Synthesis and Characterization of Co-Doped Ceria Ceramics by Sol ...

Trans. Ind. Ceram. Soc., 70 (3) 143-147 (2011).© 2011 The Indian Ceramic SocietySynthesis and Characterization of Co-Doped CeriaCeramics by Sol-Gel MethodS. Ramesh*, K. C. James Raju and C. Vishnuvardhan Reddy**School of Physics, University of HyderabadHyderabad – 500 046, AP, India[MS received June 14, 2011; revised copy received August 24, 2011]Doped and co-doped ceria electrolyte materials are very useful in solid oxide fuel cells. The Ce 0.9 Sm 1–x Sr x O 2(x = 0-0.09) compositions were synthesized through the sol-gel method. Dense Ce 0.9 Sm 1–x Sr x O 2 ceramics wereobtained through sintering the pellets at 1300 o C for 8 h. XRD measurements indicate that all synthesized materialscrystallized in cubic fluorite-type structure. Average crystallite size of the samples was in the range 21-27 nm. Therelative density of Ce 0.9 Sm 1–x Sr x O 2 samples was over 94% of the theoretical density. The lattice parameter increasedlinearly with increasing Sr concentration in Ce 0.9 Sm 1–x Sr x O 2 following Vegard’s rule. Surface morphology wasanalyzed using SEM. It was observed that the thermal expansion increased linearly with increasing temperature.The two-probe a.c. impedance spectroscopy was used to study the grain, grain boundary and total ionic conductivityof doped and co-doped ceria in the temperature range 250 o -500 o C. The Ce 0.9 Sm 0.07 Sr 0.03 O 2 composition showedhigher grain ionic conductivity and minimum activation energy at 500 o C.[Keywords : Oxides, Sol-gel process, Impedance spectroscopy, Ionic conduction, Activation energy]IntroductionSolid oxide fuel cells are the energy converters; theyconvert chemical energy to electrical energy. Recentlysolid oxide fuel cells (SOFC) have been attracting moreattention due to high efficiency and clean energyconversion. Yttria-stabilized zirconia (YSZ) is a standardelectrolyte for solid oxide fuel cell applications. Highoperating temperatures (1000 o C) are required toincrease the ionic conductivity of YSZ. However, at suchhigh operating temperatures there are some problemslike thermal mismatch between cell components,chemical instability, selection of materials, etc. 1, 2Doped ceria electrolytes showed higher ionicconductivity in the temperature range 350 o -700 o C. Amongthe various dopants studied samarium doped ceria (SDC)has been reported to have the highest ionic conductivity. 1Ceria doped with heterovalent cations, such as rare earthand alkaline earth ions, have been extensively studied asthe most promising electrolyte materials for intermediatetemperature solid oxide fuel cells (IT-SOFC). 1As reported in the literature 3–9 it is evidenced that codopingcould improve the ionic conductivity. Dopant ion,dopant concentration, oxygen vacancy concentration,defect association energy and local defect structure arethe factors, which can influence the ionic conductivity indoped ceria. 1, 2, 10 As reported, 1 for single doped ceria incase of rare earth materials, Ce-Sm or Ce-Gd are the bestcombinations; similarly in case of alkaline earth materials(Ca, Sr), Ce-Ca or Ce-Sr are the best combinations. In thepresent study we have selected Ce-Sm-Sr (co-doped)*Corresponding author; e-mail: ramesh_ou10@rediffmail.com**Department of Physics, Osmania University, Hyderabad – 500 007,AP, Indiacombination and changed different dopant concentrationsto identify proper electrolyte composition for SOFCapplication.In the present paper, Sm 3+ and Sr 2+ co-doped ceriabased materials, Ce 0.9 Sm 1–x Sr x O 2 (x = 