- Email: [email protected]
La1 − xBaxCo0.2Fe0.8O3 − δ perovskites for application in intermediate temperature SOFCs
Solid State Ionics 225 (2012) 437–442
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
La1 − xBaxCo0.2Fe0.8O3 − δ perovskites for application in intermediate temperature SOFCs Bartłomiej Gędziorowski, Konrad Świerczek ⁎, Janina Molenda AGH University of Science and Technology, Faculty of Energy and Fuels, Department of Hydrogen Energy, al. A. Mickiewicza 30, 30‐059 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 9 September 2011 Received in revised form 17 May 2012 Accepted 25 May 2012 Available online 17 June 2012 Keywords: Crystal structure Transport properties Chemical stability LBCF IT-SOFC
a b s t r a c t Physicochemical properties of La1 − xBaxCo0.2Fe0.8O3 − δ (LBCF) oxides are presented in aspect of their possible application as cathode materials in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). The obtained results show that the increase of concentration of Ba2 + leads to an increase of unit cell volume and to a decrease of crystal structure distortion, which is accompanied by an increase of the level of oxygen nonstoichiometry in the studied compounds. For samples with x ≥ 0.6 cubic structure was observed, which is particularly worth noting for BaCo0.2Fe0.8O3 − δ composition, as both BaCoO3 − δ and BaFeO3 − δ often, but depending on oxygen nonstoichiometry δ, adopt hexagonal-type structure. Electrical conductivity and Seebeck coefficient data of the materials are quite similar, comparing to La1 − xSrxCo1 − yFeyO3 − δ oxides. For samples with barium content ≥0.6, at high temperatures, a clearly visible maximum on electrical conductivity dependence on temperature can be seen, which may be related to a significant increase of the oxygen nonstoichiometry. The highest conductivity in the intermediate temperature range (600–800 °C) was observed for La0.6Ba0.4Co0.2Fe0.8O3 − δ composition, however obtained values are significantly lower, comparing to strontium analog: La0.6Sr0.4Co0.2Fe0.8O3 − δ. La0.6Ba0.4Co0.2Fe0.8O3 − δ oxide was used as the cathode material in custom-made button-type IT-SOFCs, which properties were evaluated in 600–800 °C temperature range. Additionally, chemical stability of that material in relation to Ce0.8Gd0.2O1.9 electrolyte was studied. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A-site and B-site doped perovskites with A1 − xA'xB1 − xB'yO3 − δ general formula and various chemical compositions have attracted considerable attention due to their possible application as cathode materials in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) related to their mixed ionic-electronic conductivity and catalytic activity toward oxygen reduction [1–3]. In particular, La1 − xSrxCo1 − yFeyO3 − δ (LSCF) and Ba1 − xSrxCo1 − yFeyO3 − δ (BSCF) oxides with appropriate chemical composition, as well as other similar materials (e.g. cationordered perovskites, Ruddlesden–Popper oxides) show suitably high, mixed conductivity, high activity toward oxygen reduction, suitable thermomechanical properties and good chemical stability in relation to ceria-based or La1 − xSrxGa1 − yMgyO3 −(x + y) / 2 electrolytes [1,4]. It is believed that application of cathode materials with mixed ionicelectronic conductivity should be beneficial for operation of SOFCs in the intermediate temperature range, as in such case the cathodic reaction is not limited to the triple phase boundary, but is extended to the whole surface of the cathode [2,3]. Comparing to relatively well known LSCF and BSCF perovskites, La1 − xBaxCo1 − yFeyO3 − δ (LBCF) materials are less studied, and literature data, especially regarding their application and tests in SOFCs, are limited ⁎ Corresponding author. E-mail address: [email protected] (K. Świerczek). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.05.025
[5–9], nevertheless, there are also studies about La1 − x − zSrxBazCo1 − y FeyO3 − δ compounds [10,11]. The main advantage of application of LBCF cathode materials is related to their higher resistance for chromium poisoning [7,11,12], which makes them especially suitable for application in cells with Fe–Cr metallic interconnects. They, however, exhibit lower total electrical conductivity, comparing to LSCF analogs [5]. The electrical conductivity of LBCF oxides is strongly dependent on the chemical composition, i.e. the amount of barium and iron [5,13,14]. Generally, it can be stated that Fe-rich LBCF materials exhibit lower conductivity than Co-rich ones, and the maximum conductivity is observed for materials with comparable contents of La and Ba. Electrical properties of these compounds, depending on the chemical composition, must be interpreted taking into account complex relationship resulting from coexistence of Fe3 +, Fe4 +, Co3 +, Co4 + with preferential oxidation of Fe [14], composition and temperature-dependent concentration of oxygen vacancies, and composition-dependent changes of crystal symmetry and unit cell parameters. The oxygen vacancies, concentration of which increases significantly for Ba-rich samples, especially at higher temperatures [5], are responsible for an apparent decrease of the average oxidation state of 3d metals M, which may be described by the following reaction: x x 2B•B þ OO →2BB þ V•• O þ 1=2O2 ↑:
ð1Þ
Significant changes of the concentration of oxygen vacancies are a cause of so called chemical expansion effect [14], which was also observed for LSCF oxides [15]. However, until now there are no systematic
438
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
studies showing oxygen nonstoichiometry, thermal expansion and electrical conductivity in a wide temperature range for all materials from La1 − xBaxCo1 − yFeyO3 − δ system. There is also lack of detailed information regarding chemical stability of these materials in relation to common solid electrolytes and their stability in CO2 and H2O containing atmospheres, especially that materials with high Ba content may suffer from limited stability in such atmospheres. The aim of this work is to evaluate physicochemical properties in the La1 − xBaxCo0.2Fe0.8O3 − δ series (crystal structure, electrical properties, changes of the oxygen stoichiometry) from the point of view of possible application of such materials in IT-SOFCs. Among examined LBCF compounds one material was chosen (La0.6Ba0.4Co0.2Fe0.8O3 − δ) and its chemical reactivity toward Ce0.8Gd0.2O1.9 electrolyte and electrochemical properties in IT-SOFCs were studied. 