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Perovskite electrodes for sensors
SOLID STATE
Solid State lonics 51 (1992) 281-289 North-Holland
IOHICS Perovskite electrodes for sensors C.B. Alcock, R.C. D o s h i a n d Y. Shen Center for Sensor Materials, University of Notre Dame, Notre Dame, IN 46556, USA Received 22 October 1991; accepted for publication 5 February 1992
The data published on perovskite oxides La~ _,SrxBO3_a (B = Co, Cr, Fe, Mn ) are compared to assess their use as electrodes in applications such as fuel cells, low temperature oxygen meters, etc. The oxides are reviewed in terms of their stability in temperature and oxygen partial pressure of operation, nonstoichiometry, conductivity and catalytic properties and the effect of the strontium substitution content, x. From this comparison it is found that La~_xSrxCrO3_ 6 is better suited for applications in reducing conditions, while Lat _~SrxCoO3_a is better suited for applications at higher oxygen potentials. While noble metals like platinum are currently being used for these applications, the perovskite oxides offer better kinetics at lower temperatures. To compare the kinetics, the electrochemical cell: air, electrode/Bi-based or Zr-based electrolyte/electrode, air was used and the overpotential of the interfaces was measured. The electrodes used for this comparison are Pt, Lao.7Sro.3MnO3_,~, and Lao.7Sro.3CoO3_6. The results show that for temperatures lower than 800 ° C, the oxide electrodes performed better than platinum. This is attributed to the ease of transfer of oxygen potential between the electrodes and the electrolyte.
1. Introduction
Platinum has been the ideal electrode material for most applications where sensor electrodes and catalysts are used. It is an expensive material and its supply is limited, therefore, a substitute for platinum is desirable. Secondly, when used in the finely divided state on an oxide support it is prone to sintering [ 1 ], and requires frequent replacement with new material. In recent years there has been considerable interest in perovskite oxides for application as electrodes in solid oxide fuel cells (SOFC), magnetohydrodynamic ( M H D ) generators, exhaust gas sensors in automobiles, and as catalysts for CO and hydrocarbon oxidation. Many of the requirements of electrodes for chemical sensors, galvanic and amperometric, are similar to those for the above applications namely: ( 1 ) Stability in the temperature range of operation; (2) Stability in the environment of operation; (3) High electronic conductivity; (4) High oxide ion diffusion coefficients for oxygen sensors. In this study, perovskite oxides, Lal_xMxBO3_6 ~r Presented at SSI-8, Lake Louise, Canada, Oct. 20-26, 1991.
( M = S r , B = C o , Cr, Fe, Mn) are evaluated for use as electrodes for chemical sensors in comparison to platinum from 400 to 800°C. The electrolytes considered are (ZrO2)o.92(Y203)o.o8 (YSZ) and (Bi203)o.s-(SrO)0.2 systems. A comparison is then made between perovskite oxides in terms of their stability and conductivity, and reported literature data for the number of electrons transferred in electrode reactions with each perovskite oxide in conjunction with an electrolyte. The perovskite oxide electrodes under consideration are all refractory oxides with melting points above 2000°C. Sintering of the electrode is therefore not a major problem with oxides. Also, due to the availability of oxygen in the oxide electrode which is able to participate in the process, a three phase boundary may not be essential, unlike the case of platinum electrodes, especially when used at high temperatures ( T > 700 ° C). The reaction mechanism for perovskite oxide electrodes has been much less studied than the noble metal electrodes. In fact they are believed to be similar [2] to the noble metal electrodes but show improved gas/solid exchange coupled with rapid diffusion of oxygen through the oxide electrodes.
0167-2738/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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2. Preparation of electrolytes and electrodes The bismuth based electrolyte, (Bi203)o.8 (SrO)o.2, was prepared by the solid state method. Bi203 and S r C o 3 powders were mixed in proper proportions, pressed into a pellet and sintered at 800 °C in air for 15 h. The pellets were crushed, ground, repressed and sintered again at 800°C for 15 h in air. The YSZ electrolyte, (ZrO2)0.92(Y203)o.o8 , w a s made by the Pechini process [3]. Nitrates of zirconium and yttrium were mixed with citric acid (2 moles acid per mole of electrolyte), The solution was heated to 90°C and ethylene glycol ( 1 mole per mole acid) was added while stirring the solution. Further heating caused the water to evaporate accompanied by the emission of NO\ fumes. This was followed by the formation of a gel which was charred at 400°C and calcined at 700°C for 8 h. The oxide powder was finally pressed under 20000 lb. load and sintered at 1450°C in air for 5 h, crushed, ground, re-pressed and sintered again at 1450°C for 5 h in air. The electrolyte discs were 10.2 mm in diameter and 1.2 mm thick. The porous platinum electrodes were prepared by applying a platinum paste on the surface of the electrolyte and firing at 800°C in air for three hours. The perovskite electrodes were made by the Pechini method using nitrates of lanthanum, strontium, manganese (or cobalt) in the proper proportions. The preparation method was similar to the YSZ electrolyte except for the final sintering which was done at 1000 ° C in air for 15 h. The oxides were then ball milled and applied as a paste on the whole electrolyte surface on both sides. The paste was then fired at 800°C for three hours in air. X-ray diffraction patterns of each of the materials revealed that they were the desired single phase.
3. Results of conductivity and overpotential measurements A schematic of the cell arrangements is shown in fig. 1. For conductivity measurement, the galvanostat was replaced by an impedance analyzer (Solartron 1260). The real and imaginary parts of the impedance were plotted as - Z " versus Z'. The high frequency intercept of the curve with the Z' axis gave
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Fig. 2. Comparisonof the conductivityof YSZ and bismuth-based electrolytes. the resistance across the electrolyte, E R. A comparison of conductivity measurements for the two electrolytes is given in fig. 2 and compared with published data on (Bi203) o.s (SrO) 0.2. The conductivity of the bismuth-based electrolyte is about an order of magnitude greater than the YSZ electrolyte in the temperature interval 300 to 800 °C. The overpotential (t/) was calculated as the difference between the measured potential drop across
283
C.B. Alcock et al. /Perovskite electrodes for sensors
the electrolyte I E R and the measured potential across the whole electrode/electrolyte/electrode system. A plot ofq versus current density at 400, 600 and 800°C for oxide electrodes compared with platinum is shown in fig. 3a (for (Bi203) o.8 ( S r O ) 0.2 electrolyte)
and fig. 3b (for YSZ electrolyte). For both electrolytes, the oxide electrodes all allowed higher current than platinum at low overpotential below 800°C. Consequently, the oxide electrodes exhibited lower overpotential for the same current density. There-
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Fig. 3(a). Overpotential behavior for Lao.TSro.3MnO3_6, Lao.TSro.3CoO3_ 6 and Pt electrodes on (BizO3)o.a(SrO)o.2 electrolyte at three temperatures; (b) overpotential behavior for Lao.7Sro.3MnO3_,s, Lao.TSro.3CoO3_6 and Pt electrodes on 8 mol% yttria-stabilized zirconia electrolyte at three temperatures.