0-0.09), areprepared and characterized. The effect of co-doping onstructure and electrical conductivity is studied incomparison with samarium-doped ceria, Ce 0.9 Sm 0.1 O 2 , inthe temperature range 250 o -500 o C.ExperimentalThe samples with the general formula Ce 0.9 Sm 1–x Sr x O 2(x = 0-0.09) were synthesized through sol-gel method. Highpurity cerium nitrate hexahydrate (Ce(NO 3 ) 3 6H 2 O),samarium oxide (Sm 2 O 3 ) and strontium nitrate(Sr(NO 3 ) 2 .4H 2 O) were used as the starting materials.Stoichiometric amount of samarium oxide powder wasmixed with nitric acid in a beaker labelled as ‘solution A’and placed on a heater at 45 o C; slowly the evolution ofwhite gas fumes occurred and disappeared resulting in athick transparent solution. This was an indication that thesamarium oxide has been converted into samarium nitrate.Stoichiometric amounts of Ce and Sr nitrates weredissolved in distilled water under continuous stirring in abeaker labelled as ‘solution B’. ‘Solution A’ was then addedto ‘solution B’ and stirred properly. Citric acid was addedto the whole solution in 1:1 molar ratio. Then ammoniumhydroxide was added drop by drop to the solution to adjustthe pH to ?7. After adjusting pH, the whole mixture wasstirred at 70 o C for 10 to 14 h and a homogeneous solutionwas then formed. With the evaporation of water, ethyleneglycol was added to the solution in the molar ratio 1:1.2and heated at 85 o C. After 8-12 h a brownish sponge typeviscous gel was formed. The gel was placed in a burner.VOL. 70 (3) JULY – SEPTEMBER 2011 143

Slowly the gel started to foam, swell and finally burn withglowing flints and the evolution of large amounts of gasoccurred. This auto ignition was completed within a fewseconds, giving rise to a light yellow coloured ash, whichwas calcined at 600 o C for 2 h to remove the carbonaceousmaterials and to obtain the most stable mixed oxide phase.Oxidation of Ce 3+ to Ce 4+ occurred during this stage. 11 Theresultant ash was ground in agate mortar to get a finehomogeneous powder.The powders were pressed uniaxially with the help ofa hydraulic press under a continuous pressure of 5 MPainto a circular pellet (10 mm in diameter and 2 mm inthickness). Then the pellets were sintered in air at 1300 o Cfor 8 h. The densities of sintered samples were measuredin xylene using the Archimedes method. The sinteredsamples have more than 94% of the theoretical density.X-ray diffraction patterns of the samples were measuredby PANalytical X’Pert Pro X-ray diffractometer (XRD) usingCuK? radiation in the Bragg’s angle range of 20 o ? 2? ?80 o . Lattice parameter was calculated by XLAT software.ZEISS FE-SEM was used to scan the surface. The thermalexpansion measurements were carried out with Netzschpush-rod dilatometer (DIL; model: Netzsch DIL 402 PC,Germany) at a constant heating rate of 5 o C.min –1 in thetemperature range 30 o to 600 o C. The rectangular samplesof dimension 25 ? 6 ? 6 mm 3 were used for thesemeasurements. Electrical impedance (Z) and phase angle(?) were measured using LCR Hi-Tester (HIOKI 3531Z) inthe frequency range from 42 Hz to 5 MHz. Z-view softwarewas used to analyze the impedance data and to calculatethe grain resistance of the samples. Electrical conductivitymeasurements were conducted at various temperaturesin the range 250 o -500 o C in air. Silver paste was brushedto each side of the samples, which was subsequently driedat 600 o C for 20 min, producing solid silver electrode onboth sides of the pellet.Results and DiscussionStructure AnalysisFigure 1 shows XRD patterns of Ce 0.9 Sm 1–x Sr x O 2system. It is clear that the powders contain only a cubic–fluorite structure with the space group Fm3m (JCPDSpowder diffraction file no. 34-0394). The crystallite size(D XRD ) is calculated using the Scherrer equation,D XRD = 0.9? / ?cos? ..