- Email: [email protected]
Characteristics of novel BaZr0.4Ce0.4In0.2O3 proton conducting ceramics and their application to hydrogen sensors
Solid State Ionics 176 (2005) 2979 – 2983 www.elsevier.com/locate/ssi
Characteristics of novel BaZr0.4Ce0.4In0.2O3 proton conducting ceramics and their application to hydrogen sensors Noboru Taniguchi *, Tomohiro Kuroha, Chiharu Nishimura, Kenji Iijima Battery Research Laboratory, Matsushita Electric Industrial Co., Ltd., 1006 Kadoma, Kadoma City, Osaka 571-8501, Japan
Abstract The characteristics of novel BaZr0.4Ce0.4In0.2O3 (BZCI) proton conducting ceramics and their application to hydrogen sensors were investigated. Their properties were examined by the EMF responses of a hydrogen concentration cell and an electrochemical hydrogen permeation test. The BZCI ceramic exhibited good proton conduction in an atmosphere containing hydrogen in the temperature range of 300 – 700 -C. To verify that these ceramics are practical protonic conductors, limiting-current-type hydrogen sensors using BZCI ceramics as electrolytes were constructed. They exhibited good sensing properties in a reducing atmosphere and hardly degraded on repeated use. BZCI seems to be a very useful protonic conductor in terms of reliability and conduction. D 2005 Elsevier B.V. All rights reserved. Keywords: Proton; Conductor; Perovskite; Ceramic; Hydrogen sensor
1. Introduction
2. Experimental
It is very important for high performance SOFCs and new sensors to develop good protonic conductive ceramics. A good protonic conductor needs high proton conductivity and high reliability. We have developed BaCe0.8Gd0.2O3 (i.e., BCG) ceramics [1– 3] which have high ionic conductivity [4 – 6]. However, these barium cerate ceramics were affected by steam or CO2 and have met with problems in practical use [7– 10]. We have since found novel protonic conductors that have practical durability in the presence of steam and have relatively high conductivity, such as BaZr0.4Ce0.4In0.2O3 (i.e., BZCI) [11 – 13]. However, the conductive properties and practical issues of BZCI ceramics had not been clarified. In this study, their conductive properties were investigated by the EMF response of a hydrogen concentration cell and an electrochemical hydrogen permeation test. Application of BZCI ceramics to a limiting-current-type hydrogen sensor was tested.
BZCI ceramics were prepared by a solid-state reaction and were cut and polished into thin disks (0.5 mm thickness, 13 mm diameter) and plates (10 10 4.5 mm). The samples were made by baking the disks coated on the face with platinum paste (TR7905 made by Tanaka International). Conductivities of the sample were measured in the temperature range 200– 1000 -C using an ac impedance analyzer (Solatron Schlumberger 1286 electrochemical interface and 1255 HF frequency response analyzer). Protonic conduction in the BZCI ceramics was investigated by the EMF response of a hydrogen concentration cell and an electrolytic permeation test for hydrogen. The hydrogen concentration cell was constructed using BZCI as the electrolyte and pure hydrogen at 1 105 Pa; a mixture of hydrogen and argon was supplied to each electrode. During the hydrogen permeation test, hydrogen gas and dry argon gas were supplied to the ceramic electrolyte, and the amount of hydrogen which permeated from anode to cathode was measured. To examine the practical problems with BZCI ceramics, limiting-current-type hydrogen sensors were constructed in the form of a limiting-current oxygen sensor (Fig. 1) [3,14 –20]. The sensor element consisted of a BZCI electrolyte plate with a
* Corresponding author. Tel.: +81 6 6906 5097; fax: +81 6 6904 7461. E-mail address: [email protected] (N. Taniguchi). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.035
2980
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983
Current I
Output
A
Diffusion determining
H2 H 2O
O2
Cathode
hole HC
Electrolyte CO2 H2O
H+ O2
H-C
Ceramics plate HC Anode Fig. 1. Schematic construction of the sensor element with a hydrogen diffusion passage.
pair of platinum electrodes and a diffusion resistant plate. A hydrogen diffusion path was created through the glass between the BZCI and the diffusion resistant plate. The current – potential characteristics, hydrogen concentration dependence, and sensing properties were investigated at several temperatures. The amount of hydrogen was also measured using a gas chromatograph.
