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Structural and electrochemical properties of the Sr0.8Ce0.1Fe0.7Co0.3O3−δ perovskite as cathode material for ITSOFCs
Solid State Ionics 147 (2002) 41 – 48 www.elsevier.com/locate/ssi
Structural and electrochemical properties of the Sr0.8Ce0.1Fe0.7Co0.3O3d perovskite as cathode material for ITSOFCs M.T. Colomer *, B.C.H. Steele, J.A. Kilner Centre for Ion Conducting Membranes, Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP, UK Received 1 October 2001; received in revised form 17 December 2001; accepted 17 December 2001
Abstract Powders of the perovskite of nominal composition Sr0.8Ce0.1Fe0.7Co0.3O3 d (SCFC) were prepared by a modified citrate route in order to determine its structural and microstructural characteristics. In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell M2ap 2ap M2ap. Two reflections in the selected area electron diffraction may be interpreted as first-order satellites, of different basic reflections, arising from an ˚ ). Symmetrical electrodes of Sr0.8Ce0.1Fe0.7Co0.3O3 d powders incommensurate modulation along the b* direction (b 2.4ap A were deposited on Ce0.9Gd0.1O1.95 (CGO) ceramic pellets, and the electrochemical properties of the interfaces between the porous cathodes (SCFC) and the electrolyte (CGO) have been investigated at intermediate temperatures (500 – 766 C) using AC impedance spectroscopy. SCFC electrodes exhibited an area-specific resistivity (ASR) at 700 C of 0.86 V cm2. The behaviour of the perovskite cathodes was interpreted in terms of available oxygen ion kinetic data. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Intermediate temperature solid oxide fuel cells; Cathode; Orthorhombic perovskite; Structural study; Mixed conductor
1. Introduction Traditionally, fuel cell systems aimed at large scale power production have been based on stabilised zirconia (YSZ) electrolytes operating at high temperature (1000 C), which places considerable restrictions on the materials that can be used in both the cell construc*
Corresponding author. Present address: Instituto de Cera´mica y Vidrio, CSIC, Antigua Crtra. Valencia Km. 24,300, 28500 Arganda del Rey, Madrid, Spain. Tel.: +34-91-871-1800; fax: +3491-870-0550. E-mail address: [email protected] (M.T. Colomer).
tion and in the balance-of-plant. To overcome these problems, it has been widely accepted that the operating temperature of the device should be lowered in order to minimise the stringent requirements placed on the balance-of-plant at the high temperature operation. This has lead to the development of intermediate temperature SOFC (IT-SOFC) operating at 500– 700 C. The performance of IT-SOFCs is strongly dependent on the cathode/electrolyte interface since the interfacial polarisation of solid-state cells increases rapidly as the temperature is decreased. At these reduced operating temperatures, it is necessary to use electrolytes with higher ionic conductivity than YSZ.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 0 2 - 4
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M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
The electrodes for IT-SOFC should have high electrical conductivities, adequate porosity for gas transport, good compatibility with the electrolyte and longterm stability. There is a need to understand the behaviour of electrodes in terms of microstructural and electrochemical parameters in order to optimise performance. More particularly, because of the large cathode polarisation at these low operating temperatures, the need for considerable improvements in cathode materials and microstructures has resulted. Interest is currently being expressed in porous ceramic structures fabricated from perovskites oxides with high electronic conductivities referred as mixed electronic and ionic conductors (MIECs). Many of the perovskites structured oxides show a wide range of oxygen stoichiometry, indicating the tendency to ionic and electronic defect structures. These are the basis for a variety of electrical properties ranging from insulating materials through ionic to electronic conductors. Additional doping on A and B sites makes it easy to change the electrical characteristics of perovskite-type oxides. Yokokawa et al. [1,2] observed a strong correlation between the formation enthalpies of perovskite phases and the Goldschmidt numbers with the thermodynamic stability increasing with larger Goldschmidt number values. Strategies to optimise the ionic conduction are provided in recent surveys by Kilner [3], Cook and Sammells [4], and Paulsen [5]. The work of Teraoka et al. [6], and reports by Mazanec et al. [7] indicated that mixed conducting compositions close to the brown-millerite stoichiometry [A2B2O5, e.g. Sr2(Fe,Co)2O5] often exhibited very high oxygen permeability at high temperatures ( > 800 C). However, A2B2O5 phases usually undergo a disorder (cubic) Z ordered structure at intermediate temperatures (650 –750 C), which is accompanied by a significant decrease in oxygen ion conductivity. The report by Trofimenko et al. [8] that the high temperature disordered cubic structure could be stabilised at lower temperatures by the addition of small amounts of CeO2 encouraged us to examine this stabilised phase as a possible cathode material for IT-SOFC operation in the temperature range 500 –700 C. It was hoped that many of the anion vacancies incorporated into the disordered A2B2O5 phase would remain mobile at lower temperatures. Moreover, the stabilised phase exhibited an acceptable electronic conductivity at 500 C ( f 300 S cm 1).
