Electrical degradation of LSM–YSZ interfaces

Solid State Ionics 138 (2000) 1–17 www.elsevier.com / locate / ssi

Electrical degradation of LSM–YSZ interfaces M.C. Brant, L. Dessemond* Laboratoire d’ Electrochimie et de Physico-chimie des Materiaux et des Interfaces ( INPG and CNRS), ENSEEG, BP 75, ` , France 38402 Saint Martin d’ Heres Received 26 October 1998; received in revised form 26 July 2000; accepted 4 August 2000

Abstract The impedance of an interface between La 12x Sr x MnO 3 (x 5 0.55 2 0.6) and yttria-stabilized zirconia has been analyzed as a function of time during high temperature annealings in air, and at medium temperatures. For curing below 11008C, an improvement of intimate contacts between the electrode and the electrolyte materials is evidenced. Even at 11008C, the electrode characteristic proper shows a marked degradation which increases with time. It mainly comes from an additional semicircle located on the low frequency side of the electrode impedance diagram. The additional low frequency satellite is ascribed to an alteration of the electrode reaction mechanism itself. The interface properties are shown to degrade only from 12008C. Up to 12008C, the degradation of both contributions appears reversible when cooling down the electrode. This is interpreted in terms of a narrowing of the contact area between the insulating phase and yttria-stabilized zirconia. Above 12008C, the interface aging obeys linear laws. As could be expected, the growth of a reaction product along the LSM–YSZ interface results in an increased blocking effect. A remaining degradation of the reaction rate is also detected. The results suggest that the cathodic degradation mainly proceeds in the vicinity of the triple phase boundary (TPB) line, while the alteration of interface properties acts on a larger electrode area. The formation and the growth of an insulating layer results in ohmic and polarization losses.  2000 Elsevier Science B.V. All rights reserved. Keywords: Lanthanum strontium manganite (LSM); Impedance spectroscopy; Aging effect; High temperature annealing; Electrical properties

1. Introduction The chemical reactivity between perovskite type oxides, intended to be used as cathodes in solid oxide fuel cells (SOFCs), and yttria-stabilized zirconia (YSZ), which still represents the most useful electrolyte material, has been widely investigated *Corresponding author. Tel.: 133-47-6826-565; fax: 133-476826-670. E-mail address: [email protected] (L. Dessemond).

[1–17]. From all the available electrode materials, lanthanum cobaltites can be viewed as the most reactive [18–21]. The chemical compatibility towards YSZ of strontium-doped lanthanum manganites (LSM) has been thoroughly examined both experimentally and theoretically, for temperatures at least equal to the operating temperature of SOFCs. For conventional SOFCs, it is around 10008C and their components must be sintered at higher temperatures to achieve a good adhesion between the electrode and the electrolyte materials. Yokokawa and co-workers [22–27] have made a series of

0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00769-4

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thermodynamic analysis on the LSM–YSZ interactions and have clarified their chemical features. Redox reactions seem to be the dominating contributions to the formation of additional phases. Most of the experimental studies were carried out after curing at temperatures typically higher than 12008C for times depending on the annealing temperature. The experimental procedures performed to characterise chemical reactivity are based on the use of powder mixtures (composites) and bi-layered specimen configurations. For strontium contents up to 30 mol%, the reaction product is lanthanum zirconate La 2 Zr 2 O 7 . At higher concentrations, the formation of strontium zirconate SrZrO 3 increasingly dominates. It is the major secondary phase above 50 mol% Sr. The results concerning the chemical compatibility between LSM and YSZ at lower temperatures are scarce. Van Roosmalen and Cordfunke [12] and Chen et al. [28] have detected strontium zirconate after firing at, respectively, 1100 and 10008C. At 10008C, the formation of the La 2 Zr 2 O 7 phase was evidenced at the interface between La 0.85 Sr 0.15 MnO 3 and YSZ [29]. The formation of another insulating phase, Sr 2 ZrO 4 , has been reported in the literature [20]. Recently, a microscopic observation has revealed interesting features concerning the microstructural morphology of a La 2 Zr 2 O 7 phase after high temperature bonding and after a cell operation at 10008C [29]. In agreement with other microscopic observations of the interface in bi-layers, the lanthanum zirconate phase was formed mainly along the LSM–YSZ interface. Taimatsu et al. [30] have proposed a reaction mechanism for the La 2 Zr 2 O 7 formation where manganese diffusion proceeds through grain boundaries in YSZ. Impedance spectroscopy is useful to characterize the kinetics of the LSM–YSZ interface [31–35]. Several experimental factors related to the preparation technique, such as microstructure and / or composition of the electrode material are of great importance for the properties of the cathode and thus for the performance of SOFCs [36–40]. Accordingly, the diagram shape depends on the electrode morphology and can be composed of one or two more or less overlapping semicircles [41–46]. Impedance spectroscopy has been widely used for the determination of the electric and dielectric prop-