(1)where ? is the wavelength of the radiation, ? is thediffraction angle and ? is the full wave half width maxima.The crystallite size of Ce 0.9 Sm 1–x Sr x O 2 samples was in therange 21-27 nm. Figure 2 shows the lattice parameter asa function of dopant concentration (x). It is noticed thatthere is a linear relationship between lattice parameter (a)and dopant concentration (x) in Ce 0.9 Sm 1–x Sr x O 2 . Therelationship can be expressed asa(x) = 5.4232 + 0.2233x (Å) ..(2)INTENSITY (a.u.)(111)(111)(200)x = 0.09x = 0.06x = 0.03x = 0.00(220)(311)(222)(400)(331)(420)20 30 40 50 60 70 802? (degree)Fig. 1 – XRD patterns of Ce 0.9 Sm 1–x Sr x O 2 systemLATTICE PARAMETER (Å)5.4455.4405.4355.4305.4255.420R-square: 0.99930.00 0.03 0.06 0.09DOPANT CONCENTRATION (x)Fig. 2 – Lattice parameter of Ce 0.9 Sm 1–x Sr x O 2 systemThis shows the validation of Vegard’s rule. The introductionof Sm 3+ and Sr 2+ into Ce 4+ can cause a small shift in theceria peaks. This shift is indicative of a change in the latticeparameter. This is due to the difference in ionic radii ofCe 4+ (0.96 Å), Sr 2+ (1.21 Å) and Sm 3+ (1.08 Å) 12 in an oxidesolid solution. Results are consistent with the reportedliterature. 5–7Figure 3 shows the relative density versus differentsintering temperatures of Ce 0.9 Sm 1–x Sr x O 2 specimens.It can be seen from the figure that Sr and Sm codopedceria powders shrinks very fast since itsrelative density increases from 63-69% to over 93%in the temperature range 1100 o -1300 o C. Figure 4shows the SEM picture of Ce 0.9 Sm 0.07 Sr 0.03 O 2–?sample. It is very clear that the surface of the sampleis high dense. This is in good agreement with therelative density of the test samples.144 TRANSACTIONS OF THE INDIAN CERAMIC SOCIETY

1000.8RELATIVE DENSITY (%)908070600.000.030.060.09dL/L (%)0.60.40.20.000.030.060.09500.01000 1100 1200 1300TEMPERATURE ( o C)100 200 300 400 500 600TEMPERATURE ( o C)Fig. 3 – Variation of relative density as a function of temperaturefor Ce 0.9 Sm 1–x Sr x O 2 systemFig. 5 – Thermal expansion of Ce 0.9 Sm 1–x Sr x O 2 systemwhere ? ? T is the average of TEC in the temperature rangeof ? T, dL is the change of sample’s length in ? T, L is theoriginal length of sample. The calculated TECs arepresented in Table I. The decrease of TEC (after x = 0) isaccompanied by increase of lattice parameter. Presentresults are well matched with the literature. 5, 7Table I : Thermal expansion coefficients (TECs)of Ce 0.9 Sm 1–x Sr x O 2 systemCompositionTEC (?10 –6 o C –1 ) at 600 o Cx = 0.00 12.01x = 0.03 12.32x = 0.06 12.17Fig. 4 – SEM picture of Ce 0.9 Sm 0.07 Sr 0.03 O 2 sampleThermal ExpansionFigure 5 shows the thermal expansion of Ce 0.9 Sm 1–x Sr x O 2 .The electrolyte materials for SOFC must have matchedthermal expansion coefficients for cathode and anodematerials to avoid a micro crack between anodeelectrolyte-cathodeat the operation temperature. FromFig. 5 it is noticed that the thermal expansion curvesare linear. Kumar et al. 5 have reported that the thermalexpansion depends on the electrostatic forces within thelattice, which depend on the concentration of positiveand negative charges and their distances within thelattice. The thermal expansion coefficients (TEC) arecalculated from the expansion curves using followingexpressiondL? ? T = ..