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Proton conduction in sintered oxides and its application to steam electrolysis for hydrogen production
Solid State Ionics 3/4 (1981) 359-363 North-Holland Publishing Company
P R O T O N C O N D U C T I O N IN SINTERED OXIDES AND ITS A P P L I C A T I O N TO STEAM ELECTROLYSIS FOR H Y D R O G E N P R O D U C T I O N H. I W A H A R A , T. E S A K A , H. U C H I D A and N. M A E D A Department of Environmental Chemistry and Technology, Faculty of Engineering, Tottori University, Koyamacho, Tottori, Japan Some sintered oxides based on SrCeO3 were found to exhibit proton conduction on exposing them to a hydrogencontaining atmosphere at high temperature. The verification of proton conduction was made by studying the emf of various gas cells using the specimen diaphragm as an electrolyte. These materials could be applied to the electrolyte for steam electrolysis to produce hydrogen gas.
I. Introduction Although many investigators have been working on proton conduction in solids at temperatures below 100°C, high-temperature solid proton conductors have not been widely studied [1]. Wagner et al. [2] have discussed the existence of protons in CuzO, CuO, NiO and some stabilized zirconias at high temperatures in the presence of water vapor, and Shores et ai. [3] reported proton conduction in thoria-based sinters. Several investigators have studied proton conduction in SiO2 [4,5] some perovskite-type oxides [6-8] and hydroxyapatites [9] at temperatures above several hundred degrees centigrade. These studies, however, do not provide a direct demonstration of proton conduction, and the conductivities are not sufficiently high for practical use. In the present study, some SrCeO3-based sinters were found to exhibit appreciably high protonic conduction in hydrogen-containing atmospheres at high temperature. Confirmation of proton conduction and demonstration of hydrogen generation by steam electrolysis using these oxides as the electrolyte are described in this paper.
2. Experimental The ceramics tested were strontium cerium
trioxide and its derivatives SrCel_xMxO3_~ in which Ce in SrCeO3 was partially substituted by the aliovalent cation M. These were prepared by the solid-state reaction of cerium dioxide, strontium carbonate and the oxides or carbonates of various metals. The powders of raw materials were mixed and calcined at 1300-1450°C for 5-10 h in air. The calcined oxides were ground and pressure molded hydrostatically into the column (diameter 13 mm) and sintered at 1350--1450°C for 5-10 h in air. The sinters thus obtained were sliced into thin discs (thickness ~0.5 mm) to provide test specimens. Protonic conduction was confirmed primarily by studying the emf of the following gas concentration cell at high temperatures using the specimen ceramics as the electrolyte diaphragm, gas I, Ptlceramic disclPt, gas I I .
(1)
Air, oxygen or hydrogen at 1 atm was used as gas I or gas II. They were used in wet or dry states. Wet gas was prepared by saturating the water vapor at room temperature and dry gas by passing through phosphoruspentoxide powder. In the steam electrolysis, the steam at 1 atm was supplied to the anode compartment and argon was passed through the cathode compartment to carry the gas generated during electrolysis to a detector. The determination of the generated gas was made by gas chromatography using active carbon as adsorbent.
0167-2738/81/0000-0000/$02.50 © North-Holland Publishing Company
360
H. l w a h a r a et al. / Proton conduction in sintered oxides
Table 1 emf of various gas cells, gas I, Pt[specimen oxide[ Pt, gas 11 Cell no.
1 2 3 4 5
Cell type a) gas Illgas II
dry wet wet wet dry
air][dry 02 air[[wet O: airlldry air airf[dry Oz air]lwet 02
emf (mV) u) SrCeo 95Ybo 0503-~
SrCe~}95Mg0.0503-
600°C
800°C
600°C
800°C
0.5 25.0 59.0 69.5 -43.5
1.0 14.0 30.0 30.5 13.0
0.1 16.0 39.0 46.8 -25.0
-0.3 12.0 17.0 19.5 -10.5
a) Dry g a s : - dried with P205; wet g a s : - saturated with H 2 0 at room temperature (21-22°C). u) Negative sign shows that the electrode of gas II is negative.
