- Email: [email protected]
Influence of the transition-metal doping on conductivity of a BaCeO3-based protonic conductor
Solid State Ionics 176 (2005) 2945 – 2950 www.elsevier.com/locate/ssi
Influence of the transition-metal doping on conductivity of a BaCeO3-based protonic conductor Tetsuo Shimura a,*, Hiroomi Tanaka a, Hiroshige Matsumoto b, Toshinobu Yogo a b
a EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1, 1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan
Abstract The effect of transition metal (Mn, Fe and Co) doping to the Ce-site of the perovskite-type protonic conductor BaCe0.90Y0.10O3d was examined. Single-phase samples of BaCe0.90x Y0.10Mx O3d were obtained at 0 x 0.10 for M = Mn and Co and at 0 x 0.075 for M = Fe. The conductivity of the samples in air decreased with transition-metal doping. Among the doped samples, Mn-doped solutions showed the lowest conductivity and the highest activation energy of conduction. The conductivity of these samples was independent of the concentration of the transition metals (x). This behavior of the conductivity could be explained by supposing the generation of the impurity state by transition-metal doping inside the band gap. In hydrogen, samples showed ionic conduction. The influence of transition-metal doping on the conduction behavior in hydrogen is small. D 2005 Elsevier B.V. All rights reserved. PACS: 66.10.Ed Keywords: Perovskite; Protonic conduction; Transition metal
1. Introduction High temperature protonic conduction in the perovskite-type ACeO3, and AZrO3 has gathered much attention since its first discovery [1 –3]. When the lower valence ions are doped at the Ce4+- or Zr4+-site in such perovskites, a vacancy is introduced into the oxide ionic sub lattice to keep electric neutrality. Through the reaction between such vacancies and water vapor in the atmosphere, protons are absorbed into the lattice and oxides show protonic conduction. At present, protonic conduction is confirmed in several perovskites [4,5] and perovskite-related [6– 10] oxides. The position of protons in the perovskites has been determined by neutron diffraction analysis [11]. There are many possible applications for high temperature protonic conductors. Among them, the electrolyte of the solid oxide fuel cell (SOFC) is the most promising. Since the generation of water occurs at the cathode, an SOFC using a protonic conductor does not need water management in the fuel gas. This may be an advantage in the construction of a large scale SOFC system.
* Corresponding author. Tel.: +81 52 789 2750; fax: +81 52 789 2133. E-mail address: [email protected] (T. Shimura). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.027
At present, however, there are some problems in proton conducting oxides for this actual application. One is the low chemical stability at elevated temperatures. Another problem is the low ionic transport number under high oxygen partial pressure condition like in air. In such an atmosphere, the dominant conduction carriers in protonic conductors are electron holes [6,12 –14]. The low ionic transport number causes a decrease in the open circuit voltage of the H2-air fuel cell using a protonic conductor as electrolyte [15]. The generation mechanism of the electron hole under high oxygen partial pressure ( PO2) conditions is expressed as follows; VP þ 1 = 2 O2 ¼ Oox þ 2h:
ð1Þ
It is apparent from this relation that the concentration of the electron hole (h &) depends on PO2 with a slope of PO21 / 4 if the concentration of the oxide ionic vacancy can be regarded as constant. Since the mobility of the hole is larger than that of ions, holes become dominant conduction carrier and total conductivity depends on PO2 with a slope of PO21 / 4. In this paper, a decrease in the hole conduction in a perovskite-type protonic conductor with transition-metal doping is reported. Doping with a transition metal into ionic conduction materials has rarely been examined [16,17]. Such
2946
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
T/oC
doping has been considered to lower the chemical stability and to enhance electronic conductivity. However, in our results, the electron hole conductivity in BaCeO3-based protonic conductor apparently decreases with Mn-, Fe- or Co-doping, and there is no apparent decrease in stability of the material.
1000
900
10 2
800
(i) Mn-doped solutions; BaCe0.90x Y0.10Mnx O3d (x = 0, 0.05, 0.075 and 0.10), BaCe0.90Y0.05Mn0.05O3d . (ii) Fe-doped solutions; BaCe0.90x Y0.10Fex O3d (x = 0, 0.05, 0.075 and 0.10). (iii) Co-doped solutions; BaCe0.90x Y0.10Cox O3d (x = 0, 0.05, 0.075 and 0.10), BaCe0.85Sc0.10Co0.05O3d . (iv) Mn- and Fe-doped solution; BaCe 0.80 Y 0.10 Mn 0.05 Fe0.05O3d . The samples were prepared by conventional solid state reaction methods. Starting materials were BaCO3, CeO2, Y2O3, Sc2O3, MnO, Fe2O3 and Co3O4. The powders of the starting materials were mixed in an agate mortar with ethanol and then pressed into a pellet. The pellet was calcined at 1000 -C (Cocontaining samples) or 1200 -C (Mn- or Fe-containing samples) in air for 10 h. Samples were reground and then pressed into a pellet by isostatic pressing of 200 MPa. Dense ceramic samples were obtained by sintering the pellets at 1300 -C (Co-containing samples), 1500 -C (Fe-containing samples) or 1550 -C (Mn containing samples). Phase relations of sintered samples were examined by X-ray diffraction (XRD) analysis at room temperature. Electric conductivities of the samples were measured by a dc 4-probe method at between 600 and 1000 -C in wet air and
σ T/Scm-1K
In this study, samples with the following compositions were prepared and their properties were examined.
