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La and Sc co-doped SrTiO3 as novel anode materials for solid oxide fuel cells
Electrochemistry Communications 10 (2008) 1567–1570
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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
La and Sc co-doped SrTiO3 as novel anode materials for solid oxide fuel cells Xue Li a, Hailei Zhao a,b,*, Feng Gao a, Ning Chen a,b, Nansheng Xu a a b
Department of Inorganic Nonmetallic Materials, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Material and Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 6 July 2008 Received in revised form 10 August 2008 Accepted 12 August 2008 Available online 19 August 2008 Keywords: Solid oxide fuel cells Anode Oxygen ion migration energy SrTiO3 Ionic conductivity Charge compensation mechanism
a b s t r a c t La and Sc co-doped SrTiO3 was synthesized via solid state reaction. The oxygen ion migration energy was investigated by first-principles calculations in SrBO3 systems (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) with perovskite structure. Structure with Sc showed the lowest oxygen migration energy, and thus Sc was selected as B-site dopant with the primary aim to improve the ionic conductivity of SrTiO3-based anode materials. With increasing Sc-doping amount, the electrical conductivity of La0.3Sr0.7ScxTi1xO3d decreased in 25–1000 °C, while the ionic conductivity increased significantly between 500 and 1000 °C. The ionic conductivity for La0.3Sr0.7Sc0.10Ti0.90O3d was 1 102 S cm1 and increased about 230% compared with La0.3Sr0.7TiO3d at 800 °C and under oxygen partial pressure of 1019 atm. Sc-doping increased the oxygen vacancy concentration and decreased the oxygen migration energy, thus facilitating the conduction process of oxygen ions in La and Sc codoped SrTiO3. The possible charge compensation mechanism of Sc-doped La0.3Sr0.7TiO3d can be 4þ 3þ described as La0:3 Sr0:7 Sc3þ x Ti0:72dx1 Ti0:3þ2dx2 O3ðdþx1 =2Þ (x = x1 + x2). Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs), which convert chemical energy directly into electrical energy, have been regarded as a promising energy conversion and generation system due to their high efficiency, fuel adaptability and low pollution [1–3]. The conventional Ni-based (Ni/YSZ or Ni/GDC) cermet anodes produce excellent performance with hydrogen fuel at SOFCs operating conditions, while they show low tolerance to sulfur and carbon deposition when exposing to practical hydrocarbon fuel such as natural gas, leading to catalysis degradation [4–6]. Therefore, new anode materials are in demand for further development of SOFCs. Strontium titanates (SrTiO3) with perovskite structure shows high chemical stability at high temperatures under both oxidizing and reducing atmospheres, and has strong resistance to carbon deposition and suffer poison [7]. However, its low electronic and ionic conductivities prevent SrTiO3 as a practical anode for SOFCs. Doping donors such as La3+ or Y3+ on Sr2+ site and Nb5+ on Ti4+ site convert SrTiO3 into a highly semi-conducting n-type material, and doping acceptors such as Fe3+, Co3+ and Al3+ on Ti4+ site can improve the oxygen vacancy concentration [8–10]. According to Hui and Petric [10], the electrical conductivities of Sr11.5xYxTiO3d decreased by acceptors (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Al or Ga) and the system Sr0.85Y0.10Ti0.95Co0.05O3d had the highest conductivity * Corresponding author. Address: Department of Inorganic Nonmetallic Materials, University of Science and Technology Beijing, Beijing 100083, China. Tel./fax: +86 10 82376837. E-mail address: [email protected] (H. Zhao). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.017
of 45 S/cm at 800 °C and oxygen partial pressure of 1019 atm. The ionic conductivity of both SrTi0.95Ga0.05O3d and SrTi0.95Co0.05O3d at 800 °C was about 3 104 S/cm by estimating from the total conductivity. LaxSr1xTiO3 (x = 0.1–0.4) also has comparatively high electrical conductivity, with conductivities of 100–400 S/cm in a reducing atmosphere between 700 and 1000 °C [11]. La-doped SrTiO3, however, shows lower oxygen ion conductivity and poor electro-catalytic performance for hydrogen oxidation as Ni-cermet [7]. Doping acceptor on Ti-site of La0.3Sr0.7TiO3d has the possibility to enhance the ionic conduction process. The relevant studies indicated that donor-doping on A-site of SrTiO3 material mainly affected the electronic conductivity rather than ionic conductivity [8,9,12]. Accordingly, SrBO3 (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) structure models were established in this work to simply the calculation process for estimating the effect of B-site elements on the oxygen ion migration energy. Sc was selected as Ti-site dopant according to the calculation results with the lowest energy criterion with the primary aim to improve the ionic conductivity of SrTiO3-based materials. The effects of Sc-doping amount on the electrical and ionic conductivities of La0.3Sr0.7ScxTi1xO3d with temperature were investigated. The charge compensation mechanism in La0.3Sr0.7ScxTi1xO3d was proposed.
