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Electrical properties of ZrO2-In2O3-Y2O3 and its application to a membrane for gas separation
SOLID STATE ELSEVIE?R
Solid State
Ionics 90 (1996) 173-176
IOMCS
and its application to a Electrical properties of DO,- In,O,-Y,O, membrane for gas separation Hitoshi Naito*, Hiroo Yugami, Haruo Arashi Faculty of Engineering, Tohoku University, Aoba, Aramaki, Sendai 980-77, Japan
Received 3 December 1995; accepted 18 April 1996
Abstract Electrical properties of In,O,-doped yttria-stabilized zirconia (In-YSZ) were investigated. The solubility limit of In,O, in YSZ (10 mol% Y,O,) is 17.5 mol%. The total conductivity depended on the concentration of $0,. The activation energy of In-YSZ was higher than that of YSZ. From the oxygen partial pressure (PO,) dependence of the total conductivity of In-YSZ, the electronic conductivity increased with increasing In,O, concentration at low oxygen partial pressures and at high temperature. From the results, we discussed the applicability of In-YSZ to a membrane for hydrogen production from direct water splitting at high temperature. Keywords: ZrO,-In,O,-Y,O,;
Membrane for gas separation; Mixed conductor
1. Introduction Mixed conductors have many applications, e.g. as electrodes, electrocatalytic reactors and membranes for gas separation. Hydrogen, which is a renewable and clean energy resource, can be produced from water using a mixed conductor as a membrane for gas separation. Several researchers discussed the hydrogen production system [ 1,2]. Yttria-stabilized zirconia (YSZ)-based mixed conductors are durable up to high temperature and have high ionic conductivity. We are investigating YSZ-based mixed conducting membranes to produce hydrogen from direct water splitting at high temperatures. As YSZ has a high ionic conduction, additives which are doped into YSZ are required to exhibit a high *Corresponding author.
electronic conduction to promote hydrogen production. The electrical properties of ZrO,-TiO,-Y,O, (Ti-YSZ) have been investigated [3-61, and we demonstrated hydrogen production using this mixed conductor [7]. From this result, it is revealed that the YSZ-based mixed conductor is required to have high electronic conductivity to promote the hydrogen production at relatively high oxygen partial pressures. An In ion can exhibit a mono- or trivalent state. The ZrO,-In,O, system shows several types of conduction depending on the concentration of In,O, system and its phase [8,9]. When In,O, is doped into YSZ and maintains a fluorite-type structure, In-YSZ can be one of the most promising materials for mixed conductors. We investigated the electrical properties of In-YSZ and discussed its applicability to a membrane for a hydrogen production.
0167-2738/96/$15.00 Copyright 01996 Elsevier Science B.V.All rights reserved P/I SO167-2738(96)00405-5
174
H. Naito et al. I Solid State Ionics 912 (1996) 173-176
2. Experimental Samples were synthesized by means of a solid state reaction. ZrO,, In,O, and Y,O, powders, with a level of purity of 99.99%, were used as starting materials. The Y,O, concentration ratio was set to 10 mol% to keep the ionic conductivity high. The In,O, concentration ratio was varied from 0 to 20 mol%. Appropriate amounts of the powders were weighed, mixed and ball-milled in methanol for 24 h with a zirconia ball. After being dried at 353 K, mixtures were ground and isostatically pressed into rods at 200 MPa and sintered at 1973 K for 5 h. The crystal structures of the synthesized samples were identified using an X-ray diffractometer (JEOL, JDX-3530). Sintered rods were cut into 1.5 X 3 X 15 mm3 parallelepipeds to measure electrical conductivity. The electrical conductivity was measured using a standard four-probe DC. technique in the temperature range from 873-1673 K in air. The PO, dependence of the electrical conductivity was also measured in the PO, range from 2.1 X lo4 (air) to lo-l6 Pa. The PO, was controlled in flowing H,CO, gas mixtures and by changing the H,-CO, flow ratio.
