Combinatorial characterisation of mixed conducting perovskites

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Solid State Ionics 179 (2008) 1085 – 1089 www.elsevier.com/locate/ssi

Combinatorial characterisation of mixed conducting perovskites Jeremy C.H. Rossiny a,⁎, Jennifer Julis a , Sarah Fearn a , John A. Kilner a , Yong Zhang b , Lifeng Chen b , Shoufeng Yang b , Julian R.G. Evans b a b

Department of Materials, Imperial College London, Prince Consort Road, London SW7 2AZ, UK Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, UK Received 12 July 2007; received in revised form 25 January 2008; accepted 31 January 2008

Abstract One of the most common types of structure which displays Mixed Electronic Ionic Conductivity is the ABO3 perovskite. The perovskite structure defines a very large range of possible oxide compositions and thus, for screening as potential SOFC cathodes, new methods must be used to obtain a complete picture of these MIEC materials. Combinatorial methods offer a route to identify the major trends in functional properties prior to full scale experiment. The system La1 − xSrxCo1 − y − zMnyFezO3 + δ (LSCMF) has been synthesised by combinatorial methods using a robot ink-dip printer. A systematic set of experiments was performed to characterise the trends in crystal structure, and non-stoichiometry with composition. It is the first time that this LSCMF pseudo-ternary has been measured with a 10% B-site substitution of cobalt, manganese and iron. © 2008 Elsevier B.V. All rights reserved. Keywords: Cathode; SOFC; Combinatorial; LSM; LSC

1. Introduction In order to lower the operating temperature of the Solid Oxide Fuel Cell (SOFC), there has been an intensive search for new cathode materials in the last ten years. This is because, in the range of intermediate temperature operation between 500 °C and 700 °C, the SOFC cathode has the largest Area Specific Resistance (ASR) compared to those of the electrolyte and the anode. In commercially available SOFC technology, only three cathode materials are widely used: La0.8Sr0.2MnO3 + δ (LSM), La0.6Sr0.4Co0.2Fe0.8O3 + δ and Sm0.5Sr0.5CoO3 + δ The choice of cathode has been dictated by the need for high activity and for compatibility with the SOFC electrolyte materials. LSM is the conventional cathode ceramic and is coupled with yttria stabilised-zirconia (YSZ) for high temperature SOFC applications. As Ce0.9Gd0.1O2 + δ and La0.8Sr0.6Ga0.8Mg0.2O3 + δ exhibit higher conductivity than YSZ at intermediate temperatures, a search for new cathode materials, compatible with these electrolytes and showing high activity at low temperatures, is required. ⁎ Corresponding author. E-mail address: [email protected] (J.C.H. Rossiny). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.098

The conventional way to characterise and develop ceramic materials is limited by the synthesis and characterisation process, which is time-consuming. Effort is normally expended to obtain high quality samples and accurate measurements, and consequently, a laborious and meticulous one-sample-at-a-time approach leads to slow progress. As an example, developing the two electrolyte materials previously mentioned required a period of two decades. It is unfortunate that our knowledge of how to select new cathode materials from the wide range of available compositions of perovskite oxide by substitution on either A-sites and or B-sites is still limited. In order to speed up the search for new multi-functional cathode materials, a combinatorial approach has been implemented recently [1,2]: an automated robot has been built to rapidly synthesise large numbers of small ceramic samples. In this work, a systematic characterisation of the La0.8Sr0.2Co1 − y − zMnyFezO3 + δ system (LSCMF) has been carried out to measure how phase purity and oxygen non-stoichiometry varies with composition in this ternary. This preliminary work is performed before characterising the oxygen ion diffusivity in the ceramic [1]. For simplicity, in what follows, the composition is reduced to the following notation, LSCMF 82325 for La0.8Sr0.2Co0.3Mn0.2Fe0.5O3 + δ and

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LSCMF 82046 for La0.8Sr0.2Mn0.4Fe0.6O3 + δ etc. La0.8Sr0.2 CoO3 + δ and are noted LSC and LSF respectively. 2. Experimental details High purity oxide and carbonate powders were used as sources for the metal cations: La2(CO3)3, SrCO3, MnCO3, Co3O4 and Fe2O3. Since the powders were relatively coarse for ink preparation, they were milled in a high-energy Dyno mill (Model KDLA, Glen Creston Ltd., Middlesex, U.K.) with