Trofimenko and Ullmann [9] studied the structure, oxygen stoichiometry, ionic and total electrical conductivities of Sr0.9Ce0.1Co1 xFexO3 d, where x = 0 – 0.8, and Sr0.8Ce0.2Co1 xFexO3 d, where x = 0.2. They showed that the total conductivity was enhanced by the Co content and reached 200 S cm 1 at 600 C. This fact encourage us to study the structural and electrochemical features of the perovskites in the above-mentionedsystem. Single-phase Sr0.9Ce0.1Fe0.7Co0.3O3 d proved difficult to make by a modified Pechini route. For this reason, an A-site-deficient composition Sr0.8Ce0.1 Fe0.7Co0.3O3-d (SCFC) with a reduced SrO thermodynamic activity was chosen in this work. Structural and microstructural studies of that perovskite were carried out. The electrochemical response as cathode of that composition on CGO electrolytes was studied and correlated with its microstructure.
2. Experimental 2.1. Preparation of materials Ce0.9Gd0.1O1.95 powder from Rhodia was used for fabrication of the electrolytes. The powder was uniaxially pressed at 15 MPa pressure and subsequently isostatically pressed at 300 MPa to ensure a high green body density. Sintering was carried out at 1450 C for 6 h. Final densities were measured by the Archimedes’ immersiontechniqueinwater. Samples of the nominal composition Sr0.8Ce0.1Fe0.7 Co0.3O3 d were prepared using a modified citrate route [10]. Solutions of the relevant nitrates, previously standardised by atomic absorption spectroscopy, were combined with an excess of citric acid and heated. The resulting product was then dehydrated and calcined at 500 C/5 h to remove any remaining organics, before being calcined at 1000 C/1 h, sieved, micro-milled and sieved again. Calcinations at 1100 C/10 h and 1200 C/20 h were performed in air to obtain the perovskite solid solution. 2.2. Structural and microstructural characterisation A Phillips PW1710 X-ray diffractometer using CuKa radiation was used to determine the structure of the SCFC electrode powders with a scanning rate
M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
of 0.0752h min 1, using silicon as internal standard. The XRD patterns of the powders were used to calculate lattice parameters. The powders calcined at 1200 C/20 h were ultrasonically dispersed in acetone. A few drops of the resulting suspension were deposited on a carbon-coated grid. Selected area electron diffraction (SAED) studies were performed with an electron microscope JEOL 2000FX (double tilt F 45 C) working at 200 kV. The microstructure of the electrode and the electrode/electrolyte interface was studied by scanning electron microscopy (SEM, JEOL T200).
43
thermodynamic activity, a single phase was identified as an orthorhombic perovskite structure having lattice parameter values of: a = 0.54911 F 0.00005 nm, b = 0.77136 F 0.00005 nm, c = 0.54642 F 0.00005 nm and cell volume 0.23144 F 0.00001 nm3. Trofimenko and Ullmann [9] reported cubic symmetries for different perovskites with nominal compositions Sr0.9 Ce0.1 Co 1 x Fe x O 3 d, where x = 0 – 0.8, and Sr 0.8 Ce0.2Co1 xFexO3 d, where x = 0.2. In our case, a deficient A-site could provoke a change in the lattice symmetry. 3.2. SEM
2.3. Fabrication of cathodes. Porous thick structures A thick and porous layer on top of the CGO electrolyte was fabricated by brushing of the relevant perovskite slurry. After deposition, the cathode was dried and fired at 1000 C/2 h. The porosity of the sintered cathode was determined by mercury porosimetry (Model Autopore II 9220, Micromeritics, Norcross, Ga) with an intrusion pressure between 1.33 and 414 MPa and a resolution power of F 1% of total scale, i.e. 0.207 MPa. A 6.12-cm3, 1-Torr (133 Pa) penetrometer was used to study degasified samples at room temperature.
A SEM micrograph of a fractured surface (Fig. 1) of a SCFC/CGO cross-section indicates that the thickness of the electrode is about 130 mm and the grain size of the SCFC is of the order of 9 mm. Porous electrodes were apparent (Fig. 2). The average porosity value was about 30% (measured by mercury porosimetry), whereas the CGO electrolyte was dense (98% theoretical density). The electrode microstructure appeared uniform and showed good bonding and continuous contact with the dense electrolyte pellet.
2.4. Electrical and electrochemical characterisation
3.3. TEM
AC impedance spectroscopy measurements of CGO pellets coated with Pt were carried out using a Solartron SI 1260 Impedance/Gain-Phase Analyzer with a 50mV signal over a frequency range of 20 MHz to 1 mHz in the temperature range from 150 to 700 C. AC impedance spectroscopy measurements were used to obtain the electrode resistivity of symmetrical perovskite electrodes on CGO electrolytes in the temperature range from 500 to 766 C. The sample was mechanically contacted with spring-loaded platinum gauze and held in a purpose built sample holder that fitted within a tube furnace.
