ORIGINAL PAPER

Preparation and characterization of co-doped(Ce0.80La0.15Al0.05O1.90) and multiple-doped(Ce0.80Sm0.10Gd0.05Al0.05O1.90

and Ce0.80Gd0.10Sm0.05Al0.05O1.90) ceria

Namrata Singh & Nitish Kumar Singh & Om Parkash &

Devendra Kumar

Received: 24 September 2011 /Revised: 17 November 2011 /Accepted: 27 November 2011 /Published online: 23 December 2011# Springer-Verlag 2011

Abstract Attempts have been made to synthesize a few compo-sitions Ce0.80La0.15Al0.05O1.90, Ce0.80Sm0.10Gd0.05Al0.05O1.90,and Ce0.80Gd0.10Sm0.05Al0.05O1.90 by citrate–nitrate auto-combustion method. The aim of the present investigationwas to study the effect of co-doping and multiple doping onthe ionic conductivity of CeO2 for its use as solid electrolyte inintermediate temperature solid oxide fuel cells. XRD patternsshowed that all the samples have fluorite-type crystal structuresimilar to undoped ceria. Microstructures of thermally etchedsamples have been studied by scanning electron microscopy.Contributions of grains σg and grain boundaries σgb to thetotal conductivity σT, have been determined using impedanceanalysis. Impedance measurements were made in the frequen-cy range 1 Hz–1 MHz and temperature range 250–500 °C.Our experimental results show that multiple doping is moreeffective than co-doping for improving the oxide ion conduc-tivity of ceria in the materials investigated in the present study.

Keywords Ceria . Impedance spectroscopy . Ionicconductivity . Solid oxide fuel cells

Introduction

Doped ceria is considered as one of the most reliable oxideion electrolyte for the development of intermediate tempera-ture solid oxide fuel cells (IT-SOFCs) because of its high ionicconductivity. It is also very much compatible with the

electrodes [1–5]. These materials exhibit much higher ionicconductivity at relatively lower temperatures as compared toyttria-stabilized zirconia which is currently used as a solidelectrolyte. Application of yttria-stabilized zirconia as a solidelectrolyte puts lot of constraints on the materials used aselectrodes, interconnects and those used in construction ofplant. Ionic conductivity in ceria is closely related to formationand migration of oxygen vacancies [5, 6]. Oxygen vacanciesare generated to compensate the lower charge of the dopantcations. Ionic conductivity resulting from oxygen vacan-cies depends on the nature and amount of dopant [7, 8].Undoped ceria is basically a poor oxide ion conductor(σ600°C∼10−5 S cm−1) [9]. Its ionic conductivity issignificantly increased due to substitution of lower valentdopants which increase the concentration of oxygen vacanciesaccording to the reaction.

MO����!CeO2 M0 0CeþOOþV ��

O ð1Þor

M2O3�����!2CeO2 2M0Ce þ 3OO þ V ��

O ð2Þ

where M is a divalent (Ca2+, Sr2+, etc.) or a trivalent (Sm3+,Gd3+, Al3+, etc.) ion. Such aliovalent dopants form solidsolutions with ceria and introduce vacancies in the anionsub-lattice as charge-compensating defects. It has been demon-strated that the electrical properties of doped ceria can besignificantly influenced by the dopant type [10]. There existsan optimum radius of the dopant for an optimum ionic conduc-tion.With the dopant radius deviating from this optimum value,the conductivity of doped ceria decreases dramatically [5, 11,12]. This phenomenon has been attributed to the strain

N. Singh :N. K. Singh :O. Parkash (*) :D. KumarDepartment of Ceramic Engineering, Institute of Technology,Banaras Hindu University,Varanasi 221005, Indiae-mail: oprakash.cer@itbhu.ac.in

Ionics (2012) 18:473–478DOI 10.1007/s11581-011-0647-y

developed due to mismatch of ionic radius of the dopant andCe4+ and association energy of the dopant and oxygen vacancy.Maximum conductivity is observed for ceria doped with Sm orGd because these dopant cations have the optimum radius and,thereby, a smaller association enthalpy and minimum strain [5,11, 12].

