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High-temperature steam electrolysis using SrCeO3-based proton conductive solid electrolyte
0360 3199/87 $3.00 + 0.00 Pergamon Journals Ltd. © 1987 International Association for Hydrogen Energy.
Int. J. Hydrogen Energy, Vol. 12, No. 2, pp. 73-77, 1987. Printed in Great Britain.
HIGH-TEMPERATURE STEAM ELECTROLYSIS USING SrCeO3-BASED PROTON CONDUCTIVE SOLID ELECTROLYTE H.
IWAHARA,
H. UCHIDAand I. YAMASAKI
Department of EnvironmentalChemistry and Technology, Faculty of Engineering, Tottori University, Minami 4-101, Koyama-cho, Tottori, 680 Japan
(Received for publication 17 July 1986) Abstraet--A new type of steam electrolyzer was fabricated using high-temperature protonic conductors as the solid electrolytes, and its performance was examined at 700-900°C. As a solid electrolyte, Y- or Yb-doped SrCeO3 ceramic was used, which was found to be a protonic conductor in the presence of hydrogen or water vapor at elevated temperatures. The electrolyzer worked stably. The current efficiencyfor hydrogen production was above 95% at a current density of 0.2 A cm -2 at 800°C. Nickel and platinum were promising materials for the cathode. The major limitation of the electrolyzer was the resistance of the solid electrolyte.
INTRODUCTION
EXPERIMENTAL
High-temperature steam electrolysis using a solid electrolyte is expected to be an effective method for large-scale hydrogen generation. An advantage of the high temperature electrolyzer is the saving of electric energy due to favorable thermodynamic and kinetic conditions [1-7]. This device can be inversely operated as a fuel cell--a well-known effective power generator. Such cell devices have been intensively studied [1-17] using stabilized zirconia (oxide ion conductor) as the electrolyte. High-temperature proton conductive solids are also favorable materials as electrolytes for such solid electrolyte cells. The advantage of a proton conductor electrolyzer is that it produces pure hydrogen, free of water vapor. In the oxide ion conductor cell, steam is supplied to the cathode compartment and the hydrogen produced is accompanied by unreacted water vapor, whereas in the proton conductor electrolyzer, steam is introduced to the anode compartment and pure hydrogen free from water vapor is obtained at the cathode compartment. Before our work, few good high-temperature proton conductors were known, but recently we found that some sintered oxides based on SrCeO3 exhibit appreciable proton conduction at high temperatures [18]. We confirmed that, using SrCe0.90Sc0.1003_a or SrCe0.95Yb0.0sOa-a as a solid electrolyte, steam could be electrolyzed at the anode compartment at a rate close to the theoretical as calculated from Faraday's law [18-21]. These ceramics could also be used as the electrolyte in a hydrogen-air fuel cell and it worked stably [18-23]. The objective of this research was to obtain fundamental information about the steam electrolyzer with SrCeO3-based proton conductive ceramics. We fabricated a small steam electrolyzer by way of experiment and its performance was examined. The current efficiency for hydrogen evolution, the stability of the electrolyzer and the polarization characteristics of some electrode materials are reported in this paper.