BaCe 0.9Y 0.1O 3-δ
10 0
BaCe 0.90-xY 0.10Fe xO 3-δ x = 0.05 x = 0.075
10 -1
10 -2
0.8
0.9 1.0 T-1/kK-1
1000 10 2
900
800
1.1
Fig. 2. The conductivity of BaCe0.90x Y0.10Fex O3d (x = 0, 0.05 and 0.075) in air.
in wet hydrogen. In some samples, ionic transport numbers were estimated from the electromotive force (emf) of the gas concentration cell measurements using the samples as an electrolyte. For these measurements, porous platinum was used as electrode. 3. Results and discussions From XRD analysis, the formation of perovskite-type phases are confirmed in all samples. There are no extra diffraction peaks from impurity phase in any XRD patterns, except BaCe0.80Y0.10Fe0.10O3d , where a small diffraction peak from BaY2O4 is observed, indicating the low solubility
T/oC
T/oC 700
600
1000 10 2
BaCe 0.9Y 0.1O 3-δ
10 1
900
800
700
600
BaCe0.9Y0.1O3-δ
10 1
BaCe 0.90-xY 0.10Mn xO 3-δ x = 0.05 x = 0.075 x = 0.10
10 0
10 -1
σ T/Scm-1K
σ T/Scm-1K
600
10 1
2. Experimental
10 -2
700
10 0
BaCe 0.90-xY0.10Co xO 3-δ x = 0.05 x = 0.075 x = 0.10
10 -1
0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 1. The conductivity of BaCe0.90x Y0.10Mnx O3d (x = 0, 0.05, 0.075 and 0.10) in air.
10 -2
0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 3. The conductivity of BaCe0.90x Y0.10Cox O3d (x = 0, 0.05, 0.075 and 0.10) in air.
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
2947
Table 1 The activation energy (E a) of conduction in air Sample
BaCe0.90x Y0.10O3d
BaCe0.85Y0.10Co0.05O3d
BaCe0.85Y0.10Fe0.05O3d
BaCe0.85Y0.10Mn0.05O3d
E a (eV) E a (103 K)
0.61 7.07
0.71 8.23
0.81 9.39
1.19 13.80
limit of yttrium in the Fe-doped BaCeO3. The color of all the samples is black. The conductivities of BaCe0.90x Y0.10Mnx O3d , BaCe0.90x Y0.10Fex O3d and BaCe0.90x Y0.10Cox O3d in air are shown in Figs. 1 –3, respectively. The content of yttrium is the same in all samples shown in the figures. It is apparent that transitionmetal doping clearly decreases the conductivity in air. The magnitude of the decrease is largest in Mn-doped solutions and smallest in Co-doped solutions. In addition, conductivity of BaCe0.90x Y0.10Mx O3d (M = Mn, Fe and Co) in air is nearly independent of x, as shown in the figures. The activation energy of conductivity changes with doping. Table 1 summarizes the activation energy calculated from the conductivity above 700 -C. The activation energy depends only on the type of doping metal. These results on BaCe0.90x Y0.10Mx O3d can be explained by the following mechanism. The valence band of the perovskite-type oxide is, if it contains only the ions with no d electrons ( like Ce4+ and Y3+ ) at B-site, considered to be originated from 2p orbital of O2 (Fig. 4a) [18 –22]. When the oxide ions occupy the vacancy Conduction band Ce4f
sites through the reaction in Eq. (1), a new impurity band with O2p character will appear just above the top of the valence band (Fig. 4b). Since there is no electron to occupy it, this level is empty and it behaves as a hole. This is the case for holes in BaCe0.90Y0.10O3d. The activation energy of the conduction in BaCe0.90Y0.10O3d will thus, represent the energy difference between the top of the valence band and impurity level that appears through the reaction [21]. In the transition-metal doped oxides, if the concentration of transition metal is not high enough to form the new valence band, the impurity level originating from the 3d orbital will be formed in the band structure of the original material (Fig. 4c). When this level is in the band gap, the introduced holes will occupy this impurity level if the density-of-states of this impurity level is higher than the concentration of holes (Fig. 4d). In this case, the activation energy of conduction will be the energy difference between the top of the valence band and the bottom of the impurity band with 3d orbital character. These energy differences estimated from the conductivities are summarized in Table 1. The reason for the very small dependency of conductivity on transition-metal content is thus explained. The concentration of the holes introduced through the reaction with atmosphere is determined by the equilibrium constant of Eq. (1) as follows: ½h ¼ K½Oox 1=2 ½V¨o 1 = 2 PO2 : 1=4
K is the equilibrium constant of Eq. (1). This is not the concentration of the conductive holes. Hole conduction is caused by the holes in the valence band. The density of the
Valence band O2p
(a) BaCe 0.90Y0.10 O 2.95
(b) BaCe 0.90Y 0.10O 3-δ ( δ < 0.05)
1000 900 10
M3d
T/oC 800 700
600
2
10 1
(d) BaCe 0.90-xY 0.10M xO 3-δ ( δ < 0.05)
Fig. 4. Schematic illustrations of the proposed electronic structures of: (a) BaCe0.90Y0.10O2.95; the conduction band has Ce4+ 4f orbital character and valence band originating from the O2 2p orbital. (b) BaCe0.90Y0.10O3d (d < 0.05); some of the oxide vacancies are occupied by oxide ions absorbed from the atmosphere, leading an increase in the O2p density-of-states with the holes located at the top of the valence band. (c) BaCe0.90x Y0.10Mx O2.95; a small impurity state forms in the band gap with M 3d orbital character. (d) BaCe0.90x Y0.10Mx O3d (d < 0.05); the holes are trapped in the impurity state with the conduction process including the excitation of carriers from the impurity state to the valence band.
σ T/Scm-1K
(c) BaCe 0.90-xY 0.10M xO 2.95
ð2Þ
10 0
BaCe 0.85Y0.10 Co 0.05 O 3-δ BaCe 0.85 Sc 0.10Co 0.05 O 3-δ
10 -1
10 -2
0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 5. The conductivity of BaCe0.85Y0.10Co0.05O3d and BaCe0.85Sc0.10 Co0.05O3d in air.