Ionic conductivity is affected by ionic radius mismatchbetween the host and the dopant because of lattice strain.This is the basic principle for the choice of a dopant estab-lished by many researchers to get large value of oxygen ionconductivity [13–17]. It has been reported that co-dopedceria, i.e., ceria doped with two or more heterovalent cationsexhibits higher ionic conductivity than that of singly dopedceria in the temperature range 500–700 °C. Herle et al. [18]found that co-doping of ceria with alkaline earth and rareearth ion showed significantly higher conductivity in airthan the best singly doped materials having the same con-centration of oxygen vacancies. There are many reports onthe double doped ceria compositions such as (Ca–Sm),(Gd–Sm), (Gd–Pr), (Sm–La), and (La–Y) as a means toimprove ionic conductivity [19–23]. Although many single-and double-doped compositions have been investigated indetail, only a few triply doped ceria compositions (Ca–Sm–Gd) have been studied so far. [24]. We have chosen Alas a dopant along with Sm and Gd so that the averageradius of the dopant becomes close to the critical radiusproposed for multiple doping [5]. This multicomponent sys-tem has not been studied so far to the best of our knowledge.Further in case, conductivity of these materials is higher thanthat of ceria doped singly or co-doped with Sm and Gd, thesecan be potential candidate for IT-SOFCs at much lower cost.A few compositions viz Ce0.80La0.15Al0.05O1.90 (CL15A5),Ce0 .80Sm0.10Gd0.05Al0 .05O1.90 (CS10G5A5), andCe0.80Gd0.10Sm0.05Al0.05O1.90 (CG10S5A5) have been pre-pared by auto-combustion method and are characterized usingpowder X-ray diffraction (XRD), scanning electron micros-copy and impedance measurements.

Experimental procedure

Ce0.80La0.15Al0.05O1.90, Ce0.80Sm0.10Gd0.05Al0.05O1.90, andCe0.80Gd0.10Sm0.05Al0.05O1.90 powders were synthesizedby citrate–nitrate auto-combustion method. Ammonium cericnitrate (>99% purity, G.S Chemical Testing & Allied Industries,New Delhi), samarium oxide (99% purity, Thomas Baker(Chemicals) Limited, Mumbai), gadolinium oxide (99% purity,Central DrugHouse (P) Ltd,Mumbai-Delhi), lanthanum oxalate(99% purity), and citric acid (99% purity, Polypharm PrivateLtd., Mumbai) were used as starting materials. Lanthanumoxalate, samarium oxide, and gadolinium oxide were convertedinto corresponding nitrate with the help of nitric acid. Aqueoussolutions of required metal nitrates were mixed in appropriate

amounts and calculated amount of citric acid was added to themixed nitrate solution maintaining the citrate to nitrate (C/N)ratio as 0.3 to have a controlled combustion [25]. The mixedsolution was evaporated by heating on a hot plate using amagnetic stirrer at approximately 200 °C. The homoge-neously mixed solution became viscous and turned into a gelduring heating. The gel slowly foamed and finally burnt on itsown to produce light brown powder. This ash was calcined at1,100 °C for 2 h. The calcined powder was pelletized under aload of 60 kN into cylindrical pellets (diameter ∼13 mm,thickness ∼2.5 mm). Pellets were sintered at 1,350 °C for 4 h.

Powder X-ray diffraction patterns of calcined powderswere recorded using Rigaku X-ray diffractometer employ-ing CuKα radiation with Ni filter for determination ofcrystal structure. Theoretical density was calculated fromthe molecular weight and the unit cell volume (a3). Experi-mental density was determined using Archimedes principle.Percentage porosity was calculated from the theoretical andexperimental density using the formula.

%Porosity ¼ theoretical density� experimental density

theoretical density� 100

ð3Þ

a

b

c

Fig. 1 X-ray diffraction patterns of compositions a CS10G5A5, bCG10S5A5, c CL15A5

474 Ionics (2012) 18:473–478

For microstructural studies, sintered pellets were polished andthen thermally etched at 1,250 °C. Samples were coated withgold by sputtering technique. Micrographs were taken usingInspect S-50, FP 2017/12 Scanning Electron Microscope.Two probe AC impedance measurements were made on elec-troded pellets using Novocontrol Alpha-A high-performancefrequency analyzer in the frequency range 1 Hz–1MHz and inthe temperature range 200–500 °C in air.

Results and discussion

XRD patterns of various compositions after calcination areshown in Fig. 1. Absence of characteristic lines of constituentoxides in the patterns confirmed the formation of single phase.There is a slight shift in 2θ values from corresponding 2θvalues of undoped ceria. XRD data could be indexed on thebasis of a cubic unit cell similar to CeO2. Lattice constant forall the compositions was calculated using the program ‘UNITCEL’. Lattice parameter for all the three compositions is givenin Table 2. Lattice parameter is slightly more than that ofundoped ceria (5.4019 Å) [26] for all the three compositions.Change in lattice parameter occurs due to difference in ionicradius of the substituent and substituted ions. Further, it hasbeen reported recently that oxygen vacancies created due tosubstitution of lower valence ion also leads to expansion of theunit cell [27]. In the present compounds, difference in the sizeof host ion and substituted ion leads to increase or decrease ofthe unit cell dimensions. In all the cases, presence of sameconcentration of vacancies leads to equal expansion of thelattice parameter. One more factor which also contributes tothe value of the lattice parameter is the fact that some oxygen

vacancies are associated with the substituted ions. Their num-ber depends on the difference between the electropositivenature of the host and substituted ion. Due to the complexinterplay of these factors, change in lattice parameter withcomposition is not systematic. It is observed that X-ray