2. Calculation for oxygen ion migration in SrBO3 The aim of calculation was to choose a better element as a B-site dopant in SrTiO3 to lower the oxygen ion migration energy and
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Fig. 1. Effect of different elements on the oxygen ion migration energies in SrBO3, B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb.
thus increase the oxygen ion conductivity. The ion migration energy was referred to as the activation energy for oxygen ion conduction. The ion migration energies in SrBO3 (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As in the fourth period and Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn, Sb in the fifth period) were estimated by first-principles calculations [13] via the software of ABINIT, which is based on the density-functional theory (DFT) and plane wavepseudopotential (PWP). SrTiO3 has perovskite structure, whose space group is Pm3m and lattice parameter is 3.905 Å. The super-lattice with 2 2 2 was introduced as the model for calculations. The degree of convergence of the results was 105 with generalized gradient approximation (GGA). The cut-off energy of plane wave was 380 eV. The ion migration energy in SrBO3 was defined as the lowest energy for lattice oxygen jumping barrier from original site to oxygen vacancy [14]. Fig. 1 shows the calculated oxygen ion migration energies in SrBO3. Sc was shown to be a good choice of all the present elements to improve the ionic conductivity of SrTiO3-based materials. Therefore, the system of La0.3Sr0.7ScxTi1xO3d was investigated in this paper. 3. Experimental La0.3Sr0.7ScxTi1xO3d (x = 0, 0.03, 0.05, 0.08, 0.10) powders were prepared by solid state reaction from high purity La2O3, SrCO3, Sc2O3 and TiO2 in forming gas (5 vol% hydrogen in argon) at 1300 °C for 10 h after ball-milling for 12 h. The synthesized pow-
ders were uniaxially pressed into bars (40 mm–7 mm–3 mm) and pellets (diameter 13 mm). The green bars and pellets were densified at 1500 °C for 10 h in forming gas for the measurement of electrical and ionic conductivities. The phase purity of sintered samples was examined by X-ray diffraction (XRD, Rigaku D/maxA X-ray diffractometer) using Cu Ka radiation. The total electrical conductivity was measured by the standard four terminal DC methods at 25–1000 °C in flowing forming gas; and ionic conductivity was measured by electron-blocking method at 500–1000 °C in flowing forming gas saturated with room-temperature water, as previously described in detail [15,16]. The oxygen partial pressure of flowing forming gas saturated with room-temperature water at 800 °C was about 1019 atm. The oxygen vacancy concentration (d) was estimated from the average oxygen contents of 10 micro-areas in the fracture surface of each sample using an electron probe microanalyzer (EPMA, JXA-8100, JEOL). The samples used for EPMA examination were quenched from 1000 °C to ensure the oxygen vacancy concentration of sample at high temperature. The bulk densities of sintered samples were measured by the Archimedes method. All the relative densities of La0.3Sr0.7ScxTi1xO3d samples were higher than 90%.
4. Results and discussion 4.1. Phase development Single-phase samples were observed by XRD for SrTiO3 and La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) after sintering at 1500 °C for 10 h in forming gas, as shown in Fig. 2. The samples with x 6 0.10 showed a single cubic perovskite structure. Compared with that of SrTiO3, the XRD peaks of La0.3Sr0.7TiO3d had no obvious shift. However, with increasing Sc-doping amount, the XRD peaks of La0.3Sr0.7ScxTi1xO3d shifted to the left gradually. The lattice parameters calculated by Rietveld refinement for La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) were 3.905, 3.914, 3.915, and 3.916 Å, respectively. The lattice parameter changed little when La was doped in SrTiO3 (3.905 Å), while it increased with Sc-doping in La0.3Sr0.7TiO3d. This mainly resulted from that the ionic radius of XIILa3+ (1.36 Å) is close to that of XIISr2+ (1.44 Å) but the radius of VISc3+ (0.745 Å) is larger than both of VITi4+ (0.605 Å) and VITi3+ (0.670 Å) [17].
Fig. 2. XRD patterns of La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) after sintering at 1500 °C for 10 h in forming gas: (a) 10 6 2h 6 90° and (b) 30 6 2h 6 47°.
