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Chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2, solid oxide fuel cell cathode and electrolyte materials
SOLID STATE IOlUCS
Solid State Ionics 52 (1992) 303-312 North-Holland
Chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2, solid oxide fuel cell cathode and electrolyte materials J.A.M. van R o o s m a l e n a n d E.H.P. C o r d f u n k e Netherlands Energy Research Foundation ECN, P. 0. Box 1, 1755 ZG Petten, The Netherlands
Received 17 September 1991; accepted for publication 18 March 1992
The chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2 were studied from 1110 to 1755 K. Reaction of LaMnO3 with (Zr, Y)O2 was already observed at 1170 K, whereas reactions between (La, Sr)MnO3 with 30 at.% Sr and (Zr, Y)O2 with 8 at.% Y were not observed at 1365 K. The reaction products observed in the experiments are L a 2 Z r 2 0 7 and/or SrZrO3. It is proposed that reaction layers are formed by diffusion of La and/or Sr into (Zr, Y)O2 via a vacancy diffusion mechanism. The composition of the layers depends on the La203 and SrO activities in (La, Sr)MnO3. The activation energy for the formation of a La2Zr207 reaction layer was determined to be 17.5_+1.8 kJ- mol- ~, the activation energy for the formation of a SrZrO3 reaction layer was determined to be 18.8 _+1.9 kJ. mol- 1. On the basis of the experiments it was calculated that at 1273 K it would take about 29100 h to grow a reaction layer of SrZrO3 from (Lao.sSr0.5)MnO3and (Zro.97Yo.o3)OI.985,and about 82000 h to grow a reaction layer o f La2Zg20 7 from LaMnO3 and ( Z r o . 9 2 Y o . o s ) O i . 9 6 , with a thickness of I ~tm for both layers, It is proposed that the reaction layers might result in both ohmic and polarization losses of the SOFC.
1. Introduction The d e v e l o p m e n t o f Solid Oxide Fuel Cells ( S O F C ' s ) has been given new attention during the last few years. Especially the i n t r o d u c t i o n o f the flat plate S O F C concept has stimulated research activities, for instance, at E C N [ 1,2 ]. Some o f the advantages o f the flat plate design are a high power output and short conductivity paths with respect to the tubular or the h o n e y c o m b design. To commercialize S O F C reactors a n u m b e r o f technological and chemical p r o b l e m s still have to be solved. State-of-the-art cathode material is (La, Sr)MnO3, but it is not yet clear what percentage o f strontium should be used, The state-of-the-art electrolyte material consists o f ZrO2 solid solutions with Y203. One o f the requirements for successful operation is the t h e r m o d y n a m i c stability o f the cathode with respect to the electrolyte. A n u m b e r o f materials that have been tested as cathode materials app e a r to react with the electrolyte [ 3 - 1 4 ] , forming badly conducting c o m p o u n d s [ 11,15 ]. F o r a number o f c o m p o u n d s this reaction has been p r e d i c t e d by means o f t h e r m o d y n a m i c considerations [ 16-18 ].
In this p a p e r results are reported o f an investigation on the reaction between (La, Sr)MnO3 [19] with 0, 15, 30, and 50 at.% Sr and (Zr, Y)O2 [20] with 3 and 8 at.% Y as a function o f time and temperature. The results can be interpreted on the basis o f layer growth a n d chemical t h e r m o d y n a m i c considerations. F o r c o m p a r i s o n some experiments were conducted with (La, C a ) M n O 3 and (La, Ba)MnO3. Throughout the p a p e r (La, Sr)MnO3 with 0, 15, 30 a n d 50 at.% Sr will be referred to as LSM-0, LSM15, LSM-30 and LSM-50, respectively, likewise for La substitution by Ca and Ba. The electrolyte (Zr, Y)O2 with 3 and 8 at.% Y will be referred to as ZY3 and ZY-8, respectively.
2. Experimental The (La, A ) M n O 3 ( A = C a , St, Ba) samples were prepared by a co-precipitation method, similar to that described by H a s h i m o t o [21 ]. According to the Xray diffraction patterns all samples were single phase perovskite-type oxides. Z Y - 3 a n d ZY-8 were obt a i n e d from TOSOH. Dense pellets (>_ 90% o f the-
0167-2738/92/$ 05,00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
304
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
oretical density) of all materials were obtained by sintering at 1655 K in air. Sintering was not performed at the highest reaction temperature ( 1755 K), because in some cases excessive grain growth occurred. Two types of experiments were performed: (a) diffusion experiments, in which pellets were put together in the order ZY-3 ILSM-01ZY-81LSM-151 etc., and held together in the hot part of the furnace by applying an external pressure. In some experiments a small strip of platinum was sputtered on the surface of the (Zr, Y)O2 pellets as a marker for the diffusion reaction. To analyse the experiments with SEM and EDS the pellets were cut perpendicular to the surface as well as to the platinum strip; (b) reactivity experiments, in which (La, Sr)MnO3 and (Zr, Y)O2 powders ( 1:1 ) were thoroughly mixed, pressed into pellets and heated at various temperatures in air. X-ray patterns were taken to follow the formation of the reaction products. To analyse the X-ray patterns obtained after the experiments, SrZrO3 and La2Zr207 were synthesized. The structure of SrZrO3 is cubic with a doubled axis with respect to the simple perovskite structure [ 22 ]. La2Zr207 has a cubic pyrochlore structure [23]. All intensities in the X-ray pattern corresponded with cubic La2Zr207, except for three small intensities which could not be attributed to impurities. It is suggested that these intensities are due to a small tetragonal distortion. Since the sample was synthesized at 2070 K in an argon atmosphere, a small oxygen deficiency may cause the extra intensities, although they did not disappear after retiring at 1420 K in air. The colour of the sample after both treatments was white. Despite the small extra intensities the sample is suitable for comparison with possible reaction products. Chemical analyses were performed by Inductively Coupled Emission Spectroscopy (ICP), the results are listed in table 1. SEM photographs were recorded on a JEOL JXA-840 scanning microanalyzer, equipped with energy dispersive X-ray spectroscopy (EDS) for identification of the reaction products. With this technique it is not possible to detect Y in ZY-3 and ZY-8 because of band-broadening. In this article it is assumed that Y has the same behaviour as Zr, since it is in solid solution. It is also difficult to detect Sr, because of overlap between a small Zr-
Table 1 Chemical analysis (ICP) of (La,Sr)MnO3 materials obtained from co-precipitation, values relative to the Mn-content. (La,Sr)MnO3
La/Mn
Sr/Mn
(La,Sr)/Mn
LSM-O LSM-15 LSM-30 LSM-50
1.001 0.858 0.704 0.501
O.148 0.301 0.506
1.001 1.006 1.005 1.007
adsorption and the major Sr-adsorption. In recordings of elemental distribution this can be confusing, but fortunately relatively large amounts of Sr can be identified without problems. X-ray patterns of starling materials and reaction products were recorded on a Guinier camera using Cu Kcq.2 radiation (0.15418 nm).
3. Results The kinetics of the reaction between (La, Sr) MnO3 and (Zr, Y ) 0 2 were studied by powder experiments in the lower temperature region ( 1110-1505 K), and pressed pellets experiments at higher temperatures (1455-1755 K). The reaction time for the powder experiments was 110 h, and 260 or 596 h for the pressed pellets experiments. The reaction products which were observed at the various temperatures are listed in table 2. The phases in the reaction layers were identified to be SrZrO3 and La2Zr207 by X-ray diffraction. The thicknesses of the diffusion layers, which were observed in the various experiments, were determined from the SEM recordings and are listed in table 3 for (La, Sr)MnO3 and in table 4 for (La, Ca)MnO3 and (La, Ba)MnO3. The SEM recordings cover only a small part of the entire reaction layer. Care was taken to select a representative part of the layer. The thickness of the layer was determined by hand from the SEM recordings. The error that can be made by this procedure is estimated to be 10% for thick layers to 15% for thin layers. Information about the diffusion behaviour was obtained from SEM backscatter recordings in combination with energy dispersive X-ray spectroscopy. Some SEM/EDS recordings of observed diffusion layers (with and without platinum markers) are
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and inlerdiffusion
305
Table 2 Results of reactivity experiments. The products of the reaction between (La,Sr)MnO3 and (Zr, Y)02 are indicated. The powder experiments were conducted at 1110-1505 K, the pressed pellets experiments were conducted at 1455-1755 K. T(K)
ZY-3
ZY-8
LSM-0
LSM- 15 .