The stability of the BZCI ceramics in a reducing atmosphere was then examined. The faces of the sample disks were coated with platinum paste, and their current changes were measured by applying a potential of 1.0 V in a dry pure hydrogen atmosphere at 500 -C, 700 -C and 800 -C for 100 h. Fig. 3 shows the output current curve at 500 -C. Degradation of the output current at 700 -C was hardly observed on rebaking the electrodes, but some degradation at 800 -C appeared even after
3. Results and discussion 500
The X-ray diffraction patterns of the BZCI ceramics obtained by sintering are shown in Fig. 2. The peak at 25- is background contribution and the small shoulders at the lowangle side of the main peak are indicative of impurities. The peaks of the diffraction patterns were indexed according to a double cubic unit cell of the crystal lattice because the BZCI ceramics were guessed to be composed of a double cell of a barium zirconate and a barium cerate. The BZCI ceramics consisted of nearly single phases of a perovskite-type structure and their crystal structure was expected to have a cubic cell. The observed densities of the sintered samples were in excess of 96% of the calculated density.
Output current / µA
3.1. Characteristics and stability in a reducing atmosphere
200 100
0
10
(400) •
(422) • (440) •
(222) •
(620) •
0
Conductivity σ / S / cm
2000
peak
10 20 30 40 50 60 70 80 90 100 110 Applied potential time / hour
0
1000
600 400
30
100
0 (°C)
10
-2
10
-3
BCG
10
-4
10
-5
10
YSZ
-6
10 20
200
-1
3000
Background
Temperature:500°C Applied Voltage:1.0 V In dry pure hydrogen
Fig. 3. Output current vs. applied potential time for the BZCI cell in dry pure hydrogen at 500 -C.
• (220)
CPS
300
0
4000
1000
400
40
50
60
70
2θ / θ / degree
BZCI
-7
10
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1000/ T ⁄ 1 / K Fig. 2. Typical X-ray diffraction patterns of a BaZr0.4Ce0.4In0.2O3 ceramic indexed according to a cubic unit cell.
Fig. 4. Arrhenius plots of the conductivity of BZCI ceramics in wet hydrogen.
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983
200
20 1000°C 800°C 600°C
temperature:400°C
18 Output current / µA
150 E.M.F. / mV
2981
100
50
16
5.0%H2
14
3.9%H2
12
2.8%H2
10
1.7%H2
8 6
0%H2 in N2
4 2
0 -2 10
-1
0 0.0
0
10 PH2(c) / PH2(a)
10
Fig. 5. EMF response of the hydrogen concentration cell to ( P H2O: 1%), P H2(c); H2:Pt/BZCI/Pt:P H2(a), (H2 + Ar(1 105 Pa)). The lines show the theoretical EMF at each temperature.
0.2
0.4 0.6 Potential / V
0.8
1.0
Fig. 8. Typical current – potential characteristics of a BZCI sensor over a hydrogen concentration range of 0 – 5 % at 400 -C.
Output current / µA
500
700°C 500°C 350°C
400 300 in pure (100%) hydrogen
200
Limiting current / µA
14 600
12 10 8 6 Temperature:400°C Applied voltage:0.1V in N2 1L/min
4 2
100
0 0 0.0
0 0.4
0.8 1.2 Potential / V
1.6
Fig. 6. Typical current – potential characteristics of a limiting-current hydrogen sensor using BZCI ceramic as an electrolyte in pure hydrogen (thickness 0.45 mm, anode area 0.36 cm2).
rebaking. There was very little degradations in the output current at 800 -C and 1000 -C in a wet pure hydrogen atmosphere. The crystal structure and the composition of all samples did not change during a test with hydrogen. The faces of the BZCI ceramic materials seemed to be reduced at an oxygen partial pressure of less than 3.9 10 17 Pa in wet pure hydrogen at 1000 -C [21] but were not decomposed. In other
5
words, it was suggested that the BZCI ceramics were able to be used under the environment over an oxygen partial pressure of 3.9 10 17 Pa. 3.2. Conductivity and protonic conduction Fig. 4 shows Arrhenius plots of the conductivity of the BZCI ceramics in wet hydrogen. The slope of the line was almost linear, which would indicate the constancy of the 12
100% 50% 10% 0% in N2
Output current / µA
Output current / µA
2 3 4 H2 concentration / %
Fig. 9. Limiting current vs. the hydrogen concentration for a BZCI sensor with an applied potential of 0.1 V at 400 -C.
