Effect of Cu doping on the electrochemical properties and structural phases of La0.8Sr0.2Mn1 − xCuxO3 (0 ≤ x ≤ 0.2) at elevated temperature

Solid State Ionics 260 (2014) 30–35

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Effect of Cu doping on the electrochemical properties and structural phases of La0.8Sr0.2Mn1 − xCuxO3 (0 ≤ x ≤ 0.2) at elevated temperature Taimin Noh a, Jiseung Ryu a, Ryan O'Hayre b, Heesoo Lee a,b,⁎ a b

School of Materials Science & Engineering, Pusan National University, Busan 609-735, Republic of Korea Metallurgical & Materials Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA

a r t i c l e

i n f o

Article history: Received 16 October 2013 Received in revised form 8 March 2014 Accepted 10 March 2014 Available online 28 March 2014 Keywords: La0.8Sr0.2Mn1 − xCuxO3 (0 ≤ x ≤ 0.2) Area specific resistance Valence change Phase transition Oxygen reduction reaction Solid oxide fuel cell

a b s t r a c t The electrochemical properties of La0.8Sr0.2Mn1 − xCuxO3 (0 ≤ x ≤ 0.2)–Gd0.2Ce0.8O2 − δ (GDC) composite cathodes are determined in the intermediate-temperature range (600–850 °C), and their cathode performance is analyzed in the context of their phase transition behavior at elevated temperatures. The addition of Cu to LSM forces Mn3+ to Mn4+, thereby increasing the Mn4+ concentration with increasing Cu doping. The increased Mn4+ concentration increases the cubic phase fraction, and thus enhances the low-temperature performance of the LSM cathode. The La0.8Sr0.2Mn0.8Cu0.2O3–GDC yields the greatest low-temperature cubic phase fraction and shows the smallest area specific resistance (0.49 Ω cm2 at 750 °C) of the compositions tested. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Lanthanum strontium manganite (La1 − xSrxMnO3 ± δ or LSM) is a well-known solid-oxide fuel cell (SOFC) cathode material, due to its thermal and chemical stability, as well as its high electro-catalytic activity for oxygen reduction reaction (ORR) at high temperatures. When compared to mixed conductors containing cobalt or iron, LSM provides significantly better thermodynamic stability, although the low ionic conductivity of LSM reduces its efficiency at intermediate temperatures of 800 °C and below. For SOFCs operating at 800–1000 °C, LSM is often the cathode material of choice [1–4]. When a La3+ ion is replaced by a Sr2+ ion on the A-site in LSM, an electronic hole (Mn4 + cation) is formed on the B-site to maintain charge neutrality. Increasing the amount of Mn4+ increases the number of Mn3+–Mn4+ hopping sites and thus improves the electrical conductivity. The amount of Mn4+ can also affect the phase transformation and the transition temperature [5,6]. Under typical SOFC operating temperatures, most perovskite-type oxides, including LSM, can undergo deleterious phase transitions that can impact their performance in cathode applications. For LSM, this elevated-temperature phase transformation behavior is usually dependent on the valence state and the composition of the constituent A- and ⁎ Corresponding author at: School of Materials Science & Engineering, Pusan National University, Busan 609-735, Republic of Korea. Tel.: +82 51 510 2388; fax: +82 51 512 0528. E-mail address: [email protected] (H. Lee).

http://dx.doi.org/10.1016/j.ssi.2014.03.012 0167-2738/© 2014 Elsevier B.V. All rights reserved.

B-site cations [5–8]. To investigate the performance of LSM, it is therefore imperative to study the high-temperature structural and valence state dynamics through in-situ analysis. We previously reported the effects of Cu doping on the structure, electrical conductivity, and area specific resistance (ASR) of LSM. We confirmed that Cu doping on the B-site resulted in additional Mn4+ by charge compensation, and that this change improved the performance of the LSM cathode [9,10]. The effect of the addition of Cu to LSM can be expressed by the following defect reaction [11].

X

þCuO;−MnO

I

•

2MnMn → CuMn þ MnMn

ð1Þ

Similar to the effect of A-site Sr substitution, the increased Mn4 + concentration due to B-site Cu substitution increases the number of Mn3+–Mn4+ hopping sites, and improves electrical conductivity as a result. As with Sr substitution, Cu substitution can also be expected to affect the phase transformation and the transition temperature of the material. The present work focuses on the structural and electrochemical properties of the La0.8Sr0.2Mn1 − xCuxO3 (LSMCu, 0 ≤ x ≤ 0.2) system at high temperatures. In-situ high temperature X-ray diffraction (HTXRD) is conducted, and the obtained diffraction patterns are quantitatively refined to investigate the structure and phase-transition behavior. The electrochemical properties are characterized by impedance analysis and ASR is used as a metric of electrode performance. The correlations

T. Noh et al. / Solid State Ionics 260 (2014) 30–35

between the structural changes and the cathode performance are discussed in the context of SOFC applications.