In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell M2ap 2ap M2ap. Fig. 3 corresponds to a selected area electron diffraction (SAED) pattern of a Sr0.8Ce0.1 Co0.7Fe0.3O3 d crystallite taken along [1 0 1] zone axis while Fig. 4 depicts a schematic representation of a possible indexation of the observed electron diffraction (ED). The contribution of different microdomains where the double lattice parameter is oriented in different perpendicular directions can explain the existence of two apparent doubled axes. In any case, the doubled parameter is not commensurate since, as it can be seen, along the b* axis, two weak reflections are present instead of one only spot coming from 010 planes which are in fact not observed. Both reflections may be interpreted as first-order satellites, of different basic reflections,
3. Results and discussion 3.1. X-ray characterisation By preparing an A-site-deficient composition Sr0.8 Ce0.1Fe0.7Co0.3O3 d (SCFC) with a reduced SrO
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Fig. 1. SEM micrograph of the fractured surface of a Sr0.8Ce0.1Fe0.7Co0.3O3 d/CGO interface.
Fig. 2. Microstructure of the porous cathode.
M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
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quently performing a nonlinear least squares fit to the bulk and grain boundary semicircle. Above 400 C, the grain boundary and bulk processes could not be measured, as they became indistinguishable. Our data coincide perfectly with previous studies on CGO [11 – 13]. 3.5. Impedance studies of the electrodes
Fig. 3. SAED pattern along [1 0 1] zone axis of Sr0.8Ce0.1Fe0.7 Co0.3O3 d.
arising from an incommensurate modulation along the ˚ ). b* direction (b 2.4ap A 3.4. Conductivity of CGO electrolytes The conductivity of the samples was determined by plotting the data on the complex plane and subse-
The evolution of the impedance spectra measured at different temperatures in air is shown in Fig. 5. The increase of the measurement temperature resulted in a significant reduction of the area-specific resistivity (ASR) typically from 5.33 V cm2 at 600 C to 0.86 V cm2 at 700 C and to 0.23 V cm2 at 766 C. The shape of the complex impedance arcs of the cathode material is similar to that found for LSCF [13], although in the latter case, two arcs are distinguishable at 590 C and, in our case, the high and low frequency arcs overlap. For LSCF calcined at 800 C, the spectrum at 590 C depicts an arc at high frequency (peak at 6 kHz) with a capacitance of 10 5 F and is attributed to charge transfer of O2 ions between the electrode and the electrolyte, and the low frequency arc (peak at 158 Hz) with a capacitance of the order of 10 4 F is due to concentration polarisation caused by the diffusion and exchange of oxygen species to the electrode/electrolyte interface [14]. The latter process dominates at lower temperatures and decreases with increasing temperature.
Fig. 4. Schematic representation of the SAED pattern.
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M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
For the SCCF cathode, at 700 C, the low frequency arc (peak at 0.4 Hz) with a capacitance of the order of 10 1 F is due to concentration polarisation caused by the diffusion and exchange of oxygen species to the electrode/electrolyte interface. This process dominates at all studied temperatures and the associated capacity increases as a function of temperature. Therefore, this process at the SCCF electrode is much slower than that of LSCF. In principle, the polarisation resistance of a mixed conducting electrode can be decreased in two ways: by control of the microstucture and their three-phase boundary length is increased; by improving the kinetics of oxygen exchange and diffusion. For an electrode, a certain amount of pore volume is necessary in order to facilitate gas transport and to maximise the number of active reaction site. It is widely accepted [15,16] that there are actually three macroscopic pathways available for oxygen reduction proc-
Fig. 5 (continued).
ess as shown in Eq. (1) to occur on porous cathode/ solid electrolyte structures. 1=2O2 þ 2e ! O2
Fig. 5. Evolution of the impedance spectra measured at different temperatures in air of the symmetrical electrodes onto CGO.