erties of low conductivity materials as yttria-stabilized zirconia [47,48]. Measurements were performed on several types of microstructure defects such as grain boundaries, pores and second phases. It is worth stressing that nothing in the experimental technique imposes a steady arrangement of the microstructure defects for their detection. The blocking effect of a lonely microstructure defect should also be examined. The corresponding results have clearly shown that all microstructure defects behave very similarly to the regular grain boundaries in YSZ [49]. Clearly, any blocking effect is indeed geometric and depends only on the matrix conductivity. It is independent of the chemical nature of the microstructure defect [50]. Several studies have been devoted to the electrical characterization of interfaces between two solids at a macroscopic scale [51–56]. The observed effects were strictly geometric and were not associated with electrochemical reactions or polarisation effects. One important feature of experimental interest is that any microstructural change at the interface between two solids will be detected by a modification of the impedance diagram shape [57–59]. Impedance measurements on different contacts and weldings between two YSZ single crystals have clearly shown that impedance spectroscopy is sufficiently sensitive to detect the blocking responses of different families of microstructure defects located at only one interface or within a small volume beneath the working electrode [60]. The impedance characterization of the electrode behaviour reported here was performed by using cone shaped dense lanthanum manganite electrodes in order to alleviate, as far as possible, any interference with an evolution of the morphology of the electrode on the observed phenomena. The use of point electrodes has been initiated by Kleitz [61]. This kind of geometry has been developed to determine the elementary steps involved in oxygen electrode reaction by means of metallic [62,63] or lanthanum manganite based electrodes [32,64]. The cone shaped electrodes have also been used by other groups [35,68]. This paper is mainly focused on the impedance spectroscopy investigation of the electrical properties of the interface between strontium-doped lanthanum manganites with yttria-stabilized zirconia and of the

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

electrochemical behaviour of the cathode materials. Strontium-rich manganites have been selected in this study as electrode materials to favour the formation of SrZrO 3 . Impedance measurements are currently performed on the investigated cathodes at higher temperatures than those used in this first approach and they will be correlated with microscopic analysis. The corresponding results will be discussed in a forthcoming paper.

2. Experimental procedure

2.1. Sample preparation The electrolyte pellet was prepared from a YSZ commercial powder containing 8 mol% Y 2 O 3 (TZ8Y from Tosoh, Japan). The compacted powder was isostatically pressed under 400 MPa and sintered at 15008C for 2 h in air. The final density of the sample was around 98% of the theoretical value. The electrode material compositions under study were La 0.45 Sr 0.55 MnO 3 and La 0.40 Sr 0.60 MnO 3 . For the former composition, electrical measurements were performed on two point electrodes of different radius. The investigated cathodes will be called, respectively, L55L (L for a ‘low’ contact area), L55H (H for a ‘high’ contact area) and L60L. The samples were synthesized by calcination of appropriate mixtures of SrCO 3 , MnCO 3 and La 2 O 3 , at 14008C for 4 h. The products were then ground, compacted at 0.2 MPa and isostatically pressed at 300 MPa. They were sintered at 14008C for 4 h in air. This procedure was repeated twice in order to obtain single phased samples of relative density higher than 95%. The final grain size was less than 5 mm. The working electrodes were cone shaped with diamond tools. The investigated tips are typically 0.4 cm in height and 0.4 cm in diameter on their flat bases. One side of the YSZ pellet was polished with diamond paste down to 1 mm to lower, as far as possible, the influence that the interface roughness could have on impedance spectroscopy measurements. The counter and reference electrodes, deposited on the opposite side of the electrolyte pellet, were made of platinum paint (Degussa 308 A) and were baked at 10008C for 2 h in air prior to any

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measurement. The electrodes were contacted by platinum paste, grid and wires.

2.2. Impedance measurements Impedance spectroscopy measurements were carried out in a five-electrode cell in air. Three working electrodes can be investigated simultaneously. The counter electrode is annular and broad enough to cover the area opposite the working electrodes. The reference electrode is central and small. The impedance diagrams were recorded under zero dc conditions. The electrode characteristics were obtained with an Autolab potentiostat / impedance frequency analyser (Eco-Chemie, The Netherlands) in the frequency range 10 23 –10 4 Hz. The amplitude of the measuring signal was 30 mV. The electrolyte responses corresponding to the two-electrode cells formed by one working and the counter electrodes were plotted by using a Hewlett-Packard impedancemeter (HP 4192 A LF) in the frequency range 5–1.3 3 10 7 Hz. The amplitude of the measuring signal was around 100 mV to obtain well-defined diagrams. It was checked that both pieces of apparatus gave the same impedances for the same cells in the common frequency interval. The amplitude of the ac signal was varied to verify the linear behaviour of the electrical properties of the LSM–YSZ interfaces and to ensure that the observed relaxations are related to charge transport through and in the vicinity of these interfaces. Under zero dc conditions, the low frequency intercept of the electrode characteristics with the real axis (in the Nyquist plane) was found to be in agreement with the slope of the steady state I(h ) curve at h 50. This ensures that there are no more semicircles at lower frequencies. We checked this agreement several times during thermal aging and thermal history. On both the interface and electrode impedance diagrams presented hereafter, the numbers indicate the logarithm of the measuring frequency. Fig. 1 summarizes the thermal history of the examined electrodes. The electrical properties of the LSM–YSZ interfaces were characterized at two reference temperatures: 550 and 4508C. The former was chosen to describe the overall blocking response of the interface defects and the high frequency part

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

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not obey a straightforward law, they were simply plotted as a function of the curing time (see Section 3.2).