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4.2. Effect of La and Sc co-doping on the electrical characteristics of SrTiO3 The electrical conductivities and the ionic conductivities of SrTiO3 and La0.3Sr0.7ScxTi1xO3d samples were measured in forming gas from 25 to 1000 °C and in flowing forming gas saturated with room-temperature water from 500 to 1000 °C, respectively. The electrical conductivity of SrTiO3 without doping was very low, while it was remarkably enhanced by La-doping (Fig. 3). The La0.3Sr0.7TiO3d sample exhibited an electrical conductivity on the order of 130–397 S cm1 in 50–1000 °C and 216 S cm1 at 800 °C. With the increase of Sc-doping, the electrical conductivity of La0.3Sr0.7ScxTi1xO3d decreased (Fig. 3), which suggested the decreased concentration of Ti3+ in La0.3Sr0.7ScxTi1xO3d. With increasing temperature, the electrical conductivity increased through a maximum then decreased, indicating a polaron conduction behavior of La0.3Sr0.7ScxTi1xO3d anode [18,19]. The electrical conductivities of La0.3Sr0.7Sc0.05Ti0.95O3d and La0.3Sr0.7Sc0.10Ti0.90O 1 at 800 °C, respectively. On the other 3d were 118 and 49 S cm hand, the ionic conductivity of La0.3Sr0.7ScxTi1xO3d increased with increasing Sc-doping amount remarkably (Fig. 4). The ionic conductivity of La0.3Sr0.7TiO3d was only 3 103 S cm1 at 800 °C and oxygen partial pressure of 1019 atm, while it increased to 4 103 S cm1 for La0.3Sr0.7Sc0.05Ti0.95O3d and 1 102 S cm1 for La0.3Sr0.7Sc0.10- Ti0.90O3d. Compared with La0.3Sr0.7TiO3d, the ionic conductivity of La0.3Sr0.7Sc0.10Ti0.90O3d increased about 230%. The ionic conductivity is primarily dependent on the oxygen vacancy concentration and the migration energy of oxygen ion at a certain temperature. The oxygen vacancy concentration of La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.10) were estimated to be about 0.12, 0.14 and 0.18, respectively, by EPMA. It showed an increasing trend with increasing Sc-doping amount. Additionally, the activation energy for oxygen ion migration (Ea) was calculated by Arrhenius rule, according to the results shown in Fig. 4. As shown in inset of Fig. 4, Ea of La0.3Sr0.7ScxTi1xO3d decreased with increasing Sc amount, indicating that Sc-doping facilitates the conduction of oxygen ions, which agreed well with the calculation results shown in Section 2. 4.3. Charge compensation mechanism of La, Sc co-doped SrTiO3 For La-doped SrTiO3, La0.3Sr0.7TiO3d, Ti3+ ions will be produced as the electrovalent compensation in reducing condition. The defects formed in La donor doped SrTiO3 can be expressed as:
Fig. 4. Temperature dependence of ionic conductivity of La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) measured in flowing forming gas saturated with roomtemperature water. La and Sc co-doped SrTiO3 as novel anode materials for solid oxide fuel cells.
SrO
3þ0
mLa2 O3 þ 2mTiO2 $ 2mLaSr þ 2mTiTi4þ þ 6mOO þ m=2O2 ðgÞ
ð1Þ
3+
Oxygen vacancies and Ti ions also may occur at low oxygen partial pressure, which can be described as: 3þ0
nTiO2 $ nTiTi4þ þ ð2n n=2ÞOO þ n=2VO þ n=4O2 ðgÞ
ð2Þ
As a whole result, the defects formed in La-doped SrTiO3 will be: SrO
3þ0
mLa2 O3 þ ð2m þ nÞTiO2 $ 2mLaSr þ ð2m þ nÞTiTi4þ þ ð6m þ ð2n n=2ÞÞOO þ n=2VO þ ð2m þ nÞ=4O2 ðgÞ
ð3Þ
In this case, electrons bonded on Ti ions (Ti3+) and oxygen vacancies are the main defects. Due to the low mobility of oxygen vacancies compared with electrons, oxygen vacancies make little contribution to the total conductivity of La-doped SrTiO3. As a result, the electrical conductivity of La-doped SrTiO3 in reducing condition mostly depends on the concentration of Ti3+. The concentration of Ti3+ at a constant oxygen partial pressure can be expressed as:
h
i 3þ0 TiTi4þ ¼ LaSr þ 2 VO
ð4Þ
The number of compensated Ti3+ with localized electrons depends on the amount of La incorporated in SrTiO3 and oxygen vacancy concentration related to the oxygen partial pressure. Thus the electrical conductivity of La0.3Sr0.7TiO3d is much higher than that of SrTiO3. For La0.3Sr0.7TiO3d, there are ½LaSr ¼ 0:3 and 3þ0 4þ ½VO ¼ d, then ½TiTi4þ ¼ 0:3 þ 2d and ½Ti ¼ 0:7 2d. The solid 4þ 3þ solution formula can be described as La0:3 Sr0:7 Ti0:72d Ti0:3þ2d O3d . With respect to Sc-doping sample La0.3Sr0.7ScxTi1xO3d, there are three possible charge compensation mechanisms. The first possibility is Sc3+ ions in place of Ti4+ ions, which can be expressed as 4þ 3þ La0:3 Sr0:7 Sc3þ x Ti0:72dx Ti0:3þ2d O3ðdþx=2Þ . The defects produced in Sc acceptor doping La0.3Sr0.7TiO3d can be expressed as: TiO2
1=2Sc2 O3 $ Sc0Ti4þ þ 3=2OO þ 1=2VO Fig. 3. Temperature dependence of electrical conductivity of SrTiO3 and La0.3Sr0.7ScxTi1xO3d (x = 0, 0.03, 0.05, 0.08, 0.10) measured in flowing forming gas.