ð1Þ
The kinetics of this reaction is influenced by several factors. Firstly, the reaction of molecular oxygen with the CGO electrolyte surface can be neglected, as the surface exchange coefficient values are very low at these temperatures. Secondly, dissociative-adsorption of oxygen molecules followed by surface diffusion of oxygen ions toward the tpb, and finally, surface reaction followed by dissolution in the cathode and diffusion of oxygen ions includes normal bulk lattice diffusion together with contribution from the grain boundary and dislocation core pathways depending on the level of bulk diffusivity. Although there is agreement about the reaction pathways, there remains uncertainty and disagreement about the rate-controlling
M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
mechanisms. This is partly due to difficulties in separating the relative effects of microstructure and electrocatalytic activity. The excellent performance of LSCF is due to the fact that it is a mixed conducting oxide, which provides multiple pathways for the oxygen ions to migrate to the electrode/electrolyte interface. Although much debate has surrounded the mechanism of the oxygen-reduction reaction at a porous mixed conducting oxide electrode, a recent model [14], together with experimental data [17] has provided much evidence that the predominant cathodic overpotential is associated with heterogeneous chemical reactions within the electrode microstructure. The actual charge transfer step across the electrode/electrolyte interface is relatively facile and only makes a small contribution to the total over-potential. The model clearly demonstrates the importance of the oxygen surface exchange coefficient [k (cm s 1)] and oxygen diffusion coefficient [D* (cm2 s 1)] values for the oxide cathode. These parameters are incorporated in the following expression [14,18] for cathode ASR values: Rchem ¼
RT 2F 2
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s ð1 eÞaC 20 D*0 k
ð2Þ
Although the electrode arcs observed in these experiments do not entirely resemble those predicted
47
by the model of Adler et al. [14], the calculation of Rchem provides a method of the estimation of the expected electrode response of such a material. The attraction of using the model is that it allows an estimation of the DC component of the resistance using microstructural and physicochemical parameters, which can be estimated independently, i.e. no equivalent circuit fitting of the electrode arc is necessary. This of course will only work for a material where oxygen diffusion and exchange are the limiting processes governing the electrode response, which for this material is suspected but has not been conclusively demonstrated. The value for D* (1.1 10 7 cm2 s 1) at 700 C was calculated, using the Nernst – Einstein equation, from the ionic conductivity data derived from permeability experiments [19] carried out on SCFC compositions. There is no information about surface exchange coefficients values for SCFC compositions, and so a value of 5 10 7 cms 1 was assumed which is comparable to the value measured by Benson et al. [20] for La0.6Sr0.4Co0.2Fe0.8O3 d (LSCF) at 700 C. The value of C0 (7.4 10 2 mol cm 3) was determined from the X-ray data assuming an oxygen stoichiometry of O2.6. Values for i (tortuosity), e (fractional porosity) and a (internal surface area/unit volume) were taken as 1.5, 0.3 and 5000 cm 1, res-
Fig. 6. Area electrode resistance for Sr0.8Ce0.1Fe0.7Co0.3O3 d cathode as a function of reciprocal temperature.
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pectively. With these values, the cathode ASR (Rchem) was calculated to be 0.2 F 0.1 V cm2. This value is reasonably close to the experimental value of 0.8 V cm2 to suggest that the Adler model, with the chosen values, provides a plausible interpretation for the cathode behaviour. It is interesting to note that using the same microstructural parameters, the calculated value of Rchem for La0.6Sr0.4Co0.2Fe0.8O3 d (LSCF) is 1.5 V cm2 at 700 C. It would appear that the enhanced oxygen ion diffusion in SCFC does provide superior cathodic performance compared to LSCF at 700 C. It is now important to investigate whether the large concentration of anion vacancies in the stabilised SCFC material continue to be mobile at lower temperatures, or whether ordered complexes trap these vacancies as in SrO doped LaCoO3 d. If these investigations are positive, then it will be appropriate to optimise the cathodic structure by using duplex composite assemblies incorporating fine SCFC and CGO powders [13]. The logarithm of the ASR (in V cm2) in air as a function of temperature for SCFC is shown in Fig. 6. A single slope implies that the same reaction mechanism controls the overall electrode behaviour in the temperature range studied. The activation energy of the electrode resistivities examined in the present investigation was 1.44 eV. This value is of the same order as that of the LSCF electrode (1.60 eV) [18].
4. Conclusions Powders of Sr0.8Ce0.1Fe0.7Co0.3O3 d prepared by a modified citrate route showed an orthorhombic symmetry by XRD. In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell M2ap 2ap M2ap. Two reflections that appear in the selected area electron diffraction may be interpreted as first-order satellites, of different basic reflections, arising from an incommensurate modulation ˚ ). along the b* direction (b 2.4ap A AC impedance spectroscopy measurements with SCFC/CGO/SCFC symmetrical cells demonstrated a cathodic electrochemical impedance of 0.8 V cm2 at 700 C, which could be interpreted in terms of the
kinetic properties of the SCFC material. Microstructural optimisation involving duplex layer composite cathode structures and an increase of porosity could be successful in improving the performance of these SCFC cathodes.
Acknowledgements This work was sponsored by the Spanish government (EX51666889 scholarship) and Johnson Matthey. The authors wish to acknowledge Prof. F. Garcı´a-Alvarado for helpful discussions about TEM.
