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Oxide ion transport in undoped and Cr-doped LaCoO3−δ
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Solid State Ionics 178 (2007) 1458 – 1462 www.elsevier.com/locate/ssi
Oxide ion transport in undoped and Cr-doped LaCoO3−δ D.S. Tsvetkov, A.Yu. Zuev ⁎, A.I. Vylkov, A.N. Petrov Department of Chemistry, Ural State University, Lenin av.51, 620083 Yekaterinburg, Russia Received 12 January 2007; received in revised form 9 July 2007; accepted 26 August 2007
Abstract The results of chemical and self-diffusion coefficient of oxygen vacancies, oxygen ionic conductivity and ionic transport numbers measured as a function of oxygen partial pressure pO2 and temperature in the ranges − 4 ≤ log(pO2, atm) ≤ 0 and 900 ≤ T, °C ≤ 1050, respectively, are presented for the perovskite-type undoped LaCoO3−δ and doped with chromium LaCo0.7Cr0.3O3−δ cobaltites. The dependencies of these properties on oxygen partial pressures were shown to have monotonic character. This can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete pO2 range investigated. The predominant charge carriers in both undoped and Cr-doped lanthanum cobaltite are electron defects, since oxygen vacancies transport numbers do not exceed 2 · 10−3%. The substitution of Cr for Co was found to lead to decrease of both ionic conductivity and oxygen chemical diffusion coefficient. Activation energy of ionic conductivity of the oxides studied decreases with the oxygen nonstoichiometry increase. Self-diffusion coefficient of oxygen vacancies and their mobility were shown to be independent of oxygen partial pressure and nonstoichiometry most likely due to lack of the defects interaction in the oxide studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Lanthanum cobaltite; Polarization; Oxygen chemical diffusion coefficient; Oxygen ionic conductivity; Self-diffusion coefficient
1. Introduction Mixed ionic- and electronic-conducting perovskite-type oxides are the-state-of-the-art materials for high temperature electrochemical devices such as solid oxide fuel cells (SOFCs) [1–3], oxygen membranes [4–6]. They also find an application as cathodes [7] for CO2-lasers and catalysts [8,9]. Many of such materials are lanthanum cobaltite-based oxides. Undoped and doped lanthanum cobaltites were extensively studied since the 1950s [4,10–17]. However, the data on such key properties as oxygen ionic conductivity and chemical diffusion coefficient of oxygen vacancies are very restricted to date. Oxygen ionic conductivity of undoped LaCoO3 was measured [5,18], but results obtained are not consistent with each other despite the same oxygen permeation technique was used in both studies. Oxygen chemical diffusion was studied by Ishigaki et al. [19]
and Fueki et al. [20] in undoped LaCoO3 using isotope exchange and thermogravimetric methods, respectively, but these authors did not study an oxygen partial pressure dependence of oxygen chemical diffusion coefficient. The polarization technique first introduced by Wagner [21,22] and Yokota [23] allows to determine both oxygen chemical diffusion coefficient and oxygen ionic conductivity. Recently Bucher et al. [24] described the experimental polarization set up suitable for perovskite oxides, which have tendency to react with yttria stabilized zirconia-based blocking electrodes. The main goal of the present work was to determine oxygen chemical diffusion coefficient and oxygen ionic conductivity of undoped LaCoO3 and Cr-doped LaCr0.3Co0.7O3 cobaltites as a function of oxygen partial pressure at different temperatures by means of polarization measurements. 2. Experimental
⁎ Corresponding author. Tel.: +7 343 251 7927; fax: +7 343 261 5978. E-mail address: [email protected] (A.Y. Zuev). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.08.005
Powder samples of LaCoO3 and LaCo0.7Cr0.3O3 were prepared by pyrolysis of the corresponding polymer-salt
D.S. Tsvetkov et al. / Solid State Ionics 178 (2007) 1458–1462
1459
compositions according to the technique described elsewhere [25]. The phase composition of all samples prepared was determined by means of X-ray diffraction, XRD, with a Dron-6 diffractometer using Cu Kα radiation. XRD showed no indication for the presence of a second phase. The single phase powders were pressed into tablets of 6 mm diameter and 0.9 mm thickness and then sintered at 1500 °C for 24 h in air. Relative density of sintered samples was 95%. For the determination of chemical diffusion coefficient and ionic conductivity the following electrochemical cell was used: O2 ; PtjZrO2 ðY2 O3 ÞjPtporous jsamplejPtporous jZrO2 ðY2 O3 ÞjPt; O2 ð1Þ The scheme of the experimental setup is shown in Fig. 1. The sample is located between two ionic conducting electrodes (O2,Pt/ZrO2(Y2O3)). Specimen is polarized by passing a constant current through the ionic electrodes. In order to prevent any side reaction between the sample and YSZ both surfaces of the sample were covered with thin porous platinum layers. Internal platinum layers were also used to measure oxygen partial pressure inside the cell. The whole device was sealed with a glass to prevent direct oxygen penetration from the ambient atmosphere. The mixture of 30 wt.% BaO and 70 wt.% magnesium silicate Mg3H2Si4O12 was applied as a glass. In order to make sure that this glass does not react with lanthanum cobaltites investigated calculation of equilibrium composition was implemented using the module Equilibrium of the thermodynamic software FactSage 5.3.1. This calculation showed obviously that there are no any equilibrium products in the temperature range investigated except MgSiO3, SiO2, BaO, and LaCoO3.