2. Experimental procedure Powders of three La0.8Sr0.2Mn1 − xCuxO3 compositions (LSM: x = 0; LSMCu10: x = 0.1; and LSMCu20: x = 0.2) were synthesized by the EDTA combined citrate process using nitrate solution containing La, Sr, Mn and Cu, and then calcined at 750 °C for 12 h in air. A detailed description of the synthesis process has been given in the literature [9,10]. All powders were pressed uniaxially into pellets under a hydraulic pressure of 20 MPa. The pellets were then sintered at 1050 °C for 4 h in air, and some were ground into sintered powders for additional analysis. Powder X-ray diffraction patterns of the sintered powder were recorded at room temperature using a step scan procedure (0.02°/2θ step, 0.5 s time per step) in the 2θ range of 10–90° (X'Pert-Pro MPD PW3040/60, PANalytical). All compositions showed a pure perovskite phase with the rhombohedral space group (No. 167). A high temperature X-ray diffraction (HT-XRD) analysis was carried out in order to investigate the details during phase transition upon heating from 550 to 850 °C with an interval of 50 °C. The samples were heated using a rate of 10 °C/min and kept at various temperatures for 30 min until temperature equilibrium was established. Then the XRD-patterns were recorded using a step scan procedure (0.02°/2θ step, 0.5 s time per step) in the 2θ range of 10–90°. A Rietveld analysis program, MAUD, was used for structural analysis by using the full-profile fitting method. The XRD peak shape was fitted to a pseudo-Voigt function, and the background was fitted to a fifthdegree polynomial. In the final run, the following parameters were refined: five background coefficients, scale factors, and unit cell parameters. Site occupancies were fixed according to the results of the chemical analysis. Thermal gravity analysis (TGA, STA 409 PC/PG, Netzsch) of the powders was carried out in air to 1000 °C (10 °C/min) to characterize the mass change as a function of temperature. TGA was used in this study to monitor the formation of oxygen vacancies at elevated temperature. Symmetric cells were prepared to study the electrochemical properties of the LSM–Gd0.2Ce0.8O2 − δ (GDC), the LSMCu10–GDC and the LSMCu20–GDC composite cathodes on GDC electrolytes. The GDC powder (Gd0.2Ce0.8O2 − x, Fuel Cell Materials, USA) was sintered at 1500 °C

31

for 10 h. The GDC electrolyte pellet was 15 mm in diameter and 1.5 mm thick. The LSMCu–GDC composite powders were ball-milled and mixed with binder prepared from α-terpineol and ethyl-cellulose; cathodes with an area of 0.49 cm2 were brushed onto both sides of the GDC pellets. After drying, the cathodes were sintered at 1050 °C for 2 h in air. Pt-paste current-collectors were applied on both sides, and Pt-mesh connected with Pt-wires was attached to each electrode. Impedance measurements were carried out using an IviumStat (Ivium, The Netherlands) over the frequency range from 106 to 0.1 Hz and 10 mV excitation voltage at operating temperatures from 700 to 850 °C, with intervals of 50 °C, in air. The impedance measurements were acquired under zero DC bias (i.e., open circuit condition). The electrochemical impedance spectra (EIS) results have been multiplied by 0.5 to account for the two electrodes. The impedance spectra were analyzed using an equivalent circuit via the program Z-view.

3. Results and discussion Fig. 1 shows SEM images of the composite cathodes, with varying Cu contents, deposited on GDC electrolytes; the composite electrodes produced in this study are 30–40 μm thick. For all compositions, a porous structure was observed with 0.1–0.2 μm electrode particles of LSM, LSMCu10, and LSMCu20 intermixed with somewhat larger (~ 1 μm) GDC particles. The composite electrodes are also well adhered to the GDC electrolytes. The similar microstructures of all three cathodes suggest that differences in their EIS behavior can be attributed to chemical and structural factors, rather than to morphological factors. Fig. 2 shows the EIS plots of the symmetric cells. The impedance spectra were fitted to an equivalent circuit consisting of an inductance, a serial resistance, and two resistance/constant phase element (R/Q) units. R1 was attributed to the electrolyte, electrodes and the connection wires; L was the inductance, which was attributed to the Pt–lead connection and the instrumentation; (R2, Q1) and (R3, Q2) corresponded to the high- and low-frequency arcs, respectively. The high frequency arc (R2, Q1) was interpreted as oxygen ion transfer from the electrode to the oxygen ion vacancies of the cathode or to the grain boundary resistance of the GDC component. The low-frequency arc (R3, Q2) reflected the oxygen adsorption or dissociation process. From Table 1 and Fig. 3, it can be seen that the LSMCu20–GDC composite cathode demonstrates significantly lower R2 values than the