3. Results and discussion

3.1. Microstructure defects at the LSM–YSZ interface

Fig. 1. Thermal history of LSM electrodes.

of the electrode characteristic. The latter was selected to give a fair description of the high frequency specific contribution of the YSZ matrix. The electrochemical properties of the LSM electrodes were investigated at 9008C. This is the lowest temperature where we can get a reasonably complete description of the electrode characteristic in the frequency range extending to 10 23 Hz (sometimes 10 24 Hz). Furthermore, at this temperature the electrode characteristic and the interface behaviour were stable after heating from room temperature. Preliminary measurements were carried out from 500 up to 8508C (impedance diagrams were recorded every 12 h for 2 days) in order to record reproducible electrode characteristics (the corresponding results are not presented here). Then impedance measurements were performed systematically during heating and cooling cycles between medium temperatures and the chosen curing temperatures (Table 1). During each annealing, the diagrams were plotted, over all the frequency range of our equipment every 24 h. After each annealing, diagrams were also plotted at the three reference temperatures. The heating and cooling rates were of the order of 2008C / h. The correlation factors deduced from the linear and square root laws were at least equal to 0.97. Accordingly, the aging rate was defined as the increase of the measured additional resistance per unit of curing time. When the recorded variations did

Fig. 2 shows impedance diagrams recorded at 5508C in air for the L55L and L60L electrodes before any heat treatment. Impedance diagrams are

Fig. 2. (a) Interface impedances of L55L and L60L electrodes recorded at 5508C in air. (b) Blow up of the high frequency part of (a).

Table 1 High temperature curing conditions of LSM electrodes Annealing Curing temperature (8C) Time of exposure (min)

HT1

HT2

HT3

HT4

1100

1100

1200

1100

1165

1200

1250

15 625

18 385

15 770

11 070

15 470

22 555

12 550

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composed of three components: two are easily detectable, the third one is located at the high frequency end of the diagrams. It can be viewed by magnifying this part and more easily on diagrams recorded at lower temperatures (Fig. 3). The high frequency end of the electrode characteristic is depicted typically below 10 4 Hz (in experimental conditions of Fig. 2). To assign these three components, we referred to the relaxation frequency criteria [49]. According to these criteria, the relaxation frequencies of both the specific semicircle of the YSZ and the blocking effect contributions are independent of the geometrical factor of the cell. Therefore, at a given temperature, the semicircle which characterizes the bulk of the electrolyte should relax at a frequency of the order of that of the bulk semicircles obtained with symmetrical cells [47]. These criteria indicate unambiguously that the bulk component is the high frequency semicircle. It also indicates that the low frequency (LF) contribution visible in Fig. 2 relaxes at frequencies slightly lower than those of regular grain boundaries in YSZ. Moreover, its magnitude is far too large, compared to the bulk one, to be viewed as the conventional grain boundary effect in YSZ. This can be related to an interference between this regular blocking effect and that of thinner voids located at the LSM–YSZ interface, because of poor mechanical contacts between the two materials. Both high and low frequency blocking effects overlap strongly (Fig. 2). According to the relaxation frequency criteria, the relaxation frequency of the high frequency blocking effect is close to that of 1-mm pores at the interface between both two contacted YSZ single crystals (by assuming close specific conductivities of polycrystal-

Fig. 3. Interface impedances of L55L and L60L electrodes recorded at 4508C in air after an annealing at 11008C (HT1).

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line and single crystals of YSZ) [48]. Such a thickness is of the order of the estimated roughness of the electrolyte surface after polishing. The high total blocking effect indicates that a perfect contact between two hard materials is not readily achieved since both materials are not expected to have congruent surface roughnesses. The contact area is likely to consist of a large number of individual point contacts. It further evidences that the contact morphology between the electrode and the electrolyte strongly influence the impedance diagram shape [55,56]. If one refers to the characterization of cracks in YSZ [65], a ready contact between platinum point electrodes and YSZ was reached because of metal ductility (the electrode is likely to match the shape of the electrolyte surface perfectly). A good separation between the specific properties of the YSZ matrix and the additional blocking effect was achieved without any interference of the contact geometry. In the case of LSM–YSZ interfaces, the separation of both elementary contributions to the total blocking effect was not so easy. Therefore, we will consider hereafter only the blocking resistance R bl , which was calculated from the difference between the interface resistance R t and the specific resistance R s of the bulk of the electrolyte (Fig. 2). Electrode interface radii were calculated by applying the Newman equation [66]. In case of a perfect contact between the electrode and the electrolyte, this equation can be applied either with the bulk or the dc parameters. By applying Newman formula with interface resistances, electrode interface radii are far too small, further suggesting poor mechanical contacts between the two materials [67,68]. Its application with the R s values, determined at medium temperatures, gives radii varying between 400 and 600 mm which are in concordance with optical observations prior to electrical measurements. The variation of the interface resistance R t (and therefore of the blocking resistance R bl ) during high temperature annealings has been evaluated by using the additional interface resistance DR t determined by mathematical difference between the interface characteristic, measured at time t, and the initial one. A positive DR t value is related to an aging effect on the electrical properties of the interface. The change of the interface response with thermal history has been quantified by using both R t and R bl . Its

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influence on the specific resistance R s will be discussed afterwards.