LSM-50
LSM-0
-
LZ
LSM-50
-
-
-
-
-
-
. LZ
1270
LZ
LZ
LZ
SZ b )
LZ
LZ
-
SZ
1360
LZ LZ LZ LZ LZ
LZ LZ LZ LZ/SZ LZ/SZ
LZ LZ LZ/SZ LZ/SZ LZ/SZ
SZ SZ SZ LZ/SZ LZ/SZ
LZ LZ LZ LZ LZ
LZ LZ LZ LZ/SZ LZ/SZ
LZ LZ LZ/SZ LZ/SZ LZ/SZ
SZ SZ SZ LZ/SZ LZ/SZ
a) LZ=La:Zr2OT;
¢)
LSM-30
1110
")
.
LSM- ! 5
1170
1505 1555 1655 1755
.
LSM-30
-
b) SZ=SrZrO3;
¢) No reaction products observed.
Table 3 Diffusion layers that are formed in (Zr, Y)O2 by the reaction of (La, Sr)MnO3 with (Zr, Y)O2 at various reaction times and temperatures. Layer thickness is listed in micrometers. T(K)
t (h)
ZY-3
ZY-8
LSM-0
LSM- 15
1455 1555 1655
260 596 260 596
c) 1.8 a) 2.7 `') 3.5 ")
c) 0.9 a) c) 2.5 `')'b)
1755
260 596
5.0`') 7.3 a) 7.4 a)
c) 8.2 ") 8.5 ~)
LSM-30
LSM-50
LSM-0
0.3 a) 1.4 a)'b) 4.1 ,,).b) 2.5 `')
¢) 3.0 b) c) 7.3 ")'b)
c) 1.6 ~) 2.0 ~) 2.9 `0
8,6 ~).b) 13.0 `')'b) 16.4 `'). b)
¢) 15.5 TM 16.0 ~). b)
4.5,) 6.5 ~) 7.4`')
LSM- 15 ¢) 0.7 `') ¢) 1.8 ~) c) 5.6 a) 10.4 .), b)
LSM-30 _a) 0.5 a)'b) 2.7,0.b) 3.6 a)'b) 5.5 a). b) 7.3 a).b) 6.8 ~) 10.0 a). b)
LSM-50 c) 2.7 b) ¢) 8.0 b) ¢) 12.4 b) 12.7 b)
`') Formation ofLa2Zr207 ; b) Formation ofSrZrO3 ; ¢) No experiment ; d) NO reaction products observed. Table 4 Diffusion layers that are formed in (Zr, Y)O2 by the reaction of (La, Ca)MnO3 and (l_a, Ba)MnO3 with (Zr, Y)O2 during 110 h at 1715 K. Layer thickness is listed in micrometers.
ZY-3 ZY-8
LCM-15
LCM-30
LBM-15
LBM-30
8.9 `') 6.1 b)
12.1 `'),b) 22.3 a)
5.4 b) 7.2 a), b)
8.5 ¢) 7.3 ¢)
a) Formation of CaZrO3, mation of BaZrO 3.
b) Formation ofLa2Zr207 ; ¢) For-
shown in fig. 1. From the SEM backscatter photographs and EDS recordings there are a number of indications for the reaction and diffusion mechanism: (a) The platinum markers were sputtered on the surface of the (Zr, Y)O2 pellets. After the reaction they are located on top of the reaction layers (fig.
lg). It is evident that there is no diffusion of Zr(Y) into (La, Sr)MnO3 (see also fig. la, b); only diffusion of La and Sr into iZr, Y)O2 was observed (fig. la-c, g-j). (b) Diffusion of Mn from (La, Sr)MnO3 into (Zr, Y)O2 reported by Lau and Singhal [9] and Milliken et al. [ 11 ], could not be observed in our experiments (fig. la, c). The reaction mechanism will be discussed in paragraph 4.2; (c) There is a simultaneous diffusion of La and Sr resulting in a peculiar diffusion pattern with La-rich and Sr-rich regions (fig. l d - g ) . However, depending on the St-concentrations, diffusion of La or Sr apart, resulting in La2Zr207 (fig. la, h) or SrZrO3 layers (fig. 1i, j ) is also observed. We will come back to this later. (d) At 1755 K excessive grain growth was ob-
306
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
;
•
~.~i
'
i. ¸
r ',
,
Fig. 1. (a) Backscatter recording of the reaction layer formed in the reaction between ZY-8 (left-hand side) and LSM-30 (right-hand side ) at 1755 K, 596 h, showing a La2Zr207 reaction layer (no SrZrO3 ); (b) zirconium distribution belonging to I a. It is very clear that the Zr-concentration in the reaction layer is lower than in bulk ZY-8; (c) manganese distribution belonging to la. There is no Mn in the reaction layer and no diffusion of Mn into ZY-8; (d) backscatter recording of the reaction layer formed in the reaction between ZY-3 and LSM-30 at 1755 K, 596 h. Only the ZY-3 part is shown, showing a reaction layer of La2Zr207 (bright pattern) and SrZrO 3, a dense layer of (Zr, Y)O2, and grain growth in bulk ZY-3; (e) strontium distribution belonging to ld (overlap of Zr) showing the distribution of Sr in the reaction layer. No Sr is observed in the La2Zr207 regions; (f) lanthanum distribution belonging to 1d, showing the distribution of La over the reaction layer. La is also observed in the SrZrO3 regions.
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
307
0~
.......
2 r ~ ¢
;".; i .. O~ O
i 0 l: r~,
2 '37f~
20!::~i
Fig. 1. Continued. (g) Backscatter recording of the reaction layer formed in the reaction between ZY-8 (upper part ) and LSM-30 (lower part) at 1755 K, 596 h, showing Pt-markers (bright spots) and a reaction layer of La2Zr207 (clearly visible) and SrZrO3 (slightly different colour); (h) backscatter recording of the reaction layer formed in the reaction between ZY-8 (right-hand side) and LSM-0 (left-hand side ) at 1755 K, 596 h, showing a La2Zr207 reaction layer and holes due to the Kirkendal! effect; (i) backscatter recording of the reaction layer formed in the reaction between ZY-8 (right-hand side) and LSM-50 (left-hand iside) at 1655 K, 596 h, showing a SrZrO3 reaction layer; (j) strontium distribution belonging to l i. Due to overlap between a small reflection of Zr with the major reflection of Sr there also appears to be Sr in bulk ZY-8. The reaction layer, however, is clearly resolved.
served in ZY-3 during the reaction. However, between the reaction layer a n d the grains a dense layer o f (Zr, Y)O2 is formed. This indicates Zr-diffusion against its chemical potential gradient (fig. l d - f ) . (e) In some cases pores are f o r m e d on the (La, Sr)MnO3 side o f the reaction layer (fig. l h ) . This is known as the Kirkendall effect [24] a n d is indicative for a vacancy diffusion mechanism, resulting in a net mass flow. ( f ) In the case where there is m i x e d diffusion o f La a n d Sr, La-rich a n d Sr-rich regions are f o r m e d in
the reaction layer. There is almost no Sr in the Larich regions, whereas there is a significant a m o u n t o f La in the Sr-rich region (fig. l d - f ) . This indicates that tho solubility o f Sr in La2Zr207 is very small, while there is a certain solubility o f La in SrZrO3. The solubility limit o f La in SrZrOa is r e p o r t e d to be a p p r o x i m a t e l y 6 at.% [25].