2. Experimental La1 − xBaxCo0.2Fe0.8O3 − δ (0 ≤ x ≤ 1) perovskites were synthesized using soft chemistry method. High purity (≥99.9%) La, Ba, Co and Fe nitrates were dissolved in stoichiometric proportions in minimal amount of deionized water. After dissolution, ammonium salt of ethylenediaminetetraacetic acid (EDTA) was added as a complexing agent. The solutions were heated in quartz evaporator, until sol–gel transition, decomposition of excess of ammonium nitrate and oxidation of residual carbon were observed. Further details of the procedure can be found elsewhere [16]. Depending on the chemical composition, the final annealings were performed at 900 °C for BaCo0.2Fe0.8O3 − δ, at 1100 °C for La0.2Ba0.8Co0.2Fe0.8O3 − δ and at 1200 °C for other materials. Additional annealing for La0.2Ba0.8Co0.2Fe0.8O3 − δ was performed at 1300 °C. Crystal structure of the samples was identified using XRD technique with Panalytical X'PERT Pro diffractometer in 10–90° range using CuKα radiation. Obtained data were refined using Rietveld method and GSAS/EXPGUI set of software [17,18]. Goldschmidt tolerance factor tS was calculated on the basis of Shannon's ionic radii using well known equation: tS = (rA + rO) / (20.5 · (rB + rO)), where r stands for ionic radius of respectively: A and B cations and oxygen in ABO3 perovskite. A following simplifications were assumed: oxidation state of cobalt and iron cations is +3 and +4 with ratio depending directly on the Ba 2 + content; oxidation of both cations in not preferential; all cations possess high spin configuration. An influence of oxygen nonstoichiometry on tolerance factor was also considered: in case of tδ, the oxidation state of Co and Fe cations was appropriately lowered, in order to maintain charge balance in La1 − xBaxCo1 − yFeyO3 − δ. This caused a decrease of the calculated values of tδ, as compared to tS. In another approach for determination of tolerance factor, the actual LaBa–O and CoFe–O distances, refined from XRD data, were used instead of ionic radii. Such approach shows real values of t (marked in this work as tR) and was previously presented in literature for other perovskite materials [19]. Transport properties of the studied compounds were characterized by the electrical conductivity σ (4-probe DC) and thermoelectric power α (dynamical method, with up to 2–3 °C temperature gradient) measurements in 25–900 °C temperature range. The measurements were carried out simultaneously. For determination of Seebeck coefficient α, measured values of thermoelectric voltage as a function of temperature gradient on the sample were plotted, with α being calculated as a slope of this dependence. Thermogravimetric (TG) studies were performed in synthetic air and in 0.1% H2 in Ar mixture using TA Q5000IR thermogravimetric apparatus with 1 °C/min heating/cooling rate. Equilibrium values of the oxygen nonstoichiometry δ, calculated from TG measurements in air and Ref. [5], allowed for evaluation of the enthalpy of formation of oxygen vacancies (ΔH). Values of ΔH were calculated for materials with x ≥ 0.4 from the linear portion of logδ dependence on T− 1. Button-type electrolyte-supported IT-SOFC cells were constructed using Ni-8YSZ cermet anode, Ce0.8Gd0.2O1.9 electrolyte and La0.6Ba0.4Co0.2Fe0.8O3 − δ cathode material. Gas tight electrolyte pellets with about 0.7 mm thickness were sintered at 1500 °C. Typical,
40 wt.% 8YSZ–60 wt.% NiO paste was used for preparation of the anode. The paste was painted on one side of the electrolyte sinter and fired at 1400 °C in air, with additional step at 400 °C, at which organic binder from the paste decomposes. Cathode paste was prepared by mixing thoroughly grinded La0.6Ba0.4Co0.2Fe0.8O3 − δ with appropriate amount (about 1:2 wt. ratio) of commercial texanol-based binder (ESL ElectroScience) in order to obtain desired consistence. After painting, the cathode paste was fired at 1100 °C, with additional step at 400 °C. Area of both electrodes was approx. 0.25 cm2 with thickness of about 0.3 mm. Such cells were sealed using glass sealer to alumina tube and mounted in testing unit. Platinum wires were used as current collectors. The anode was fuelled with 10 cm3·min− 1 flow of dry hydrogen using mass-flow controller. At the cathode side a constant flow of air was provided. Impedance spectra and the current–voltage characteristics of the cells were measured in the 600–800 °C temperature range using custom-made SOFC testing unit equipped with Solartron SI 1287 electrochemical interface and Solartron 1252A frequency response analyzer. The impedance spectroscopy measurements were performed in 0.1– 300 kHz range with 25 mV amplitude at open circuit conditions. Interpretation of the results was done in the same way as presented in [20]: the ohmic resistance Rohm and the total interfacial polarization resistance Ri were evaluated by fitting the impedance spectra to an equivalent circuit consisting of L–Rohm–(RQ)HF–(RQ)LF, where L is an inductance, R—resistance, Q—constant phase element, while HF and LF stand for respectively high-frequency and low-frequency contributions. 3. Results and discussion The performed XRD studies showed that with the increasing amount of Ba in La1 − xBaxCo0.2Fe0.8O3 − δ, crystal structure changes from lower symmetry orthorhombic through rhombohedral to cubic one. This behavior is analogous to the one found in La1 − xSrxCo0.2Fe0.8O3 − δ compounds [16]. Detailed structural data for synthesized LBCF materials are presented in Table 1. Rietveld refinement of XRD data for La0.8Ba0.2Co0.2Fe0.8O3 − δ composition revealed coexistence of orthorhombic and rhombohedral phases, with 50:50 wt.% ratio. Different heating temperatures (1200 °C and 1300 °C) did not have any visible influence on the structural properties of this compound. In case of La0.6Ba0.4Co0.2Fe0.8O3 − δ material, the observed rhombohedral distortion is minimal (Fig. 1), but is supported by better statistics of Rietveld refinement, comparing to refinement performed for Pm-3m space group (Fig. 1 inset). Interestingly, we were able to obtain single phase BaCo0.2Fe0.8O3 − δ with cubic symmetry (Fig. 1). In case of endmembers: BaFeO3 − δ and BaCoO3 − δ hexagonal-type structure is often observed, but depending on nonstoichiometry δ, the materials can have perovskite-like structure or be mixtures of phases [21–25]. Analyzing these data, the presence of cubic symmetry in BaCo0.2Fe0.8O3 − δ can be attributed to a very high concentration of oxygen vacancies of the order of 0.4 mol/mol. This was confirmed by TG reduction experiments performed in 0.1 H2 in Ar atmospheres: at 900 °C sample decomposed into Ba3Fe2O6, Ba2Fe2O5 and Fe (as confirmed by XRD studies), with molar ratio indicating initial δ ≈0.4 mol/mol. In Fig. 2 the dependence of normalized to simple perovskite unit cell volume, as well as calculated and refined values of Goldschmidt's tolerance factor t are shown. As can be noticed, there is a rather big discrepancy between the calculated (tS, tδ, see Fig. 2) and the refined (tR) values of tolerance factor, with tR values being 1 or almost 1 for all the materials. This can be expected, as Shannon's ionic radii do not properly describe non purely ionic compounds, and was observed in LSCF perovskites [26]. The nonlinear dependence of normalized unit cell volume on composition can be rationalized on the basis of two effects: the first one is related to an increase of average ionic radii of La1 − xBax with the increase of x (bigger Ba2 + cations substituting smaller La3 + cations), which however causes an increase of the average oxidation state, and therefore decrease of radii of 3d metals; the second effect is related to the oxygen nonstoichiometry, which decreases average oxidation state of Co and Fe
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
439
Table 1 Structural parameters of synthesized La1 − xBaxCo0.2Fe0.8O3 − δ perovskites. Composition
Space group
a [Å]
b [Å]
c [Å]
Vp [Å3]
δ [mol/mol] at 300 K
LaCo0.2Fe0.8O3 − δ La0.8Ba0.2Co0.2Fe0.8O3 − δ
Pbnm 50 wt.% Pbnm 50 wt.% R-3c R-3c (almost cubic) Pm-3mb Pm-3mb Pm-3m
5.5123(1) 5.5200(2) 5.5466(1) 5.5422(4) 3.9339(1) 3.9846(1) 4.0721(1)
5.5334(2) 5.5585(2) – – – – –
7.8013(1) 7.8090(2) 13.4845(4) 13.5148(15) – – –
237.95(4) 239.60(4) 359.27(6) 359.50(6) 60.88(1) 63.26(1) 67.52(1)
0a 0a
La0.6Ba0.4Co0.2Fe0.8O3 − δ La0.4Ba0.6Co0.2Fe0.8O3 − δ La0.2Ba0.8Co0.2Fe0.8O3 − δ BaCo0.2Fe0.8O3 − δ a b c
0.05a 0.1a 0.25a ~ 0.4c
Values of oxygen nonstoichiometry were taken from Ref. [5], but were also confirmed by our thermogravimetric experiments. Secondary phase (b3 wt.%) A2BO4 type with I4/mmm symmetry was present in the samples. Approximate value obtained from thermogravimetric reduction experiment.