ð5Þ
In this case, the ionic conductivity of La0.3Sr0.7TiO3d may increase because of the increased oxygen vacancy concentration. On the other hand, the electronic conductivity will not change
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due to the unchanged concentration of Ti3+ ions. As a result, the total conductivity should increase slightly, which is inconsistent with the experimental results. The second possible mechanism can be expressed as La0:3 Sr0:7 4þ 3þ 3+ Sc3þ ions take the place of Ti3+ ions. x Ti0:72d Ti0:3þ2dx O3d , i.e., all Sc The defects produced by Sc acceptor doping in La0.3Sr0.7TiO3d can be described as: Ti2 O3
1=2Sc2 O3 $ ScTi3þ þ 3=2OO
ð6Þ
In this case, the electronic conductivity of La0.3Sr0.7TiO3d may decrease for the reduced amount of Ti3+ and oxygen vacancy concentration will keep unchanged. This conflicts apparently with oxygen vacancy concentration estimated by EPMA. The last possible mechanism can be described as La0:3 Sr0:7 4þ 3þ Sc3þ x Ti0:72dx1 Ti0:3þ2dx2 O3ðdþx1 =2Þ (x = x1 + x2), i.e., Eqs. (5) and (6) may occur simultaneously. Sc3+ ions replace both Ti4+ and Ti3+ sites. The electronic conductivity of sample should decrease due to the lowered Ti3+ content, while the ionic conductivity ought to increase owing to the increased oxygen vacancy concentration. Due to the lower mobility of oxygen vacancy compared with electron, oxygen vacancy will make less contribution to the total conductivity of Sc-doped La0.3Sr0.7TiO3d material. Therefore, the total electrical conductivity decreases with increasing Sc-doping amount, which agrees well with the experimental results. 5. Conclusions The oxygen ion migration energy in SrScO3 was the lowest among SrBO3 (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) according to first-principles calculations. Accordingly, Sc was selected as B-site dopant to enhance the ionic conductivity of SrTiO3-based anode materials. With increasing Sc-doping amount, the electrical conductivity of La0.3Sr0.7ScxTi1xO3d decreased in 25–1000 °C, while the ionic conductivity increased significantly between 500 and 1000 °C. The electrical and ionic conductivities were 118 S cm1 and 4 103 S cm1 for La0.3Sr0.7Sc0.05Ti0.95O3d and 49 S cm1 and
1 102 S cm1 for La0.3Sr0.7Sc0.10Ti0.90O3d at 800 °C, respectively. Compared with La0.3Sr0.7TiO3d, the ionic conductivity of La0.3 Sr0.7Sc0.10Ti0.90O3d increased about 230%. Sc-doping increased oxygen vacancy concentration and decreased the oxygen migration energy, thus promoting the conduction of oxygen ions in La and Sc co-doped SrTiO3 anode materials. The possible charge compensation mechanism of Sc-doped La0.3Sr0.7TiO3d can be described as 4þ 3þ La0:3 Sr0:7 Sc3þ x Ti0:72dx1 Ti0:3þ2dx2 O3ðdþx1 =2Þ (x = x1 + x2). Acknowledgements This work has been funded by National Nature Science Foundation of China (No. 50672009) and 863 Program of National High Technology Research Development Project of China (No. 2006AA11A189). References [1] J. Huang, Z. Mao, Z. Liu, C. Wang, Electrochem. Commun. 9 (2007) 2601. [2] B. Zhu, I. Albinsson, C. Andersson, K. Borsand, M. Nilsson, B. Mellander, Electrochem. Commun. 8 (2006) 495. [3] B. Zhu, X.R. Liu, T. Schober, Electrochem. Commun. 6 (2004) 378. [4] S.P. Jiang, S.H. Chan, Mater. Sci. Technol. 20 (2004) 1109. [5] X.J. Chen, Q.L. Liu, S.H. Chan, N.P. Brandon, K.A. Khor, Electrochem. Commun. 9 (2007) 767. [6] J.H. Wang, M. Liu, Electrochem. Commun. 9 (2007) 2212. [7] M. Gong, X. Liu, J. Trembly, C. Johnson, J. Power Sources 168 (2007) 89. [8] O.N. Tufte, P.W. Chapman, Phys. Rev. 155 (1967) 796. [9] H.P.R. Frederikse, W.R. Hosler, Phys. Rev. 161 (1967) 822. [10] S.Q. Hui, A. Petric, Mater. Res. Bull. 37 (2002) 1215. [11] O.A. Marina, N.L. Canfield, J.W. Stevenson, Solid State Ionics 149 (2002) 21. [12] M.A. Peña, J.L.G. Fierro, Chem. Rev. 101 (2001) 981. [13] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, J. Phys.: Condens. Mater. 14 (2002) 2717. [14] J. Carrasco, N. Lopez, F. Illas, Phys. Rev. Lett. 93 (2004) 225502. [15] X. Li, H. Zhao, W. Shen, F. Gao, X. Huang, Y. Li, Z. Zhu, J. Power Sources 166 (2007) 47. [16] X. Li, H. Zhao, F. Gao, Z. Zhu, N. Chen, W. Shen, Solid State Ionics 179 (2008) 1588. [17] R.D. Shannon, Acta Crystallogr. 32 (1976) 751. [18] M. Sogaars, P.V. Hendriksen, M. Mogensen, J. Solid State Chem. 180 (2007) 1489. [19] P. Blennow, K.K. Hansen, L.R. Wallenberg, M. Mogensen, Electrochim. Acta 52 (2006) 1651.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
La and Sc co-doped SrTiO3 as novel anode materials for solid oxide fuel cells Xue Li a, Hailei Zhao a,b,*, Feng Gao a, Ning Chen a,b, Nansheng Xu a a b
