The BSCF compositions analyzed in this study and their corresponding notations.
The mixed conducting perovskite-type oxides BaxSr1-xCo1-yFeyO3-δ (BSCF) are intensively studied as potential high-performance solid oxide fuel cell cathode materials. The effect of different compositional variables and oxygen stoichiometry on the structure and thermodynamic stability of the BaxSr1-xCo1-yFeyO3-δ (x = 0.2, 0.4, 0.5, 0.6, 0.8; y = 0.2, 0.4, 0.6, 0.8, 1) perovskite-type compositions were investigated by solid electrolyte electrochemical cells method and scanning electron microscopy (SEM). The thermodynamic quantities represented by the partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen were obtained in the temperature range of 823–1273 K. The in situ change of oxygen stoichiometry and the determination of thermodynamic parameters of the new oxygen-deficient BSCF compositions were studied via coulometric titration technique coupled with electromotive force (EMF) measurements. The effect of A- and B-site dopants concentration correlated to the variation of oxygen stoichiometry on the thermodynamic stability and morphology of the BSCF samples was evidenced.
- perovskite-type compounds
- oxygen stoichiometry
- thermodynamic data
- electromotive force measurements
- scanning electron microscopy
- cathodes SOFC
The series BaxSr1-xCo1-yFeyO3-δ (BSCF) perovskites are well known for their good oxygen catalytic activity and mixed ionic-electronic conductivity (MIEC) and gained attention as promising electrode materials for solid oxide fuel cells (SOFCs) and oxygen-permeable membranes. Depending on temperature and oxygen partial pressure the BSCF perovskites exhibit high oxygen non-stoichiometry (0.3 <
Despite the interest and the research effort in this field, many aspects of finding appropriate processing parameters and, above all, the fundamental understanding of the correlations between all the factors that ensure the optimization of the SOFC cathodes are not yet elucidated.
The aim of the study is to evidence the effect of the composition, and oxygen stoichiometry change on the thermodynamic properties and morphology of perovskite-type oxides in the BaxSr1-xCo1-yFeyO3-δ (
2. Materials and methods
The details of the sample preparation method are presented elsewhere . Briefly, powder specimens of BSCF were obtained by solid state reaction starting from barium carbonate, strontium carbonate, iron oxide and cobalt oxide raw materials. In order to reach the phase equilibrium of the desired perovskite, the powder mixture was ground and calcined for several times at 1273 K for 10 h. The X-ray diffraction analysis of the as prepared powder samples (shown elsewhere ) demonstrates the formation of a predominant cubic phase for all the BSCF investigated compositions, although small amounts of hexagonal phase could be present in the BSCF 8282 sample [20, 21].
The morphology of BSCF powders was analyzed by SEM using a FEI Quanta 3D equipment operated at low acceleration voltage (maximum 5 kV) and using the backscatter detector in beam deceleration mode. This SEM mode enables high resolution imaging and high surface sensitivity , and it was used in this study to evidence the surface features of the BSCF particles.
The electrochemical cell method was employed to obtain the thermodynamic properties of the BSCF samples. The experimental setups as well as theoretical aspects were described in detail in previous papers [23, 24, 25]. The electrochemical cell contains a yttria stabilized zirconia solid electrolyte and an iron- wüstite reference electrode:
(−) Fe, wustite/ZrO2 (Y2O3) /BSCF (+)
where BSCF denotes the BaxSr1-xCo1-yFeyO3-δ (
Measurements were performed in vacuum at a residual gas pressure of 10−5 atm. The EMF was measured with a Keithley 2000 multimeter, at 50 K intervals between 823 and 1273 K, each time waiting until equilibrium conditions were obtained. Equilibrium conditions were achieved when EMF values for increasing and decreasing temperatures agreed within ±1 mV within a-30 minutes interval.
