J. Phys. Chem. 1995,99, 3282-3287
3282
Effects of Ionic Conductivities of Zirconia Electrolytes on Polarization Properties of Platinum Anodes in Solid Oxide Fuel Cells Hiroyuki Uchida, Manabu Yoshida, and Masahiro Watanabe" Laboratory of Electrochemical Energy Conversion, Faculty of Engineering, Yamanashi University, Takeda 4-3, KO@ 400, Japan Received: July 20, 1994; In Final Form: December 8, 1994@
To find a clue for the design of high-performance electrodes for solid oxide fuel cells (SOFCs), the polarization properties of Pt electrodes attached to zirconia electrolytes with various ionic conductivities were investigated at 800- 1000 "C. The IR-free anodic polarization in hydrogen was greatly affected by the ionic conductivity of the electrolyte, and it obeyed the Tafel equation. The exchange current density increased in proportion to the square of the ionic conductivity for all electrolytes operated at 800-1000 "C,while the transfer coefficient ( n a , = 2) was independent of the temperature and of the conductivity of electrolytes. According to our analysis, the rate-determining step is not a simple electron transfer from oxide ions but a recombination step involving discharged oxygen atoms adsorbed on the Pt electrode/electrolyte interface; an increase in the rate of transport of oxide ions to the interface, for example, by using an electrolyte with higher-ionic conductivity, reduces the anodic overpotential greatly.
Introduction
Experimental Section
Recently, solid oxide fuel cells (SOFCs) have been intensively investigated because, in principle, their energy conversion efficiency is fairly high. Lowering the operating temperature of SOFCs from 1000 to around 800 "C is desirable for reducing serious problems such as physical and chemical degradation of the constructing materials. To reduce the ohmic loss in the solid electrolyte for medium-temperature operation of an SOFC, methods of fabrication of thin-film zirconia electrolytes or of syntheses of various model solid electrolytes with higher ionic cond~ctivityl-~ have been investigated. It is very important to develop high-performance electrodes because the electrode reaction rates at the anode or cathode are low in such a temperature region. Recently, we have reported an approach for preparation of high-performance catalyzed-reaction layers for SOFCs operating at medium t e m p e r a t ~ r e . ~ , ~ The object of the present study is to find a clue for developing the electrodes with higher performance than those of the present level. Thus, it is essential to investigate the factors which control the polarization properties of electrodes in SOFC. Many researchers have investigated electrochemical reaction kinetics or mechanisms at some or o ~ i d e ~ Oelectrodes -~~ interfaced with zirconia electrolytes. Although it has been demonstrated that the polarization loss at the electrodes can be reduced by using mixed conducting solid electrolytes26 or introducing mixed conducting layers on zirconia surface^,^^^^^^^^ no reports are available on the effects of the purely ionic conductivity of the electrolyte on the electrodepolarization. The higher the ionic conductivity of the electrolyte, of course, the lower is the ohmic loss. However, we found that the IR-free polarization of a platinum anode is greatly influenced by the ionic conductivity of the zirconia electrolyte. In this paper, we present the experimental results and our analysis of mechanism of the hydrogen oxidation at the Pt-Zr02 (doped with Y203 or Yb203) interface at 800-1000 "c.
