4596(90)90066-7 Get rights and content

Abstract

The oxygen nonstoichiometry (δ) of La_1−xSr_xCoO_3−δ (0 < x ≤ 0.6) has been studied as a function of temperature (573–1673 K) and oxygen partial pressure (1–10⁻³ atm) using TGA and coulometric titration techniques. The absolute values of oxygen nonstoichiometry were determined by the direct reduction in a flux of hydrogen (TGA) or by decreasing P_O₂ in a coulometric titration cell. The boundaries of phase stability of La_0.7Sr_0.3CoO_3−δ and (La_0.7Sr_0.3)₂CoO_4±v were evaluated. A possible mechanism of disordering processes has been discussed.

References (12)

I. Kojima et al.
Surf. Sci.
(1983)
T. Nakamura et al.
J. Catal.
(1983)
A.N. Petrov et al.
J. Solid State Chem.
(1988)
N.I. Lipatov et al.
Pis'ma GTF (Russian J. Teor. Phys. Lett.)
(1987)
L.Ya. Gavrilova et al.
Pis'ma GTF (Russian J. Teor. Phys. Lett.)
(1988)
A.N. Petrov et al.
Izv. Akad. Nauk SSSR. Neorg. Mater.
(1988)

There are more references available in the full text version of this article.

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Phase equilibria and defect chemistry of the La-Sr-Co-O system
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Strontium doped lanthanum cobaltite (LSC, La_1−xSr_xCoO_3−δ) is a promising oxygen electrode material for intermediate temperature solid oxide cells (SOCs) and oxygen membranes. Its application in service is however impeded by its thermodynamic instability. The present work is aiming to identify the phase stability of LSC by thermodynamic modeling of the La-Sr-Co-O system using the CALculation of PHAse Diagrams (CALPHAD) method coupled with experiments. The calculated results based on the developed database are validated using experimental data both from literature and the present work. General agreement is achieved between model predictions and experimental results. Other properties of the LSC perovskite, such as oxygen non-stoichiometry and cation distribution, are also calculated and predicted based on the developed La-Sr-Co-O database. These calculations can provide important guidance in designing LSC-based materials for various applications and the established thermodynamic functions can serve as input for further thermodynamic calculations on altered and even more complex perovskites.
Oxygen reduction reaction in solid oxide fuel cells
2022, Oxygen Reduction Reaction: Fundamentals, Materials, and Applications
Solid oxide fuel cells (SOFCs) remain one of the most commercially well-established technologies. In recent years, they have gained enormous attention because of their excellent efficiency and fuel flexibility. At the heart of a typical SOFC cell lies is a ceramic membrane electrode assembly which consists of an air cathode and a fuel anode separated by a dense solid electrolyte. The chemistry of SOFCs essentially involves a gas–solid electrode interface operational at high temperatures (600–1000 °C). The presence of a dense oxygen ion-conducting solid electrolyte which is ionically connected to two catalytically active electrodes results in complexities at the level of material synthesis and interface engineering. As in other fuel cell systems, the oxygen reduction reaction remains one of the most challenging reactions within the SOFC. But, by the sheer nature of the solid-ceramic design, additional challenges such as compatibility of thermal expansion coefficients of materials, temperature-sensitive ionic conductivity arise. The materials, operational principles, degradation mechanisms, and strategies to mitigate them are very different from conventional ambient temperature liquid electrolyte-based designs. This has significant consequences in both material design and measurement strategies. A considerable amount of material research has gone into developing materials and interfaces that can carry out the desired catalytic reactions besides conducting ions and electrons to the required interfaces. A host of materials that include perovskites such as