400
300
1
2.0
200 temperature:500°C 100
5%H2 in N2
10 8 6 4
Temperature:400°C Applied voltage:0.1V
2 0%H2 in N2 0 0.0
0.4
0.8 1.2 Potential / V
1.6
2.0
Fig. 7. Typical current – potential characteristics of a BZCI sensor at a hydrogen concentration in the range of 0 – 100 % at 500 -C.
0
0
20 40 60 80 100 Cycle number (2 times / day)
120
Fig. 10. Cycling characteristics of the response to hydrogen concentrations between 0 % and 5 % at an applied potential of 0.1 V at 400 -C.
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983
conduction mechanism. BZCI ceramics showed higher conductivities than that of YSZ and lower than that of BCG: 2 10 2 S cm 1 at 800 -C and 3 10 3 S cm 1 at 500 -C. The conductivities of the BZCI ceramics in wet air were lower than those in wet hydrogen below 800 -C. It seems that protonic conduction is more dominant than oxygen ion conduction. Proton conduction in BZCI ceramics was then investigated using a hydrogen concentration cell. Fig. 5 shows the EMF response of the cell to various hydrogen partial pressures ( P H2). The relation between the EMF and the log( P H2(C)) was linear, as predicted by the Nernst equation :
500 Temperature:500°C Output current / µA
2982
400 100%H2 100%CH4 100%C3H8 100%C4H10
300 200 100 0 0.0
0.5
1.0
1.5
2.0
2.5
Potential / V
EMF ¼ ð RT =2F ÞlnPH2 ðaÞ=PH2 ðC Þ: The actual emfs almost coincided with the theoretical values below 800 -C, but were lower than the theoretical values at 1000 -C. Protonic conduction in BZCI was confirmed below 800 -C and seems to decrease at higher temperature. Hydrogen permeation tests were made to verify proton conduction. The evolution of hydrogen in the cell was hardly observed at 1000 -C. When a constant current of 30 mA was sent through the cell at 800 -C, a little hydrogen evolution was confirmed, but its volume was not in accordance with Faraday’s law. The ohmic resistance of the cell was too high at 600 -C to measure the evolution of hydrogen in this study. However, it was verified in these tests that protons were the charge carriers in BZCI ceramics. It is concluded that electronic conduction occurs at high temperatures (above 800 -C) and proton conduction increases with decreasing temperature in BZCI in the absence of oxygen. Although hydrogen permeation could not be observed below 600 -C in the electrolysis, it was expected that the BZCI ceramics could be used as proton conductors. 3.3. Application of the limiting-current hydrogen sensor Application of a limiting-current hydrogen sensor using the hydrogen pumping ability of a BZCI cell was tested. A limiting-current hydrogen sensor using BZCI ceramics with a thickness of 0.45 mm as an electrolyte was constructed according to the principle shown in Fig. 1, and the hydrogen sensing properties were examined. The current – potential characteristics of these sensors were first investigated in pure hydrogen gases. The limiting current resulting from the hydrogen diffusion was evaluated at several temperatures. Limiting currents were observed by applying a potential of 0.8 to 1.6 V in dry pure hydrogen gases in the temperature range of 350 – 700 -C (Fig. 6). It was found that these materials could be applied to a limiting-current hydrogen sensor. The current –potential characteristics of the BZCI sensors at various hydrogen concentrations at 500 -C are shown in Fig. 7. Nearly flat limiting currents, shown as plateaus, were observed for every hydrogen concentration; the sensors were able to respond to the hydrogen concentration in the range 0 –100%, although the currents are not really horizontal because of slight electric conductive contribution. Fig. 8 shows the current – potential characteristics at hydrogen concentration in the range
Fig. 11. Current – potential characteristics of a BZCI sensor with contaminant gases at 500 -C.