3. Results and discussion
3.1. emf of various gas cells The cells using undoped SrCeO3 as a diaphragm showed an unstable emf in all cases examined. However, when doped ceramics SrCel_xMxO3_~ were applied as the diaphragm, the ceils showed a peculiar emf, which could not be explained by the specimen being either an oxide ion conductor or simple electronic conductor. The emfs of the cells on applying various gases are tabulated in table 1 for the cases of Srfe0.95Yb0.0503-,~ and SrCe0.95Mg0.0503-~ as typical examples. When dry oxygen gases of different partial pressures (gas I, air; gas II, pure oxygen) were introduced to each electrode compartment, only a very small unstable emf was observed as shown in cell 1 in table 1, suggesting that the conduction in the oxides was mainly electronic. However, when each gas of the above cell was moistened, a stable emf of the oxygen concentration cell appeared (cell 2 in table 1) although the values were low c o m p a r e d to the theoretical values (29.5 m V at 600°C and 35.7 m V at 800°C). A stable emf was also observed when air with different humidities was supplied to the electrode c o m p a r t m e n t s (cell 3). In this case, the electrode with the higher humidity was the negative pole. This is a type of steam concen-
tration cell. A steady stable current could be drawn from this cell. Cells 4 and 5 show that the electrodes with higher humidity are also negative poles, and that the absolute emf values at 600°C are higher than those of oxygen concentration cells. Table 2 shows the effect of water vapor on the emf of the hydrogen-oxygen cell using SrCe0.959c0.0503-,~ a s the electrolyte. Higher emfs were observed when the oxygen gas contained a smaller amount of steam than did the hydrogen gas. This p h e n o m e n o n is inverse to the hydrogen--oxygen cell with an oxide ion conductor. Some other specimens containing Y, In, Zn, Nd, Sm and Dy as M in SrCel_xM/O3_~ (x = 0.05-0.10) were observed to behave in the same manner as described above.
Table 2 Effect of water vapor on the e m f of the cell, H2, PtlSrCe095Sc00503 ~[Pt, O2 H 2 0 content (Tort)
emf (mV)
H2 electrode
700°C
800°C
100(1°C
1119 1108 1108 0.99
1057 1047 1047 0.95
9(13 897 894 0.84
02 electrode
15.5 4.6 4.6 15.5 15.5 15.5 ion transference n u m b e r
H. Iwahara et al. / Proton conduction in sintered oxides
3.2. Confirmation of protonic conduction The cell behavior described above cannot be elucidated by oxide ion conduction or normal electronic conduction in the ceramics. However, if the specimen has proton conduction in the moistened atmosphere, these p h e n o m e n a can be rationally explained as follows. (1) If the specimen diaphragm in cell 3 is a proton conductor, the difference in partial pressure of water vapor between two electrodes can be a driving force of the following electrode reactions. Electrode with higher vapor pressure: H 2 0 --* 2H + + 102 -[- 2 e - .
(2)
Electrode with lower vapor pressure: 2H ÷ + ½02 + 2e- ~ H 2 0 .
(3)
For this reason, this cell may give a stable emf, with the electrode of lower vapor pressure being the cathode in spite of the negligible pressure difference of oxygen between the two electrodes. In fig. 1, the electrode reactions for this steam concentration cell are illustrated schematically. (2) When two electrode gases have different oxygen pressures but equal v a p o r pressures like cell 2 in table 1, the equilibrium partial pressures of hydrogen at high t e m p e r a t u r e differ from each other; the gas having the higher oxygen pressure includes a lower partial pres-
Solid proton conductor wet air
dry air _~H+~I ',~
H20 ~'~ 2
q
--
02
2
_
L;0- 0-TI Fig. 1. Concept of steam concentration cell.