The solid electrolytes employed were zirconia doped with yttria or ytterbia. Zirconias with the composition of (Zr02)1-x(Y203)x (X = 0.03, 0.04, 0.08) were prepared from Y-doped zirconia powders, Tosoh TZ-3Y, TZ4Y, and TZ-8Y. These powders were pressed hydrostatically (2 tons/cm2)and sintered at 1600 "C for 4 h in air. The Y-doped zirconia specimens will be denoted as 3Y, 4Y, and 8Y, respectively. Ytterbiastabilized zirconia (zfi2)0.92(Yb203)0,0S(denoted as 8Yb) was prepared from ZrOz(TZ-0) and Yb2O3 (99.95%, Kanto Chemical). The powders of the well-mixed raw materials were pressed and sintered in the same manner as described above. X-ray difiaction was performed to c o n f i i the formations of partiallystabilized (for 3Y and 4Y) and fully stabilized (for 8Y and 8Yb) zirconia phases. The relative density of each specimen, defined as the ratio of the apparent density to the theoretical one, was more than 96% and the values are presented in Table 1. Thus, very dense zirconia sinters with different ionic conductivities were obtained. The construction of experimental fuel cell is as described in ref 5. The zirconia sinters were sliced into thin disks (diameter 13 mm, thickness 1 mm) to serve as the solid electrolytes for the test cells. Porous Pt electrodes were attached to both faces by screen printing a fritless platinum paste (TKI, TR-7905) followed by firing at 1050 "C for 4 h. The surface area and amount of Pt attached for each Pt electrode were 0.26 cm2 and 1.3 mg, respectively. Observations of the Pt electrodes by scanning electron microscope showed that very uniform porous Pt layers with a thickness of ca. 5 pm were produced on the electrolyte and the morphology was the same for all specimens. A gold mesh current collector was attached to each electrode. Two gold wires (current supply and potential probe) were contacted to the current collector. A platinum wire, would laterally around the disk, served as the reference electrode. Platinum paste was applied to improve the contact between the reference electrode and the electrolyte. The reference electrode was exposed to air in the electric furnace and exhibited a reversible oxygen potential in air. The anode and cathode compartments were separated by the electrolyte disk and each compartment was sealed by a glass ring gasket.
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, February 15, 1995.
0022-3654/95/2099-3282$09.00/0 0 1995 American Chemical Society
J, Phys. Chem., Vol. 99, No. 10, 1995 3283
Polarization Properties of Pt Anodes
TABLE 1: Properties of Zirconia Electrolytes (ZrOt)l-~)(M203)~ Prepared S cm-'
tic
sample 8Yb 8Y 4Y 3Y
dopant, M Yb Y Y Y
X 0.08 0.08 0.04 0.03
crystalline phase"
re1 density,b %
1000 "C
900 "C
800 "C
1000 OC
900 'C
800 "C
C C C+T C+T
99 99 97 96
1.01 1.00 1.01 1.02
1.02 1.02 1.01 1.02
1.00 1.01 1.02 1.02
0,1598 0.1386 0.0856 0.0431
0.1122 0.0949 0.0499 0.0240
0.0614 0.0495 0.0260 0.0116
+
+
Determined by XRD; C, cubic (fully stabilized); C T, cubic tetragonal (partially stabilized). Ratio of apparent density to the theoretical one. Ionic transport number determined as a ratio of the observed emf of H2(P[H2O] = 0.042 atm)/02( 1 atm) to the theoretical one. Apparent ionic conductivity for the disk-shape electrolyte (diameter, 13 mm; thickness, 1 mm; area of Pt electrode, 0.26 cm2) used in the fuel cell experiments.
Oxygen gas at 1 atm was supplied to the cathode compartment at a flow rate of 30 cm3/min. Hydrogen gas saturated with water vapor at 30 "C (P[H20] = 0.042 atm) was introduced to the anode compartment (flow rate = 30 cm3/min). The IR-free polarization characteristics of the electrodes were measured by a current interruption method at 800-1000 "C. The resistance of the solid electrolyte disk, under the operating condition of the fuel cell, was evaluated from both the impedance measured by an ac impedance meter (YHP, 4276A) and the ohmic drop during current interruption. Both methods showed resistance values in good agreement.