(3)L ??? Tx = 0.09 12.09Impedance SpectroscopyImpedance spectroscopy is a powerful tool to studythe electrical properties of solid electrolytes. The grain ionicconductivity is the fundamental material property, whereasthe total conductivity is related with the contributions ofmicrostructure, impurity and other factors to the samples’resistance. In the present work grain ionic conductivity ofmaterials is reported. Figure 6a shows the Nyquist plot ofCe 0.9 Sm 0.07 Sr 0.03 O 2 ceramic sample at 300 o C. Incompletedepressed arc, complete semi-circle and a part of arc areclearly seen. High frequency depressed arc correspondsto grain resistance (R b ), intermediate frequency semi-circlecorresponds to grain boundary resistance (R gb ) and lowfrequency incomplete arc corresponds to electroderesistance (R e ). Grain and grain boundary resistance arcsare well-resolved up to 300 o C. In this case the equivalentcircuit (Fig. 6a) is used to fit the impedance data and tocalculate grain (R b ) resistance. After 350 o C grain arc isVOL. 70 (3) JULY – SEPTEMBER 2011 145

not well resolved due to decrease in relaxation time and itshifts towards the higher frequency region exceeding theequipment limit (higher frequencies are required to observegrain contribution). Figure 6b shows the Nyquist plot at500 o C. In this case the equivalent circuit (Fig. 6b) is usedto fit the impedance data. The use of simple capacitor isnot sufficient to model the electrical response of thematerials due to depression of arcs in some cases. Forthis purpose a constant phase element (CPE) is used tofit the results. 13Electrical ConductivityThe main contribution of the conductivity of ceria-basedcompounds in air is the ionic conductivity and contributionof the electronic conductivity is negligible. 1, 13 The grainionic conductivity (? ) is then calculated from the grainresistance (R b ), measured thickness (l) and cross sectionalarea (A) usingThe activation energy for the conduction is calculated usingequationE a?T = ? 0 exp(– ) ..(5)kTwhere E a is the activation energy for conduction, T is theabsolute temperature, k is the Boltzmann’s constant and? 0 is a pre-exponential factor.Figure 7 shows the Arrhenius plots of the grain ionicconductivity for Ce 0.9 Sm 1–x Sr x O 2 ceramics. In ceriumoxide (CeO 2 ), oxide vacancies (V Oº º ) may be introducedby ceria reduction or by doping with oxides of metalswith lower valences. These equations are written inKroger-Vink notation. 1, 2CeO 2Sm 2 O 3 ???????????? 2Sm? Ce + 3O o x + V Oº º..(6)–Z? (? )500040003000200010000l??RA b0(a)FIT fit300 o CCPE 1 CPE 2 CPE 3R b R gb R e(a)1000 2000 3000 4000..(4)5000CeO 2SrO Sr? Ce + O o x + V Oº º..(7)º ºOne mole Sm 2 O 3 or SrO can produce one mole V O(Eqns 6 and 7). Due to this, the concentration of freeoxygen vacancies bound at various traps in electrolytecontributes to high ionic conductivity. However, at aº ºparticular dopant concentration (x), V O interacts with14, 15doped cations to form a defect complex (e.g. Sr? Ce V O ),which results in the decrease in ionic conductivity.It can be seen from Fig. 7 that Sm and Sr co-dopedceria samples have higher conductivities for theCe 0.9 Sm 1–x Sr x O 2 system than the single doped ceria. Itcan be seen from Table II that the activation energy isminimum for the composition Ce 0.9 Sm 0.07 Sr 0.03 O 2 . Thisdecrease in activation energy is due to the presence ofattractive interactions between dopant cations and–Z? (? )140120100806040(b)R b R gb R eCPE 1 CPE 2500 o CFIT fit(b)log (? g T) (S.cm –1 .K)oxygen vacancies. 2 0.001.51.00.50.0–0.5–1.00.000.030.060.09200020 40 60 80 100 120 140Z? (? )–1.5–2.01.31.4 1.5 1.6 1.7 1.8 1.91000/T (K –1 )Fig. 6 – Nyquist plots of Ce 0.9 Sm 0.07 Sr 0.03 O 2 sample at (a) 300 o C,(b) 500 o CFig. 7 – Temperature dependence of grain ionic conductivity ofCe 0.9 Sm 1–x Sr x O 2 system146 TRANSACTIONS OF THE INDIAN CERAMIC SOCIETY