1 mm zirconia grinding media for 1 h. To obtain well-dispersed ceramic inks, a dispersant, Darvan 821A (R.T. Vanderbilt Co. Inc., Norwalk, CT), was used to stabilize the suspensions. A thixotropic agent, Acrysol 12 W (Rohm and Haas Company, Philadelphia, USA) was used to prevent compositional segregation during drying of the ink mixtures [3]. The sedimentation behaviour of the inks was observed as an indication of suspension stability. The London University Search Instrument (LUSI), an aspirating–dispensing ink-jet printer work station (Modified ProSys

Fig. 1. Phase purity analysis: (a) ICP-MS of lanthanum and cobalt content in the composition range of La0.8Sr0.2Co1 − yFeyO3 + δ; (b) Perovskite and second phase composition and distribution in the LSCMF observed from the room temperature XRD patterns.

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4510 manufactured by Cartesian Ltd., Huntingdon, Cambridge, U.K. [4]), was employed to deposit arrays of droplets. To produce the LSCMF system, LUSI printed a series of 66 compositions of powder mixtures onto an alumina substrate, which were first dried in ambient air and then heated at 1250 °C for 1 h to promote interdiffusion and reaction to give the perovskite phase. The perovskite La0.8Sr0.2Co1 − y − zMnyFezO3 + δ system (LSCMF), in which y and z vary from 0 to 1 with incremental step of 0.1, was eventually synthesised. Two dots of each composition were ground to powder, which were then analysed by X-ray diffraction (XRD) and by Induction Coupled Plasma Mass Spectrometry (ICP-MS). ICP-MS was carried out using a Varian instrument VISTA PRO ICP-AES at the Natural History Museum, London. X-Ray diffraction (XRD) was carried out on a Phillips PW1710 automated powder diffractometer equipped with a secondary graphite crystal monochromator using CuKα radiation (λ = 1.54178 Å). Measurements were made relative to an external silicon standard. For standard phase identification analysis, samples were scanned in the region 2θ = 20–90° using a step size of 0.04°/s. The composition and processing conditions used to fabricate the perovskite dots determine the final oxygen content of the ceramic oxide. In order to obtain some measure of the degree of non-stoichiometry, the materials were subjected to TGA analysis. A measure of the oxygen stoichiometry was therefore estimated from thermogravimetric analysis in flowing air using a Netzsch STA 449C Jupiter simultaneous TG-DTA / DSC instrument. Two dots of each sample were ground to powders, which were then measured from room temperature through to 1040 °C. In order to estimate the non-stoichiometry of the dots, it was assumed that every dot synthesised by the LUSI robot initially had an oxygen stoichiometry close to 3. 3. Results and discussion 3.1. Composition and phase purity The ICP-MS measurements on the La0.8Sr0.2Co1 − yFeyO3 + δ (LSCF) from the LSCMF series synthesized from oxide/carbonate inks shows that the intended compositions were correctly reproduced, as shown in Fig. 1a. Moreover, any aluminium contamination from the substrate was less than 2000 ppm, suggesting that substrate contamination was minimal. Room temperature XRD over the pseudo-ternary LSCMF system indicates that most of the compositions were single phase perovskites. Only four out of 55 samples showed presence of substantial second phases, whereas XRD patterns of twelve other compositions indicate traces of second phases. The second phase distribution and composition are summarized in Fig. 1b. The high cobalt content mixtures have resulted in a single phase, whereas as the substitution of cobalt with iron or manganese increases, the synthesis is not completed as either binary oxides or the thermodynamical less favorable K2NiF4 phase are detected as traces. When 90% of the cobalt is substituted, most of the samples have second phases when processed at 1250 °C. From the XRD patterns, a hexagonal structure was deduced [5] and lattice parameters were calculated using the freeware

Fig. 2. Variation of the lattice parameters (a) a (Å) and (b) c (Å) with composition in the LSCMF pseudo-ternary determined by XRD at room temperature.