The solid-state coulometric titration technique was used to accurately change the oxygen stoichiometry of BSCF pellets. The titrations were performed
For every new composition obtained, the equilibrium EMF’s values at 50 K intervals between 1073 and 1273 K are recorded in the open circuit condition.
3. Results and discussions
The BSCF samples with different cation compositions analyzed in this study are listed in Table 1. In order to study the effect of the A- and B- site composition of the perovskite structure on the thermodynamic properties and particles’ morphologies, the samples were grouped in two sets corresponding to the variation of Ba (
|Ba0.2Sr0.8Co0.8Fe0.2O3-δ||BSCF 2882||Ba0.5Sr0.5Co0.8Fe0.2O3-δ||BSCF 5582|
|Ba0.4Sr0.6Co0.8Fe0.2O3-δ||BSCF 4682||Ba0.5Sr0.5Co0.6Fe0.4O3-δ||BSCF 5564|
|Ba0.5Sr0.5Co0.8Fe0.2O3-δ||BSCF 5582||Ba0.5Sr0.5Co0.4Fe0.6O3-δ||BSCF 5546|
|Ba0.6Sr0.4Co0.8Fe0.2O3-δ||BSCF 6482||Ba0.5Sr0.5Co0.2Fe0.8O3-δ||BSCF 5528|
|Ba0.8Sr0.2Co0.8Fe0.2O3-δ||BSCF 8282||Ba0.5Sr0.5FeO3-δ||BSCF 5501|
3.1 A-site effect: Ba/Sr variation
The micrographs of the as prepared BSCF powders obtained for increasing Ba content, a)
The variation of partial molar free energy () and oxygen partial pressure () with temperature is presented in Figure 2 for the five selected BSCF compositions corresponding to different Ba/Sr ratios with
The variation of and log
To get insights into the energetics of oxygen vacancy formation, the partial molar enthalpy and entropy of oxygen dissolution in the perovskite lattice (and , respectively) were calculated in the temperature domains in which the partial molar free energies are linear functions of temperature. For each sample, these particular temperature domains are: 973–1123 K and 1123–1223 K for BSCF 2882; 973–1073 K and 1073–1173 K for BSCF 4682; 973–1073 K and 1073–1223 K for BSCF 5582; 923–1023 K and 1023–1173 K for BSCF 6482; 923–1023 K and 1023–1123 K for BSCF 8282 (Figure 3a). The thermodynamic investigation pointed out that the temperature of structural transformations (
The variation of and with Ba content is depicted in Figure 3b. At temperatures lower than
This decrease of and is associated with an increase of the binding energy of oxygen and an increase of order in the oxygen sublattice of the perovskite-type structure, respectively. At temperatures higher than the points of transitions, both and increased as the Ba-content increases. For BSCF 5582 and BSCF 6482 specimens these values are similar. Thus, besides the oxygen vacancy concentration, the ordering of the oxygen vacancy has a remarkable influence on the entropy values being an indication that the oxygen vacancies are distributed randomly on the oxygen sublattice.
In the intermediate temperature range below 1173 K the variations of the thermodynamic data show some anomalies, which could be correlated with the transition to the cubic BSCF structures . These structural transformations are connected to the charge compensation mechanism. Crystal structure and electrical conductivity of several selected compositions in the Ba–Sr–Co–Fe–O system indicate that doping with more Ba into the system increases the ability for lattice oxygen exchange [13, 20, 21, 33]. A reversible phase transition from cubic to mixed phase of cubic and hexagonal at 973–1173 K for the BSCF 5582 compositions was pointed out both experimentally (employing coulometric titrations and thermal analysis [14, 20, 21, 32, 33, 34, 35] and theoretically, by applying the density functional theory calculations [36, 37].