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fore, oxide electrodes are superior to platinum electrode at 600°C and below. Another important fact is that the total current density at a given applied voltage obtained with all the three electrodes was much higher with the (Bi203)o.s(SrO)o.2 electrolyte than with the YSZ electrolyte. For example at 400 ° C for Lao vSro.3MnO3_~ and at an overpotential of 0.2 V, the current density for YSZ electrolyte was 2 txA/cm 2 compared to 80 ~A/cm 2 for the (Bi203)o.8(SrO)o.2 electrolyte. The current density-overpotential relationship for the perovskite electrodes is linear when used with either electrolyte but nonlinear when the electrode is platinum at T~< 600 ° C. In the latter case it is marked by a decreasing resistance as the current density is increased. This could be explained as being due to a reduction of the electrolyte accompanied by an increase in electronic conduction contribution from the electrolyte. It has been shown earlier [ 5 ] that when a high current density (0.129 A / c m 2) is passed through a YSZ electrolyte ( 12 mol% Y 2 0 3 in ZrO2), sandwiched between two platinum disc electrodes, there is a blackening of the electrolyte originating from the cathode region• This was attributed to an imbalance of oxygen being transported from the cathode and oxygen replenishment at the anode, the former being greater than the latter suggesting a rate controlling cathode reaction• Our results (fig. 3a, b) further indicate that such a phenomenon should not occur with the perovskite oxide electrodes. In order to further understand the phenomena, a simple experiment to measure the ionic transport number of the electrolyte as a function of applied current density was devised.
theoretical and experimentally observed rates of oxygen evolution are shown in fig. 4. The ionic transport number was then calculated from the theoretical amount of oxygen evolved for the applied current compared with the experimentally determined amount of oxygen evolved. Several measurements were made for each applied current. Fig. 5 is a plot of the ionic transport number of the electrolyte as a function of applied current. Since the oxygen evolution rates are small at low applied currents, there is considerable scatter in transport number calculations at low current densities. Within experimental errors, however, the transport number was found to decrease from a value of approximately 1 to about 0.6. Therefore it is probable that the platinum electrode kinetics of oxygen reduction from the atmo-
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A ( B i 2 0 3 ) o . s ( S r O ) o . 2 electrolyte pellet was coated with platinum paste on both sides as before. One side of the pellet was sealed to a quartz tube through which air was passed continuously. The second side also contained air but of a known volume. When a current was applied across the electrolyte, the volume of gas on the second side increased due to oxygen evolution. This increase in volume was measured by the displacement of a liquid column in a capillary. The
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C.B. Alcock et al. / Perovskite electrodes for sensors
sphere is slower than the oxygen evolution at the cathode. Further experiments on oxide electrodes using (Bi203)o.8(SrO)0.2 electrolyte and on both types of electrodes using YSZ electrolyte are currently being conducted and the results will be reported later. Having shown the advantage of materials of the conductive perovskite family as electrodes over platinum, a comparison can now be made between different perovskite oxide materials from available literature for use as electrode materials in sensors.
5. Stability and nonstoichiometry of perovskite oxides All the lanthanum-based perovskite oxides under consideration with a transition metal ion without a divalent ion (strontium) substitution are refractory with melting points above 2000°C. They can be used as electrodes for continuous operation up to 1200 ° C and for short times at 1600-1700°C (SOFC application requires operation in the 1000 to 1200°C range ). Their stability in reducing environments depends on the transition metal species. Table 1 lists the range of oxygen partial pressure in which each perovskite oxide is stable. Addition of a divalent ion like Sr on the lanthanum site slightly increases the oxygen partial pressure at which the oxide is reduced to new phases [ 7,9 ]. The substitution of 0.3 moles of La by Sr in LaFeO3 increases the decomposition pressure from 10 -16"95 (for LaFeO3) to 10 -14 [9] at 1000°C. A similar substitution in LaCoO3 increases the decomposition pressure from 1 0 - 7 t o 1 0 - 4 [ 7 ] . Table 1 Stability range o f oxygen partial pressure at 1000°C. P e r o v s k i t e c o m p o s i t i o n Po2 ( b a r ) range (at 1 0 0 0 ° C )
Refs.
LaCoO3 LaCrO3 LaFeO3 LaMnO3 Lao.TSro.3CoO 3_ 6 Lao.TSro.3CrO3_ ~ Lao.TSro.3FeO3 _ ~ Lao.7Sro.3MnO3-a
[6] [6] [6] [6 ] [7] [8] [9l [ 10]
1-10 -7 1 - < 10 -20 1- 10-16.95 1- 10-15.05 l - 10-4 1- 1 0 - 20 1-- 10 -- 14 1-10 -14
285
LaFeO~ and LaCrO3 are better suited for application in reducing conditions requiring low oxygen partial pressure, due to their wider stability range. LaCoO3 has the smallest range of stability among the oxides under consideration. With substitutions of Sr for La, the operating range in terms of oxygen partial pressure becomes narrower therefore the cobaltites can be used only in the high Po2 range of 1 to 10-4 arm for T> 1000 ° C. If the temperature is decreased, the operating range is broadened and the oxides can be used at lower oxygen pressures. At sufficiently low temperatures, the slower kinetics allow the use of oxides at oxygen pressures even lower than their stability range. For example, when the oxides are used as electrodes in low temperature (room temperature to 200°C) oxygen meters [ 11 ], the decomposition of the oxides at very low oxygen pressures will be too slow to have any significant effect. In order to quantify the point we can assume a square slab of material 1 mm thick which undergoes a decrease in Po: on one side. Since oxygen vacancies are the most mobile species in the material we can use D = 10-8 cm2/s as an upper estimate at 200°C, then the time required for reduction of the sample is about 50 d. The pure oxides are all stoichiometric in their stability range (table 1 ) except LaMnO3 which has a stoichiometry of LaMnO3.12 in 1 atm. oxygen at 1000°C. When a divalent ion such as Sr 2+ is partially substituted for La 3+ on the A site of the perovskite, all the perovskites exhibit nonstoichiometry under reducing conditions. The maximum nonstoichiometry, ~, is a function of the Sr content. For the composition La~_xSrxBO3_a, the maximum nonstoichiometry before decomposition is Lal_zSrxBo3_x/2 (O=x/2). For Lal_xSrxCrO3_a, 6=x/2 at P o 2 = 1 0 - 2 ° for T>1200 °C [8]. The nonstoichiometry is a function of Po2. Therefore ~', corresponding to P~2 at the surface, will be transmitted to the electrode-electrolyte interface. The higher current density for oxides compared to platinum at 400 and 600°C (fig. 3a and b) could be due to the ease of transfer of oxygen potential between the electrodes and the electrolyte.