ConclusionsTable II : Activation energy of Ce 0.9 Sm 1–x Sr x O 2 systemCompositionx = 0.00x = 0.03x = 0.06x = 0.09Figure 8 showsCe 0.9 Sm 1–x Sr x Ograin ionic conductivitydopant concentrationdopant concentration= 0.03, the grainformation of localmobile oxygenCe 0.9 Sm 0.07 Sr 0.03Table III. FromCe 0.9 Sm 0.07 Sr 0.036, 7, 16values.10 –210 –310 –40.00Fig. 8 – GrainCe 0.9 Sm 1–x Sr x OTable III : ComparisonCompositionCe 0.9 Sm 0.07 Sr 0.03Ce 0.78 Gd 0.2 Sr 0.02Ce 0.8 Gd 0.2 O 1.9grainat differentincreasesand it0.03.conductivitystructures,2, 10comparedis noticedcompared0.03CONCENTRATIONconductivitiesof grainSr 0.03ReferencePresent[6][16]Activation energy (eV)at 250 o -500 o C0.9830.9610.9720.980ionic conductivitytemperatures.with an increasereaches the maximumAfter dopant concentrationdecreases duewhich lowersGrain conductivitieswith the literaturethat the compositionwith the reported500500 o450450 400400 o350350 o300300 o0.06(x)versus Sr contentconductivity (? g ) ofO 2 system? g (S.cm –1 ) at 3002.57 ? 10 –41.73 ? 10 –49.09 ? 10 –5 Samarium and strontium co-doped ceria samplesCe 0.9 Sm 1–x Sr x O 2 (x = 0-0.09) were successfully preparedthrough sol-gel method. The sample pellets were sinteredat 1300 o C for 8 h to obtain dense ceramics and thecalculated relative densities were over 94% of thetheoretical densities. Lattice parameter increased linearlywith increasing dopant concentration. Thermal expansioncoefficients were in the range (12.01-12.32) ? 10 –6 o C –1 .The composition Ce 0.9 Sm 0.07 Sr 0.03 O 2 showed highest grainionic conductivity and minimum activation energy in theseries.Acknowledgements : Dr S. Ramesh thanks the University GrantsCommission (UGC), New Delhi, India, for financial assistance underDSKPDF scheme.References1. H. Inaba and H. Tagawa, Solid State Ionics, 83, 1-16 (1996).2. J. A. Kilner, Solid State Ionics, 129, 13-23 (2000).3. S. Lubke and H. D. Wiemhofer, Solid State Ionics, 117, 229-243 (1999).4. N. Kim, B. H Kim and D. Lee, J. Power Sources, 90, 139-143 (2000).5. V. P. Kumar, Y. S. Reddy, P. Kistaiah, G. Prasad and C. V.Reddy, Mater. Chem. Phys., 112, 711-718 (2008).6. S. Ramesh and C. V. Reddy, Acta Phys. Polon. A, 115, 909-913 (2009).7. S. Ramesh, V. P. Kumar, P. Kistaiah and C. V. Reddy, SolidState Ionics, 181, 86-91 (2010).8. F. Y. Wang, B. Z. Wan and S. Cheng, J. Solid StateElectrochem., 9, 168-173 (2005).9. F. Y. Wang, S. Chen, Q. Wang, S. Yu and S. Cheng, Catal.Today, 97, 189-194 (2004).10. H. Yoshida, T. Inagaki, K. Miura, M. Inaba and Z Ogumi,Solid State Ionics, 160, 109-116 (2003).11. B. C. H. Steele, Solid State Ionics, 129, 95-110 (2000).12. R. D. Shannon, Acta Crystall. A, 32, 751-767 (1976).13. W. Lai and S. M. Haile, J. Am. Ceram. Soc., 89, 2979-2997(2005).14. Q. Dong, Z. H. Du, T. S. Zhang, J. Lu, X. C. Song and J. Ma,Int. J. Hydrogen Energy, 34, 7903-7909 (2009).15. L. P. Li, G. S. Li, J. Xiang, Jr R. L. Smith and H. Inomata,Chem. Mater., 15, 889-898 (2003).16. H. Duncan and A. Lasia, Solid State Ionics, 176, 1429-1437the of2 samples Theinatx =x ionic to thedefect thevacancy. ofO 2 are inTable III itO 2 is well o CCo CCoCCo CCo CC0.09DOPANTionicin 2 systemCe 0.9 Sm 0.07 o CO 2O 2–?(2005).? g (S.cm –1 )VOL. 70 (3) JULY – SEPTEMBER 2011 147