References [1] H. Yokokawa, N. Sakai, T. Kawada, M. Dokiya, Solid State Ionics 52 (1992) 43. [2] H. Yokokawa, T. Kawada, M. Dokiya, J. Am. Ceram. Soc. 72 (1989) 152. [3] J.A. Kilner, Solid State Ionics 129 (2000) 13. [4] R.L. Cook, A.F. Sammells, Solid State Ionics 45 (1991) 311. [5] J. Paulsen, Thermodynamics, oxygen stoichiometric effects and transport properties of ceramic materials in the system Sr – Ce – M – O (M = Co, Fe). PhD Thesis, Technical University of Dresden, 1998. [6] Y. Teraoka, H.M. Zang, S. Furukawa, N. Yamazoe, Chem. Lett. (1985) 1743. [7] T. Mazenec, et al., US Patent 5,591,315 (7 Jan. 1997). [8] N.E. Trofimenko, J. Paulsen, H. Ullmann, R. Muller, Solid State Ionics 100 (1997) 183. [9] N. Trofimenko, H. Ullmann, J. Eur. Ceram. Soc. 20 (2000) 1241. [10] M. Pechini, US Patent 3,330,697 (11 July 1967). [11] I. Reiss, D. Braunshtein, D.S. Tannhauser, J. Am. Ceram. Soc. 64 (1981) 479. [12] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 81 (2) (1998) 357. [13] V. Dusastre, J.A. Kilner, Solid State Ionics 126 (1999) 163. [14] S.B. Adler, J.A. Lane, B.C.H. Steele, J. Electrochem. Soc. 143 (1996) 3554. [15] B.C.H. Steele, Solid Sate Ionics 86 – 88 (1996) 1223. [16] B.C.H. Steele, Solid State Ionics 94 (1997) 239. [17] S.B. Adler, in: T.A. Ramanarayanan, W.L. Worrell, H.L. Tuller (Eds.), Proc. 3rd Intl. Symp. on Ionic and Mixed Conducting Ceramics, ECS Proc., vol. 97-24. Electrochemical Society, NJ, USA, 1997, pp. 539. [18] J.-M. Bae, B.C.H. Steele, J. Electroceram. 3 (1999) 37. [19] H. Ullmann, N. Trofimenko, Solid State Ionics 119 (1999) 1. [20] S.J. Benson, R.J. Chater, J.A. Kilner, in: T.A. Ramanarayanan, W.L. Worrell, H.L. Tuller (Eds.), Proc. 3rd Intl. Symp. on Ionic and Mixed Conducting Ceramics, ECS Proc., vol. 97-24. Electrochemical Society, NJ, USA, 1997, pp. 596.
Structural and electrochemical properties of the Sr0.8Ce0.1Fe0.7Co0.3O3d perovskite as cathode material for ITSOFCs M.T. Colomer *, B.C.H. Steele, J.A. Kilner Centre for Ion Conducting Membranes, Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP, UK Received 1 October 2001; received in revised form 17 December 2001; accepted 17 December 2001
Abstract Powders of the perovskite of nominal composition Sr0.8Ce0.1Fe0.7Co0.3O3 d (SCFC) were prepared by a modified citrate route in order to determine its structural and microstructural characteristics. In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell M2ap 2ap M2ap. Two reflections in the selected area electron diffraction may be interpreted as first-order satellites, of different basic reflections, arising from an ˚ ). Symmetrical electrodes of Sr0.8Ce0.1Fe0.7Co0.3O3 d powders incommensurate modulation along the b* direction (b 2.4ap A were deposited on Ce0.9Gd0.1O1.95 (CGO) ceramic pellets, and the electrochemical properties of the interfaces between the porous cathodes (SCFC) and the electrolyte (CGO) have been investigated at intermediate temperatures (500 – 766 C) using AC impedance spectroscopy. SCFC electrodes exhibited an area-specific resistivity (ASR) at 700 C of 0.86 V cm2. The behaviour of the perovskite cathodes was interpreted in terms of available oxygen ion kinetic data. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Intermediate temperature solid oxide fuel cells; Cathode; Orthorhombic perovskite; Structural study; Mixed conductor
1. Introduction Traditionally, fuel cell systems aimed at large scale power production have been based on stabilised zirconia (YSZ) electrolytes operating at high temperature (1000 C), which places considerable restrictions on the materials that can be used in both the cell construc*
Corresponding author. Present address: Instituto de Cera´mica y Vidrio, CSIC, Antigua Crtra. Valencia Km. 24,300, 28500 Arganda del Rey, Madrid, Spain. Tel.: +34-91-871-1800; fax: +3491-870-0550. E-mail address: [email protected] (M.T. Colomer).
tion and in the balance-of-plant. To overcome these problems, it has been widely accepted that the operating temperature of the device should be lowered in order to minimise the stringent requirements placed on the balance-of-plant at the high temperature operation. This has lead to the development of intermediate temperature SOFC (IT-SOFC) operating at 500– 700 C. The performance of IT-SOFCs is strongly dependent on the cathode/electrolyte interface since the interfacial polarisation of solid-state cells increases rapidly as the temperature is decreased. At these reduced operating temperatures, it is necessary to use electrolytes with higher ionic conductivity than YSZ.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 0 2 - 4
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M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
The electrodes for IT-SOFC should have high electrical conductivities, adequate porosity for gas transport, good compatibility with the electrolyte and longterm stability. There is a need to understand the behaviour of electrodes in terms of microstructural and electrochemical parameters in order to optimise performance. More particularly, because of the large cathode polarisation at these low operating temperatures, the need for considerable improvements in cathode materials and microstructures has resulted. Interest is currently being expressed in porous ceramic structures fabricated from perovskites oxides with high electronic conductivities referred as mixed electronic and ionic conductors (MIECs). Many of the perovskites structured oxides show a wide range of oxygen stoichiometry, indicating the tendency to ionic and electronic defect structures. These are the basis for a variety of electrical properties ranging from insulating materials through ionic to electronic conductors. Additional doping on A and B sites makes it easy to change the electrical characteristics of perovskite-type oxides. Yokokawa et al. [1,2] observed a strong correlation between the formation enthalpies of perovskite phases and the Goldschmidt numbers with the thermodynamic stability increasing with larger Goldschmidt number values. Strategies to optimise the ionic conduction are provided in recent surveys by Kilner [3], Cook and Sammells [4], and Paulsen [5]. The work of Teraoka et al. [6], and reports by Mazanec et al. [7] indicated that mixed conducting compositions close to the brown-millerite stoichiometry [A2B2O5, e.g. Sr2(Fe,Co)2O5] often exhibited very high oxygen permeability at high temperatures ( > 800 C). However, A2B2O5 phases usually undergo a disorder (cubic) Z ordered structure at intermediate temperatures (650 –750 C), which is accompanied by a significant decrease in oxygen ion conductivity. The report by Trofimenko et al. [8] that the high temperature disordered cubic structure could be stabilised at lower temperatures by the addition of small amounts of CeO2 encouraged us to examine this stabilised phase as a possible cathode material for IT-SOFC operation in the temperature range 500 –700 C. It was hoped that many of the anion vacancies incorporated into the disordered A2B2O5 phase would remain mobile at lower temperatures. Moreover, the stabilised phase exhibited an acceptable electronic conductivity at 500 C ( f 300 S cm 1).