6. Reaction with YSZ electrolyte Addition of a divalent ion such as Sr for La causes
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the generation of a large number of defects [Vo +B~]. Therefore the electrode performance is improved due to increased Vo at low Po2 and increased BB at high Po2. However the electrode materials are then found to react with Z r O 2 electrolyte [12-16] with the formation of Sr2ZrO4 above 1000°C for Sr substituted perovskites or Ca2ZrO4 above 900°C for La~_~Ca~CoO3 at reduced oxygen partial pressures. At high oxygen pressures CaZrO3 is formed. Highly reducing atmospheres and/or high temperatures ( T > 1200 °C) also cause the formation of reaction products between the perovskite and zirconia resulting in the formation of La2Zr2P7. Among the unsubstituted perovskites, only LaCoO3 was found to react with Z r O 2 tO form L a 2 Z r 2 0 7 below 1000°C. Inoue et al. [17] detected the formation of L a z Z r 2 0 7 at 800°C for Lao.6Sro.4CoO3_ 6 in contact with Z r O 2 . However the manganite equivalent, Lao.6Sro.4MnO3_6, was found to be inert under the same conditions. There is therefore a need to obtain more reliable data on the reactivity of the perovskites with the electrolyte phase.
7. Reaction with bismuth-based electrolyte At present there are no reports of significant interaction between lanthanum-based perovskite electrodes and Bi203. However, bismuth based oxides, BiFeO3 [18], BiMnO3 [19], and BiCrO3 [19] are known to exist in the perovskite form at 700°C and high oxygen pressure. It is quite possible then that the lanthanum-based perovskite electrodes react with the bismuth based electrolyte at the interface. Since the interface product and the electrode are both perovskites with close matched lattice constant ( a = c = 3 . 9 3 A for BiMnO3 [19] and a = 3 . 9 3 / k for LaCrO3 in a pseudocubic approximation) it may result in a lower interface resistance compared to YSZ electrolyte.
8. Electrode processes At the three-phase boundary between gas, electrode, and electrolyte, the oxygen in the gas phase interacts with the oxygen in the electrolyte phase [2,20-22] according to the total reaction:
(l)
O2(g ) + 4 e - + 2 V o ~ 2 0 ~ .
The EMF across the electrolyte subjected to different oxygen potentials at the two ends is given by the Nernst equation:
E=RTLn~ nF Po2
(2)
with the well known value of n = 4 for reaction ( 1 ). This is the case for high temperature operation using zirconia electrolytes since 4 electrons are involved in the reaction. When the sensor is operated at temperatures below 200 ° C, the value of n has been found to be less than 4 [ 11 ]. This is attributed to the formation of other oxygen species. The successive steps of the oxygen reaction at the electrode have been described by Kleitz [23 ] with the corresponding value of n. For example: n = 1 for the formation of Oi- from 02, n = 2 for the formation of 022-, n = 3 for the formation of 2 0 - , n = 4 for the formation of 202-. Among the perovskite electrode materials, values of n ranging from 1 to 4 have been observed [ 2,11 ]. Table 2 lists the values of n for each perovskite composition. An n value of 4/3 can be explained by a mixed potential of 1/n= 1 / 4 + 1/2 arising from the formation of two oxygen species. Note that the value of n for platinum is 2 compared to the La~_~MxCoO3_a (M=Sr, Ca) oxides where it is 4. In this respect the perovskite oxides are comparable or better than platinum as electrodes. Table 2 Values of n for different perovskite compositions. Composition
n at 30°C (ref. [ I l l )
Pt LaCrO3 LaMnO3 Lao.vSro.3CrO3_a LaCoO3 Lao.vSro.3FeO3 a Lao.TSro.3MnO3 a Lao.TSro.3CoO3 a Lao.sCao.2CoO3_a Lao.6Cao.4CoO3_ a Lao.~Sr0.sCoO3_a Lao.3Sro.TCo03-6
2 1 2 2
4 4
n at 600°C (ref. [2])
2 4 4/3 4/3 4
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C.B. Alcock et al. /Perovskite electrodes for sensors Table 3 Oxygen ion diffusion coefficients in perovskite oxides. Composition
CSZ Lao.9Sro.lCoO3_a Lao.9Sro.lCoOs_a La0.79Sro.2MnO3_ a LaMgo.osCro.9503_a
Temperature ( ° C)
1000 850 900 860 1000
The catalytic property of the electrodes for carbon monoxide oxidation has been measured and compared with platinum [24]. It was shown that even at the low temperatures of CO oxidation ( T < 200°C), these oxides were comparable to Pt in catalytic activity. Therefore, perovskite oxides are viable alternatives to platinum for catalytic as well as electrode applications. The reasons for the formation of different oxygen ion species on different perovskite materials which are very similar to each other have been unexplained so far. The values of n for each perovskite oxide should be compared with bulk properties of the perovskites to shed further light on this problem. As was explained earlier, the substitution of a divalent ion like Sr or Ca gives rise to oxygen vacancies in perovskites. The concentration of oxygen vacancies (given by 6) depends on the amount of the substitution, the temperature, and oxygen partial pressure. However, nonstoichiometry data below 600 °C are not available due to the long equilibration times required at low temperature therefore a quantitative comparison between different perovskites cannot be made. The conductivities of perovskites oxides have been reported in a number of studies [25-30]. Conductivity data on La~ _xCaxCoO3_6 are not available. In table 2, the perovskite compositions have been arranged in descending order of conductivity estimated from the above mentioned studies. LaCrO3 has the lowest conductivity and compositions in the La~_ x S r x C o 0 3 _ 6 system have the highest conductivities. Lao.sCao.2CoO3_6 and Lao.6Cao.4CoO3_a have been placed in the order roughly and may be one step up or down. The qualitative picture, however, is not changed. As the conductivity is increased, the value
Tracer diff. coefficient
Chemical diff. coefficient
(cm2/s)
(cm2/s)
8.99)< I 0 - io 3.22)< 10 9
8)<10 -6 6.3)< 10 -6 7.41)<10 -6 1.7)<10 -6 3)< 10 -4
Refs.
