Materials for electrodes based on rare earth manganites

Materials for electrodes based on rare earth manganites

SOLID STATE ELSEVIER Solid State Ionics 68 (1994) 177-184 IONICS Materials for electrodes based on rare earth manganites P. Shuk, L. Tichonova Rese...

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SOLID STATE ELSEVIER

Solid State Ionics 68 (1994) 177-184

IONICS

Materials for electrodes based on rare earth manganites P. Shuk, L. Tichonova Research Institute for Physico-Chemical Problems, Byelorussian University, 220080 Minsk, Belarus U. Guth InstitutJ~r Physikalische Chemic, Ernst-Moritz-Arndt-University of Greifswald, 2200 Greifswald, Germany Received 18 May 1992; accepted for publication 8 November 1993

Abstract

For application in high temperature fuel cells and oxygen sensors, alkaline earth doped lanthanoide manganites have been studied systematically with regard to preparation conditions, structures, electrical conductivities and thermal expansion coefficients, respectively. La, _xSr(Ca)xMnO3, (x= 0-0.5 ) show a cubic perovskite structure for Ca-containing at x = 0.2-0.5 and for Sr-containing at x=0.5. For Gd(Nd)~_xCaxMnO3 (x=0-0.5) an orthorhombic distorted perovskite structure was found. Yb(Y)~_xCaxMn03 shows in the range of x=0.3-0.5 an orthorhombic structure, whereas for the pure Yb(Y)MnO3 the hexagonal structure was determined. A correlation between the conductivity of doped lanthanum manganites and the concentration of Mn 4+ could be found. The suitability of these materials as an electrode for zirconia cells has also been studied in detail. At temperatures under 1073 K the polarization resistivity of the best electrodes based upon Lao.TCa0.3MnO3 is much smaller than such made of active platinum. Electrodes were tested in solid electrolyte fuel cells.

1. Introduction

Over the last 30 years, binary oxides with a perovskite-like structure o f the type ABO3 (B = Cr, Mn, Fe, Co, N i ) have become increasingly important, not only as an electrode material for electrochemical cells with both aqueous and solid electrolytes, but also as a catalyst for various oxidation and reduction processes [ 1-8 ]. F o r high t e m p e r a t u r e application in fuel cells and sensors, the character o f the gas phase a n d the working t e m p e r a t u r e represent i m p o r t a n t limiting factors in the use o f such perovskite-type materials. In spite o f the high electrical conductivity o f rare earth cobaltites a n d nickelites (higher than 1000 S / c m ) , these materials are unsuitable as an electrode material, both because o f their low stability under reducing conditions, a n d because o f their high thermal ex-

pansion coefficient as c o m p a r e d with solid electrolyte materials based on stabilized zirconia [ 9,10 ]. Rare earth chromites a n d ferrites exhibit a low conductivity (below 1 S / c m ) [ 11,12 ]. At 1273 K the chemical stability as expressed by the d e c o m p o s i t i o n pressure progressively falls in the o r d e r LaCrO3 (up to 10-16 Pa ), LaFeO3, LaMnO3 ( 1 0 - i I Pa ), LaCoO3 ( 10 -2 P a ) , and LaNiO3 ( 104 P a ) [ 13]. F o r application as an electrode material, rare earth manganites with an electrical conductivity o f more than 200 S / cm a n d thermal expansion coefficients similar to that o f d o p e d zirconia exhibit the best properties [ 14 ]. Rare earth manganites o f the type LaMnO3 can be prepared by well-established ceramic procedures from mixtures o f oxides [ 15-20 ] and from aqueous solutions o f nitrates [ 15,20], carbonates [ 2 1 - 2 3 ], citrates [24] and oxalates [ 16,25 ]. The precursors can