Fig. 1. SEM images of fractured cross-sections showing the features in the interface region between composite cathodes and the GDC electrolyte. All composite cathodes on GDC electrolytes were sintered at 1050 °C for 2 h.

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T. Noh et al. / Solid State Ionics 260 (2014) 30–35

-6

-4

Z'' (Ω cm2)

R1

-3

R3 QPE2

1k

10 k

-2

R2 QPE1

-0.8 L1

-0.6

100 10 1

-1

750 °C 800 °C 850 °C

b) LSM-GDC

Z'' (Ω cm2)

-5

L1

-1.0

600 °C 650 °C 700 °C

a) LSM-GDC

R1

R2

R3

QPE1

QPE2

10 k

-0.4

1k

-0.2

0

1

0.0

1 0.2 2

3

4

5

6

7

8

9

1.4

1.6

Z' (Ω cm2)

-5 -4

L1

R1

R2

Z'' (Ω cm2)

L1

QPE2

100

-2

2.2

2.4

10 1

-1

750 °C 800 °C 850 °C

-0.8

1k

10 k

2.0

d) LSMCu10-GDC

R3

QPE1

-3

-1.0

600 °C 650 °C 700 °C

c) LSMCu10-GDC

Z'' (Ω cm2)

-6

1.8

Z' (Ω cm2)

R2

R3

-0.6

R1

QPE1

QPE2

-0.4

10 k

1k 1

-0.2

0

0.0

1 0.2 2

3

4

5

6

7

8

9

1.4

1.6

Z' (Ω cm2)

-5

Z'' (Ω cm2)

-4

L1

R1

-1.0

600 °C 650 °C 700 °C

e) LSMCu20-GDC

R2

R3

QPE1

QPE2

L1

100

1k

2.2

2.4

10 k

10

-1

750 °C 800 °C 850 °C

f) LSMCu20-GDC

R1

-0.6

-3 -2

2.0

-0.8

Z'' (Ω cm2)

-6

1.8

Z' (Ω cm2)

-0.4

R2

R3

QPE1

QPE2

10 k

1k 1

-0.2

1

0

0.0

1 0.2 2

3

4

5

6

7

8

9

Z' (Ω cm2)

1.4

1.6

1.8

2.0

2.2

2.4

Z' (Ω cm2)

Fig. 2. Nyquist plots and equivalent circuits of the symmetric cells with the composite cathodes on the GDC electrolyte obtained in air for different temperatures with zero DC bias. The active cathode area is approximately 0.49 cm2: (a, b) LSM–GDC, (c, d) LSMCu10–GDC and (e, f) LSMCu20–GDC.

other composite cathodes (LSM–GDC and LSMCu10–GDC) over the entire temperature range studied. LSMCu10–GDC showed lower R2 values than the LSM–GDC above 700 °C. Overall, the high-frequency resistance

decreases with increasing Cu content, suggesting that O2 reduction on the composite cathode is improved by the charge-transfer process due to Cu-doping. Among the composition, LSMCu10 showed the lowest

T. Noh et al. / Solid State Ionics 260 (2014) 30–35

33

Table 1 Results of fitting of LSMCu–GDC composite cathodes at measured temperatures (standard deviation ranges from 0.1% to 5%).