2948
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
T/ o C 1000
900
800
T/o C 700
600
1000 900
10 2
10
2
10
1
10
0
800
700
600
10
10
0
10 -1
10 -2
σ T/Scm-1 K
σ T/Scm-1K
BaCe 0.85Y 0.10Fe 0.05O 3-δ 1
BaCe 0.85Y0.10 Mn0.05O3-δ
10
BaCe 0.85Y 0.10Mn 0.05 O 3-δ BaCe 0.80Y 0.10Mn 0.05Fe 0.05O 3-δ
BaCe 0.90Y0.05Mn 0.05 O3-δ 10
0.8
0.9
1.0
-1
1.1
-2
0.8
0.9
1.0
1.1
T -1/kK-1
T -1/kK-1 Fig. 6. The conductivity of BaCe0.80Y0.10Mn0.05Fe0.05O3d compared to that of BaCe0.85Y0.10Mn0.05O3d and BaCe0.85Y0.10Fe0.05O3d in air.
holes excited into the valence band from impurity level is given by: ½h expð Ea = T Þ¼ K½Oox 1 = 2 ½V¨o 1 = 2 PO2 1 = 4 expð Ea=T Þ ð3Þ This means that the hole conductivity depends on the oxide ionic vacancy concentration introduced through doping with yttrium ion (Y3+) and is independent of the transition-metal concentration. This is supported in the following results. Fig. 5 compares the conductivity of BaCe0.90Sc0.05Co0.05O3d with that of BaCe0.85Y0.10Co0.05O3d in air. Since Sc3+ has no d electrons, the role of Sc3+ in BaCe0.90Sc0.05Co0.05O3d is the introduction of oxide ionic vacancy, and is the same as that of Y3+ in BaCe0.85Y0.10Co0.05O3d . In this case, the hole conductivity of BaCe0.90Sc0.05Co0.05O3d is determined by the E a and the concentration of oxide ionic vacancy introduced by Sc3+ doping, and it must be equal to that of BaCe0.85Y0.10 Co0.05O3d . It is clear from the figure that the conductivity of BaCe0.90Sc0.05Co0.05O3d is close to that of BaCe0.85 Y0.10Co0.05O3d . Above 700 -C both conductivities are
Fig. 8. The conductivity of BaCe0.85Y0.10Mn0.05O3d and BaCe0.90Y0.05 Mn0.05O3d in air.
equivalent. This result supports our band structure. In addition, the result indicates that the equilibrium constant K is not sensitive to the type of doping element. Fig. 6 shows the conductivity of BaCe0.80Y0.10Mn0.05 Fe 0.05 O 3d with that of BaCe 0.85 Y 0.10 Mn 0.05 O 3d and BaCe0.85Y0.10Fe0.05O3d . It is clear that the conductivity of Mn- and Fe-substituted solution is same as that of only Mnsubstituted solution. This result also supports the proposed band structure. In both Mn- and Fe-doped oxides, two impurity levels will appear in the band gap. One has Mn 3d and other Fe 3d character (Fig 7a). The hole generated via reaction given in Eq. (1) enters the highest energy Mn 3d band (Fig. 7b) because this level has the lowest energy for positive charged hole. Since the conduction is caused by the holes excited to the valence band with O2p character, the activation energy of the co-doped oxides becomes the same as that of the Mn-doped oxide. As given in Eq. (3), the conductivity linearly depends on the concentration of the oxide ionic vacancy. Fig. 8 compares the conductivity of BaCe 0.90 Y 0.05 Mn 0.05 O 3d with that of BaCe0.85Y0.10Mn0.05O3d . It is clear from the figure that BaCe0.90Y0.05Mn0.05O3d shows lower conductivity than
Conduction band Ce4f
Mn3d Fe3d Valence band O2p
(a) BaCe 0.80Y0.10 Mn0.05 Fe0.05O2.95 (b) BaCe 0.80 Y0.10Mn 0.05Fe 0.05O 3-δ (δ < 0.05) Fig. 7. A schematic illustration of the electronic structure of BaCe0.80Y0.10Mn0.05Fe0.05O3d . (a) No hole state. (b) Hole-doped state. Holes enter the impurity state with Mn 3d character.
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
T/ o C 1000 900 800 700 10 2 BaCe 0.90-xY0.10 M xO3-δ
BaCe0.85Y0.10Mn0.05O3d . This low conductivity may be due to the low concentration of vacancies. Ionic transport number is estimated in some samples by an oxygen concentration experiment using the sample disc as an electrolyte. In Fig. 9, the emfs of the cells using BaCe0.85Y0.10Mn0.05O3d and BaCe0.825Y0.10Co0.075O3d are shown:
The solid line indicates the theoretical emf of the pure ionic conductor given the by Nernst equation. Above 900 -C, the emfs of both cells are small, indicating that the dominant carriers in these oxides are electron holes. Below 800 -C, there is a difference between the two emfs. The emf of the cell using BaCe0.85Y0.10Mn0.05O3d shows an apparent increase with temperature decrease. The transport number in BaCe0.85 Y0.10Mn0.05O3d at 600 -C is 0.60. In the cell using BaCe0.825Y0.10Co0.075O3d , only a small increase in the emf appears at 600 -C and the estimated transport number at 600 -C is 0.27. This means that, by reducing the electronic conductivity, the ionic transport number in a BaCeO3-based ionic conductor can be enhanced. The conductivity of the BaCe0.90x Y0.10Mx O3d (M = Mn, Fe and Co) in hydrogen is not the same as that of BaCe0.90Y0.10O3d . However, in hydrogen, the change in conductivity caused by substitution of a transition metal is not large compared to that in air. The profile of the temperature dependence of the conductivity is same for all samples. Fig. 10 shows the conductivities of BaCe0.90x Y0.10Mx O3d . It is clear that there is no systematic change in the activation energy of conductivity by substitution. Among all samples, the Fe-doped solution shows the lowest conductivity in hydrogen. It is about one order of magnitude lower than that of BaCe0.90Y0.10O3d at each temperature measured. The highest conductivity is for 50
air, Pt I BaCe 0.90-xY0.10 MxO3-δ I Pt, O2
emf/mV
40
theoretical
30 M = Mn, x = 0.05 20
M = Co, x = 0.075
10
0 600
700
800
900
1000
T/IoC Fig. 9. The emf of the oxygen concentration cell using BaCe0.85Y0.10 Mn0.05O3d and BaCe0.825Y0.10Co0.075O3d .