Department of Inorganic Nonmetallic Materials, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Material and Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 6 July 2008 Received in revised form 10 August 2008 Accepted 12 August 2008 Available online 19 August 2008 Keywords: Solid oxide fuel cells Anode Oxygen ion migration energy SrTiO3 Ionic conductivity Charge compensation mechanism
a b s t r a c t La and Sc co-doped SrTiO3 was synthesized via solid state reaction. The oxygen ion migration energy was investigated by first-principles calculations in SrBO3 systems (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) with perovskite structure. Structure with Sc showed the lowest oxygen migration energy, and thus Sc was selected as B-site dopant with the primary aim to improve the ionic conductivity of SrTiO3-based anode materials. With increasing Sc-doping amount, the electrical conductivity of La0.3Sr0.7ScxTi1xO3d decreased in 25–1000 °C, while the ionic conductivity increased significantly between 500 and 1000 °C. The ionic conductivity for La0.3Sr0.7Sc0.10Ti0.90O3d was 1 102 S cm1 and increased about 230% compared with La0.3Sr0.7TiO3d at 800 °C and under oxygen partial pressure of 1019 atm. Sc-doping increased the oxygen vacancy concentration and decreased the oxygen migration energy, thus facilitating the conduction process of oxygen ions in La and Sc codoped SrTiO3. The possible charge compensation mechanism of Sc-doped La0.3Sr0.7TiO3d can be 4þ 3þ described as La0:3 Sr0:7 Sc3þ x Ti0:72dx1 Ti0:3þ2dx2 O3ðdþx1 =2Þ (x = x1 + x2). Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs), which convert chemical energy directly into electrical energy, have been regarded as a promising energy conversion and generation system due to their high efficiency, fuel adaptability and low pollution [1–3]. The conventional Ni-based (Ni/YSZ or Ni/GDC) cermet anodes produce excellent performance with hydrogen fuel at SOFCs operating conditions, while they show low tolerance to sulfur and carbon deposition when exposing to practical hydrocarbon fuel such as natural gas, leading to catalysis degradation [4–6]. Therefore, new anode materials are in demand for further development of SOFCs. Strontium titanates (SrTiO3) with perovskite structure shows high chemical stability at high temperatures under both oxidizing and reducing atmospheres, and has strong resistance to carbon deposition and suffer poison [7]. However, its low electronic and ionic conductivities prevent SrTiO3 as a practical anode for SOFCs. Doping donors such as La3+ or Y3+ on Sr2+ site and Nb5+ on Ti4+ site convert SrTiO3 into a highly semi-conducting n-type material, and doping acceptors such as Fe3+, Co3+ and Al3+ on Ti4+ site can improve the oxygen vacancy concentration [8–10]. According to Hui and Petric [10], the electrical conductivities of Sr11.5xYxTiO3d decreased by acceptors (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Al or Ga) and the system Sr0.85Y0.10Ti0.95Co0.05O3d had the highest conductivity * Corresponding author. Address: Department of Inorganic Nonmetallic Materials, University of Science and Technology Beijing, Beijing 100083, China. Tel./fax: +86 10 82376837. E-mail address: [email protected] (H. Zhao). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.017
of 45 S/cm at 800 °C and oxygen partial pressure of 1019 atm. The ionic conductivity of both SrTi0.95Ga0.05O3d and SrTi0.95Co0.05O3d at 800 °C was about 3 104 S/cm by estimating from the total conductivity. LaxSr1xTiO3 (x = 0.1–0.4) also has comparatively high electrical conductivity, with conductivities of 100–400 S/cm in a reducing atmosphere between 700 and 1000 °C [11]. La-doped SrTiO3, however, shows lower oxygen ion conductivity and poor electro-catalytic performance for hydrogen oxidation as Ni-cermet [7]. Doping acceptor on Ti-site of La0.3Sr0.7TiO3d has the possibility to enhance the ionic conduction process. The relevant studies indicated that donor-doping on A-site of SrTiO3 material mainly affected the electronic conductivity rather than ionic conductivity [8,9,12]. Accordingly, SrBO3 (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) structure models were established in this work to simply the calculation process for estimating the effect of B-site elements on the oxygen ion migration energy. Sc was selected as Ti-site dopant according to the calculation results with the lowest energy criterion with the primary aim to improve the ionic conductivity of SrTiO3-based materials. The effects of Sc-doping amount on the electrical and ionic conductivities of La0.3Sr0.7ScxTi1xO3d with temperature were investigated. The charge compensation mechanism in La0.3Sr0.7ScxTi1xO3d was proposed.
