Electrical behaviour of LSGM–LSM composite cathode materials

Solid State Ionics 177 (2006) 1991 – 1996 www.elsevier.com/locate/ssi

Electrical behaviour of LSGM–LSM composite cathode materials Giovanni Dotelli a,⁎, C.M. Mari b , R. Ruffo b , R. Pelosato c , I. Natali Sora c a

Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, p.zza L. da Vinci 32, I-20133, Milano, Italy b Department of Materials Science, University of Milano-Bicocca, via Cozzi 53, I-20125, Milano, Italy c Engineering Faculty, University of Bergamo, INSTM R.U. – Unibg, Viale Marconi 5, 24044 Dalmine, Italy Received 1 July 2005; received in revised form 5 May 2006; accepted 17 May 2006

Abstract Composite cathode materials were prepared by mixing La0.83Sr0.17Ga0.83Mg0.17O2.83 (LSGM) and La0.8Sr0.2MnO3 (LSM) powders fired at 1300 °C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSM, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSM, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 °C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM–LSM interface and electrode polarizations are attributed above 600 °C. © 2006 Elsevier B.V. All rights reserved. Keywords: Composites; Mixed ionic–electronic conductors; Cathodes materials; IT-SOFCs

1. Introduction The research in the field of intermediate-temperature solid oxide fuel cells (IT-SOFCs), i.e. working below 800 °C, is constantly increasing, and one of the electrolytes which has attracted the greatest interest of researchers is Sr- and Mg-doped LaGaO3 (LSGM) [1,2]. Indeed, this material has an high oxygen-ion conductivity at intermediate temperatures, i.e. approximately 10− 1 S cm− 1 at 750 °C. At present, one major limit to a real advancement in IT-SOFCs technology is probably the availability of suitable electrode materials. In the case of LSGM electrolytes, the use of composite cathodes, obtained by physically mixing an electronic and an ionic conductor, e.g. LSM/LSGM [3] and LSM/YSZ [4], has been recently suggested. Contradictory results have been reported on the elemental interdiffusion and/or the chemical reactions between LSGM and LSM. Huang et al. [5] found no significant diffusion of Mn into ⁎ Corresponding author. E-mail address: [email protected] (G. Dotelli). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.05.033

LSGM and Ga in LSM at the LSGM/LSM interface of two pellets sintered at 1470 °C for 36 h, so they concluded that LSM could be an appropriate cathode material for LSGM-based fuel cells. Yi et al. [6] sintered powder mixtures of (1 − x)LSGM– xLSM (0 ≤ x ≤ 1) at 1500 °C for 6 h and observed harmful effects on the conductivity of LSGM electrolyte for x ≤ 0.16. Rozumek et al. [7] studied the chemical reactions between LSGM and LSM powders annealed at 650 °C and 1000 °C in air for 1000 h. By energy dispersive X-ray analysis (EDX), they found about 2 mol% of Mn that had substituted for Ga cation in LSGM samples annealed at 1000 °C, indicating a moderate solid solubility of Mn in LSGM. Yaroslavtsev et al. [8] found that systems with an LSGM/LSM interface exhibited low electrochemical activity toward the oxygen reaction due to formation of a poorly conducting product: the interaction between LSGM and LSM occurred, albeit in a thin surface layer, even if it was not detectable by X-ray powder diffraction (XRPD). Pelosato et al. [9] performed the electrical characterisation of the LSGM thick (about 100 μm) films sandwiched between a pellet and a film of LSM. Both LSGM and LSM coatings were deposited on the porous LSM sintered substrate

1992

G. Dotelli et al. / Solid State Ionics 177 (2006) 1991–1996

In this work the microstructural and electrical behaviour of mixtures prepared by mixing La0.83Sr0.17Ga0.83Mg0.17O2.83 (LSGM) and La0.8Sr0.2MnO3 (LSM) powders fired at 1300 °C was investigated by XRPD, SEM/EDS and EIS. Different compositions were prepared, containing 1, 5, 25, 50, 75 wt.% of LSM. Preliminary results about these materials have been recently presented [10]. 2. Experimental 2.1. Materials La0.83Sr0.17Ga0.83Mg0.17O2.83 was synthesised via a sol–gel route, La0.8Sr0.2MnO3 via common solid state mixing and firing; powder mixtures were prepared by dry mixing the appropriate amount of sintered LSGM and LSM powders and firing at 1300 °C for 2 h. For La0.83Sr0.17Ga0.83Mg0.17 the reagents were La(OCOCH3)3 (Sigma Aldrich, 99.9%), Sr (OCOCH3)2 (Sigma Aldrich, 99.995%), Ga(NO3)3 (Sigma Aldrich, 99.9%), Mg(OCOCH3)2 (Sigma Aldrich, 99.999%); for La0.8Sr0.2MnO3 were La2O3, SrCO3 and MnCO3 (Sigma Aldrich, 99.9%). Details of the synthesis procedure and composite preparation are reported elsewhere [10]. 2.2. Techniques