Fig. 2. Typical depolarization curves of the cell (1).
Polarization Up and depolarization (relaxation) Ud voltages of aforementioned cell (1) can be expressed according to [22– 24] as follows (long time solution of Fick's second law): jL jLte 8 Up ðtÞ ¼ þ expðt=sÞ ð2Þ ri ri p2 and jLte 8 Ud ðtÞ ¼ expðt=sÞ ; ri p2
ð3Þ
where j, L, te are current density, sample thickness, and electronic defects transport number, respectively. τ is a relaxap2 tion time defined as s ¼ 2 ~ and t is time. D˜ is oxygen vacancies LD
chemical diffusion coefficient. As can be seen ~ ln(Ud −U∞) vs. 1 p2D t is straight line with slope ratio k ¼ s ¼ L2 and intercept A e on y-axis defined as A ¼ ln 8jLt ri p2 . Oxygen vacancies chemical diffusion coefficient and oxygen ionic conductivity can be, therefore, calculated from the plot ln(Ud − U∞) vs. t. Nevertheless, it should be noted that calculation of oxygen vacancies self-diffusion coefficient and, therefore, oxygen ionic conductivity according to Eqs. (4) and (5) is more suitable than that on the basis of Eqs. (2) and (3), since chemical diffusion coefficient and oxygen ionic conductivity are calculated from the slope and intercept, respectively, of the plot ln(Ud − U∞) vs. t, but accuracy of the slope determination is higher than that of the intercept. ~ DVO•• ¼ DVO••
Aln PO2 ; Aln½VO•• T¼const
ð4Þ
~ where D VO•• and DVO•• are oxygen vacancies chemical diffusion and self-diffusion coefficient, respectively. ri ¼
Fig. 1. The scheme of the polarization cell used for measurements: 1) glass sealing, 2) sample, 3) YSZ electrode, 4) porous platinum layers.
ðzi FÞ2 Di B d ¼ zi FUi Bd; RT
ð5Þ
where δ, B, σi, zi = 2, F, Ui are oxygen nonstoichiometry, a conversion constant, oxide ion conductivity, oxygen vacancy charge, Faraday constant, oxygen vacancies mobility, respectively.
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Fig. 3. Isothermal dependences of chemical diffusion coefficient and ionic conductivity of oxygen vacancies for LaCoO3−δ.
AlnP Derivative Aln½VO••2 was determined from the isotherO T ¼const mal dependencies of oxygen nonstoichiometry vs. oxygen partial pressure measured in our previous works by coulometric titration technique [27,29]. Other advantage of the choice of a slope coefficient for calculations consists in the opportunity to ignore even a weak interaction between a glass and a sample. Indeed the slope coefficient contains only the sample length, which does not depend on this interaction and remains, therefore, a constant during the measurements. The polarization current 0.1–0.3 mA/cm2 was used in the present work and voltage drop on the cell did not exceed 10– 100 mV. Depolarization voltage was measured by Agilent 34401A voltmeter.
3. Results and discussion Fig. 2 shows typical depolarization curves of cell (1) with sample and without it in the “U–t”coordinates and with sample
Fig. 4. Isothermal dependences of chemical diffusion coefficient and ionic conductivity of oxygen vacancies for LaCo0.7Cr0.3O3−δ on oxygen partial pressure.
Fig. 5. Oxygen nonstoichiometry dependences of oxygen ionic conductivity of LaCoO3−δ at different temperatures.
in the “ln|Ud − U∞| − t” coordinates, respectively. It is obvious from the comparison of “U–t” plots for cell (1) with and without sample that YSZ electrodes and porous platinum layers do not affect the total polarization or depolarization and oxygen vacancies chemical diffusion coefficient and oxygen ionic conductivity of the sample can be, therefore, calculated. Fig. 2 also shows that plot ln|Ud − U∞| vs. t is a straight line when t → ∞. The latter is in full agreement with Eq. (3). Figs. 3 and 4 show results of polarization measurements. As seen oxygen chemical diffusion coefficient and oxygen ionic conductivity increase with temperature and decrease with oxygen partial pressure, since oxygen vacancies concentration increases at the same time [26–29]. From the comparison of these figures it follows that a substitution of Cr for Co in LaCoO3−δ leads to significant decrease of oxygen ionic conductivity. This seems to be explained, first of all, by the particularities of the defect structure of the oxides investigated. Recently the doping with chromium was shown by Vylkov [27] to result in oxygen vacancies concentration drop in LaCo0.7Cr0.3O3−δ as compared to LaCoO3−δ at the same oxygen partial pressure and temperature. This drop, in turn, leads to oxygen ionic conductivity depreciation according to Eq. (5).