Trofimenko and Ullmann [9] studied the structure, oxygen stoichiometry, ionic and total electrical conductivities of Sr0.9Ce0.1Co1 xFexO3 d, where x = 0 – 0.8, and Sr0.8Ce0.2Co1 xFexO3 d, where x = 0.2. They showed that the total conductivity was enhanced by the Co content and reached 200 S cm 1 at 600 C. This fact encourage us to study the structural and electrochemical features of the perovskites in the above-mentionedsystem. Single-phase Sr0.9Ce0.1Fe0.7Co0.3O3 d proved difficult to make by a modified Pechini route. For this reason, an A-site-deficient composition Sr0.8Ce0.1 Fe0.7Co0.3O3-d (SCFC) with a reduced SrO thermodynamic activity was chosen in this work. Structural and microstructural studies of that perovskite were carried out. The electrochemical response as cathode of that composition on CGO electrolytes was studied and correlated with its microstructure.
2. Experimental 2.1. Preparation of materials Ce0.9Gd0.1O1.95 powder from Rhodia was used for fabrication of the electrolytes. The powder was uniaxially pressed at 15 MPa pressure and subsequently isostatically pressed at 300 MPa to ensure a high green body density. Sintering was carried out at 1450 C for 6 h. Final densities were measured by the Archimedes’ immersiontechniqueinwater. Samples of the nominal composition Sr0.8Ce0.1Fe0.7 Co0.3O3 d were prepared using a modified citrate route [10]. Solutions of the relevant nitrates, previously standardised by atomic absorption spectroscopy, were combined with an excess of citric acid and heated. The resulting product was then dehydrated and calcined at 500 C/5 h to remove any remaining organics, before being calcined at 1000 C/1 h, sieved, micro-milled and sieved again. Calcinations at 1100 C/10 h and 1200 C/20 h were performed in air to obtain the perovskite solid solution. 2.2. Structural and microstructural characterisation A Phillips PW1710 X-ray diffractometer using CuKa radiation was used to determine the structure of the SCFC electrode powders with a scanning rate
M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
of 0.0752h min 1, using silicon as internal standard. The XRD patterns of the powders were used to calculate lattice parameters. The powders calcined at 1200 C/20 h were ultrasonically dispersed in acetone. A few drops of the resulting suspension were deposited on a carbon-coated grid. Selected area electron diffraction (SAED) studies were performed with an electron microscope JEOL 2000FX (double tilt F 45 C) working at 200 kV. The microstructure of the electrode and the electrode/electrolyte interface was studied by scanning electron microscopy (SEM, JEOL T200).
43
thermodynamic activity, a single phase was identified as an orthorhombic perovskite structure having lattice parameter values of: a = 0.54911 F 0.00005 nm, b = 0.77136 F 0.00005 nm, c = 0.54642 F 0.00005 nm and cell volume 0.23144 F 0.00001 nm3. Trofimenko and Ullmann [9] reported cubic symmetries for different perovskites with nominal compositions Sr0.9 Ce0.1 Co 1 x Fe x O 3 d, where x = 0 – 0.8, and Sr 0.8 Ce0.2Co1 xFexO3 d, where x = 0.2. In our case, a deficient A-site could provoke a change in the lattice symmetry. 3.2. SEM
2.3. Fabrication of cathodes. Porous thick structures A thick and porous layer on top of the CGO electrolyte was fabricated by brushing of the relevant perovskite slurry. After deposition, the cathode was dried and fired at 1000 C/2 h. The porosity of the sintered cathode was determined by mercury porosimetry (Model Autopore II 9220, Micromeritics, Norcross, Ga) with an intrusion pressure between 1.33 and 414 MPa and a resolution power of F 1% of total scale, i.e. 0.207 MPa. A 6.12-cm3, 1-Torr (133 Pa) penetrometer was used to study degasified samples at room temperature.