361
sure of hydrogen. As a result, if the diaphragm is a proton conductor, the emf of the hydrogen concentration cell appears making the electrode with the higher oxygen pressure the cathode. Thus, the stable emf of cell 2 can be explained assuming the diaphragm has proton conduction. (3) Generally, when gas I and gas II in the cell include oxygen and water vapor at differential partial pressures, the emf E of the steam concentration cell can be given as
R T In PH2O(I) (Po2(II),~u2 E = ~ ~ ) \p----~2(i)j ,
(4)
where P.2o and Po: are partial pressures of water and oxygen, respectively, and R, F and T have their usual meaning. The emf behavior of cells 4 and 5 can also be explained qualitatively by eq. (4) provided the diaphragm is a proton conductor. The protonic conduction could also be verified by the emf behavior of a h y d r o g e n oxygen cell. If the electrolyte of this cell is a proton conductor, the proton has a tendency to migrate across the conductor to the oxygen electrode, where it discharges to form water vapor. Therefore, the emf given by
E = E ° - ~R-TI n
PH20 pH2p82
(5)
must be controlled by the vapor pressure PH20 at the oxygen electrode; the higher the PH20 at the cathode, the lower the emf of the cell, but the change in vapor pressure at the hydrogen electrode will not influence the emf. On the contrary, if the electrolyte is an oxide ion conductor, the situation is inverse. The results listed in table 2 coincide with that for the proton conductor. In table 2, the proton transference numbers, which were determined by the ratio of the measured emf to the theoretical emf of this cell, are also given. Proton conductivities were not separately determined in the present experiment. However, the conductivities measured in hydrogen atmosphere shown in fig. 2 will be close to the proton conductivities, since the proton transference numbers given in table 2 are close to unity.
362
H. l w a h a r a et al. / Proton conduction in sintered oxides
Temperature ( °C ) I000 900 800 /00 i
~
i
Temperature 000 9UO 800
600
i
I
i
10-2
I
~'C ) 7DO
I
600
I
1
i
E
I
E
i0 -2
i
u
i
E
\©
g
4_a
O
10-3
~zJ
g ¢_)
IO .3
0,8
I
I
I
0,9
1,0
I,I
I000 / T
0',8 1.2
OI
,9
i090 /
I
1,0
I
1,1
( K -I )
( K -I )
Fig. 2. Conductivities in hydrogen atmosphere. (1) SrCe095Yb~u~5Ox ~ (2) SrCe09Ycj10~ ,, (3) SrCe09~Sc~ t~O~ ,.
3.3. Possible mechanism of proton formation Fig. 3 shows the conductivities of SrCe0.95Yb0.0503 ,~ measured in various atmospheres. The conductivities increased as the oxygen pressure increased indicating that the conduction mode is p-type (conduction by electron holes). The electron holes may be formed by charge compensation on substituting the aliovalent cation Yb 3+ for C e 4+ in SrCeO3. The addition of water vapor made the conductivity decrease, probably due to the decrease in the hole concentration. The protonic conduction appears in this condition as described above. These phenomena suggest that the protons in the oxide may be provided from H20 at the expense of the electron holes [2,3]. One of the possible mechanisms for proton formation is H 2 0 + 2h + (oxides) ~ 2H ÷ (oxides) + ½02
The small emfs in table 1 compared to the
(6)
Fig. 3. The conductivities of SrCe4~95Yb.0503 ~ in various gases. (1) In dry O2, (2) in dry air, (3) in wet air, (4) in dry N2.
theoretical values calculated from eq. (4) may be ascribed to conduction by electron holes surviving in the oxide.
3.4. Electrolysis of steam Using the ceramic diaphragm as the electrolyte, we could electrolyze the steam at high temperature to get hydrogen gas. The steam at 1 arm was supplied to the anode compartment, and argon gas was passed through the cathode compartment to carry the generated gas to a detector. On passing direct current through the cell at 600-1000°C, evolution of hydrogen gas at the cathode was recognized by gas chromatography. This fact is a direct demonstration of protonic conduction, since the protons formed at the steam electrode must migrate across the solid specimen to discharge at the cathode. Current efficiencies for hydrogen evolution were 50-95% in the range of 0.1-0.8 A/cm 2
H. Iwahara et al. / Proton conduction in sintered oxides
//// //////~
E
u
It._
5
-d 4 3
, / / / ~
0
2
2 >
!