Fig. 6. Oxygen nonstoichiometry dependences of oxygen ionic conductivity of LaCoO3−δ and LaCo0.7Cr0.3O3−δ at 1000 °C.
D.S. Tsvetkov et al. / Solid State Ionics 178 (2007) 1458–1462
Fig. 7. Temperature dependences of self-diffusion coefficient and mobility of oxygen vacancies in LaCoO3−δ and LaCo0.7Cr0.3O3−δ.
Oxygen ionic conductivity of LaCoO3−δ is replotted vs. oxygen nonstoichiometry in Fig. 5 at all temperatures investigated. For the sake of comparison oxygen ionic conductivity of both LaCoO3−δ and LaCo0.7Cr0.3O3−δ is given in Fig. 6 at 1000 °C. As seen in this figure oxygen ionic conductivity of LaCoO3−δ exceeds that of Cr-doped cobaltite over the whole oxygen nonstoichiometry range investigated. This indicates obviously that the oxygen vacancies mobility of undoped LaCoO3−δ is larger then that of Cr-doped LaCoO3−δ at the same oxygen vacancies concentration. The decrease of oxygen vacancies mobility can be explained by the oxygen-3d-metal binding energy increase due to the substitution of Cr for Co in LaCoO3−δ. Fig. 5 also obviously shows that the dependences σi = f(δ)T are really straight lines at all temperatures investigated and hence according to Eq. (5) oxygen vacancies self-diffusion coefficient and mobility depend only on temperature. Oxygen vacancies self-diffusion coefficients calculated accordingly and their mobilities in LaCoO3−δ and LaCo0.7 Cr0.3O3−δ are given in Fig. 7 as a function of reciprocal temperature. The dependencies of the oxygen vacancies self-
Fig. 8. Comparison of the oxygen vacancies self-diffusion coefficient for LaCoO3−δ with Fueki et al. [20] data.
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Fig. 9. Oxygen nonstoichiometry dependence of the activation energy of oxygen ionic conductivity in LaCo0.7Cr0.3O3−δ.
diffusion coefficient and their mobility on temperature were found to be given as log DVO•• ¼ 1:5ðF0:4Þ
5429ðF505Þ T
ð6Þ
and logUVO•• ¼ 0:68ðF0:4Þ
4888ðF500Þ ; T
ð7Þ
respectively. Calculated activation energy of oxygen vacancies selfdiffusion comes to value of 104 ± 10 kJ/mol (1.08 ± 0.1 eV) for undoped lanthanum cobaltite and 121.3 ± 10 kJ/mol (1.26 ± 0.1 eV) for that doped with chromium. It is worth to note that the value of activation energy of oxygen vacancies selfdiffusion in LaCoO3−δ is quite consistent with that reported by Fueki et al. [20] within the range of experimental errors. Experimentally determined values of oxygen vacancies selfdiffusion coefficient for LaCoO3−δ alone with those reported by Fueki et al. [20] are shown in Fig. 8 It is clear that they are quite consistent with each other within the experimental error. The activation energy of oxygen ionic conductivity was calculated as a function of oxygen nonstoichiometry. Its values
Fig. 10. Isothermal dependences of oxygen vacancies transport numbers for LaCoO3−δ.
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since oxygen vacancies concentration grows at the same time. The predominant charge carriers in both undoped LaCoO3−δ and Cr-doped LaCo0.7Cr0.3O3−δ were shown to be electronic defects. Doping with Cr results in the drop of ionic conductivity and oxygen chemical diffusion coefficient. Acknowledgment This work was supported by Russian Foundation for Basic Research (grant Nos 07-03-00840 and 06-08-08120-ofi). References
Fig. 11. Isothermal dependences of oxygen vacancies transport numbers for LaCo0.7Cr0.3O3−δ.