Fig. 1. X-ray diffraction patterns of composites with 1, 5, 25, 50, 75 wt.% of LSM. The 2θ positions of the LSGM, LSM and LaSrGa3O7 reflections are plotted on the bottom.

one after the other by screen printing technique. Although the LSGM thick film exhibited good bulk conductivity, the total conductivity was about two orders of magnitude smaller than the total conductivity of LSGM pellet specimens. This behaviour was accounted for by the formation of a new lowconducting phase, whose growth could be ascribed to the migration of the Mn cations from the electrode material to the electrolyte.

Ground pellets of LSGM and of composites with 1, 5, 25, 50 and 75 wt.% of LSM (named M01, M05, M25, M50 and M75, respectively) were characterized by X-ray diffraction (XRD) (Philips PW1830, Cu Kα radiation). The patterns were collected at room temperature, in the 2θ range 4°–70°, using graphite monochromated Cu–Kα radiation; the step scan was 0.02° 2θ and the counting time of 2 s per step. The XRD data were refined by the Rietveld method using the GSAS program of Larson and Von Dreele. Cell parameters of the starting LSGM and LSM powders were used for the calculation of the theoretical densities; the mixture rule was applied for the calculation of theoretical densities of composites. The densities of the samples were measured by the Archimedes' method in water.

Table 1 Composition, relative density and LSGM electrical conductivity, calculated by removing the LSGM/LSM interface polarization contribution from the total composite conductivity; activation energy (Ea) and capacitances at high (HT) and low (LT) temperature are shown, where applicable ID

Composition (wt.%)

M00

100

0

92

6.34 × 10− 2

M01

99

1

89

8.5 × 10− 3

M05

95

5

86

1.05 × 10− 2

M25

75

25

87

1.55 × 10− 2

M50 M75 M100

50 25 0

50 75 100

90 85 89

3.81 × 10− 1 1.05 13.5

LSGM

LSM

Relative density (%)

Conductivity 700 °C (S cm− 1) HT LT HT LT HT LT HT LT – – –

Ea (eV)

C1 (F)

C2 (F)

C3 (F)

C4 (F)

0.77 ± 0.2 0.94 ± 0.2 0.71 ± 0.3 1.01 ± 0.2 0.75 ± 0.3 1.06 ± 0.4 0.67 ± 0.2 1.04 ± 0.3 0.24 ± 0.2 0.26 ± 0.5 0.18 ± 0.2

– 1.7 × 10− 10 – 1.9 × 10− 10 – 2.2 × 10− 10 – 2.8 × 10− 10

– 1.6 × 10− 8 – 1.7 × 10− 9 – 1.9 × 10− 9 – 3.7 × 10− 9 – – –

3 × 10− 4 – 8 × 10− 6 4 × 10− 7 1 × 10− 6 2 × 10− 7 5 × 10− 6 8 × 10− 7

4 × 10− 3 – 8 × 10− 4 – 1 × 10− 4 – 2 × 10− 4 –

G. Dotelli et al. / Solid State Ionics 177 (2006) 1991–1996

Morphology observations and elemental analysis of composites polished sections were performed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The electrical characterisation was performed by electrochemical impedance spectroscopy (EIS) measurements in the temperature range 300–900 °C, in air. The current collectors were made by painting the opposite pellet faces with platinum paste and heating at 850 °C for 1.5 h.

1993

3. Results and discussion 3.1. Phase composition As shown by XRD data, the LSGM powder was not pure and a secondary phase, recognized as the melilite-type compound LaSrGa3O7 (ICDD PDF-4 card # 00-045-0637), was always present. The weight fraction for LaSrGa3O7 was 8.7(2)%; this value was calculated from structural refinements, performed in

Fig. 2. SEM micrographs of M25 (left) and M50 (right) composites at three different magnifications; colours are applied based on EDX analysis: Ga element in blue, Mn element in orange.

