Solid State Ionics 184 (2011) 39–41
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Electrochemical properties of novel SOFC dual electrode La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3−δ T. Delahaye a, T. Jardiel b, O. Joubert b, R. Laucournet a, G. Gauthier a,c, M.T. Caldes b,⁎ a b c
CEA, LITEN, France (LITEN), 17, Rue des Martyrs, F-38054 Grenoble cedex 9, France Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2, Rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France Universidad Industrial de Santander (UIS), Bucaramanga, Colombia
a r t i c l e
i n f o
Article history: Received 16 May 2010 Received in revised form 6 October 2010 Accepted 15 October 2010 Available online 13 November 2010 Keywords: SOFC electrodes Electrochemical performances Electrical conductivity Perovskite LSCM
a b s t r a c t The perovskite La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3−δ (LSCMMn0.30Ni0.20) was evaluated as potential electrode for solid oxide fuel cells. The electrochemical performances of LSCMMn0.30Ni0.20 for hydrogen oxidation and oxygen reduction reactions were studied at 800 °C. Symmetrical cells LSCMMn0.30Ni0.20/YSZ/LSCMMn0.30Ni0.20 were studied by electrochemical impedance spectroscopy. The total conductivity of LSCMMn0.30Ni0.20 is 22 S cm−1 in air and 0.8 S cm−1 under wet 5% H2/Ar at 800 °C. The area specific resistance (ASR) at 800 °C for hydrogen oxidation reaction is 1 Ω cm2. The ASR for oxygen reduction reaction is 1.6 Ω cm2. An ageing study during 24 h shows a relatively good stability of the electrochemical performances. LSCMMn0.3Ni0.2 seems to be a promising dual electrode for a symmetrical SOFC. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The perovskite (La0.75Sr0.25)(Cr0.5Mn0.5)O3−δ (called LSCM) was largely explored as a potential anode for solid oxide fuel cells (SOFCs) [1–7]. However, the electrocatalytic properties of LSCM towards direct oxidation of fuel have to be improved for device application. Electrochemical characteristics of ceramic electrodes can be enhanced by precipitation of metal nanoparticles on electrode surfaces during the initial stages of SOFC operation [8–10]. Instead of adding metal particles as a separate phase, they can be dissolved in the electrode phase in its oxidized form. The metal nanoparticles precipitate from the electrode phase upon heating in hydrogen at the start of SOFC operation. In previous paper [11], we used this approach to improve the electrocatalytic properties of LSCM. The series of compounds La0.75Sr0.25Cr0.5Mn0.5−xNixO3−δ and La0.75Sr0.25Cr0.5−xNixMn0.5O3−δ were prepared in air. A partial exsolution of Ni was obtained from the reduction of LSCM nickel-substituted compounds. In all cases the substituted phases showed conductivities slightly higher than those corresponding to the undoped materials. The activity and selectivity of La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3−δ (called LSCMMn0.30Ni0.20 ) toward total oxidation of fuel were also studied. This compound supports total oxidation of methane for all oxygen stoichiometries studied while the selectivity of LSCM towards total oxidation products decreases with decreasing lattice oxygen. To obtain the maximum SOFC efficiency an anode material should selectively catalyze the
⁎ Corresponding author. Tel.: +33 2 40 37 39 36; fax: +33 2 40 37 39 95. E-mail address:
[email protected] (M.T. Caldes). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.10.015
complete oxidation of fuel, so LSCMMn0.30Ni0.20 seems to be a good candidate for this application. However, electrochemical measurements are needed to confirm this assessment. Consequently, in this paper we describe the manufacture of symmetrical cells LSCMMn0.30Ni0.20/ YSZ/LSCMMn0.30Ni0.20 and their characterization by electrochemical impedance spectroscopy (EIS). Since LSCM has been reported as an efficient redox stable dual electrode (anode and cathode) [12–14], the electrochemical properties of LSCMMn0.30Ni0.20 were studied under both oxidizing and reducing atmospheres. 