0– 5% at 400 -C; Fig. 9 shows the limiting current vs. hydrogen concentration at a potential of 0.1 V applied to the sensor. The limiting current increased almost linearly with increased hydrogen concentration. The cyclic characteristics of the response to hydrogen concentrations between 0% and 5% in nitrogen gases were finally investigated. The cycling characteristics of the BZCI sensors at an applied potential of 0.1 V at 400 -C are shown in Fig. 10. There was hardly any output degradation over 100 cycles for 52 days. The BZCI ceramics were thus shown to be hardly degraded under an applied potential in a reducing atmosphere over a long period. They can be seen as very useful proton conductors in terms of reliability and conductivity. It can be added that the influence of contaminant gases such as CH4, C3H8 and C4H10 in hydrogen sensors using BZCI ceramics were investigated. The output currents of the sensors in various hydrocarbon gases appeared slightly by the occurrence of electric conduction, but affected hardly the sensing properties, as shown in Fig. 11. 4. Conclusions It is verified that BZCI ceramics show proton conduction in hydrogen from 350 -C to 700 -C and were hardly degraded. A limiting-current hydrogen sensor using this ceramic as an electrolyte exhibited good sensing properties in a reducing atmosphere. Its output currents were proportional to hydrogen concentration in the range 0 – 100% at each temperature. There was hardly any output degradation over a long time under the reducing atmosphere. The BZCI ceramic would seem to be a very useful proton conductor in terms of reliability and conductivity. References [1] N. Taniguchi, K. hatoh, J. Niikura, T. Gamo, H. Iwahara, Solid State Ionics 53 (1992) 998. [2] N. Taniguchi, E. Yasumoto, T. Gamo, J. Electrochem. Soc. 143 (1996) 1886. [3] N. Taniguchi, E. Yasumoto, Y. Nakagiri, T. Gamo, J. Electrochem. Soc. 145 (5) (1998) 1744.
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983 [4] H. Iwahara, H. Uchida, K. Ono, K. Ogaki, J. Electrochem. Soc. 135 (1988) 529. [5] N. Bonanos, B. Ellis, M.N. Mahmood, Solid State Ionics 44 (1991) 305. [6] T. Norby, Nature 410 (2001) 877. [7] N. Taniguchi, T. Gamo, Denki Kagaku 62 (1992) 326. [8] M.J. Scholten, J. Schoonman, J.C. van Miltenburg, H.A.J. Oonk, Solid State Ionics 61 (1993) 83. [9] J.J Lander, J. Am. Chem. Soc. 73 (1951) 5794. [10] E.H. Baker, J. Chem. Soc. (1964) 699. [11] N. Taniguchi, C. Nishimura, J. Kato, Solid State Ionics 145 (2001) 349. [12] K.D. Kreuer, E. Schonherr, J. Maier, Solid State Ionics 70/71 (1994) 278.
[13] [14] [15] [16] [17] [18]
2983
K.S. Knight, Solid State Ionics 127 (2000) 43. H. Fukuda, T. Nagai, Tech. Dig. 7th Sensor Symp., 1988, p. 33. T. Suzuki, H. Futata, Tech. Dig. 13th Sensor Symp, 1995, p. 89. E.N. Fuller, K. Ensley, J.C. Giddings, J. Phys. Chem. 73 (1969) 3679. K. Nakahara, A. Takami, Nainen Kikan 31 (1992) 63. T. Kamo, Y. Chujo, T. Akatsuka, J. Nakano, M. Suzuki, Lean Mixture Sensor, SAE Paper, vol. 850380, 1985. [19] T. Usui, Y. Kurumiya, K. Nuri, M. Nakazawa, Sens. Actuators 16 (1989) 345. [20] H. Dietz, Solid State Ionics 6 (1982) 175. [21] K. Nishikinuki, Science Forum, New Trigger Series, 1992, p. 82.