The proton conductors used in this study were sinters composed of SrCel-xMxO3_~, (M = Y, Yb; x = 0.05-0.10), where a is the number of oxygen deficiencies per perovskite-type oxide unit cell. The preparation of the specimen ceramics was similar to that in a previous study [18]. The sinters obtained were sliced into thin discs (thickness: 0.5 ram; diameter: 12 mm) to provide test specimens. The construction of the steam electrolyzer is illustrated in Fig. 1. The electrolyzer was situated in the electric furnace. The electrode compartments were separated by the ceramic electrolyte, each face of which was covered with porous electrode. The electrode materials used were platinum or nickel for the cathode and the mixture of platinum and La0.4Sr0.6CoO3_ a (3-5:1 by weight) for the anode. The addition of the oxide stabilized the cell performance. The electrode area was 0.5 cm 2 (projected area). Each electrode compartment was sealed by a glass ring gasket. A platinum wire was attached to the lateral of the electrolyte disc as the reference electrode. This reference electrode may act as a hydrogen electrode, the potential of which depends on the partial pressure of hydrogen produced by the thermal dissociation equilibrium between water vapor and oxygen in the atmosphere at elevated temperature. Therefore, this electrode exhibited a different potential value depending on the experimental conditions (cell temperature, humidity of the air etc.). Water vapor at 1 arm was supplied to the anode compartment and the flow rate of water vapor was measured from the amount of water condensed at the outlet of the anode. In order to carry the evolved hydrogen to a detector, argon gas, dried by P205, was passed through the cathode compartment (30 cm 3 m i n - 1). The exit gas from the cathode compartment was introduced to the gas sampler for a gas chromatograph to analyze the hydrogen content. A gas concentration cell type oxygen meter using stabilized zirconia was also 73
74
H. IWAHARA, H. UCHIDA and I. YAMASAKI to gas chromatograph
Porous electrode S°lidM 0
I
II
electrolyte SrCe, ...... ~
iL~ _ _ Y S02Zmeter
Electric
furnace
Reference _ ~ elect rode
L~H20 ~
.
02
+ E
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Fig. 1. Schematic diagram of the experimental steam electrolyzer.
used at 800°C in order to check the hydrogen generation. When exit gas from the cathode contains hydrogen, the oxygen meter indicates a large e.m.f, since the partial pressure of oxygen in the gas goes down. After the e.m.f, of the oxygen meter had become stable, the hydrogen content was analyzed quantitatively by a gas chromatograph (Shimazu, GC-3BT, carrier gas: argon; column packing: molecular sieves 5A 30-60 mesh). The hydrogen content in the exit gas was 0.3-6% in this experiment. The hydrogen evolution rate (ml min -1 cm -2 [STP]) was calculated from the concentration of hydrogen and the flow rate of the argon carrier gas [19]. The oxygen evolution rate at the anode was determined in a similar manner. Polarization characteristics of the electrolyzer were 3
studied by the current interruption method as in the previous study [23]. RESULTS A N D DISCUSSION
Electrolysis of steam Electrolysis was carried out at 700-900°C under various conditions. The electrolyzers using SrCe0.95Yb0.0sO3-a and SrCe0.90Y0.]003-,~ ceramics as solid electrolytes exhibited stable characteristics. The performances of the electrolyzers depended to some extent on the density of the ceramic electrolyte and the morphology of the electrodes. The dense ceramic oxides (with a density above 95% of the theoretical value) exhibited good performances (high current efficiency and high protonic conductivity). Figure 2 shows the hydrogen evolution rate as a function of the electrolytic current density. The hydrogen evolution rate is close to the theoretical value calculated from Faraday's law (broken line) in the low current densities, but it deviates gradually from this as the current density increases. The current efficiency for hydrogen evolution (the ratio of the observed value to the theoretical value) was above 90% at 700-800°C and became somewhat lower at 900°C due to electronic conduction [18, 24]. Figure 3 shows the relation between the electrolytic current and applied voltage. A high voltage was necessary for electrolysis due to insufficient conductivity of the electrolyte ceramics. However, the overvoltage, excluding for ohmic loss, was very small (dotted lines); e.g. it was less than 0.3 V at a current density of 0.4 A c m - 2 at 900 o C. This suggests that the thinner the ceramic electrolyte, the better the performance of the electrolyzer. Although the conductivities of these protonic conductors are not sufficiently high for thick ceramic tubes
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Fig. 3. Electrolytic current vs applied voltage of the steam Fig. 2. Hydrogen evolution rate vs current density. Electro- electrolyzer. Electrolyte: SrCeo.9oY0.~oOa-a; anode: Pt + lyte: SrCeo.9oYojoO3_, • = 700°C, /X = 800°C, C) = 900°C; Lao.4Sro.6CoO3_c~;cathode: Pt; temperature: z%, • = 800°C, SrCeo.95YboosO3_,~ [] = 800°C (broken line shows the C), • = 900°C (dotted lines show the voltage excluding the ohmic loss of the electrolyte). theoretical rate).
