This work reports the modified Pechini preparation and properties of an A-site deficient perovskite Sr0.86Y0.08TiO3-δ (SYT). Good chemical homogeneity and electrical conductivity were determined, making SYT a potential conductive component material in a composite anode for solid oxide fuel cells. As an oxide with inherent resistance to oxidation and hydroxylation, SYT could be a substitute for the metallic Ni component in composite anodes for solid oxide fuel cells, particularly for reversible solid oxide fuel cells with attendant high steam concentrations. © 2007 Elsevier B.V. All rights reserved

Brouwer, J

Lu, X

Mumm, DR

Pine, TS

Lu, Xinyu

Pine, Tom S

Mumm, Daniel R

Brouwer, Jacob

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleModified Pechini synthesis and characterization of Y-doped strontium titanate perovskitePermalinkhttps://escholarship.org/uc/item/51t0g79cJournalSolid State Ionics, 178(19-20)ISSN0167-2738AuthorsLu, XinyuPine, Tom SMumm, Daniel Ret al.Publication Date2007-07-15DOI10.1016/j.ssi.2007.05.018Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of California007) 1195–1199www.elsevier.com/locate/ssiSolid State Ionics 178 (2Modified Pechini synthesis and characterization of Y-doped strontiumtitanate perovskiteXinyu Lu a, Tom S. Pine a, Daniel R. Mumm a,b, Jacob Brouwer a,⁎a National Fuel Cell Research Center, University of California, Irvine, Irvine, CA 92697-3550, United Statesb Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575, United StatesReceived 26 April 2007; received in revised form 10 May 2007; accepted 21 May 2007AbstractThis work reports the modified Pechini preparation and properties of an A-site deficient perovskite Sr0.86Y0.08TiO3−δ (SYT). Good chemicalhomogeneity and electrical conductivity were determined, making SYT a potential conductive component material in a composite anode for solidoxide fuel cells. As an oxide with inherent resistance to oxidation and hydroxylation, SYT could be a substitute for the metallic Ni component incomposite anodes for solid oxide fuel cells, particularly for reversible solid oxide fuel cells with attendant high steam concentrations.© 2007 Elsevier B.V. All rights reserved.Keywords: Modified Pechini process; Doped strontium titanate; Perovskite; Conductivity; Solid oxide fuel cell1. IntroductionSolid oxide fuel cell (SOFC) technology based on the yttria-stabilized zirconia (YSZ) materials set is being advanced for avariety of applications. This technology typically employs acomposite nickel-YSZ anode, which limits the electrochemicalreaction zone to the region very near the triple-phase-boundary(TPB). At typical SOFC operating temperatures (700–1000 °C),Ni particles tend to coarsen over time, which results in greaterpolarization losses. Some intermediate temperature SOFCtechnologies, such as that based on strontium and magnesium-doped lanthanum gallate (LSGM) electrolyte, cannot use acomposite Ni-LSGM anode because sintering produces undesir-able insulating phases, such as lanthanum nickelate [1].Replacing the metallic Ni-containing anode with a ceramicmixed-conducting anode that possesses similar electrical andelectrocatalytic properties could significantly improve the long-term stability of SOFC technology. In addition, use of a mixedionic- and electronic-conductor (MIEC) in the place of nickel⁎ Corresponding author. Tel.: +1 949 824 1999x221; fax: +1 949 824 7423.E-mail address: jb@nfcrc.uci.edu (J. Brouwer).0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2007.05.018may reduce the tendency of carbon formation and depositionwhen operating an SOFC on a hydrocarbon fuel.In addition, reversible SOFC (R-SOFC) technology could alsobenefit from an MIEC composite anode. Several investigatorshave studied the YSZ materials set in R-SOFC stacks using a Ni-YSZ anode and a strontium-doped lanthanum manganite (LSM)cathode [2–4]. These studies have advanced understanding of R-SOFC technology and provided useful insights into electrodeperformance optimization. These studies generally confirm therobust performance of theYSZmaterials set in fuel cellmode, but,suggest high rates of degradation in electrolyzer mode.The observed degraded electrolyzer mode performance isbelieved to be due toNi oxidation or hydroxylation in the Ni-YSZanode. Oxidation or hydroxylation can readily occur at hightemperatures in the presence of high steam concentration. Whenpure steam is used for electrolysis, the thermodynamic equilib-rium oxygen partial pressure (PO2) is 7.0×10−8 atm at 700 °C and4.7×10−7 atm at 800 °C. Thermodynamic calculations indicatethat the Ni/NiO equilibrium PO2 is 10−14 atm at 800 °C for unitactivities