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G. Dotelli et al. / Solid State Ionics 177 (2006) 1991–1996

Fig. 3. Nyquist plots of composite series at low temperature (325 °C, column a) and high temperature (620 °C, column b); numbers indicate different samples: (1) M00 (pure LSGM); (2) M01; (3) M05; (4) M25. The equivalent circuits used to interpret the experimental data (×) are printed on each diagram; discrete electrical elements (i.e. R and C) in round brackets are connected in parallel, then in series to each other.

G. Dotelli et al. / Solid State Ionics 177 (2006) 1991–1996

the orthorhombic Imma space group for LSGM [11] and in the tetragonal P-421 m space group for LaSrGa3O7 [12]. X-ray diffraction patterns of M01, M05, M25, M50 and M75 composites are presented in Fig. 1. Diffraction peaks belonging to the LSGM, LSM and LaSrGa3O7 phases were detected in all composites; however, upon decreasing the LSGM content (i.e. from M01 to M75) the intensity of LaSrGa3O7 diffraction peaks decreases (see Fig. 1). No reaction products were observed in the XRD patterns of the composites. 3.2. Densities The secondary phase LaSrGa3O7, detected in the XRD pattern of starting LSGM powder, was taken into account in density calculations. It is worth noting that both the low sintering temperature (1300 °C) and the addition of the organic binder in the preparation of the pellets have affected the porosity (Table 1). Indeed PEG 8000, added in the extent of 2 wt.%, has a density of 1.21 g cm− 3, much lower than theoretical densities of LSGM (6.648 g cm− 3), LSM (6.557 g cm− 3) and LaSrGa3O7 (5.28 g cm− 3); therefore it accounts for about 9–10% of the total volume of the unfired samples. 3.3. Microstructure For brevity, only SEM micrographs of M25 and M50 samples are shown in Fig. 2, at three different magnifications, yet the same microstructural features can be found in the other specimens. Coloured images are presented, where blue pixels indicate Ga element and identify LSGM, while orange pixels indicate Mn element and identify LSM phase. Large porosities are visible in the matrix constituted of well-separated aggregates of LSGM and LSM; single grains, wherever they can be picked up, show an average size in the 1 to 5 μm range. In the SEM micrographs two opposite situations are evidenced: In (a) specimen M25, the purely ionic conductor LSGM (in blue) is dominant, with embedded LSM islands; so, a typical ionic conductor-type impedance spectra can be predicted. In (b) M50, the LSM phase forms continuous conduction paths and it is likely that the electronic percolation threshold has been reached. Hence metallic-like behaviour is expected in the electrical measurements.

1995

arises from the electrode polarization effects (here modelled with a Warburg element). Instead, in view of the very large values of C3 and C4, both HT arcs are ascribed to electrode polarization effects. Therefore grain boundary and bulk conductivities can be evaluated separately only at the lower temperatures. In the LT IS spectra of M01, M05 and M25 composites, a third arc, whose assignment is not straightforward, appears in the low frequency range. The corresponding capacity value (C3), around 10− 7 F, suggests a polarization phenomenon at the LSGM/LSM interface. Also at higher temperatures these composites show, besides the electrode polarization effect (C4 ≈ 10− 4 F), an arc due to the LSGM/LSM interface polarization (C3 ≈ 10− 6 F). As for M00, the ascription to each contribution was based on the capacitance values. The specific capacitance values are reported in Table 1, where C1 to C4 indicate bulk (≈10− 10 F), grain boundary (≈ 10− 9 F), LSGM/ LSM interface polarization (10− 7 ÷ 10− 6 F) and electrode polarization capacitances (≈ 10− 4 F), respectively. The M50, M75 and M100 samples showed a single point lying along Zreal axis over a wide range of frequencies. The high LSM content led to a semiconductor behaviour due to electron conduction path percolation (see Fig. 2). The composites conductivity is presented and compared with that of pure LSGM (M00) (Fig. 4). When LSM ratio is larger than 25 wt.%, the electron conduction path percolation leads to conductivity values much higher than both other composites and LSGM (see also Table 1). The total conductivity (i.e. the sum of LSGM ionic conductivity and LSGM/LSM polarization effect) decreases with the LSM amount in the range 1–25% and it is lower than pure LSGM (Fig. 4). Removing the LSGM/LSM interface polarization effect to the total conductivity, one can

3.4. Impedance spectroscopy The impedance spectra of LSGM (M00) and M01, M05, and M25 composites, measured at 325 °C and 620 °C, are reported in Fig. 3. The low temperature (LT) Nyquist diagrams of compound M00 show two arcs followed by a portion of a straight line (at low frequency), while at higher temperatures (HT), besides the two arcs, a resistance element has to be considered. Impedance spectra have been interpreted using the equivalent circuit modelling and arc attributions have been done on the basis of capacitance values listed in Table 1: C1 (≈ 10− 10 F), C2 (≈ 10− 8 F), C3 (≈ 10− 4 F) and C4 (≈ 10− 3 F). So, the LT arcs (labelled 1 and 2) are assigned to bulk and grain boundary contributions, respectively, while the straight line

Fig. 4. Total conductivity (S cm− 1) of pure LSGM (M00) and LSM–LSGM composites M01, M05, M25, M50 vs. 1000/T (K− 1).

