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Oxygen ion transport and electrode properties of La(Sr)MnO3
.__ __ k!!B
CBS
SOLID STATE
ELSEVIER
IONICS
Solid State Ionics 8 1 (1995) 201-209
Oxygen ion transport and electrode properties of La(Sr)MnO, V.V. Kharton, A.V. Nikolaev, E.N. Naumovich,
A.A. Vecher
Institute of Physico-Chemical Problems, Belarus State University, 14 Leningrad&y Sfr., 220080, Minsk, Belarus
Received 31 March 1994; accepted for publication
13 June 1995
Abstract
Cathodic polarization of La,Sr,MnO, (n = 0.3-0.6; y = 0.3-0.5) electrodes, doped by the sintering agents Bi,O,, PbO, Sb,O,, CuO, or Bi,CuO,, on a zrrconia solid electrolyte has been studied. The electrochemical activity of Laa,,Sra,,MnO, electrodes with sintering agent Bi,CuO, was found to be the highest among the solid solutions based on La(Sr)MnO,. The positive effect of the bismuth cuprate addition is ascribed to the sufficiently high mixed ionic and electronic conductivity of Bi,CuO,. The obtained results are characterized by the absence of a direct correlation between the electrochemical parameters of the electrode layers and oxygen permeability of the manganite ceramics. It was found that oxygen transport through the manganite ceramics is limited by the sorption on ceramics surface. Keywords: Manganites;
Oxygen
permeability;
Electrode
materials;
Cathodic
1. Introduction
Perovskite-type oxides derived from LaMnO, by partial substitution of La3+ by strontium ions are promising materials for electrodes of high-temperature solid electrolyte fuel cells, electrochemical oxygen pumps, sensors and catalysts [l-5]. Electrical and electrochemical properties of the manganites depend on the concentration of cation vacancies on the dodecahedral La-sites (sublattice A of the perovskite crystal structure ABO,) [l]. Along with the other factors, cation vacancies affect the electronic state of the Mn ions [3], which defines the structure, electrical conductivity, and other properties of manganites. In the present work, we have investigated defect perovskite-type manganites containing cation vacancies. The general formula of these materials is where c] is a cation vaLa,Sr,. 0 l_x-vMnO,_,, cancy on the A-site and S is the oxygen nonstoichiometry. The data on ceramics, in which the oxy0167-2738/95/$09.50 0 1995 Elsevier Science B.V. AI1 rights reserved SSDf 0167-2738(95)00179-4
polarization
gen content is as close as possible to the equilibrium with the gas phase at the temperature of experiment are presented in this paper. Some results of our studies of lanthanum-strontium manganites were published previously [4]. Ionic transport of oxygen in electrode materials leads to an increase of the effective electrode reaction zone [ 1,2]. In order to determine whether the mixed oxide ion and electron conduction of the electrode oxides affects the electrochemical properties of the electrode, oxygen semipermeability of solid solutions La,Sr,MnO, was determined at 960 to 1200 K. The characteristic feature of the perovskite-type oxides containing lanthanum and strontium ions is their interaction with zirconia-based solid electrolytes, which is accompanied by a sharp increase in the resistance of the oxide electrodes [2,6]. This solid state reaction is of considerable importance at temperatures above 1500 K [6]. To prepare manganite
V. V. Kharton et al. /Solid
202
electrodes, we used sintering of a paste with different sintering additives at 1470 rt 10 K for two hours. These conditions of the electrode production minimized the formation of barrier layers. Only the electrodes, obtained by this method, were investigated in the present paper, whereas use of other methods of electrode preparation (for example, the screen printing technique) makes it possible to reduce the electrode resistance. However, the method used here is the most simple and the most available. The other purpose of our work was to choose the optimal electrode and sintering agent materials for application in high-temperature fuel cells.
2. Experimental 2.1. Preparation
and investigation
of the ceramics
Synthesis of La,Sr,.MnO, (x = 0.3-0.6; y = 0.3-0.5) polycrystalline samples has been carried out by the standard ceramic technology from corresponding oxides at temperatures between 1570 and 1620 K for 15-30 h in air with repeated powder milling. The total impurities content in the starting materials was below 0.05 at%. The X-ray diffraction patterns were obtained by the DRON-3M diffractometer using Cu K a-radiation (Ni-filter). The error in the lattice parameters was *O.OOOl nm. Specimens were annealed at 1370 K in air and then cooled at a rate below 100 K/h with the aim to obtain the equilibrium oxygen content before the X-ray diffraction studies. Quantitative analysis of cations was carried out by X-ray fluorescence analysis (XFA) on a “Microscan-5” and a scanning electron microscope
Table I Properties
of La,Sr,.MnO,
State Ionics 81 (1995) 201-209
“Nanolab-7” with a micro X-ray fluorescence spectrometer SR-860-2. The data were processed using the ZAF-4/FLS program. Ceramic samples were compacted from the synthesized powders under pressures of 300-600 MPa. Samples had the shape of bars 4 X 4 X 30 mm3, or tablets (15 or 28 mm in diameter, 3-4 mm in thickness). The ceramic samples were annealed at 1620-1770 K in air during 20-35 h. The sample density (d,,,) was 80-95% of the theoretical value (Table 1). The obtained ceramics were tested for gas tightness (absence of open porosity) using a solution of “Sudan” dye-stuff and by the helium-vacuum method with a leak-detector GTI-3. A quartz dilatometer DKV-5A was used to investigate the thermal expansion of the ceramics. The electrical conductivity was measured by the four-probe dc method in air at temperatures of 300 to 1100 K. The relative error of the conductivity measurements did not exceed 3%. The distinctions between the data, obtained during the heating and cooling, from dilatometry and conductivity measurements, did not exceed 2%. Consequently, the measurements were conducted under conditions close to equilibrium. The methods, used to investigate the crystal structure, thermal expansion, and electrical conductivity are described in detail in Ref. [7]. 2.2. Oxygen permeability
investigations
Measurements of oxygen permeability were carried out with the help of the electrochemical solid electrolyte cell equipped with an oxygen pump and a sensor (Fig. 1). The oxygen permeability was deter-
ceramics
x
Y
Structure
a (nm) +O.OOOl nm
CY(“1
dexp (kg/m31
Z ‘XIOh(K-‘) 300-11OOK
0.6
0.4 0.5 0.4 0.3 0.5
Rb CC R C C
0.5458 0.3850 0.5452 0.3858 0.3829
60.28
5440 5260 5690 4960 5570
11.31 12.07 11.30 12.25 11.89
0.5 0.5 0.5 0.3
’ E = mean values of the thermal expansion coefficients. h R = perovskite structure having rhombohedric distortion. ’ C = cubic perovskite structure.