g
::::I
, , , ,,,,,,
, , , ,,,,,)
, ,
X -0.4
,,,,I
L
$ -0.5 0 0.001
2
-
0.01
0.1
1
0.01
0.1
1
0 -0.1
3.- -0.2 -0.3 I
Results and Discussion
0
-0.4
9
-0.5
0
Properties of Zirconia Electrolytes. Properties of various zirconia electrolytes are summarized in Table 1. The ionic transport number ti for each of the electrolytes was obtained from the ratio of the measured electromotive force (emf) to the theoretical one for the fuel cell with the Zr02 sample as the electrolyte diaphragm.28 Since the ti value for all the electrolytes was unity at 800-1000 "C, there is no contribution of electronic conduction to the polarization of the electrodes. As is wellkn0wn,*~3~~ the ionic conductivity (aion) of the zirconia doped with Y203 increases with increasing amount of the dopant up to about 10 mol %, further the same amount of a YbzO3 dopant gives a higher conductivity than Y2O3. The conductivities decrease in the order 8Yb > 8Y > 4Y > 3Y; the conductivities evaluated in this study were somewhat higher than those r e p ~ r t e dprobably * ~ ~ ~ ~because of an indefinite size-factor of the Pt electrode which covered a part of each surface. We were able to obtain the same value of the conductivity as the published value for a cylindrical sample, both faces of which were fully coated with Pt electrodes. In this study, however, the apparent conductivity of the disk-shape electrolyte was used as a measure of conductivity, mainly because it was evaluated under the actual operating condition of fuel cell. Effects of Ionic Conductivity on the Polarization of Pt Electrodes. Figure 1 shows the overpotentials (IR free) for oxygen reduction at Pt cathodes on 8Yb, 8Y, 4Y, and 3Y at 800-1000 "C as a function of logarithm of current densities. At 1000 and 900 "C, no distinct difference in the polarization was noted in the fuel cells with all the electrolytes. This result indicates that porous Pt electrodes with equivalent microstmctures were prepared using the screen-printing method. At 800 "C, the polarization loss at the Pt cathode was very large and increased with decreasing conductivity of the electrolyte in the order 8Yb < 8Y < 4Y < 3Y. Much more dramatic effects were seen at the Pt anodes in wet hydrogen, as shown in Figure 2, where the IR-free overpotentials 7 are plotted against logarithm of current densities for Pt anodes on 8Yb, 8Y, 4Y, and 3Y. In the temperature range between 1000 and 800 "C, the 7 for the Pt anode at a given current density increased with decreasing conductivity of the electrolyte; the Pt anode on the 3Y electrolyte with the lowest conductivity exhibited the largest polarization. Figure
0.001
2 O e -0.1
3.-c -0.2 C -0.3 d 0 -0.4 E
F
-0.5
0.001
0.01 0.1 Current density, j I A cm-2
1
Figure 1. Polarization curves (IR free) of Pt cathode attached to various zirconia solid electrolytes at (a) 1O00, (b) 900, and (c) 800 "C measured in oxygen atmosphere at 1 atm. Zirconia electrolyte: 0, 8Yb; 0, 8Y; A, 4Y; 0 , 3Y.
3 shows the Arrhenius plots of anodic current densities at a constant 7 of 0.1 V for Pt anodes on various zirconia electrolytes. The slope of each plot is almost same and the activation energy is about 1.O eV. This indicates that the ratedetermining step on the Pt anode for hydrogen oxidation is not changed by the type of electrolyte. It is very striking that the current density for hydrogen oxidation with 8Yb even at 800 "C (~i0,[8Yb, 800 "C] = 0.061 S cm-') is much higher than that for 3Y at 1000 "C (oion[3Y, 1000 "C] = 0.043 S cm-I). These results show that the polarization properties of the Pt anodes are significantly affected by the ionic conductivities of the electrolytes at all temperatures, while similar effects were noticeably observed for the Pt cathode only at low temperatures around 800 "C. In the following sections, we will focus on the anodic polarization at the interface of Wzirconia electrolytes as a function of the electrolytic conductivities and will consider the cathode polarization in a further publication. Dependence of Kinetic Parameters for Hydrogen Oxidation on the Conductivity of Electrolytes. To display the performance of the Pt anode on each electrolyte at 800-1000 "C, kinetic parameters were calculated by applying the Tafel equation to the polarization curves shown in Figure 2: logo/jo) = aanF7/2.303RT
(1)
where j , j o , a,,and n are current density at overpotential 7, exchange current density, anodic transfer coefficient, and
Uchida et al.
3284 J. Phys. Chem., Vol. 99, No. 10, 1995
2 . P
0.8
0.6
(a)
2.v)
0:001
0.01
0.1
1
8
I
I
2 10-37 5 0, cn
m' 2
.c
10-4
uL 10-2
.0.81 > P
.-m 5 4-
Figure 4. Plots of exchange current density jo for Pt anode against
0.6 0.4
Ef 0.2
9
n
OlbOl
Conductivity, r~ ,on / S cm-'
A I
c
O
10-1
0.01
0.1
1
ionic conductivity qonof zirconia solid electrolyte measured in wet hydrogen (P[H20] = 0.042 atm). Solid line is the least-squares fitting for all the data. Zirconia electrolyte: 0 e, 8Yb; 0 0 0 , 8Y; A A A,4 Y; 0 0 W, 3Y. Temperature: open symbols, 1000 "C; semisolid symbols, 900 "C; solid symbols, 800 "C.