UNITCELL [6]. Two lattice parameters a and c, coupled with the cell volume were then extracted for each composition. In this first report, not all compositions of the LSCMF system have been studied and compositions where the cobalt substitution is greater than 90% have not been characterized. A contour plot representation of the data has been employed as this facilitates easy identification of regions of compositional interest. The contours are generated by plotting the manganese and iron contents along x and y axes, respectively. Compounds for which a property has been calculated therefore form a grid of points. The lattice parameters are displayed by the contours of varying colour. Warm colours represent high values, whilst cool colours represent low values [7]. Data values that fall between experimental results were interpolated

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via the Kriging method, which is a modified weighted average interpolation approach. The weights are calculated by solving sets of linear equations based on the variance of the data being interpolated, while the original dataset remains unchanged after the interpolation has been performed. The lattice parameters trends are displayed in Fig. 2a & b. In these figures, the LSC composition is in the bottom left corner, the La0.8Sr0.2Mn1 − yCoyO3 + δ (LSMC) series lies on the bottom horizontal line, while the LSCF series lies in the left vertical line. Substituting cobalt for either manganese or iron in the Bsites of LSC causes the cell parameters to increase slowly until 30–40% of substitution. Then the hexagonal cell volume increases sharply with further substitution of cobalt, i.e., a strong deviation from Vegard's law. In our initial work [1,2], the pseudo binary La0.8Sr0.2Mn1 − yCoyO3 + δ (LSMC) was synthesised by the same automated route, but using perovskite inks of La0.8Sr0.2MnO3 + δ and La0.8Sr0.2CoO3 + δ. In this previous investigation a single phase perovskite was synthesised with no second phases detected. Lattice parameters trends calculated in the pseudo-ternary LSCMF are similar to those found in LSMC series. Fig. 3 shows the fair agreement between trends of lattice parameter a of perovskite samples synthesized by the two routes, confirming this change in cell parameters. This example demonstrates that combinatorial methods could be undertaken to screen property trends, even if the measured values would not necessarily be the exact ones. At larger substitution of cobalt around 90%, the lattice parameters seem to be varying sharply with the manganese/iron ratio, however, LSMCF 82145 and LSMCF 82136 and those with higher content of iron are in the compositional space where second phases are observed; i.e., the cell parameters for these compositions are more likely to have significant errors. 3.2. Variation of oxygen non-stoichiometry Oxygen contents in air at 1000 °C have been calculated from DTA data for a selected range of composition (LSMC series),

Fig. 4. Oxygen loss at 1000 °C in the composition range of La0.8Sr0.2Mn1 − y CoyO3 + δ inside the LSCMF series determined by thermo gravimetric analysis in air.

we have assumed that each composition starts with δ ≈ 0 to observe the major trends across the series, in particular the onset of hyperstoichiometry. It can be seen by inspection of Fig. 4 that the apparent oxygen non-stoichiometry slowly decreases from a δ value close to 0, for the region LSM to LSCMF 82460 (40% Co doped) to a final value of approximately δ = −0.13 for LSC. In the LSMC series, the substitution of manganese by cobalt enhances the formation of oxygen vacancies at around 50% substitution on the B-site, which has been related to the enhancement of oxygen ion diffusivity with level of 40–50% cobalt substitution [2]. 4. Conclusions Perovskite oxide samples can be produced by a thick film combinatorial dip-printing method from mixtures of oxides and carbonates. With these oxide/carbonate parent inks, the potential range of compositions produced by the LUSI robot is now enlarged to a encompass A-/B-sites pseudo-ternary or -quaternary systems. A systematic study has been initiated to identify potential cathode materials: phase purity and oxygen non-stoichiometry at 1000 °C were evaluated as screening tools. Lattice parameter trends of combinatorially produced samples indicate some deviation from Vegard's law and oxygen content measurements confirm the onset of hypostoichiometry of high cobalt substitutions. Acknowledgements J.C.H.R. would like to thank EPSRC for funding this project (grant number GR/S8 5245/01). J. J. would like to thank the IDEA league for funding her visit to Imperial College.

Fig. 3. Variation of the lattice parameter a (Å), determined by XRD at room temperature, in the LSMC series synthesized from perovskite inks (black circle) and LSCMF ternary synthesized from oxide/carbonate inks (red circle: single phase perovskite; green circle: perovskite + second phases:).

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