are shown as an example for LaCo0.7Cr0.3O3−δ in Fig. 9 As can be seen the activation energy of ionic conductivity decreases with oxygen nonstoichiometry, since ionic transport becomes easier if oxygen vacancies concentration increases. From Figs. 3 and 4 it also follows that the pO2 dependences of oxygen chemical diffusion coefficient and oxygen ionic conductivity have monotonic character. The latter can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete pO2 range investigated. It is well known that defect clusters formation is unlikely, if defects concentration is really small [6,10,28]. It is of interest to note that the results of the defect structure analysis presented by us recently do not confirm defect interaction in LaCoO3−δ [26,29] and LaCo0.7Cr0.3O3−δ [27] as well. Oxygen vacancies transport number was calculated as a function of oxygen partial pressure using our recent data on overall conductivity [26] and shown in Figs. 10 and 11 for LaCoO3−δ and LaCo0.7Cr0.3O3−δ, respectively. The transport number increases with oxygen partial pressure diminution due to simultaneous oxygen ionic conductivity gain whereas electronic conductivity drops at the same time [27,29]. Figs. 10 and 11 show obviously that the predominant charge carriers in both undoped LaCoO3−δ and Cr-doped LaCo0.7Cr0.3O3−δ are electronic defects as well. 4. Conclusion Oxygen vacancies chemical diffusion coefficient, oxygen ionic conductivity and ionic transport number were determined as a function of oxygen partial pressure and temperature in the ranges − 4 ≤ log(pO2, atm) ≤ 0 and 900 ≤ T, °C ≤ 1050, respectively, by means of the polarization technique. The dependencies of these properties on oxygen partial pressures were shown to have monotonic character. This can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete pO2 range investigated. Oxygen vacancies chemical diffusion coefficient, oxygen ionic conductivity and oxide-ionic transport number increase with temperature growth and oxygen partial pressure diminution,
[1] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. [2] K. Yasumoto, Y. Inagaki, M. Shiono, M. Dokiya, Solid State Ionics 148 (2002) 545. [3] N.Q. Minh, Solid State Ionics 174 (2004) 271. [4] Y. Teraoka, H.M. Zhang, K. Okamoto, N. Yamazoe, Materials Research Bulletin 23 (1988) 51. [5] V.V. Kharton, A.P. Viskup, E.N. Naumovich, N.M. Lapchuk, Solid State Ionics 104 (1997) 67. [6] E. Bucher, W. Jantscher, A. Benisek, W. Sitte, W. Preis, I. Row, F. Hofer, Solid State Ionics 141–142 (2001) 141. [7] N.I. Lipatov, P.P. Pashinin, A.N. Petrov, A.M. Prokhorov, V.Yu. Yurov, Letters in Russian Journal of Theoretical Physics 13 (1987) 1209. [8] S.C. Sorenson, I.B. Wroukiewicz, L.B. Sis, Ceramic Bulletin 53 (1974) 446. [9] E.D. Wachsman, P. Jayaweera, G. Krishnan, A. Sanjurjo, Solid State Ionics 136–137 (2000) 775. [10] J.J. Janecek, G.P. Wirtz, Journal of the American Chemical Society 61 (1978) 242. [11] M. Seppanen, M. Kyto, P. Taskinen, Scandinavian Journal of Metallurgy 8 (1979) 199. [12] A.N. Petrov, V.A. Cherepanov, E.M. Novitsky, V.M. Zhukovsky, Russian Journal of Physical Chemistry 58 (1984) 1618. [13] A.N. Petrov, V.A. Cherepanov, A.Yu. Zuev, V.M. Zhukovsky, Russian Journal of Physical Chemistry 75 (1988) 1. [14] F. Askham, J. Fancuohen, R. Ward, Journal of the American Chemical Society 72 (1950) 3799. [15] S.R. Sehlin, H.U. Anderson, D.M. Sparlin, Physical Review B 52 (1995) 11681. [16] A.N. Petrov, O.F. Kononchuk, A.V. Andreev, V.A. Cherepanov, P. Kofstad, Solid State Ionics 80 (1995) 189. [17] J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, Journal of Solid State Chemistry 80 (1989) 102. [18] C.H. Chen, H. Kruidhoff, H.J.M. Bouwmeester, A.J. Burggraaf, Journal of Applied Electrochemistry 27 (1997) 71. [19] T. Ishigaki, S. Yamauchi, J. Mizusaki, K. Kishio, K. Fueki, Y. Mima, Annual Report of the Engineering 43 (1984) 153. [20] K. Fueki, J. Mizusaki, S. Yamauchi, T. Ishigaki, Y. Mima, Materials Science Monographs 28(A) (1985) 339. [21] C. Wagner, Zeitschrift f ü¯ r Physikalische Chemie. Abteilung B, Chemie der Elementarprozesse, Aufbau der Materie 21 (1933) 25. [22] C. Wagner, Zeitschrift f ü¯ r Elektrochemie 60 (1956) 4. [23] I. Yokota, Journal of the Physical Society of Japan 16 (1961) 2213. [24] E. Bucher, A. Benisek, W. Sitte, Solid State Ionics 157 (2003) 39. [25] A.A. Ostroushko, Mendeleev Chemistry Journal 42 (1998) 153. [26] A.N. Petrov, A.Yu. Zuev, A.I. Vylkov, D.S. Tsvetkov, Journal of Materials Science 42 (2007) 1901. [27] A.I. Vylkov, PhD thesis, Ekaterinburg 2006. [28] V.V. Kharton, A.V. Nikolaev, E.N. Naumovich, A.A. Vecher, Solid State Ionics 81 (1995) 201. [29] A.Yu. Zuev, A.N. Petrov, A.I. Vylkov, D.S. Tsvetkov, Journal of Materials Science 42 (2007) 1909.