3.1.1. Effect of high temperature annealing Fig. 4 shows the variation of the additional interface resistance DR t with curing time for both annealings at 11008C (HT1 and HT2). The interface resistance R t typically decreases with time during annealing HT1 for all the investigated samples suggesting an improvement of intimate contacts between LSM and YSZ. A larger contact area results in a lower decrease rate of R t . In the case of the L55L electrode, R t decreases by a factor of 1.4

Fig. 4. Variation of the additional interface resistance for LSM electrodes at 11008C. (a) Annealing HT1; (b) annealing HT2.

during a period of 260 h while it is lowered only by a factor of 1.1 for the L55H sample. For the second annealing at 11008C (HT2), the observed decreases are smaller than the previous ones. As depicted below, the observed decrease of R t is likely to be due mainly to a smaller blocking resistance. During the annealing at 12008C, an increasing blocking effect developed on the low frequency end of the interface response (Fig. 5). By assuming that the relaxation frequencies of the bulk of the YSZ matrix and the additional semicircles vary with an activation energy close to 1 eV as a function of temperature (i.e. the observed blocking effects are geometric in nature), one can deduce that the detected low frequency semicircle is also located on the low frequency side of the interface response at medium temperatures. A point we want to stress here is that, at high temperature, the relaxation frequency of the developing blocking effect is close to the upper limit of our equipment (see Fig. 5). Accordingly, this additional contribution cannot be regarded as fully significant of only one low frequency contribution. Nevertheless, the additional interface resistance DR t represents the most suitable parameter to describe an increasing blocking effect. The additional resistance DR t was found to increase linearly with the square root of curing time for all the electrodes (Fig. 6). The values determined from the recorded variation laws are summarized in Table 2. For instance, R t increases by a factor of 3.9 for the L60L electrode whereas it is enhanced by a factor of 1.5 for others over a period of 200 h. Umemura et al. [69] detected a slight increase of the interface resistance between a La 0.9 Sr 0.1 MnO 3 –YSZ composite cathode and YSZ (co-sintered at 13008C

Fig. 5. Interface impedances of the L60L electrode at 12008C (annealing HT3).

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

Fig. 6. Variation of the additional interface resistance for LSM electrodes at 12008C (annealing HT3).

for 5 h). The results of Kaneko et al. [70] on La 12x Ca x MnO 3 –YSZ interfaces (0#x#0.2) suggest a square root dependence of R t . These authors have directly related the increase of the interface resistance above 12008C to the growth of lanthanum zirconate at the interface. The recorded square root law suggests a diffusion controlled mechanism for the phenomenon described. Diffusion of ionic species could be invoked. During the initial stage of reaction between lanthanum manganites and YSZ, diffusion of manganese ions is likely to occur [29,30]. The formation of the insulating phase (La 2 Zr 2 O 7 or SrZrO 3 depending on the strontium content) by solid state reaction is likely to proceed by diffusion of other cationic species through the phase formed at the electrode / electrolyte interface [3,11,12,16,19]. In agreement with thermodynamic predictions

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[25], X-ray diffraction study of powder mixtures of La 12x Sr x MnO 3 (0#x#0.5) and YSZ showed that SrZrO 3 formed at the interface after 50 h at 12008C for strontium contents higher than 40 mol% [71]. The higher the Sr content, the larger was the amount of SrZrO 3 . From experimental data of van Roosmalen and Cordfunke [12], the estimated thickness of the SrZrO 3 phase at the investigated LSM–YSZ interface should only be of the order of 0.5 mm over 260 h during curing at 12008C. By assuming that the dielectric permittivity of strontium zirconate is higher than that of air, the blocking effect of the SrZrO 3 phase, if any, is thought to relax at the lowest frequencies [49]. By considering the slight variations of the specific resistance at medium temperatures (see below), the recorded variations of the interface resistance are thought to be mainly due to the blocking effect. At this stage, the aging effect detected at 12008C is likely to be connected to the formation of a thin SrZrO 3 layer at the LSM–YSZ interface, at least for the L60L electrode. At 11008C (during annealing HT4), the interface resistance rises with time, as opposed to that previously observed at the same curing temperature. The additional DR t was found to increase linearly with the square root of curing time (Fig. 7). The aging rates are lower than those determined during a former annealing at 12008C, further suggesting a diffusion controlled process. For temperatures above 11008C, a linear dependence of the additional interface resistance on the curing time was observed for all the electrodes (Fig. 8). The aging is an increasing function of the curing temperature and varies in inverse ratio to the contact area of the electrode (Table 2). The obeyed linear laws suggest that the described phenomenon is not chiefly controlled by

Table 2 Aging rates of the additional interface resistance for LSM electrodes at high temperatures Curing temperature (8C) 1200 (HT3) 1100 (HT4) 1165 (HT4) 1200 (HT4) 1250 (HT4)

L55L electrode

L55H electrode

L60L electrode

DR t /Dt (V min 21 )

DR t /Dt 1 / 2 (V min 21 / 2 )

DR t /Dt (V min 21 )

DR t /Dt 1 / 2 (V min 21 / 2 )

DR t /Dt (V min 21 )

DR t /Dt 1 / 2 (V min 21 / 2 )

– – 1.7310 23 2.8310 23 4.3310 23

0.45 0.22 – – –

– – 4.2310 24 6.9310 24 5.7310 24

0.23 4.62310 22 – – –

– –

2.97 0.44 – – –

8.4310 23 9.7310 23 12.9310 23

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M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

trode is increased by a factor of 1.3 at 12008C over a period of 200 h during annealing HT4. For the L55H electrode, R t increases only by a factor of 1.1.