(Eq. (1)). This can be followed in greater detail in Fig. 3, in which refined interatomic distances for LaBa–O and also for CoFe–O are presented. While bLaBa–O>g increases with the increasing x in whole range of chemical composition, in case of bCoFe–O>g, the minimum can be noticed for intermediate compositions. Unfortunately, for materials with orthorhombic structure, due to low sensitivity of XRD technique for oxygen, the obtained data is not precise enough to evaluate bCoFe–O>g with higher accuracy. Also, in case of calculated values of lattice free volume (Fig. 3), a minimum can be noticed for samples with x = 0.4 and 0.6. Results of the performed thermogravimetric studies are presented in Fig. 4. With the increasing amount of barium, mass loss, which can be related to the formation of oxygen vacancies, increases. Nevertheless, for materials with x ≥0.6 the temperature, at which such loss begins remains almost constant and is close to 300 °C. For BaCo0.2Fe0.8O3 − δ compound mass loss occurs also around 100 °C. These results together with the literature data [5], regarding values of the oxygen nonstoichiometry δ present in the samples at room temperature (Table 1), allowed for calculation of enthalpy of formation of oxygen vacancies ΔH (Fig. 5). With the increasing barium content, ΔH decreases significantly and for materials with x= 0.8 and 1 is of the order of 0.04– 0.05 eV. Similar conclusion may be drawn analyzing TG data published for various LBCF and LSCF oxides [5]. This is a clear proof that the increase of concentration of Ba2 + (or Sr2 + in the case of LSCF oxides) favors formation of oxygen vacancies, which in consequence influence average oxidation state of cobalt and iron (Eq. (1)). For comparison, previous studies indicate ΔH = 0.28 eV for La0.5Ba0.5Co0.5Fe0.5O3 − δ composition [27]. It is known that for oxygen vacancy-related ionic conduction activation energy consists of two terms: activation energy for vacancy diffusion and the second term related (but not equivalent) to the enthalpy
of formation. Interestingly, the calculated values of ΔH match well the difference between total activation energy of ionic conductivity and diffusion-related activation energy published for respective LBCF oxides in work [5], for which an increase of the ionic conductivity with increasing barium content was also shown. Fig. 6 shows total electrical conductivity of measured materials with 0.2 ≤x ≤ 0.8. Because ionic component of the conductivity is more than 2 orders of magnitude lower that the electronic component [5], the presented values can be identified with the electronic conductivity. Similarly to LSCF compounds, with the increase of x conductivity first increases and then decreases, reflecting changes of the average oxidation state of cobalt and iron [5,28]. Additionally, barium rich samples show maximum on the electrical conductivity dependence on temperature. The appearance of these maxima can be related to the formation of oxygen vacancies at elevated temperatures and that effect is very similar to the one observed for LSCF perovskites [3]. The highest values of the electrical conductivity were measured for La0.6Ba0.4Co0.2Fe0.8O3 − δ, but nevertheless, were less than 100 S·cm− 1, which is substantially lower, comparing to La0.6Sr0.4Co0.2Fe0.8O3 − δ [3] and comparing to other cobalt-rich LBCF oxides, for which conductivities exceeding 500 S·cm− 1 were observed [14]. In general, it may be stated that studied LBCF compounds exhibit lower electrical conductivity comparing to respective LSCF materials. Seebeck coefficient measurement (Fig. 7) indicates that in case of all the materials the dominant charge carriers are holes (positive sign of α). For oxides with x= 0.6 and 0.8 at temperatures exceeding 400 °C an increase of α can be noticed, which corresponds to the increase of the oxygen vacancy concentration. Similar behavior was previously observed for LSCF oxides [16]. Interesting characteristics of σ(T) were measured for BaCo0.2Fe0.8O3 − δ material (Fig. 8). On the two consecutive heating and cooling cycles unexpected features appear at around 650 °C
Fig. 1. Exemplary XRD patterns with Rietveld refinement for La0.6Ba0.4Co0.2Fe0.8O3 − δ and BaCo0.2Fe0.8O3 − δ perovskites.
Fig. 2. Calculated (tS using Shannon's ionic radii, tδ with correction for oxygen nonstoichiometry) and refined (tR as geometric average using interatomic distances) values of tolerance factor, together with normalized unit cell volume as a function of barium content in La1 − xBaxCo0.2Fe0.8O3 − δ.
440
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
Fig. 3. Interatomic distances and lattice free volume as a function of barium content in La1 − xBaxCo0.2Fe0.8O3 − δ. Distances were calculated as geometric averages.
Fig. 6. High temperature electrical conductivity of La1 − xBaxCo0.2Fe0.8O3 − δ (0.2≤ x ≤ 0.8) perovskites.
and 750 °C. We believe that they are related to the existence of structural phase transition, but further studies are needed to explain this phenomenon. La0.6Ba0.4Co0.2Fe0.8O3 − δ composition was chosen as the cathode material for preparation of custom-made electrolyte-supported IT-SOFC cells. This material exhibits the highest electrical conductivity among studied LBCF oxides, low nonstoichiometry at room temperature
and moderate changes of δ at elevated temperatures (Table 1, Fig. 6). These features are beneficial in terms of application, due to expected low resistance associated with the cathode and small influence of chemical expansion on the total thermal expansion of the material, which, in turn, helps with preparation of the cathode layers on the electrolyte. Fig. 9 shows performance of an exemplary IT-SOFC cell. At 800 °C the maximum power density reaches 0.14 W·cm − 2, which however is a rather low value, comparing to literature data. Impedance spectroscopy studies (Figs. 10 and 11) revealed that in the whole temperature range (600–800 °C) the cell's power output is limited by its charge transfer and diffusional resistances, summarized as Ri. This is in opposition to our previous studies, showing much smaller values of Ri, which are lower than ohmic resistance (Rohm) at high temperatures, in case of similarly constructed cells with La0.5Sr0.5Co0.5Fe0.5O3 − δ or Pr2NiO4 + δ based cathodes [27,29]. While the sintered cathodes were not especially optimized in terms of their microstructural features, it can be stated that such low power densities may be rather originate from not sufficient catalytic activity of La0.6Ba0.4Co0.2Fe0.8O3 − δ for the oxygen reduction reaction. In order to obtain higher power outputs of constructed IT-SOFCs it would be necessary to decrease activation and diffusion related polarizations considerably. Unfortunately, due to thermomechanical incompatibility we were not able to sinter cathode layers using LBCF materials with x ≥ 0.6, which are expected to possess higher catalytic activity. Their application would require preparation of intermediate layers, which allow for better matching of the expansion between used materials.
Fig. 5. Arrhenius-type plot for calculation of enthalpy of formation ΔH of oxygen vacancies for La1 − xBaxCo0.2Fe0.8O3 − δ. Equilibrium δ values taken from TG measurements in air.
Fig. 7. High temperature Seebeck coefficient of La1 − xBaxCo0.2Fe0.8O3 − δ (0.2 ≤ x ≤ 0.8) perovskites.
Fig. 4. Mass loss related to the formation of oxygen vacancies measured by TG technique for La1 − xBaxCo0.2Fe0.8O3 − δ.
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
Fig. 8. High temperature electrical conductivity and Seebeck coefficient of BaCo0.2Fe0.8O3 − δ material.
441
Fig. 11. Temperature dependence of calculated values of Rohm and Ri. Inset shows equivalent circuit used for impedance data analysis.