75
STEAM ELECTROLYSIS WITH SrCeO3-BASED ELECTROLYTE to be utilized as electrolytes in a practical electrolyzer, they may be prospective materials if they can be used as thin films to reduce ohmic loss. We tried to use slightly Y-doped SrCeO3 ceramics such as SrCeo.95Yo.0503_a for the electrolyzer. In that case, because of insufficient protonic conductivities, the voltage required was higher than that described above, and the current efficiency was low due to partial electronic conduction.
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Effect of oxygen partial pressure at the anode on current efficiency As shown in Fig. 2, the current efficiency had a tendency to decrease with increasing current density above 0.2 A cm -2. Such a decrease may be ascribed to electronic conduction in the electrolyte. In order to clarify this phenomena, the following experiments were carried out. Using SrCe0.95Yb0.0503_a as the electrolyte, the hydrogen evolution rate was measured as a function of the current density in three experiments. In the first experiment, steam was fed to the anode compartment at a rate of 364 cm 3 m i n - i. In the second, oxygen gas (flow rate = 40 cm 3 min - l ) was added to the steam. In the third, hydrogen gas (flow rate = 10 cm 3 min -1) was added to the steam. The current efficiency decreased on introducing oxygen into the anode compartment whereas it increased on introducing hydrogen. As reported previously [18, 24], these oxides exhibited p-type conduction in the absence of water vapor or hydrogen, and on introducing water vapor, proton conduction appeared at the expense of electron holes. H20(g) + 2 h+(oxides) ~__ 2 H+(oxides)
+ 1/2 02' (g)
( 1)
Therefore, accumulation of the evolved oxygen at the anode should give rise to electron holes in the oxide, which results in current loss during electrolysis. From these results, some counterplots against the accumulation of oxygen (e.g. geometry of the electrolyzer) must be taken into consideration in order to operate at high current densities.
Continuous electrolysis test A constant current electrolysis (0.2 A cm -2) was carried out for 6 h at 800°C using SrCe0.95Yb0.0503-~ as the electrolyte. Figure 4 shows the time dependence of cell performances during the electrolysis. The hydrogen evolution rate was almost constant and the current efficiency was above 95% ((a) in Fig. 4). The applied voltage was stable and about 2.8 V, whereas the voltage excluding ohmic loss was low (1.1 V, (b) in Fig. 4). After an overnight break, this electrolyzer was operated similarly for another 8 h and worked stably. Little deterioration was observed in the cell performance compared with the initial performance. Similar experimental results were
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Fig. 4. Cell performances for continuous electrolysis of 0.2 A cm -2 at 800°C using SrCeo.95Ybo.o503_,~ as the electrolyte. (a) H2 evolution rate (broken line shows the theoretical rate); (b) cell voltage (open symbol shows the voltage excluding ohmic loss of the electrolyte).
also obtained for the electrolyzer with SrCe0.90YoA003-~ electrolyte. This indicates that these proton conductors are steady and stable when used as the solid electrolyte for the steam electrolyzer under these conditions.
Steam electrolysis using a nickel cathode In the actual operation mode, pure hydrogen gas must be obtained without using carrier gas for the cathode. Since it was difficult to measure the volume of the evolved hydrogen in our experimental cell, we examined the cell performance with pure hydrogen at the cathode. Supplying hydrogen gas at 1 atm, instead of Ar carrier gas, into the cathode compartment, the water vapor accompanied by carrier gas (air) was electrolyzed at the anode and the oxygen evolution rate was analyzed. Porous nickel or platinum was used as the cathode material and SrCe0.95Yb0.0503_a as the electrolyte in this experiment. These cells also worked stably. Oxygen was observed to evolve at a rate close to the theoretical at the anode, irrespective of the cathode materials. This result suggests that the proton conductor can be used for the electrolyzer, producing pure hydrogen gas at the cathode. This may be reasonable because SrCeO3-based ceramics exhibited as stable characteristics as the proton
H. IWAHARA, H. UCHIDA and I. YAMASAKI
76 0.4
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cathode was also stable, but the polarization was somewhat larger (Fig. 6) probably due to the nickel adhering unsatisfactorily to the electrolyte surface. However, in place of expensive platinum, nickel may be a promising cathode material because of its stable properties and low cost.