LSM–GDC

LSMCu10–GDC

LSMCu20–GDC

T (°C)

L (H cm2)

600 650 700 750 800 850 600 650 700 750 800 850 600 650 700 750 800 850

3.26 3.48 3.55 3.60 3.71 3.71 3.09 3.41 3.52 3.58 3.61 3.64 2.51 2.89 3.55 3.61 3.64 3.66

× × × × × × × × × × × × × × × × × ×

10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7 10−7

R1 (Ω cm2)

R2 (Ω cm2)

QPE1-Q (F cm−2)

3.13 2.34 1.91 1.64 1.45 1.36 3.47 2.59 2.08 1.78 1.60 1.47 3.86 2.63 2.05 1.78 1.58 1.46

2.07 1.28 0.76 0.44 0.33 0.26 3.11 1.35 0.57 0.31 0.18 0.12 1.09 0.60 0.19 0.15 0.10 0.08

1.47 2.22 3.26 4.23 4.34 4.97 1.79 2.76 4.44 6.47 1.12 2.21 1.76 2.27 7.17 1.28 3.69 6.14

low-frequency resistance (R3) over the measured temperature range, although R3 appears to be relatively insensitive to the amount of Cu doping. LSM–GDC and LSMCu10–GDC composite cathodes showed a higher value for R3 than for R2 above 650 °C, which suggests that O2 reduction was limited primarily by the charge-transfer process for these cathodes. For the LSMCu20–GDC composite cathodes, the highfrequency resistance was smaller than that of the low-frequency resistance, indicating that the limiting step was likely the oxygen adsorption or dissociation process for this cathode. The ASR was calculated as the sum of these resistances (ASR = R2 + R3). In general, the ASR embodies the overall cathodic resistance with respect to oxygen reduction, oxygen surface/bulk diffusion, and gasphase oxygen diffusion. The ASR of the LSM–GDC cathode is 1.34 Ω cm2 at 700 °C and 0.79 Ω cm2 at 750 °C; these values are higher than either of the Cu-doped cathodes (LSMCu10–GDC and the LSMCu20–GDC). Among the compositions, the LSMCu20–GDC has the lowest ASR over the entire temperature range studied. Table 1 tabulates the detailed ASR values. The relative performance improvement of the

: R2 3

: R3

LSMCu20-GDC

2

0 3

10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−5 10−5 10−6 10−6 10−6 10−5 10−5 10−5

R3 (Ω cm2)

QPE2-Q (F cm−2)

0.73 0.77 0.85 0.92 0.84 0.82 0.58 0.62 0.69 0.71 0.71 0.65 0.60 0.55 0.76 0.66 0.67 0.65

3.11 1.23 0.63 0.32 0.18 0.10 2.51 1.19 0.56 0.29 0.14 0.07 3.32 1.73 0.87 0.40 0.19 0.08

1.23 2.88 4.44 6.28 1.07 6.67 1.89 1.29 8.39 8.22 8.65 1.06 1.11 1.50 1.55 1.69 1.71 2.01

10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−5 10−5 10−5 10−4 10−5 10−5 10−5 10−5 10−5 10−5

ASR (R2 + R3) (Ω cm2)

0.63 0.69 0.71 0.71 0.56 0.50 0.84 0.82 0.82 0.78 0.70 0.64 0.64 0.64 0.65 0.64 0.62 0.70

5.18 2.51 1.39 0.76 0.51 0.36 5.62 2.54 1.13 0.60 0.32 0.19 4.41 2.33 1.06 0.55 0.29 0.16

3

LSMCu10-GDC 2

2 1 0 3

× × × × × × × × × × × × × × × × × ×

QPE2-n

Cu-doped LSM composite cathodes increases with increasing temperature. These results suggest that the intermediate and high-temperature electrode performance of the LSM is improved by the addition of Cu. All the Arrhenius plots of the ASR values derived from EIS measurements in Fig. 4 show a single slope, which implies that the same reaction mechanisms control the overall electrode behavior in the temperature range studied for each cathode. The activation energy (Ea) of LSM– GDC is 0.91 eV, which is consistent with previous reports in the literature [12]. Ea increases noticeably upon Cu doping. However, Ea appears to be relatively insensitive to the amount of Cu doping, as both the LSMCu10–GDC and the LSMCu20–GDC compositions show approximately the same Ea (1.15 vs. 1.14 eV). The increased activation energy for the Cu-doped compositions suggests a shift in the relative electronic/ionic transference numbers, and/or a change in the ORR mechanism for these cathodes, compared to the undoped cathode. Fig. 5 shows the detailed diffraction peaks around 2θ = 32° for the LSMCu powders, as measured via in-situ HT-XRD. The strong doublet of the main peak is observed at room temperature around 2θ = 32°; a slight shift in the room temperature peak positions to higher angle is observed with an increasing Cu doping level. This shift may be due to the increase in the concentration of smaller Mn4+ ions compared with Mn3+ ions, which would result in unit cell contraction [9]. For all compositions, the doublet-peaks increasingly change to single-peaks as the measurement temperature increases. At 800 °C, the peak also broadens and shifts to a higher angle with increasing Cu content. This change indicates that the (110) and (104) peaks of the rhombohedral system

ln (ASR) (Ω cm2)

R2 and R3 (Ω cm2)

1

× × × × × × × × × × × × × × × × × ×

QPE1-n

LSM-GDC

2

LSM-GDC 0.91 eV LSMCu10-GDC 1.15 eV LSMCu20-GDC 1.14 eV

1 0 -1 -2

1 -3 0.85

0 0.9

1.0

1.1

1000/T (K-1) Fig. 3. Temperature dependence of R2 and R3 of each composition.