[17] [33,34] [33] [35] [36]
of n increases from 1 for LaCrO3 to 4 for Lal _xSrxCoO3_a. From this comparison, increase in n values corresponds to an increase in the conductivity. This correlation can be made at 30°C and even at 600 °C. In the case of LaCoO3, the value of n at 30°C was 2 [ l l ] while at 600°C it was 4 [2]. This can also be explained by an increase in conductivity in LaCoO3 with increasing temperature. All the strontium and calcium substituted oxides under consideration are p-type semiconductors [26,28,29,31 ]. Some even exhibit metallic behavior as in Lal_xSr~CoO3_~ [28] and Lal_xSrxFeO3_a [ 31 ]. They all show high electronic conductivity with small activation energies (0.2 to 0.4 eV [25,26]). The conduction mechanism is hopping [ 26 ] of holes. In this mechanism it has been shown that the concentration of charge carriers at ambient oxygen pressures is fixed by strontium substitution at low temperatures where intrinsic conduction is not dominant [26]. Therefore, if the n value is dependent on the concentration of charge carriers, it would be independent of temperature for substituted perovskites as is observed. For unsubstituted perovskites this would not hold true since the generation of charge carriers can be assumed to be temperature dependent. Therefore the increase in the value of n for LaCoO3 with increasing temperature is consistent. Among the unsubstituted perovskites LaCoO3 [25 ] is an n-type conductor and LaCrO3 [26] and LaMnO3 [25] are p-type semiconductors. In the case of LaFeO3, both n-type [25] and p-type [32] conductivity have been reported. The literature data on conductivity in these unsubstituted perovskites are therefore not very reliable. One reason for this may be the presence of either donor or acceptor impur-
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ities g i v i n g rise to n-type or p - t y p e c o n d u c t i o n respectively. O n e a n o m a l y exists in o u r c o r r e l a t i o n . L u k a s z e w icz et al. [ 11 ] f o u n d t h a t w i t h an LaFeO3 e l e c t r o d e t h e r e was no significant E M F g e n e r a t e d . A c c o r d i n g to o u r c o m p a r i s o n , h o w e v e r , the n v a l u e for LaFeO3 s h o u l d be b e t w e e n 1 a n d 2 since its c o n d u c t i v i t y lies b e t w e e n that o f L a C r O 3 a n d L a M n O 3 . N o c o r r e l a t i o n can be f o u n d b e t w e e n the d i f f u s i o n coefficient o f o x i d e ions in perovskites w i t h the v a l u e o f n. T h e o x i d e ion d i f f u s i o n c o e f f i c i e n t s for perovskites, listed in table 3, are all o f the s a m e order.
9. Conclusions O x i d e e l e c t r o d e s are f o u n d to h a v e b e t t e r characteristics for sensor a p p l i c a t i o n s t h a n p l a t i n u m at low t e m p e r a t u r e ( T < 600 ° C ) . At high t e m p e r a t u r e their p e r f o r m a n c e is c o m p a r a b l e to p l a t i n u m but they are not p r o n e to s i n t e r i n g or v o l a t i l i z a t i o n like Pt. A s i m p l e c o r r e l a t i o n was f o u n d b e t w e e n the cond u c t i v i t y o f the p e r o v s k i t e o x i d e s a n d the v a l u e o f n in the N e r n s t e q u a t i o n for the o x y g e n d i s s o l u t i o n rea c t i o n for all p e r o v s k i t e s e x c e p t LaFeO3. F r o m the l i t e r a t u r e d a t a a v a i l a b l e so far, d i f f e r e n t p e r o v s k i t e s fit the r e q u i r e m e n t s for use as e l e c t r o d e s d e p e n d i n g o n the c o n d i t i o n s o f m e a s u r e m e n t : ( 1 ) W h e n m e a s u r e m e n t s m u s t be m a d e at v e r y low o x y g e n partial pressures (Po2 < 1 0 - ~4 a t m ) , the c h r o m i t e is the best suitable for the p u r p o s e . (2) W h e n t h e e l e c t r o d e s are to be o p e r a t e d at int e r m e d i a t e or high o x y g e n pressures ( P o 2 > 10 -~4 a r m ) a n d at high t e m p e r a t u r e s ( T > 800 ° C ) , the ferrite a n d the m a n g a n i t e are b o t h stable b u t the m a n ganite has h i g h e r c o n d u c t i v i t y especially with a s t r o n t i u m s u b s t i t u t i o n o f 0.3. In this case, h o w e v e r , r e a c t i o n b e t w e e n the s t r o n t i u m a n d the z i r c o n i a electrolyte c a n n o t be r u l e d out. ( 3 ) F o r o p e r a t i o n at high o x y g e n partial pressures (Po2>10-5 atm.) a n d at low t e m p e r a t u r e s (T<800°C) the cobaltite, L a l _ x S r x C o O 3 _ 6 is the best suited d u e to its h i g h e r c o n d u c t i v i t y . T h e use m u s t be r e s t r i c t e d w h e n the o x i d e elect r o d e s are in c o n t a c t w i t h Z r O : to 9 0 0 ° C p a r t i c u larly in the case o f C o - b a s e d p e r o v s k i t e s due to t h e i r r e p o r t e d r e a c t i v i t y w i t h ZrO2 resulting in the form a t i o n o f La2Zr207.
References [ 1 ] S.P.S. Badwal, M.J. Bannister and W.G. Garrett, Advances in Ceramics, Vol. 12, Science and Technology of Zirconia II (1984) pp. 598-606. [2] Y. Takeda, R. Kanno, M. Noda and O. Yamamoto, Bull. Inst. Chem. Res., Kyoto Univ. 64 (1986) 157. [3] M.P. Pechini, U.S. Patent 3, 330, 697 (1967). [4]T. Takahashi, H. Iwahara and Y. Nagai, J. Appl. Electrochem. 2 (1972) 97. [5] R.E.W. Casselton in: Proc. Symp. by Nuffield Research Group, London, ed. C.B. Alcock (1968) p. 151. [6] T. Nakamura, G. Petzow and L.J. Gauckler, Mat. Res. Bull. 14 (1979) 649. [7 ] A.N. Petrov, V.A. Cherepanov, O.F. Kononchuk and L.Ya. Gavrilova, J. Solid State Chem. 87 (1990) 69. [ 8 ] J. Mizusaki, S. Yamauchi, K. Fueki and A. Ishikawa, Solid State Ionics 12 (1984) 119. [9] J. Mizusaki, M. Yoshihiro, S. Yamauchi and K. Fueki, J. Solid State Chem. 58 (1985) 257. [ 10] J. Mizusaki, H. Tagawa, K. Naraya and T. Sasamoto, Solid State Ionics 49 ( 1991 ) 111. [ 11 ] J.P. Lukaszewicz, N. Miura and N. Yamazoe, J. Appl. Phys. 28 (1989) L711. [ 12 ] O. Yamamoto, Y. Takeda, R. Kanno and M. Noda, Solid State Ionics 22 (1987) 241. [ 13 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Denki Kagaku 58 (1990) 489. [ 14] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Proc. Intern. Symp. on Solid Oxide Fuel Cells, Nagoya, Japan, 1989. [ 15 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Fuel Cell Seminar, Phoenix, AZ ( 1990 ) pp. 156-159. [ 16 ] H. Yokokawa, N. Sakai, T. Kawada and M. Dokiya, Solid State Ionics 40/41 (1990) 398. [ 17] T. Inoue, K. Eguchi, T. Setoguchi and H. Arai, Solid State Ionics 40/41 (1990) 407. [ 18] A.1. Zaslavskii and A.G. Tutov, Dokl. Akad. Nauk. SSSR 135 (1960) 815. [ 19] F. Sugawara and S. Iida, J, Phys. Soc. Japan. 20 (1965) 1529. [ 20 ] H. Tanabe and S. Fukushima, Electrochim. Acta 31 ( 1986 ) 801. [21]D.Y. Wang and A.S. Nowick, J. Electrochem. Soc. 126 (1979) 1155. [22]T.H, Etsell and S.N. Flengas, J. Electrochem. Soc. 118 (1971) 1890. [ 23 ] M. Kleitz and E. Siebert in: Chemical Sensor Technology, ed. T. Seiyama (Elsevier, Amsterdam, 1989)pp. 151-171. [24] J.J. Carberry, S. Rajadurai, C.B. AIcoek and B. Li, Catal. Lett. 4 (1990) 43. [ 25 ] K. Gaur, S.C. Verma and H.B. Lal, J. Mater. Sci. 23 ( 1988 ) 1725. [26 ] D.P. Karim and A.T. Aldred, Phys. Rev. B 20 (1979) 2255. [27]D.B. Meadowcroft, Brit. J. Appl. Phys. (J. Phys. D) 2 ( 1969 ) 1225, and references therein. [28] J. Mizusaki, Kikan Kaguku Sosetsu 3 (1988) 1.