Fig. 7. Variation of the additional interface resistance for LSM electrodes at 11008C (annealing HT4).

Fig. 8. Variation of the additional interface resistance for the L60L electrode for curing temperatures above 11008C (annealing HT4).

diffusion. The increase of the interface resistance is likely to be due to the growth of a crystalline phase by solid-state reaction. After consecutive annealings, the average thickness of the SrZrO 3 phase is expected to be of the order of 1 mm [12]. The growth of that reaction product is thought to proceed at the expense of its extension along the interface. This should result in a reduced aging rate of DR T . For instance, the interface resistance of the L60L elec-

3.1.2. Effect of thermal history In agreement with impedance measurements performed below 12008C (annealings HT1 and HT2), a decrease of the interface resistance is observed after cooling down to 5508C (Table 3). After annealing HT2, the values reported at 5508C have been recorded during cooling down to 4508C. No thermal hysteresis was detected at this reference temperature (the standard deviation is of the order of 15–20%). It is mainly due to a reduced blocking effect. The decrease of R t can be closely related to those observed at high temperatures. For instance, the blocking resistance for the L55L electrode is decreased by a factor of 4.5 at 5508C after annealing HT1, while it is slightly decreased in the case of the L60L electrode. After annealing HT1, a marked decrease of R bl is recorded for the L55H electrode. The frequency distribution of the interface blocking contributions remains nearly identical (Fig. 9). The average thickness of both families of microstructure defects are not thought to be significantly altered. On the other hand, there is a contradiction between the medium temperature measurements and the high temperatures ones after annealing HT3. For all the electrodes, the values of both interface resistance and blocking resistances are lower than those determined prior to any annealing (Table 3). In fact, cooling down to medium temperatures reduces the blocking effect previously raised at high temperatures. It looks like a ‘cleaning’ of the LSM–YSZ interface as depicted by Tricker and Stobbs [29] for the La 2 Zr 2 O 7 phase formed at the interface between a La 0.85 Sr 0.15 MnO 3 electrode and YSZ. Consequently, the area of the insulating phase became narrower. As manganese dissolution is required to form strontium zirconate, and as the average thickness of this insulating phase, if any, is thought to be low as quoted above, the observed reduced blocking effect could be connected to a ‘dissolution’ of the SrZrO 3 phase during cooling [26]. After annealing HT4, the electrical properties of the LSM–YSZ interface determined at medium temperatures can be closely related to those recorded at high temperatures. The aging of the interface

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Table 3 Specific, blocking and interface resistances for LSM electrodes recorded at medium temperatures as a function of high temperature curing conditions a Curing conditions

T55508C L55L electrode

None HT1 HT2 HT3 HT4

L55H electrode

L60L electrode

Rs (10 2 V)

R bl (10 4 V)

Rt (10 4 V)

Rs (10 2 V)

R bl (10 4 V)

Rt (10 4 V)

Rs (10 2 V)

R bl (10 4 V)

Rt (10 4 V)

7.6 8.3 7.3 9.1 7.2

11.3 2.5 2.9 2.7 5.2

11.3 2.6 2.9 2.8 5.3

8.4 9.2 7.4 8.0 7.0

1.8 1.4 1.4 1.0 1.9

1.9 1.5 1.5 1.1 2.0

9.3 10.0 9.6 9.1 8.9

4.4 3.9 3.2 4.5 10.1

4.5 4.1 3.3 4.6 10.1

R bl 5 (10 V)

Rt 5 (10 V)

Rs 3 (10 V)

R bl 4 (10 V)

Rt 4 (10 V)

Rs 3 (10 V)

R bl 5 (10 V)

Rt 5 (10 V)

6.6 4.5 4.2

8.2 5.5 8.6

8.9 6.0 9.0

10.3 8.7 8.9

T54508C L55L electrode Rs 3 (10 V) HT2 HT3 HT4

7.4 8.7 8.4

L55H electrode

1.8 1.5 2.2

1.8 1.6 2.3

L60L electrode

2.2 2.3 3.8

2.3 2.4 3.9

a

After annealing HT2, the resistances at 5508C have been recorded during cooling down to 4508C. The accuracy on the resistance values is about 5%.