Solid State Ionics 178 (2007) 1458 – 1462 www.elsevier.com/locate/ssi
Oxide ion transport in undoped and Cr-doped LaCoO3−δ D.S. Tsvetkov, A.Yu. Zuev ⁎, A.I. Vylkov, A.N. Petrov Department of Chemistry, Ural State University, Lenin av.51, 620083 Yekaterinburg, Russia Received 12 January 2007; received in revised form 9 July 2007; accepted 26 August 2007
Abstract The results of chemical and self-diffusion coefficient of oxygen vacancies, oxygen ionic conductivity and ionic transport numbers measured as a function of oxygen partial pressure pO2 and temperature in the ranges − 4 ≤ log(pO2, atm) ≤ 0 and 900 ≤ T, °C ≤ 1050, respectively, are presented for the perovskite-type undoped LaCoO3−δ and doped with chromium LaCo0.7Cr0.3O3−δ cobaltites. The dependencies of these properties on oxygen partial pressures were shown to have monotonic character. This can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete pO2 range investigated. The predominant charge carriers in both undoped and Cr-doped lanthanum cobaltite are electron defects, since oxygen vacancies transport numbers do not exceed 2 · 10−3%. The substitution of Cr for Co was found to lead to decrease of both ionic conductivity and oxygen chemical diffusion coefficient. Activation energy of ionic conductivity of the oxides studied decreases with the oxygen nonstoichiometry increase. Self-diffusion coefficient of oxygen vacancies and their mobility were shown to be independent of oxygen partial pressure and nonstoichiometry most likely due to lack of the defects interaction in the oxide studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Lanthanum cobaltite; Polarization; Oxygen chemical diffusion coefficient; Oxygen ionic conductivity; Self-diffusion coefficient
1. Introduction Mixed ionic- and electronic-conducting perovskite-type oxides are the-state-of-the-art materials for high temperature electrochemical devices such as solid oxide fuel cells (SOFCs) [1–3], oxygen membranes [4–6]. They also find an application as cathodes [7] for CO2-lasers and catalysts [8,9]. Many of such materials are lanthanum cobaltite-based oxides. Undoped and doped lanthanum cobaltites were extensively studied since the 1950s [4,10–17]. However, the data on such key properties as oxygen ionic conductivity and chemical diffusion coefficient of oxygen vacancies are very restricted to date. Oxygen ionic conductivity of undoped LaCoO3 was measured [5,18], but results obtained are not consistent with each other despite the same oxygen permeation technique was used in both studies. Oxygen chemical diffusion was studied by Ishigaki et al. [19]
and Fueki et al. [20] in undoped LaCoO3 using isotope exchange and thermogravimetric methods, respectively, but these authors did not study an oxygen partial pressure dependence of oxygen chemical diffusion coefficient. The polarization technique first introduced by Wagner [21,22] and Yokota [23] allows to determine both oxygen chemical diffusion coefficient and oxygen ionic conductivity. Recently Bucher et al. [24] described the experimental polarization set up suitable for perovskite oxides, which have tendency to react with yttria stabilized zirconia-based blocking electrodes. The main goal of the present work was to determine oxygen chemical diffusion coefficient and oxygen ionic conductivity of undoped LaCoO3 and Cr-doped LaCr0.3Co0.7O3 cobaltites as a function of oxygen partial pressure at different temperatures by means of polarization measurements. 2. Experimental
⁎ Corresponding author. Tel.: +7 343 251 7927; fax: +7 343 261 5978. E-mail address: [email protected] (A.Y. Zuev). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.08.005
Powder samples of LaCoO3 and LaCo0.7Cr0.3O3 were prepared by pyrolysis of the corresponding polymer-salt
D.S. Tsvetkov et al. / Solid State Ionics 178 (2007) 1458–1462
1459
compositions according to the technique described elsewhere [25]. The phase composition of all samples prepared was determined by means of X-ray diffraction, XRD, with a Dron-6 diffractometer using Cu Kα radiation. XRD showed no indication for the presence of a second phase. The single phase powders were pressed into tablets of 6 mm diameter and 0.9 mm thickness and then sintered at 1500 °C for 24 h in air. Relative density of sintered samples was 95%. For the determination of chemical diffusion coefficient and ionic conductivity the following electrochemical cell was used: O2 ; PtjZrO2 ðY2 O3 ÞjPtporous jsamplejPtporous jZrO2 ðY2 O3 ÞjPt; O2 ð1Þ The scheme of the experimental setup is shown in Fig. 1. The sample is located between two ionic conducting electrodes (O2,Pt/ZrO2(Y2O3)). Specimen is polarized by passing a constant current through the ionic electrodes. In order to prevent any side reaction between the sample and YSZ both surfaces of the sample were covered with thin porous platinum layers. Internal platinum layers were also used to measure oxygen partial pressure inside the cell. The whole device was sealed with a glass to prevent direct oxygen penetration from the ambient atmosphere. The mixture of 30 wt.% BaO and 70 wt.% magnesium silicate Mg3H2Si4O12 was applied as a glass. In order to make sure that this glass does not react with lanthanum cobaltites investigated calculation of equilibrium composition was implemented using the module Equilibrium of the thermodynamic software FactSage 5.3.1. This calculation showed obviously that there are no any equilibrium products in the temperature range investigated except MgSiO3, SiO2, BaO, and LaCoO3.