A SEM micrograph of a fractured surface (Fig. 1) of a SCFC/CGO cross-section indicates that the thickness of the electrode is about 130 mm and the grain size of the SCFC is of the order of 9 mm. Porous electrodes were apparent (Fig. 2). The average porosity value was about 30% (measured by mercury porosimetry), whereas the CGO electrolyte was dense (98% theoretical density). The electrode microstructure appeared uniform and showed good bonding and continuous contact with the dense electrolyte pellet.
2.4. Electrical and electrochemical characterisation
3.3. TEM
AC impedance spectroscopy measurements of CGO pellets coated with Pt were carried out using a Solartron SI 1260 Impedance/Gain-Phase Analyzer with a 50mV signal over a frequency range of 20 MHz to 1 mHz in the temperature range from 150 to 700 C. AC impedance spectroscopy measurements were used to obtain the electrode resistivity of symmetrical perovskite electrodes on CGO electrolytes in the temperature range from 500 to 766 C. The sample was mechanically contacted with spring-loaded platinum gauze and held in a purpose built sample holder that fitted within a tube furnace.
In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell M2ap 2ap M2ap. Fig. 3 corresponds to a selected area electron diffraction (SAED) pattern of a Sr0.8Ce0.1 Co0.7Fe0.3O3 d crystallite taken along [1 0 1] zone axis while Fig. 4 depicts a schematic representation of a possible indexation of the observed electron diffraction (ED). The contribution of different microdomains where the double lattice parameter is oriented in different perpendicular directions can explain the existence of two apparent doubled axes. In any case, the doubled parameter is not commensurate since, as it can be seen, along the b* axis, two weak reflections are present instead of one only spot coming from 010 planes which are in fact not observed. Both reflections may be interpreted as first-order satellites, of different basic reflections,
3. Results and discussion 3.1. X-ray characterisation By preparing an A-site-deficient composition Sr0.8 Ce0.1Fe0.7Co0.3O3 d (SCFC) with a reduced SrO
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Fig. 1. SEM micrograph of the fractured surface of a Sr0.8Ce0.1Fe0.7Co0.3O3 d/CGO interface.
Fig. 2. Microstructure of the porous cathode.
M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
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quently performing a nonlinear least squares fit to the bulk and grain boundary semicircle. Above 400 C, the grain boundary and bulk processes could not be measured, as they became indistinguishable. Our data coincide perfectly with previous studies on CGO [11 – 13]. 3.5. Impedance studies of the electrodes
Fig. 3. SAED pattern along [1 0 1] zone axis of Sr0.8Ce0.1Fe0.7 Co0.3O3 d.
arising from an incommensurate modulation along the ˚ ). b* direction (b 2.4ap A 3.4. Conductivity of CGO electrolytes The conductivity of the samples was determined by plotting the data on the complex plane and subse-
The evolution of the impedance spectra measured at different temperatures in air is shown in Fig. 5. The increase of the measurement temperature resulted in a significant reduction of the area-specific resistivity (ASR) typically from 5.33 V cm2 at 600 C to 0.86 V cm2 at 700 C and to 0.23 V cm2 at 766 C. The shape of the complex impedance arcs of the cathode material is similar to that found for LSCF [13], although in the latter case, two arcs are distinguishable at 590 C and, in our case, the high and low frequency arcs overlap. For LSCF calcined at 800 C, the spectrum at 590 C depicts an arc at high frequency (peak at 6 kHz) with a capacitance of 10 5 F and is attributed to charge transfer of O2 ions between the electrode and the electrolyte, and the low frequency arc (peak at 158 Hz) with a capacitance of the order of 10 4 F is due to concentration polarisation caused by the diffusion and exchange of oxygen species to the electrode/electrolyte interface [14]. The latter process dominates at lower temperatures and decreases with increasing temperature.
Fig. 4. Schematic representation of the SAED pattern.
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For the SCCF cathode, at 700 C, the low frequency arc (peak at 0.4 Hz) with a capacitance of the order of 10 1 F is due to concentration polarisation caused by the diffusion and exchange of oxygen species to the electrode/electrolyte interface. This process dominates at all studied temperatures and the associated capacity increases as a function of temperature. Therefore, this process at the SCCF electrode is much slower than that of LSCF. In principle, the polarisation resistance of a mixed conducting electrode can be decreased in two ways: by control of the microstucture and their three-phase boundary length is increased; by improving the kinetics of oxygen exchange and diffusion. For an electrode, a certain amount of pore volume is necessary in order to facilitate gas transport and to maximise the number of active reaction site. It is widely accepted [15,16] that there are actually three macroscopic pathways available for oxygen reduction proc-
Fig. 5 (continued).
ess as shown in Eq. (1) to occur on porous cathode/ solid electrolyte structures. 1=2O2 þ 2e ! O2
Fig. 5. Evolution of the impedance spectra measured at different temperatures in air of the symmetrical electrodes onto CGO.