M=Mn
M=Co
σ T/Scm-1K
Air, Pt | BaCe0.85Y0.10Mn0.05O3d | Pt, O2 Air, Pt | BaCe0.825Y0.10Co0.075O3d | Pt, O2.
2949
600 x = 0.05 x = 0.075 x = 0.10 x = 0.05 x = 0.075 x = 0.10
10 1
M=Fe
10 0
x = 0.05 x = 0.075
BaCe 0.9Y0.1O3-δ 0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 10. The conductivity of BaCe0.90x Y0.10Mx O3d in hydrogen.
the 7.5% Co-doped one, and it is slightly higher than that of BaCe0.90Y0.10O3d . The transport numbers of ions in hydrogen are estimated from a hydrogen concentration cell experiment. The emf of the cell: wet H2, Pt I BaCe0.9x Y0.1Mx O3d I Pt, wet 5% H2 95% Aris equal to the theoretical emf given by the Nernst equation. Since humidified gases are used, the emf of this cell represents the sum of protonic and oxide ionic contributions to the conduction. This result thus means that the conduction carriers in BaCe0.9x Y0.1Mx O3d in hydrogen are ions. There is no sign of decomposition in the samples after the experiments in hydrogen. No diffraction peaks from impurity phases are observed in XRD analysis after the measurement. This indicates that the transition-metal substitution does not lower the chemical stability in a reducing atmosphere, if their concentration is not greater than 10%. These tendencies in hydrogen indicate that transition-metal doping does not change the conduction mechanism of ions in hydrogen. They may change only the concentration of the carriers. The reason for this change is not clear at present. Further investigations are necessary to interpret the effect of transition-metal substitution on conduction properties in hydrogen. 4. Summary The conduction behavior of Mn-, Fe- and Co-doped BaCe0.90Y0.10O3d has been investigated. In air, the conductivity decreases with transition-metal substitution of the B-site. Among all the samples examined, Mn-doped BaCe 0.90 Y0.10O3d shows the lowest conductivity and the highest activation energy. The conductivity is independent of concentration of the transition metal. This behavior is interpreted by the hole-trapping effect of the transition metal 3d orbital. In hydrogen, transition-metal doped BaCe0.90Y0.10O3d shows protonic conduction; the ionic transport number is unity.
2950
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
Acknowledgement This study was supported by an Industrial Technology Research Grant Program (02B65001c) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [2] H. Iwahara, H. Uchida, K. Ono, K. Ogaki, Journal of the Electrochemical Society 135 (1988) 529. [3] T. Yajima, H. Kazeoka, T. Yogo, H. Iwahara, Solid State Ionics 47 (1991) 271. [4] K.C. Liang, A.S. Nowick, Solid State Ionics 61 (1993) 77. [5] K.C. Liang, Y. Du, A.S. Nowick, Solid State Ionics 69 (1994) 177. [6] P. Murugaraj, K.D. Kreuer, T. He, T. Schober, J. Maier, Solid State Ionics 98 (1997) 1. [7] T. Schober, Solid State Ionics 109 (1998) 1. [8] I. Animitsa, T. Norby, S. Marion, R. Glo¨ckner, A. Neiman, Solid State Ionics 145 (2001) 357. [9] D.J.D. Corcoran, J.T.S. Irvine, Solid State Ionics 145 (2001) 307.
[10] T. Shimura, K. Suzuki, H. Iwahara, Solid State Ionics 113 – 115 (1998) 355. [11] N. Sata, S. Shin, S. Hosoya, N. Niimura, K. Hiramoto, M. Ishigame, Physical Review, B 54 (1996) 15795. [12] S. Shin, H.H. Huang, M. Ishigame, H. Iwahara, Solid State Ionics 40/41 (1990) 910. [13] J. Mu¨ller, K.D. Kreuer, J. Maier, S. Matsuo, M. Ishigame, Solid State Ionics 97 (1997) 421. [14] H. Iwahara, Solid State Ionics 125 (1999) 271. [15] H. Iwahara, T. Yajima, T. Hibino, H. Ushida, Journal of the Electrochemical Society 140 (1993) 1687. [16] T. Shimura, K. Suzuki, H. Iwahara, Solid State Ionics 125 (1999) 313. [17] T. Shimura, K. Esaka, H. Matsumoto, H. Iwahara, Solid State Ionics 149 (2002) 237. [18] L.F. Matheiss, Physical Review 181 (1972) 987. [19] L.F. Matheiss, Physical Review., B 6 (1972) 4718. [20] T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, Y. Tezuka, S. Shin, Physical Review, B 57 (1998) 6978. [21] T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, K. Kobayashi, S. Yamaguchi, Y. Ishiwata, T. Yokota, M. Fujiwara, S. Shin, Solid State Ionics 136 – 137 (2000) 261. [22] T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, K. Kobayashi, S. Yamaguchi, S. Shin, Solid State Ionics 154 – 155 (2002) 735.