5
10
15
20
111~0~ concentration [mol%] Fig. 1. The dependence centration of In,O,.
of the lattice
parameter
-d-
on the con-
5mol%ln20,
-
10mol%ln20,
-e-
15mol%In~o,
3. Results and discussion The crystal structures of the samples were determined by using XRD. Samples in which the In,O, concentration was up to 17.5 mol% exhibit a single cubic fluorite structure. Fig. 1 shows the relationship between the lattice parameter of the samples and In,O, concentration. The lattice parameter was calculated by using the (331) peak of the cubic fluorite structure. As the ionic radius of In3+ is smaller than that of Zr4+ [lo], the lattice parameter decreases linearly with increasing In,O, concentration, up to 17.5 mol%. A relative density of 95% was obtained for each sample. The temperature dependence of the total electrical conductivity of In-YSZ is shown in Fig. 2. The conductivity decreased with increasing In,O, concentration. The 15 mol% In,O,-doped YSZ shows almost the same conductivity as that of cubic phase 35 mol% InO,,,-ZrO, from 873 to 1000 K [8]. The activation energy increases with increasing In,O,
Fig. 2. The dependence air on the temperature.
of the electrical conductivity
of In-YSZ
in
concentration. This phenomenon corresponds to the activation energy change with the Y,O, concentration in YSZ [ 111. This indicates that the decrease in the electrical conductivity and the increase in the activation energy by doping In,O, are due to the association and the clustering of oxygen vacancies. The electrical conductivity of 10 mol% In,O,-doped YSZ was almost comparable to that of YSZ at 1673 K because of its higher activation energy. The oxygen partial pressure dependence of the total electrical conductivity at 1273 and 1473 K are shown in Fig. 3. The conductivity of samples containing over 10 mol% In,O, increased with decreasing PO,. This indicates that the n-type elec-
H. Nairo et al. I Solid State Ionics
lo-’ ’ ’ ’ ’ ’ ’ ;,I,:’ ;;-i ’ ;;-a ’ ‘,bJ ’ ’ ‘4 10-m dL Oxygea Partial Pressure PO, rp:; Fig. 3. The oxygen
partial pressure
dependence
In;, + 0;
+
Inlr + Vo + +O,,
(1)
where Kroger and Vi&s notation is used [12]. When we use a mixed conductor as a membrane for hydrogen production from direct water splitting at high temperatures, some characteristics mentioned in the introduction are required. As In-YSZ satisfies these requirements, this mixed conductor is consid-’ ered as one of the candidates. The electrical properties of In-YSZ are compared with those of TiYSZ [6]. The electrical conductivity of In-YSZ (10 mol% In,O,) is almost the same as that of Ti-YSZ (10 mol% TiO,). The slope derived from the relationship between the conductivity and the PO, is small. That is, the electronic conductivity is small. In-YSZ with 15 mol% In,O, shows low electrical conductivity. The electronic conduction of this sam-
175
173-176
]($A ’ ;$a ’ ;;.I: ’ ‘Id-s’’ id-1 ’ ’ ‘(1 ’ ’ ‘I Oxygen Partial Pm.wm Pz &
of the electrical
tronic conduction which occurs is of the same order of magnitude as ionic conduction at low PO,. As the electrical conductivity of YSZ does not change in this PO, range [6], the increase in the conductivity is caused by the doping of In,O,. At higher values of Po2, as in air, ionic conduction is dominant. The PO at which the conductivity change occurs depends 0; the In,O, concentration. When In-YSZ was heattreated in a reducing atmosphere at 1273 K, the color changed from white to pale gray. This color change is attributed to the valence change of the In ion from trivalent to monovalent. The increase in the electron concentration is caused by the valence change of the In ion at reducing atmosphere, as expressed by the following equation:
90 (19%)
conductivity
of In-YSZ.
(a) 1273 K. (b) 1473
K.
ple is, however, comparable with that of Ti-YSZ and appears to be in a relatively higher PO, range. This sample is promising for use as a membrane at higher levels of PO,. Further investigations into areas such as the measurment of the ionic transference number and the oxygen permeability are required to confirm its applicability.