100 h are gathered in Table 2. No secondary phases were observed for both cathode and electrolyte powders, suggesting good mutual stability of the materials in these conditions. Only slight increase of normalized unit cell volume for La0.6Ba0.4Co0.2Fe0.8O3 − δ after heatings at 800 °C and 1000 °C was recorded, comparing to the initial material (see Tables 1 and 2). Additionally, for the compound annealed with Ce0.8Gd0.2O1.9 at 800 °C, rhombohedral distortion present in the “as prepared” material disappeared, but it was again visible after annealing with the electrolyte at 1000 °C. The observed results may be explained taking into account possible cation exchange between phases and influence of the oxygen nonstoichiometry. Occurrence, to some degree, cation exchange reaction may be further supported by very small, but visible separation of XRD peaks of Ce0.8Gd0.2O1.9 material after annealing at 1000 °C. Summarizing, the observed degree of change of unit cell parameters in both cathode and electrolyte phases is relatively small and comparable to our previous studies for pure and Ni-doped LSCF perovskite oxides and ceria-based electrolytes [30]. Fig. 9. Current density–voltage and corresponding power density characteristics in 600–800 °C range of exemplary IT-SOFC cell with La0.6Ba0.4Co0.2Fe0.8O3 − δ based cathode.
Chemical stability of La0.6Ba0.4Co0.2Fe0.8O3 − δ in relation to Ce0.8Gd0.2O1.9 electrolyte was tested by XRD method. The obtained results of Rietveld refinements of mixtures containing electrolyte and cathode powders, which were heated at 800 °C and 1000 °C for
4. Conclusions Crystal structure, oxygen nonstoichiometry and transport properties of La1 − xBaxCo0.2Fe0.8O3 − δ perovskites were evaluated, revealing quite similar characteristics, comparing to La1 − xSrxCo0.2Fe0.8O3 − δ series. With increasing amount of barium, distortion of perovskite structure decreases, and disappears for materials with higher values of x. Interestingly, we were able to synthesize cubic BaCo0.2Fe0.8O3 − δ with δ≈0.4 mol/mol. Electrical conductivity of the synthesized LBCF compounds was found to be smaller than for LSCF materials with corresponding composition. The highest values of σ were measured for La0.6Ba0.4Co0.2Fe0.8O3 − δ oxide. This composition was selected as the cathode material for custom-made Table 2 Structural parameters evolution of La0.6Ba0.4Co0.2Fe0.8O3 − δ and Ce0.8Gd0.2O1.9 in the 50/50 wt.% mixture heated in air for 100 h at 800 °C and 1000 °C.
Fig. 10. Exemplary impedance spectra recorded for IT-SOFC with La0.6Ba0.4Co0.2Fe0.8O3 − δ based cathode.
c [Å]
Vp [Å3]
Composition
Heating temperature
a [Å]
Ce0.8Gd0.2O1.9 (Fm-3m) La0.6Ba0.4Co0.2Fe0.8O3 − δ (Pm-3m)a Ce0.8Gd0.2O1.9 (Fm-3m) La0.6Ba0.4Co0.2Fe0.8O3 − δ (R-3c)
800 °C 800 °C
5.4250(1) – 3.9180(1) –
1000 °C 1000 °C
5.4285(1) – 159.97(1) 5.5418(1) 13.5865(5) 361.36(2) 60.23(1)b
159.66(1) 60.14(1)
a After heating for 100 h at 800 °C small rhombohedral distortion present in the “as prepared” La0.6Ba0.4Co0.2Fe0.8O3 − δ sample disappeared. It was accompanied by a small increase of the unit cell volume: 60.14 Å3 in cubic, as compared to 359.50 / 6 = 59.92 Å3 (see Table 1) in rhombohedral phase. b Value normalized to cubic unit cell.
442
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
IT-SOFC cells, which electrochemical properties were evaluated in 600–800 °C range. The obtained maximum power density at 800 °C, however, did not exceed 0.14 W·cm− 2. Lack of significant structural changes of La0.6Ba0.4Co0.2Fe0.8O3 − δ and Ce0.8Gd0.2O1.9, as shown in chemical stability experiments, indicates rather good compatibility between the studied materials. However, further studies are needed to determine long-term stability of LBCF cathode material, especially in terms of its reactivity with CO2. Acknowledgment This work was supported by the AGH statutory grant no. 11.11.210.201. The authors acknowledge a financial support from the European Institute of Innovation and Technology, under the KIC InnoEnergy NewMat project. References [1] [2] [3] [4] [5]
E.V. Tsipis, V.V. Kharton, J. Solid State Electrochem. 12 (2008) 1367–1391. T. Ishihara (Ed.), Perovskite Oxide for Solid Oxide Fuel Cells, Springer, 2009. J. Molenda, K. Świerczek, W. Zając, J. Power Sources 173 (2007) 657–670. W. Zhou, R. Ran, Z. Shao, J. Power Sources 192 (2009) 231–246. J.W. Stevenson, T.R. Armstrong, R.D. Carneim, L.R. Pederson, W.J. Weber, J. Electrochem. Soc. 143 (1996) 2722–2729. [6] S. Diethelm, J. Van Herle, Solid State Ionics 174 (2004) 127–134. [7] Y. Zhen, S.P. Jiang, J. Power Sources 180 (2008) 695–703.
[8] S. Li, W. Jin, N. Xu, J. Shi, J. Mater. Sci. 35 (2000) 4329–4335. [9] S. Lee, Y. Lim, E.A. Lee, H.J. Hwang, J.-W. Moon, J. Power Sources 157 (2006) 848–854. [10] X. Ding, X. Kong, J. Jiang, C. Cui, X. Guo, Mater. Res. Bull. 45 (2010) 1271–1277. [11] X. Chen, L. Zhang, S.P. Jiang, J. Electrochem. Soc. 155 (2008) B1093–B1101. [12] Y. Zhen, A.I.Y. Tok, F.Y.C. Boey, S.P. Jiang, Electrochem. Solid-State Lett. 11 (2008) B42–B46. [13] M.S. Khalil, Z. Phys. Chem. 217 (2003) 1387–1398. [14] X. Li, X. Jiang, S. Pang, Q. Wang, Z. Su, Q. Zhang, Int. J. Hydrogen Energy 36 (2011) 13850–13857. [15] K. Świerczek, Solid State Ionics 179 (2008) 126–130. [16] K. Świerczek, M. Gozu, J. Power Sources 173 (2007) 695–699. [17] A.C. Larson, R.B. Von Dreele, Los Alamos Natl. Lab. Rep.—LAUR, 2004, pp. 86–748. [18] B.H. Toby, J. Appl. Crystallogr. 34 (2001) 210–213. [19] B. Dabrowski, O. Chmaissem, J. Mais, S. Kolesnik, J.D. Jorgensen, S. Short, J. Solid State Chem. 170 (2003) 154–164. [20] Q.L. Liu, K.A. Khor, S.H. Chan, J. Power Sources 161 (2006) 123–128. [21] K.S. Roh, K.H. Ryu, C.H. Yo, J. Mater. Sci. 30 (1995) 1245–1250. [22] J.M. Gonzalez-Calbet, M. Parras, M. Valletregi, J.C. Grenier, J. Solid State Chem. 86 (1990) 149–159. [23] A.J. Jacobson, Acta Crystallogr. B 32 (1976) 1087–1090. [24] K. Yamaura, H.W. Zandbergen, K. Abe, R.J. Cava, J. Solid State Chem. 146 (1999) 96–102. [25] M. Parras, A. Varela, H. Seehofer, J.M. González-Calbet, J. Solid State Chem. 120 (1995) 327–331. [26] K. Świerczek, B. Dabrowski, L. Suescun, S. Kolesnik, J. Solid State Chem. 182 (2009) 280–288. [27] K. Świerczek, J. Power Sources 196 (2011) 7110–7116. [28] A. Petric, P. Huang, F. Tietz, Solid State Ionics 135 (2000) 719–725. [29] J. Han, K. Zheng, K. Świerczek, Funct. Mater. Lett. 4 (2011) 151–155. [30] W. Zając, K. Świerczek, J. Molenda, J. Power Sources 173 (2007) 675–680.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
La1 − xBaxCo0.2Fe0.8O3 − δ perovskites for application in intermediate temperature SOFCs Bartłomiej Gędziorowski, Konrad Świerczek ⁎, Janina Molenda AGH University of Science and Technology, Faculty of Energy and Fuels, Department of Hydrogen Energy, al. A. Mickiewicza 30, 30‐059 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 9 September 2011 Received in revised form 17 May 2012 Accepted 25 May 2012 Available online 17 June 2012 Keywords: Crystal structure Transport properties Chemical stability LBCF IT-SOFC