308
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
4. Discussion 4.1. Diffusion e x p e r i m e n t s
It is very likely that an initial reaction layer is formed by reaction of La (and/or Sr) from the surface of (La, Sr)MnO3 with Zr from the surface of (Zr, Y)O2. The experiments with the Pt markers (fig. I g) indicate that the growth of the layers can be described by a diffusion of La and/or Sr through the reaction layer into the ZrO2 lattice. From the location of the Pt markers at the top of the reaction layers it is concluded that there is probably no Zr diffusing through the layer. Apart from the reaction layer, another layer is formed between the reaction layer and (Zr, Y)O2 (fig. ld). The formation of this dense (Zr, Y)O2 layer, that does not contain La and/ or Sr, is caused by the diffusion of La and/or Sr into the reaction layer. This layer is probably formed also in the other reactions (fig. la, g-i), but cannot be seen separately, because the (Zr, Y)O2-pellet already was dense in these experiments. The composition of the initial reaction layer depends on the La and Sr-activity in (La, Sr)MnO3. At low Sr-activities the formation of La2Zr207 is more favourable then that of SrZrO3, whereas at 50% Sr the formation of SrZrO3 is more favourable. In intermediate situations the initial reaction layer can be composed of both La and Sr. Because of the different diffusion rates for Sr and La, and the relatively small solubility of Sr in La2Zr207 (fig. l d - f ) , La- and Srrich regions can be formed in the diffusion layer. It should be noted, however, that in some cases, reaction of both La and Sr was expected on the basis of the experimental results (table 2), while only a La2Zr207 layer was observed (compare fig. la with fig. lg). There is no obvious explanation for these differences. The layer growth is determined by the smallest diffusion constant of the processes that are necessary to form the reaction layer. If the anionic conductivity is supposed to be several orders of magnitude higher than the cationic conductivity the rate determining processes could be the supply of La and/or Sr to the reaction layer, by diffusion of cations in (La, Sr)MnO3, or the diffusion of La and/or Sr into the reaction layer. From the absence of the Kirkendall effect (fig. lh) in the majority of the observed re-
actions it is concluded that the diffusion of La (and/ or Sr) through the reaction layer (La2Zr207 and/or SrZrO3) is the rate determining step. The analysis of the diffusion layers is complicated in the situation where both La2Zr207 and SrZrO3 are present: it is difficult to establish the relative amounts of both phases. It is suggested that the diffusion in the La-rich regions is determined by the diffusion coefficient of La alone and in the Sr-rich regions by the diffusion coefficient of Sr alone, although there is a slight solubility of La in SrZrO3. Fortunately, the reaction between LSM-50 and (Zr, Y)O2 is almost only the result of Sr-diffusion. The diffusion coefficient (growth constant of the reaction layer) of La can be determined from the reaction between LSM-0 and (Zr, Y )02. Diffusion can be described by Fick's law, which can be simplified to: (l)
x2=D.t ,
in which x is the layer thickness in cm, D is the chemical diffusion coefficient, here growth constant, t, the reaction time in seconds. Although cracks and porosity were observed in the reaction layers of the dense pellets used, the results, nevertheless, indicate that Fick's law can be used within experimental error. The activation energy for the diffusion can be obtained from the Tafel plot of log D versus l / T. The experimental results (table 3) have been evaluated and the calculated growth constants are listed in table 5 and plotted in fig. 2. The activation energy is Table 5 Growth constants for the formationof La2Zr207and SrZrOareaction layersat varioustemperatures,calculatedfrom the experimental results in table 3. T(K)
log(D s/cm 2) La2Zr207
1555 1655 1755
SrZrO3
LSM-0 ZY-3
LSM-0 ZY-8
LSM-50 ZY-3
LSM-50 ZY-8
-13.82 -13.11 - 13.24 -12.57 - 12.60 -12.59
-13.92 -13.37 -13.41 - 12.66 -12.71 -12.59
-13.38 -12.60
-13.47 -12.53
-11.95 -11.92
-12.14 -12.12
J.A.M. van Roosmalen, E.H.P. Cordfunke/Chemical reactivity and interdiffusion
For this reason only reactions involving ZrO2 are considered. The chemical potential of ZrO2 is by definition:
14.50 © = reaction with ZY-3 [] = reaction with ZY-8
309
/ ~ ///////
gZrO2 =/~Oro2 + R T In (azro2) •
13.50
(2)
Since (Zr, Y ) O 2 is regarded as an ideal solid solution the activity equals the concentration ( 1 - z) in Zrl _zYzO2_ i/2z.
~0
_o t
12.50
11.50 5.5
i
6.0
6.5
104K/T Fig. 2. Growth constants for the formation of a La2Zr207 layer (solid line ) by the reaction between LSM-0 and ZY-3, ZY-8 and for the formation ofa SrZrO3 layer (broken line) by the reaction between LSM-50 and ZY-3, ZY-8.
supposed to be the same for the reaction with ZY-3 and ZY-8. On these grounds the activation energy is determined to be ( 17.5 _ 1.8) k J . m o l - t for the formation ofa La2Zr207reaction layer, and ( 18.8 _+1.9) kJ-mol -~ for the formation of a SrZrO3 reaction layer. These values are somewhat lower than the activation energy for the diffusion of La 3+ or Zr 4+ in La2Zr207, obtained from powder experiments by Yamamoto et al. [ 10 ]. Although the activation energy for the formation of the reaction layer is supposed to be the same in the case of ZY-3 and ZY-8, somewhat higher growth constants are found for ZY3. This is probably due to the metastable nature of the tetragonal phase in ZY-3. From the experiments with Ca and Ba (table 4) it can be concluded that, in principle, they show the same behaviour. At low Ca and Ba concentrations La2Zr207 is formed, whereas at higher concentrations there is only reaction of Ca or Ba, yielding CaZrO3 or BaZrO3.
4.2. Chemical thermodynamic considerations (Zr, Y)O2 is regarded here as an ideal solid solution. The amount of yttrium is small and no reaction products containing yttrium were observed.
The most likely defect model for oxygen excess in (La, Sr)MnO3 is cation deficiency [26 ]. No diffusion of manganese into ZrO2 and no phases containing manganese, other than the perovskite-type phase, were observed. This means that due to the reaction, La and/or Sr vacancies are formed in (La, Sr)MnO3. The pellets that were used in the experiments had a thickness of approximately 1.5 mm, while the reaction layers were smaller than 15 ~tm. The number of La a n d / o r Sr vacancies in (La, Sr)MnO3 will therefore be not larger than about 1%. The reaction between (La, Sr)MnO3 and (Z, Y)O2 can be described as: La~ _xSrxMnOs + (a-~-~)ZrO 2 = Lal_x_~Srx_#MnO3 4- o~/2LaEZr207 + f l S r Z r O 3 .
(3) Due to the rather high mobility of the vacancies in (La, Sr)MnO3 no La a n d / o r Sr depletion is found near the reaction layers. Lau and Singhal [ 9 ], and Milliken et al. [ 11 ] found Mn-rich zones, probably because the ratio of bulk (La, Sr)MnO3 to reaction products was significantly smaller in their experiments. The thermodynamic properties of (La, Sr)MnO3 are not accurately known. To be able to describe the reactions between (La, Sr)MnO3 and (Zr, Y ) O 2 thermodynamically, (La, Sr)MnO3 can be considered, in first approximation, as a solid solution of LaOL5 and SrO in (La, Sr)MnO3, since only La and/ or Sr react with ZrO2. Thus, the reactions between Lal_xSrxMn03 and Zrl_xYzOE_l/2z can be described by: [LaO,.5 ]ss + [ZrO2]ss = ½La2Zr207,
(4)
[SrO]ss + [ZrO2]ss = SrZrO3.
(5)
The activities (a) of LaO1.5 and SrO in (La, Sr)MnO3 are probably small. At equilibrium:
31 o
!
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
0 /-LLa2Zr207
I2 ILLa203 o +RTln(aL~o,~)+/zOo2+RTln(l_z),
(6) and 0 /(/SrZrO3
(7)
=l~o+RTln(as~o)+#°~oe+RTln(1-z).