Characteristics of novel BaZr0.4Ce0.4In0.2O3 proton conducting ceramics and their application to hydrogen sensors Noboru Taniguchi *, Tomohiro Kuroha, Chiharu Nishimura, Kenji Iijima Battery Research Laboratory, Matsushita Electric Industrial Co., Ltd., 1006 Kadoma, Kadoma City, Osaka 571-8501, Japan
Abstract The characteristics of novel BaZr0.4Ce0.4In0.2O3 (BZCI) proton conducting ceramics and their application to hydrogen sensors were investigated. Their properties were examined by the EMF responses of a hydrogen concentration cell and an electrochemical hydrogen permeation test. The BZCI ceramic exhibited good proton conduction in an atmosphere containing hydrogen in the temperature range of 300 – 700 -C. To verify that these ceramics are practical protonic conductors, limiting-current-type hydrogen sensors using BZCI ceramics as electrolytes were constructed. They exhibited good sensing properties in a reducing atmosphere and hardly degraded on repeated use. BZCI seems to be a very useful protonic conductor in terms of reliability and conduction. D 2005 Elsevier B.V. All rights reserved. Keywords: Proton; Conductor; Perovskite; Ceramic; Hydrogen sensor
1. Introduction
2. Experimental
It is very important for high performance SOFCs and new sensors to develop good protonic conductive ceramics. A good protonic conductor needs high proton conductivity and high reliability. We have developed BaCe0.8Gd0.2O3 (i.e., BCG) ceramics [1– 3] which have high ionic conductivity [4 – 6]. However, these barium cerate ceramics were affected by steam or CO2 and have met with problems in practical use [7– 10]. We have since found novel protonic conductors that have practical durability in the presence of steam and have relatively high conductivity, such as BaZr0.4Ce0.4In0.2O3 (i.e., BZCI) [11 – 13]. However, the conductive properties and practical issues of BZCI ceramics had not been clarified. In this study, their conductive properties were investigated by the EMF response of a hydrogen concentration cell and an electrochemical hydrogen permeation test. Application of BZCI ceramics to a limiting-current-type hydrogen sensor was tested.
BZCI ceramics were prepared by a solid-state reaction and were cut and polished into thin disks (0.5 mm thickness, 13 mm diameter) and plates (10 10 4.5 mm). The samples were made by baking the disks coated on the face with platinum paste (TR7905 made by Tanaka International). Conductivities of the sample were measured in the temperature range 200– 1000 -C using an ac impedance analyzer (Solatron Schlumberger 1286 electrochemical interface and 1255 HF frequency response analyzer). Protonic conduction in the BZCI ceramics was investigated by the EMF response of a hydrogen concentration cell and an electrolytic permeation test for hydrogen. The hydrogen concentration cell was constructed using BZCI as the electrolyte and pure hydrogen at 1 105 Pa; a mixture of hydrogen and argon was supplied to each electrode. During the hydrogen permeation test, hydrogen gas and dry argon gas were supplied to the ceramic electrolyte, and the amount of hydrogen which permeated from anode to cathode was measured. To examine the practical problems with BZCI ceramics, limiting-current-type hydrogen sensors were constructed in the form of a limiting-current oxygen sensor (Fig. 1) [3,14 –20]. The sensor element consisted of a BZCI electrolyte plate with a
* Corresponding author. Tel.: +81 6 6906 5097; fax: +81 6 6904 7461. E-mail address: [email protected] (N. Taniguchi). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.035
2980
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983
Current I
Output
A
Diffusion determining
H2 H 2O
O2
Cathode
hole HC
Electrolyte CO2 H2O
H+ O2
H-C
Ceramics plate HC Anode Fig. 1. Schematic construction of the sensor element with a hydrogen diffusion passage.
pair of platinum electrodes and a diffusion resistant plate. A hydrogen diffusion path was created through the glass between the BZCI and the diffusion resistant plate. The current – potential characteristics, hydrogen concentration dependence, and sensing properties were investigated at several temperatures. The amount of hydrogen was also measured using a gas chromatograph.