a b s t r a c t Physicochemical properties of La1 − xBaxCo0.2Fe0.8O3 − δ (LBCF) oxides are presented in aspect of their possible application as cathode materials in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). The obtained results show that the increase of concentration of Ba2 + leads to an increase of unit cell volume and to a decrease of crystal structure distortion, which is accompanied by an increase of the level of oxygen nonstoichiometry in the studied compounds. For samples with x ≥ 0.6 cubic structure was observed, which is particularly worth noting for BaCo0.2Fe0.8O3 − δ composition, as both BaCoO3 − δ and BaFeO3 − δ often, but depending on oxygen nonstoichiometry δ, adopt hexagonal-type structure. Electrical conductivity and Seebeck coefficient data of the materials are quite similar, comparing to La1 − xSrxCo1 − yFeyO3 − δ oxides. For samples with barium content ≥0.6, at high temperatures, a clearly visible maximum on electrical conductivity dependence on temperature can be seen, which may be related to a significant increase of the oxygen nonstoichiometry. The highest conductivity in the intermediate temperature range (600–800 °C) was observed for La0.6Ba0.4Co0.2Fe0.8O3 − δ composition, however obtained values are significantly lower, comparing to strontium analog: La0.6Sr0.4Co0.2Fe0.8O3 − δ. La0.6Ba0.4Co0.2Fe0.8O3 − δ oxide was used as the cathode material in custom-made button-type IT-SOFCs, which properties were evaluated in 600–800 °C temperature range. Additionally, chemical stability of that material in relation to Ce0.8Gd0.2O1.9 electrolyte was studied. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A-site and B-site doped perovskites with A1 − xA'xB1 − xB'yO3 − δ general formula and various chemical compositions have attracted considerable attention due to their possible application as cathode materials in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) related to their mixed ionic-electronic conductivity and catalytic activity toward oxygen reduction [1–3]. In particular, La1 − xSrxCo1 − yFeyO3 − δ (LSCF) and Ba1 − xSrxCo1 − yFeyO3 − δ (BSCF) oxides with appropriate chemical composition, as well as other similar materials (e.g. cationordered perovskites, Ruddlesden–Popper oxides) show suitably high, mixed conductivity, high activity toward oxygen reduction, suitable thermomechanical properties and good chemical stability in relation to ceria-based or La1 − xSrxGa1 − yMgyO3 −(x + y) / 2 electrolytes [1,4]. It is believed that application of cathode materials with mixed ionicelectronic conductivity should be beneficial for operation of SOFCs in the intermediate temperature range, as in such case the cathodic reaction is not limited to the triple phase boundary, but is extended to the whole surface of the cathode [2,3]. Comparing to relatively well known LSCF and BSCF perovskites, La1 − xBaxCo1 − yFeyO3 − δ (LBCF) materials are less studied, and literature data, especially regarding their application and tests in SOFCs, are limited ⁎ Corresponding author. E-mail address: [email protected] (K. Świerczek). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.05.025
[5–9], nevertheless, there are also studies about La1 − x − zSrxBazCo1 − y FeyO3 − δ compounds [10,11]. The main advantage of application of LBCF cathode materials is related to their higher resistance for chromium poisoning [7,11,12], which makes them especially suitable for application in cells with Fe–Cr metallic interconnects. They, however, exhibit lower total electrical conductivity, comparing to LSCF analogs [5]. The electrical conductivity of LBCF oxides is strongly dependent on the chemical composition, i.e. the amount of barium and iron [5,13,14]. Generally, it can be stated that Fe-rich LBCF materials exhibit lower conductivity than Co-rich ones, and the maximum conductivity is observed for materials with comparable contents of La and Ba. Electrical properties of these compounds, depending on the chemical composition, must be interpreted taking into account complex relationship resulting from coexistence of Fe3 +, Fe4 +, Co3 +, Co4 + with preferential oxidation of Fe [14], composition and temperature-dependent concentration of oxygen vacancies, and composition-dependent changes of crystal symmetry and unit cell parameters. The oxygen vacancies, concentration of which increases significantly for Ba-rich samples, especially at higher temperatures [5], are responsible for an apparent decrease of the average oxidation state of 3d metals M, which may be described by the following reaction: x x 2B•B þ OO →2BB þ V•• O þ 1=2O2 ↑:
ð1Þ
Significant changes of the concentration of oxygen vacancies are a cause of so called chemical expansion effect [14], which was also observed for LSCF oxides [15]. However, until now there are no systematic
438
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
studies showing oxygen nonstoichiometry, thermal expansion and electrical conductivity in a wide temperature range for all materials from La1 − xBaxCo1 − yFeyO3 − δ system. There is also lack of detailed information regarding chemical stability of these materials in relation to common solid electrolytes and their stability in CO2 and H2O containing atmospheres, especially that materials with high Ba content may suffer from limited stability in such atmospheres. The aim of this work is to evaluate physicochemical properties in the La1 − xBaxCo0.2Fe0.8O3 − δ series (crystal structure, electrical properties, changes of the oxygen stoichiometry) from the point of view of possible application of such materials in IT-SOFCs. Among examined LBCF compounds one material was chosen (La0.6Ba0.4Co0.2Fe0.8O3 − δ) and its chemical reactivity toward Ce0.8Gd0.2O1.9 electrolyte and electrochemical properties in IT-SOFCs were studied. 2. Experimental La1 − xBaxCo0.2Fe0.8O3 − δ (0 ≤ x ≤ 1) perovskites were synthesized using soft chemistry method. High purity (≥99.9%) La, Ba, Co and Fe nitrates were dissolved in stoichiometric proportions in minimal amount of deionized water. After dissolution, ammonium salt of ethylenediaminetetraacetic acid (EDTA) was added as a complexing agent. The solutions were heated in quartz evaporator, until sol–gel transition, decomposition of excess of ammonium nitrate and oxidation of residual carbon were observed. Further details of the procedure can be found elsewhere [16]. Depending on the chemical composition, the final annealings were performed at 900 °C for BaCo0.2Fe0.8O3 − δ, at 1100 °C for La0.2Ba0.8Co0.2Fe0.8O3 − δ and at 1200 °C for other materials. Additional annealing for La0.2Ba0.8Co0.2Fe0.8O3 − δ was performed at 1300 °C. Crystal structure of the samples was identified using XRD technique with Panalytical X'PERT Pro diffractometer in 10–90° range using CuKα radiation. Obtained data were refined using Rietveld method and GSAS/EXPGUI set of software [17,18]. Goldschmidt tolerance factor tS was calculated on the basis of Shannon's ionic radii using well known equation: tS = (rA + rO) / (20.5 · (rB + rO)), where r stands for ionic radius of respectively: A and B cations and oxygen in ABO3 perovskite. A following simplifications were assumed: oxidation state of cobalt and iron cations is +3 and +4 with ratio depending directly on the Ba 2 + content; oxidation of both cations in not preferential; all cations