2. Calculation for oxygen ion migration in SrBO3 The aim of calculation was to choose a better element as a B-site dopant in SrTiO3 to lower the oxygen ion migration energy and
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Fig. 1. Effect of different elements on the oxygen ion migration energies in SrBO3, B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb.
thus increase the oxygen ion conductivity. The ion migration energy was referred to as the activation energy for oxygen ion conduction. The ion migration energies in SrBO3 (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As in the fourth period and Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn, Sb in the fifth period) were estimated by first-principles calculations [13] via the software of ABINIT, which is based on the density-functional theory (DFT) and plane wavepseudopotential (PWP). SrTiO3 has perovskite structure, whose space group is Pm3m and lattice parameter is 3.905 Å. The super-lattice with 2 2 2 was introduced as the model for calculations. The degree of convergence of the results was 105 with generalized gradient approximation (GGA). The cut-off energy of plane wave was 380 eV. The ion migration energy in SrBO3 was defined as the lowest energy for lattice oxygen jumping barrier from original site to oxygen vacancy [14]. Fig. 1 shows the calculated oxygen ion migration energies in SrBO3. Sc was shown to be a good choice of all the present elements to improve the ionic conductivity of SrTiO3-based materials. Therefore, the system of La0.3Sr0.7ScxTi1xO3d was investigated in this paper. 3. Experimental La0.3Sr0.7ScxTi1xO3d (x = 0, 0.03, 0.05, 0.08, 0.10) powders were prepared by solid state reaction from high purity La2O3, SrCO3, Sc2O3 and TiO2 in forming gas (5 vol% hydrogen in argon) at 1300 °C for 10 h after ball-milling for 12 h. The synthesized pow-
ders were uniaxially pressed into bars (40 mm–7 mm–3 mm) and pellets (diameter 13 mm). The green bars and pellets were densified at 1500 °C for 10 h in forming gas for the measurement of electrical and ionic conductivities. The phase purity of sintered samples was examined by X-ray diffraction (XRD, Rigaku D/maxA X-ray diffractometer) using Cu Ka radiation. The total electrical conductivity was measured by the standard four terminal DC methods at 25–1000 °C in flowing forming gas; and ionic conductivity was measured by electron-blocking method at 500–1000 °C in flowing forming gas saturated with room-temperature water, as previously described in detail [15,16]. The oxygen partial pressure of flowing forming gas saturated with room-temperature water at 800 °C was about 1019 atm. The oxygen vacancy concentration (d) was estimated from the average oxygen contents of 10 micro-areas in the fracture surface of each sample using an electron probe microanalyzer (EPMA, JXA-8100, JEOL). The samples used for EPMA examination were quenched from 1000 °C to ensure the oxygen vacancy concentration of sample at high temperature. The bulk densities of sintered samples were measured by the Archimedes method. All the relative densities of La0.3Sr0.7ScxTi1xO3d samples were higher than 90%.
4. Results and discussion 4.1. Phase development Single-phase samples were observed by XRD for SrTiO3 and La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) after sintering at 1500 °C for 10 h in forming gas, as shown in Fig. 2. The samples with x 6 0.10 showed a single cubic perovskite structure. Compared with that of SrTiO3, the XRD peaks of La0.3Sr0.7TiO3d had no obvious shift. However, with increasing Sc-doping amount, the XRD peaks of La0.3Sr0.7ScxTi1xO3d shifted to the left gradually. The lattice parameters calculated by Rietveld refinement for La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) were 3.905, 3.914, 3.915, and 3.916 Å, respectively. The lattice parameter changed little when La was doped in SrTiO3 (3.905 Å), while it increased with Sc-doping in La0.3Sr0.7TiO3d. This mainly resulted from that the ionic radius of XIILa3+ (1.36 Å) is close to that of XIISr2+ (1.44 Å) but the radius of VISc3+ (0.745 Å) is larger than both of VITi4+ (0.605 Å) and VITi3+ (0.670 Å) [17].
Fig. 2. XRD patterns of La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) after sintering at 1500 °C for 10 h in forming gas: (a) 10 6 2h 6 90° and (b) 30 6 2h 6 47°.