60.16
Iko.03 f 0.07 A 0.04 + 0.05 * 0.09
V. V. Kharton et al. /Solid State Ionics 81 (I 995) 201-206
4-
Fig. 1. Electrochemical cell for the oxygen permeability measurements: 1 - zirconia solid electrolyte; 2 - electrodes of the oxygen sensor; 3 - electrodes for the oxygen pump; 4 - manganite sample; 5 - high temperature glass.
mined at the condition of equality of the oxygen flows, being removed from the cell by the pump and entering it through the hermetically attached sample by the ionic transport. Specially designed adhesives were used to attach the ceramics. The oxygen permeability has been studied at temperatures from 960 to 1200 K with a difference in oxygen partial pressures between 2 X lo4 and 1 X lo3 Pa in the internal and external spaces of the measuring cell. More than 5 values of the flow through the sample, corresponding to different values of oxygen chemical potential gradient, were determined isothermally during each measurement. The time of attainment of the steadystate in the cell was 1 to 6 hours. The reproducibility dispersion of the oxygen permeability values was within 5% of the measured value. The method of investigation of the oxygen permeability is described in detail in Ref. [7]. 2.3. Preparation
and study of the electrodes
In the present work, the electrodes were prepared using pastes containing the manganite powder, sintering agent (2-6 wt%), and an organic binder (1 wt%). The manganite powder used for the electrode preparation had particle sizes smaller than lo-15 pm. Bi,O,, PbO, Sb,O,, CuO and Bi,CuO, were used as sintering agents. The minimal quantity of sintering agent, sufficient to provide the proper mechanical strength of electrodes, was added to the paste.
203
The powder of Bi,CuO, was synthesized by standard ceramic techniques from the bismuth and copper oxides at temperatures varying from 870-1000 K for 25 h in air. Bi,CuO, was identified by means of X-ray diffraction studies and XFA. Oxygen ion transference numbers of bismuth cuprate were determined by the EMF method using an electrochemical cell with separated gas compartments. A 2-probe ac method (1, 5, 10 and 50 kHz) was used for the measurements of the conductivity of Bi,CuO, in air. The electrode layers, obtained by applying the pastes on Zr,,,,Y, tOO,,s, ceramics, were annealed at 1470 + 10 K in ‘air during two hours. The layer density was 100 + 10 mg/cm2. The used technology provided the maximum longitudinal conductivity of the electrode layers. The overpotential values 77 were determined using the formula: v=U-IR,
(1) where U is the total potential drop across the electrodes, and ZR the potential drop across the solid electrolyte. The R values were measured by the two methods: (1) using an alternating current (20 kHz); (2) by the oscilloscopic method. The ac frequency was chosen on the basis of the results arising from the investigations of the frequency dependence of the electrochemical cell resistance. The procedure of the ac overpotential measurements has been described in Ref. [7]. The overpotential of the manganite electrodes was determined under steady cathodic polarization in air at 1000-1300 K. The time of the steady-state attainment in the electrochemical cell was 0.2-l h. The 71 values were in the range of - 10 to - 800 mV. We shall adduce the absolute values of the overpotential in the discussion. The reproducibility of the overpotential values was within 10% of the measured values for the different batches of electrodes having the same composition.
3. Results and discussion 3.1. Properties
of the ceramics
X-ray diffraction studies of the La,Sr?MnO, samples showed the formation of perovskite-like solid
V.V. Kharton et al. /Solid State Ionics 81 (199.5) 201-209
204
0.3 7
1 12
I 17
/ 22
27
I 12
1 32
10*/T, It-'Fig. 2. Temperature dependence of the conductivity of La,Sr,.MnO, ceramics: 1 - (x = 0.5, y = 0.5); 2 - (X = 0.5, y = 0.4); 3 - (x = 0.5, y = 0.3); 4 - (x = 0.3, y = 0.5); 5 - (x = 0.6, y = 0.4).
solutions. The crystal symmetry of La,,,Sr,,MnO,, La,,Sr,,,MnO, and La,,Sr,,,MnO, was identified as pseudocubic. The solid solutions La,,Sr,,MnO, and La,,Sr,,MnO, had the rhombohedric perovskite-type structure. All materials were singlephased, with one exception. X-ray diffraction studies demonstrated the presence of impurities (manganese oxides) in La,,Sr,,MnO,. Crystal lattice parameters of manganites (a and a> are given in Table 1. The mA results showed that the deviation of the cationic composition for each sublattice of the perovskite structure from the nominal composition did not exceed 3%. The overall impurities concentration was below 0.4 at%. The temperature dependencies of the conductivity of La(Sr)MnO, (Fig. 2) are in agreement with literature data [8]. The conductivity increases with substitution of La3+ by strontium ions. These results may be ascribed to the increase of the Mn4+ concentration [8]. The maximum conductivity was found for La,,Sr,,,MnO, and La0,3Sr,,5Mn03 ceramics. Ceramic samples of La0,,Sr,,4Mn03 containing Bi,O,, CuO or Bi,CuO, were prepared for studying the effect of sintering agents on the electrical properties of the manganite. These ceramics were annealed at 1470 + 20 K in air during 2-5 h. XFA showed
1 17
,
I
I
22
27
32
104/T, It-'Fig. 3. Temperature dependence of the conductivity of La,, ST~,~MnO, ceramics containing 2, 4 and 6 wt% of Bi,CuO, (curves 1, 2 and 3, respectively) or 3 wt% of CuO (4) before annealing.
that copper oxide evaporates substantially less than bismuth oxide during annealing. For example, the atomic ratio Cu : Bi for a sample containing initially 2 wt% Bi,O, and 2 wt% CuO were: 2.93 (before annealing), 3.5 (after two hours of annealing), 8.2 (after five hours of annealing). X-ray diffraction studies of samples with the sintering agents revealed the presence of perovskite phase only. Addition of sintering agents to the sample composition leads to an increase in the electrical conductivity as shown in Fig. 3. A fact which is well explained by the increase of the density of the ceramics (Table 2) and the implantation of copper ions into the sublattice B of the perovskite structure. The life service test of the samples containing the sintering agents revealed an increase of l-2% in the conductivity during annealing at 1170 K for 2.50 h.
Table 2 Properties of La,,Sro,,Mn03 ceramics annealed at 1470+20 in air during two hours with addition of h wt% of Bi,CuO, h
a(nm) kO.001 nm
a (“1
dexp (kg/m31
E X IO6 (K- ‘) 5OO- 1100K
2 4 6
0.546 0.547 0.546
60.29 60.21 60.16
5670 5810 5930
11.35*0.03 10.64 * 0.09 10.08 * 0.05
K
V. V. Kharton et al. / Solid State Ionics 81 (I 995) 201-209
205
This effect is caused by the increase of the density of the ceramics which is due to the additional sintering agents. It was found that the conductivity of the ceramics without sintering agents does not vary during 100-200 h of annealing. 3.2. Oxygen permeability
of the manganite
ceramics
The results of the oxygen permeability investigations are given in Figs. 4 and 5. The oxygen permeability values were calculated from the formula [9]: Jo2 = (RTCf/16SP)
. (Z/E),
(2)
where Jo, is the oxygen permeability (mol . s- ’ cm-‘), I is the current through the oxygen pump of the electrochemical cell (Fig. l), E is the EMF of the oxygen sensor, d, the sample thickness, and S the effective surface area of the manganite ceramics. The oxygen partial pressure dependencies of Jo, are non-linear and have the typical shape, illustrated by Fig. 4. This suggests that equation [9] j, = 2. JoZd-’
ln( P,/P,),
-11.5
+ 8.0
8.5
9.0
9.5
10,o
10,5
11.0
104/T,It-'v Fig. 5. Temperature dependence of the oxygen permeability of La,Sr,Mn03 ceramics at E = 50 mV: 1 - (X = 0.6, y = 0.4); 2 - (X = 0.5, y = 0.5); 3,4 - (x = 0.5, y = 0.4); 5 - (X = 0.5, y = 0.3); 4 - ceramic with platinum layers.