C.B. Alcock et aL / Perovskite electrodes for sensors
[29] J.H. Kuo, H.U. Anderson and D.M. Sparlin, J. Solid State Chem. 87 (1990) 55. [ 30 ] A. Hammouche, E.J.L. Schouler and M. Henault, Solid State Ionics 28-30 ( 1988 ) 1203. [31 ] J. Mizusaki, T. Sasamoto, W.R. Cannon and H.K. Bowen, J. Am. Ceram. Soc. 66 (1983) 247. [ 32] J. Mizusaki, T. Sasamoto, W.R. Cannon and H.K. Bowen, J. Am. Ceram. Soc. 65 (1982) 363. [33] T. Ishigaki, S. Yamauchi, K. Kishio, J. Mizusaki and K. Fueki, J. Solid State Chem. 73 (1988) 179.
289
[34] K. Feuki, J. Mizusaki, S. Yamauchi, T. Ishigaki and Y. Mima, in: Mater. Sci. Monographs 28A, Reactivity of Solids, eds. P. Barret and L.-C. Dufour (Elsevier, Amsterdam, 1985) pp. 339-343. [35] A. Belzner, T.M. Gur and R.A. Huggins, Proc. 1st Intern. Symp. SOFCs, The Electrochem. Sot., Vol. 89-11, ed. S.C. Singhal (1989) pp. 214-217. [36] C.J. Yu, D.M. Sparlin and H.U. Anderson, J. Am. Ceram. Soc. 70 (1987) C-189.
Solid State lonics 51 (1992) 281-289 North-Holland
IOHICS Perovskite electrodes for sensors C.B. Alcock, R.C. D o s h i a n d Y. Shen Center for Sensor Materials, University of Notre Dame, Notre Dame, IN 46556, USA Received 22 October 1991; accepted for publication 5 February 1992
The data published on perovskite oxides La~ _,SrxBO3_a (B = Co, Cr, Fe, Mn ) are compared to assess their use as electrodes in applications such as fuel cells, low temperature oxygen meters, etc. The oxides are reviewed in terms of their stability in temperature and oxygen partial pressure of operation, nonstoichiometry, conductivity and catalytic properties and the effect of the strontium substitution content, x. From this comparison it is found that La~_xSrxCrO3_ 6 is better suited for applications in reducing conditions, while Lat _~SrxCoO3_a is better suited for applications at higher oxygen potentials. While noble metals like platinum are currently being used for these applications, the perovskite oxides offer better kinetics at lower temperatures. To compare the kinetics, the electrochemical cell: air, electrode/Bi-based or Zr-based electrolyte/electrode, air was used and the overpotential of the interfaces was measured. The electrodes used for this comparison are Pt, Lao.7Sro.3MnO3_,~, and Lao.7Sro.3CoO3_6. The results show that for temperatures lower than 800 ° C, the oxide electrodes performed better than platinum. This is attributed to the ease of transfer of oxygen potential between the electrodes and the electrolyte.
1. Introduction
Platinum has been the ideal electrode material for most applications where sensor electrodes and catalysts are used. It is an expensive material and its supply is limited, therefore, a substitute for platinum is desirable. Secondly, when used in the finely divided state on an oxide support it is prone to sintering [ 1 ], and requires frequent replacement with new material. In recent years there has been considerable interest in perovskite oxides for application as electrodes in solid oxide fuel cells (SOFC), magnetohydrodynamic ( M H D ) generators, exhaust gas sensors in automobiles, and as catalysts for CO and hydrocarbon oxidation. Many of the requirements of electrodes for chemical sensors, galvanic and amperometric, are similar to those for the above applications namely: ( 1 ) Stability in the temperature range of operation; (2) Stability in the environment of operation; (3) High electronic conductivity; (4) High oxide ion diffusion coefficients for oxygen sensors. In this study, perovskite oxides, Lal_xMxBO3_6 ~r Presented at SSI-8, Lake Louise, Canada, Oct. 20-26, 1991.
( M = S r , B = C o , Cr, Fe, Mn) are evaluated for use as electrodes for chemical sensors in comparison to platinum from 400 to 800°C. The electrolytes considered are (ZrO2)o.92(Y203)o.o8 (YSZ) and (Bi203)o.s-(SrO)0.2 systems. A comparison is then made between perovskite oxides in terms of their stability and conductivity, and reported literature data for the number of electrons transferred in electrode reactions with each perovskite oxide in conjunction with an electrolyte. The perovskite oxide electrodes under consideration are all refractory oxides with melting points above 2000°C. Sintering of the electrode is therefore not a major problem with oxides. Also, due to the availability of oxygen in the oxide electrode which is able to participate in the process, a three phase boundary may not be essential, unlike the case of platinum electrodes, especially when used at high temperatures ( T > 700 ° C). The reaction mechanism for perovskite oxide electrodes has been much less studied than the noble metal electrodes. In fact they are believed to be similar [2] to the noble metal electrodes but show improved gas/solid exchange coupled with rapid diffusion of oxygen through the oxide electrodes.
0167-2738/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
282
C.B. A/cock et al. / Perovskite electrodes for sensors
2. Preparation of electrolytes and electrodes The bismuth based electrolyte, (Bi203)o.8 (SrO)o.2, was prepared by the solid state method. Bi203 and S r C o 3 powders were mixed in proper proportions, pressed into a pellet and sintered at 800 °C in air for 15 h. The pellets were crushed, ground, repressed and sintered again at 800°C for 15 h in air. The YSZ electrolyte, (ZrO2)0.92(Y203)o.o8 , w a s made by the Pechini process [3]. Nitrates of zirconium and yttrium were mixed with citric acid (2 moles acid per mole of electrolyte), The solution was heated to 90°C and ethylene glycol ( 1 mole per mole acid) was added while stirring the solution. Further heating caused the water to evaporate accompanied by the emission of NO\ fumes. This was followed by the formation of a gel which was charred at 400°C and calcined at 700°C for 8 h. The oxide powder was finally pressed under 20000 lb. load and sintered at 1450°C in air for 5 h, crushed, ground, re-pressed and sintered again at 1450°C for 5 h in air. The electrolyte discs were 10.2 mm in diameter and 1.2 mm thick. The porous platinum electrodes were prepared by applying a platinum paste on the surface of the electrolyte and firing at 800°C in air for three hours. The perovskite electrodes were made by the Pechini method using nitrates of lanthanum, strontium, manganese (or cobalt) in the proper proportions. The preparation method was similar to the YSZ electrolyte except for the final sintering which was done at 1000 ° C in air for 15 h. The oxides were then ball milled and applied as a paste on the whole electrolyte surface on both sides. The paste was then fired at 800°C for three hours in air. X-ray diffraction patterns of each of the materials revealed that they were the desired single phase.