The stability of the BZCI ceramics in a reducing atmosphere was then examined. The faces of the sample disks were coated with platinum paste, and their current changes were measured by applying a potential of 1.0 V in a dry pure hydrogen atmosphere at 500 -C, 700 -C and 800 -C for 100 h. Fig. 3 shows the output current curve at 500 -C. Degradation of the output current at 700 -C was hardly observed on rebaking the electrodes, but some degradation at 800 -C appeared even after
3. Results and discussion 500
The X-ray diffraction patterns of the BZCI ceramics obtained by sintering are shown in Fig. 2. The peak at 25- is background contribution and the small shoulders at the lowangle side of the main peak are indicative of impurities. The peaks of the diffraction patterns were indexed according to a double cubic unit cell of the crystal lattice because the BZCI ceramics were guessed to be composed of a double cell of a barium zirconate and a barium cerate. The BZCI ceramics consisted of nearly single phases of a perovskite-type structure and their crystal structure was expected to have a cubic cell. The observed densities of the sintered samples were in excess of 96% of the calculated density.
Output current / µA
3.1. Characteristics and stability in a reducing atmosphere
200 100
0
10
(400) •
(422) • (440) •
(222) •
(620) •
0
Conductivity σ / S / cm
2000
peak
10 20 30 40 50 60 70 80 90 100 110 Applied potential time / hour
0
1000
600 400
30
100
0 (°C)
10
-2
10
-3
BCG
10
-4
10
-5
10
YSZ
-6
10 20
200
-1
3000
Background
Temperature:500°C Applied Voltage:1.0 V In dry pure hydrogen
Fig. 3. Output current vs. applied potential time for the BZCI cell in dry pure hydrogen at 500 -C.
• (220)
CPS
300
0
4000
1000
400
40
50
60
70
2θ / θ / degree
BZCI
-7
10
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1000/ T ⁄ 1 / K Fig. 2. Typical X-ray diffraction patterns of a BaZr0.4Ce0.4In0.2O3 ceramic indexed according to a cubic unit cell.
Fig. 4. Arrhenius plots of the conductivity of BZCI ceramics in wet hydrogen.
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983
200
20 1000°C 800°C 600°C
temperature:400°C
18 Output current / µA
150 E.M.F. / mV
2981
100
50
16
5.0%H2
14
3.9%H2
12
2.8%H2
10
1.7%H2
8 6
0%H2 in N2
4 2
0 -2 10
-1
0 0.0
0
10 PH2(c) / PH2(a)
10
Fig. 5. EMF response of the hydrogen concentration cell to ( P H2O: 1%), P H2(c); H2:Pt/BZCI/Pt:P H2(a), (H2 + Ar(1 105 Pa)). The lines show the theoretical EMF at each temperature.
0.2
0.4 0.6 Potential / V
0.8
1.0
Fig. 8. Typical current – potential characteristics of a BZCI sensor over a hydrogen concentration range of 0 – 5 % at 400 -C.
Output current / µA
500
700°C 500°C 350°C
400 300 in pure (100%) hydrogen
200
Limiting current / µA
14 600
12 10 8 6 Temperature:400°C Applied voltage:0.1V in N2 1L/min
4 2
100
0 0 0.0
0 0.4
0.8 1.2 Potential / V
1.6
Fig. 6. Typical current – potential characteristics of a limiting-current hydrogen sensor using BZCI ceramic as an electrolyte in pure hydrogen (thickness 0.45 mm, anode area 0.36 cm2).
rebaking. There was very little degradations in the output current at 800 -C and 1000 -C in a wet pure hydrogen atmosphere. The crystal structure and the composition of all samples did not change during a test with hydrogen. The faces of the BZCI ceramic materials seemed to be reduced at an oxygen partial pressure of less than 3.9 10 17 Pa in wet pure hydrogen at 1000 -C [21] but were not decomposed. In other
5
words, it was suggested that the BZCI ceramics were able to be used under the environment over an oxygen partial pressure of 3.9 10 17 Pa. 3.2. Conductivity and protonic conduction Fig. 4 shows Arrhenius plots of the conductivity of the BZCI ceramics in wet hydrogen. The slope of the line was almost linear, which would indicate the constancy of the 12
100% 50% 10% 0% in N2
Output current / µA
Output current / µA
2 3 4 H2 concentration / %
Fig. 9. Limiting current vs. the hydrogen concentration for a BZCI sensor with an applied potential of 0.1 V at 400 -C.