(3)
where j, is the oxygen ion flux density (mol. SC’ cmT2>, P, and P2 the oxygen partial pressures
c
outside and inside the cell (atm) respectively, can be used only for certain values of the oxygen partial pressure. Regression analysis of the experimental data on the permeability was used to ascertain the limiting stage of the oxygen transport through the manganites. The following regression models were the most adequate: jo(p2)
=ki
.ln(WW
jo(P*)=Q(P;n-P?)
+& +k,,
jo(P2)=kl*(Py2-Py2)+k2, jo(P2)
(4) (5) (6)
=k,-(k2/2).ln(P2)+(k,/4).ln2(P2) (7)
/
I
I
40 E, mV
60
I
I
80
I
1d0
-
Fig. 4. Dependence of oxygen permeability of La,.%,,MnO, ceramics on the EMF of the oxygen sensor at 1170 + 2 K: 1 - (x = 0.6, y = 0.4); 2 - (n = 0.5, y = 0.5); 3,4 - (x = 0.5, y = 0.4); 5 - (x = 0.5, y = 0.3); 4 - ceramics with platinum layers.
where k, and m are the regression parameters and P, is kept constant. Model (4) corresponds to Eq. (3). Models (5) and (6) can be obtained on the assumptions that the ion flow is proportional to the 02- concentration gradient and that the 02- concentration in the ceramic surface layers (cr and c2) depends linearly on P;” and ‘PT, respectively (Freindlich’s adsorption isotherm). For model (6) m equals l/2.
206
V. V. Kharton et al. / Solid State Ionics 81 (I 995) 201-209
Model (7) can be obtained on the following assumptions: The concentration of oxygen ions in the surface layers of the manganite can be expressed in terms of Temkin’s adsorption isotherm [lo]: C[ = K,
ln( Pi) + K,.
~c is the oxygen ion conductivity, p0 the chemical potential of oxygen ions, p, and p2 the values of ~a in the surface layers, v. the mean frequency of oxygen ions vibrations, r the distance of the elementary ion translation, iJ,, the migration energy, N the concentration of the oxygen ion sites, and c the concentration of oxygen ions. Model (7) can be derived by integration of E%J.(9) and substitution of formula (8) on the condition that
(8)
Since the electronic conductivity of the manganites is much higher than the oxygen ion conductivity ( C~x=-aa), the oxygen ion flux density can be given by the following [l I]: j, = (4F2d)-1
(%a
k, =K,K,.[(N-2KJ
dpu,
‘112
= KZflc(
N - c) d[ln~],
(9)
=K,K,N-2K,K,K,,
(k,/4)
=2KfK,.
The oxygen oxygen square
E2
where K, = vor2( Nd)-’
(k,/2)
exp( - U,/RT),
ln(P,)
-2K,
*ln”(P,)],
regression analysis of the dependence of the flow through the manganite samples on the partial pressure was carried out by the least method using BFGS-formula for minimiza-
Table 3 Parameters of the regression models of the oxygen flux density through the manganites of the oxygen partial pressure dependences Sample
L4”
Model 980
II00
1170
W”
980
Coefficients (mol/(s * cm))
6a
Sb (%I
PC
k, = t2.0 * 0.1) x 10-s kl=(-1.7f0.2)X10-8 k, = (2.1 -+ 0.2) x 10-7
5.7 x lo- HI
3.1
0.989
6.7 x IO- I0
4.0
0.987
4.2 X IO- I0
1.4
0.998
5.8 X lo- I0
2.0
0.996
3.9 x lo- ‘O
1.3
0.999
3.4 x lo- ‘0
1.3
0.998
3.9 x 10-a
13.8
0.94
6.3 X IO-”
2.1
0.997
7.6 x lo- ”
4.4
0.998
k, = (-6.5 rf:0.8) x lo+ k, =(-8.1~0.8)x 10-8 k, = (6.1 f 0.1) x 1O-8 k,=(-1.0+0.1)x lO-s k, =(1.8rtO.l)xlO-’ k, = (-3.8 f 0.3) x lo-* k, =(-9.2&0.8)x lo-* k, = (8.8 + 0.1) x lo-* k, = (- 1.6 k 0.3) x lo+ k, =(-1.6&0.2)x 1O-7 k2=(1.8f0.2)x10-7 k, = (-4.5 % 0.4) x IO-’ k, = (1.7 f 0.5) x 10-S k,=(-3.2f2)XlO-’ k, =(2.1 zb0.7) x 1O-7 m = 0.56 f 0.09 k, =(-4.3,0.3)X
IO-’
k,=~1.2+0.5)~10-’ k,=(--2.1 *00.1)x IO-* kq =fl.l + 0.2) x lo-* a 6 = adequacy dispersion. ’ s = relative error of the model. E p = correlation coefficient. ’ L4 = LaO,,Sro,.,MnO,. ’ L5 = La,,Sr,~,MnO,.
V.V. Kharton et al. /Solid State Ionics 81 (1995) 201-209
tion of the non-linear adequacy dispersion model [12]. Statistical parameters of the non-linear models were calculated using the method proposed in Ref. [ 131. A special program was designed for this purpose. In addition to models (4)-(71, the solutions of the sets of equations, which included Wagner’s law, and Langmuir’s and Freinlich’s adsorption isotherms, were used as transport models. Certain results of the regression analysis, according to models (4)-(7), are presented in Table 3. Model (7) was the most adequate among the used models. The obtained results suggest that oxygen transport through the lanthanum-manganite-based ceramics is limited by the sorption of oxygen on the ceramic surface. Application of platinum layers (5 mg/cm’) on both surfaces of the manganite ceramics, which was done to verify the premise that oxygen sorption is the limiting step of oxygen transport, leads to a considerable increase in the oxygen permeability and to a change of the character of the dependence of Jo on the oxygen partial pressure. Most probably, so& tion hampers the oxygen transport less as the temperature increases; since at T > 1100 K the permeability of ceramic samples covered with platinum, approaches that of ceramic samples without platinum (Fig. 5). The temperature dependencies of the oxygen semipermeability at E = 50 mV are presented in Fig. 5. The maximum Jo, at T > 1000 K was found for La,,Sr,,MnO, ceramics. Oxygen permeability values for the other investigated manganites are close to each other. The oxygen semipermeability of the manganites was found to be 100-1000 times lower than that of the lanthanum-strontium cobaltites [5,7].