Influence of the transition-metal doping on conductivity of a BaCeO3-based protonic conductor Tetsuo Shimura a,*, Hiroomi Tanaka a, Hiroshige Matsumoto b, Toshinobu Yogo a b
a EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1, 1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan
Abstract The effect of transition metal (Mn, Fe and Co) doping to the Ce-site of the perovskite-type protonic conductor BaCe0.90Y0.10O3d was examined. Single-phase samples of BaCe0.90x Y0.10Mx O3d were obtained at 0 x 0.10 for M = Mn and Co and at 0 x 0.075 for M = Fe. The conductivity of the samples in air decreased with transition-metal doping. Among the doped samples, Mn-doped solutions showed the lowest conductivity and the highest activation energy of conduction. The conductivity of these samples was independent of the concentration of the transition metals (x). This behavior of the conductivity could be explained by supposing the generation of the impurity state by transition-metal doping inside the band gap. In hydrogen, samples showed ionic conduction. The influence of transition-metal doping on the conduction behavior in hydrogen is small. D 2005 Elsevier B.V. All rights reserved. PACS: 66.10.Ed Keywords: Perovskite; Protonic conduction; Transition metal
1. Introduction High temperature protonic conduction in the perovskite-type ACeO3, and AZrO3 has gathered much attention since its first discovery [1 –3]. When the lower valence ions are doped at the Ce4+- or Zr4+-site in such perovskites, a vacancy is introduced into the oxide ionic sub lattice to keep electric neutrality. Through the reaction between such vacancies and water vapor in the atmosphere, protons are absorbed into the lattice and oxides show protonic conduction. At present, protonic conduction is confirmed in several perovskites [4,5] and perovskite-related [6– 10] oxides. The position of protons in the perovskites has been determined by neutron diffraction analysis [11]. There are many possible applications for high temperature protonic conductors. Among them, the electrolyte of the solid oxide fuel cell (SOFC) is the most promising. Since the generation of water occurs at the cathode, an SOFC using a protonic conductor does not need water management in the fuel gas. This may be an advantage in the construction of a large scale SOFC system.
* Corresponding author. Tel.: +81 52 789 2750; fax: +81 52 789 2133. E-mail address: [email protected] (T. Shimura). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.027
At present, however, there are some problems in proton conducting oxides for this actual application. One is the low chemical stability at elevated temperatures. Another problem is the low ionic transport number under high oxygen partial pressure condition like in air. In such an atmosphere, the dominant conduction carriers in protonic conductors are electron holes [6,12 –14]. The low ionic transport number causes a decrease in the open circuit voltage of the H2-air fuel cell using a protonic conductor as electrolyte [15]. The generation mechanism of the electron hole under high oxygen partial pressure ( PO2) conditions is expressed as follows; VP þ 1 = 2 O2 ¼ Oox þ 2h:
ð1Þ
It is apparent from this relation that the concentration of the electron hole (h &) depends on PO2 with a slope of PO21 / 4 if the concentration of the oxide ionic vacancy can be regarded as constant. Since the mobility of the hole is larger than that of ions, holes become dominant conduction carrier and total conductivity depends on PO2 with a slope of PO21 / 4. In this paper, a decrease in the hole conduction in a perovskite-type protonic conductor with transition-metal doping is reported. Doping with a transition metal into ionic conduction materials has rarely been examined [16,17]. Such
2946
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
T/oC
doping has been considered to lower the chemical stability and to enhance electronic conductivity. However, in our results, the electron hole conductivity in BaCeO3-based protonic conductor apparently decreases with Mn-, Fe- or Co-doping, and there is no apparent decrease in stability of the material.
1000
900
10 2
800
(i) Mn-doped solutions; BaCe0.90x Y0.10Mnx O3d (x = 0, 0.05, 0.075 and 0.10), BaCe0.90Y0.05Mn0.05O3d . (ii) Fe-doped solutions; BaCe0.90x Y0.10Fex O3d (x = 0, 0.05, 0.075 and 0.10). (iii) Co-doped solutions; BaCe0.90x Y0.10Cox O3d (x = 0, 0.05, 0.075 and 0.10), BaCe0.85Sc0.10Co0.05O3d . (iv) Mn- and Fe-doped solution; BaCe 0.80 Y 0.10 Mn 0.05 Fe0.05O3d . The samples were prepared by conventional solid state reaction methods. Starting materials were BaCO3, CeO2, Y2O3, Sc2O3, MnO, Fe2O3 and Co3O4. The powders of the starting materials were mixed in an agate mortar with ethanol and then pressed into a pellet. The pellet was calcined at 1000 -C (Cocontaining samples) or 1200 -C (Mn- or Fe-containing samples) in air for 10 h. Samples were reground and then pressed into a pellet by isostatic pressing of 200 MPa. Dense ceramic samples were obtained by sintering the pellets at 1300 -C (Co-containing samples), 1500 -C (Fe-containing samples) or 1550 -C (Mn containing samples). Phase relations of sintered samples were examined by X-ray diffraction (XRD) analysis at room temperature. Electric conductivities of the samples were measured by a dc 4-probe method at between 600 and 1000 -C in wet air and
σ T/Scm-1K
In this study, samples with the following compositions were prepared and their properties were examined.