Fig. 9. Interface impedances of the L60L electrode recorded at 5508C in air. (a) Prior to any annealing; (b) during cooling down from 12008C (annealing HT3); (c) during cooling down from 12508C (annealing HT4).

resistance is mainly due to the predominant blocking resistance R bl (Table 3). The adverse effect of the annealing on the transport properties of the interface is an increasing function of the aging effect recorded at high temperatures. The values of R bl and R t are lower than their initial ones (prior to any annealing) for electrodes containing 55 mol% Sr, whereas they increase by a factor of 1.8–1.9 at 5508C. On the other hand, both resistances are 2.1 times higher for the L60L electrode. The additional blocking effect

observed at medium temperatures is likely to be the electrical response of an insulating phase formed at the LSM–YSZ interface at high temperatures. The specific resistance of the YSZ matrix can roughly be regarded as constant during thermal history (Table 3). This suggests that the thickness of the SrZrO 3 phase is sufficiently small to not interfere with the specific contribution of the electrolyte. This result also evidences the frequency-dependence of the current-carrying area [49]. Accordingly, the specific resistance cannot be viewed as a suitable parameter for detection and characterization of microstructure evolution in the vicinity of an interface between two solids involving YSZ and microstructure defects of 1–2 mm average thickness. However, the formation of a sufficiently thick insulating phase should result in an additional semicircle which could interfere strongly with the specific response of the matrix [55].

3.2. Electrode characteristic Fig. 10 shows the impedance diagrams of the L55L and L60L electrodes recorded at 9008C in air

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3.2.1. Effect of high temperature annealing Fig. 11 shows the additional overpotential resistance DR h during the course of two annealings at 11008C (HT1 and HT2). A marked aging effect was detected as already reported for La 12x Ca x MnO 3 – YSZ (0#x#0.2) [70] and La 0.9 Sr 0.1 MnO 3 –YSZ interfaces [71,72]. The higher aging rate was always determined for the L60L electrode. For instance, the overpotential resistance of this electrode was increased by a factor of 1.4 over 100 h during annealing HT1. A significant shift in the frequency

Fig. 10. Electrode impedances of (a) L55L and (b) L60L electrodes recorded at 9008C in air before any heat treatment. The interface resistance has been subtracted.

before any heat treatment. The diagram shapes are typical of electrode characteristics determined with loaded pin-shaped electrodes [32,59,68]. This assignment was not regarded as questionable. In a first approximation, the diagrams presented can be typically resolved into two contributions whose magnitudes both seem to depend on the contact area and the strontium content of the electrode material. At this stage, no interpretation was proposed to explain the influence of both parameters upon the observed electrode overpotentials. The determination of the elementary steps of the electrode reaction is outside the scope of this paper. Accordingly, any variation of the overpotential resistance R h , calculated from the difference between the dc resistance of the cell and the corresponding interface resistance, has been examined by using the additional overpotential resistance DR h determined by the mathematical difference between the loop characteristics measured at time t and the initial ones. A positive DR h value can be related to an increasing overpotential resistance. Hereafter, an increasing DR h value with curing time will correspond to an aging effect of the electrodes. The effect of thermal history has been determined at 9008C by using the overpotential resistance R h .

Fig. 11. Variation of the additional overpotential resistance for LSM electrodes at 11008C. (a) Annealing HT1; (b) annealing HT2.

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

Fig. 12. Electrode impedances of the L55L electrode at 11008C (annealing HT2). The interface resistance has been subtracted.

distribution was detected on the low frequency part of the electrode characteristics (Fig. 12). No significant modification of the high frequency part of the impedance diagram was detected. The increase of R h is caused by an increasing magnitude of a low frequency satellite. The existence of this additional semicircle was further confirmed at higher temperatures. In the same curing conditions, the interface resistance was reduced (Fig. 4). Therefore, the recorded evolution cannot be solely related to a simple reduction of the active electrode reaction zone. By assuming that the ratio of the specific conductivity of YSZ to that of SrZrO 3 is about 1500 at 11008C [12] and that the dielectric constants of both oxides are of the same order, the additional low frequency loop must not be related to the electrical contribution of an insulating phase. Accordingly, the recorded aging effect can be regarded as an alteration of the electrode reaction mechanism itself. The variation laws of the steady increase of DR h depend on the heat treatment. The higher aging rate was still observed for the L60L electrode in both

11

cases (Table 4). In the course of heat treatment HT1, the aging effect is a linear function of curing time, whereas a square root law was obeyed during annealing HT2. This latter variation suggests a diffusion controlled aging effect. Data of Kaneko et al. [70] also suggest a square root dependence of the overpotential resistance of La 12x Ca x MnO 3 electrodes (0#x#0.2) (but at a higher annealing temperature). In the course of annealing HT2, the aging rates are enhanced, with respect to those determined during HT1 (Table 4). The overpotential resistance of the L60L electrode was increased by a factor of 5 over a period of 100 h. The recorded aging seems to be activated by a former annealing at the same temperature. However, the explanation of different time dependences is still unclear. At the beginning of annealing HT3, the evolution of the additional overpotential resistance can be roughly compared to that of the additional interface one (Fig. 13). Conversely, this comparison is not consistent within the whole annealing period for all the electrodes. A levelling off was detected above approximately 150 h, while a steady aging of the interface was observed (see Fig. 6). Moreover, the aging effect of R h is higher than for R t . For instance, the overpotential resistance was increased by a factor of 7.2 over a period of 150 h, whereas the interface resistance is only 3 times higher. This was confirmed by further annealings (HT4). During a subsequent annealing at 11008C (HT4) and in agreement with the DR t variations, a square root law was obeyed over the whole curing period (Table 4). The behaviours of both interface and overpotential resistances can be related to a diffusion