Current density,j / A cm-2 Figure 2. Polarization curves (IR free) of Pt anode attached to various zirconia solid electrolytes at (a) 1O00, (b) 900, and (c) 800 "C measured in wet hydrogen (P[H20] = 0.042 atm). Zirconia electrolyte: 0, 8Yb; 0,8Y; A, 4 Y; 0 , 3Y.
N
I
I
10-2
I
1
10-1
Conductivity, o ion / S cm-' Figure 5. Plots of nu, obtained from the Tafel slope for Pt anode
against ionic conductivity ulOnof zirconia solid electrolyte measured in wet hydrogen (P[H2O] = 0.042 atm). Symbols are same as in Figure 4.
Figure 3. Arrhenius plots of anodic current densities at = 0.1 V for Pt anodes. Conditions and symbols are same as in Figure 2.
number of electrons transferred, respectively. F, R, and T have their usual meanings. Since linear relationships are seen between log j and 11 ranged from 0.1 to 0.3 V, each jo is obtained by extrapolating the plot to 11 = 0. At a given temperature, all the Tafel lines with the various electrolytes have approximately the same slope, i.e., 125 mV/decade at 1000 "C, 118 mV/decade at 900 "C, and 104 mV/decade at 800 "C, respectively. The Tafel slope at 1000 "C is close to those reported for pt3' or Nils anode in a hydrogen atmosphere. The value of na, was calculated from the Tafel slope, and the parameters log j o and na, are plotted as a function of logarithm of the ionic conductivity, aion,of the electrolyte disks in Figures 4 and 5, respectively. It must be emphasized that these plots correspond to the data obtained at different temperatures and different electrolytes. In Figure 4, the solid line represents the leastsquares fitting for all the data. The correlation factor of the line was 0.95, and the slope was 1.97. Therefore, it becomes clear that the exchange current density on pt anode increases
linearly with the square of the ionic conductivity for all of the zirconia electrolytes. At 800- 1000 "C, the polarization of the Pt anode was not affected simply by the operating temperature itself. On the other hand, the value of na, was approximately 2 and independent of the conductivity or the temperature, as shown in Figure 5, suggesting that the rate-determining step is independent of the experimental conditions in our work. Mechanism of the Anodic Reaction at WZirconia Interface. Let us consider the reaction mechanism for the anodic oxidation of hydrogen at the PtJzirconia interface based on our experimental results, i.e.
na, = 2
(3) Assuming that the transfer coefficient a, is 0.5, the number of electrons transferred during the overall anodic reaction is four. The reaction can be expressed by the following equation: 202-(electrolyte)
+ 2H2(g)
-
2H20(g)
+ 4e-(electrode) (4)
Polarization Properties of Pt Anodes
J. Phys. Chem., Vol. 99,No. 10,1995 3285
On the other hand, the ionic conductivity is expressed as
oion = 2FpiV(02-)
(5)
where p is the ionic mobility of oxide ions [cm2 V-‘ s-l 1 and N ( 0 2 - ) is the concentration of oxide ions in the electrolyte [mol ~ m - ~ ] The . mobility ,M is defined as a velocity of ionic movement, v[cm s-I], per unit electric field, U[V cm-I]:
p = v/u
(8)
(Sion
Next, we consider the following elementary reaction steps for the overall anodic reaction eq 4: (9
2[02-
-
oad
+ 2e-1 electron transfer at phase boundary
(ii)
(iii)
Oad + Oad
O,(g)
-
O2(g)
+ 2H,(g)
recombination of Oad
-
0.1
1
0.01
0.1
1
(b)
(7)
where [02-]sud, the product of v and N(02-), represents the supplying rate of oxide ions to unit cross-sectional area with the dimension of mol cm-2 s-l. Since the thickness of the electrolyte was kept constant at 0.1 cm for all experiments, the electric field U is constant. Hence, we obtain the following relation: OC
0.01
0.4