BaCe 0.9Y 0.1O 3-δ
10 0
BaCe 0.90-xY 0.10Fe xO 3-δ x = 0.05 x = 0.075
10 -1
10 -2
0.8
0.9 1.0 T-1/kK-1
1000 10 2
900
800
1.1
Fig. 2. The conductivity of BaCe0.90x Y0.10Fex O3d (x = 0, 0.05 and 0.075) in air.
in wet hydrogen. In some samples, ionic transport numbers were estimated from the electromotive force (emf) of the gas concentration cell measurements using the samples as an electrolyte. For these measurements, porous platinum was used as electrode. 3. Results and discussions From XRD analysis, the formation of perovskite-type phases are confirmed in all samples. There are no extra diffraction peaks from impurity phase in any XRD patterns, except BaCe0.80Y0.10Fe0.10O3d , where a small diffraction peak from BaY2O4 is observed, indicating the low solubility
T/oC
T/oC 700
600
1000 10 2
BaCe 0.9Y 0.1O 3-δ
10 1
900
800
700
600
BaCe0.9Y0.1O3-δ
10 1
BaCe 0.90-xY 0.10Mn xO 3-δ x = 0.05 x = 0.075 x = 0.10
10 0
10 -1
σ T/Scm-1K
σ T/Scm-1K
600
10 1
2. Experimental
10 -2
700
10 0
BaCe 0.90-xY0.10Co xO 3-δ x = 0.05 x = 0.075 x = 0.10
10 -1
0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 1. The conductivity of BaCe0.90x Y0.10Mnx O3d (x = 0, 0.05, 0.075 and 0.10) in air.
10 -2
0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 3. The conductivity of BaCe0.90x Y0.10Cox O3d (x = 0, 0.05, 0.075 and 0.10) in air.
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
2947
Table 1 The activation energy (E a) of conduction in air Sample
BaCe0.90x Y0.10O3d
BaCe0.85Y0.10Co0.05O3d
BaCe0.85Y0.10Fe0.05O3d
BaCe0.85Y0.10Mn0.05O3d
E a (eV) E a (103 K)
0.61 7.07
0.71 8.23
0.81 9.39
1.19 13.80
limit of yttrium in the Fe-doped BaCeO3. The color of all the samples is black. The conductivities of BaCe0.90x Y0.10Mnx O3d , BaCe0.90x Y0.10Fex O3d and BaCe0.90x Y0.10Cox O3d in air are shown in Figs. 1 –3, respectively. The content of yttrium is the same in all samples shown in the figures. It is apparent that transitionmetal doping clearly decreases the conductivity in air. The magnitude of the decrease is largest in Mn-doped solutions and smallest in Co-doped solutions. In addition, conductivity of BaCe0.90x Y0.10Mx O3d (M = Mn, Fe and Co) in air is nearly independent of x, as shown in the figures. The activation energy of conductivity changes with doping. Table 1 summarizes the activation energy calculated from the conductivity above 700 -C. The activation energy depends only on the type of doping metal. These results on BaCe0.90x Y0.10Mx O3d can be explained by the following mechanism. The valence band of the perovskite-type oxide is, if it contains only the ions with no d electrons ( like Ce4+ and Y3+ ) at B-site, considered to be originated from 2p orbital of O2 (Fig. 4a) [18 –22]. When the oxide ions occupy the vacancy Conduction band Ce4f
sites through the reaction in Eq. (1), a new impurity band with O2p character will appear just above the top of the valence band (Fig. 4b). Since there is no electron to occupy it, this level is empty and it behaves as a hole. This is the case for holes in BaCe0.90Y0.10O3d. The activation energy of the conduction in BaCe0.90Y0.10O3d will thus, represent the energy difference between the top of the valence band and impurity level that appears through the reaction [21]. In the transition-metal doped oxides, if the concentration of transition metal is not high enough to form the new valence band, the impurity level originating from the 3d orbital will be formed in the band structure of the original material (Fig. 4c). When this level is in the band gap, the introduced holes will occupy this impurity level if the density-of-states of this impurity level is higher than the concentration of holes (Fig. 4d). In this case, the activation energy of conduction will be the energy difference between the top of the valence band and the bottom of the impurity band with 3d orbital character. These energy differences estimated from the conductivities are summarized in Table 1. The reason for the very small dependency of conductivity on transition-metal content is thus explained. The concentration of the holes introduced through the reaction with atmosphere is determined by the equilibrium constant of Eq. (1) as follows: ½h ¼ K½Oox 1=2 ½V¨o 1 = 2 PO2 : 1=4
K is the equilibrium constant of Eq. (1). This is not the concentration of the conductive holes. Hole conduction is caused by the holes in the valence band. The density of the
Valence band O2p
(a) BaCe 0.90Y0.10 O 2.95
(b) BaCe 0.90Y 0.10O 3-δ ( δ < 0.05)
1000 900 10
M3d
T/oC 800 700
600
2
10 1
(d) BaCe 0.90-xY 0.10M xO 3-δ ( δ < 0.05)
Fig. 4. Schematic illustrations of the proposed electronic structures of: (a) BaCe0.90Y0.10O2.95; the conduction band has Ce4+ 4f orbital character and valence band originating from the O2 2p orbital. (b) BaCe0.90Y0.10O3d (d < 0.05); some of the oxide vacancies are occupied by oxide ions absorbed from the atmosphere, leading an increase in the O2p density-of-states with the holes located at the top of the valence band. (c) BaCe0.90x Y0.10Mx O2.95; a small impurity state forms in the band gap with M 3d orbital character. (d) BaCe0.90x Y0.10Mx O3d (d < 0.05); the holes are trapped in the impurity state with the conduction process including the excitation of carriers from the impurity state to the valence band.
σ T/Scm-1K
(c) BaCe 0.90-xY 0.10M xO 2.95
ð2Þ
10 0
BaCe 0.85Y0.10 Co 0.05 O 3-δ BaCe 0.85 Sc 0.10Co 0.05 O 3-δ
10 -1
10 -2
0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 5. The conductivity of BaCe0.85Y0.10Co0.05O3d and BaCe0.85Sc0.10 Co0.05O3d in air.