Table 4 Aging rates of the additional overpotential resistance for LSM electrodes at high temperatures Curing temperature (8C) 1100 (HT1) 1100 (HT2) 1100 (HT4) 1165 (HT4)

L55L electrode

L55H electrode

L60L electrode

DR h /Dt (V min 21 )

DR h /Dt 1 / 2 (V min 21 / 2 )

DR h /Dt (V min 21 )

DR h /Dt 1 / 2 (V min 21 / 2 )

DR h /Dt (V min 21 )

DR h /Dt 1 / 2 (V min 21 / 2 )

0.14 – – –

– 27.6 83.6 42.7

0.13 – – –

–

0.28 – – –

–

30.3 139.7 16.6

68.8 140.7 162.2

12

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

Fig. 13. Variation of the additional overpotential resistance for LSM electrodes at 12008C (annealing HT3).

controlled process. However, the aging effect on R h is higher than the previous ones at 11008C. This indicates that the aging is more efficient after a prior initiation of the degradation of the reaction rate. Above 11008C (annealing HT4), the aging rate decreases for the L55L and L55H electrodes and only a slightly increased one is recorded for the L60L electrode. This is in contradiction with a diffusion controlled phenomenon expected to be thermally activated. For curing temperatures from 12008C, a levelling off occurs above typically 100–150 h (Fig. 14), in agreement with the variation of DR h during annealing HT3. However, a peculiar feature is that the aging effect of R h is lower than during annealing HT3. Moreover, this aging was decreased at 12508C. In the same conditions, the aging effect of the interface resistance was seen to be thermally activated (see Fig. 8). Nevertheless, the aging effect of the overpotential resistance remains higher than that of R t . It further evidences that the degradation of the electrode performance cannot be regarded as only a reduction of its active area. According to thermodynamic calculations [23], evolving of oxygen is required during the formation of any insulating phase between LSM and YSZ. In the case of dense electrodes, it is expected to occur in the vicinity of the triple phase boundary (TPB) line. Therefore, the growth of a new phase, if any, should proceed by spreading along the interface

Fig. 14. Variation of the additional overpotential resistance for LSM electrodes at (a) 12008C and (b) 12508C (annealing HT4).

towards its centre. The fact that the overpotential resistance reaches saturation values, while the interface resistance exhibits a steady increase within the whole curing period, suggests that the additional low frequency satellite in the electrode impedance diagram is related to a part of the volume of the electrode material in the vicinity of TPB line (by assuming a small inward expansion of the electrode reaction [73]). Consequently, the higher the curing temperature, the higher the aging of the interface, as observed up to annealing HT3. The lowering of the aging effect during consecutive annealing periods supports this assumption. Without any cooling down (which results in a reversibility of the observed aging

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

as discussed below), a further aging could be expected by raising the temperature. But, the subsequent alteration of the reaction rate should proceed from a previously aged one. Accordingly, by assuming a dominating TPB process at low frequencies, the aging should be reduced.

3.2.2. Effect of thermal history The values of the overpotential and interface resistances determined at 9008C during cooling down to medium temperatures and re-heating are summarized in Table 5. A peculiar feature that we could verify several times is the reversibility of the aging of the electrode after a cooling at medium temperatures whereas no significant thermal hysteresis was detected for the interface and blocking resistances. For annealing temperatures up to 12008C (HT1 to HT3), the general trend can be summarized as follows: in

13

agreement with high temperature measurements, the overpotential resistance R h is increased when cooling down (where the interface characteristic was recorded) and then, by re-heating to the reference temperature, R h was found lower (Fig. 15). For instance, after annealing HT3, the overpotential resistance of the L60L electrode is 1.4 times that prior to heat treatment and it is lowered by a factor of 10.3 after re-heating. The latter value is smaller than the initial one (before any annealing) (Table 5). The increase of the overpotential resistance (after curing) results always in a increased magnitude of the low frequency part of the impedance diagram (Fig. 15). For curing temperatures below 12008C, this additional contribution is removed when reheating. The frequency distribution is then identical to that determined before annealing. Accordingly, this can be connected to an improvement of intimate contacts between LSM and YSZ as indicated by the

Table 5 Overpotential and interface resistances for LSM electrodes recorded at 9008C as a function of high temperature curing conditions a Curing conditions

b a b a b a b a

HT1 HT1 HT2 HT2 HT3 HT3 HT4 HT4

L55L electrode

L55H electrode

L60L electrode

Rh (10 4 V)

Rt (10 2 V)

Rh (10 4 V)

Rt (10 2 V)

Rh (10 4 V)

Rt (10 2 V)

1.9 4.8 1.7 6.5 1.5 1.5 0.7 1.6

18.3 4.9 4.6 4.6 4.9 5.8 5.5 24.2

7.3 3.5 0.4 6.9 0.3 0.5 0.2 0.4

5.0 4.4 2.5 2.4 2.9 3.4 3.3 9.5

9.8 7.1 1.1 13.9 1.3 2.0 1.4 10.2

10.8 5.1 5.4 4.9 5.4 9.1 9.4 54.2

The letters a and b indicate that measurements were performed, respectively, during cooling down to medium temperatures and during heating up to high temperatures. The accuracy on the resistance values is about 5%.