4. Conclusions The electrical properties of In-YSZ have been studied. The n-type electronic conduction appears in the low PO, range. It increased with increasing In,O, concentration and temperature. The conductivity is rather small compared to that of Ti-YSZ. However, the electronic conduction of 15 mol% In,O,-doped YSZ appears in the higher PO, range. It is promising as a membrane for hydrogen production for use at higher Po2.
Acknowledgments The authors would like to thank Mr. M. Chiba for his skillful technical assistance and Mr. T. Takahashi and A. Shimizu for their assistance throughout this study. This work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Project No. 07239204) and by Toyota Physical and Chemical Research Institute.
H. Naito et al. / Solid State Ionics 90 (1996)
176
References [II K.W. Browall and R.H. Doremus, J. Am. Ceram. Sot. 60 (1977) 262. Sot. 13 1 (1984) 2407. t31 S.S. Liou and W.L. Worrell, Appl. Phys. A 49 (1989) 25. r41 N. Matsui, Denki Kagaku 58 (1990) 716. [51 H. Arashi and H. Naito, Solid State Ionics 53-56 (1992) 431. [61 H. Naito and H. Arashi, Solid State Ionics 53-56 (1992) 436.
121B. Cales and J.F. Baumard, J. Electrochem.
173- I76
[7] H. Naito and H. Arashi,
Solid State Ionics 79 (1995) 366. [8] L.J. Gauckler, K. Sasaki, H. Heinrich, P. Bhoac and A. Oriukas, Science and Technology of Zirconia V (1993) 555. [9] K. Sasaki, H.P. Seifert and L.J. Gauckler, J. Electrochem. Sot. 141 (1994) 2759. [lo] R.D. Shannon and R.C.T. Prewitt, Acta Crystallogr. B 26 (1970) 1046. [I 11 D.W. Strickler and W.G. Carlson, J. Am. Ceram. Sot. 47 (1964) 122. [I21 F.A. Krcger and H.J. Vink, Solid State Phys. 3 (1956) 310.
Solid State
Ionics 90 (1996) 173-176
IOMCS
and its application to a Electrical properties of DO,- In,O,-Y,O, membrane for gas separation Hitoshi Naito*, Hiroo Yugami, Haruo Arashi Faculty of Engineering, Tohoku University, Aoba, Aramaki, Sendai 980-77, Japan
Received 3 December 1995; accepted 18 April 1996
Abstract Electrical properties of In,O,-doped yttria-stabilized zirconia (In-YSZ) were investigated. The solubility limit of In,O, in YSZ (10 mol% Y,O,) is 17.5 mol%. The total conductivity depended on the concentration of $0,. The activation energy of In-YSZ was higher than that of YSZ. From the oxygen partial pressure (PO,) dependence of the total conductivity of In-YSZ, the electronic conductivity increased with increasing In,O, concentration at low oxygen partial pressures and at high temperature. From the results, we discussed the applicability of In-YSZ to a membrane for hydrogen production from direct water splitting at high temperature. Keywords: ZrO,-In,O,-Y,O,;
Membrane for gas separation; Mixed conductor
1. Introduction Mixed conductors have many applications, e.g. as electrodes, electrocatalytic reactors and membranes for gas separation. Hydrogen, which is a renewable and clean energy resource, can be produced from water using a mixed conductor as a membrane for gas separation. Several researchers discussed the hydrogen production system [ 1,2]. Yttria-stabilized zirconia (YSZ)-based mixed conductors are durable up to high temperature and have high ionic conductivity. We are investigating YSZ-based mixed conducting membranes to produce hydrogen from direct water splitting at high temperatures. As YSZ has a high ionic conduction, additives which are doped into YSZ are required to exhibit a high *Corresponding author.
electronic conduction to promote hydrogen production. The electrical properties of ZrO,-TiO,-Y,O, (Ti-YSZ) have been investigated [3-61, and we demonstrated hydrogen production using this mixed conductor [7]. From this result, it is revealed that the YSZ-based mixed conductor is required to have high electronic conductivity to promote the hydrogen production at relatively high oxygen partial pressures. An In ion can exhibit a mono- or trivalent state. The ZrO,-In,O, system shows several types of conduction depending on the concentration of In,O, system and its phase [8,9]. When In,O, is doped into YSZ and maintains a fluorite-type structure, In-YSZ can be one of the most promising materials for mixed conductors. We investigated the electrical properties of In-YSZ and discussed its applicability to a membrane for a hydrogen production.