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CONCLUSION
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High temperature steam electrolysis was studied using solid proton conductors based on SrCeO3 at 700-900°C. SrCeo.95Ybo.o503-a and SrCeo.9oYo.loO3-a ceramics were successfully used for the electrolyzer. The current efficiency for hydrogen evolution was above 95% at a current density of 0.2 A cm -2 at 800°C but it decreased with increasing cell temperature, probably due to the increase in electronic conduction in the ceramic electrolyte. The mixture of Lao.aSr0.6CoO3_ a and Pt exhibited stable characteristics for the anode material. Nickel as well as platinum was a promising material for the cathode. Since the major limitation was the resistance of the solid electrolyte in these cells, the SrCeO3-based proton conductors may be prospective materials for hydrogengenerating steam electrolyzers if used as thin films to reduce ohmic loss. Bench-scale tests on steam electrolyzers using SrCeO3-based proton conductive solid electrolyte are in progress in our laboratory.
-0.3 -0.4
Cathode I ~ I i -0.5 0 0.5 }.0 1.5 Electrode potential / Vvs R.E.
Fig. 5. Polarization of the electrodes in the cell (H20 + air / / H2 (1 atm)) at 800°C (A) and at 900°C (O). Electrolyte: SrCeo.95Ybo.osO3-a; cathode: Pt (ohmic loss is excluded in this figure).
conductor in the hydrogen-air fuel cell [18-231 and the hydrogen concentration cell [25]. Typical polarization characteristics of the electrodes are shown in Figs 5 and 6. The magnitude of the overvoltage depended largely upon the electrode condition at each particular stage. For example, the anodic polarization was different in Figs 5 and 6 in spite of using the same electrode material. The platinum cathode exhibited a little polarization (Fig. 5). The nickel
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REFERENCES
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a
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-0.4
tion, Science and Culture of Japan for supporting part of this work by a grant from the Special Research Project on the Effective Use of Energy (320).
~-
0.2
<~ 0.1
Acknowledgements--The authors thank the Ministry of Educa-
rhode/ J i -1.5 -1.0 -0.5 0 0.5 Electrode potential/V vs R.E.
Fig. 6. Polarization of the electrodes in the cell as in Fig. 5 at 800°C (A) and a t 900°C (©). Cathode: Ni (ohmic loss is excluded in this figure).
1. H. S. Spacil and C. S. Tedmon, Jr, J. Electrochem. Sac. 116, 1618 (1969). 2. H. S. Spacil and C. S. Tedmon, Jr, J. Electrochem. Soc. 116, 1627 (1969). 3. S. P. S. Badwal, D. J. M. Bevan and J. O'M. Bockris, Electrochim. Acta 25, 1115 (1980). 4. W. Doenitz, R. Schmidberger, E. Steinheil and R. Streincher, Int. J. Hydrogen Energy 5, 55 (1980). 5. R. Accorsi and E. Bergmann, J. Electrochem. Sac. 127, 804 (1980). 6. M. Takemori and I. Abe, Denki Kagaku 47, 204 (1979). 7. F. J. Rohr. In T. Takahashi and A. Kozawa (eds), Applications o f Solid Electrolytes p. 196. JEC Press, Cleveland, Ohio (1980). 8. A. O. Isenberg, Solid State lonics 3/4, 431 (1981). 9. B. G. Pound, D. J. M. Bevan and J. O'M. Bockris, Int. J. Hydrogen Energy 6, 473 (1981). 10. H. S. Isaacs, L. J. Olmer, E. J. L. Schouler and C. Y. Yang, Solid State lonics 3/4, 503 (1981). 11. E. J. L. Schouler and H. S. Isaacs, Solid State lonics 5, 555 (1981).