Keeping in mind the key role of oxygen vacancy ordering on the crystalline phase formation, the less symmetrical non-cubic phases are expected to have highly ordered oxygen vacancies. When the temperature increases above 1023 K, the vacancy ordering starts to disappear, the oxygen vacancies become more mobile and the crystalline phase of the material tends to exhibit higher symmetry, but lower stability. Both, the thermodynamic data and the phase symmetry results let us conclude that low symmetric BSCF perovskites, like BSCF 5582, BSCF 6482 and BSCF 4682 are thermodynamically more stable than the high symmetric BSCF perovskite.
3.2 B-site effect: Co/Fe variation
The morphological evolution of the as prepared BSCF powders with increasing Fe content, is shown in Figure 4 for a)
In Figure 5, the variation with temperature of the partial molar free energy (a) and of the oxygen partial pressure (b) is shown for the BSCF compositions with variable Fe content (0.2 ≤
The heated samples exhibited a complex behavior in the entire investigated temperature domain. One can observe that, in the intermediate temperature range from 823 K to 973 K, the BSCF 5501 sample has the highest recorded values for both and , while, in the high temperature range from 1073 K to 1273 K, the highest values were obtained for the BSCF 5564 composition. This suggests the highest concentration of oxygen vacancies in the specified temperature ranges for these samples. The results could be correlated to the high oxygen non-stoichiometry , high ionic conductivity , as well as with the lattice expansion (volume effect) of the crystalline structure  reported for these compositions. For the BSCF 5582 it has been reported that about 60% of the Co3+ ions are oxidized to Co4+ (), while, at the same time, all the Fe3+ ions are into high valence state (Fe4+ with ), thus this effect has to be compensated by oxygen vacancy formation [40, 41]. Increasing iron content in the heat-treated samples at 1173 K and 1223 K, the decrease of the non-stoichiometry and of the total electrical conductivity is expected [1, 40]. This effect was also evidenced for various substituted ABO3 perovskites solid solution series (e.g. La1-xSrxCo1-yFeyO3-δ [42, 43] and La1-xSrxMn1-yFeyO3-
The values for the partial molar enthalpy and entropy, and , respectively, were calculated in the temperature ranges in which the partial molar free energies are linear functions of temperature (Figure 5a). These specific temperature domains are 973–1073 K and 1073–1223 K for BSCF 5582; 973–1273 K for BSCF 5564; 923–1023 K and 1023–1123 K for BSCF 5546 and 923–1173 K for BSCF 5501. The temperatures of transition (
The BSCF 5528 sample has a distinct thermodynamic behavior which is further discussed. At temperatures lower than 1023 K, the values of enthalpy and entropy of 264 kJ mol−1 and 362 J mol−1 K−1, respectively were obtained (Figure 6b). In the interval 1023–1123 K a strong decrease of the partial molar enthalpy and entropy was observed to values as low as −215 kJ mol−1 and -58.9 J mol−1 K−1, respectively. In this temperature domain the BSCF 5528 sample exhibits high thermodynamic stability. Between 1173 and 1223 K, the variation of the partial molar free energy is observed (Figure 5a), which can be due to further structural transformation related to the charge compensation mechanism. The result is in accordance with the literature indicating the presence of secondary phases in the X-ray diffraction patterns of the samples following the thermal cycle at 1173 K . At the same time, a sharp decrease in the permeation flux was reported for BSCF membranes with the increase of iron concentration from 60 to 80% . The results suggest that the increase of iron concentration in BSCF might be hindered more by the slow oxygen bulk diffusion than by the surface exchange kinetics of the oxides. This could also explain the behavior of the BSCF 5528 sample at 1273 K for which the partial molar free energy increases with ∼40 kJ mol−1 above the values corresponding to all the other investigated samples.
3.3 Oxygen non-stoichiometry effect
In order to further evaluate the previous results, the influence of the change of the oxygen stoichiometry on the thermodynamic properties was examined by solid state coulometric titration technique coupled with EMF measurements. The oxygen stoichiometry was modified by decreasing the stoichiometry with the same relative deviation of Δ
Two sets of data representing the change before and after the isothermal titration experiments, in the temperature range 1123–1273 K, for BSCF compositions are plotted in Figure 7(a) (for the samples with Ba-content increasing) and Figure 7(b) (for the samples with variable Fe concentration).