207
___
400 -
300 -
I 2 E
200 -
F’
_ EE::
100 -
zi
lJ, , / , , , , / , 1 OO
40
80 j,
rnA.~rn-~
120
160
200
-
Fig. 6. Overpotential versus current density plots for La,SryMnO, electrodes with sintering agent Bi,O, (4 wt%) at 1260 + 8 K under stationary cathodic polarization in air: 1 - (x = 0.6, y = 0.4); 2 - (x = 0.5, y = 0.3); 3 - (x = 0.5, y = 0.4); 4 - (x = 0.3, y = 0.5).
These dependencies can be described adequately by exponential models such as Tafel’s equation or the slow discharge equation [2]. The attribute of the
3.3. Properties of the electrode layers According to the results of the life service tests of the manganite electrodes at 1170 to f3 K, the overpotential does not change with time during 70 h when the current density (j) is kept constant. The total potential drop across the electrodes (U) decreased by lo-25% during a given time interval due to additional sintering effects of the electrode layers and the corresponding increase in the electrode conductivity. The overpotential vs current density plots for the investigated electrodes are presented in Figs. 6-8.
Fig. 7. Overpotential versus current density plots for electrode Lao,,Sra,MnO, at 126Ok 8 K in air. Sintering agents: (wt%): I 2% of Bi,O, and 2% of CuO, 2 - 4% of CuO, 3 - 4% of PbO, 4 - 4% of Sb,O,.
208
V.V. Kharton et al. /Solid State Ionics 81 (1995) 201-209
obtained results is the absence of a direct correlation between the oxygen semipermeability of the ceramics and the electrochemical parameters of the electrode layers. The regularities of the change of the electrode properties, illustrated in Figs. 6-8, are observed for all temperatures in the range studied, i.e. 1000-1300 K in air. The polarizability of the La,,Sr,,MnO, electrode is lower than that of the electrodes based on the other solid solutions of La(Sr)MnO,. The lowest electrode resistance can be achieved when the electrode material LaO,,Sr,,dMnO, is used in combination with the sintering agent Bi,CuO,. The positive effect of addition of the bismuth cuprate is due, probably, to both the sufficiently high mixed oxygen ion and electronic conductivity of Bi,CuO,, and the decrease in the resistance of the manganite owing to the implantation of copper ions into the crystal lattice of the perovskite-type oxide. Some electric properties of synthesized Bi,CuO, samples are given in Table 4. The obtained results are in agreement with literature data [14]. Bi,CuO, has a sufficiently high mixed conductivity. The oxygen ion partial conductivity of bismuth cuprate increases with increasing temperature. The crystal
500,
1
Table 4 Properties
of Bi,CuO,
ceramics b
dexP (kg/m31
T (K)
K * (S/cm)
r”
6850
900 800 700
1.6x lo-* 9.0x 1o-3 5.4x 10-3
0.05 0.03
* K = specific conductivity of the samples. b to = ionic oxygen transference number.
structure of this complex oxide is tetragonal (the space group is P4/mmm). The Bi,CuO, lattice parameters at room temperature are: a = 0.8502 f 0.0001 nm, c = 0.5818 + 0.0001 nm. DTA results showed that the temperature of the congruent melting of Bi,CuO, is 1123 f 5 K. The mean values of the thermal expansion coefficient of this compound are 4.2 X 10m6 K-’ (400-800 K) and 10.1 X 10m6 K-’ (800-1000 K). The high mixed conductivity of Bi,CuO, offers, probably, good electrochemical properties of electrode layers with bismuth cuprate. Bismuth cuprate is pulverized by ceramic crushing in order to give powders which were used to study the effect of the preparation conditions on its properties as a sintering agent. The electrochemical parameters of the electrodes with powdered Bi,CuO, are significantly worse than those of the other electrodes (Fig. S), because of the distinctions between the specific surfaces of the sintering agents. Probably, the manganite La,,Sr,,dMnO, with added Bi,O, and CuO has also good electrode properties (Fig. 7) due to the formation of Bi,CuO,.
4. Conclusion
I 50
100 j,
I 150
I ZOO
I 250
300
mA.~rn-~ -
Fig. 8. Overpotential versus current density plots for electrodes La,,Sr,,,MnO, with sintering agent Bi,CuO, at 1260*3 K in air: 1 - 2 wt% of Bi,CuO,, 2,3 - 4 wt% of Bi,CuO,, 4 - 6 wt% of Bi,CuO,, 3 - powdered Bi,CuO, was used.
The oxygen semipermeability of the solid solutions La(Sr)MnO, was found to be limited by oxygen sorption on the manganite ceramic surface. Applying the platinum layers to the ceramic surfaces result in an increase of the oxygen permeability by a factor of 2 to 4. It was shown that introduction of a sintering agent Bi,CuO, improves the electrochemical parameters of the manganite electrodes markedly. The solid solution La,,Sr,,MnO, in combination with the sintering agent Bi,CuO, has good properties as an electrode material.
V. V. Kharton et al. /Solid State Ionics 81 (I 99.5)201-209
References [I] T. Takahashi
[2]
[3] [4] [5]
and H. Iwahara, Report of Special Project Research Under Grant in Aid of Scientific Research of The Ministry of Education, Science and Culture (Japan, 1980) p. 727. M.V. Perlllyev, A.K. Demin, B.L. Kuzin and AS. Lipilin, High Temperature Electrolysis of Gases (Science, Moscow, 1988) [in Russian]. B.C. Totield and W.R. Scott, J. Solid State Chem. 10 (1974) 183. A.V. Nikolaev, V.V. Kharton, E.N. Naumovich and A.A. Vecher, Proc. I Europ. SOFC Forum (Lucerne, 1994) p. 415. P.P. Shuk, A.A. Vecher, V.V. Kharton, L.A. Tichonova, H.-D. Wiemhoefer and W. Goepel, Sensors Actuators. B1516 (1993) 401.
209
[6] B.L. Kuzin and A.N. Vlasov, Elektrokhimya 20 (1984) 1636. [7] V.V. Kharton, E.N. Naumovich, P.P. Shuk, A.K. Demin and A.V. Nikolaev, Elektrokhimya 28 (1992) 1693. [8] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [9] H.-H. Moehius, Ext. Abstr., 37th Meeting ISE, Vol. 1 (Vilnius, 1986) p. 136. [lo] M.I. Temkin, J. Phys. Chem. 1.5 (1941) 296. [ll] V.N. Chebotin, The Chemical Diffusion in the Solid State (Science, Moscow, 1989) [in Russian]. [12] J.E. Dennis, Jr. and R.B. Schnabel, Numerical Methods for Unconstrained Optimisation and Nonlinear Equations (Prentice-Hall, New York, 1983). [13] N.R. Draper and H. Smith, Applied Regression Analysis (Wiley, New York, 1981). [14] A.F. Poluyan and A.G. Gusakov, Ion Melts Solid Electrol. 2 (1987) 78 [in Russian].