2948
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
T/ o C 1000
900
800
T/o C 700
600
1000 900
10 2
10
2
10
1
10
0
800
700
600
10
10
0
10 -1
10 -2
σ T/Scm-1 K
σ T/Scm-1K
BaCe 0.85Y 0.10Fe 0.05O 3-δ 1
BaCe 0.85Y0.10 Mn0.05O3-δ
10
BaCe 0.85Y 0.10Mn 0.05 O 3-δ BaCe 0.80Y 0.10Mn 0.05Fe 0.05O 3-δ
BaCe 0.90Y0.05Mn 0.05 O3-δ 10
0.8
0.9
1.0
-1
1.1
-2
0.8
0.9
1.0
1.1
T -1/kK-1
T -1/kK-1 Fig. 6. The conductivity of BaCe0.80Y0.10Mn0.05Fe0.05O3d compared to that of BaCe0.85Y0.10Mn0.05O3d and BaCe0.85Y0.10Fe0.05O3d in air.
holes excited into the valence band from impurity level is given by: ½h expð Ea = T Þ¼ K½Oox 1 = 2 ½V¨o 1 = 2 PO2 1 = 4 expð Ea=T Þ ð3Þ This means that the hole conductivity depends on the oxide ionic vacancy concentration introduced through doping with yttrium ion (Y3+) and is independent of the transition-metal concentration. This is supported in the following results. Fig. 5 compares the conductivity of BaCe0.90Sc0.05Co0.05O3d with that of BaCe0.85Y0.10Co0.05O3d in air. Since Sc3+ has no d electrons, the role of Sc3+ in BaCe0.90Sc0.05Co0.05O3d is the introduction of oxide ionic vacancy, and is the same as that of Y3+ in BaCe0.85Y0.10Co0.05O3d . In this case, the hole conductivity of BaCe0.90Sc0.05Co0.05O3d is determined by the E a and the concentration of oxide ionic vacancy introduced by Sc3+ doping, and it must be equal to that of BaCe0.85Y0.10 Co0.05O3d . It is clear from the figure that the conductivity of BaCe0.90Sc0.05Co0.05O3d is close to that of BaCe0.85 Y0.10Co0.05O3d . Above 700 -C both conductivities are
Fig. 8. The conductivity of BaCe0.85Y0.10Mn0.05O3d and BaCe0.90Y0.05 Mn0.05O3d in air.
equivalent. This result supports our band structure. In addition, the result indicates that the equilibrium constant K is not sensitive to the type of doping element. Fig. 6 shows the conductivity of BaCe0.80Y0.10Mn0.05 Fe 0.05 O 3d with that of BaCe 0.85 Y 0.10 Mn 0.05 O 3d and BaCe0.85Y0.10Fe0.05O3d . It is clear that the conductivity of Mn- and Fe-substituted solution is same as that of only Mnsubstituted solution. This result also supports the proposed band structure. In both Mn- and Fe-doped oxides, two impurity levels will appear in the band gap. One has Mn 3d and other Fe 3d character (Fig 7a). The hole generated via reaction given in Eq. (1) enters the highest energy Mn 3d band (Fig. 7b) because this level has the lowest energy for positive charged hole. Since the conduction is caused by the holes excited to the valence band with O2p character, the activation energy of the co-doped oxides becomes the same as that of the Mn-doped oxide. As given in Eq. (3), the conductivity linearly depends on the concentration of the oxide ionic vacancy. Fig. 8 compares the conductivity of BaCe 0.90 Y 0.05 Mn 0.05 O 3d with that of BaCe0.85Y0.10Mn0.05O3d . It is clear from the figure that BaCe0.90Y0.05Mn0.05O3d shows lower conductivity than
Conduction band Ce4f
Mn3d Fe3d Valence band O2p
(a) BaCe 0.80Y0.10 Mn0.05 Fe0.05O2.95 (b) BaCe 0.80 Y0.10Mn 0.05Fe 0.05O 3-δ (δ < 0.05) Fig. 7. A schematic illustration of the electronic structure of BaCe0.80Y0.10Mn0.05Fe0.05O3d . (a) No hole state. (b) Hole-doped state. Holes enter the impurity state with Mn 3d character.
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
T/ o C 1000 900 800 700 10 2 BaCe 0.90-xY0.10 M xO3-δ
BaCe0.85Y0.10Mn0.05O3d . This low conductivity may be due to the low concentration of vacancies. Ionic transport number is estimated in some samples by an oxygen concentration experiment using the sample disc as an electrolyte. In Fig. 9, the emfs of the cells using BaCe0.85Y0.10Mn0.05O3d and BaCe0.825Y0.10Co0.075O3d are shown:
The solid line indicates the theoretical emf of the pure ionic conductor given the by Nernst equation. Above 900 -C, the emfs of both cells are small, indicating that the dominant carriers in these oxides are electron holes. Below 800 -C, there is a difference between the two emfs. The emf of the cell using BaCe0.85Y0.10Mn0.05O3d shows an apparent increase with temperature decrease. The transport number in BaCe0.85 Y0.10Mn0.05O3d at 600 -C is 0.60. In the cell using BaCe0.825Y0.10Co0.075O3d , only a small increase in the emf appears at 600 -C and the estimated transport number at 600 -C is 0.27. This means that, by reducing the electronic conductivity, the ionic transport number in a BaCeO3-based ionic conductor can be enhanced. The conductivity of the BaCe0.90x Y0.10Mx O3d (M = Mn, Fe and Co) in hydrogen is not the same as that of BaCe0.90Y0.10O3d . However, in hydrogen, the change in conductivity caused by substitution of a transition metal is not large compared to that in air. The profile of the temperature dependence of the conductivity is same for all samples. Fig. 10 shows the conductivities of BaCe0.90x Y0.10Mx O3d . It is clear that there is no systematic change in the activation energy of conductivity by substitution. Among all samples, the Fe-doped solution shows the lowest conductivity in hydrogen. It is about one order of magnitude lower than that of BaCe0.90Y0.10O3d at each temperature measured. The highest conductivity is for 50
air, Pt I BaCe 0.90-xY0.10 MxO3-δ I Pt, O2
emf/mV
40
theoretical
30 M = Mn, x = 0.05 20
M = Co, x = 0.075
10
0 600
700
800
900
1000
T/IoC Fig. 9. The emf of the oxygen concentration cell using BaCe0.85Y0.10 Mn0.05O3d and BaCe0.825Y0.10Co0.075O3d .