The experiments have been performed at sufficiently high temperatures and long times to assume that kinetics does not play a role. The results obtained on both ZY-3 and ZY-8 (table 2) are used to estimate the temperatures at which the reactions start for pure ZrO2. The formation of LaeZr207 from (La, Sr)MnO3 and ZrO2 is estimated to start at 1150, 1210, 1300, and 1650 K for LSM=0, LSM-I 5, LSM30 and LSM-50, respectively. The formation of SrZrO3 from (La, Sr)MnO3 and ZrO2 is estimated to start at 1250, 1520, and 1650 K f o r LSM-50, LSM30, and LSM-15, respectively. The temperatures at which the reactions start are plotted in fig. 3, up to x=0.70, the solid solution limit of Sr in (La, Sr)MnO3 [27]. Fig. 3 shows that at 1300 K equilibrium is reached between La~ _xSrxMnO 3 and ZrO2 for x is about 0.30 for the reaction of LaOL5 (eq. ( 6 ) ) , and for x is about 0.45 for the reaction of SrO
1700
1500
1300
1100 0.00
0.20
0.40
0.60
x in Lal_xSrxMnO3 Fig. 3. Temperatures at which the reactions between La~_~rxMnO3 and ZrO2, forming La2Zr207or SrZrO3, are estimated to start, for various amounts of strontium (x).
(eq. (7) ). The thermodynamic functions of the pure compounds are accurately known [28], except for La2Zr207 [ 16 ]. These functions have been used to calculate that at 1300 K the Gibbs energies of for0 marion (in kJ.mol -~) are: AGLa2O~=-I425.1, AGs°~o= -458.4, AG°~o2= - 8 5 4 . 0 , AG oLa2Zr207------3229, and AG°rz~o3 = - 1414.0. Using these data the activity of LaOL5 in Lao.7Sro.3MnO3 at 1300 K is calculated to be ( 1.2 + 0.1 ) × 10- 2 from eq. ( 6 ), and the activity of SrO in Lao.55Sro.45MnO3 to be ( 8.3 + 1.2 ) × 10- 5 from eq. ( 7 ). The relatively small activities, especially for SrO, indicate large deviations from ideality.
5. Consequences for long term SOFC applications The powder experiments and thermodynamic considerations have shown that at the SOFC operation temperature, about 1300 K, (La, Sr)MnO3 reacts with ZrO2 via La up to about 30 at.% St, and via Sr above about 45 at.% Sr, forming La2Zr207 and SrZrO3, respectively. From fig. 3 it can be seen that above about 1400 K (La, Sr)MnO3 reacts with ZrO2 at all compositions. Therefore, low fabrication temperatures (up to 1400 K for x=0.37, or even lower temperatures for other compositions) should be used to prevent a reaction to occur. By extrapolation from the high-temperature growth constants (fig. 2, table 4) it can be calculated what time it would take to grow reaction layers at 1273 K for dense materials. Because LSM-15 and LSM-30 show mixed reactivity of La and Sr, which depends on temperature, extrapolation to 1273 K for this composition is not possible. For LSM-0 and LSM-50 extrapolation is possible. The time it would take to grow a layer of 1 ~tm at 1273 K is listed in table 6. In the case of LSM-15 and LSM-30 there is no reaction of Sr at 1273 K. The lower concentration of LaOL5 in LSM-I 5 and LSM-30 with respect to LSM0 will probably reduce the diffusion rate. It must be mentioned that the uncertainties have increased upon extrapolating to 1273 K. For long term operation LSM-30 seems to be the best candidate of the investigated materials, in combination with ZY-8, because no reaction was observed at 1270 for this combination. There are probably two contributions to the deg-
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion Table 6 Reaction times to form a La2Zr207 or SrZrO 3 reaction layer of 1 lam at 1273 K, from LSM-0 and LSM-50 and ZY-3 and ZY-8. The reaction times are calculated from the extrapolation of the growth constants measured at higher temperatures. t (h) to form a reaction layer of 1 lam
ZY-3 ZY-8
La2Zr207
SrZrO 3
LSM-0
LSM-50
62200 82000
29100 36600
radation of the cell performance by the formation of the reaction layers. The first contribution is due to the low conductivity of the reaction products (ohmic losses). The second contribution is due to the blocking of the oxygen transfer at the three-phase boundary between cathode, electrolyte and oxygen (polarization losses). At present it is not clear what contribution is the most severe. The present investigation shows that SOFC-cell performance can be improved in a number of possible ways. If the blocking of the oxygen transfer by the reaction layer is severe it is important to use low fabrication temperatures. The reaction rate between (La, Sr)MnO3 and (Zr, Y)O2 could be reduced further if the La and/or Sr activities in (La, Sr)MnO3 and the Zr-activity in (Zr, Y)02 is reduced. The La and/or Sr activity can be reduced by decreasing the (La, Sr):Mn ratio in (La, Sr)~_yMnO3. However, the extent o f y in (I_a, Sr)t_yMnO3 has not yet been determined experimentally. The Zr-activity in (Zr, Y)O2 can be reduced by increasing the Y-content; this will reduce the ionic conductivity only slightly.
Acknowledgement The authors wish to thank G. Hamburg for his advice concerning electron microscopy photographs and energy dispersive X-ray spectroscopy.
311
References [ 1 ] J.P.P. Huijsmans, E.J. Siewers, F.H. van Heuveln and J.P. de Jong, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 113-118. [2]J.A.M. van Roosmalen, J.P.P. Huijsmans and E.H.P. Cordfunke, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 507-516. [ 3 ] C.S. Tedmon Jr., H.S. Spacil and S.P. Mitoff, J. Electrochem. Soc. 116 (1969) ll70. [4] O. Yamamoto, Y. Takeda, R. Kanno and M. Noda, Solid State Ionics 22 (1987) 241. [5] J. Echigoya, S. Hiratsuka and H. Suto Mater. Trans., JIM 30 (1989) 789. [6] T. Setoguchi, T. Inoue, H. Takebe, K. Eguchi, K. Morinaga and H. Arai, Solid State Ionics 37 (1990) 217. [ 7 ] W. Baukal, W. Kuhn, H. Kleinsehmager and F.J. Rohr, J. Power Sources l (1976/1977) 203. [8] M. Moil, N. Sakai, T. Kawada, H. Yokokawa and M. Dokiya, Denki Kagaku 58 (1990) 528. [ 9 ] S.K. Lau and S.C. Singhal, Extended Abstracts of 1985 Fuel Cell Seminar (Tucson, AZ, 1985) pp. 107-110. [ 10] O. Yamamoto, Y. Takeda, R. Kanno and T. Kojima, Proc. First Intern. Symp. Solid Oxide Fuel Cells, ed. S.C. Singhal (Hollywood, FL, 1989) pp. 242-253; Proc. SOFC-Nagoya, (Nagoya, 1989) pp. 87-92. [ I 1]C. Milliken, D. Tucker, S. Elangovan and A. Khandkar, Extended Abstracts of 1990 Fuel Cell Seminar, (Phoenix, AZ, Nov. 22-25, 1990) pp. 218-221. [ 12 ] H. Kaneko, H. Taimatsu, K. Wada and E. Iwamoto, Proc. Second Intern. Syrup. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 673-680. [ 13 ] H. Tagawa, J. Mizusaki, M. Katou, K. Hirano, A. Sawata and K. Tsuneyoshi, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 681-688. [14]J.A. Labrincha, J.R. Frade and F.M.B. Marques, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ). pp. 689-696. [ 15 ] T. Sasamoto, W.R. Cannon and H.K. Bowen, Ceram. Int. 7 (1981) 3. [ 16 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 57 (1989) 821. [ 17 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 58 (1990) 162. [ 18 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 58 (1990) 489. [ 19 ] A better formulation is Lat_~SrxMnO3+6, but for simplicity this will not be used. [ 20 ] A better formulation is Zr~_xYzO2_ t~, but for simplicity this will not be used. [211T. Hashimoto, N. Ishizawa, N. Mizutani and M. Kato, J. Mater. Sci. 23 (1988) 1102.
312
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
[22]J.A.M. van Roosmalen, P. van Vlaanderen and E.H.P. Cordfunke, J. Solid State Chem., accepted for publication. [23] J. Roth, J. Res. Nat. Bur. Std. 56 (1956) 17. [24] A.D. Smigelskas and E.O. Kirkendall, Trans. AIME 171 (1947) 130. [25 ] N. Suriyayothin and N.G. Eror, J. Mater. Sci. 19 (1984) 2775.
[ 26 ] B.C. Tofield and W.R. Scott, J. Solid State Chem. 10 (1974) 183. [27] G.H. Jonker, Physica 22 (1956) 707. [28 ] E.H.P. Cordfunke and R.J.M. Konings, eds. Thermochemical Data for Reactor Materials and Fission Products (North-Holland, Amsterdam, 1990).