CBS
SOLID STATE
ELSEVIER
IONICS
Solid State Ionics 8 1 (1995) 201-209
Oxygen ion transport and electrode properties of La(Sr)MnO, V.V. Kharton, A.V. Nikolaev, E.N. Naumovich,
A.A. Vecher
Institute of Physico-Chemical Problems, Belarus State University, 14 Leningrad&y Sfr., 220080, Minsk, Belarus
Received 31 March 1994; accepted for publication
13 June 1995
Abstract
Cathodic polarization of La,Sr,MnO, (n = 0.3-0.6; y = 0.3-0.5) electrodes, doped by the sintering agents Bi,O,, PbO, Sb,O,, CuO, or Bi,CuO,, on a zrrconia solid electrolyte has been studied. The electrochemical activity of Laa,,Sra,,MnO, electrodes with sintering agent Bi,CuO, was found to be the highest among the solid solutions based on La(Sr)MnO,. The positive effect of the bismuth cuprate addition is ascribed to the sufficiently high mixed ionic and electronic conductivity of Bi,CuO,. The obtained results are characterized by the absence of a direct correlation between the electrochemical parameters of the electrode layers and oxygen permeability of the manganite ceramics. It was found that oxygen transport through the manganite ceramics is limited by the sorption on ceramics surface. Keywords: Manganites;
Oxygen
permeability;
Electrode
materials;
Cathodic
1. Introduction
Perovskite-type oxides derived from LaMnO, by partial substitution of La3+ by strontium ions are promising materials for electrodes of high-temperature solid electrolyte fuel cells, electrochemical oxygen pumps, sensors and catalysts [l-5]. Electrical and electrochemical properties of the manganites depend on the concentration of cation vacancies on the dodecahedral La-sites (sublattice A of the perovskite crystal structure ABO,) [l]. Along with the other factors, cation vacancies affect the electronic state of the Mn ions [3], which defines the structure, electrical conductivity, and other properties of manganites. In the present work, we have investigated defect perovskite-type manganites containing cation vacancies. The general formula of these materials is where c] is a cation vaLa,Sr,. 0 l_x-vMnO,_,, cancy on the A-site and S is the oxygen nonstoichiometry. The data on ceramics, in which the oxy0167-2738/95/$09.50 0 1995 Elsevier Science B.V. AI1 rights reserved SSDf 0167-2738(95)00179-4
polarization
gen content is as close as possible to the equilibrium with the gas phase at the temperature of experiment are presented in this paper. Some results of our studies of lanthanum-strontium manganites were published previously [4]. Ionic transport of oxygen in electrode materials leads to an increase of the effective electrode reaction zone [ 1,2]. In order to determine whether the mixed oxide ion and electron conduction of the electrode oxides affects the electrochemical properties of the electrode, oxygen semipermeability of solid solutions La,Sr,MnO, was determined at 960 to 1200 K. The characteristic feature of the perovskite-type oxides containing lanthanum and strontium ions is their interaction with zirconia-based solid electrolytes, which is accompanied by a sharp increase in the resistance of the oxide electrodes [2,6]. This solid state reaction is of considerable importance at temperatures above 1500 K [6]. To prepare manganite
V. V. Kharton et al. /Solid
202
electrodes, we used sintering of a paste with different sintering additives at 1470 rt 10 K for two hours. These conditions of the electrode production minimized the formation of barrier layers. Only the electrodes, obtained by this method, were investigated in the present paper, whereas use of other methods of electrode preparation (for example, the screen printing technique) makes it possible to reduce the electrode resistance. However, the method used here is the most simple and the most available. The other purpose of our work was to choose the optimal electrode and sintering agent materials for application in high-temperature fuel cells.
2. Experimental 2.1. Preparation
and investigation
of the ceramics
Synthesis of La,Sr,.MnO, (x = 0.3-0.6; y = 0.3-0.5) polycrystalline samples has been carried out by the standard ceramic technology from corresponding oxides at temperatures between 1570 and 1620 K for 15-30 h in air with repeated powder milling. The total impurities content in the starting materials was below 0.05 at%. The X-ray diffraction patterns were obtained by the DRON-3M diffractometer using Cu K a-radiation (Ni-filter). The error in the lattice parameters was *O.OOOl nm. Specimens were annealed at 1370 K in air and then cooled at a rate below 100 K/h with the aim to obtain the equilibrium oxygen content before the X-ray diffraction studies. Quantitative analysis of cations was carried out by X-ray fluorescence analysis (XFA) on a “Microscan-5” and a scanning electron microscope
Table I Properties
of La,Sr,.MnO,
State Ionics 81 (1995) 201-209
“Nanolab-7” with a micro X-ray fluorescence spectrometer SR-860-2. The data were processed using the ZAF-4/FLS program. Ceramic samples were compacted from the synthesized powders under pressures of 300-600 MPa. Samples had the shape of bars 4 X 4 X 30 mm3, or tablets (15 or 28 mm in diameter, 3-4 mm in thickness). The ceramic samples were annealed at 1620-1770 K in air during 20-35 h. The sample density (d,,,) was 80-95% of the theoretical value (Table 1). The obtained ceramics were tested for gas tightness (absence of open porosity) using a solution of “Sudan” dye-stuff and by the helium-vacuum method with a leak-detector GTI-3. A quartz dilatometer DKV-5A was used to investigate the thermal expansion of the ceramics. The electrical conductivity was measured by the four-probe dc method in air at temperatures of 300 to 1100 K. The relative error of the conductivity measurements did not exceed 3%. The distinctions between the data, obtained during the heating and cooling, from dilatometry and conductivity measurements, did not exceed 2%. Consequently, the measurements were conducted under conditions close to equilibrium. The methods, used to investigate the crystal structure, thermal expansion, and electrical conductivity are described in detail in Ref. [7]. 2.2. Oxygen permeability
investigations
Measurements of oxygen permeability were carried out with the help of the electrochemical solid electrolyte cell equipped with an oxygen pump and a sensor (Fig. 1). The oxygen permeability was deter-
ceramics
x
Y
Structure
a (nm) +O.OOOl nm
CY(“1
dexp (kg/m31
Z ‘XIOh(K-‘) 300-11OOK
0.6
0.4 0.5 0.4 0.3 0.5
Rb CC R C C
0.5458 0.3850 0.5452 0.3858 0.3829
60.28
5440 5260 5690 4960 5570
11.31 12.07 11.30 12.25 11.89
0.5 0.5 0.5 0.3
’ E = mean values of the thermal expansion coefficients. h R = perovskite structure having rhombohedric distortion. ’ C = cubic perovskite structure.