0167-2738/96/$15.00 Copyright 01996 Elsevier Science B.V.All rights reserved P/I SO167-2738(96)00405-5
174
H. Naito et al. I Solid State Ionics 912 (1996) 173-176
2. Experimental Samples were synthesized by means of a solid state reaction. ZrO,, In,O, and Y,O, powders, with a level of purity of 99.99%, were used as starting materials. The Y,O, concentration ratio was set to 10 mol% to keep the ionic conductivity high. The In,O, concentration ratio was varied from 0 to 20 mol%. Appropriate amounts of the powders were weighed, mixed and ball-milled in methanol for 24 h with a zirconia ball. After being dried at 353 K, mixtures were ground and isostatically pressed into rods at 200 MPa and sintered at 1973 K for 5 h. The crystal structures of the synthesized samples were identified using an X-ray diffractometer (JEOL, JDX-3530). Sintered rods were cut into 1.5 X 3 X 15 mm3 parallelepipeds to measure electrical conductivity. The electrical conductivity was measured using a standard four-probe DC. technique in the temperature range from 873-1673 K in air. The PO, dependence of the electrical conductivity was also measured in the PO, range from 2.1 X lo4 (air) to lo-l6 Pa. The PO, was controlled in flowing H,CO, gas mixtures and by changing the H,-CO, flow ratio.
5
10
15
20
111~0~ concentration [mol%] Fig. 1. The dependence centration of In,O,.
of the lattice
parameter
-d-
on the con-
5mol%ln20,
-
10mol%ln20,
-e-
15mol%In~o,
3. Results and discussion The crystal structures of the samples were determined by using XRD. Samples in which the In,O, concentration was up to 17.5 mol% exhibit a single cubic fluorite structure. Fig. 1 shows the relationship between the lattice parameter of the samples and In,O, concentration. The lattice parameter was calculated by using the (331) peak of the cubic fluorite structure. As the ionic radius of In3+ is smaller than that of Zr4+ [lo], the lattice parameter decreases linearly with increasing In,O, concentration, up to 17.5 mol%. A relative density of 95% was obtained for each sample. The temperature dependence of the total electrical conductivity of In-YSZ is shown in Fig. 2. The conductivity decreased with increasing In,O, concentration. The 15 mol% In,O,-doped YSZ shows almost the same conductivity as that of cubic phase 35 mol% InO,,,-ZrO, from 873 to 1000 K [8]. The activation energy increases with increasing In,O,
Fig. 2. The dependence air on the temperature.