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P. Shuk et al. / Solid State lonics 68 (1994) 177-184

be obtained by sublimation [26] and disturbance drying [27 ] and classified by the sedimentation process [21,24,28]. The nonstoichiometry of oxygen in rare earth manganites LnMnO3+y depends on both temperature and oxygen partial pressure and decreases in the order La, Sin, Y, Dy, Er, Yb in the range y-- 0.132 to y-- 0 for YbMnO3 at ambient temperature [ 29,30 ]. Practically all manganites of rare earths crystallize in a distorted orthorhombic perovskite-like structure [ 18,31,32 ], caused by a specific arrangement of the four free coplanar orbitals of Mn 3+ [ 33,37 ]. Substitution of the B cation by Cr, Co or Fe and of the A cation by Ca or Sr diminishes the distortion of the lattice. The manganites of the heavy rare earth metals LnMnO3 ( L n = H o , Er, Tm, Yb, Lu, Y) show a hexagonal structure [ 36,37 ]. Aspects concerning the electrical and electrochemical properties of electrodes consisting of the manganite LaMnO3 have been investigated thoroughly. Other manganites, however, have been studied only, to a small extent, with regard to their application in electrochemical cells. Furthermore, parameters of practical interest such as the thermal expansion coefficient and the electrokinetic properties are not known. For application in high-temperature fuel cells and oxygen sensors, alkaline earth-doped lanthanoide manganites have been studied systematically. This is with special regard to preparation conditions, electrical conductivities and thermal expansion coefficients. The suitability of these materials as an electrode for zirconia cells has also been studied in detail.

2. Experimental For the preparation of rare earth manganites the oxides La203, Nd203, Gd203, Yb203 and Y203 (more than 99.9%, USSR) and Mn203, SrO and CaO (more than 99.99%) were used. In a few cases the nitrates were also used. The desired amounts of substances were mixed carefully in a mortar. The mixtures obtained were heated in an oven in air at 1500-1650 K for 4-25 h. Determination of the structure of all substances prepared was accomplished by X-ray powder diffraction patterns using Cu Kct-radiation and a nickel ill-

ter at a rate of 0.25-0.5 °/min. Computations of lattice constants were performed by a computer program using a least square refinement. The analysis of cations of the rare earth manganites was done by atom absorption spectroscopy using a CzHz-flame for Mn and alkaline earth elements, and a NO-C2H2-flame for rare earth elements. The determination of the Mn4+-concentration was carried out by a permanganometric method [ 40 ]. For the determination of thermal expansion coefficients, bar-shaped samples were pressed at about 600 MPa. The sintered bars were polished with a diamond disk and diamond paste. The thermal expansion was measured by means of a quartz dilatometer in air or nitrogen, with a heating rate of 3.5 K / m i n in the temperature range of 300-1100 K. On rod-shaped samples the conductivity was measured both isothermally at 673,873 and 1073 K, and non-isothermally with a heating rate of 3.5 K / m i n up to 1100 K. This was in both cases by means of 4-terminal dc and 2-terminal ac method in the frequency range 50 Hz-5 MHz. The oxygen partial pressure could be fixed in the range of 105 to 10-15 Pa with different mixtures of oxygen and argon and by using a solid electrolyte oxygen pump. The thermoelectric power was measured on rod-shaped samples with a temperature difference of 30 K from one end to the other. The samples were heated at a rate of 3 K / m i n up to a maximum of 1150 K and then slowly cooled down to ambient temperature. The polarization resistivity as electrochemical important parameter of manganite electrodes on stabilized zirconia was determined using the steady state current density overpotential curves measured with the well known three electrode arrangement. The reference and counter electrode consist of porous platinum layers. The overpotential was measured currentless between the manganite electrode and the reference electrode over a small electrolyte bridge of thickness d x [ 41 ]. For the determination of polarization resistivity rp, a straight line near equilibrium (0-10 mV overpotential) according to equation rp = (Otl/Oj)j~ o

( 1)

was used. The polarization resistivities were measured by varying the thickness of electrode layers ( 1560 mg/cm 2), the temperature ( 800-1100 K), and the oxygen partial pressure ( 105-10-1 Pa ).