Solid State Ionics 52 (1992) 303-312 North-Holland
Chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2, solid oxide fuel cell cathode and electrolyte materials J.A.M. van R o o s m a l e n a n d E.H.P. C o r d f u n k e Netherlands Energy Research Foundation ECN, P. 0. Box 1, 1755 ZG Petten, The Netherlands
Received 17 September 1991; accepted for publication 18 March 1992
The chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2 were studied from 1110 to 1755 K. Reaction of LaMnO3 with (Zr, Y)O2 was already observed at 1170 K, whereas reactions between (La, Sr)MnO3 with 30 at.% Sr and (Zr, Y)O2 with 8 at.% Y were not observed at 1365 K. The reaction products observed in the experiments are L a 2 Z r 2 0 7 and/or SrZrO3. It is proposed that reaction layers are formed by diffusion of La and/or Sr into (Zr, Y)O2 via a vacancy diffusion mechanism. The composition of the layers depends on the La203 and SrO activities in (La, Sr)MnO3. The activation energy for the formation of a La2Zr207 reaction layer was determined to be 17.5_+1.8 kJ- mol- ~, the activation energy for the formation of a SrZrO3 reaction layer was determined to be 18.8 _+1.9 kJ. mol- 1. On the basis of the experiments it was calculated that at 1273 K it would take about 29100 h to grow a reaction layer of SrZrO3 from (Lao.sSr0.5)MnO3and (Zro.97Yo.o3)OI.985,and about 82000 h to grow a reaction layer o f La2Zg20 7 from LaMnO3 and ( Z r o . 9 2 Y o . o s ) O i . 9 6 , with a thickness of I ~tm for both layers, It is proposed that the reaction layers might result in both ohmic and polarization losses of the SOFC.
1. Introduction The d e v e l o p m e n t o f Solid Oxide Fuel Cells ( S O F C ' s ) has been given new attention during the last few years. Especially the i n t r o d u c t i o n o f the flat plate S O F C concept has stimulated research activities, for instance, at E C N [ 1,2 ]. Some o f the advantages o f the flat plate design are a high power output and short conductivity paths with respect to the tubular or the h o n e y c o m b design. To commercialize S O F C reactors a n u m b e r o f technological and chemical p r o b l e m s still have to be solved. State-of-the-art cathode material is (La, Sr)MnO3, but it is not yet clear what percentage o f strontium should be used, The state-of-the-art electrolyte material consists o f ZrO2 solid solutions with Y203. One o f the requirements for successful operation is the t h e r m o d y n a m i c stability o f the cathode with respect to the electrolyte. A n u m b e r o f materials that have been tested as cathode materials app e a r to react with the electrolyte [ 3 - 1 4 ] , forming badly conducting c o m p o u n d s [ 11,15 ]. F o r a number o f c o m p o u n d s this reaction has been p r e d i c t e d by means o f t h e r m o d y n a m i c considerations [ 16-18 ].
In this p a p e r results are reported o f an investigation on the reaction between (La, Sr)MnO3 [19] with 0, 15, 30, and 50 at.% Sr and (Zr, Y)O2 [20] with 3 and 8 at.% Y as a function o f time and temperature. The results can be interpreted on the basis o f layer growth a n d chemical t h e r m o d y n a m i c considerations. F o r c o m p a r i s o n some experiments were conducted with (La, C a ) M n O 3 and (La, Ba)MnO3. Throughout the p a p e r (La, Sr)MnO3 with 0, 15, 30 a n d 50 at.% Sr will be referred to as LSM-0, LSM15, LSM-30 and LSM-50, respectively, likewise for La substitution by Ca and Ba. The electrolyte (Zr, Y)O2 with 3 and 8 at.% Y will be referred to as ZY3 and ZY-8, respectively.
2. Experimental The (La, A ) M n O 3 ( A = C a , St, Ba) samples were prepared by a co-precipitation method, similar to that described by H a s h i m o t o [21 ]. According to the Xray diffraction patterns all samples were single phase perovskite-type oxides. Z Y - 3 a n d ZY-8 were obt a i n e d from TOSOH. Dense pellets (>_ 90% o f the-
0167-2738/92/$ 05,00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
304
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
oretical density) of all materials were obtained by sintering at 1655 K in air. Sintering was not performed at the highest reaction temperature ( 1755 K), because in some cases excessive grain growth occurred. Two types of experiments were performed: (a) diffusion experiments, in which pellets were put together in the order ZY-3 ILSM-01ZY-81LSM-151 etc., and held together in the hot part of the furnace by applying an external pressure. In some experiments a small strip of platinum was sputtered on the surface of the (Zr, Y)O2 pellets as a marker for the diffusion reaction. To analyse the experiments with SEM and EDS the pellets were cut perpendicular to the surface as well as to the platinum strip; (b) reactivity experiments, in which (La, Sr)MnO3 and (Zr, Y)O2 powders ( 1:1 ) were thoroughly mixed, pressed into pellets and heated at various temperatures in air. X-ray patterns were taken to follow the formation of the reaction products. To analyse the X-ray patterns obtained after the experiments, SrZrO3 and La2Zr207 were synthesized. The structure of SrZrO3 is cubic with a doubled axis with respect to the simple perovskite structure [ 22 ]. La2Zr207 has a cubic pyrochlore structure [23]. All intensities in the X-ray pattern corresponded with cubic La2Zr207, except for three small intensities which could not be attributed to impurities. It is suggested that these intensities are due to a small tetragonal distortion. Since the sample was synthesized at 2070 K in an argon atmosphere, a small oxygen deficiency may cause the extra intensities, although they did not disappear after retiring at 1420 K in air. The colour of the sample after both treatments was white. Despite the small extra intensities the sample is suitable for comparison with possible reaction products. Chemical analyses were performed by Inductively Coupled Emission Spectroscopy (ICP), the results are listed in table 1. SEM photographs were recorded on a JEOL JXA-840 scanning microanalyzer, equipped with energy dispersive X-ray spectroscopy (EDS) for identification of the reaction products. With this technique it is not possible to detect Y in ZY-3 and ZY-8 because of band-broadening. In this article it is assumed that Y has the same behaviour as Zr, since it is in solid solution. It is also difficult to detect Sr, because of overlap between a small Zr-
Table 1 Chemical analysis (ICP) of (La,Sr)MnO3 materials obtained from co-precipitation, values relative to the Mn-content. (La,Sr)MnO3
La/Mn
Sr/Mn
(La,Sr)/Mn
LSM-O LSM-15 LSM-30 LSM-50
1.001 0.858 0.704 0.501
O.148 0.301 0.506
1.001 1.006 1.005 1.007
adsorption and the major Sr-adsorption. In recordings of elemental distribution this can be confusing, but fortunately relatively large amounts of Sr can be identified without problems. X-ray patterns of starling materials and reaction products were recorded on a Guinier camera using Cu Kcq.2 radiation (0.15418 nm).
3. Results The kinetics of the reaction between (La, Sr) MnO3 and (Zr, Y ) 0 2 were studied by powder experiments in the lower temperature region ( 1110-1505 K), and pressed pellets experiments at higher temperatures (1455-1755 K). The reaction time for the powder experiments was 110 h, and 260 or 596 h for the pressed pellets experiments. The reaction products which were observed at the various temperatures are listed in table 2. The phases in the reaction layers were identified to be SrZrO3 and La2Zr207 by X-ray diffraction. The thicknesses of the diffusion layers, which were observed in the various experiments, were determined from the SEM recordings and are listed in table 3 for (La, Sr)MnO3 and in table 4 for (La, Ca)MnO3 and (La, Ba)MnO3. The SEM recordings cover only a small part of the entire reaction layer. Care was taken to select a representative part of the layer. The thickness of the layer was determined by hand from the SEM recordings. The error that can be made by this procedure is estimated to be 10% for thick layers to 15% for thin layers. Information about the diffusion behaviour was obtained from SEM backscatter recordings in combination with energy dispersive X-ray spectroscopy. Some SEM/EDS recordings of observed diffusion layers (with and without platinum markers) are
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and inlerdiffusion
305
Table 2 Results of reactivity experiments. The products of the reaction between (La,Sr)MnO3 and (Zr, Y)02 are indicated. The powder experiments were conducted at 1110-1505 K, the pressed pellets experiments were conducted at 1455-1755 K. T(K)
ZY-3
ZY-8
LSM-0
LSM- 15 .