363
However, over-voltage (except ohmic loss) was less than 0.3 V at a current density of 0.4 A/cm 2. This e l e c t r o l y t i c cell c o u l d b e o p e r a t e d inv e r s e l y as a h y d r o g e n fuel cell. In p r i n c i p l e , this cell m a y b e a p p l i e d t o ' a r e c i p r o c a l direct e n e r g y c o n v e r t e r for h y d r o g e n ~ electricity which is i m p o r t a n t in a h y d r o g e n e n e r g y system in t h e future. A n a d v a n t a g e of p r o t o n c o n d u c t o r s for such c o n v e r t e r s lies in t h e n e e d l e s s n e s s of fuel c i r c u l a t i o n since p u r e h y d r o g e n w i t h o u t w a t e r v a p o r can b e o b t a i n e d by e l e c t r o l y s i s a n d since h y d r o g e n fuel is not d i l u t e d with w a t e r v a p o r in t h e fuel cell.
I
0,2
0,4
0,6
0,8
Current density ( A / cm 2 )
Fig. 4. Hydrogen evolution rate versus current density at 900°C. Electrolyte: (1) SrCe0.90Sc0.1003-,,(2) S r f e 0 . 9 5 5 c 0 . 0 5 0 3 a (broken line shows theoretical rate). d e p e n d i n g on t h e t y p e of s p e c i m e n s a n d elect r o d e c o n d i t i o n s . C u r r e n t loss m a y b e a s c r i b e d to t h e c o n d u c t i o n d u e to e l e c t r o n h o l e s in the s p e c i m e n o x i d e s . Fig. 4 shows t h e r e l a t i o n s between hydrogen evolution rate and electrolytic c u r r e n t at 900°C for t h e case of SrCe0.955c0.0503-,~ a n d SrCe0.90Sc0.1003-,~. C o n s i d e r a b l y high v o l t a g e was n e c e s s a r y to e l e c t r o l y z e d u e to insufficient c o n d u c t i v i t y of t h e s p e c i m e n d i a p h r a g m .
References [1] L. Gasser, Chem. Rev. 75 (1975) 21. [2] S. Stotz and C. Wagner, Ber. Bunsenges. Physik. Chem. 70 (1966) 781. [3] D.A. Shores and R.A. Rapp, J. Electrochem. Soc. 119 (1972) 300. [4] P.J. Jorgensen and F.J. Norton, Phys. Chem. Glasses 10 (1969) 23. [5] S. White, Nature 225 (1970) 375. [6] F. Forrat, R. Jansen and P. Tr6voux, Compt. Rend. Acad. Sci. (Paris) 257 (1963) 1271. [7] F. Forrat, G. Dauge, P. Tr6voux, G. Danner and M. Christen, Compt. Rend. Acad. Sci. (Paris) 259 (1964) 2813. [8] T. Takahashi and H. Iwahara, Rev. Chim. Min6rale 17 (1980) 243. [9] T. Takahashi, S. Tanase and O. Yamamoto, Electrochim. Acta 23 (1978) 369.
P R O T O N C O N D U C T I O N IN SINTERED OXIDES AND ITS A P P L I C A T I O N TO STEAM ELECTROLYSIS FOR H Y D R O G E N P R O D U C T I O N H. I W A H A R A , T. E S A K A , H. U C H I D A and N. M A E D A Department of Environmental Chemistry and Technology, Faculty of Engineering, Tottori University, Koyamacho, Tottori, Japan Some sintered oxides based on SrCeO3 were found to exhibit proton conduction on exposing them to a hydrogencontaining atmosphere at high temperature. The verification of proton conduction was made by studying the emf of various gas cells using the specimen diaphragm as an electrolyte. These materials could be applied to the electrolyte for steam electrolysis to produce hydrogen gas.
I. Introduction Although many investigators have been working on proton conduction in solids at temperatures below 100°C, high-temperature solid proton conductors have not been widely studied [1]. Wagner et al. [2] have discussed the existence of protons in CuzO, CuO, NiO and some stabilized zirconias at high temperatures in the presence of water vapor, and Shores et ai. [3] reported proton conduction in thoria-based sinters. Several investigators have studied proton conduction in SiO2 [4,5] some perovskite-type oxides [6-8] and hydroxyapatites [9] at temperatures above several hundred degrees centigrade. These studies, however, do not provide a direct demonstration of proton conduction, and the conductivities are not sufficiently high for practical use. In the present study, some SrCeO3-based sinters were found to exhibit appreciably high protonic conduction in hydrogen-containing atmospheres at high temperature. Confirmation of proton conduction and demonstration of hydrogen generation by steam electrolysis using these oxides as the electrolyte are described in this paper.