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P. Shuk et al. / Solid State lonics 68 (1994) 177-184

a n d Ca at high concentrations. According to the equation

3. Results and discussion

Lal_xSGMnX_xMn~O3

X-ray investigations o f L a l _ ~ r (Ca)xMnO3 ( x = 0 0.5) show a cubic perovskite structure for Ca-containing at x = 0.2-0.5 and for Sr-containing at x = 0.5. In all other cases a rhombohedrally distorted perovskite structure was built up. For G d ( N d l _ x C a ~ M n O 3 ( x = 0 - 0 . 5 ) an orthorhombic distorted perovskite structure was found. Yb (Y) ~_~CaxMnO3 shows in the range of x = 0.3-0.5 an orthorhombic structure, whereas for the pure Y b ( Y ) M n O 3 the hexagonal structure was determined. U p to x = 0 . 3 a mixture of both phases was detected. The determined parameters of structures are s u m m a r i z e d in Table 1. As can be seen, the symmetry of the structure increases with increasing content of Ca and Sr respectively. The concenlration of M n 4+ (Mn/~n using K r 6 g e r - V i n k n o t a t i o n ) d e t e r m i n e d at room temperature in quenched samples increases up to 56% with increases in the Ca and Sr content. These values do not correspond to the doped a m o u n t of Sr

La~ _~ Sr" M n ~_~+ 2yMn~_ 2yVo'y03_ y + y / 2 0 2

(2) or 2MnMn + 0 ~ ~-2MnX~ +V6" + ½02,

(3)

M n 4+ as well as oxygen vacancies (Vb') were formed. For the system Lal_xSr(Ca)xMnO3, the temperature dependence of conductivity shown in Figs. 1 a n d 2 shows a semiconducting behavior with a low activation energy in the temperature range of 300-1150 K. The intercept at 500-530 K for LaMnO3 suggests a phase transforming process from rhombohedric distorted to the cubic structure that could also be found by high temperature X-ray investigation. A correlation between the conductivity of doped lant h a n u m manganites and the concentration of M n a+ could be found. With increasing temperature the mo-

Table 1 Structure parameter of Ln~_xMnxMnO3. x

0

0.1

0.2

0.3

0.4

0.5

La/Sr a/nm or/°

0.5472 60.71

0.5470 60.61

0.5470 60.65

0.5467 60.51

0.5448 60.10

0.3885

La/Ca a/nm a/o

0.5468 60.57

0.5460 60.40

0.3877

0.3873

0.3862

0.3848

a Nd/Ca b/nm c

0.5415 0.5633 0.7627

0.5411 0.5616 0.7652

0.5419 0.5525 0.7660

0.5402 0.5464 0.7680

0.5365 0.5437 0.7693

0.5358 0.5420 0.7675

a Gd/Ca b/nm c

0.5300 0.5870 0.7440

0.5310 0.5780 0.7490

0.5320 0.5650 0.7490

0.5340 0.5550 0.7460

0.5320 0.5440 0.7550

0.5370 0.5430 0.7360

Yb/Ca a/nm c/nm

0.6059 1.1357

0.6053 1.1362

0.6048 1.1365

0.6052 1.1367

0.5270 0.5603 0.7459

0.5267 0.5541 0.7459

0.5280 0.5518 0.7441

0.5287 0.5519 0.7428

0.5298 0.5520 0.7450

0.6139 1.1381

0.6134 1.1409

0.5267 0.5542 0.7474

0.5264 0.5712 0.7419

0.5281 0.5585 0.7427

0.5298 0.5526 0.7438

0.5301 0.5463 0.7437

Ln/Me

a Yb/Ca b/nm c Y/Ca a/nm c/nm a Y/Ca b/nm c

0.6147 1.1368

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P. Shuk et al. / Solid State lonics 68 (1994) 177-184

"[/K 1000 800 • I

600

4.00

+

I

,

300 J

I

1

2

t

"7E

1

0

8

116

£

2~4 4 10 K/T

32

Fig. 1. Arrhenius plots of the conductivity of Lal_xCaxMnO3 in air for various compositions x = 0 ( 1 ), 0.1 (2), 0.2 (3), 0.3 (4), 0.4 (5), 0.5 (6).