possess high spin configuration. An influence of oxygen nonstoichiometry on tolerance factor was also considered: in case of tδ, the oxidation state of Co and Fe cations was appropriately lowered, in order to maintain charge balance in La1 − xBaxCo1 − yFeyO3 − δ. This caused a decrease of the calculated values of tδ, as compared to tS. In another approach for determination of tolerance factor, the actual LaBa–O and CoFe–O distances, refined from XRD data, were used instead of ionic radii. Such approach shows real values of t (marked in this work as tR) and was previously presented in literature for other perovskite materials [19]. Transport properties of the studied compounds were characterized by the electrical conductivity σ (4-probe DC) and thermoelectric power α (dynamical method, with up to 2–3 °C temperature gradient) measurements in 25–900 °C temperature range. The measurements were carried out simultaneously. For determination of Seebeck coefficient α, measured values of thermoelectric voltage as a function of temperature gradient on the sample were plotted, with α being calculated as a slope of this dependence. Thermogravimetric (TG) studies were performed in synthetic air and in 0.1% H2 in Ar mixture using TA Q5000IR thermogravimetric apparatus with 1 °C/min heating/cooling rate. Equilibrium values of the oxygen nonstoichiometry δ, calculated from TG measurements in air and Ref. [5], allowed for evaluation of the enthalpy of formation of oxygen vacancies (ΔH). Values of ΔH were calculated for materials with x ≥ 0.4 from the linear portion of logδ dependence on T− 1. Button-type electrolyte-supported IT-SOFC cells were constructed using Ni-8YSZ cermet anode, Ce0.8Gd0.2O1.9 electrolyte and La0.6Ba0.4Co0.2Fe0.8O3 − δ cathode material. Gas tight electrolyte pellets with about 0.7 mm thickness were sintered at 1500 °C. Typical,
40 wt.% 8YSZ–60 wt.% NiO paste was used for preparation of the anode. The paste was painted on one side of the electrolyte sinter and fired at 1400 °C in air, with additional step at 400 °C, at which organic binder from the paste decomposes. Cathode paste was prepared by mixing thoroughly grinded La0.6Ba0.4Co0.2Fe0.8O3 − δ with appropriate amount (about 1:2 wt. ratio) of commercial texanol-based binder (ESL ElectroScience) in order to obtain desired consistence. After painting, the cathode paste was fired at 1100 °C, with additional step at 400 °C. Area of both electrodes was approx. 0.25 cm2 with thickness of about 0.3 mm. Such cells were sealed using glass sealer to alumina tube and mounted in testing unit. Platinum wires were used as current collectors. The anode was fuelled with 10 cm3·min− 1 flow of dry hydrogen using mass-flow controller. At the cathode side a constant flow of air was provided. Impedance spectra and the current–voltage characteristics of the cells were measured in the 600–800 °C temperature range using custom-made SOFC testing unit equipped with Solartron SI 1287 electrochemical interface and Solartron 1252A frequency response analyzer. The impedance spectroscopy measurements were performed in 0.1– 300 kHz range with 25 mV amplitude at open circuit conditions. Interpretation of the results was done in the same way as presented in [20]: the ohmic resistance Rohm and the total interfacial polarization resistance Ri were evaluated by fitting the impedance spectra to an equivalent circuit consisting of L–Rohm–(RQ)HF–(RQ)LF, where L is an inductance, R—resistance, Q—constant phase element, while HF and LF stand for respectively high-frequency and low-frequency contributions. 3. Results and discussion The performed XRD studies showed that with the increasing amount of Ba in La1 − xBaxCo0.2Fe0.8O3 − δ, crystal structure changes from lower symmetry orthorhombic through rhombohedral to cubic one. This behavior is analogous to the one found in La1 − xSrxCo0.2Fe0.8O3 − δ compounds [16]. Detailed structural data for synthesized LBCF materials are presented in Table 1. Rietveld refinement of XRD data for La0.8Ba0.2Co0.2Fe0.8O3 − δ composition revealed coexistence of orthorhombic and rhombohedral phases, with 50:50 wt.% ratio. Different heating temperatures (1200 °C and 1300 °C) did not have any visible influence on the structural properties of this compound. In case of La0.6Ba0.4Co0.2Fe0.8O3 − δ material, the observed rhombohedral distortion is minimal (Fig. 1), but is supported by better statistics of Rietveld refinement, comparing to refinement performed for Pm-3m space group (Fig. 1 inset). Interestingly, we were able to obtain single phase BaCo0.2Fe0.8O3 − δ with cubic symmetry (Fig. 1). In case of endmembers: BaFeO3 − δ and BaCoO3 − δ hexagonal-type structure is often observed, but depending on nonstoichiometry δ, the materials can have perovskite-like structure or be mixtures of phases [21–25]. Analyzing these data, the presence of cubic symmetry in BaCo0.2Fe0.8O3 − δ can be attributed to a very high concentration of oxygen vacancies of the order of 0.4 mol/mol. This was confirmed by TG reduction experiments performed in 0.1 H2 in Ar atmospheres: at 900 °C sample decomposed into Ba3Fe2O6, Ba2Fe2O5 and Fe (as confirmed by XRD studies), with molar ratio indicating initial δ ≈0.4 mol/mol. In Fig. 2 the dependence of normalized to simple perovskite unit cell volume, as well as calculated and refined values of Goldschmidt's tolerance factor t are shown. As can be noticed, there is a rather big discrepancy between the calculated (tS, tδ, see Fig. 2) and the refined (tR) values of tolerance factor, with tR values being 1 or almost 1 for all the materials. This can be expected, as Shannon's ionic radii do not properly describe non purely ionic compounds, and was observed in LSCF perovskites [26]. The nonlinear dependence of normalized unit cell volume on composition can be rationalized on the basis of two effects: the first one is related to an increase of average ionic radii of La1 − xBax with the increase of x (bigger Ba2 + cations substituting smaller La3 + cations), which however causes an increase of the average oxidation state, and therefore decrease of radii of 3d metals; the second effect is related to the oxygen nonstoichiometry, which decreases average oxidation state of Co and Fe
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
439
Table 1 Structural parameters of synthesized La1 − xBaxCo0.2Fe0.8O3 − δ perovskites. Composition
Space group
a [Å]
b [Å]
c [Å]
Vp [Å3]
δ [mol/mol] at 300 K
LaCo0.2Fe0.8O3 − δ La0.8Ba0.2Co0.2Fe0.8O3 − δ
Pbnm 50 wt.% Pbnm 50 wt.% R-3c R-3c (almost cubic) Pm-3mb Pm-3mb Pm-3m
5.5123(1) 5.5200(2) 5.5466(1) 5.5422(4) 3.9339(1) 3.9846(1) 4.0721(1)
5.5334(2) 5.5585(2) – – – – –
7.8013(1) 7.8090(2) 13.4845(4) 13.5148(15) – – –
237.95(4) 239.60(4) 359.27(6) 359.50(6) 60.88(1) 63.26(1) 67.52(1)
0a 0a
La0.6Ba0.4Co0.2Fe0.8O3 − δ La0.4Ba0.6Co0.2Fe0.8O3 − δ La0.2Ba0.8Co0.2Fe0.8O3 − δ BaCo0.2Fe0.8O3 − δ a b c
0.05a 0.1a 0.25a ~ 0.4c
Values of oxygen nonstoichiometry were taken from Ref. [5], but were also confirmed by our thermogravimetric experiments. Secondary phase (b3 wt.%) A2BO4 type with I4/mmm symmetry was present in the samples. Approximate value obtained from thermogravimetric reduction experiment.