2. Experimental La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3-δ was synthesized as described previously [11]. XRD data were obtained using a Brüker “D8 Advance” powder diffractometer operated in Bragg–Brentano reflection geometry with a Cu anode X-ray source. Total conductivity measurements (4-probe DC) were carried out in air and under wet (PH2O = 0.023 atm) 5% H2/95% Ar between 200 °C and 800 °C. In order to promote Ni exsolution, the samples were maintained at 800 °C in a reducing atmosphere during 12 h before beginning measurements. EIS was carried out using a frequency response analyzer Solartron 1260. The impedance spectra were recorded over a frequency range 106 to 0.01 Hz with a signal amplitude of 10 mV under open circuit conditions. Symmetrical cells were fabricated by screen-printing the electrode on a YSZ dense electrolyte using a DEK245 apparatus. The inks are based on a terpineol-ethyl cellulose vehicle. Powders of LSCMMn0.30Ni0.20, Gd-doped ceria (GDC) and NiO were mixed with the vehicle in the following proportions: 60 wt.% powder/40 wt.% binder for LSCMMn0.30Ni0.20 and 50/50 for the two others materials. The obtained ink was screen printed on both faces of the YSZ dense
40
T. Delahaye et al. / Solid State Ionics 184 (2011) 39–41
pellets (~16 mm in diameter and 6 mm thick), dried at 100 °C for 10 min and annealed at 1150 °C during 3 h. 3. Results and discussion 3.1. Electrical conductivity Table 1 shows the total conductivity of LSCMMn0.3Ni0.2 at 800 °C in air and wet 5% H2. The corresponding activation energies are also given. The diminution of the conductivity under a reducing atmosphere suggests a p-type behaviour. As described before [11], the electronic conductivity of LSCMMn0.3Ni0.2 is related to the mixed valence of manganese Mn3+/Mn4+. Under a reducing atmosphere the total number of carriers decreases because of Mn4+ reduction, leading to the total conductivity drop. 3.2. Chemical compatibility Evidence of chemical reaction between LSCMMn0.3Ni0.2 with YSZ was observed by XRD (Fig. 1a). After firing for 3 h at the sintering temperature of the electrode/electrolyte interface (1150 °C), SrZrO3 was detected as a minor impurity. Nevertheless, no reaction was observed between GDC and LSCMMn0.3Ni0.2 as shown in Fig. 1b. Despite this feature, YSZ was kept as the electrolyte in order to facilitate the comparison with the electrochemical properties of LSCM [15]. However a thin diffusion barrier layer of GDC (3 nm) was used to fabricate symmetrical cells. 3.4. Symmetrical cell manufacturing Symmetrical cells were prepared by screen-printing the elctrode on the YSZ electrolyte. Hydrogen-electrode was prepared with three distinct layers: a diffusion barrier of GDC (5 μm), a functional layer of LSCMMn0.3Ni0.2 (thickness range 15–20 μm) and a thin porous Nicollector layer. For the oxygen-electrode fabrication only barrier and functional layers were used. Slurry preparation is described in the experimental section. The thermal expansion coefficient (TEC) of LSCMMn0.3Ni0.2 was inferred from X-ray diffraction patterns recorded from room temperature to 1000 °C in air. The TEC value was 13.3 × 10−6 K−1 for the temperature range 500–1000 °C. Fig. 2 shows the Scanning Electron Microscopy (SEM) micrograph of the LSCMMn0.3Ni0.2/GDC/YSZ interfaces after sintering at 1150 °C for 3 h. Although LSCMMn0.3Ni0.2 has a quite higher thermal expansion coefficient than YSZ (10.5–11 × 10−6 K−1) an adequate interface adhesion is observed. In fact, the use of a barrier layer of GDC, which exhibits a TEC value (12.5 × 10−6 K−1) intermediate between those of YSZ and LSCMMn0.3Ni0.2, decreases the mechanical mismatch between the electrolyte and the functional layer. 3.4. Electrochemical characterization The electrochemical properties of the LSCMMn0.3Ni0.2 electrode were investigated by EIS at 800 °C in wet (PH2O = 0.023 atm) 5% H2/95%
Table 1 Total conductivity of LSCM and LSCMMn0.3Ni0.2 at 800 °C and activation energies in air and reducing atmosphere. σ (S/cm) (800 °C)
Air
5% H2/Ar, 0.025 atm H2O
LSCM LSCM Mn0.30Ni0.20
13 20
9.6 · 10−1 7.5 · 10−1
Activation Energy (eV)
Air (800–500 °C)
Air (400–25 °C)
5% H2/Ar, 0.025 atm H2O (800–600 °C)
LSCM LSCM Mn0.30Ni0.20
0.33 0.30
0.23 0.25
0.39 0.49
Fig. 1. X-ray powder diffraction patterns of a) LSCMMn0.3Ni0.2, YSZ and their mixture after reaction, b) LSCMMn0.3Ni0.2, GDC and their mixture after reaction.