dK 1000 800 i



i

600 ,

400

300

i

2,5

8 5 ~LOG

l 2,0 . "7E ? U3 1,5

6

taining and at x = 0 . 1 for Ca-containing compounds. This can be correlated with the formation of a maximum concentration of Mn 4+. Oxygen vacancies are also formed to a certain extent, but have a negligible influence on the total conductivity because of their smaller mobility. Low activation energies, 2.1-11.8 kJ/mol for Srcontaining and 8.1-16.9 kJ/mol for Ca-containing manganites, are in agreement with the suggestion of a hopping mechanism in the theory of localized electrons [33-35]. Similar behavior could be observed in the case of Nd~_xCaxMnO3 (Fig. 3 ). The intersection at 540 K is probably caused by a Jahn-Teller phase transformation [42 ]. At high concentrations of Mn 4+ this temperature is shifted from 1100 K to 600 K. The maximum conductivity was found to occur at x = 0.2; this is because in addition to the carrier concentration the mobility of Mn 4+ also plays an important role. For Gd~ _xCaxMnO3 the maximum conductivity at x = 0 . 4 could be detected (Fig. 4). From the Arrhenius plots of the conductivity of YbMnO3 two ranges can be distinguished. Below 600 K the conductivity is practically independent of the temperature, whereas in the higher range the conductivity is increased by 3 orders of magnitude due to increasing intrinsic disorder with generation of Mn/a.. With increasing concentration of Ca the conductivity also increases strongly by 4 to 6 orders of magnitude. T/K

1000 800 i

,

600

i

400

,

300

T

2 1,0

~

2 "

t 1

1

0,5

1'0

1'5

1 20 a 215 10 K/T

3t0

3'5

Fig. 2. Plots of the logarithm of conductivity versus 1 / T of Lal_~,SrxMnO3 in air for various compositions x = 0 (1), 0.1 (2, 7), 0.2 (3), 0.3 (4, 8), 0.4 (5), 0.5 (6). Samples 7 and 8 are annealed in oxygen at 1370 K.

bility and the chemical equilibrium Mn 3+ / M n 4+ play an important role. The mobility, in turn, depends on both temperature and defect concentration that can additionally be influenced by interaction between negatively and positively charged defects. The maximum conductivity was found at x = 0 . 3 for Sr-con-

"7E u 0

-1 -2

lr0

i

i

15

2 0

1

3'5 4 10 K/T Fig. 3. Plots of lgcr versus 1 / T of Ndt_xCaxMnO3 measured in air for various compositions x = 0 ( 1 ), 0.1 ( 2 ), 0.2 ( 3 ), 0.3 (4), 0.4 (5), 0.5 (6). 2'5

3 0

P. Sh uk et al. / Solid State lonics 68 (1994) 177-184

181

T/K 1000 800 i , i ,

600 i

T/K 400 j

,

300 i

1000 800 ,

t

,

I

600 ,

400

I

300

r

t

2

2

tl

1

t

~7E u

& o

"TE 0 t,o

-e -1

-2 1

8

1J6

2~4 4 10 K / T

32

Fig. 4. Plots of lg tr versus 1/ T of Gd~_xCaxMnO3 measured in air for x=0 (1), 0.1 (2), 0.15 (3), 0.2 (4), 0.25 (5), 0.3 (6), 0.4 (7), 0.5 (8).

r 10

, 15

t 20

2'5 z, 10 K / T

~ 30

5

Fig. 6. Plots of lg a versus 1/ T of YI _xCaxMnO3 measured in air for x=0 (1), 0.1 (2), 0.2 (3), 0.3 (4), 0.4 (5), 0.5 (6).