(Eq. (1)). This can be followed in greater detail in Fig. 3, in which refined interatomic distances for LaBa–O and also for CoFe–O are presented. While bLaBa–O>g increases with the increasing x in whole range of chemical composition, in case of bCoFe–O>g, the minimum can be noticed for intermediate compositions. Unfortunately, for materials with orthorhombic structure, due to low sensitivity of XRD technique for oxygen, the obtained data is not precise enough to evaluate bCoFe–O>g with higher accuracy. Also, in case of calculated values of lattice free volume (Fig. 3), a minimum can be noticed for samples with x = 0.4 and 0.6. Results of the performed thermogravimetric studies are presented in Fig. 4. With the increasing amount of barium, mass loss, which can be related to the formation of oxygen vacancies, increases. Nevertheless, for materials with x ≥0.6 the temperature, at which such loss begins remains almost constant and is close to 300 °C. For BaCo0.2Fe0.8O3 − δ compound mass loss occurs also around 100 °C. These results together with the literature data [5], regarding values of the oxygen nonstoichiometry δ present in the samples at room temperature (Table 1), allowed for calculation of enthalpy of formation of oxygen vacancies ΔH (Fig. 5). With the increasing barium content, ΔH decreases significantly and for materials with x= 0.8 and 1 is of the order of 0.04– 0.05 eV. Similar conclusion may be drawn analyzing TG data published for various LBCF and LSCF oxides [5]. This is a clear proof that the increase of concentration of Ba2 + (or Sr2 + in the case of LSCF oxides) favors formation of oxygen vacancies, which in consequence influence average oxidation state of cobalt and iron (Eq. (1)). For comparison, previous studies indicate ΔH = 0.28 eV for La0.5Ba0.5Co0.5Fe0.5O3 − δ composition [27]. It is known that for oxygen vacancy-related ionic conduction activation energy consists of two terms: activation energy for vacancy diffusion and the second term related (but not equivalent) to the enthalpy
of formation. Interestingly, the calculated values of ΔH match well the difference between total activation energy of ionic conductivity and diffusion-related activation energy published for respective LBCF oxides in work [5], for which an increase of the ionic conductivity with increasing barium content was also shown. Fig. 6 shows total electrical conductivity of measured materials with 0.2 ≤x ≤ 0.8. Because ionic component of the conductivity is more than 2 orders of magnitude lower that the electronic component [5], the presented values can be identified with the electronic conductivity. Similarly to LSCF compounds, with the increase of x conductivity first increases and then decreases, reflecting changes of the average oxidation state of cobalt and iron [5,28]. Additionally, barium rich samples show maximum on the electrical conductivity dependence on temperature. The appearance of these maxima can be related to the formation of oxygen vacancies at elevated temperatures and that effect is very similar to the one observed for LSCF perovskites [3]. The highest values of the electrical conductivity were measured for La0.6Ba0.4Co0.2Fe0.8O3 − δ, but nevertheless, were less than 100 S·cm− 1, which is substantially lower, comparing to La0.6Sr0.4Co0.2Fe0.8O3 − δ [3] and comparing to other cobalt-rich LBCF oxides, for which conductivities exceeding 500 S·cm− 1 were observed [14]. In general, it may be stated that studied LBCF compounds exhibit lower electrical conductivity comparing to respective LSCF materials. Seebeck coefficient measurement (Fig. 7) indicates that in case of all the materials the dominant charge carriers are holes (positive sign of α). For oxides with x= 0.6 and 0.8 at temperatures exceeding 400 °C an increase of α can be noticed, which corresponds to the increase of the oxygen vacancy concentration. Similar behavior was previously observed for LSCF oxides [16]. Interesting characteristics of σ(T) were measured for BaCo0.2Fe0.8O3 − δ material (Fig. 8). On the two consecutive heating and cooling cycles unexpected features appear at around 650 °C
Fig. 1. Exemplary XRD patterns with Rietveld refinement for La0.6Ba0.4Co0.2Fe0.8O3 − δ and BaCo0.2Fe0.8O3 − δ perovskites.
Fig. 2. Calculated (tS using Shannon's ionic radii, tδ with correction for oxygen nonstoichiometry) and refined (tR as geometric average using interatomic distances) values of tolerance factor, together with normalized unit cell volume as a function of barium content in La1 − xBaxCo0.2Fe0.8O3 − δ.
440
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
Fig. 3. Interatomic distances and lattice free volume as a function of barium content in La1 − xBaxCo0.2Fe0.8O3 − δ. Distances were calculated as geometric averages.
Fig. 6. High temperature electrical conductivity of La1 − xBaxCo0.2Fe0.8O3 − δ (0.2≤ x ≤ 0.8) perovskites.
and 750 °C. We believe that they are related to the existence of structural phase transition, but further studies are needed to explain this phenomenon. La0.6Ba0.4Co0.2Fe0.8O3 − δ composition was chosen as the cathode material for preparation of custom-made electrolyte-supported IT-SOFC cells. This material exhibits the highest electrical conductivity among studied LBCF oxides, low nonstoichiometry at room temperature
and moderate changes of δ at elevated temperatures (Table 1, Fig. 6). These features are beneficial in terms of application, due to expected low resistance associated with the cathode and small influence of chemical expansion on the total thermal expansion of the material, which, in turn, helps with preparation of the cathode layers on the electrolyte. Fig. 9 shows performance of an exemplary IT-SOFC cell. At 800 °C the maximum power density reaches 0.14 W·cm − 2, which however is a rather low value, comparing to literature data. Impedance spectroscopy studies (Figs. 10 and 11) revealed that in the whole temperature range (600–800 °C) the cell's power output is limited by its charge transfer and diffusional resistances, summarized as Ri. This is in opposition to our previous studies, showing much smaller values of Ri, which are lower than ohmic resistance (Rohm) at high temperatures, in case of similarly constructed cells with La0.5Sr0.5Co0.5Fe0.5O3 − δ or Pr2NiO4 + δ based cathodes [27,29]. While the sintered cathodes were not especially optimized in terms of their microstructural features, it can be stated that such low power densities may be rather originate from not sufficient catalytic activity of La0.6Ba0.4Co0.2Fe0.8O3 − δ for the oxygen reduction reaction. In order to obtain higher power outputs of constructed IT-SOFCs it would be necessary to decrease activation and diffusion related polarizations considerably. Unfortunately, due to thermomechanical incompatibility we were not able to sinter cathode layers using LBCF materials with x ≥ 0.6, which are expected to possess higher catalytic activity. Their application would require preparation of intermediate layers, which allow for better matching of the expansion between used materials.
Fig. 5. Arrhenius-type plot for calculation of enthalpy of formation ΔH of oxygen vacancies for La1 − xBaxCo0.2Fe0.8O3 − δ. Equilibrium δ values taken from TG measurements in air.
Fig. 7. High temperature Seebeck coefficient of La1 − xBaxCo0.2Fe0.8O3 − δ (0.2 ≤ x ≤ 0.8) perovskites.
Fig. 4. Mass loss related to the formation of oxygen vacancies measured by TG technique for La1 − xBaxCo0.2Fe0.8O3 − δ.
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
Fig. 8. High temperature electrical conductivity and Seebeck coefficient of BaCo0.2Fe0.8O3 − δ material.
441
Fig. 11. Temperature dependence of calculated values of Rohm and Ri. Inset shows equivalent circuit used for impedance data analysis.