2. Experimental The ceramics tested were strontium cerium
trioxide and its derivatives SrCel_xMxO3_~ in which Ce in SrCeO3 was partially substituted by the aliovalent cation M. These were prepared by the solid-state reaction of cerium dioxide, strontium carbonate and the oxides or carbonates of various metals. The powders of raw materials were mixed and calcined at 1300-1450°C for 5-10 h in air. The calcined oxides were ground and pressure molded hydrostatically into the column (diameter 13 mm) and sintered at 1350--1450°C for 5-10 h in air. The sinters thus obtained were sliced into thin discs (thickness ~0.5 mm) to provide test specimens. Protonic conduction was confirmed primarily by studying the emf of the following gas concentration cell at high temperatures using the specimen ceramics as the electrolyte diaphragm, gas I, Ptlceramic disclPt, gas I I .
(1)
Air, oxygen or hydrogen at 1 atm was used as gas I or gas II. They were used in wet or dry states. Wet gas was prepared by saturating the water vapor at room temperature and dry gas by passing through phosphoruspentoxide powder. In the steam electrolysis, the steam at 1 atm was supplied to the anode compartment and argon was passed through the cathode compartment to carry the gas generated during electrolysis to a detector. The determination of the generated gas was made by gas chromatography using active carbon as adsorbent.
0167-2738/81/0000-0000/$02.50 © North-Holland Publishing Company
360
H. l w a h a r a et al. / Proton conduction in sintered oxides
Table 1 emf of various gas cells, gas I, Pt[specimen oxide[ Pt, gas 11 Cell no.
1 2 3 4 5
Cell type a) gas Illgas II
dry wet wet wet dry
air][dry 02 air[[wet O: airlldry air airf[dry Oz air]lwet 02
emf (mV) u) SrCeo 95Ybo 0503-~
SrCe~}95Mg0.0503-
600°C
800°C
600°C
800°C
0.5 25.0 59.0 69.5 -43.5
1.0 14.0 30.0 30.5 13.0
0.1 16.0 39.0 46.8 -25.0
-0.3 12.0 17.0 19.5 -10.5
a) Dry g a s : - dried with P205; wet g a s : - saturated with H 2 0 at room temperature (21-22°C). u) Negative sign shows that the electrode of gas II is negative.
3. Results and discussion
3.1. emf of various gas cells The cells using undoped SrCeO3 as a diaphragm showed an unstable emf in all cases examined. However, when doped ceramics SrCel_xMxO3_~ were applied as the diaphragm, the ceils showed a peculiar emf, which could not be explained by the specimen being either an oxide ion conductor or simple electronic conductor. The emfs of the cells on applying various gases are tabulated in table 1 for the cases of Srfe0.95Yb0.0503-,~ and SrCe0.95Mg0.0503-~ as typical examples. When dry oxygen gases of different partial pressures (gas I, air; gas II, pure oxygen) were introduced to each electrode compartment, only a very small unstable emf was observed as shown in cell 1 in table 1, suggesting that the conduction in the oxides was mainly electronic. However, when each gas of the above cell was moistened, a stable emf of the oxygen concentration cell appeared (cell 2 in table 1) although the values were low c o m p a r e d to the theoretical values (29.5 m V at 600°C and 35.7 m V at 800°C). A stable emf was also observed when air with different humidities was supplied to the electrode c o m p a r t m e n t s (cell 3). In this case, the electrode with the higher humidity was the negative pole. This is a type of steam concen-
tration cell. A steady stable current could be drawn from this cell. Cells 4 and 5 show that the electrodes with higher humidity are also negative poles, and that the absolute emf values at 600°C are higher than those of oxygen concentration cells. Table 2 shows the effect of water vapor on the emf of the hydrogen-oxygen cell using SrCe0.959c0.0503-,~ a s the electrolyte. Higher emfs were observed when the oxygen gas contained a smaller amount of steam than did the hydrogen gas. This p h e n o m e n o n is inverse to the hydrogen--oxygen cell with an oxide ion conductor. Some other specimens containing Y, In, Zn, Nd, Sm and Dy as M in SrCel_xM/O3_~ (x = 0.05-0.10) were observed to behave in the same manner as described above.