Fig. 15. Electrode impedances of the L55L electrode recorded at 9008C in air. (a) During heating up to 11008C (annealing HT2); (b) during cooling down from 11008C (annealing HT2); (c) during heating up to 12008C (annealing HT3). The interface resistance has been subtracted.

14

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

electrical behaviour of the interface. After an annealing at 12008C (HT3), the situation is quite similar in terms of overpotential resistance. Nevertheless, a significant shift in the frequency distribution of the electrode characteristic was detected (Fig. 16). The electrode reaction rate is then lowered. A simple modification of the electrode contact area is not solely implicated in the electrode reaction mechanism. As quoted above for the interface resistance, the reversibility is no longer complete after annealing HT4 which is in agreement with the high temperature measurements. Moreover, the overpotential resistance of all electrodes was higher than those recorded before this annealing (Table 5). For instance, the overpotential resistance of the L60L electrode increased by a factor of 7.4 when cooling down to medium temperatures. R h is further increased when re-heating to 9008C. The degradation of the electrode performance can be viewed as a result of the remaining low frequency satellite whose magnitude was increased during annealing (Fig. 17). The cathodic degradation of perovskite type oxide electrodes in contact with YSZ was reported to come

mainly from the growth of an interfacial product layer, whose nature depends on the composition of the electrode material [74]. The formation of an insulating phase results in a low frequency additional contribution in the electrode characteristic [75]. Its magnitude increasingly dominates the reaction rate by raising the annealing temperature [16]. In the case of an La 0.85 Sr 0.15 Mn 0.98 O 3 –YSZ interface, Mitterdorfer and Gauckler [76] suggest that this additional relaxation depicts diffusion of oxygen intermediates along the lanthanum zirconate surface, formed along the interface, prior to incorporation into the electrolyte. In this study, any change of the electrode morphology cannot be invoked. After annealing HT4, the values of the overpotential resistance of the low radius electrodes are close to those initially recorded, in spite of improvement of intimate contacts expected to occur before. This further suggests the formation of a SrZrO 3 phase at the interface above 12008C. The recorded variations of R h indicate that the insulating phase fades during cooling down from curing temperatures up to 12008C. Above 12008C, the remaining phase induces

Fig. 16. Electrode impedances of the L60L electrode recorded at 9008C in air. (a) During heating up to 12008C (annealing HT3); (b) during cooling down from 12008C (annealing HT3); (c) during heating up to 12508C (annealing HT4). The interface resistance has been subtracted.

Fig. 17. Electrode impedances of the L55L electrode recorded at 9008C in air. (a) During heating up to 11008C (annealing HT2); (b) during cooling down from 12508C (annealing HT4). The interface resistance has been subtracted.

M.C. Brant, L. Dessemond / Solid State Ionics 138 (2000) 1 – 17

an irreversible alteration of the electrode mechanism. At this stage, a modification of the electrode in the vicinity of the TPB line is likely to be the most suitable explanation for the degradation of the cathodic activity of LSM electrodes.

4. Conclusions The technique presented here allows us to monitor the interface chemical reaction as it develops from its very start and to determine its effects on both the electrical properties of an LSM–YSZ interface and the electrode reaction rate. By using dense pinshaped electrodes, no pre-sintering or annealing is required. Furthermore, any significant modification of the electrode morphology cannot be taken into account to explain the recorded alterations. The impedance diagram of the interface exhibits additional semicircles, compared to the regular grain boundary blocking effect in YSZ, which characterize voids present along the interface or reaction product. For curing temperatures below 12008C, the variations determined during heat treatments can be compared to those recorded at medium temperatures. Intimate contacts with YSZ are likely to be improved. From 12008C, a marked aging of the interface properties was detected. It mainly comes from an increasing magnitude of a low frequency satellite in the interface impedance diagram. The formation of the expected SrZrO 3 appears reversible after cooling down to medium temperatures. A disappearance of the reaction product from the interface to the inside of the electrode is suggested. Annealing temperatures above 12008C are required to enhance the blocking effect. As could be expected, the specific resistance of the YSZ matrix cannot be regarded as a suitable parameter for the detection of a thin insulating layer along the interface. Nevertheless, the blocking response of a thicker layer is likely to interfere more with the specific semicircle of the YSZ matrix. For curing temperatures from 11008C, the variations of the overpotential resistance cannot be directly connected to those of the interface resistance. Even at 11008C, a marked aging of the reaction rate was determined. Irrespective of the thermal history, the higher aging effects are always recorded for low

15

radius electrodes. This could not be interpreted in terms of a reduction of current pathways at the LSM–YSZ interface. It indicates an alteration of the electrode reaction mechanism itself. On the other hand, a reversibility of the electrode degradation was detected for curing temperatures up to 12008C. Above 12008C, an additional semicircle located on the low frequency side of the electrode impedance diagram evidences an irreversible degradation of the reaction rate. It is likely to occur in the vicinity of the TPB line. Formation and growth of an insulating layer at the LSM–YSZ interface results in both ohmic and polarization losses.

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