60.16
Iko.03 f 0.07 A 0.04 + 0.05 * 0.09
V. V. Kharton et al. /Solid State Ionics 81 (I 995) 201-206
4-
Fig. 1. Electrochemical cell for the oxygen permeability measurements: 1 - zirconia solid electrolyte; 2 - electrodes of the oxygen sensor; 3 - electrodes for the oxygen pump; 4 - manganite sample; 5 - high temperature glass.
mined at the condition of equality of the oxygen flows, being removed from the cell by the pump and entering it through the hermetically attached sample by the ionic transport. Specially designed adhesives were used to attach the ceramics. The oxygen permeability has been studied at temperatures from 960 to 1200 K with a difference in oxygen partial pressures between 2 X lo4 and 1 X lo3 Pa in the internal and external spaces of the measuring cell. More than 5 values of the flow through the sample, corresponding to different values of oxygen chemical potential gradient, were determined isothermally during each measurement. The time of attainment of the steadystate in the cell was 1 to 6 hours. The reproducibility dispersion of the oxygen permeability values was within 5% of the measured value. The method of investigation of the oxygen permeability is described in detail in Ref. [7]. 2.3. Preparation
and study of the electrodes
In the present work, the electrodes were prepared using pastes containing the manganite powder, sintering agent (2-6 wt%), and an organic binder (1 wt%). The manganite powder used for the electrode preparation had particle sizes smaller than lo-15 pm. Bi,O,, PbO, Sb,O,, CuO and Bi,CuO, were used as sintering agents. The minimal quantity of sintering agent, sufficient to provide the proper mechanical strength of electrodes, was added to the paste.
203
The powder of Bi,CuO, was synthesized by standard ceramic techniques from the bismuth and copper oxides at temperatures varying from 870-1000 K for 25 h in air. Bi,CuO, was identified by means of X-ray diffraction studies and XFA. Oxygen ion transference numbers of bismuth cuprate were determined by the EMF method using an electrochemical cell with separated gas compartments. A 2-probe ac method (1, 5, 10 and 50 kHz) was used for the measurements of the conductivity of Bi,CuO, in air. The electrode layers, obtained by applying the pastes on Zr,,,,Y, tOO,,s, ceramics, were annealed at 1470 + 10 K in ‘air during two hours. The layer density was 100 + 10 mg/cm2. The used technology provided the maximum longitudinal conductivity of the electrode layers. The overpotential values 77 were determined using the formula: v=U-IR,
(1) where U is the total potential drop across the electrodes, and ZR the potential drop across the solid electrolyte. The R values were measured by the two methods: (1) using an alternating current (20 kHz); (2) by the oscilloscopic method. The ac frequency was chosen on the basis of the results arising from the investigations of the frequency dependence of the electrochemical cell resistance. The procedure of the ac overpotential measurements has been described in Ref. [7]. The overpotential of the manganite electrodes was determined under steady cathodic polarization in air at 1000-1300 K. The time of the steady-state attainment in the electrochemical cell was 0.2-l h. The 71 values were in the range of - 10 to - 800 mV. We shall adduce the absolute values of the overpotential in the discussion. The reproducibility of the overpotential values was within 10% of the measured values for the different batches of electrodes having the same composition.
3. Results and discussion 3.1. Properties
of the ceramics
X-ray diffraction studies of the La,Sr?MnO, samples showed the formation of perovskite-like solid
V.V. Kharton et al. /Solid State Ionics 81 (199.5) 201-209
204
0.3 7
1 12
I 17
/ 22
27
I 12
1 32
10*/T, It-'Fig. 2. Temperature dependence of the conductivity of La,Sr,.MnO, ceramics: 1 - (x = 0.5, y = 0.5); 2 - (X = 0.5, y = 0.4); 3 - (x = 0.5, y = 0.3); 4 - (x = 0.3, y = 0.5); 5 - (x = 0.6, y = 0.4).
solutions. The crystal symmetry of La,,,Sr,,MnO,, La,,Sr,,,MnO, and La,,Sr,,,MnO, was identified as pseudocubic. The solid solutions La,,Sr,,MnO, and La,,Sr,,MnO, had the rhombohedric perovskite-type structure. All materials were singlephased, with one exception. X-ray diffraction studies demonstrated the presence of impurities (manganese oxides) in La,,Sr,,MnO,. Crystal lattice parameters of manganites (a and a> are given in Table 1. The mA results showed that the deviation of the cationic composition for each sublattice of the perovskite structure from the nominal composition did not exceed 3%. The overall impurities concentration was below 0.4 at%. The temperature dependencies of the conductivity of La(Sr)MnO, (Fig. 2) are in agreement with literature data [8]. The conductivity increases with substitution of La3+ by strontium ions. These results may be ascribed to the increase of the Mn4+ concentration [8]. The maximum conductivity was found for La,,Sr,,,MnO, and La0,3Sr,,5Mn03 ceramics. Ceramic samples of La0,,Sr,,4Mn03 containing Bi,O,, CuO or Bi,CuO, were prepared for studying the effect of sintering agents on the electrical properties of the manganite. These ceramics were annealed at 1470 + 20 K in air during 2-5 h. XFA showed
1 17
,
I
I
22
27
32
104/T, It-'Fig. 3. Temperature dependence of the conductivity of La,, ST~,~MnO, ceramics containing 2, 4 and 6 wt% of Bi,CuO, (curves 1, 2 and 3, respectively) or 3 wt% of CuO (4) before annealing.
that copper oxide evaporates substantially less than bismuth oxide during annealing. For example, the atomic ratio Cu : Bi for a sample containing initially 2 wt% Bi,O, and 2 wt% CuO were: 2.93 (before annealing), 3.5 (after two hours of annealing), 8.2 (after five hours of annealing). X-ray diffraction studies of samples with the sintering agents revealed the presence of perovskite phase only. Addition of sintering agents to the sample composition leads to an increase in the electrical conductivity as shown in Fig. 3. A fact which is well explained by the increase of the density of the ceramics (Table 2) and the implantation of copper ions into the sublattice B of the perovskite structure. The life service test of the samples containing the sintering agents revealed an increase of l-2% in the conductivity during annealing at 1170 K for 2.50 h.
Table 2 Properties of La,,Sro,,Mn03 ceramics annealed at 1470+20 in air during two hours with addition of h wt% of Bi,CuO, h
a(nm) kO.001 nm
a (“1
dexp (kg/m31
E X IO6 (K- ‘) 5OO- 1100K
2 4 6
0.546 0.547 0.546
60.29 60.21 60.16
5670 5810 5930
11.35*0.03 10.64 * 0.09 10.08 * 0.05
K
V. V. Kharton et al. / Solid State Ionics 81 (I 995) 201-209
205
This effect is caused by the increase of the density of the ceramics which is due to the additional sintering agents. It was found that the conductivity of the ceramics without sintering agents does not vary during 100-200 h of annealing. 3.2. Oxygen permeability
of the manganite
ceramics
The results of the oxygen permeability investigations are given in Figs. 4 and 5. The oxygen permeability values were calculated from the formula [9]: Jo2 = (RTCf/16SP)
. (Z/E),
(2)
where Jo, is the oxygen permeability (mol . s- ’ cm-‘), I is the current through the oxygen pump of the electrochemical cell (Fig. l), E is the EMF of the oxygen sensor, d, the sample thickness, and S the effective surface area of the manganite ceramics. The oxygen partial pressure dependencies of Jo, are non-linear and have the typical shape, illustrated by Fig. 4. This suggests that equation [9] j, = 2. JoZd-’
ln( P,/P,),
-11.5
+ 8.0
8.5
9.0
9.5
10,o
10,5
11.0
104/T,It-'v Fig. 5. Temperature dependence of the oxygen permeability of La,Sr,Mn03 ceramics at E = 50 mV: 1 - (X = 0.6, y = 0.4); 2 - (X = 0.5, y = 0.5); 3,4 - (x = 0.5, y = 0.4); 5 - (X = 0.5, y = 0.3); 4 - ceramic with platinum layers.