of the electrical conductivity
of In-YSZ
in
concentration. This phenomenon corresponds to the activation energy change with the Y,O, concentration in YSZ [ 111. This indicates that the decrease in the electrical conductivity and the increase in the activation energy by doping In,O, are due to the association and the clustering of oxygen vacancies. The electrical conductivity of 10 mol% In,O,-doped YSZ was almost comparable to that of YSZ at 1673 K because of its higher activation energy. The oxygen partial pressure dependence of the total electrical conductivity at 1273 and 1473 K are shown in Fig. 3. The conductivity of samples containing over 10 mol% In,O, increased with decreasing PO,. This indicates that the n-type elec-
H. Nairo et al. I Solid State Ionics
lo-’ ’ ’ ’ ’ ’ ’ ;,I,:’ ;;-i ’ ;;-a ’ ‘,bJ ’ ’ ‘4 10-m dL Oxygea Partial Pressure PO, rp:; Fig. 3. The oxygen
partial pressure
dependence
In;, + 0;
+
Inlr + Vo + +O,,
(1)
where Kroger and Vi&s notation is used [12]. When we use a mixed conductor as a membrane for hydrogen production from direct water splitting at high temperatures, some characteristics mentioned in the introduction are required. As In-YSZ satisfies these requirements, this mixed conductor is consid-’ ered as one of the candidates. The electrical properties of In-YSZ are compared with those of TiYSZ [6]. The electrical conductivity of In-YSZ (10 mol% In,O,) is almost the same as that of Ti-YSZ (10 mol% TiO,). The slope derived from the relationship between the conductivity and the PO, is small. That is, the electronic conductivity is small. In-YSZ with 15 mol% In,O, shows low electrical conductivity. The electronic conduction of this sam-
175
173-176
]($A ’ ;$a ’ ;;.I: ’ ‘Id-s’’ id-1 ’ ’ ‘(1 ’ ’ ‘I Oxygen Partial Pm.wm Pz &
of the electrical
tronic conduction which occurs is of the same order of magnitude as ionic conduction at low PO,. As the electrical conductivity of YSZ does not change in this PO, range [6], the increase in the conductivity is caused by the doping of In,O,. At higher values of Po2, as in air, ionic conduction is dominant. The PO at which the conductivity change occurs depends 0; the In,O, concentration. When In-YSZ was heattreated in a reducing atmosphere at 1273 K, the color changed from white to pale gray. This color change is attributed to the valence change of the In ion from trivalent to monovalent. The increase in the electron concentration is caused by the valence change of the In ion at reducing atmosphere, as expressed by the following equation:
90 (19%)
conductivity
of In-YSZ.
(a) 1273 K. (b) 1473
K.
ple is, however, comparable with that of Ti-YSZ and appears to be in a relatively higher PO, range. This sample is promising for use as a membrane at higher levels of PO,. Further investigations into areas such as the measurment of the ionic transference number and the oxygen permeability are required to confirm its applicability.
4. Conclusions The electrical properties of In-YSZ have been studied. The n-type electronic conduction appears in the low PO, range. It increased with increasing In,O, concentration and temperature. The conductivity is rather small compared to that of Ti-YSZ. However, the electronic conduction of 15 mol% In,O,-doped YSZ appears in the higher PO, range. It is promising as a membrane for hydrogen production for use at higher Po2.
Acknowledgments The authors would like to thank Mr. M. Chiba for his skillful technical assistance and Mr. T. Takahashi and A. Shimizu for their assistance throughout this study. This work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Project No. 07239204) and by Toyota Physical and Chemical Research Institute.
H. Naito et al. / Solid State Ionics 90 (1996)
176
References [II K.W. Browall and R.H. Doremus, J. Am. Ceram. Sot. 60 (1977) 262. Sot. 13 1 (1984) 2407. t31 S.S. Liou and W.L. Worrell, Appl. Phys. A 49 (1989) 25. r41 N. Matsui, Denki Kagaku 58 (1990) 716. [51 H. Arashi and H. Naito, Solid State Ionics 53-56 (1992) 431. [61 H. Naito and H. Arashi, Solid State Ionics 53-56 (1992) 436.
121B. Cales and J.F. Baumard, J. Electrochem.
173- I76
[7] H. Naito and H. Arashi,
Solid State Ionics 79 (1995) 366. [8] L.J. Gauckler, K. Sasaki, H. Heinrich, P. Bhoac and A. Oriukas, Science and Technology of Zirconia V (1993) 555. [9] K. Sasaki, H.P. Seifert and L.J. Gauckler, J. Electrochem. Sot. 141 (1994) 2759. [lo] R.D. Shannon and R.C.T. Prewitt, Acta Crystallogr. B 26 (1970) 1046. [I 11 D.W. Strickler and W.G. Carlson, J. Am. Ceram. Sot. 47 (1964) 122. [I21 F.A. Krcger and H.J. Vink, Solid State Phys. 3 (1956) 310.