STEAM ELECTROLYSIS WITH SrCeO3-BASED ELECTROLYTE 12. E. J. L. Schouler, M. Kleitz, E. Forest, E. Fernandez and P. Fabry, Solid State lonics 5,559 (1981). 13. G. B. Barbi and C. M. Mari, Solid State Ionics 6, 341 (1982). 14. L. J. Olmer, J. C. Viguie and E. J. L. Schouler, Solid State lonics 7, 23 (1982). 15. J. C. Viguie, Ear. Rep. Commun. (LUX) [EUR-8054], p. 356 (1982). 16. G. Dietrich, W. Donitz, H. Hermeking, G. Oehme and J. Wahl, Eur. Rep. Commun. [LUX) [EUR-8054], p. 371 (1982). 17. G. B. Barbi and C. M. Mari, Solid State lonics 9/10,979 (1983). 18. H. Iwahara, T. Esaka, H. Uchida and N. Maeda, Solid State lonics 3/4, 359 (1981).
77
19. H. Iwahara, H. Uchida and N. Maeda, J. Power Sources 7,293 (1982). 20. H. lwahara, H. Uchida and T. Esdaka, Prog. Batteries Sol. Cells 4, 279 (1982). 21. H. Iwahara, H. Uchida and S. Tanaka, Denki Kagaku 51,187 (1983). 22. H. Iwahara, H. Uchida and S. Tanaka, Solid State lonics 9/10, 1021 (1983). 23. H. Uchida, S. Tanaka and H. Iwahara, J. Appl. Electrochem. 15, 93 (1985). 24. H. Uchida, N. Maeda and H. lwahara, Solid State lonics 11,117 (1983). 25. H. Iwahara, H. Uchida and N. Maeda, Solid State lonics l l , 109 (1983).
Int. J. Hydrogen Energy, Vol. 12, No. 2, pp. 73-77, 1987. Printed in Great Britain.
HIGH-TEMPERATURE STEAM ELECTROLYSIS USING SrCeO3-BASED PROTON CONDUCTIVE SOLID ELECTROLYTE H.
IWAHARA,
H. UCHIDAand I. YAMASAKI
Department of EnvironmentalChemistry and Technology, Faculty of Engineering, Tottori University, Minami 4-101, Koyama-cho, Tottori, 680 Japan
(Received for publication 17 July 1986) Abstraet--A new type of steam electrolyzer was fabricated using high-temperature protonic conductors as the solid electrolytes, and its performance was examined at 700-900°C. As a solid electrolyte, Y- or Yb-doped SrCeO3 ceramic was used, which was found to be a protonic conductor in the presence of hydrogen or water vapor at elevated temperatures. The electrolyzer worked stably. The current efficiencyfor hydrogen production was above 95% at a current density of 0.2 A cm -2 at 800°C. Nickel and platinum were promising materials for the cathode. The major limitation of the electrolyzer was the resistance of the solid electrolyte.
INTRODUCTION
EXPERIMENTAL
High-temperature steam electrolysis using a solid electrolyte is expected to be an effective method for large-scale hydrogen generation. An advantage of the high temperature electrolyzer is the saving of electric energy due to favorable thermodynamic and kinetic conditions [1-7]. This device can be inversely operated as a fuel cell--a well-known effective power generator. Such cell devices have been intensively studied [1-17] using stabilized zirconia (oxide ion conductor) as the electrolyte. High-temperature proton conductive solids are also favorable materials as electrolytes for such solid electrolyte cells. The advantage of a proton conductor electrolyzer is that it produces pure hydrogen, free of water vapor. In the oxide ion conductor cell, steam is supplied to the cathode compartment and the hydrogen produced is accompanied by unreacted water vapor, whereas in the proton conductor electrolyzer, steam is introduced to the anode compartment and pure hydrogen free from water vapor is obtained at the cathode compartment. Before our work, few good high-temperature proton conductors were known, but recently we found that some sintered oxides based on SrCeO3 exhibit appreciable proton conduction at high temperatures [18]. We confirmed that, using SrCe0.90Sc0.1003_a or SrCe0.95Yb0.0sOa-a as a solid electrolyte, steam could be electrolyzed at the anode compartment at a rate close to the theoretical as calculated from Faraday's law [18-21]. These ceramics could also be used as the electrolyte in a hydrogen-air fuel cell and it worked stably [18-23]. The objective of this research was to obtain fundamental information about the steam electrolyzer with SrCeO3-based proton conductive ceramics. We fabricated a small steam electrolyzer by way of experiment and its performance was examined. The current efficiency for hydrogen evolution, the stability of the electrolyzer and the polarization characteristics of some electrode materials are reported in this paper.