(3)
where j, is the oxygen ion flux density (mol. SC’ cmT2>, P, and P2 the oxygen partial pressures
c
outside and inside the cell (atm) respectively, can be used only for certain values of the oxygen partial pressure. Regression analysis of the experimental data on the permeability was used to ascertain the limiting stage of the oxygen transport through the manganites. The following regression models were the most adequate: jo(p2)
=ki
.ln(WW
jo(P*)=Q(P;n-P?)
+& +k,,
jo(P2)=kl*(Py2-Py2)+k2, jo(P2)
(4) (5) (6)
=k,-(k2/2).ln(P2)+(k,/4).ln2(P2) (7)
/
I
I
40 E, mV
60
I
I
80
I
1d0
-
Fig. 4. Dependence of oxygen permeability of La,.%,,MnO, ceramics on the EMF of the oxygen sensor at 1170 + 2 K: 1 - (x = 0.6, y = 0.4); 2 - (n = 0.5, y = 0.5); 3,4 - (x = 0.5, y = 0.4); 5 - (x = 0.5, y = 0.3); 4 - ceramics with platinum layers.
where k, and m are the regression parameters and P, is kept constant. Model (4) corresponds to Eq. (3). Models (5) and (6) can be obtained on the assumptions that the ion flow is proportional to the 02- concentration gradient and that the 02- concentration in the ceramic surface layers (cr and c2) depends linearly on P;” and ‘PT, respectively (Freindlich’s adsorption isotherm). For model (6) m equals l/2.
206
V. V. Kharton et al. / Solid State Ionics 81 (I 995) 201-209
Model (7) can be obtained on the following assumptions: The concentration of oxygen ions in the surface layers of the manganite can be expressed in terms of Temkin’s adsorption isotherm [lo]: C[ = K,
ln( Pi) + K,.
~c is the oxygen ion conductivity, p0 the chemical potential of oxygen ions, p, and p2 the values of ~a in the surface layers, v. the mean frequency of oxygen ions vibrations, r the distance of the elementary ion translation, iJ,, the migration energy, N the concentration of the oxygen ion sites, and c the concentration of oxygen ions. Model (7) can be derived by integration of E%J.(9) and substitution of formula (8) on the condition that
(8)
Since the electronic conductivity of the manganites is much higher than the oxygen ion conductivity ( C~x=-aa), the oxygen ion flux density can be given by the following [l I]: j, = (4F2d)-1
(%a
k, =K,K,.[(N-2KJ
dpu,
‘112
= KZflc(
N - c) d[ln~],
(9)
=K,K,N-2K,K,K,,
(k,/4)
=2KfK,.
The oxygen oxygen square
E2
where K, = vor2( Nd)-’
(k,/2)
exp( - U,/RT),
ln(P,)
-2K,
*ln”(P,)],
regression analysis of the dependence of the flow through the manganite samples on the partial pressure was carried out by the least method using BFGS-formula for minimiza-
Table 3 Parameters of the regression models of the oxygen flux density through the manganites of the oxygen partial pressure dependences Sample
L4”
Model 980
II00
1170
W”
980
Coefficients (mol/(s * cm))
6a
Sb (%I
PC
k, = t2.0 * 0.1) x 10-s kl=(-1.7f0.2)X10-8 k, = (2.1 -+ 0.2) x 10-7
5.7 x lo- HI
3.1
0.989
6.7 x IO- I0
4.0
0.987
4.2 X IO- I0
1.4
0.998
5.8 X lo- I0
2.0
0.996
3.9 x lo- ‘O
1.3
0.999
3.4 x lo- ‘0
1.3
0.998
3.9 x 10-a
13.8
0.94
6.3 X IO-”
2.1
0.997
7.6 x lo- ”
4.4
0.998
k, = (-6.5 rf:0.8) x lo+ k, =(-8.1~0.8)x 10-8 k, = (6.1 f 0.1) x 1O-8 k,=(-1.0+0.1)x lO-s k, =(1.8rtO.l)xlO-’ k, = (-3.8 f 0.3) x lo-* k, =(-9.2&0.8)x lo-* k, = (8.8 + 0.1) x lo-* k, = (- 1.6 k 0.3) x lo+ k, =(-1.6&0.2)x 1O-7 k2=(1.8f0.2)x10-7 k, = (-4.5 % 0.4) x IO-’ k, = (1.7 f 0.5) x 10-S k,=(-3.2f2)XlO-’ k, =(2.1 zb0.7) x 1O-7 m = 0.56 f 0.09 k, =(-4.3,0.3)X
IO-’
k,=~1.2+0.5)~10-’ k,=(--2.1 *00.1)x IO-* kq =fl.l + 0.2) x lo-* a 6 = adequacy dispersion. ’ s = relative error of the model. E p = correlation coefficient. ’ L4 = LaO,,Sro,.,MnO,. ’ L5 = La,,Sr,~,MnO,.
V.V. Kharton et al. /Solid State Ionics 81 (1995) 201-209
tion of the non-linear adequacy dispersion model [12]. Statistical parameters of the non-linear models were calculated using the method proposed in Ref. [ 131. A special program was designed for this purpose. In addition to models (4)-(71, the solutions of the sets of equations, which included Wagner’s law, and Langmuir’s and Freinlich’s adsorption isotherms, were used as transport models. Certain results of the regression analysis, according to models (4)-(7), are presented in Table 3. Model (7) was the most adequate among the used models. The obtained results suggest that oxygen transport through the lanthanum-manganite-based ceramics is limited by the sorption of oxygen on the ceramic surface. Application of platinum layers (5 mg/cm’) on both surfaces of the manganite ceramics, which was done to verify the premise that oxygen sorption is the limiting step of oxygen transport, leads to a considerable increase in the oxygen permeability and to a change of the character of the dependence of Jo on the oxygen partial pressure. Most probably, so& tion hampers the oxygen transport less as the temperature increases; since at T > 1100 K the permeability of ceramic samples covered with platinum, approaches that of ceramic samples without platinum (Fig. 5). The temperature dependencies of the oxygen semipermeability at E = 50 mV are presented in Fig. 5. The maximum Jo, at T > 1000 K was found for La,,Sr,,MnO, ceramics. Oxygen permeability values for the other investigated manganites are close to each other. The oxygen semipermeability of the manganites was found to be 100-1000 times lower than that of the lanthanum-strontium cobaltites [5,7].