3. Results of conductivity and overpotential measurements A schematic of the cell arrangements is shown in fig. 1. For conductivity measurement, the galvanostat was replaced by an impedance analyzer (Solartron 1260). The real and imaginary parts of the impedance were plotted as - Z " versus Z'. The high frequency intercept of the curve with the Z' axis gave
IGalvanostat I
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Fig. 2. Comparisonof the conductivityof YSZ and bismuth-based electrolytes. the resistance across the electrolyte, E R. A comparison of conductivity measurements for the two electrolytes is given in fig. 2 and compared with published data on (Bi203) o.s (SrO) 0.2. The conductivity of the bismuth-based electrolyte is about an order of magnitude greater than the YSZ electrolyte in the temperature interval 300 to 800 °C. The overpotential (t/) was calculated as the difference between the measured potential drop across
283
C.B. Alcock et al. /Perovskite electrodes for sensors
the electrolyte I E R and the measured potential across the whole electrode/electrolyte/electrode system. A plot ofq versus current density at 400, 600 and 800°C for oxide electrodes compared with platinum is shown in fig. 3a (for (Bi203) o.8 ( S r O ) 0.2 electrolyte)
and fig. 3b (for YSZ electrolyte). For both electrolytes, the oxide electrodes all allowed higher current than platinum at low overpotential below 800°C. Consequently, the oxide electrodes exhibited lower overpotential for the same current density. There-
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Fig. 3(a). Overpotential behavior for Lao.TSro.3MnO3_6, Lao.TSro.3CoO3_ 6 and Pt electrodes on (BizO3)o.a(SrO)o.2 electrolyte at three temperatures; (b) overpotential behavior for Lao.7Sro.3MnO3_,s, Lao.TSro.3CoO3_6 and Pt electrodes on 8 mol% yttria-stabilized zirconia electrolyte at three temperatures.
284
C.B. Alcock et al. / Perovskite electrodes for sensors
fore, oxide electrodes are superior to platinum electrode at 600°C and below. Another important fact is that the total current density at a given applied voltage obtained with all the three electrodes was much higher with the (Bi203)o.s(SrO)o.2 electrolyte than with the YSZ electrolyte. For example at 400 ° C for Lao vSro.3MnO3_~ and at an overpotential of 0.2 V, the current density for YSZ electrolyte was 2 txA/cm 2 compared to 80 ~A/cm 2 for the (Bi203)o.8(SrO)o.2 electrolyte. The current density-overpotential relationship for the perovskite electrodes is linear when used with either electrolyte but nonlinear when the electrode is platinum at T~< 600 ° C. In the latter case it is marked by a decreasing resistance as the current density is increased. This could be explained as being due to a reduction of the electrolyte accompanied by an increase in electronic conduction contribution from the electrolyte. It has been shown earlier [ 5 ] that when a high current density (0.129 A / c m 2) is passed through a YSZ electrolyte ( 12 mol% Y 2 0 3 in ZrO2), sandwiched between two platinum disc electrodes, there is a blackening of the electrolyte originating from the cathode region• This was attributed to an imbalance of oxygen being transported from the cathode and oxygen replenishment at the anode, the former being greater than the latter suggesting a rate controlling cathode reaction• Our results (fig. 3a, b) further indicate that such a phenomenon should not occur with the perovskite oxide electrodes. In order to further understand the phenomena, a simple experiment to measure the ionic transport number of the electrolyte as a function of applied current density was devised.
theoretical and experimentally observed rates of oxygen evolution are shown in fig. 4. The ionic transport number was then calculated from the theoretical amount of oxygen evolved for the applied current compared with the experimentally determined amount of oxygen evolved. Several measurements were made for each applied current. Fig. 5 is a plot of the ionic transport number of the electrolyte as a function of applied current. Since the oxygen evolution rates are small at low applied currents, there is considerable scatter in transport number calculations at low current densities. Within experimental errors, however, the transport number was found to decrease from a value of approximately 1 to about 0.6. Therefore it is probable that the platinum electrode kinetics of oxygen reduction from the atmo-
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A ( B i 2 0 3 ) o . s ( S r O ) o . 2 electrolyte pellet was coated with platinum paste on both sides as before. One side of the pellet was sealed to a quartz tube through which air was passed continuously. The second side also contained air but of a known volume. When a current was applied across the electrolyte, the volume of gas on the second side increased due to oxygen evolution. This increase in volume was measured by the displacement of a liquid column in a capillary. The
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C.B. Alcock et al. / Perovskite electrodes for sensors
sphere is slower than the oxygen evolution at the cathode. Further experiments on oxide electrodes using (Bi203)o.8(SrO)0.2 electrolyte and on both types of electrodes using YSZ electrolyte are currently being conducted and the results will be reported later. Having shown the advantage of materials of the conductive perovskite family as electrodes over platinum, a comparison can now be made between different perovskite oxide materials from available literature for use as electrode materials in sensors.
5. Stability and nonstoichiometry of perovskite oxides All the lanthanum-based perovskite oxides under consideration with a transition metal ion without a divalent ion (strontium) substitution are refractory with melting points above 2000°C. They can be used as electrodes for continuous operation up to 1200 ° C and for short times at 1600-1700°C (SOFC application requires operation in the 1000 to 1200°C range ). Their stability in reducing environments depends on the transition metal species. Table 1 lists the range of oxygen partial pressure in which each perovskite oxide is stable. Addition of a divalent ion like Sr on the lanthanum site slightly increases the oxygen partial pressure at which the oxide is reduced to new phases [ 7,9 ]. The substitution of 0.3 moles of La by Sr in LaFeO3 increases the decomposition pressure from 10 -16"95 (for LaFeO3) to 10 -14 [9] at 1000°C. A similar substitution in LaCoO3 increases the decomposition pressure from 1 0 - 7 t o 1 0 - 4 [ 7 ] . Table 1 Stability range o f oxygen partial pressure at 1000°C. P e r o v s k i t e c o m p o s i t i o n Po2 ( b a r ) range (at 1 0 0 0 ° C )
Refs.