of condensed phases. Therefore, NiO will be readilyformed from surface oxidation of metallic Ni particles in theelectrolysis environment, even at intermediate operating tem-peratures of 700–800 °C. This oxidation results in severeelectrode polarization and deactivation.Fig. 1. Flow chart of a modified Pechini process to synthesize Sr0.86Y0.08TiO3−δ (SYT) powder.1196 X. Lu et al. / Solid State Ionics 178 (2007) 1195–1199Yttrium-doped strontium titanate (Sr1–1.5xYxTiO3−δ) dis-plays relatively high electrical conductivity and good chemicalstability under reducing conditions [5,6]. An optimizedcomposition with A-site deficient Sr0.86Y0.08TiO3−δ (SYT)shows electrical conductivity as high as 82 S/cm at 800 °Cwith oxygen partial pressure of 10−19 atm. This material hasbeen demonstrated to have high structural stability over a broadrange of temperatures (up to 1400 °C) and oxygen partialpressures (1–10−20 atm). No phase change was found to occurfor mixtures of SYT and LSGM sintered at 1400 °C for 10 h.Finally, the thermal expansion of SYT was determined to becompatible with those of LSGM and YSZ [7,8]. All thesefeatures suggest that SYT could be used as an active ceramicoxide component in the hydrogen-side electrode for SOFCs aswell as R-SOFCs.However, to-date the synthesis of SYT material haspredominantly relied upon conventional solid-state reactionprocesses. Synthesis begins with mixtures of precursor oxidesand carbonates, which are subjected to prolonged heating atelevated temperatures (1500–1700 °C) to obtain the desiredperovskite phase. This process is straightforward, but it isenergy intensive and the materials produced are oftensubstandard, exhibiting non-homogeneous phase distribution,large grain sizes, strongly bonded agglomerates, and chemicalcontamination from repetitive grinding and milling processes.In order to avoid such undesirable aspects of SYT synthesis, alow-cost modified Pechini process, based on the originalwork of Pechini [9], has been developed to synthesize ultrafine SYT powder with desired chemical homogeneity. In thiswork, we describe the synthesis approach, and subsequentlycharacterize SOFC-relevant properties of the as-synthesizedSYT.2. Experimental2.1. Modified Pechini ProcessThe SYT powder was synthesized using the followingprecursors: Sr(NO3)2 (≥99.0% Fluka), Y(NO3)3·6H2O (99.9%,Sigma-Aldrich), and Ti(OC3H7)4 (98%, Sigma-Aldrich). Stoi-chiometric amounts of Sr(NO3)2 and Y(NO3)3·6H2O weredissolved into deionized water to make a nitrate solution. Theminimum amount of water necessary to fully dissolve thenitrates was utilized (e.g. 25 ml of H2O for 3.640 g of Sr nitrateand 0.613 grams of Y-nitrate). Quantities of citric acid (99.5+%,Sigma-Aldrich), in a 3:1 molar ratio with respect to the totalamount of cations, and ethylene glycol (99+%, Alfa Aesar), in a4:1 molar ratio with respect to the citric acid were prepared.Ethylene glycol was first added into a three-neck flask. Thestoichiometric amount of Ti(OC3H7)4 was then carefully addedinto the ethylene glycol. The flask containing this mixture wasthen placed in a water bath, held at a temperature between 70and 80 °C, and stirred with a magnetic stir bar. An amount ofabsolute ethanol representing a 2:1 molar ratio with thedeionized water used in making the nitrate solution was thenadded to the three-neck flask. The previously measured citricacid was gradually added, continuing to stir the solution. Uponcomplete mixing of the citric acid, the nitrate solution wasgradually added. The pH of the resulting solution was adjustedto 6–7 by adding appropriate quantities of NH4OH. With theflask left open (one or two necks unsealed), the solution washeld at the bath temperature (70–80 °C) and continuouslystirred for 4–5 h, yielding a viscous clear solution of metal–citrate complexes. The resulting solution was then transferred toan alumina crucible to be dried in a convection oven at 100–Fig. 2. Simultaneous TG/DTA curves of SYT gel precursor at a heating rate of10 °C/min in air.Fig. 4. XRD patterns of the SYT powders fired at 900 °C in air for 4 h and 8 h.1197X. Lu et al. / Solid State Ionics 178 (2007) 1195–1199110 °C for 10–15 h. After drying, the solution was transformedinto a thick purple-brown gel. Finally, the resulting gel wasfurther calcined at 900 °C in air for 8 h to generate a foam-likepower. This modified Pechini synthesis procedure is summa-rized in the flow chart illustrated in Fig. 1.2.2. Physicochemical and Structural CharacterizationThermal analysis of the conversion of SYT gel precursor (fromthe above modified Pechini process) to ceramic powder wasconducted on a thermogravimetric/differential thermal analysissystem (TG/DTA, PerkinElmer Diamond) running PYRISsoftware. The gel was heated in an open-topped platinum crucible,from 40 °C to 1400 °C at a heating rate of 10 °C/min in air. TheBrunauer–Emmett–Teller (BET) specific surface area of the as-calcined SYTpowderwasmeasuredwith an automated volumetricsorption analyzer (Quantachrome, Autosorb-1) using high surfacearea sample cells and nitrogen as the adsorbate at −196 °C.The crystalline structure of a series of SYT powders fired for3 h at temperatures ranging from 800 to 1400 °C was examinedusing an X-ray diffractometer (XRD, Bruker D5000). XRDpatterns were taken using a Ni-filtered Cu-Kα radiation sourceFig. 