207
___
400 -
300 -
I 2 E
200 -
F’
_ EE::
100 -
zi
lJ, , / , , , , / , 1 OO
40
80 j,
rnA.~rn-~
120
160
200
-
Fig. 6. Overpotential versus current density plots for La,SryMnO, electrodes with sintering agent Bi,O, (4 wt%) at 1260 + 8 K under stationary cathodic polarization in air: 1 - (x = 0.6, y = 0.4); 2 - (x = 0.5, y = 0.3); 3 - (x = 0.5, y = 0.4); 4 - (x = 0.3, y = 0.5).
These dependencies can be described adequately by exponential models such as Tafel’s equation or the slow discharge equation [2]. The attribute of the
3.3. Properties of the electrode layers According to the results of the life service tests of the manganite electrodes at 1170 to f3 K, the overpotential does not change with time during 70 h when the current density (j) is kept constant. The total potential drop across the electrodes (U) decreased by lo-25% during a given time interval due to additional sintering effects of the electrode layers and the corresponding increase in the electrode conductivity. The overpotential vs current density plots for the investigated electrodes are presented in Figs. 6-8.
Fig. 7. Overpotential versus current density plots for electrode Lao,,Sra,MnO, at 126Ok 8 K in air. Sintering agents: (wt%): I 2% of Bi,O, and 2% of CuO, 2 - 4% of CuO, 3 - 4% of PbO, 4 - 4% of Sb,O,.
208
V.V. Kharton et al. /Solid State Ionics 81 (1995) 201-209
obtained results is the absence of a direct correlation between the oxygen semipermeability of the ceramics and the electrochemical parameters of the electrode layers. The regularities of the change of the electrode properties, illustrated in Figs. 6-8, are observed for all temperatures in the range studied, i.e. 1000-1300 K in air. The polarizability of the La,,Sr,,MnO, electrode is lower than that of the electrodes based on the other solid solutions of La(Sr)MnO,. The lowest electrode resistance can be achieved when the electrode material LaO,,Sr,,dMnO, is used in combination with the sintering agent Bi,CuO,. The positive effect of addition of the bismuth cuprate is due, probably, to both the sufficiently high mixed oxygen ion and electronic conductivity of Bi,CuO,, and the decrease in the resistance of the manganite owing to the implantation of copper ions into the crystal lattice of the perovskite-type oxide. Some electric properties of synthesized Bi,CuO, samples are given in Table 4. The obtained results are in agreement with literature data [14]. Bi,CuO, has a sufficiently high mixed conductivity. The oxygen ion partial conductivity of bismuth cuprate increases with increasing temperature. The crystal
500,
1
Table 4 Properties
of Bi,CuO,
ceramics b
dexP (kg/m31
T (K)
K * (S/cm)
r”
6850
900 800 700
1.6x lo-* 9.0x 1o-3 5.4x 10-3
0.05 0.03
* K = specific conductivity of the samples. b to = ionic oxygen transference number.
structure of this complex oxide is tetragonal (the space group is P4/mmm). The Bi,CuO, lattice parameters at room temperature are: a = 0.8502 f 0.0001 nm, c = 0.5818 + 0.0001 nm. DTA results showed that the temperature of the congruent melting of Bi,CuO, is 1123 f 5 K. The mean values of the thermal expansion coefficient of this compound are 4.2 X 10m6 K-’ (400-800 K) and 10.1 X 10m6 K-’ (800-1000 K). The high mixed conductivity of Bi,CuO, offers, probably, good electrochemical properties of electrode layers with bismuth cuprate. Bismuth cuprate is pulverized by ceramic crushing in order to give powders which were used to study the effect of the preparation conditions on its properties as a sintering agent. The electrochemical parameters of the electrodes with powdered Bi,CuO, are significantly worse than those of the other electrodes (Fig. S), because of the distinctions between the specific surfaces of the sintering agents. Probably, the manganite La,,Sr,,dMnO, with added Bi,O, and CuO has also good electrode properties (Fig. 7) due to the formation of Bi,CuO,.
4. Conclusion
I 50
100 j,
I 150
I ZOO
I 250
300
mA.~rn-~ -
Fig. 8. Overpotential versus current density plots for electrodes La,,Sr,,,MnO, with sintering agent Bi,CuO, at 1260*3 K in air: 1 - 2 wt% of Bi,CuO,, 2,3 - 4 wt% of Bi,CuO,, 4 - 6 wt% of Bi,CuO,, 3 - powdered Bi,CuO, was used.
The oxygen semipermeability of the solid solutions La(Sr)MnO, was found to be limited by oxygen sorption on the manganite ceramic surface. Applying the platinum layers to the ceramic surfaces result in an increase of the oxygen permeability by a factor of 2 to 4. It was shown that introduction of a sintering agent Bi,CuO, improves the electrochemical parameters of the manganite electrodes markedly. The solid solution La,,Sr,,MnO, in combination with the sintering agent Bi,CuO, has good properties as an electrode material.
V. V. Kharton et al. /Solid State Ionics 81 (I 99.5)201-209
References [I] T. Takahashi
[2]
[3] [4] [5]
and H. Iwahara, Report of Special Project Research Under Grant in Aid of Scientific Research of The Ministry of Education, Science and Culture (Japan, 1980) p. 727. M.V. Perlllyev, A.K. Demin, B.L. Kuzin and AS. Lipilin, High Temperature Electrolysis of Gases (Science, Moscow, 1988) [in Russian]. B.C. Totield and W.R. Scott, J. Solid State Chem. 10 (1974) 183. A.V. Nikolaev, V.V. Kharton, E.N. Naumovich and A.A. Vecher, Proc. I Europ. SOFC Forum (Lucerne, 1994) p. 415. P.P. Shuk, A.A. Vecher, V.V. Kharton, L.A. Tichonova, H.-D. Wiemhoefer and W. Goepel, Sensors Actuators. B1516 (1993) 401.
209
[6] B.L. Kuzin and A.N. Vlasov, Elektrokhimya 20 (1984) 1636. [7] V.V. Kharton, E.N. Naumovich, P.P. Shuk, A.K. Demin and A.V. Nikolaev, Elektrokhimya 28 (1992) 1693. [8] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [9] H.-H. Moehius, Ext. Abstr., 37th Meeting ISE, Vol. 1 (Vilnius, 1986) p. 136. [lo] M.I. Temkin, J. Phys. Chem. 1.5 (1941) 296. [ll] V.N. Chebotin, The Chemical Diffusion in the Solid State (Science, Moscow, 1989) [in Russian]. [12] J.E. Dennis, Jr. and R.B. Schnabel, Numerical Methods for Unconstrained Optimisation and Nonlinear Equations (Prentice-Hall, New York, 1983). [13] N.R. Draper and H. Smith, Applied Regression Analysis (Wiley, New York, 1981). [14] A.F. Poluyan and A.G. Gusakov, Ion Melts Solid Electrol. 2 (1987) 78 [in Russian].