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1000/T (K-1) Fig. 4. Arrhenius plots of the area specific resistance under air for the composite cathodes on the GDC electrolyte.

34

T. Noh et al. / Solid State Ionics 260 (2014) 30–35

360

LSMCu20

R3c

800 °C 600 °C RT

LSMCu10

800 °C 700 °C 600 °C RT

(110)

800 °C

LSM

700 °C (110)

(104)

600 °C

31.5

32.0

32.5

33.0

33.5

LSM LSMCu10 LSMCu20

356

59.4 59.2 59.0 58.8 58.6 58.4 58.2

Pm3m 550

RT

31.0

358

Unit cell volume (Å3)

Intensity [Arb. Unit]

700 °C

34.0

Diffraction angle (2θ)

600

650

700

750

800

850

Temperature (°C) Fig. 7. Refined unit cell volume as a function of temperature for LSMCu (550–850 °C).

Fig. 5. Detail of the (110) and (104) diffraction peaks around 2θ = 32° for the LSMCu powders, measured via in-situ HT-XRD.

R3c phase fraction (%) Pm3m phase fraction (%)

change to the single (110) peak of the cubic system [8,13]. The rhombohedral-cubic phase transition was reported for the case of Sr substitution of A-site cations in the LSM [6]; this transition also clearly occurs in the LSMCu system at high temperatures. Undoped LaMnO3 is orthorhombic at room temperature and shows a transformation from orthorhombic to rhombohedral at ~600 °C. This transformation is attributed to the oxidation of some of the Mn3+ to Mn4+ ions, which reduces the energy for producing long-range Jahn– Teller ordering, and is dependent on the Mn4+ content. Doping lowervalence cations such as Sr2 + and Ca2 + at the La sites increases the Mn4+ concentration in LaMnO3, and thus similarly affects the transformation [5,6,8]. By analogy, we speculate that the increased concentration of Mn4 + formed by the addition of Cu to the LSM system could affect the temperature of the phase transformation, and that this change could enhance electrode performance. To obtain more detailed structural information, the structural parameters of the analyzed powders were calculated by Rietveld refinement. When combined rhombohedral R3c and cubic Pm3m models were applied to the patterns, the reliability factor for refined parameters indicated the goodness-of-fits from 1.43 to 1.52 between

100 LSM

80

LSMCu10 LSMCu20

60 40 20 LSM

80

LSMCu10

60

LSMCu20

40 20 0 550

600

650

700

750

800

850

Temperature (°C) Fig. 6. Refined phase fraction as a function of the temperature for LSMCu (550–850 °C).