LaSrMnO_x, double perovskites (e.g., Sr₂FeMoO_x), Riddlesden–Popper phases (e.g., La₂NiO₄), pyrochlores, and many more phases have been discussed in the chapter. The chapter introduces the readers to basic concepts and measurement strategies in SOFCs. Special attention has been paid to material design principles, synthesis techniques, and deposition strategies to obtain the desired interfaces. The chapter also covers challenges to ORR activity and electrode stability arising out of material instability or environmental factors such as the presence of H₂O and CO₂. Wherever possible, an attempt has been made to emphasize the chemical principles behind the choice and design of materials.
Influence of strontium content on the oxygen surface exchange kinetics and oxygen diffusion in La<inf>1–x</inf>Sr<inf>x</inf>CoO<inf>3–δ</inf> oxides
2019, Solid State Ionics
Citation Excerpt :
When doping lanthanum cobaltite, ions of alkaline earth metals produced the formation of oxygen vacancies. The oxygen concentration in La1-xSrxCoO3-δ oxides is characterized by oxygen nonstoichiometry, see Fig. 1 [21–24,33–45]. The oxygen nonstoichiometry increases noticeably as the strontium acceptor concentration in the lanthanum sublattice increases, it also depends on the temperature and oxygen partial pressure due to the change in the degree of oxidation of cobalt.
Oxygen surface exchange kinetics between the gas phase oxygen and La_1–xSr_xCoO_3–δ (x = 0, 0.3, 0.4, 0.6) oxides at 0.67 kPa oxygen pressure in the 600–850°°С range were investigated. The mechanism within the two-step model including two consecutive stages, oxygen dissociative adsorption and oxygen adatom incorporation into the oxide lattice, was analysed. The oxygen diffusion coefficient (D), the rate of heterogeneous oxygen exchange (r_H) and the elementary rates of this process — dissociative adsorption (r_a) and incorporation (r_i) — were determined. The effect of oxygen vacancy concentration, strontium segregation on the surface, temperature and oxygen pressure on r_H, r_a and r_i are discussed.
Crystal structure, oxygen nonstoichiometry, and mass and charge transport properties of the Sr-free SOFC/SOEC air electrode material La <inf>0.75</inf> Ca <inf>0.25</inf> FeO <inf>3-δ</inf>
2019, Journal of Solid State Chemistry
Citation Excerpt :
The oxygen nonstoichiometry δ of LCF7525 increases with increasing temperature and decreasing oxygen partial pressure at 20 ≤ T/°C ≤ 800 and 2 × 10−4 ≤ pO2/bar ≤0.2 (Fig. 5 and Figure S-5 (supplementary material)). LCF7525 shows smaller oxygen deficiencies than other SOFC cathode materials with high Co- and/or Sr-content, such as La0.40Sr0.60CoO3-δ [58,59] and La0.40Sr0.60FeO3-δ [37,60]. By comparing the data obtained for LCF7525 in the present work with previous studies on La1-xCaxFeO3-δ, (0.10 ≤ x ≤ 0.20) [24,25], it is evident that δ increases with increasing Ca-concentration.
The mixed ionic-electronic conducting perovskite La_0.75Ca_0.25FeO_3-δ (LCF7525) was synthesized via a citric acid – ethylenediaminetetraacetate sol-gel route. Crystal structure, phase purity, and lattice constants were determined by powder X-ray diffraction and Rietveld refinement. The oxygen exchange kinetics (chemical surface exchange coefficients and chemical diffusion coefficients of oxygen) and the electronic conductivity were studied by in-situ dc-conductivity (relaxation) measurements at 600–800 °C and 1 × 10⁻³ ≤ pO₂/bar ≤ 0.1. The thermal expansion coefficient was determined by dilatometry at 30–1000 °C and 1 × 10⁻³ ≤ pO₂/bar ≤1. The oxygen nonstoichiometry was measured as a function of temperature and oxygen partial pressure by thermogravimetry and could be described by a point defect model. Experimental data of the chemical diffusion coefficient of oxygen and results from defect chemical modelling were used to estimate self-diffusion coefficients of oxygen and oxygen vacancies, as well as the ionic conductivity. Based on the results obtained for the mass and charge transport properties and the thermal expansion behaviour, it can be concluded that LCF7525 may be an attractive Sr- and Co-free material for air electrodes in intermediate temperature solid oxide fuel cells and solid oxide electrolyser cells.