LSM-50
LSM-0
-
LZ
LSM-50
-
-
-
-
-
-
. LZ
1270
LZ
LZ
LZ
SZ b )
LZ
LZ
-
SZ
1360
LZ LZ LZ LZ LZ
LZ LZ LZ LZ/SZ LZ/SZ
LZ LZ LZ/SZ LZ/SZ LZ/SZ
SZ SZ SZ LZ/SZ LZ/SZ
LZ LZ LZ LZ LZ
LZ LZ LZ LZ/SZ LZ/SZ
LZ LZ LZ/SZ LZ/SZ LZ/SZ
SZ SZ SZ LZ/SZ LZ/SZ
a) LZ=La:Zr2OT;
¢)
LSM-30
1110
")
.
LSM- ! 5
1170
1505 1555 1655 1755
.
LSM-30
-
b) SZ=SrZrO3;
¢) No reaction products observed.
Table 3 Diffusion layers that are formed in (Zr, Y)O2 by the reaction of (La, Sr)MnO3 with (Zr, Y)O2 at various reaction times and temperatures. Layer thickness is listed in micrometers. T(K)
t (h)
ZY-3
ZY-8
LSM-0
LSM- 15
1455 1555 1655
260 596 260 596
c) 1.8 a) 2.7 `') 3.5 ")
c) 0.9 a) c) 2.5 `')'b)
1755
260 596
5.0`') 7.3 a) 7.4 a)
c) 8.2 ") 8.5 ~)
LSM-30
LSM-50
LSM-0
0.3 a) 1.4 a)'b) 4.1 ,,).b) 2.5 `')
¢) 3.0 b) c) 7.3 ")'b)
c) 1.6 ~) 2.0 ~) 2.9 `0
8,6 ~).b) 13.0 `')'b) 16.4 `'). b)
¢) 15.5 TM 16.0 ~). b)
4.5,) 6.5 ~) 7.4`')
LSM- 15 ¢) 0.7 `') ¢) 1.8 ~) c) 5.6 a) 10.4 .), b)
LSM-30 _a) 0.5 a)'b) 2.7,0.b) 3.6 a)'b) 5.5 a). b) 7.3 a).b) 6.8 ~) 10.0 a). b)
LSM-50 c) 2.7 b) ¢) 8.0 b) ¢) 12.4 b) 12.7 b)
`') Formation ofLa2Zr207 ; b) Formation ofSrZrO3 ; ¢) No experiment ; d) NO reaction products observed. Table 4 Diffusion layers that are formed in (Zr, Y)O2 by the reaction of (La, Ca)MnO3 and (l_a, Ba)MnO3 with (Zr, Y)O2 during 110 h at 1715 K. Layer thickness is listed in micrometers.
ZY-3 ZY-8
LCM-15
LCM-30
LBM-15
LBM-30
8.9 `') 6.1 b)
12.1 `'),b) 22.3 a)
5.4 b) 7.2 a), b)
8.5 ¢) 7.3 ¢)
a) Formation of CaZrO3, mation of BaZrO 3.
b) Formation ofLa2Zr207 ; ¢) For-
shown in fig. 1. From the SEM backscatter photographs and EDS recordings there are a number of indications for the reaction and diffusion mechanism: (a) The platinum markers were sputtered on the surface of the (Zr, Y)O2 pellets. After the reaction they are located on top of the reaction layers (fig.
lg). It is evident that there is no diffusion of Zr(Y) into (La, Sr)MnO3 (see also fig. la, b); only diffusion of La and Sr into iZr, Y)O2 was observed (fig. la-c, g-j). (b) Diffusion of Mn from (La, Sr)MnO3 into (Zr, Y)O2 reported by Lau and Singhal [9] and Milliken et al. [ 11 ], could not be observed in our experiments (fig. la, c). The reaction mechanism will be discussed in paragraph 4.2; (c) There is a simultaneous diffusion of La and Sr resulting in a peculiar diffusion pattern with La-rich and Sr-rich regions (fig. l d - g ) . However, depending on the St-concentrations, diffusion of La or Sr apart, resulting in La2Zr207 (fig. la, h) or SrZrO3 layers (fig. 1i, j ) is also observed. We will come back to this later. (d) At 1755 K excessive grain growth was ob-
306
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
;
•
~.~i
'
i. ¸
r ',
,
Fig. 1. (a) Backscatter recording of the reaction layer formed in the reaction between ZY-8 (left-hand side) and LSM-30 (right-hand side ) at 1755 K, 596 h, showing a La2Zr207 reaction layer (no SrZrO3 ); (b) zirconium distribution belonging to I a. It is very clear that the Zr-concentration in the reaction layer is lower than in bulk ZY-8; (c) manganese distribution belonging to la. There is no Mn in the reaction layer and no diffusion of Mn into ZY-8; (d) backscatter recording of the reaction layer formed in the reaction between ZY-3 and LSM-30 at 1755 K, 596 h. Only the ZY-3 part is shown, showing a reaction layer of La2Zr207 (bright pattern) and SrZrO 3, a dense layer of (Zr, Y)O2, and grain growth in bulk ZY-3; (e) strontium distribution belonging to ld (overlap of Zr) showing the distribution of Sr in the reaction layer. No Sr is observed in the La2Zr207 regions; (f) lanthanum distribution belonging to 1d, showing the distribution of La over the reaction layer. La is also observed in the SrZrO3 regions.
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
307
0~
.......
2 r ~ ¢
;".; i .. O~ O
i 0 l: r~,
2 '37f~
20!::~i
Fig. 1. Continued. (g) Backscatter recording of the reaction layer formed in the reaction between ZY-8 (upper part ) and LSM-30 (lower part) at 1755 K, 596 h, showing Pt-markers (bright spots) and a reaction layer of La2Zr207 (clearly visible) and SrZrO3 (slightly different colour); (h) backscatter recording of the reaction layer formed in the reaction between ZY-8 (right-hand side) and LSM-0 (left-hand side ) at 1755 K, 596 h, showing a La2Zr207 reaction layer and holes due to the Kirkendal! effect; (i) backscatter recording of the reaction layer formed in the reaction between ZY-8 (right-hand side) and LSM-50 (left-hand iside) at 1655 K, 596 h, showing a SrZrO3 reaction layer; (j) strontium distribution belonging to l i. Due to overlap between a small reflection of Zr with the major reflection of Sr there also appears to be Sr in bulk ZY-8. The reaction layer, however, is clearly resolved.
served in ZY-3 during the reaction. However, between the reaction layer a n d the grains a dense layer o f (Zr, Y)O2 is formed. This indicates Zr-diffusion against its chemical potential gradient (fig. l d - f ) . (e) In some cases pores are f o r m e d on the (La, Sr)MnO3 side o f the reaction layer (fig. l h ) . This is known as the Kirkendall effect [24] a n d is indicative for a vacancy diffusion mechanism, resulting in a net mass flow. ( f ) In the case where there is m i x e d diffusion o f La a n d Sr, La-rich a n d Sr-rich regions are f o r m e d in
the reaction layer. There is almost no Sr in the Larich regions, whereas there is a significant a m o u n t o f La in the Sr-rich region (fig. l d - f ) . This indicates that tho solubility o f Sr in La2Zr207 is very small, while there is a certain solubility o f La in SrZrO3. The solubility limit o f La in SrZrOa is r e p o r t e d to be a p p r o x i m a t e l y 6 at.% [25].