400
300
1
2.0
200 temperature:500°C 100
5%H2 in N2
10 8 6 4
Temperature:400°C Applied voltage:0.1V
2 0%H2 in N2 0 0.0
0.4
0.8 1.2 Potential / V
1.6
2.0
Fig. 7. Typical current – potential characteristics of a BZCI sensor at a hydrogen concentration in the range of 0 – 100 % at 500 -C.
0
0
20 40 60 80 100 Cycle number (2 times / day)
120
Fig. 10. Cycling characteristics of the response to hydrogen concentrations between 0 % and 5 % at an applied potential of 0.1 V at 400 -C.
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983
conduction mechanism. BZCI ceramics showed higher conductivities than that of YSZ and lower than that of BCG: 2 10 2 S cm 1 at 800 -C and 3 10 3 S cm 1 at 500 -C. The conductivities of the BZCI ceramics in wet air were lower than those in wet hydrogen below 800 -C. It seems that protonic conduction is more dominant than oxygen ion conduction. Proton conduction in BZCI ceramics was then investigated using a hydrogen concentration cell. Fig. 5 shows the EMF response of the cell to various hydrogen partial pressures ( P H2). The relation between the EMF and the log( P H2(C)) was linear, as predicted by the Nernst equation :
500 Temperature:500°C Output current / µA
2982
400 100%H2 100%CH4 100%C3H8 100%C4H10
300 200 100 0 0.0
0.5
1.0
1.5
2.0
2.5
Potential / V
EMF ¼ ð RT =2F ÞlnPH2 ðaÞ=PH2 ðC Þ: The actual emfs almost coincided with the theoretical values below 800 -C, but were lower than the theoretical values at 1000 -C. Protonic conduction in BZCI was confirmed below 800 -C and seems to decrease at higher temperature. Hydrogen permeation tests were made to verify proton conduction. The evolution of hydrogen in the cell was hardly observed at 1000 -C. When a constant current of 30 mA was sent through the cell at 800 -C, a little hydrogen evolution was confirmed, but its volume was not in accordance with Faraday’s law. The ohmic resistance of the cell was too high at 600 -C to measure the evolution of hydrogen in this study. However, it was verified in these tests that protons were the charge carriers in BZCI ceramics. It is concluded that electronic conduction occurs at high temperatures (above 800 -C) and proton conduction increases with decreasing temperature in BZCI in the absence of oxygen. Although hydrogen permeation could not be observed below 600 -C in the electrolysis, it was expected that the BZCI ceramics could be used as proton conductors. 3.3. Application of the limiting-current hydrogen sensor Application of a limiting-current hydrogen sensor using the hydrogen pumping ability of a BZCI cell was tested. A limiting-current hydrogen sensor using BZCI ceramics with a thickness of 0.45 mm as an electrolyte was constructed according to the principle shown in Fig. 1, and the hydrogen sensing properties were examined. The current – potential characteristics of these sensors were first investigated in pure hydrogen gases. The limiting current resulting from the hydrogen diffusion was evaluated at several temperatures. Limiting currents were observed by applying a potential of 0.8 to 1.6 V in dry pure hydrogen gases in the temperature range of 350 – 700 -C (Fig. 6). It was found that these materials could be applied to a limiting-current hydrogen sensor. The current –potential characteristics of the BZCI sensors at various hydrogen concentrations at 500 -C are shown in Fig. 7. Nearly flat limiting currents, shown as plateaus, were observed for every hydrogen concentration; the sensors were able to respond to the hydrogen concentration in the range 0 –100%, although the currents are not really horizontal because of slight electric conductive contribution. Fig. 8 shows the current – potential characteristics at hydrogen concentration in the range
Fig. 11. Current – potential characteristics of a BZCI sensor with contaminant gases at 500 -C.