= (~F/v>[o~-I~,,
[O2-ls,,
0.1 0 0.001
(6)
Then, eq 5 may be rewritten as oion
0.3 0.2
2H,O(g) formation of water vapor
In steps i and ii, Oad means an oxygen atom adsorbed on Pt surface. If the electron-transfer step of (i) is fast enough compared to the following steps and in the quasi-equilibrium, the concentration of Oad generated, [Oad], is related to the [O2-Isurf:
0:001
0.6 0.5
0:001
The substitution of eqs 9 and 8 into eq 10 gives
The experimental results expressed by eqs 2 and 3 strongly indicate that the step ii is rate determining at the Pt anode and that the rate is controlled by the supply rate of oxide ions to the Pt anode/zirconia interface, as expressed by eq 11. It is also reasonable to consider that the step iii may be fast enough due to the presence of sufficient Pt surface area for the catalytic reaction of 02(g) with H2(g). To clarify the effect of the conductivity ai,, on the polarization of the Pt anode, another experiment was carried out. If eq 11 is valid for the present system, the kinetics at the Pt anode must not be affected by changing the thicknesses of the
1
0.01 0.1 Current density, j / A cm-‘
Figure 6. Polarization curves (IR free) of Pt anode attached to 8Yzirconia solid electrolytes with different thicknesses measured at (a) 1000, (b) 900, and (c) 800 “C in wet hydrogen (P[H20] = 0.042 atm). Thickness of 8Y-electrolyte: 1.5 mm; 0, 1.0 mm; V, 0.5 mm.
+,
electrolytes because a,,, is same. Figure 6 shows plots of the IR-free overpotentials against logarithm of current densities for Pt anodes on 8Y-electrolytes with different thicknesses of 0.5, 1.0, and 1.5 mm, where qonis the same but the conductances are different. The anodic polarization curves show fairly good agreement with each other at a given temperature. This supports that the anodic polarization is controlled by the [O2-Isufiand not by the thickness of the electrolytes after the pure ohmic drop of the electrolyte is corrected. On the basis of the Tafel slope experimentally obtained, it is possible to analyze the above multistep reaction.32 For the anodic charge transfer
+
Z = (ya/u) r - rp
(12)
+
5. = (y,/v) rp 5 6.= n/v
+
(9) where K is the equilibrium constant. If the step ii is the ratedetermining step (rds), the reaction rate, r, with the rate constant k can be expressed as
(c)
(13)
(14)
where ya and yc are, respectively, the number of electrons transferred before and after the rate-determining step (rds), r is the number of electrons transferred in the rds, 8 , is the symmetry factor (usually taken as ‘h),u is the number of times the rds must repeat itself in order to complete the overall reaction, and n is the total number of electrons transferred. Since naa determined above corresponds to 8,we can obtain two reasonable solutions for eqs 12-14. The first solution is 8 = 2, ’a = 0, ya = 4, yc = r = 0, and Y = 2, i.e., four electrons transferred before the rds and none in afterward steps including the rds: 2[02-
-
oad
+ 2e-1
(15, fast)
following reactions with 2 H,(g) The pair of (Oad
+ Oad)
stands for two
oad
(17, fast) adsorbed on
Uchida et al.
3286 J. Phys. Chem., Vol. 99, No. IO, 1995 neighboring Pt sites. This solution is essentially the same as the above discussion, where step ii is the rds. However, a surface diffusion of Oad atoms on Pt into neighboring sites preferable for the following recombination step (16') is suggested as a slow step. The second one is Zi = 'I= 2, y a = yc = 1, r = 2, and Y = 1, Le., one electron transferred before and after the rds and two electrons during the rds:
-
+ e0- + 0,- o,- + 2 e0,- - O,(g) + e-
(20, fast)
following reactions with 2H2(g)
(21, fast)
02- 0--.