M=Mn
M=Co
σ T/Scm-1K
Air, Pt | BaCe0.85Y0.10Mn0.05O3d | Pt, O2 Air, Pt | BaCe0.825Y0.10Co0.075O3d | Pt, O2.
2949
600 x = 0.05 x = 0.075 x = 0.10 x = 0.05 x = 0.075 x = 0.10
10 1
M=Fe
10 0
x = 0.05 x = 0.075
BaCe 0.9Y0.1O3-δ 0.8
0.9
1.0
1.1
T -1/kK-1 Fig. 10. The conductivity of BaCe0.90x Y0.10Mx O3d in hydrogen.
the 7.5% Co-doped one, and it is slightly higher than that of BaCe0.90Y0.10O3d . The transport numbers of ions in hydrogen are estimated from a hydrogen concentration cell experiment. The emf of the cell: wet H2, Pt I BaCe0.9x Y0.1Mx O3d I Pt, wet 5% H2 95% Aris equal to the theoretical emf given by the Nernst equation. Since humidified gases are used, the emf of this cell represents the sum of protonic and oxide ionic contributions to the conduction. This result thus means that the conduction carriers in BaCe0.9x Y0.1Mx O3d in hydrogen are ions. There is no sign of decomposition in the samples after the experiments in hydrogen. No diffraction peaks from impurity phases are observed in XRD analysis after the measurement. This indicates that the transition-metal substitution does not lower the chemical stability in a reducing atmosphere, if their concentration is not greater than 10%. These tendencies in hydrogen indicate that transition-metal doping does not change the conduction mechanism of ions in hydrogen. They may change only the concentration of the carriers. The reason for this change is not clear at present. Further investigations are necessary to interpret the effect of transition-metal substitution on conduction properties in hydrogen. 4. Summary The conduction behavior of Mn-, Fe- and Co-doped BaCe0.90Y0.10O3d has been investigated. In air, the conductivity decreases with transition-metal substitution of the B-site. Among all the samples examined, Mn-doped BaCe 0.90 Y0.10O3d shows the lowest conductivity and the highest activation energy. The conductivity is independent of concentration of the transition metal. This behavior is interpreted by the hole-trapping effect of the transition metal 3d orbital. In hydrogen, transition-metal doped BaCe0.90Y0.10O3d shows protonic conduction; the ionic transport number is unity.
2950
T. Shimura et al. / Solid State Ionics 176 (2005) 2945 – 2950
Acknowledgement This study was supported by an Industrial Technology Research Grant Program (02B65001c) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [2] H. Iwahara, H. Uchida, K. Ono, K. Ogaki, Journal of the Electrochemical Society 135 (1988) 529. [3] T. Yajima, H. Kazeoka, T. Yogo, H. Iwahara, Solid State Ionics 47 (1991) 271. [4] K.C. Liang, A.S. Nowick, Solid State Ionics 61 (1993) 77. [5] K.C. Liang, Y. Du, A.S. Nowick, Solid State Ionics 69 (1994) 177. [6] P. Murugaraj, K.D. Kreuer, T. He, T. Schober, J. Maier, Solid State Ionics 98 (1997) 1. [7] T. Schober, Solid State Ionics 109 (1998) 1. [8] I. Animitsa, T. Norby, S. Marion, R. Glo¨ckner, A. Neiman, Solid State Ionics 145 (2001) 357. [9] D.J.D. Corcoran, J.T.S. Irvine, Solid State Ionics 145 (2001) 307.
[10] T. Shimura, K. Suzuki, H. Iwahara, Solid State Ionics 113 – 115 (1998) 355. [11] N. Sata, S. Shin, S. Hosoya, N. Niimura, K. Hiramoto, M. Ishigame, Physical Review, B 54 (1996) 15795. [12] S. Shin, H.H. Huang, M. Ishigame, H. Iwahara, Solid State Ionics 40/41 (1990) 910. [13] J. Mu¨ller, K.D. Kreuer, J. Maier, S. Matsuo, M. Ishigame, Solid State Ionics 97 (1997) 421. [14] H. Iwahara, Solid State Ionics 125 (1999) 271. [15] H. Iwahara, T. Yajima, T. Hibino, H. Ushida, Journal of the Electrochemical Society 140 (1993) 1687. [16] T. Shimura, K. Suzuki, H. Iwahara, Solid State Ionics 125 (1999) 313. [17] T. Shimura, K. Esaka, H. Matsumoto, H. Iwahara, Solid State Ionics 149 (2002) 237. [18] L.F. Matheiss, Physical Review 181 (1972) 987. [19] L.F. Matheiss, Physical Review., B 6 (1972) 4718. [20] T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, Y. Tezuka, S. Shin, Physical Review, B 57 (1998) 6978. [21] T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, K. Kobayashi, S. Yamaguchi, Y. Ishiwata, T. Yokota, M. Fujiwara, S. Shin, Solid State Ionics 136 – 137 (2000) 261. [22] T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, K. Kobayashi, S. Yamaguchi, S. Shin, Solid State Ionics 154 – 155 (2002) 735.