100 h are gathered in Table 2. No secondary phases were observed for both cathode and electrolyte powders, suggesting good mutual stability of the materials in these conditions. Only slight increase of normalized unit cell volume for La0.6Ba0.4Co0.2Fe0.8O3 − δ after heatings at 800 °C and 1000 °C was recorded, comparing to the initial material (see Tables 1 and 2). Additionally, for the compound annealed with Ce0.8Gd0.2O1.9 at 800 °C, rhombohedral distortion present in the “as prepared” material disappeared, but it was again visible after annealing with the electrolyte at 1000 °C. The observed results may be explained taking into account possible cation exchange between phases and influence of the oxygen nonstoichiometry. Occurrence, to some degree, cation exchange reaction may be further supported by very small, but visible separation of XRD peaks of Ce0.8Gd0.2O1.9 material after annealing at 1000 °C. Summarizing, the observed degree of change of unit cell parameters in both cathode and electrolyte phases is relatively small and comparable to our previous studies for pure and Ni-doped LSCF perovskite oxides and ceria-based electrolytes [30]. Fig. 9. Current density–voltage and corresponding power density characteristics in 600–800 °C range of exemplary IT-SOFC cell with La0.6Ba0.4Co0.2Fe0.8O3 − δ based cathode.
Chemical stability of La0.6Ba0.4Co0.2Fe0.8O3 − δ in relation to Ce0.8Gd0.2O1.9 electrolyte was tested by XRD method. The obtained results of Rietveld refinements of mixtures containing electrolyte and cathode powders, which were heated at 800 °C and 1000 °C for
4. Conclusions Crystal structure, oxygen nonstoichiometry and transport properties of La1 − xBaxCo0.2Fe0.8O3 − δ perovskites were evaluated, revealing quite similar characteristics, comparing to La1 − xSrxCo0.2Fe0.8O3 − δ series. With increasing amount of barium, distortion of perovskite structure decreases, and disappears for materials with higher values of x. Interestingly, we were able to synthesize cubic BaCo0.2Fe0.8O3 − δ with δ≈0.4 mol/mol. Electrical conductivity of the synthesized LBCF compounds was found to be smaller than for LSCF materials with corresponding composition. The highest values of σ were measured for La0.6Ba0.4Co0.2Fe0.8O3 − δ oxide. This composition was selected as the cathode material for custom-made Table 2 Structural parameters evolution of La0.6Ba0.4Co0.2Fe0.8O3 − δ and Ce0.8Gd0.2O1.9 in the 50/50 wt.% mixture heated in air for 100 h at 800 °C and 1000 °C.
Fig. 10. Exemplary impedance spectra recorded for IT-SOFC with La0.6Ba0.4Co0.2Fe0.8O3 − δ based cathode.
c [Å]
Vp [Å3]
Composition
Heating temperature
a [Å]
Ce0.8Gd0.2O1.9 (Fm-3m) La0.6Ba0.4Co0.2Fe0.8O3 − δ (Pm-3m)a Ce0.8Gd0.2O1.9 (Fm-3m) La0.6Ba0.4Co0.2Fe0.8O3 − δ (R-3c)
800 °C 800 °C
5.4250(1) – 3.9180(1) –
1000 °C 1000 °C
5.4285(1) – 159.97(1) 5.5418(1) 13.5865(5) 361.36(2) 60.23(1)b
159.66(1) 60.14(1)
a After heating for 100 h at 800 °C small rhombohedral distortion present in the “as prepared” La0.6Ba0.4Co0.2Fe0.8O3 − δ sample disappeared. It was accompanied by a small increase of the unit cell volume: 60.14 Å3 in cubic, as compared to 359.50 / 6 = 59.92 Å3 (see Table 1) in rhombohedral phase. b Value normalized to cubic unit cell.
442
B. Gędziorowski et al. / Solid State Ionics 225 (2012) 437–442
IT-SOFC cells, which electrochemical properties were evaluated in 600–800 °C range. The obtained maximum power density at 800 °C, however, did not exceed 0.14 W·cm− 2. Lack of significant structural changes of La0.6Ba0.4Co0.2Fe0.8O3 − δ and Ce0.8Gd0.2O1.9, as shown in chemical stability experiments, indicates rather good compatibility between the studied materials. However, further studies are needed to determine long-term stability of LBCF cathode material, especially in terms of its reactivity with CO2. Acknowledgment This work was supported by the AGH statutory grant no. 11.11.210.201. The authors acknowledge a financial support from the European Institute of Innovation and Technology, under the KIC InnoEnergy NewMat project. References [1] [2] [3] [4] [5]
E.V. Tsipis, V.V. Kharton, J. Solid State Electrochem. 12 (2008) 1367–1391. T. Ishihara (Ed.), Perovskite Oxide for Solid Oxide Fuel Cells, Springer, 2009. J. Molenda, K. Świerczek, W. Zając, J. Power Sources 173 (2007) 657–670. W. Zhou, R. Ran, Z. Shao, J. Power Sources 192 (2009) 231–246. J.W. Stevenson, T.R. Armstrong, R.D. Carneim, L.R. Pederson, W.J. Weber, J. Electrochem. Soc. 143 (1996) 2722–2729. [6] S. Diethelm, J. Van Herle, Solid State Ionics 174 (2004) 127–134. [7] Y. Zhen, S.P. Jiang, J. Power Sources 180 (2008) 695–703.
[8] S. Li, W. Jin, N. Xu, J. Shi, J. Mater. Sci. 35 (2000) 4329–4335. [9] S. Lee, Y. Lim, E.A. Lee, H.J. Hwang, J.-W. Moon, J. Power Sources 157 (2006) 848–854. [10] X. Ding, X. Kong, J. Jiang, C. Cui, X. Guo, Mater. Res. Bull. 45 (2010) 1271–1277. [11] X. Chen, L. Zhang, S.P. Jiang, J. Electrochem. Soc. 155 (2008) B1093–B1101. [12] Y. Zhen, A.I.Y. Tok, F.Y.C. Boey, S.P. Jiang, Electrochem. Solid-State Lett. 11 (2008) B42–B46. [13] M.S. Khalil, Z. Phys. Chem. 217 (2003) 1387–1398. [14] X. Li, X. Jiang, S. Pang, Q. Wang, Z. Su, Q. Zhang, Int. J. Hydrogen Energy 36 (2011) 13850–13857. [15] K. Świerczek, Solid State Ionics 179 (2008) 126–130. [16] K. Świerczek, M. Gozu, J. Power Sources 173 (2007) 695–699. [17] A.C. Larson, R.B. Von Dreele, Los Alamos Natl. Lab. Rep.—LAUR, 2004, pp. 86–748. [18] B.H. Toby, J. Appl. Crystallogr. 34 (2001) 210–213. [19] B. Dabrowski, O. Chmaissem, J. Mais, S. Kolesnik, J.D. Jorgensen, S. Short, J. Solid State Chem. 170 (2003) 154–164. [20] Q.L. Liu, K.A. Khor, S.H. Chan, J. Power Sources 161 (2006) 123–128. [21] K.S. Roh, K.H. Ryu, C.H. Yo, J. Mater. Sci. 30 (1995) 1245–1250. [22] J.M. Gonzalez-Calbet, M. Parras, M. Valletregi, J.C. Grenier, J. Solid State Chem. 86 (1990) 149–159. [23] A.J. Jacobson, Acta Crystallogr. B 32 (1976) 1087–1090. [24] K. Yamaura, H.W. Zandbergen, K. Abe, R.J. Cava, J. Solid State Chem. 146 (1999) 96–102. [25] M. Parras, A. Varela, H. Seehofer, J.M. González-Calbet, J. Solid State Chem. 120 (1995) 327–331. [26] K. Świerczek, B. Dabrowski, L. Suescun, S. Kolesnik, J. Solid State Chem. 182 (2009) 280–288. [27] K. Świerczek, J. Power Sources 196 (2011) 7110–7116. [28] A. Petric, P. Huang, F. Tietz, Solid State Ionics 135 (2000) 719–725. [29] J. Han, K. Zheng, K. Świerczek, Funct. Mater. Lett. 4 (2011) 151–155. [30] W. Zając, K. Świerczek, J. Molenda, J. Power Sources 173 (2007) 675–680.