0– 5% at 400 -C; Fig. 9 shows the limiting current vs. hydrogen concentration at a potential of 0.1 V applied to the sensor. The limiting current increased almost linearly with increased hydrogen concentration. The cyclic characteristics of the response to hydrogen concentrations between 0% and 5% in nitrogen gases were finally investigated. The cycling characteristics of the BZCI sensors at an applied potential of 0.1 V at 400 -C are shown in Fig. 10. There was hardly any output degradation over 100 cycles for 52 days. The BZCI ceramics were thus shown to be hardly degraded under an applied potential in a reducing atmosphere over a long period. They can be seen as very useful proton conductors in terms of reliability and conductivity. It can be added that the influence of contaminant gases such as CH4, C3H8 and C4H10 in hydrogen sensors using BZCI ceramics were investigated. The output currents of the sensors in various hydrocarbon gases appeared slightly by the occurrence of electric conduction, but affected hardly the sensing properties, as shown in Fig. 11. 4. Conclusions It is verified that BZCI ceramics show proton conduction in hydrogen from 350 -C to 700 -C and were hardly degraded. A limiting-current hydrogen sensor using this ceramic as an electrolyte exhibited good sensing properties in a reducing atmosphere. Its output currents were proportional to hydrogen concentration in the range 0 – 100% at each temperature. There was hardly any output degradation over a long time under the reducing atmosphere. The BZCI ceramic would seem to be a very useful proton conductor in terms of reliability and conductivity. References [1] N. Taniguchi, K. hatoh, J. Niikura, T. Gamo, H. Iwahara, Solid State Ionics 53 (1992) 998. [2] N. Taniguchi, E. Yasumoto, T. Gamo, J. Electrochem. Soc. 143 (1996) 1886. [3] N. Taniguchi, E. Yasumoto, Y. Nakagiri, T. Gamo, J. Electrochem. Soc. 145 (5) (1998) 1744.
N. Taniguchi et al. / Solid State Ionics 176 (2005) 2979 – 2983 [4] H. Iwahara, H. Uchida, K. Ono, K. Ogaki, J. Electrochem. Soc. 135 (1988) 529. [5] N. Bonanos, B. Ellis, M.N. Mahmood, Solid State Ionics 44 (1991) 305. [6] T. Norby, Nature 410 (2001) 877. [7] N. Taniguchi, T. Gamo, Denki Kagaku 62 (1992) 326. [8] M.J. Scholten, J. Schoonman, J.C. van Miltenburg, H.A.J. Oonk, Solid State Ionics 61 (1993) 83. [9] J.J Lander, J. Am. Chem. Soc. 73 (1951) 5794. [10] E.H. Baker, J. Chem. Soc. (1964) 699. [11] N. Taniguchi, C. Nishimura, J. Kato, Solid State Ionics 145 (2001) 349. [12] K.D. Kreuer, E. Schonherr, J. Maier, Solid State Ionics 70/71 (1994) 278.
[13] [14] [15] [16] [17] [18]
2983
K.S. Knight, Solid State Ionics 127 (2000) 43. H. Fukuda, T. Nagai, Tech. Dig. 7th Sensor Symp., 1988, p. 33. T. Suzuki, H. Futata, Tech. Dig. 13th Sensor Symp, 1995, p. 89. E.N. Fuller, K. Ensley, J.C. Giddings, J. Phys. Chem. 73 (1969) 3679. K. Nakahara, A. Takami, Nainen Kikan 31 (1992) 63. T. Kamo, Y. Chujo, T. Akatsuka, J. Nakano, M. Suzuki, Lean Mixture Sensor, SAE Paper, vol. 850380, 1985. [19] T. Usui, Y. Kurumiya, K. Nuri, M. Nakazawa, Sens. Actuators 16 (1989) 345. [20] H. Dietz, Solid State Ionics 6 (1982) 175. [21] K. Nishikinuki, Science Forum, New Trigger Series, 1992, p. 82.