LaCoO3 LaCrO3 LaFeO3 LaMnO3 Lao.TSro.3CoO 3_ 6 Lao.TSro.3CrO3_ ~ Lao.TSro.3FeO3 _ ~ Lao.7Sro.3MnO3-a
[6] [6] [6] [6 ] [7] [8] [9l [ 10]
1-10 -7 1 - < 10 -20 1- 10-16.95 1- 10-15.05 l - 10-4 1- 1 0 - 20 1-- 10 -- 14 1-10 -14
285
LaFeO~ and LaCrO3 are better suited for application in reducing conditions requiring low oxygen partial pressure, due to their wider stability range. LaCoO3 has the smallest range of stability among the oxides under consideration. With substitutions of Sr for La, the operating range in terms of oxygen partial pressure becomes narrower therefore the cobaltites can be used only in the high Po2 range of 1 to 10-4 arm for T> 1000 ° C. If the temperature is decreased, the operating range is broadened and the oxides can be used at lower oxygen pressures. At sufficiently low temperatures, the slower kinetics allow the use of oxides at oxygen pressures even lower than their stability range. For example, when the oxides are used as electrodes in low temperature (room temperature to 200°C) oxygen meters [ 11 ], the decomposition of the oxides at very low oxygen pressures will be too slow to have any significant effect. In order to quantify the point we can assume a square slab of material 1 mm thick which undergoes a decrease in Po: on one side. Since oxygen vacancies are the most mobile species in the material we can use D = 10-8 cm2/s as an upper estimate at 200°C, then the time required for reduction of the sample is about 50 d. The pure oxides are all stoichiometric in their stability range (table 1 ) except LaMnO3 which has a stoichiometry of LaMnO3.12 in 1 atm. oxygen at 1000°C. When a divalent ion such as Sr 2+ is partially substituted for La 3+ on the A site of the perovskite, all the perovskites exhibit nonstoichiometry under reducing conditions. The maximum nonstoichiometry, ~, is a function of the Sr content. For the composition La~_xSrxBO3_a, the maximum nonstoichiometry before decomposition is Lal_zSrxBo3_x/2 (O=x/2). For Lal_xSrxCrO3_a, 6=x/2 at P o 2 = 1 0 - 2 ° for T>1200 °C [8]. The nonstoichiometry is a function of Po2. Therefore ~', corresponding to P~2 at the surface, will be transmitted to the electrode-electrolyte interface. The higher current density for oxides compared to platinum at 400 and 600°C (fig. 3a and b) could be due to the ease of transfer of oxygen potential between the electrodes and the electrolyte.
6. Reaction with YSZ electrolyte Addition of a divalent ion such as Sr for La causes
286
C.B. Alcock et al. / Perovskite electrodes for sensors
the generation of a large number of defects [Vo +B~]. Therefore the electrode performance is improved due to increased Vo at low Po2 and increased BB at high Po2. However the electrode materials are then found to react with Z r O 2 electrolyte [12-16] with the formation of Sr2ZrO4 above 1000°C for Sr substituted perovskites or Ca2ZrO4 above 900°C for La~_~Ca~CoO3 at reduced oxygen partial pressures. At high oxygen pressures CaZrO3 is formed. Highly reducing atmospheres and/or high temperatures ( T > 1200 °C) also cause the formation of reaction products between the perovskite and zirconia resulting in the formation of La2Zr2P7. Among the unsubstituted perovskites, only LaCoO3 was found to react with Z r O 2 tO form L a 2 Z r 2 0 7 below 1000°C. Inoue et al. [17] detected the formation of L a z Z r 2 0 7 at 800°C for Lao.6Sro.4CoO3_ 6 in contact with Z r O 2 . However the manganite equivalent, Lao.6Sro.4MnO3_6, was found to be inert under the same conditions. There is therefore a need to obtain more reliable data on the reactivity of the perovskites with the electrolyte phase.
7. Reaction with bismuth-based electrolyte At present there are no reports of significant interaction between lanthanum-based perovskite electrodes and Bi203. However, bismuth based oxides, BiFeO3 [18], BiMnO3 [19], and BiCrO3 [19] are known to exist in the perovskite form at 700°C and high oxygen pressure. It is quite possible then that the lanthanum-based perovskite electrodes react with the bismuth based electrolyte at the interface. Since the interface product and the electrode are both perovskites with close matched lattice constant ( a = c = 3 . 9 3 A for BiMnO3 [19] and a = 3 . 9 3 / k for LaCrO3 in a pseudocubic approximation) it may result in a lower interface resistance compared to YSZ electrolyte.
8. Electrode processes At the three-phase boundary between gas, electrode, and electrolyte, the oxygen in the gas phase interacts with the oxygen in the electrolyte phase [2,20-22] according to the total reaction:
(l)
O2(g ) + 4 e - + 2 V o ~ 2 0 ~ .
The EMF across the electrolyte subjected to different oxygen potentials at the two ends is given by the Nernst equation:
E=RTLn~ nF Po2
(2)
with the well known value of n = 4 for reaction ( 1 ). This is the case for high temperature operation using zirconia electrolytes since 4 electrons are involved in the reaction. When the sensor is operated at temperatures below 200 ° C, the value of n has been found to be less than 4 [ 11 ]. This is attributed to the formation of other oxygen species. The successive steps of the oxygen reaction at the electrode have been described by Kleitz [23 ] with the corresponding value of n. For example: n = 1 for the formation of Oi- from 02, n = 2 for the formation of 022-, n = 3 for the formation of 2 0 - , n = 4 for the formation of 202-. Among the perovskite electrode materials, values of n ranging from 1 to 4 have been observed [ 2,11 ]. Table 2 lists the values of n for each perovskite composition. An n value of 4/3 can be explained by a mixed potential of 1/n= 1 / 4 + 1/2 arising from the formation of two oxygen species. Note that the value of n for platinum is 2 compared to the La~_~MxCoO3_a (M=Sr, Ca) oxides where it is 4. In this respect the perovskite oxides are comparable or better than platinum as electrodes. Table 2 Values of n for different perovskite compositions. Composition
n at 30°C (ref. [ I l l )
Pt LaCrO3 LaMnO3 Lao.vSro.3CrO3_a LaCoO3 Lao.vSro.3FeO3 a Lao.TSro.3MnO3 a Lao.TSro.3CoO3 a Lao.sCao.2CoO3_a Lao.6Cao.4CoO3_ a Lao.~Sr0.sCoO3_a Lao.3Sro.TCo03-6
2 1 2 2
4 4
n at 600°C (ref. [2])
2 4 4/3 4/3 4
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C.B. Alcock et al. /Perovskite electrodes for sensors Table 3 Oxygen ion diffusion coefficients in perovskite oxides. Composition
CSZ Lao.9Sro.lCoO3_a Lao.9Sro.lCoOs_a La0.79Sro.2MnO3_ a LaMgo.osCro.9503_a
Temperature ( ° C)
1000 850 900 860 1000
The catalytic property of the electrodes for carbon monoxide oxidation has been measured and compared with platinum [24]. It was shown that even at the low temperatures of CO oxidation ( T < 200°C), these oxides were comparable to Pt in catalytic activity. Therefore, perovskite oxides are viable alternatives to platinum for catalytic as well as electrode applications. The reasons for the formation of different oxygen ion species on different perovskite materials which are very similar to each other have been unexplained so far. The values of n for each perovskite oxide should be compared with bulk properties of the perovskites to shed further light on this problem. As was explained earlier, the substitution of a divalent ion like Sr or Ca gives rise to oxygen vacancies in perovskites. The concentration of oxygen vacancies (given by 6) depends on the amount of the substitution, the temperature, and oxygen partial pressure. However, nonstoichiometry data below 600 °C are not available due to the long equilibration times required at low temperature therefore a quantitative comparison between different perovskites cannot be made. The conductivities of perovskites oxides have been reported in a number of studies [25-30]. Conductivity data on La~ _xCaxCoO3_6 are not available. In table 2, the perovskite compositions have been arranged in descending order of conductivity estimated from the above mentioned studies. LaCrO3 has the lowest conductivity and compositions in the La~_ x S r x C o 0 3 _ 6 system have the highest conductivities. Lao.sCao.2CoO3_6 and Lao.6Cao.4CoO3_a have been placed in the order roughly and may be one step up or down. The qualitative picture, however, is not changed. As the conductivity is increased, the value
Tracer diff. coefficient
Chemical diff. coefficient
(cm2/s)
(cm2/s)
8.99)< I 0 - io 3.22)< 10 9
8)<10 -6 6.3)< 10 -6 7.41)<10 -6 1.7)<10 -6 3)< 10 -4
Refs.