the experimental and calculated XRD patterns. Fig. 6 shows the resulting refined parameters for the cubic and rhombohedral phase fractions, while Fig. 7 shows the refined unit cell volume as a function of temperature. As expected, the cubic phase fraction increases with temperature, and as a result, the rhombohedral phase fraction decreases. However, for all compositions, the transition from the rhombohedral to the cubic phase is not fully completed at 850 °C. Tao et al. [14] described the transition behavior of the La0.75Sr0.25Cr0.5Mn0.5O3 − δ system. By extrapolating from the observed data, they estimated that the transition from the rhombohedral to the cubic phase should be complete around 1100 °C. For the LSM, the fractions of cubic and rhombohedral phases at 550 °C are 26.1% and 73.9%, respectively; for the LSMCu10, 45.3% and 54.7%; and for the LSMCu20, 40.1% and 59.9%. The cubic phase fraction is significantly higher in the Cu-doped materials, particularly at lower temperatures. At 850 °C, the LSMCu20 is almost completely transformed to the cubic phase (with a cubic phase of 96.9%, compared to 89.8% for LSM and 94.3% for LSMCu10). We hypothesize that this is due to additional Mn4+ formed in response to the B-site Cu substitution. Considering the changes in unit cell volume that occur with temperature, LSMCu10 exhibits the largest unit cell volume, whereas LSMCu20 exhibits the smallest unit cell volume. The LSM unit cell volumes are comparable or slightly smaller than those of the LSMCu10. For the LSMCu10, the dilation effect, due to the large ionic radius of the Cu2+ ion (0.73 Å for 6-coordinated), appears to outweigh the contraction effect due to the Mn3 +–Mn4 + transition. At the higher Cu doping level associated with LSMCu20, however, the contraction effect due to Mn4+ appears to outweigh the dilation effect of the Cu2+ ions, which suggests that these dilation/contraction effects are non-linear with composition. It has been reported that larger volume provides more space in which mobile ions may move more easily, which would therefore reduce the activation energy for anion migration [15–17]. It has also been reported that symmetric structures yield higher mobilities and reduced activation energies for ionic diffusion. The volumes of the LSM and the LSMCu10 are larger than that of the LSMCu20, but their polarization resistances are higher. On the other hand, the LSMCu20 produces a greater cubic phase fraction at lower temperature, as compared to the other compositions. We concluded that the electrode performance of the LSM system is primarily related to the increased cubic phase fraction. In other words, we hypothesize that the greater abundance of Mn4+ ions formed by the addition of Cu favors the rhombohedral-to-cubic phase transformation, and the resulting increase in the cubic phase fraction and oxygen vacancy enhances the performance of the LSM cathode.

T. Noh et al. / Solid State Ionics 260 (2014) 30–35

102

Weight loss (%)

4. Conclusions

LSM LSMCu10 LSMCu20

101

The results of HT-XRD and Rietveld refinement showed that the additional Mn4+ by the Cu doping of LSM promoted the phase transition from rhombohedral to cubic. The highest proportion of cubic phase was exhibited by the LSMCu20, followed by LSMCu10 and LSM in the intermediate temperature range. The unit cell volumes of LSM and LSMCu10 were larger than those of LSMCu20 across the temperature range. Although a larger unit cell volume might provide more space for mobile ions to move more easily, the polarization resistances of the LSM and the LSMCu10 composite cathodes were higher than that of the LSMCu20. These results showed that the oxygen reduction reaction on the charge transfer process is improved by Cu addition. We hypothesize that the electrode performance of the LSM system is improved by increasing the fraction of the cubic phase and the oxygen vacancy concentration via Cu doping. The Cu doping improved the performance of the LSM cathode due to an increase in Mn4 + ions, which promoted the transition to cubic phase at relatively lower temperature.

100 99 98

0.91 % 1.67 % 2.54 %

97 96 95 200

400

600

800

1000

Temperature (°C) Fig. 8. TG results for LSMCu in air.

Examining the relationship between cubic phase fraction and electrode performance, it is noted that the biggest performance benefit of the Cu-doped LSM is obtained at the highest temperatures although the biggest difference between the cubic and rhombohedral phase fractions is observed at the lowest temperatures. Generally, the diffusion of oxygen ions through the perovskite structure occurs through a vacancy mechanism. As a result, the concentration of vacancies plays an important role in the diffusivity of oxygen. In order to investigate the Cu-doping effect on the oxygen vacancy concentration of La0.8Sr0.2Mn1 − xCuxO3, TG analysis was carried out in air on samples LSM, LSMCu10 and LSMCu20. The results are shown in Fig. 8. Sample weight continues to decrease monotonically with increasing temperature: the weight loss at 750 °C is about 0.91% for LSM, 1.67% for LSMCu, and 2.54% for LSMCu20. The weight loss can be mainly associated with the release of lattice oxygen, which results in the formation of oxygen vacancies and the valence change of metal ions, according to the following reaction [18]: 1 X • •• X OO þ 2MnMn →VO þ 2MnMn þ O2 : 2

35

ð2Þ

Zhi et al. [19] reported that this process becomes important above 670 °C (by TPD experiments) in La0.5Sr0.5MnO3. Additionally, we have previously analyzed the Mn L2,3-edges and O K-edge spectra for the LSMCu system by using X-ray absorption spectroscopy, and confirmed that the additional Mn4+ formed in response to the B-site Cu substitution led to the generation of additional oxygen vacancies [20]. Therefore, we expect that at temperatures above 700 °C, increased vacancy concentration will lead to enhanced ionic conductivity in the materials with higher cubic phase fraction, and thus result in higher performance.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) throuh GCRC-SOP (No. 2011-0030013). We are also especially appreciative to Ms. Mary Van Tyne for her language editing of the manuscript.

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