3. XRD patterns of the SYT powders fired in a temperature range of 800 to1400 °C in air for 3 h.(operating at 40 V/30 mA), with a 2θ step of 0.02°. The nano-scale structure of the 900 °C-calcined SYT powder was observedusing transmission electron microscopy (TEM, Philips CM20).The microstructure of sintered SYT pellet samples was examinedby using scanning electron microscopy (SEM, Zeiss Ultra 55).2.3. Electrical Conductivity MeasurementFor the electrical characterization, the 900 °C-calcined SYTpowderwas first ground in an agatemortar and pestle. The powderthen was pressed into a pellet (33 mm in diameter and 2 mm inthickness) at 180 MPa using a manual uniaxial hydraulic press.The as-pressed pellet was sintered at 1400 °C for 6 h in anenvironment of forming gas (10%CO+90%N2) maintaining thatenvironment during the entire firing cycle. The sintered pellet wassubsequently machined to a bar geometry with dimensions of1.0 mm×2.5 mm×6.6 mm. Platinum paste (Engelhard, A3788A)was brush-painted onto the two ends of the SYT bar as currentcollectors, and platinum gauze (Alfa Aesar, 52 mesh woven from0.1 mm dia wire, 99.9%) with spot-welded platinum lead wires(two wires on each end) were buried in the platinum paste tofurther improve the current-collecting efficiency. The SYT barsamples with platinum current collectors were fired at 1000 °C inair for 1 h to establish good bonding between the SYT andplatinum paste. Four-probe electrical conductivity measurementswere carried out in a mixed reducing gas atmosphere (50% H2+50% N2) at temperatures ranging between 600 °C and 1000 °CFig. 5. TEM micrograph of the SYT powder calcined at 900 °C in air for 8 h.Fig. 7. Electrical conductivity of the SYT sintered at 1400 °C in forming gas(10% CO+90% N2). Measurements were carried out in a mixed gas atmosphere(50% H2+50% N2) in a temperature range of 600 to 1000 °C.1198 X. Lu et al. / Solid State Ionics 178 (2007) 1195–1199with a temperature increment of 50 °C. AC impedance spectrawere collected using a Solartron 1260A/1480A system operatingin the frequency range of 100 kHz to 0.1 Hz with an excitationvoltage amplitude of 5 mV.3. Results and discussion3.1. Thermal Properties of SYT Gel PrecursorThe simultaneous TG/DTA curves of SYT gel precursor inFig. 2 reveal rapid mass loss upon reaching a temperature of170 °C, attributed to the removal of superficial and structuralwater in the gel precursor. The mass loss around 200 °C,attributed to the combustion of nitrates and ethylene glycol,occurs with exothermic peaks in the DTA curve. Further massloss between 200 °C and 460 °C is due to the removal ofdifferent forms of chemisorbed hydroxyl groups and furthercombustion of the remaining organic groups. Alkoxide groupsbonded to metal cations can be removed by heating to 460 °C inair, accompanied by a significant endothermic peak in the DTAcurve. After this endothermic process, there are no further majorthermal events. The TG curve gradually levels off to anormalized mass of 0.044 at 1400 °C. This profile is typicalof a modified Pechini process and confirms that there are nounexpected phase transformations in the temperature range of550 to 1400 °C. It also confirms that the majority of the massloss occurs under 550 °C and allows for optimization of the heattreatment program.3.2. Characterization of SYT powder and sintered pelletThe XRD patterns of the SYT powders fired in air over atemperature range of 800 to 1400 °C (Fig. 3) indicate thatincreasing the firing temperature serves to increase the fractionof the pyrochlore phase Ti2Y2O7. The undesirable TiO2 rutilephase that is observed at low firing temperatures can beeliminated by lengthening the firing time in air from 4 h to 8 h at900 °C (Fig. 4). This treatment was found to eliminate the rutileFig. 