Table 2 Effect of water vapor on the e m f of the cell, H2, PtlSrCe095Sc00503 ~[Pt, O2 H 2 0 content (Tort)
emf (mV)
H2 electrode
700°C
800°C
100(1°C
1119 1108 1108 0.99
1057 1047 1047 0.95
9(13 897 894 0.84
02 electrode
15.5 4.6 4.6 15.5 15.5 15.5 ion transference n u m b e r
H. Iwahara et al. / Proton conduction in sintered oxides
3.2. Confirmation of protonic conduction The cell behavior described above cannot be elucidated by oxide ion conduction or normal electronic conduction in the ceramics. However, if the specimen has proton conduction in the moistened atmosphere, these p h e n o m e n a can be rationally explained as follows. (1) If the specimen diaphragm in cell 3 is a proton conductor, the difference in partial pressure of water vapor between two electrodes can be a driving force of the following electrode reactions. Electrode with higher vapor pressure: H 2 0 --* 2H + + 102 -[- 2 e - .
(2)
Electrode with lower vapor pressure: 2H ÷ + ½02 + 2e- ~ H 2 0 .
(3)
For this reason, this cell may give a stable emf, with the electrode of lower vapor pressure being the cathode in spite of the negligible pressure difference of oxygen between the two electrodes. In fig. 1, the electrode reactions for this steam concentration cell are illustrated schematically. (2) When two electrode gases have different oxygen pressures but equal v a p o r pressures like cell 2 in table 1, the equilibrium partial pressures of hydrogen at high t e m p e r a t u r e differ from each other; the gas having the higher oxygen pressure includes a lower partial pres-
Solid proton conductor wet air
dry air _~H+~I ',~
H20 ~'~ 2
q
--
02
2
_
L;0- 0-TI Fig. 1. Concept of steam concentration cell.
361
sure of hydrogen. As a result, if the diaphragm is a proton conductor, the emf of the hydrogen concentration cell appears making the electrode with the higher oxygen pressure the cathode. Thus, the stable emf of cell 2 can be explained assuming the diaphragm has proton conduction. (3) Generally, when gas I and gas II in the cell include oxygen and water vapor at differential partial pressures, the emf E of the steam concentration cell can be given as
R T In PH2O(I) (Po2(II),~u2 E = ~ ~ ) \p----~2(i)j ,
(4)
where P.2o and Po: are partial pressures of water and oxygen, respectively, and R, F and T have their usual meaning. The emf behavior of cells 4 and 5 can also be explained qualitatively by eq. (4) provided the diaphragm is a proton conductor. The protonic conduction could also be verified by the emf behavior of a h y d r o g e n oxygen cell. If the electrolyte of this cell is a proton conductor, the proton has a tendency to migrate across the conductor to the oxygen electrode, where it discharges to form water vapor. Therefore, the emf given by
E = E ° - ~R-TI n
PH20 pH2p82
(5)
must be controlled by the vapor pressure PH20 at the oxygen electrode; the higher the PH20 at the cathode, the lower the emf of the cell, but the change in vapor pressure at the hydrogen electrode will not influence the emf. On the contrary, if the electrolyte is an oxide ion conductor, the situation is inverse. The results listed in table 2 coincide with that for the proton conductor. In table 2, the proton transference numbers, which were determined by the ratio of the measured emf to the theoretical emf of this cell, are also given. Proton conductivities were not separately determined in the present experiment. However, the conductivities measured in hydrogen atmosphere shown in fig. 2 will be close to the proton conductivities, since the proton transference numbers given in table 2 are close to unity.
362
H. l w a h a r a et al. / Proton conduction in sintered oxides
Temperature ( °C ) I000 900 800 /00 i
~
i
Temperature 000 9UO 800
600
i
I
i
10-2
I
~'C ) 7DO
I
600
I
1
i
E
I
E
i0 -2
i
u
i
E
\©
g
4_a
O
10-3
~zJ
g ¢_)
IO .3
0,8
I
I
I
0,9
1,0
I,I
I000 / T
0',8 1.2
OI
,9
i090 /
I
1,0
I
1,1
( K -I )
( K -I )
Fig. 2. Conductivities in hydrogen atmosphere. (1) SrCe095Yb~u~5Ox ~ (2) SrCe09Ycj10~ ,, (3) SrCe09~Sc~ t~O~ ,.