The proton conductors used in this study were sinters composed of SrCel-xMxO3_~, (M = Y, Yb; x = 0.05-0.10), where a is the number of oxygen deficiencies per perovskite-type oxide unit cell. The preparation of the specimen ceramics was similar to that in a previous study [18]. The sinters obtained were sliced into thin discs (thickness: 0.5 ram; diameter: 12 mm) to provide test specimens. The construction of the steam electrolyzer is illustrated in Fig. 1. The electrolyzer was situated in the electric furnace. The electrode compartments were separated by the ceramic electrolyte, each face of which was covered with porous electrode. The electrode materials used were platinum or nickel for the cathode and the mixture of platinum and La0.4Sr0.6CoO3_ a (3-5:1 by weight) for the anode. The addition of the oxide stabilized the cell performance. The electrode area was 0.5 cm 2 (projected area). Each electrode compartment was sealed by a glass ring gasket. A platinum wire was attached to the lateral of the electrolyte disc as the reference electrode. This reference electrode may act as a hydrogen electrode, the potential of which depends on the partial pressure of hydrogen produced by the thermal dissociation equilibrium between water vapor and oxygen in the atmosphere at elevated temperature. Therefore, this electrode exhibited a different potential value depending on the experimental conditions (cell temperature, humidity of the air etc.). Water vapor at 1 arm was supplied to the anode compartment and the flow rate of water vapor was measured from the amount of water condensed at the outlet of the anode. In order to carry the evolved hydrogen to a detector, argon gas, dried by P205, was passed through the cathode compartment (30 cm 3 m i n - 1). The exit gas from the cathode compartment was introduced to the gas sampler for a gas chromatograph to analyze the hydrogen content. A gas concentration cell type oxygen meter using stabilized zirconia was also 73
74
H. IWAHARA, H. UCHIDA and I. YAMASAKI to gas chromatograph
Porous electrode S°lidM 0
I
II
electrolyte SrCe, ...... ~
iL~ _ _ Y S02Zmeter
Electric
furnace
Reference _ ~ elect rode
L~H20 ~
.
02
+ E
I
Fig. 1. Schematic diagram of the experimental steam electrolyzer.
used at 800°C in order to check the hydrogen generation. When exit gas from the cathode contains hydrogen, the oxygen meter indicates a large e.m.f, since the partial pressure of oxygen in the gas goes down. After the e.m.f, of the oxygen meter had become stable, the hydrogen content was analyzed quantitatively by a gas chromatograph (Shimazu, GC-3BT, carrier gas: argon; column packing: molecular sieves 5A 30-60 mesh). The hydrogen content in the exit gas was 0.3-6% in this experiment. The hydrogen evolution rate (ml min -1 cm -2 [STP]) was calculated from the concentration of hydrogen and the flow rate of the argon carrier gas [19]. The oxygen evolution rate at the anode was determined in a similar manner. Polarization characteristics of the electrolyzer were 3
studied by the current interruption method as in the previous study [23]. RESULTS A N D DISCUSSION
Electrolysis of steam Electrolysis was carried out at 700-900°C under various conditions. The electrolyzers using SrCe0.95Yb0.0sO3-a and SrCe0.90Y0.]003-,~ ceramics as solid electrolytes exhibited stable characteristics. The performances of the electrolyzers depended to some extent on the density of the ceramic electrolyte and the morphology of the electrodes. The dense ceramic oxides (with a density above 95% of the theoretical value) exhibited good performances (high current efficiency and high protonic conductivity). Figure 2 shows the hydrogen evolution rate as a function of the electrolytic current density. The hydrogen evolution rate is close to the theoretical value calculated from Faraday's law (broken line) in the low current densities, but it deviates gradually from this as the current density increases. The current efficiency for hydrogen evolution (the ratio of the observed value to the theoretical value) was above 90% at 700-800°C and became somewhat lower at 900°C due to electronic conduction [18, 24]. Figure 3 shows the relation between the electrolytic current and applied voltage. A high voltage was necessary for electrolysis due to insufficient conductivity of the electrolyte ceramics. However, the overvoltage, excluding for ohmic loss, was very small (dotted lines); e.g. it was less than 0.3 V at a current density of 0.4 A c m - 2 at 900 o C. This suggests that the thinner the ceramic electrolyte, the better the performance of the electrolyzer. Although the conductivities of these protonic conductors are not sufficiently high for thick ceramic tubes