308
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
4. Discussion 4.1. Diffusion e x p e r i m e n t s
It is very likely that an initial reaction layer is formed by reaction of La (and/or Sr) from the surface of (La, Sr)MnO3 with Zr from the surface of (Zr, Y)O2. The experiments with the Pt markers (fig. I g) indicate that the growth of the layers can be described by a diffusion of La and/or Sr through the reaction layer into the ZrO2 lattice. From the location of the Pt markers at the top of the reaction layers it is concluded that there is probably no Zr diffusing through the layer. Apart from the reaction layer, another layer is formed between the reaction layer and (Zr, Y)O2 (fig. ld). The formation of this dense (Zr, Y)O2 layer, that does not contain La and/ or Sr, is caused by the diffusion of La and/or Sr into the reaction layer. This layer is probably formed also in the other reactions (fig. la, g-i), but cannot be seen separately, because the (Zr, Y)O2-pellet already was dense in these experiments. The composition of the initial reaction layer depends on the La and Sr-activity in (La, Sr)MnO3. At low Sr-activities the formation of La2Zr207 is more favourable then that of SrZrO3, whereas at 50% Sr the formation of SrZrO3 is more favourable. In intermediate situations the initial reaction layer can be composed of both La and Sr. Because of the different diffusion rates for Sr and La, and the relatively small solubility of Sr in La2Zr207 (fig. l d - f ) , La- and Srrich regions can be formed in the diffusion layer. It should be noted, however, that in some cases, reaction of both La and Sr was expected on the basis of the experimental results (table 2), while only a La2Zr207 layer was observed (compare fig. la with fig. lg). There is no obvious explanation for these differences. The layer growth is determined by the smallest diffusion constant of the processes that are necessary to form the reaction layer. If the anionic conductivity is supposed to be several orders of magnitude higher than the cationic conductivity the rate determining processes could be the supply of La and/or Sr to the reaction layer, by diffusion of cations in (La, Sr)MnO3, or the diffusion of La and/or Sr into the reaction layer. From the absence of the Kirkendall effect (fig. lh) in the majority of the observed re-
actions it is concluded that the diffusion of La (and/ or Sr) through the reaction layer (La2Zr207 and/or SrZrO3) is the rate determining step. The analysis of the diffusion layers is complicated in the situation where both La2Zr207 and SrZrO3 are present: it is difficult to establish the relative amounts of both phases. It is suggested that the diffusion in the La-rich regions is determined by the diffusion coefficient of La alone and in the Sr-rich regions by the diffusion coefficient of Sr alone, although there is a slight solubility of La in SrZrO3. Fortunately, the reaction between LSM-50 and (Zr, Y)O2 is almost only the result of Sr-diffusion. The diffusion coefficient (growth constant of the reaction layer) of La can be determined from the reaction between LSM-0 and (Zr, Y )02. Diffusion can be described by Fick's law, which can be simplified to: (l)
x2=D.t ,
in which x is the layer thickness in cm, D is the chemical diffusion coefficient, here growth constant, t, the reaction time in seconds. Although cracks and porosity were observed in the reaction layers of the dense pellets used, the results, nevertheless, indicate that Fick's law can be used within experimental error. The activation energy for the diffusion can be obtained from the Tafel plot of log D versus l / T. The experimental results (table 3) have been evaluated and the calculated growth constants are listed in table 5 and plotted in fig. 2. The activation energy is Table 5 Growth constants for the formationof La2Zr207and SrZrOareaction layersat varioustemperatures,calculatedfrom the experimental results in table 3. T(K)
log(D s/cm 2) La2Zr207
1555 1655 1755
SrZrO3
LSM-0 ZY-3
LSM-0 ZY-8
LSM-50 ZY-3
LSM-50 ZY-8
-13.82 -13.11 - 13.24 -12.57 - 12.60 -12.59
-13.92 -13.37 -13.41 - 12.66 -12.71 -12.59
-13.38 -12.60
-13.47 -12.53
-11.95 -11.92
-12.14 -12.12
J.A.M. van Roosmalen, E.H.P. Cordfunke/Chemical reactivity and interdiffusion
For this reason only reactions involving ZrO2 are considered. The chemical potential of ZrO2 is by definition:
14.50 © = reaction with ZY-3 [] = reaction with ZY-8
309
/ ~ ///////
gZrO2 =/~Oro2 + R T In (azro2) •
13.50
(2)
Since (Zr, Y ) O 2 is regarded as an ideal solid solution the activity equals the concentration ( 1 - z) in Zrl _zYzO2_ i/2z.
~0
_o t
12.50
11.50 5.5
i
6.0
6.5
104K/T Fig. 2. Growth constants for the formation of a La2Zr207 layer (solid line ) by the reaction between LSM-0 and ZY-3, ZY-8 and for the formation ofa SrZrO3 layer (broken line) by the reaction between LSM-50 and ZY-3, ZY-8.
supposed to be the same for the reaction with ZY-3 and ZY-8. On these grounds the activation energy is determined to be ( 17.5 _ 1.8) k J . m o l - t for the formation ofa La2Zr207reaction layer, and ( 18.8 _+1.9) kJ-mol -~ for the formation of a SrZrO3 reaction layer. These values are somewhat lower than the activation energy for the diffusion of La 3+ or Zr 4+ in La2Zr207, obtained from powder experiments by Yamamoto et al. [ 10 ]. Although the activation energy for the formation of the reaction layer is supposed to be the same in the case of ZY-3 and ZY-8, somewhat higher growth constants are found for ZY3. This is probably due to the metastable nature of the tetragonal phase in ZY-3. From the experiments with Ca and Ba (table 4) it can be concluded that, in principle, they show the same behaviour. At low Ca and Ba concentrations La2Zr207 is formed, whereas at higher concentrations there is only reaction of Ca or Ba, yielding CaZrO3 or BaZrO3.
4.2. Chemical thermodynamic considerations (Zr, Y)O2 is regarded here as an ideal solid solution. The amount of yttrium is small and no reaction products containing yttrium were observed.
The most likely defect model for oxygen excess in (La, Sr)MnO3 is cation deficiency [26 ]. No diffusion of manganese into ZrO2 and no phases containing manganese, other than the perovskite-type phase, were observed. This means that due to the reaction, La and/or Sr vacancies are formed in (La, Sr)MnO3. The pellets that were used in the experiments had a thickness of approximately 1.5 mm, while the reaction layers were smaller than 15 ~tm. The number of La a n d / o r Sr vacancies in (La, Sr)MnO3 will therefore be not larger than about 1%. The reaction between (La, Sr)MnO3 and (Z, Y)O2 can be described as: La~ _xSrxMnOs + (a-~-~)ZrO 2 = Lal_x_~Srx_#MnO3 4- o~/2LaEZr207 + f l S r Z r O 3 .
(3) Due to the rather high mobility of the vacancies in (La, Sr)MnO3 no La a n d / o r Sr depletion is found near the reaction layers. Lau and Singhal [ 9 ], and Milliken et al. [ 11 ] found Mn-rich zones, probably because the ratio of bulk (La, Sr)MnO3 to reaction products was significantly smaller in their experiments. The thermodynamic properties of (La, Sr)MnO3 are not accurately known. To be able to describe the reactions between (La, Sr)MnO3 and (Zr, Y ) O 2 thermodynamically, (La, Sr)MnO3 can be considered, in first approximation, as a solid solution of LaOL5 and SrO in (La, Sr)MnO3, since only La and/ or Sr react with ZrO2. Thus, the reactions between Lal_xSrxMn03 and Zrl_xYzOE_l/2z can be described by: [LaO,.5 ]ss + [ZrO2]ss = ½La2Zr207,
(4)
[SrO]ss + [ZrO2]ss = SrZrO3.
(5)
The activities (a) of LaO1.5 and SrO in (La, Sr)MnO3 are probably small. At equilibrium:
31 o
!
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
0 /-LLa2Zr207
I2 ILLa203 o +RTln(aL~o,~)+/zOo2+RTln(l_z),
(6) and 0 /(/SrZrO3
(7)
=l~o+RTln(as~o)+#°~oe+RTln(1-z).