(18, fast) (19, rds)
This model assumes partly discharged species of 0- and 0 2 at the reaction zone. At present, we cannot decide exactly which solution is more probable since we did not measure the cathodic polarization properties to determine 'I or cl, in hydrogen atmosphere. Anyhow, both solutions for kinetic parameters in the anodic reaction are consistent with the dependence of j o on Oionz. It is, at least, clearly indicated that a quasi-equilibrium is established between oxide ions and the reaction intermediates, Le., Oad or 0-, and that the recombination of the intermediates at the Pt electrode/electrolyte interface is the rate-determining step. Mizusaki et al. reported that the interfacial conductance (UE) at R electrode/zirconia electrolyte is determined by the length of the triple-phase b0~ndary.I~Using complicated equations to analyze the dependence UE on P[O2] ranged from to 1 atm, they concluded that the surface diffusion of o a d on the Pt surface is the rds.I6 This may agree with the first solution, eq 16, of the above analyses, but our simple analyses give more direct evidence. Setoguchi et aLZ2claimed that the OE of various metal anodes in H2 can be related to the heat of oxide formation of the metal M considering the anodic reaction as the following redox cycle:
+ 02--MO + 2eMO + H,- M + H,O
M
Although the rds is not exactly specified in their this cycle may also correspond to the reaction schemes of the first solution mentioned above. The present study is the first to demonstrate that the ionic conductivity of solid oxide electrolytes plays an important role in controlling the overpotential at Pt anodes in HZat high temperature. More recently, Watanabe et al.33had demonstrated a similar effect of proton conductance in a polymer electrolyte film (Nafion) on the cathode performance in polymer electrolyte fuel cells (PEFCs), where protons are discharged to form water at the cathode. The dominant effect of electrolytic conductivity on the polarization properties probably appears when the transport of ions to the electrolyte1 electrode interface, Le., the anode in SOFC and the cathode in PEFC, controls the rates of the electrode reactions. This information is very important in the design of high-performance fuel cells, i.e., effect of the electrolyte lowering not only the internal ohmic resistance but also the overpotentials of electrode reactions.
Conclusion The present study reveals that the high ai,, in the oxide electrolyte reduces not only the ohmic loss, which is only a
linear function of ai,,, but also the polarization loss especially at the anode due to an increase in j o with aion2 at 800-1000 "C. At an operating temperature of 800 "C, the polarization at the Pt cathode in oxygen is also influenced by the aion.Thus, the use of the solid electrolyte with high ionic conductivity is essential to improve the performance of SOFC. Another solution to reduce the electrode polarization is the use of mixed ionic conducting anode^^,^^ such as doped ceria, with ai, higher than the conventional electrolyte, because the surface concentration of oxide ions, necessary to the electrode reaction, is high over the greatly enhanced effective reaction zone, brought by the mixed conduction. Although partially stabilized zirconia electrolyte (3Y-PSZ) was mainly empolyed in recent planartype S O F C S due ~ ~ to its mechanical strength, it is probable that the electrode performance is limited by the low ionic conductivity. From both electrochemical and practical view points, it is interesting to clarify effects of ui,, on the polarization of various electrodes such as the state-of-the-art Ni-cermet anode, oxide cathode, or our catalyzed-reaction layer,5 whose performances may have a different dependence on ai,, from that found for the Pt anode. Such studies are in progress.