Fig. 2. Typical depolarization curves of the cell (1).
Polarization Up and depolarization (relaxation) Ud voltages of aforementioned cell (1) can be expressed according to [22– 24] as follows (long time solution of Fick's second law): jL jLte 8 Up ðtÞ ¼ þ expðt=sÞ ð2Þ ri ri p2 and jLte 8 Ud ðtÞ ¼ expðt=sÞ ; ri p2
ð3Þ
where j, L, te are current density, sample thickness, and electronic defects transport number, respectively. τ is a relaxap2 tion time defined as s ¼ 2 ~ and t is time. D˜ is oxygen vacancies LD
chemical diffusion coefficient. As can be seen ~ ln(Ud −U∞) vs. 1 p2D t is straight line with slope ratio k ¼ s ¼ L2 and intercept A e on y-axis defined as A ¼ ln 8jLt ri p2 . Oxygen vacancies chemical diffusion coefficient and oxygen ionic conductivity can be, therefore, calculated from the plot ln(Ud − U∞) vs. t. Nevertheless, it should be noted that calculation of oxygen vacancies self-diffusion coefficient and, therefore, oxygen ionic conductivity according to Eqs. (4) and (5) is more suitable than that on the basis of Eqs. (2) and (3), since chemical diffusion coefficient and oxygen ionic conductivity are calculated from the slope and intercept, respectively, of the plot ln(Ud − U∞) vs. t, but accuracy of the slope determination is higher than that of the intercept. ~ DVO•• ¼ DVO••
Aln PO2 ; Aln½VO•• T¼const
ð4Þ
~ where D VO•• and DVO•• are oxygen vacancies chemical diffusion and self-diffusion coefficient, respectively. ri ¼
Fig. 1. The scheme of the polarization cell used for measurements: 1) glass sealing, 2) sample, 3) YSZ electrode, 4) porous platinum layers.
ðzi FÞ2 Di B d ¼ zi FUi Bd; RT
ð5Þ
where δ, B, σi, zi = 2, F, Ui are oxygen nonstoichiometry, a conversion constant, oxide ion conductivity, oxygen vacancy charge, Faraday constant, oxygen vacancies mobility, respectively.
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Fig. 3. Isothermal dependences of chemical diffusion coefficient and ionic conductivity of oxygen vacancies for LaCoO3−δ.
AlnP Derivative Aln½VO••2 was determined from the isotherO T ¼const mal dependencies of oxygen nonstoichiometry vs. oxygen partial pressure measured in our previous works by coulometric titration technique [27,29]. Other advantage of the choice of a slope coefficient for calculations consists in the opportunity to ignore even a weak interaction between a glass and a sample. Indeed the slope coefficient contains only the sample length, which does not depend on this interaction and remains, therefore, a constant during the measurements. The polarization current 0.1–0.3 mA/cm2 was used in the present work and voltage drop on the cell did not exceed 10– 100 mV. Depolarization voltage was measured by Agilent 34401A voltmeter.
3. Results and discussion Fig. 2 shows typical depolarization curves of cell (1) with sample and without it in the “U–t”coordinates and with sample
Fig. 4. Isothermal dependences of chemical diffusion coefficient and ionic conductivity of oxygen vacancies for LaCo0.7Cr0.3O3−δ on oxygen partial pressure.
Fig. 5. Oxygen nonstoichiometry dependences of oxygen ionic conductivity of LaCoO3−δ at different temperatures.
in the “ln|Ud − U∞| − t” coordinates, respectively. It is obvious from the comparison of “U–t” plots for cell (1) with and without sample that YSZ electrodes and porous platinum layers do not affect the total polarization or depolarization and oxygen vacancies chemical diffusion coefficient and oxygen ionic conductivity of the sample can be, therefore, calculated. Fig. 2 also shows that plot ln|Ud − U∞| vs. t is a straight line when t → ∞. The latter is in full agreement with Eq. (3). Figs. 3 and 4 show results of polarization measurements. As seen oxygen chemical diffusion coefficient and oxygen ionic conductivity increase with temperature and decrease with oxygen partial pressure, since oxygen vacancies concentration increases at the same time [26–29]. From the comparison of these figures it follows that a substitution of Cr for Co in LaCoO3−δ leads to significant decrease of oxygen ionic conductivity. This seems to be explained, first of all, by the particularities of the defect structure of the oxides investigated. Recently the doping with chromium was shown by Vylkov [27] to result in oxygen vacancies concentration drop in LaCo0.7Cr0.3O3−δ as compared to LaCoO3−δ at the same oxygen partial pressure and temperature. This drop, in turn, leads to oxygen ionic conductivity depreciation according to Eq. (5).