/
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~ /""
// i //
0.3
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0
I
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I
0.1 0.2 0.3 0.4 Current density ! A cn52
0
2 4 6 Applied voltage I V
Fig. 3. Electrolytic current vs applied voltage of the steam Fig. 2. Hydrogen evolution rate vs current density. Electro- electrolyzer. Electrolyte: SrCeo.9oY0.~oOa-a; anode: Pt + lyte: SrCeo.9oYojoO3_, • = 700°C, /X = 800°C, C) = 900°C; Lao.4Sro.6CoO3_c~;cathode: Pt; temperature: z%, • = 800°C, SrCeo.95YboosO3_,~ [] = 800°C (broken line shows the C), • = 900°C (dotted lines show the voltage excluding the ohmic loss of the electrolyte). theoretical rate).
75
STEAM ELECTROLYSIS WITH SrCeO3-BASED ELECTROLYTE to be utilized as electrolytes in a practical electrolyzer, they may be prospective materials if they can be used as thin films to reduce ohmic loss. We tried to use slightly Y-doped SrCeO3 ceramics such as SrCeo.95Yo.0503_a for the electrolyzer. In that case, because of insufficient protonic conductivities, the voltage required was higher than that described above, and the current efficiency was low due to partial electronic conduction.
~E u 1.5 (a) Tc ~ o E
- . = . o . . ~ . _ . . : o = ~
-
1.0 CS t_
O
go.s "6
Effect of oxygen partial pressure at the anode on current efficiency As shown in Fig. 2, the current efficiency had a tendency to decrease with increasing current density above 0.2 A cm -2. Such a decrease may be ascribed to electronic conduction in the electrolyte. In order to clarify this phenomena, the following experiments were carried out. Using SrCe0.95Yb0.0503_a as the electrolyte, the hydrogen evolution rate was measured as a function of the current density in three experiments. In the first experiment, steam was fed to the anode compartment at a rate of 364 cm 3 m i n - i. In the second, oxygen gas (flow rate = 40 cm 3 min - l ) was added to the steam. In the third, hydrogen gas (flow rate = 10 cm 3 min -1) was added to the steam. The current efficiency decreased on introducing oxygen into the anode compartment whereas it increased on introducing hydrogen. As reported previously [18, 24], these oxides exhibited p-type conduction in the absence of water vapor or hydrogen, and on introducing water vapor, proton conduction appeared at the expense of electron holes. H20(g) + 2 h+(oxides) ~__ 2 H+(oxides)
+ 1/2 02' (g)
( 1)
Therefore, accumulation of the evolved oxygen at the anode should give rise to electron holes in the oxide, which results in current loss during electrolysis. From these results, some counterplots against the accumulation of oxygen (e.g. geometry of the electrolyzer) must be taken into consideration in order to operate at high current densities.
Continuous electrolysis test A constant current electrolysis (0.2 A cm -2) was carried out for 6 h at 800°C using SrCe0.95Yb0.0503-~ as the electrolyte. Figure 4 shows the time dependence of cell performances during the electrolysis. The hydrogen evolution rate was almost constant and the current efficiency was above 95% ((a) in Fig. 4). The applied voltage was stable and about 2.8 V, whereas the voltage excluding ohmic loss was low (1.1 V, (b) in Fig. 4). After an overnight break, this electrolyzer was operated similarly for another 8 h and worked stably. Little deterioration was observed in the cell performance compared with the initial performance. Similar experimental results were
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0 (b)
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4 Time / h
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0
2
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i 6
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Fig. 4. Cell performances for continuous electrolysis of 0.2 A cm -2 at 800°C using SrCeo.95Ybo.o503_,~ as the electrolyte. (a) H2 evolution rate (broken line shows the theoretical rate); (b) cell voltage (open symbol shows the voltage excluding ohmic loss of the electrolyte).
also obtained for the electrolyzer with SrCe0.90YoA003-~ electrolyte. This indicates that these proton conductors are steady and stable when used as the solid electrolyte for the steam electrolyzer under these conditions.