6. SEM micrograph of a top-view of the SYT pellet sintered at 1400 °C for6 h in an environment of forming gas (10% CO+90% N2). The same forminggas environment was maintained during the entire firing cycle.phase while simultaneously minimizing the formation of thepyrochlore phase, leading to the final SYT powder in perovskitephase and with fine grain size and high phase purity. Rietveldrefinement of the lattice parameter using the General StructureAnalysis System (GSAS) software suite establishes that thecubic lattice parameter of the reduced SYT is a=3.905 Å.The specific surface area of the 900 °C-calcined SYT powderwas measured to be 21.3 m2/g with a standard deviation of0.9 m2/g, indicating that a highly refined powder with nanoscalegrain size is obtained.ATEMmicrograph of the SYTpowder calcined at 900 °C in airfor 8 h (Fig. 5) shows that the average particle size is 50 nm. TheSYT particles are equiaxed with a narrow size distribution. Thenanoscale dimensions of the particles and the apparent homoge-neity make the modified Pechini process-derived SYT powder agood candidate material for the SOFC anode application.The density of the sintered SYT pellet was determined to be4.75 g/cm3 using the Archimedes principle and associatedattachments to an analytical balance. This density value is94.5% of the theoretical density calculated based on the latticeparameters measured by using XRD.A SEM micrograph of the SYT pellet sintered in a reducingatmosphere (Fig. 6) shows a highly densified microstructurecomposed of polyhedral grains on a micron scale. The highdegree of control over grain size in the final SYT pellet thatresults from the highly refined grain size in the initial powdersynthesis process is vital to the practical processing of SYT asan SOFC anode component.3.3. Electrical ConductivityThe electrical conductivity of the sintered SYT – theprincipal characteristic of interest for use as an SOFC anodecomponent – was measured in a mixed gas atmosphere (50%1199X. Lu et al. / Solid State Ionics 178 (2007) 1195–1199H2+50% N2) in a temperature range of 600 to 1000 °C. Themixed gas atmosphere is used to ensure full equilibrium with thelow partial pressure of oxygen found in a typical SOFC anodecompartment. As illustrated in Fig. 7, the temperaturedependence of electrical conductivity for the sintered SYTexhibits a metallic behavior over the temperature rangeinvestigated, rather than the typical semiconductor behaviorfor a perovskite oxide. The metallic behavior can be primarilyattributed to Ti4+ reduction to Ti3+ that is facilitated by the veryhigh sintering temperature of 1400 °C and reducing atmosphere.4. ConclusionsThe synthesis of SYT using a modified Pechini process hasresulted in high phase purity perovskite oxide powder withmono-dispersed nanoscale grain size. An extended calciningperiod at 900 °C in air was found to eliminate the formation ofTiO2 rutile phase while minimizing the formation of pyrochlore.The electrical conductivity of the SYT sintered at 1400 °C in areducing atmosphere was measured, and found to exhibit ametallic behavior under reducing conditions in the testedtemperature range of 600 to 1000 °C. The high electricalconductivity of the modified Pechini process-derived SYTmakes it a promising candidate for use as a conductivecomponent in composite SOFC anodes.AcknowledgementThis work was supported by Edison Materials TechnologyCenter (EMTEC) under Award #EFC-H2-16-1C and the U.S.Department of Defense Fuel Cell Program. The authors wouldlike to acknowledge the contributions of contract managersDr. Michael Martin and Mr. Frank Holcomb. The authorsthank Prof. Ken Vecchio (UCSD) for assistance with TG/DTAmeasurements. The authors also acknowledge the use of the ZeissCenter of Excellence at UC Irvine for advanced microscopy.References[1] K. Huang, R.S. Tichy, J.B. Goodenough, C. Milliken, J. Am. Ceram. Soc.81 (1998) 2581.[2] D. Kusunoki, Y. Kikuoka, V. Yanagi, Int. J. Hydrogen Energy 20 (1995)831.[3] K. Eguchi, T. Hatagishi, H. Arai, Solid State Ion. 86–88 (1996) 1245.[4] J.S. Herring, J. O'Brien, C. Stoots, P. Lessing, R. Anderson, INEEL,Hydrogen, Fuel Cells, Infrastructure Technologies, U.S. DOE FY2003Progress Report, 2003.[5] S. Hui, A. Petric, J. Electrochem. Soc. 149 (2002) J1.[6] O.A. Marina, N.L. Canfield, J.W. Stevenson, Solid State Ion. 149 (2002) 21.[7] S. Hui, A. Petric, J. Eur. Ceram. Soc. 22 (2002) 1673.[8] A. Atkinson, S. Barnett, R.J. Gorte, J.T.S. Irvine, A.J. Mcevoy, M.Mogensen, S.C. Singhal, J. Vohs, Nat Matters 3 (2004) 17.[9] M. Pechini, Sprague Electric Co., U.S. Pat., 1967, #3,330,697.

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Modified Pechini synthesis and characterization of Y-doped strontium titanate perovskite

Modified Pechini synthesis and characterization of Y-doped strontium titanate perovskite

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