The experiments have been performed at sufficiently high temperatures and long times to assume that kinetics does not play a role. The results obtained on both ZY-3 and ZY-8 (table 2) are used to estimate the temperatures at which the reactions start for pure ZrO2. The formation of LaeZr207 from (La, Sr)MnO3 and ZrO2 is estimated to start at 1150, 1210, 1300, and 1650 K for LSM=0, LSM-I 5, LSM30 and LSM-50, respectively. The formation of SrZrO3 from (La, Sr)MnO3 and ZrO2 is estimated to start at 1250, 1520, and 1650 K f o r LSM-50, LSM30, and LSM-15, respectively. The temperatures at which the reactions start are plotted in fig. 3, up to x=0.70, the solid solution limit of Sr in (La, Sr)MnO3 [27]. Fig. 3 shows that at 1300 K equilibrium is reached between La~ _xSrxMnO 3 and ZrO2 for x is about 0.30 for the reaction of LaOL5 (eq. ( 6 ) ) , and for x is about 0.45 for the reaction of SrO
1700
1500
1300
1100 0.00
0.20
0.40
0.60
x in Lal_xSrxMnO3 Fig. 3. Temperatures at which the reactions between La~_~rxMnO3 and ZrO2, forming La2Zr207or SrZrO3, are estimated to start, for various amounts of strontium (x).
(eq. (7) ). The thermodynamic functions of the pure compounds are accurately known [28], except for La2Zr207 [ 16 ]. These functions have been used to calculate that at 1300 K the Gibbs energies of for0 marion (in kJ.mol -~) are: AGLa2O~=-I425.1, AGs°~o= -458.4, AG°~o2= - 8 5 4 . 0 , AG oLa2Zr207------3229, and AG°rz~o3 = - 1414.0. Using these data the activity of LaOL5 in Lao.7Sro.3MnO3 at 1300 K is calculated to be ( 1.2 + 0.1 ) × 10- 2 from eq. ( 6 ), and the activity of SrO in Lao.55Sro.45MnO3 to be ( 8.3 + 1.2 ) × 10- 5 from eq. ( 7 ). The relatively small activities, especially for SrO, indicate large deviations from ideality.
5. Consequences for long term SOFC applications The powder experiments and thermodynamic considerations have shown that at the SOFC operation temperature, about 1300 K, (La, Sr)MnO3 reacts with ZrO2 via La up to about 30 at.% St, and via Sr above about 45 at.% Sr, forming La2Zr207 and SrZrO3, respectively. From fig. 3 it can be seen that above about 1400 K (La, Sr)MnO3 reacts with ZrO2 at all compositions. Therefore, low fabrication temperatures (up to 1400 K for x=0.37, or even lower temperatures for other compositions) should be used to prevent a reaction to occur. By extrapolation from the high-temperature growth constants (fig. 2, table 4) it can be calculated what time it would take to grow reaction layers at 1273 K for dense materials. Because LSM-15 and LSM-30 show mixed reactivity of La and Sr, which depends on temperature, extrapolation to 1273 K for this composition is not possible. For LSM-0 and LSM-50 extrapolation is possible. The time it would take to grow a layer of 1 ~tm at 1273 K is listed in table 6. In the case of LSM-15 and LSM-30 there is no reaction of Sr at 1273 K. The lower concentration of LaOL5 in LSM-I 5 and LSM-30 with respect to LSM0 will probably reduce the diffusion rate. It must be mentioned that the uncertainties have increased upon extrapolating to 1273 K. For long term operation LSM-30 seems to be the best candidate of the investigated materials, in combination with ZY-8, because no reaction was observed at 1270 for this combination. There are probably two contributions to the deg-
J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion Table 6 Reaction times to form a La2Zr207 or SrZrO 3 reaction layer of 1 lam at 1273 K, from LSM-0 and LSM-50 and ZY-3 and ZY-8. The reaction times are calculated from the extrapolation of the growth constants measured at higher temperatures. t (h) to form a reaction layer of 1 lam
ZY-3 ZY-8
La2Zr207
SrZrO 3
LSM-0
LSM-50
62200 82000
29100 36600
radation of the cell performance by the formation of the reaction layers. The first contribution is due to the low conductivity of the reaction products (ohmic losses). The second contribution is due to the blocking of the oxygen transfer at the three-phase boundary between cathode, electrolyte and oxygen (polarization losses). At present it is not clear what contribution is the most severe. The present investigation shows that SOFC-cell performance can be improved in a number of possible ways. If the blocking of the oxygen transfer by the reaction layer is severe it is important to use low fabrication temperatures. The reaction rate between (La, Sr)MnO3 and (Zr, Y)O2 could be reduced further if the La and/or Sr activities in (La, Sr)MnO3 and the Zr-activity in (Zr, Y)02 is reduced. The La and/or Sr activity can be reduced by decreasing the (La, Sr):Mn ratio in (La, Sr)~_yMnO3. However, the extent o f y in (I_a, Sr)t_yMnO3 has not yet been determined experimentally. The Zr-activity in (Zr, Y)O2 can be reduced by increasing the Y-content; this will reduce the ionic conductivity only slightly.
Acknowledgement The authors wish to thank G. Hamburg for his advice concerning electron microscopy photographs and energy dispersive X-ray spectroscopy.
311
References [ 1 ] J.P.P. Huijsmans, E.J. Siewers, F.H. van Heuveln and J.P. de Jong, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 113-118. [2]J.A.M. van Roosmalen, J.P.P. Huijsmans and E.H.P. Cordfunke, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 507-516. [ 3 ] C.S. Tedmon Jr., H.S. Spacil and S.P. Mitoff, J. Electrochem. Soc. 116 (1969) ll70. [4] O. Yamamoto, Y. Takeda, R. Kanno and M. Noda, Solid State Ionics 22 (1987) 241. [5] J. Echigoya, S. Hiratsuka and H. Suto Mater. Trans., JIM 30 (1989) 789. [6] T. Setoguchi, T. Inoue, H. Takebe, K. Eguchi, K. Morinaga and H. Arai, Solid State Ionics 37 (1990) 217. [ 7 ] W. Baukal, W. Kuhn, H. Kleinsehmager and F.J. Rohr, J. Power Sources l (1976/1977) 203. [8] M. Moil, N. Sakai, T. Kawada, H. Yokokawa and M. Dokiya, Denki Kagaku 58 (1990) 528. [ 9 ] S.K. Lau and S.C. Singhal, Extended Abstracts of 1985 Fuel Cell Seminar (Tucson, AZ, 1985) pp. 107-110. [ 10] O. Yamamoto, Y. Takeda, R. Kanno and T. Kojima, Proc. First Intern. Symp. Solid Oxide Fuel Cells, ed. S.C. Singhal (Hollywood, FL, 1989) pp. 242-253; Proc. SOFC-Nagoya, (Nagoya, 1989) pp. 87-92. [ I 1]C. Milliken, D. Tucker, S. Elangovan and A. Khandkar, Extended Abstracts of 1990 Fuel Cell Seminar, (Phoenix, AZ, Nov. 22-25, 1990) pp. 218-221. [ 12 ] H. Kaneko, H. Taimatsu, K. Wada and E. Iwamoto, Proc. Second Intern. Syrup. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 673-680. [ 13 ] H. Tagawa, J. Mizusaki, M. Katou, K. Hirano, A. Sawata and K. Tsuneyoshi, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ) pp. 681-688. [14]J.A. Labrincha, J.R. Frade and F.M.B. Marques, Proc. Second Intern. Symp. Solid Oxide Fuel Cells, eds. F. Grosz, P. Zegers, S.C. Singhal and O. Yamamoto (Athens, 1991 ). pp. 689-696. [ 15 ] T. Sasamoto, W.R. Cannon and H.K. Bowen, Ceram. Int. 7 (1981) 3. [ 16 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 57 (1989) 821. [ 17 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 58 (1990) 162. [ 18 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 58 (1990) 489. [ 19 ] A better formulation is Lat_~SrxMnO3+6, but for simplicity this will not be used. [ 20 ] A better formulation is Zr~_xYzO2_ t~, but for simplicity this will not be used. [211T. Hashimoto, N. Ishizawa, N. Mizutani and M. Kato, J. Mater. Sci. 23 (1988) 1102.
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J.A.M. van Roosmalen, E.H.P. Cordfunke / Chemical reactivity and interdiffusion
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[ 26 ] B.C. Tofield and W.R. Scott, J. Solid State Chem. 10 (1974) 183. [27] G.H. Jonker, Physica 22 (1956) 707. [28 ] E.H.P. Cordfunke and R.J.M. Konings, eds. Thermochemical Data for Reactor Materials and Fission Products (North-Holland, Amsterdam, 1990).