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4.2. Effect of La and Sc co-doping on the electrical characteristics of SrTiO3 The electrical conductivities and the ionic conductivities of SrTiO3 and La0.3Sr0.7ScxTi1xO3d samples were measured in forming gas from 25 to 1000 °C and in flowing forming gas saturated with room-temperature water from 500 to 1000 °C, respectively. The electrical conductivity of SrTiO3 without doping was very low, while it was remarkably enhanced by La-doping (Fig. 3). The La0.3Sr0.7TiO3d sample exhibited an electrical conductivity on the order of 130–397 S cm1 in 50–1000 °C and 216 S cm1 at 800 °C. With the increase of Sc-doping, the electrical conductivity of La0.3Sr0.7ScxTi1xO3d decreased (Fig. 3), which suggested the decreased concentration of Ti3+ in La0.3Sr0.7ScxTi1xO3d. With increasing temperature, the electrical conductivity increased through a maximum then decreased, indicating a polaron conduction behavior of La0.3Sr0.7ScxTi1xO3d anode [18,19]. The electrical conductivities of La0.3Sr0.7Sc0.05Ti0.95O3d and La0.3Sr0.7Sc0.10Ti0.90O 1 at 800 °C, respectively. On the other 3d were 118 and 49 S cm hand, the ionic conductivity of La0.3Sr0.7ScxTi1xO3d increased with increasing Sc-doping amount remarkably (Fig. 4). The ionic conductivity of La0.3Sr0.7TiO3d was only 3 103 S cm1 at 800 °C and oxygen partial pressure of 1019 atm, while it increased to 4 103 S cm1 for La0.3Sr0.7Sc0.05Ti0.95O3d and 1 102 S cm1 for La0.3Sr0.7Sc0.10- Ti0.90O3d. Compared with La0.3Sr0.7TiO3d, the ionic conductivity of La0.3Sr0.7Sc0.10Ti0.90O3d increased about 230%. The ionic conductivity is primarily dependent on the oxygen vacancy concentration and the migration energy of oxygen ion at a certain temperature. The oxygen vacancy concentration of La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.10) were estimated to be about 0.12, 0.14 and 0.18, respectively, by EPMA. It showed an increasing trend with increasing Sc-doping amount. Additionally, the activation energy for oxygen ion migration (Ea) was calculated by Arrhenius rule, according to the results shown in Fig. 4. As shown in inset of Fig. 4, Ea of La0.3Sr0.7ScxTi1xO3d decreased with increasing Sc amount, indicating that Sc-doping facilitates the conduction of oxygen ions, which agreed well with the calculation results shown in Section 2. 4.3. Charge compensation mechanism of La, Sc co-doped SrTiO3 For La-doped SrTiO3, La0.3Sr0.7TiO3d, Ti3+ ions will be produced as the electrovalent compensation in reducing condition. The defects formed in La donor doped SrTiO3 can be expressed as:
Fig. 4. Temperature dependence of ionic conductivity of La0.3Sr0.7ScxTi1xO3d (x = 0, 0.05, 0.08, 0.10) measured in flowing forming gas saturated with roomtemperature water. La and Sc co-doped SrTiO3 as novel anode materials for solid oxide fuel cells.
SrO
3þ0
mLa2 O3 þ 2mTiO2 $ 2mLaSr þ 2mTiTi4þ þ 6mOO þ m=2O2 ðgÞ
ð1Þ
3+
Oxygen vacancies and Ti ions also may occur at low oxygen partial pressure, which can be described as: 3þ0
nTiO2 $ nTiTi4þ þ ð2n n=2ÞOO þ n=2VO þ n=4O2 ðgÞ
ð2Þ
As a whole result, the defects formed in La-doped SrTiO3 will be: SrO
3þ0
mLa2 O3 þ ð2m þ nÞTiO2 $ 2mLaSr þ ð2m þ nÞTiTi4þ þ ð6m þ ð2n n=2ÞÞOO þ n=2VO þ ð2m þ nÞ=4O2 ðgÞ
ð3Þ
In this case, electrons bonded on Ti ions (Ti3+) and oxygen vacancies are the main defects. Due to the low mobility of oxygen vacancies compared with electrons, oxygen vacancies make little contribution to the total conductivity of La-doped SrTiO3. As a result, the electrical conductivity of La-doped SrTiO3 in reducing condition mostly depends on the concentration of Ti3+. The concentration of Ti3+ at a constant oxygen partial pressure can be expressed as:
h
i 3þ0 TiTi4þ ¼ LaSr þ 2 VO
ð4Þ
The number of compensated Ti3+ with localized electrons depends on the amount of La incorporated in SrTiO3 and oxygen vacancy concentration related to the oxygen partial pressure. Thus the electrical conductivity of La0.3Sr0.7TiO3d is much higher than that of SrTiO3. For La0.3Sr0.7TiO3d, there are ½LaSr ¼ 0:3 and 3þ0 4þ ½VO ¼ d, then ½TiTi4þ ¼ 0:3 þ 2d and ½Ti ¼ 0:7 2d. The solid 4þ 3þ solution formula can be described as La0:3 Sr0:7 Ti0:72d Ti0:3þ2d O3d . With respect to Sc-doping sample La0.3Sr0.7ScxTi1xO3d, there are three possible charge compensation mechanisms. The first possibility is Sc3+ ions in place of Ti4+ ions, which can be expressed as 4þ 3þ La0:3 Sr0:7 Sc3þ x Ti0:72dx Ti0:3þ2d O3ðdþx=2Þ . The defects produced in Sc acceptor doping La0.3Sr0.7TiO3d can be expressed as: TiO2
1=2Sc2 O3 $ Sc0Ti4þ þ 3=2OO þ 1=2VO Fig. 3. Temperature dependence of electrical conductivity of SrTiO3 and La0.3Sr0.7ScxTi1xO3d (x = 0, 0.03, 0.05, 0.08, 0.10) measured in flowing forming gas.