3.3. Possible mechanism of proton formation Fig. 3 shows the conductivities of SrCe0.95Yb0.0503 ,~ measured in various atmospheres. The conductivities increased as the oxygen pressure increased indicating that the conduction mode is p-type (conduction by electron holes). The electron holes may be formed by charge compensation on substituting the aliovalent cation Yb 3+ for C e 4+ in SrCeO3. The addition of water vapor made the conductivity decrease, probably due to the decrease in the hole concentration. The protonic conduction appears in this condition as described above. These phenomena suggest that the protons in the oxide may be provided from H20 at the expense of the electron holes [2,3]. One of the possible mechanisms for proton formation is H 2 0 + 2h + (oxides) ~ 2H ÷ (oxides) + ½02
The small emfs in table 1 compared to the
(6)
Fig. 3. The conductivities of SrCe4~95Yb.0503 ~ in various gases. (1) In dry O2, (2) in dry air, (3) in wet air, (4) in dry N2.
theoretical values calculated from eq. (4) may be ascribed to conduction by electron holes surviving in the oxide.
3.4. Electrolysis of steam Using the ceramic diaphragm as the electrolyte, we could electrolyze the steam at high temperature to get hydrogen gas. The steam at 1 arm was supplied to the anode compartment, and argon gas was passed through the cathode compartment to carry the generated gas to a detector. On passing direct current through the cell at 600-1000°C, evolution of hydrogen gas at the cathode was recognized by gas chromatography. This fact is a direct demonstration of protonic conduction, since the protons formed at the steam electrode must migrate across the solid specimen to discharge at the cathode. Current efficiencies for hydrogen evolution were 50-95% in the range of 0.1-0.8 A/cm 2
H. Iwahara et al. / Proton conduction in sintered oxides
//// //////~
E
u
It._
5
-d 4 3
, / / / ~
0
2
2 >
!
363
However, over-voltage (except ohmic loss) was less than 0.3 V at a current density of 0.4 A/cm 2. This e l e c t r o l y t i c cell c o u l d b e o p e r a t e d inv e r s e l y as a h y d r o g e n fuel cell. In p r i n c i p l e , this cell m a y b e a p p l i e d t o ' a r e c i p r o c a l direct e n e r g y c o n v e r t e r for h y d r o g e n ~ electricity which is i m p o r t a n t in a h y d r o g e n e n e r g y system in t h e future. A n a d v a n t a g e of p r o t o n c o n d u c t o r s for such c o n v e r t e r s lies in t h e n e e d l e s s n e s s of fuel c i r c u l a t i o n since p u r e h y d r o g e n w i t h o u t w a t e r v a p o r can b e o b t a i n e d by e l e c t r o l y s i s a n d since h y d r o g e n fuel is not d i l u t e d with w a t e r v a p o r in t h e fuel cell.
I
0,2
0,4
0,6
0,8
Current density ( A / cm 2 )
Fig. 4. Hydrogen evolution rate versus current density at 900°C. Electrolyte: (1) SrCe0.90Sc0.1003-,,(2) S r f e 0 . 9 5 5 c 0 . 0 5 0 3 a (broken line shows theoretical rate). d e p e n d i n g on t h e t y p e of s p e c i m e n s a n d elect r o d e c o n d i t i o n s . C u r r e n t loss m a y b e a s c r i b e d to t h e c o n d u c t i o n d u e to e l e c t r o n h o l e s in the s p e c i m e n o x i d e s . Fig. 4 shows t h e r e l a t i o n s between hydrogen evolution rate and electrolytic c u r r e n t at 900°C for t h e case of SrCe0.955c0.0503-,~ a n d SrCe0.90Sc0.1003-,~. C o n s i d e r a b l y high v o l t a g e was n e c e s s a r y to e l e c t r o l y z e d u e to insufficient c o n d u c t i v i t y of t h e s p e c i m e n d i a p h r a g m .
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