ð1Þ
The kinetics of this reaction is influenced by several factors. Firstly, the reaction of molecular oxygen with the CGO electrolyte surface can be neglected, as the surface exchange coefficient values are very low at these temperatures. Secondly, dissociative-adsorption of oxygen molecules followed by surface diffusion of oxygen ions toward the tpb, and finally, surface reaction followed by dissolution in the cathode and diffusion of oxygen ions includes normal bulk lattice diffusion together with contribution from the grain boundary and dislocation core pathways depending on the level of bulk diffusivity. Although there is agreement about the reaction pathways, there remains uncertainty and disagreement about the rate-controlling
M.T. Colomer et al. / Solid State Ionics 147 (2002) 41–48
mechanisms. This is partly due to difficulties in separating the relative effects of microstructure and electrocatalytic activity. The excellent performance of LSCF is due to the fact that it is a mixed conducting oxide, which provides multiple pathways for the oxygen ions to migrate to the electrode/electrolyte interface. Although much debate has surrounded the mechanism of the oxygen-reduction reaction at a porous mixed conducting oxide electrode, a recent model [14], together with experimental data [17] has provided much evidence that the predominant cathodic overpotential is associated with heterogeneous chemical reactions within the electrode microstructure. The actual charge transfer step across the electrode/electrolyte interface is relatively facile and only makes a small contribution to the total over-potential. The model clearly demonstrates the importance of the oxygen surface exchange coefficient [k (cm s 1)] and oxygen diffusion coefficient [D* (cm2 s 1)] values for the oxide cathode. These parameters are incorporated in the following expression [14,18] for cathode ASR values: Rchem ¼
RT 2F 2
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s ð1 eÞaC 20 D*0 k
ð2Þ
Although the electrode arcs observed in these experiments do not entirely resemble those predicted
47
by the model of Adler et al. [14], the calculation of Rchem provides a method of the estimation of the expected electrode response of such a material. The attraction of using the model is that it allows an estimation of the DC component of the resistance using microstructural and physicochemical parameters, which can be estimated independently, i.e. no equivalent circuit fitting of the electrode arc is necessary. This of course will only work for a material where oxygen diffusion and exchange are the limiting processes governing the electrode response, which for this material is suspected but has not been conclusively demonstrated. The value for D* (1.1 10 7 cm2 s 1) at 700 C was calculated, using the Nernst – Einstein equation, from the ionic conductivity data derived from permeability experiments [19] carried out on SCFC compositions. There is no information about surface exchange coefficients values for SCFC compositions, and so a value of 5 10 7 cms 1 was assumed which is comparable to the value measured by Benson et al. [20] for La0.6Sr0.4Co0.2Fe0.8O3 d (LSCF) at 700 C. The value of C0 (7.4 10 2 mol cm 3) was determined from the X-ray data assuming an oxygen stoichiometry of O2.6. Values for i (tortuosity), e (fractional porosity) and a (internal surface area/unit volume) were taken as 1.5, 0.3 and 5000 cm 1, res-
Fig. 6. Area electrode resistance for Sr0.8Ce0.1Fe0.7Co0.3O3 d cathode as a function of reciprocal temperature.
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pectively. With these values, the cathode ASR (Rchem) was calculated to be 0.2 F 0.1 V cm2. This value is reasonably close to the experimental value of 0.8 V cm2 to suggest that the Adler model, with the chosen values, provides a plausible interpretation for the cathode behaviour. It is interesting to note that using the same microstructural parameters, the calculated value of Rchem for La0.6Sr0.4Co0.2Fe0.8O3 d (LSCF) is 1.5 V cm2 at 700 C. It would appear that the enhanced oxygen ion diffusion in SCFC does provide superior cathodic performance compared to LSCF at 700 C. It is now important to investigate whether the large concentration of anion vacancies in the stabilised SCFC material continue to be mobile at lower temperatures, or whether ordered complexes trap these vacancies as in SrO doped LaCoO3 d. If these investigations are positive, then it will be appropriate to optimise the cathodic structure by using duplex composite assemblies incorporating fine SCFC and CGO powders [13]. The logarithm of the ASR (in V cm2) in air as a function of temperature for SCFC is shown in Fig. 6. A single slope implies that the same reaction mechanism controls the overall electrode behaviour in the temperature range studied. The activation energy of the electrode resistivities examined in the present investigation was 1.44 eV. This value is of the same order as that of the LSCF electrode (1.60 eV) [18].
4. Conclusions Powders of Sr0.8Ce0.1Fe0.7Co0.3O3 d prepared by a modified citrate route showed an orthorhombic symmetry by XRD. In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell M2ap 2ap M2ap. Two reflections that appear in the selected area electron diffraction may be interpreted as first-order satellites, of different basic reflections, arising from an incommensurate modulation ˚ ). along the b* direction (b 2.4ap A AC impedance spectroscopy measurements with SCFC/CGO/SCFC symmetrical cells demonstrated a cathodic electrochemical impedance of 0.8 V cm2 at 700 C, which could be interpreted in terms of the
kinetic properties of the SCFC material. Microstructural optimisation involving duplex layer composite cathode structures and an increase of porosity could be successful in improving the performance of these SCFC cathodes.
Acknowledgements This work was sponsored by the Spanish government (EX51666889 scholarship) and Johnson Matthey. The authors wish to acknowledge Prof. F. Garcı´a-Alvarado for helpful discussions about TEM.
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