Acknowledgment. This work was partially supported by a Grant-in-Aid No. 04203113 (Priority Area Research) and No. 06650942 (Scientific Research C) from the Ministry of Education, Science and Culture, Japan. A part of this work was presented at the Spring Meeting of the Electrochemical Society of Japan, April 3-5, 1994. References and Notes (1) Kudo, I.; Obayashi, H. J . Electrochem. Soc. 1976, 123, 415. (2) Takahashi, T.; Iwahara, H. Energy Conversion 1971, 11, 359. (3) Yahiro, H.; Eguchi, Y.; Eguchi, K.; Arai, H. J . Appl. Electrochem. 1988, 18, 527. (4) Ishihara, T.; Matsuda, H.; Takita, Y. J . Am. Chem. Soc. 1994,116, 3801. (5) Watanabe, M.; Uchida, H.; Shibata, M.; Mochizuki, N.; Amikura, K. J. Electrochem. Soc. 1994, 141, 342. (6) Saeki, M. J.; Uchida, H.; Watanabe, M. Catal. Lett. 1994,26, 149. (7) Fabry, P.; Kleitz, M. J . Electroanal. Chem. 1974, 57, 165. (8) Giir, T. M.; Raistrick, J. D.; Huggins, R. A. J . Electrochem. Soc. 1980, 127, 2620. (9) Sasaki, J.; Mizusaki, J.; Yamauchi, S.; Fueki, K. Bull. Chem. Soc. Jpn. 1981, 54, 1688. (10) Badwal, S. P. S.; Bruin, H. J. J. Electrochem. Soc. 1982,129, 1921. (1 1) Verkerk, M. J.; Hammink, M. W. J.; Burggraaf, A. J. J. Electrochem. Soc. 1983, 130, 70. (12) Okamoto, H.; Kawamura, G.; Kudo, T. Electrochim. Acta 1983, 28, 379. (13) Nguyen, B. C.; Liu, T. A.; Mason, D. M. J . Electrochem. Soc. 1986, 133, 1807. (14) Shouler, E. J. L.; Kleitz, M. J . Electrochem. Soc. 1987, 134, 1045. (15) Mizusaki, J.; Amano, K.; Yamauchi, S.; Fueki, K. Nippon Kagaku Kaishi 1985, 1160. (16) Mizusala, J.; Amano, K.; Yamauchi, S.; Fueki, K. Solid State Ionics 1987, 22, 313; 1987, 22, 323. (17) Robertson, N. L.; Michaels, J. N. J . Electrochem. SOC. 1990, 137, 129. (18) Kawada, T.; Sakai, N.; Yokokawa, H.; Dokiya, M.; Mori, M.; Iwata, T. J . Electrochem. Soc. 1990, 137, 3042. (19) Nagata, M.; Iwahara, H. J . Appl. Electrochem. 1993, 23, 275. (20) Badwal, S. P. S.; Bannister, M. J.; Murray, M. J. Solid State Ionics 1984, 168, 363. (21) Liu, M.; Khandkar, A. Solid State lonics 1992, 52, 3. (22) Setoguchi, T.; Okamoto, K.; Eguchi, K.; Arai, H. J. Electrochem. Soc. 1992, 139, 2875. (23) Yamamoto, 0.;Takeda, Y.; Kanno, R.; Noda, M. SolidState Ionics 1987,22, 24 1. Takeda, Y.; Kanno, R.; Noda, M.; Tomida, Y .; Yamamoto, 0. J . Electrochem. SOC. 1987, 134, 2656. (24) Hamouche, A.; Siebert, E.; Hammou, A.; Kleitz, M.; Caneiro A. J . Electrochem. Soc. 1991, 138, 1212. (25) Mizusaki, J.; Tagawa, H.; Tsuneyoshi, K.; Sawata, A. J . Electrochem. Soc. 1991, 138, 1867.
J. Phys. Chem., Vol. 99, No. 10, 1995 3287
Polarization Properties of Pt Anodes (26) Takahashi, T.; Iwahara, H.; Ito, I. Denki Kagaku 1970, 38, 509. (27) Van Duk, M. P.; De Vries, K. J.; Burggraaf, A. J. Solid State Ionics 1986, 21, 73.
( 2 8 ) Takahashi, T.; Ito, K.; Iwahara, H. Electrochim. Acta 1967, 12, 21. (29) Strickler, D. W.; Carlson, W. G. J . Am. Ceram. Soc. 1964, 47, 122.
(30) Applications ofsolid Electrolytes;Takahashi, H.; Kozawa, A., Eds.; JEC Press: Tokyo, 1990, and references therein.
(31) Jang, S. P.; Badwal, S . P. S. Spring Meet. The Electrochem. SOC., San Francisco, 1994; Extended Abstract, 1003. (32) Bockris, J. 0. M.; Reddy, A. K. N.; Modern Electrochemistry; 1973; ‘O1. 2i Chapter 9. press: New (33) Watanabe, M.; Sakairi, K.; Inoue, M. J . Electroanal. Chem. 1994, 375, 415. (34) Proc. 3rd Int. Meet. Solid Oxide Fuel Cells. Singal, S. C., Iwahara, H., Eds.; 1993; Chap. 7, “Cell and System Development.” Jp941845Y