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G. Dotelli et al. / Solid State Ionics 177 (2006) 1991–1996

Fig. 5. Arrhenius plots of pure LSGM (M00) and LSM–LSGM composites M01, M05, M25; for the composites, the LSGM/LSM interface polarization contribution has been removed: (a) LSGM conductivity, (b) LSGM bulk conductivity, (c) LSGM grain boundary conductivity.

observe that the residual conductivity, for brevity defined as LSGM ionic conductivity, slightly increases with the LSM concentration (see conductivities at 700 °C in Table 1), even if it is always lower than M00. In the corresponding Arrhenius plots (Fig. 5a) the composites, as well as pure LSGM, show a variation of the slope at around 600 °C, pointing out a change in LSGM ionic conduction mechanism. The activation energy at temperature higher and lower than 600 °C was calculated to be about 0.71 and 1.03 eV, respectively; no significant variation was observed between the M01, M05, M25 composites and LSGM (HT and LT Ea equal to 0.77 and 0.94 eV, respectively). When the bulk and grain boundary conductivity can be separated (lower temperatures), it is possible to observe that the behaviour of each contribution follows the trend of the LSGM ionic conductivity (Fig. 5b and c), i.e. the conductivities increase from M01 to M25. The activation energies were calculated to be about 1.01 and 1.05 eV for bulk and grain boundary, respectively; again, these values are in good agreement with those of pure LSGM (0.98 and 1.02 eV). While no significant variation exists between the composites and the pure LSGM bulk conductivity (Fig. 5b), a large difference (more than one order of magnitude) appears in the grain boundary contribution (Fig. 5c).

clearly shown in the SEM micrographs. At low (≤25 wt.%) LSM concentrations the difference in electrical conductivity between composites and pure LSGM mainly lies in the presence of an additional effect arising from the LSGM/LSM interface polarization. Such a contribution, which increases with the LSM amount, was deduced by a thorough analysis of EIS spectra. Anyway, the LSGM conductivity in composites, even after removing the LSGM/LSM interface polarization contribution, is lower than that of pure LSGM; indeed, the composites have a much higher grain boundary resistivity with respect to pure LSGM and this is probably due to their lower densities. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

4. Conclusions In LSM–LSGM composites the presence of large amounts (> 25 wt.%) of LSM induces very high conductivity values and low activation energies due to LSM phase percolation, as

[10] [11] [12]

T. Ishihara, H. Matsuda, Y. Takita, J. Am. Chem. Soc. 116 (1994) 3801. M. Feng, J.B. Goodenough, Eur. J. Solid State Inorg Chem. 31 (1994) 663. T.J. Armstrong, A.V. Virkar, J. Electrochem. Soc. 149 (2002) A1565. J.Y. Yi, G.M. Choi, J. Eur. Ceram. Soc. 24 (2004) 1359. K. Huang, M. Feng, J.B. Goodenough, M. Schmerling, J. Electrochem. Soc. 143 (11) (1996) 3630. J.Y. Yi, G.M. Choi, Solid State Ionics 148 (2002) 557. M. Rozumek, P. Majewski, T. Maldener, F. Aldinger, Mater.wiss. U. Werkst.tech. 33 (2002) 348. I. Yu. Yaroslavtsev, B.L. Kuzin, D.I. Bronin, N.M. Bogdanovich, Russ. J. Electrochem. 41 (5) (2005) 527. R. Pelosato, I. Natali Sora, V. Ferrari, G. Dotelli, C.M. Mari, Solid State Ionics 175 (1–4) (2004) 87. R. Pelosato, I. Natali Sora, G. Dotelli, R. Ruffo, C.M. Mari, J. Eur. Ceram. Soc. 25 (12) (2005) 2587. M. Lerch, H. Boysen, T. Hansen, J. Phys. Chem. Solids 62 (2001) 445. M. Steins, W. Schmitz, R. Uecker, J. Doerschel, Z. Kristallogr., New Cryst. Struct. 212 (1997) 76.