[17] [33,34] [33] [35] [36]
of n increases from 1 for LaCrO3 to 4 for Lal _xSrxCoO3_a. From this comparison, increase in n values corresponds to an increase in the conductivity. This correlation can be made at 30°C and even at 600 °C. In the case of LaCoO3, the value of n at 30°C was 2 [ l l ] while at 600°C it was 4 [2]. This can also be explained by an increase in conductivity in LaCoO3 with increasing temperature. All the strontium and calcium substituted oxides under consideration are p-type semiconductors [26,28,29,31 ]. Some even exhibit metallic behavior as in Lal_xSr~CoO3_~ [28] and Lal_xSrxFeO3_a [ 31 ]. They all show high electronic conductivity with small activation energies (0.2 to 0.4 eV [25,26]). The conduction mechanism is hopping [ 26 ] of holes. In this mechanism it has been shown that the concentration of charge carriers at ambient oxygen pressures is fixed by strontium substitution at low temperatures where intrinsic conduction is not dominant [26]. Therefore, if the n value is dependent on the concentration of charge carriers, it would be independent of temperature for substituted perovskites as is observed. For unsubstituted perovskites this would not hold true since the generation of charge carriers can be assumed to be temperature dependent. Therefore the increase in the value of n for LaCoO3 with increasing temperature is consistent. Among the unsubstituted perovskites LaCoO3 [25 ] is an n-type conductor and LaCrO3 [26] and LaMnO3 [25] are p-type semiconductors. In the case of LaFeO3, both n-type [25] and p-type [32] conductivity have been reported. The literature data on conductivity in these unsubstituted perovskites are therefore not very reliable. One reason for this may be the presence of either donor or acceptor impur-
288
C.B. Alcock et al. / Perovskite electrodes for sensors
ities g i v i n g rise to n-type or p - t y p e c o n d u c t i o n respectively. O n e a n o m a l y exists in o u r c o r r e l a t i o n . L u k a s z e w icz et al. [ 11 ] f o u n d t h a t w i t h an LaFeO3 e l e c t r o d e t h e r e was no significant E M F g e n e r a t e d . A c c o r d i n g to o u r c o m p a r i s o n , h o w e v e r , the n v a l u e for LaFeO3 s h o u l d be b e t w e e n 1 a n d 2 since its c o n d u c t i v i t y lies b e t w e e n that o f L a C r O 3 a n d L a M n O 3 . N o c o r r e l a t i o n can be f o u n d b e t w e e n the d i f f u s i o n coefficient o f o x i d e ions in perovskites w i t h the v a l u e o f n. T h e o x i d e ion d i f f u s i o n c o e f f i c i e n t s for perovskites, listed in table 3, are all o f the s a m e order.
9. Conclusions O x i d e e l e c t r o d e s are f o u n d to h a v e b e t t e r characteristics for sensor a p p l i c a t i o n s t h a n p l a t i n u m at low t e m p e r a t u r e ( T < 600 ° C ) . At high t e m p e r a t u r e their p e r f o r m a n c e is c o m p a r a b l e to p l a t i n u m but they are not p r o n e to s i n t e r i n g or v o l a t i l i z a t i o n like Pt. A s i m p l e c o r r e l a t i o n was f o u n d b e t w e e n the cond u c t i v i t y o f the p e r o v s k i t e o x i d e s a n d the v a l u e o f n in the N e r n s t e q u a t i o n for the o x y g e n d i s s o l u t i o n rea c t i o n for all p e r o v s k i t e s e x c e p t LaFeO3. F r o m the l i t e r a t u r e d a t a a v a i l a b l e so far, d i f f e r e n t p e r o v s k i t e s fit the r e q u i r e m e n t s for use as e l e c t r o d e s d e p e n d i n g o n the c o n d i t i o n s o f m e a s u r e m e n t : ( 1 ) W h e n m e a s u r e m e n t s m u s t be m a d e at v e r y low o x y g e n partial pressures (Po2 < 1 0 - ~4 a t m ) , the c h r o m i t e is the best suitable for the p u r p o s e . (2) W h e n t h e e l e c t r o d e s are to be o p e r a t e d at int e r m e d i a t e or high o x y g e n pressures ( P o 2 > 10 -~4 a r m ) a n d at high t e m p e r a t u r e s ( T > 800 ° C ) , the ferrite a n d the m a n g a n i t e are b o t h stable b u t the m a n ganite has h i g h e r c o n d u c t i v i t y especially with a s t r o n t i u m s u b s t i t u t i o n o f 0.3. In this case, h o w e v e r , r e a c t i o n b e t w e e n the s t r o n t i u m a n d the z i r c o n i a electrolyte c a n n o t be r u l e d out. ( 3 ) F o r o p e r a t i o n at high o x y g e n partial pressures (Po2>10-5 atm.) a n d at low t e m p e r a t u r e s (T<800°C) the cobaltite, L a l _ x S r x C o O 3 _ 6 is the best suited d u e to its h i g h e r c o n d u c t i v i t y . T h e use m u s t be r e s t r i c t e d w h e n the o x i d e elect r o d e s are in c o n t a c t w i t h Z r O : to 9 0 0 ° C p a r t i c u larly in the case o f C o - b a s e d p e r o v s k i t e s due to t h e i r r e p o r t e d r e a c t i v i t y w i t h ZrO2 resulting in the form a t i o n o f La2Zr207.
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