Strontium-free rare earth perovskite ferrites with fast oxygen exchange kinetics: Experiment and theory
2018, Journal of Solid State Chemistry
The Sr-free mixed ionic electronic conducting perovskites La_0.8Ca_0.2FeO_3-δ (LCF82) and Pr_0.8Ca_0.2FeO_3-δ (PCF82) were synthesized via a glycine-nitrate process. Crystal structure, phase purity, and lattice constants were determined by XRD and Rietveld analysis. The oxygen exchange kinetics and the electronic conductivity were obtained from in-situ dc-conductivity relaxation experiments at 600–800 °C and 1×10⁻³≤pO₂/bar≤0.1. Both LCF82 and PCF82 show exceptionally fast chemical surface exchange coefficients and chemical diffusion coefficients of oxygen. The oxygen nonstochiometry of LCF82 and PCF82 was determined by precision thermogravimetry. A point defect model was used to calculate the thermodynamic factors of oxygen and to estimate self-diffusion coefficients and ionic conductivities. Density Functional Theory (DFT) calculations on the crystal structure, oxygen vacancy formation as well as oxygen migration energies are in excellent agreement with the experimental values. Due to their favourable properties both LCF82 and PCF82 are of interest for applications in solid oxide fuel cell cathodes, solid oxide electrolyser cell anodes, oxygen separation membranes, catalysts, or electrochemical sensors.
Interrelation of transport properties, defect structure and spin state of Ni<sup>3+</sup> in La<inf>1.2</inf>Sr<inf>0.8</inf>Ni<inf>0.9</inf>Fe<inf>0.1</inf>O<inf>4+δ</inf>
2017, Solid State Sciences
Citation Excerpt :
The dependencies of oxygen non-stoichiometry δ vs P(O2) were measured in the oxygen partial pressure range 10−5–0.21 atm at 800, 850, 900 and 950 °C. The changes of oxygen non-stoichiometry were calculated from the coulometric data according to ref. [29]. The results were recalculated to the absolute value of δ using the reference point determined from the TGA measurements.
The total conductivity, Seebeck coefficient and oxygen non-stoichiometry for La_1.2Sr_0.8Ni_0.9Fe_0.1O_4+δ have been measured vs temperature and oxygen partial pressure P(O₂). The measurements were carried out at 800, 850, 900 and 950 °C within the P(O₂) range of 10⁻⁵–0.21 atm. La_1.2Sr_0.8Ni_0.9Fe_0.1O_4+δ was shown to be oxygen deficient in all temperature and P(O₂) ranges studied. The calculated values of the partial molar enthalpy of oxygen depend very slightly on oxygen content (δ), indicating that La_1.2Sr_0.8Ni_0.9Fe_0.1O_4+δ with the oxygen deficiency can be considered an ideal solution. The model of point defect equilibria in La_1.2Sr_0.8Ni_0.9Fe_0.1O_4+δ has been proposed and fitted to experimental dependencies. Subsequent joint analysis of the defect structure and transport properties revealed that electron holes can coexist in both localized and quasi-delocalized states in the oxide: the former corresponded to high-spin state Ni³⁺ and the latter – to low-spin state Ni³⁺. The mobilities of localized electron holes were shown to be significantly lower in comparison to quasi-delocalized ones. The behavior of localized electron holes was explained in terms of a small polaron conduction mechanism; in contrast, quasi-delocalized electron holes were described in terms of a band conduction approach. The small polaron conduction mechanism was shown to be predominant in the Sr- and Fe-co-doped lanthanum nickelate.

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Regular articleOxygen nonstoichiometry of La1−xSrxCoO3−δ (0 < x ≤ 0.6)

Abstract

Surf. Sci.

J. Catal.

J. Solid State Chem.

Pis'ma GTF (Russian J. Teor. Phys. Lett.)

Pis'ma GTF (Russian J. Teor. Phys. Lett.)

Izv. Akad. Nauk SSSR. Neorg. Mater.

Regular article
Oxygen nonstoichiometry of La_1−xSr_xCoO_3−δ (0 < x ≤ 0.6)