After titration, the decrease of values with the stoichiometry change is observed for all the investigated compositions with various Ba- and Fe-dopant concentrations, except for the sample BSCF 5528 containing 80% Fe. The obtained results suggested that the energy of vacancy formation decreases with increasing non-stoichiometry while for BSCF 5528 an opposite behavior is estimated.
Considering the partial pressure of oxygen as a key parameter for the thermodynamic characterization of the materials, the variation of the with the temperature and the concentration of the dopants at the same deviation from stoichiometry was analyzed. The results are presented in Figure 8.
A decrease of with the relative non-stoichiometry change (
The changes of and with the oxygen non-stoichiometry were evaluated for the BSCF compositions (Figure 9). For BSCF 6482 and BSCF 5528, the values of enthalpy and entropy were not included in Figure 9a and 9b, respectively due to the deviation from linearity of the energy values obtained in the temperature domain 1123–1273 K. For both BSCF series with Ba- and Fe- compositional variations, the values of enthalpies and entropies decreased with stoichiometry change suggesting an increase in the binding energy of oxygen and change of order in the oxygen sublattice of the perovskite-type structure.
The and did not indicate a clear tendency of variation with barium concentration at the same deviation of stoichiometry, our results being consistent with the results of Girdauskaite
For the BSCF series with different Fe-content, a strong decrease of partial molar enthalpy with
The thermodynamic data showed that, after titration, BSCF 5501 is the most stable composition, exhibiting an increased binding energy of oxygen in the lattice but a random distribution of oxygen vacancies within the oxygen sublattice. The result is consistent with the morphological investigation as well as with the high stability of cubic perovskite phase evaluated for Sr- and Fe- rich compositions [20, 46].
The thermodynamics of solid solutions containing a mixture of cobalt and iron on the B-site is complex. Further details and measurements of the energy and the entropy of oxygen incorporation into BSCF at different values of non-stoichiometry
The thermodynamic behavior of BSCF compounds with different Ba and Fe contents, using an electrochemical cell with yttria-stabilized zirconia as solid electrolyte was investigated. The EMF measurements performed in a wide temperature range (823–1273 K) and at pressures of 10−5–10−6 Pa confirm the instability of the BSCF-perovskite phases at temperatures lower than 1123 K.
With the help of the thermodynamic data, the points of phase transformations associated with the charge compensation mechanism were highlighted, the results being important for the assessment of the long-term stability of such nonstoichiometric materials used as cathodes in IT-SOFCs. The temperature associated to the structural transformations decreases with the increase of Ba-content. The thermodynamic investigation evidenced that for the system BaxSr1-xCo0.8Fe0.2O3-δ, low symmetry BSCF-perovskites (BSCF 5582, BSCF 6482 and BSCF 4682) are thermodynamically more stable than high symmetry BSCF-perovskites (BSCF 2882 and BSCF 8282). In the case of Ba0.5Sr0.5Co1
The BSCF compounds exhibited a significant variation of the thermodynamic parameters with the oxygen non-stoichiometry change, this variation being highly dependent on temperature and dopant concentration. The thermodynamic data evidenced that after decreasing the oxygen stoichiometry with the same relative deviation of , the specimen BSCF 5501 was the most stable composition within 1123–1273 K temperature range. The results of the thermodynamic study of BSCF compounds, at temperatures of interest for the application in IT-SOFC help us to explain why the BSCF 5582 composition exhibits the highest ionic conductivity and the highest oxygen catalytic activity.
Knowing the specific thermodynamic quantities of BSCF compositions, it is possible to find new routes to modify the properties of these materials by suitable substitution and formation of oxygen vacancies in oxygen lattice. The evaluation of thermodynamic quantities is mandatory to understand the complex relationships between the defect chemistry and the material properties.
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