T/K 1000 800 ,

t

,

I

,

600

400

,

J

300 J

) m~_..-1 t3 -3

-5

1'o

2'o

4 10 K / T

Fig. 5. Plots of lg tr versus 1/T of Ybl _xCaxMnO3 measured in air for x=0 (1), 0.1 (2), 0.2 (3), 0.3 (4), 0.4 (5), 0.5 (6), 0.6 (7). The m a x i m u m conductivity has been observed also for the c o m p o s i t i o n Ybo.sCao.sMnO3. In the Arrhenius plots o f the conductivities o f both YMnO3 and YbMnO3 two ranges could be detected (Figs. 5 and 6). Below 640 K the conductivity is ind e p e n d e n t on the temperature. A b o v e this temperature the conductivity increases by 3 orders o f magnitude. Increasing concentration o f Ca causes an e n h a n c e m e n t o f c o n d u c t i v i t y by 4 to 7 orders o f magnitude. The m a x i m u m was found at x = 0.5. More recently Nasrallah et al. [43] have found the similar

conductivities in the Y~ _xCaxMnO3 system. O u r investigations o f the conductivity o f d o p e d l a n t h a n u m manganites as a function o f the oxygen partial pressure show no changes in the bulk conductivity (total conductivity) in the range o f 105 to 10-15 Pa at 1070 K. O u r analysis o f the thermoelectric power temperature d e p e n d e n c e shows that at concentrations higher than 30% M n 4+ a transition o f the conductivity m e c h a n i s m from p- to n-type takes place. Mostly the Seebeck coefficient is a r o u n d 2 0 - 4 0 IxV/K a n d decreases with increasing Ca and Sr concentration. Simultaneously the inversion t e m p e r a t u r e is shifted toward to lower temperatures. At r o o m t e m p e r a t u r e all compositions o f N d l _xCaxMnO3 exhibit p-type conduction. W i t h increasing concentration o f Ca the Seebeck-coefficient decreases. At x = 0 . 5 the whole t e m p e r a t u r e range shows n-type conduction. At all substituted n e o d y m i u m manganites the analysis o f the t e m p e r a t u r e dependence o f t h e r m o E M F shows an inversion from the p-conducting type to the n-conducting except for the c o m p o s i t i o n x = 0 . 5 . This is similar to the other substituted l a n t h a n u m manganites and is due to the increasing f o r m a t i o n of M n 3+ with increasing temperature. The inversion takes place at about 30% Mn 4+. The analysis o f t h e r m o E M F measured on Gd~_xCaxMnO3 shows that at x = 0 . 4 and 0.5, samples exhibit excess electron con-

182

P. Shuk et al. / Solid State Ionics 68 (I 994) 177-184

duction due to the high concentration o f M n 4+. Samples with a smaller a m o u n t of Ca show an inversion from p- to n-type conduction. YMnO3 exhibits p-type character whereas the Ca substituted c o m p o u n d s show n-type conduction. For samples with other compositions an inversion takes place which is shifted to the low temperature range. The investigation of thermal expansion as a function of temperature for Ca and Sr doped l a n t h a n u m manganites shows a straight line. The values determ i n e d are s u m m e r i z e d in Table 2. With regard to the thermal expansion coefficient and conductivity Ca doped l a n t h a n u m manganite has optimal properties for application in solid electrolyte fuel cells. An other i m p o r t a n t aspect is the electrokinetic behavior which can be estimated by measurement of the polarization resistivity. The lowest values could be observed for Ca-doped l a n t h a n u m manganites with x = 0 . 3 and for Sr doped ones at x = 0 . 1 . The reason

for this is that oxide ion vacancies are generated by doping with Ca and Sr. With increasing content of Ca or Sr the concentration of M n 4+ increases (Table 3) but this concentration does not correspond with the doped a m o u n t of Ca or Sr. Oxygen is generated a n d in the manganites oxide ion vacancies are produced according to Eq. (3). The electrode reaction can take place not only on the three-phase b o u n d a r y but also on the two-phase b o u n d a r y m a n g a n i t e electrode/gas. At 800°C the polarization resistivities measured on L a o . 7 C a o . 3 M n O 3 a r e in the same order as can be det e r m i n e d on cells with p l a t i n u m electrodes [41 ] b u t at lower temperatures even lower values could be obtained (Fig. 7). The polarization resistivities decrease with increasing oxygen partial pressure. The dependency can be described with the equation rp = A P r ~ exp ( E a / R T ) .