Ar as well as in air. Fig. 3a shows the impedance response of the H2 oxidation reaction on the symmetrical cells. As described before in the literature [15,16], a large overlapped arc is observed signifying that the reaction could be limited by several electrode processes. The polarization resistance (Rp) was inferred from the difference between the low and high frequency intercepts on the impedance curves. The area specific resistance (ASR) was calculated multiplying Rp by the surface of the cathode (0.6 cm2) all divided by 2 since the cell is symmetrical. Thus, the ASR value obtained at 800 °C was 1 Ω cm2. This value is comparable to that reported by Tao et al. [1] for the symmetrical cells LSCM/YSZ/ LSCM where a GDC thin film interface was also used, as in this work. An ageing study of the symmetrical cells was performed during 24 h. The ASR increases at the beginning but it stabilizes with time (see Table 2). During the ageing study Ni exsolution from LSCMMn0.3Ni0.2 can occur, what could lead to an improvement of electrocatalytic performances. Fig. 3b shows the impedance response of the O2 reduction reaction at 800 °C. Again a large overlapped arc is observed. The ASR value obtained was 1.6 Ω cm2. An ageing study was also performed during 24 h. The ASR remains stable with time. It must be noted that the series resistances in the two atmospheres are quite different. As we mentioned before, while in the hydrogen-electrode fabrication a Ni-collector layer was added, in the manufacture of the oxygen-electrode only one barrier
T. Delahaye et al. / Solid State Ionics 184 (2011) 39–41
41
Fig. 2. SEM image of the cross-section of a LSCMMn0.3Ni0.2/GDC/YSZ half-cell after sintering.
4. Conclusions The electrochemical performances of La0.75Sr0.25Cr0.5Mn0.3Ni0.2O3−δ for hydrogen oxidation and oxygen reduction were studied at 800 °C. The ASR value obtained under wet 5% H2 (1 Ωcm2) is comparable to the best results reported in the literature for LSCM. Thus, LSCMMn0.3Ni0.2 seems to be a promising dual electrode for SOFC. However, for practical applications ASRs of 0.2 Ωcm2 should be the target values. So, an optimization of the electrode microstructure coupled to a thorough study of the medium and low frequency contributions of impedance diagrams must be done to improve the polarization resistance. Acknowledgements This work was supported by the ANR (National Research Agency) project EVERESTE “Electrolyse de la Vapeur d'Eau Réalisée sur Cellules Symétriques Fonctionnant à Température Elevée”. References
Fig. 3. Impedance response of the LSCMMn0.3Ni0.2 electrode at 800 °C a) in wet 5% H2, b) in air.
[1] [2] [3] [4] [5] [6] [7]
Table 2 ASR time variation at 800 °C under wet 5% H2.
[8]
Ageing time
ASR at 800 °C (Ω cm2)
1h 2h 16 h 24 h
1 ± 0.1 1.1 ± 0.1 1.3 ± 0.1 1.3 ± 0.1
[9] [10] [11] [12]
layer and one functional layer were employed The absence of an optimized collecting layer for the oxygen-electrode could explain the high series resistance Rs measured in air.
[13] [14] [15] [16]
S. Tao, J.T.S. Irvine, Nat. Mater. 2 (2003) 320. X. Yang, J.T.S. Irvine, J. Mater. Chem. 18 (2008) 2349. S. Tao, J.T.S. Irvine, J. Electrochem. Soc. 151 (2) (2004) A252. E.S. Raj, J.A. Kilner, J.T.S. Irvine, Solid State Ionics 177 (2006) 1747. V.V. Kharton, E.V. Tsipis, I.P. Marozau, A.P. Viskup, J.R. Frade, J.T.S. Irvine, Solid State Ionics 178 (2007) 101. J.C. Ruiz-Morales, J. Canales-Vázquez, D. Marrero-López, J.T.S. Irvine, P. Núñez, Electrochim. Acta 52 (2007) 7217. B. Huang, S.R. Wang, R.Z. Liu, X.F. Ye, H.W. Nie, X.F. Sun, T.L. Wen, J. Power Sources 167 (2007) 39. B.D. Madsen, W. Kobsiriphat, Y. Wang, L.D. Marks, S.A. Barnett, ECS Trans. 7 (1) (2007) 1339. W. Kobsiriphat, B.D. Madsen, Y. Wang, L.D. Marks, S.A. Barnett, Solid State Ionics 180 (2–3) (2009) 257. A.L. Sauvet, J.T.S. Irvine, Solid State Ionics 167 (2004) 1. T. Jardiel, M.T. Caldes, F. Moser, J. Hamon, G. Gauthier, O. Joubert, Solid State Ionics 181 (2010) 894. J.C. Ruiz-Morales, J. Canales-Vázquez, J. Peña-Martínez, D. Marrero-López, P. Nuñez, Electrochim. Acta 52 (2006) 278. D.M. Bastidas, S. Tao, J.T.S. Irvine, J. Mater. Chem. 16 (2006) 1603. S.P. Jiang, J. Appl. Electrochim. 34 (2004) 1045. J.C. Ruiz-Morales, J. Canales-Vazquez, D. Marrero-Lopez, J.T.S. Irvine, P. Nuñez, Electrochim. Acta 52 (2007) 7217. L. Zhang, X. Chen, S.P. Jiang, H.Q. He, Y. Xiang, Solid State Ionics 180 (2009) 1076.