Steam electrolysis using a nickel cathode In the actual operation mode, pure hydrogen gas must be obtained without using carrier gas for the cathode. Since it was difficult to measure the volume of the evolved hydrogen in our experimental cell, we examined the cell performance with pure hydrogen at the cathode. Supplying hydrogen gas at 1 atm, instead of Ar carrier gas, into the cathode compartment, the water vapor accompanied by carrier gas (air) was electrolyzed at the anode and the oxygen evolution rate was analyzed. Porous nickel or platinum was used as the cathode material and SrCe0.95Yb0.0503_a as the electrolyte in this experiment. These cells also worked stably. Oxygen was observed to evolve at a rate close to the theoretical at the anode, irrespective of the cathode materials. This result suggests that the proton conductor can be used for the electrolyzer, producing pure hydrogen gas at the cathode. This may be reasonable because SrCeO3-based ceramics exhibited as stable characteristics as the proton
H. IWAHARA, H. UCHIDA and I. YAMASAKI
76 0.4
I
7E 0.3 u
cathode was also stable, but the polarization was somewhat larger (Fig. 6) probably due to the nickel adhering unsatisfactorily to the electrolyte surface. However, in place of expensive platinum, nickel may be a promising cathode material because of its stable properties and low cost.
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~71
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CONCLUSION
3 -0.2
High temperature steam electrolysis was studied using solid proton conductors based on SrCeO3 at 700-900°C. SrCeo.95Ybo.o503-a and SrCeo.9oYo.loO3-a ceramics were successfully used for the electrolyzer. The current efficiency for hydrogen evolution was above 95% at a current density of 0.2 A cm -2 at 800°C but it decreased with increasing cell temperature, probably due to the increase in electronic conduction in the ceramic electrolyte. The mixture of Lao.aSr0.6CoO3_ a and Pt exhibited stable characteristics for the anode material. Nickel as well as platinum was a promising material for the cathode. Since the major limitation was the resistance of the solid electrolyte in these cells, the SrCeO3-based proton conductors may be prospective materials for hydrogengenerating steam electrolyzers if used as thin films to reduce ohmic loss. Bench-scale tests on steam electrolyzers using SrCeO3-based proton conductive solid electrolyte are in progress in our laboratory.
-0.3 -0.4
Cathode I ~ I i -0.5 0 0.5 }.0 1.5 Electrode potential / Vvs R.E.
Fig. 5. Polarization of the electrodes in the cell (H20 + air / / H2 (1 atm)) at 800°C (A) and at 900°C (O). Electrolyte: SrCeo.95Ybo.osO3-a; cathode: Pt (ohmic loss is excluded in this figure).
conductor in the hydrogen-air fuel cell [18-231 and the hydrogen concentration cell [25]. Typical polarization characteristics of the electrodes are shown in Figs 5 and 6. The magnitude of the overvoltage depended largely upon the electrode condition at each particular stage. For example, the anodic polarization was different in Figs 5 and 6 in spite of using the same electrode material. The platinum cathode exhibited a little polarization (Fig. 5). The nickel
0.4
I
~
0.3 u
I
Anode
-+o+o _7-/
REFERENCES
t ! I
E -0.1
,~ -0.2
°'
~ c
a
-0.3
-0.4
tion, Science and Culture of Japan for supporting part of this work by a grant from the Special Research Project on the Effective Use of Energy (320).
~-
0.2
<~ 0.1
Acknowledgements--The authors thank the Ministry of Educa-
rhode/ J i -1.5 -1.0 -0.5 0 0.5 Electrode potential/V vs R.E.
Fig. 6. Polarization of the electrodes in the cell as in Fig. 5 at 800°C (A) and a t 900°C (©). Cathode: Ni (ohmic loss is excluded in this figure).
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