(4)

Table 2 Electrical conductivity e a t at 1100 K and thermal expansion coefficient a in the range 600-1100 K for Ln~_xMexMnO3 [a( 1100 K ) / S cm -1, 106 K ' a ]

Ln/Me

x

0

0.1

0.2

0.3

0.4

0.5

La/Ca

a

La/Sr

a

Nd/Ca

a

Gd/Ca

a

290 11.0 120 10.4 60 9.9 25

115 10.6 160 7.7 150 10.6 25

115 10.4 150 10.9 20 11.0 60

80 9.3 200 11.7 10 11.0 220

Yb/Ca

a

Y/Ca

a

60 11.8 60 11.8 70 10.0 4 6.3 0.01 8.0 0.01 3.3

2.5 6.7 10 2.3

30 7.3 70 0.5

60 7.5 130

160 8.0 140 5.4

95 10.7 160 12.3 2 11.3 160 10.0 180 10.2 180 9.1

Table 3 Mn4+-concentration in Ln t-xMexMnO3 c (Mn4+ ) + C ( M n 3+ ) = 100%. x

0

0.1

0.2

0.3

0.4

0.5

La/Ca La/Sr Nd/Ca Gd/Ca Yb/Ca Y/Ca Ln/Me

24 24 16 5 0 0

22 22 12 17 12 10

20 27 20 25 22 21

25 34 28 32 33 34

30 43 32 40 41 37

32 56 32 46 49 48

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P. Shuk et al. / Solid State lonics 68 (1994) 177-184

Table 4 Activation energies Ea/kJ mol- ~and exponents n for the polarization of the manganite-YSZ-air electrode. Composition

Lal _xCaxMnO3

x n E,

0 1/2 64

Lat _xSrxMnO3

0.1 1/2 50

0.2 1/2 77

0.3 1/2 91

0.4 1/2 84

This dependence is similar to that observed on platin u m electrodes with a n d without a layer m a d e o f mixed conducting materials. The activation energies a n d the exponent n d e t e r m i n e d after the linear regression o f the lg r p - 1 / T plots are s u m m e r i z e d in Table 4. On Ca d o p e d manganites the activation energy is much smaller than on the Sr d o p e d ones. Mostly for the exponent n = - 1 / 2 was obtained. These results suggest that for the rate-determing step a reaction with O species takes place: probably the forming o f a b s o r b e d atomic oxygen on the surface o f manganites. F o r such processes the following equations can be formulated [44 ] :

I

2,5 2,0 C

l

~

0.1 1/2 110

0.2 1/2 140

0.3 1/2 230

0.4 1/3 110

0.5 1/2 150

Table 5 Time-tests of resistivity film-thickness parameter (p/d) and polarization resistivity of Lao.7Cao.3MnO3-YSZ-electrodes at 800°C. Time (h)

p/ d

rp

(f~)

(f~cm -z)

0 10 50 100 200 500 1000 5000

0.65 0.64 0.63 0.64 0.64 0.65 0.65 0.65

12.5 12.0 12.0 12.5 13.0 12.4 12.7

O2 ~---2Oad ,

(2)

2Oa0 ~ 2 0 . a (g, se, e) ,

(3)

2 0 . d ( g , se, e ) ~ 2 O a d ( e r ) ,

(4)

2Oad + 2 e - + V/5 ~ O o ,

(5)

where g is gas, se solid electrolyte, e electrode phase, er electrode reaction zone. Electrodes based on Lao.7Cao.3MnO3 were tested in solid electrolyte fuel cells over a 5000 h period. Resuits are given in Table 5.

I 1,5

)

b) 2,5

0.5 3.4 80

2 References

2,0

1,5 I

0

I

0,2

I

i

0.4

Fig. 7. Oxygen concentration dependency of polarization resistivity of Lal_xCaxMnO3 (a)- and La,_xSrxMnO3 (b)-YSZ-air electrodes at 1073 ( 1), 1023 (2), 973 (3) and 923 K (4).

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