Fig. 6. Oxygen nonstoichiometry dependences of oxygen ionic conductivity of LaCoO3−δ and LaCo0.7Cr0.3O3−δ at 1000 °C.
D.S. Tsvetkov et al. / Solid State Ionics 178 (2007) 1458–1462
Fig. 7. Temperature dependences of self-diffusion coefficient and mobility of oxygen vacancies in LaCoO3−δ and LaCo0.7Cr0.3O3−δ.
Oxygen ionic conductivity of LaCoO3−δ is replotted vs. oxygen nonstoichiometry in Fig. 5 at all temperatures investigated. For the sake of comparison oxygen ionic conductivity of both LaCoO3−δ and LaCo0.7Cr0.3O3−δ is given in Fig. 6 at 1000 °C. As seen in this figure oxygen ionic conductivity of LaCoO3−δ exceeds that of Cr-doped cobaltite over the whole oxygen nonstoichiometry range investigated. This indicates obviously that the oxygen vacancies mobility of undoped LaCoO3−δ is larger then that of Cr-doped LaCoO3−δ at the same oxygen vacancies concentration. The decrease of oxygen vacancies mobility can be explained by the oxygen-3d-metal binding energy increase due to the substitution of Cr for Co in LaCoO3−δ. Fig. 5 also obviously shows that the dependences σi = f(δ)T are really straight lines at all temperatures investigated and hence according to Eq. (5) oxygen vacancies self-diffusion coefficient and mobility depend only on temperature. Oxygen vacancies self-diffusion coefficients calculated accordingly and their mobilities in LaCoO3−δ and LaCo0.7 Cr0.3O3−δ are given in Fig. 7 as a function of reciprocal temperature. The dependencies of the oxygen vacancies self-
Fig. 8. Comparison of the oxygen vacancies self-diffusion coefficient for LaCoO3−δ with Fueki et al. [20] data.
1461
Fig. 9. Oxygen nonstoichiometry dependence of the activation energy of oxygen ionic conductivity in LaCo0.7Cr0.3O3−δ.
diffusion coefficient and their mobility on temperature were found to be given as log DVO•• ¼ 1:5ðF0:4Þ
5429ðF505Þ T
ð6Þ
and logUVO•• ¼ 0:68ðF0:4Þ
4888ðF500Þ ; T
ð7Þ
respectively. Calculated activation energy of oxygen vacancies selfdiffusion comes to value of 104 ± 10 kJ/mol (1.08 ± 0.1 eV) for undoped lanthanum cobaltite and 121.3 ± 10 kJ/mol (1.26 ± 0.1 eV) for that doped with chromium. It is worth to note that the value of activation energy of oxygen vacancies selfdiffusion in LaCoO3−δ is quite consistent with that reported by Fueki et al. [20] within the range of experimental errors. Experimentally determined values of oxygen vacancies selfdiffusion coefficient for LaCoO3−δ alone with those reported by Fueki et al. [20] are shown in Fig. 8 It is clear that they are quite consistent with each other within the experimental error. The activation energy of oxygen ionic conductivity was calculated as a function of oxygen nonstoichiometry. Its values
Fig. 10. Isothermal dependences of oxygen vacancies transport numbers for LaCoO3−δ.
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since oxygen vacancies concentration grows at the same time. The predominant charge carriers in both undoped LaCoO3−δ and Cr-doped LaCo0.7Cr0.3O3−δ were shown to be electronic defects. Doping with Cr results in the drop of ionic conductivity and oxygen chemical diffusion coefficient. Acknowledgment This work was supported by Russian Foundation for Basic Research (grant Nos 07-03-00840 and 06-08-08120-ofi). References
Fig. 11. Isothermal dependences of oxygen vacancies transport numbers for LaCo0.7Cr0.3O3−δ.