ð5Þ
In this case, the ionic conductivity of La0.3Sr0.7TiO3d may increase because of the increased oxygen vacancy concentration. On the other hand, the electronic conductivity will not change
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due to the unchanged concentration of Ti3+ ions. As a result, the total conductivity should increase slightly, which is inconsistent with the experimental results. The second possible mechanism can be expressed as La0:3 Sr0:7 4þ 3þ 3+ Sc3þ ions take the place of Ti3+ ions. x Ti0:72d Ti0:3þ2dx O3d , i.e., all Sc The defects produced by Sc acceptor doping in La0.3Sr0.7TiO3d can be described as: Ti2 O3
1=2Sc2 O3 $ ScTi3þ þ 3=2OO
ð6Þ
In this case, the electronic conductivity of La0.3Sr0.7TiO3d may decrease for the reduced amount of Ti3+ and oxygen vacancy concentration will keep unchanged. This conflicts apparently with oxygen vacancy concentration estimated by EPMA. The last possible mechanism can be described as La0:3 Sr0:7 4þ 3þ Sc3þ x Ti0:72dx1 Ti0:3þ2dx2 O3ðdþx1 =2Þ (x = x1 + x2), i.e., Eqs. (5) and (6) may occur simultaneously. Sc3+ ions replace both Ti4+ and Ti3+ sites. The electronic conductivity of sample should decrease due to the lowered Ti3+ content, while the ionic conductivity ought to increase owing to the increased oxygen vacancy concentration. Due to the lower mobility of oxygen vacancy compared with electron, oxygen vacancy will make less contribution to the total conductivity of Sc-doped La0.3Sr0.7TiO3d material. Therefore, the total electrical conductivity decreases with increasing Sc-doping amount, which agrees well with the experimental results. 5. Conclusions The oxygen ion migration energy in SrScO3 was the lowest among SrBO3 (B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Cd, In, Sn and Sb) according to first-principles calculations. Accordingly, Sc was selected as B-site dopant to enhance the ionic conductivity of SrTiO3-based anode materials. With increasing Sc-doping amount, the electrical conductivity of La0.3Sr0.7ScxTi1xO3d decreased in 25–1000 °C, while the ionic conductivity increased significantly between 500 and 1000 °C. The electrical and ionic conductivities were 118 S cm1 and 4 103 S cm1 for La0.3Sr0.7Sc0.05Ti0.95O3d and 49 S cm1 and
1 102 S cm1 for La0.3Sr0.7Sc0.10Ti0.90O3d at 800 °C, respectively. Compared with La0.3Sr0.7TiO3d, the ionic conductivity of La0.3 Sr0.7Sc0.10Ti0.90O3d increased about 230%. Sc-doping increased oxygen vacancy concentration and decreased the oxygen migration energy, thus promoting the conduction of oxygen ions in La and Sc co-doped SrTiO3 anode materials. The possible charge compensation mechanism of Sc-doped La0.3Sr0.7TiO3d can be described as 4þ 3þ La0:3 Sr0:7 Sc3þ x Ti0:72dx1 Ti0:3þ2dx2 O3ðdþx1 =2Þ (x = x1 + x2). Acknowledgements This work has been funded by National Nature Science Foundation of China (No. 50672009) and 863 Program of National High Technology Research Development Project of China (No. 2006AA11A189). References [1] J. Huang, Z. Mao, Z. Liu, C. Wang, Electrochem. Commun. 9 (2007) 2601. [2] B. Zhu, I. Albinsson, C. Andersson, K. Borsand, M. Nilsson, B. Mellander, Electrochem. Commun. 8 (2006) 495. [3] B. Zhu, X.R. Liu, T. Schober, Electrochem. Commun. 6 (2004) 378. [4] S.P. Jiang, S.H. Chan, Mater. Sci. Technol. 20 (2004) 1109. [5] X.J. Chen, Q.L. Liu, S.H. Chan, N.P. Brandon, K.A. Khor, Electrochem. Commun. 9 (2007) 767. [6] J.H. Wang, M. Liu, Electrochem. Commun. 9 (2007) 2212. [7] M. Gong, X. Liu, J. Trembly, C. Johnson, J. Power Sources 168 (2007) 89. [8] O.N. Tufte, P.W. Chapman, Phys. Rev. 155 (1967) 796. [9] H.P.R. Frederikse, W.R. Hosler, Phys. Rev. 161 (1967) 822. [10] S.Q. Hui, A. Petric, Mater. Res. Bull. 37 (2002) 1215. [11] O.A. Marina, N.L. Canfield, J.W. Stevenson, Solid State Ionics 149 (2002) 21. [12] M.A. Peña, J.L.G. Fierro, Chem. Rev. 101 (2001) 981. [13] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, J. Phys.: Condens. Mater. 14 (2002) 2717. [14] J. Carrasco, N. Lopez, F. Illas, Phys. Rev. Lett. 93 (2004) 225502. [15] X. Li, H. Zhao, W. Shen, F. Gao, X. Huang, Y. Li, Z. Zhu, J. Power Sources 166 (2007) 47. [16] X. Li, H. Zhao, F. Gao, Z. Zhu, N. Chen, W. Shen, Solid State Ionics 179 (2008) 1588. [17] R.D. Shannon, Acta Crystallogr. 32 (1976) 751. [18] M. Sogaars, P.V. Hendriksen, M. Mogensen, J. Solid State Chem. 180 (2007) 1489. [19] P. Blennow, K.K. Hansen, L.R. Wallenberg, M. Mogensen, Electrochim. Acta 52 (2006) 1651.