are shown as an example for LaCo0.7Cr0.3O3−δ in Fig. 9 As can be seen the activation energy of ionic conductivity decreases with oxygen nonstoichiometry, since ionic transport becomes easier if oxygen vacancies concentration increases. From Figs. 3 and 4 it also follows that the pO2 dependences of oxygen chemical diffusion coefficient and oxygen ionic conductivity have monotonic character. The latter can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete pO2 range investigated. It is well known that defect clusters formation is unlikely, if defects concentration is really small [6,10,28]. It is of interest to note that the results of the defect structure analysis presented by us recently do not confirm defect interaction in LaCoO3−δ [26,29] and LaCo0.7Cr0.3O3−δ [27] as well. Oxygen vacancies transport number was calculated as a function of oxygen partial pressure using our recent data on overall conductivity [26] and shown in Figs. 10 and 11 for LaCoO3−δ and LaCo0.7Cr0.3O3−δ, respectively. The transport number increases with oxygen partial pressure diminution due to simultaneous oxygen ionic conductivity gain whereas electronic conductivity drops at the same time [27,29]. Figs. 10 and 11 show obviously that the predominant charge carriers in both undoped LaCoO3−δ and Cr-doped LaCo0.7Cr0.3O3−δ are electronic defects as well. 4. Conclusion Oxygen vacancies chemical diffusion coefficient, oxygen ionic conductivity and ionic transport number were determined as a function of oxygen partial pressure and temperature in the ranges − 4 ≤ log(pO2, atm) ≤ 0 and 900 ≤ T, °C ≤ 1050, respectively, by means of the polarization technique. The dependencies of these properties on oxygen partial pressures were shown to have monotonic character. This can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete pO2 range investigated. Oxygen vacancies chemical diffusion coefficient, oxygen ionic conductivity and oxide-ionic transport number increase with temperature growth and oxygen partial pressure diminution,
[1] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. [2] K. Yasumoto, Y. Inagaki, M. Shiono, M. Dokiya, Solid State Ionics 148 (2002) 545. [3] N.Q. Minh, Solid State Ionics 174 (2004) 271. [4] Y. Teraoka, H.M. Zhang, K. Okamoto, N. Yamazoe, Materials Research Bulletin 23 (1988) 51. [5] V.V. Kharton, A.P. Viskup, E.N. Naumovich, N.M. Lapchuk, Solid State Ionics 104 (1997) 67. [6] E. Bucher, W. Jantscher, A. Benisek, W. Sitte, W. Preis, I. Row, F. Hofer, Solid State Ionics 141–142 (2001) 141. [7] N.I. Lipatov, P.P. Pashinin, A.N. Petrov, A.M. Prokhorov, V.Yu. Yurov, Letters in Russian Journal of Theoretical Physics 13 (1987) 1209. [8] S.C. Sorenson, I.B. Wroukiewicz, L.B. Sis, Ceramic Bulletin 53 (1974) 446. [9] E.D. Wachsman, P. Jayaweera, G. Krishnan, A. Sanjurjo, Solid State Ionics 136–137 (2000) 775. [10] J.J. Janecek, G.P. Wirtz, Journal of the American Chemical Society 61 (1978) 242. [11] M. Seppanen, M. Kyto, P. Taskinen, Scandinavian Journal of Metallurgy 8 (1979) 199. [12] A.N. Petrov, V.A. Cherepanov, E.M. Novitsky, V.M. Zhukovsky, Russian Journal of Physical Chemistry 58 (1984) 1618. [13] A.N. Petrov, V.A. Cherepanov, A.Yu. Zuev, V.M. Zhukovsky, Russian Journal of Physical Chemistry 75 (1988) 1. [14] F. Askham, J. Fancuohen, R. Ward, Journal of the American Chemical Society 72 (1950) 3799. [15] S.R. Sehlin, H.U. Anderson, D.M. Sparlin, Physical Review B 52 (1995) 11681. [16] A.N. Petrov, O.F. Kononchuk, A.V. Andreev, V.A. Cherepanov, P. Kofstad, Solid State Ionics 80 (1995) 189. [17] J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, Journal of Solid State Chemistry 80 (1989) 102. [18] C.H. Chen, H. Kruidhoff, H.J.M. Bouwmeester, A.J. Burggraaf, Journal of Applied Electrochemistry 27 (1997) 71. [19] T. Ishigaki, S. Yamauchi, J. Mizusaki, K. Kishio, K. Fueki, Y. Mima, Annual Report of the Engineering 43 (1984) 153. [20] K. Fueki, J. Mizusaki, S. Yamauchi, T. Ishigaki, Y. Mima, Materials Science Monographs 28(A) (1985) 339. [21] C. Wagner, Zeitschrift f ü¯ r Physikalische Chemie. Abteilung B, Chemie der Elementarprozesse, Aufbau der Materie 21 (1933) 25. [22] C. Wagner, Zeitschrift f ü¯ r Elektrochemie 60 (1956) 4. [23] I. Yokota, Journal of the Physical Society of Japan 16 (1961) 2213. [24] E. Bucher, A. Benisek, W. Sitte, Solid State Ionics 157 (2003) 39. [25] A.A. Ostroushko, Mendeleev Chemistry Journal 42 (1998) 153. [26] A.N. Petrov, A.Yu. Zuev, A.I. Vylkov, D.S. Tsvetkov, Journal of Materials Science 42 (2007) 1901. [27] A.I. Vylkov, PhD thesis, Ekaterinburg 2006. [28] V.V. Kharton, A.V. Nikolaev, E.N. Naumovich, A.A. Vecher, Solid State Ionics 81 (1995) 201. [29] A.Yu. Zuev, A.N. Petrov, A.I. Vylkov, D.S. Tsvetkov, Journal of Materials Science 42 (2007) 1909.