Table 1 Crystallite size of calcined powder and average grain size ofsintered powder for all the compositions

Composition Crystallite size aftercalcination (nm)

Grain size of sinteredpowder (μm)

CS10G5A5 74 3.28

CG10S5A5 38 1.66

CL15A5 43 0.61

Table 2 Lattice parameter,theoretical and experimentaldensity, and percent porosity ofCeO2 and all the compositions

*Data taken from ref. [26]

Composition Lattice parameter (Ǻ) Theoreticaldensity (gm/cm3)

Experimentaldensity (g/cm3)

Percent porosity

*CeO2 5.4019±0.0016 7.26 6.89 5.0

CS10G5A5 5.4325±0.0342 6.89 6.57 4.6

CG10S5A5 5.4314±0.0345 6.93 6.41 7.5

CL15A5 5.4210±0.0231 6.86 6.15 9.4

a

b

c

Fig. 2 Scanning electron micrograph for the compositions a CS10G5A5,b CG10S5A5, c CL15A5

Ionics (2012) 18:473–478 475

diffraction lines are broad indicating fine nature of powderparticles (Fig. 1). Crystallite size of calcined powder wascalculated from X-ray line broadening analysis using Scher-rer’s formula.

d ¼ 0:9lb cos θ

ð4Þ

where β is the full width at half maximum intensity of a Braggreflection excluding instrumental broadening, l is the wave-length of the X-ray radiation and θ is the Bragg angle. β istaken for strongest Bragg’s peak corresponding to 2θ∼28° forall the three samples. Crystallite size of calcined powderobtained from X-ray line broadening is given in Table 1.Density of sintered pellets was determined by Archimedesprinciple which shows that density increases with the increasein Sm doping. Lattice parameter, theoretical density, experi-mental density and percentage porosity for all the samples aregiven in Table 2.

Scanning electron micrographs for the sintered samplesCL15A5, CS10G5A5, and CG10S5A5 are shown in Fig. 2.It can be seen that some micro-pores are present in thesesamples which is consistent with the values of relativedensity being less than theoretical density. Microstructureshows distinct grains and grain boundaries. Average grainsize of all the compositions determined by linear interceptmethod is given in Table 1. Grain size of sintered samples ismore than that of the calcined samples. This is due to graingrowth occurring during sintering.

Impedance measurements were performed in air in the tem-perature range 250–500 °C and in the frequency range 1 Hz–1 MHz. Typical complex plane impedance plots at 300 °C areshown in Fig. 3a–c for all the compositions. Generally, in acomplex plane impedance plot, three circular arcs are observed.The arc corresponding to contribution of the grains, passingthrough the origin is observed in the highest frequency range.The arc corresponding to grain boundaries is found in theintermediate frequency range. The third arc in the lowest

frequency range corresponds to the contribution of electrode–specimen interface. Intercept of these arcs on the real axis givesthe contribution of the grains, grain boundaries, and electrode–specimen interface to the total observed resistance in order ofdecreasing frequency [28].

In case of CG10S5A5, a steep line passing through originappears in the complex plane impedance plots at 250 °C.This is because of very high resistance of this sample at thistemperature. However, on plotting the data in high-frequencyrange on an expanded scale, the contribution of grains at thesetemperatures is clearly seen. With increasing temperature, abig arc was found which can be fitted to two depressed arcscorresponding to contributions of grains and grain boundaries.Third depressed arc starts appearing at and above 300 °Ccorresponding to contribution of electrode–specimen inter-face in the low-frequency range.

In case of compositions CS10G5A5 and CL15A5, two orthree depressed circular arcs are observed depending on thetemperature of measurement. The depressed circular arccorresponding to grains disappears completely above 375 °C.These arcs may appear at higher frequencies which are notavailable in the present case. The contribution of the grains tothe total resistance was calculated from the intercept of the arcin the highest frequency range on Z′ axis.

a b c

Fig. 3 Complex plane impedance plot of a CS10G5A5, b CG10S5A5, c CL15A5 at 300 °C in air

Fig. 4 Log σg.T vs. 1,000/T plots of compositions CS10G5A5,CG10S5A5, and CL15A5

476 Ionics (2012) 18:473–478

The value of capacitance corresponding to grains andgrain boundaries can be obtained from the frequency ofthe highest point in the corresponding arc where the relationωRC01 is satisfied, here ω is angular frequency, ω02Пf, f isthe applied frequency in Hertz at arc maximum. Grains havecapacitance of order of pF, grain boundaries have capaci-tance of order of nF and electrode–specimen interface hascapacitance of order of μF [29]. These values are of rightmagnitude of capacitance for grains, grain boundaries, andelectrode–specimen interface.

The conductivity of grains of the samples is determinedusing the relation

σg ¼ 1

Rg� l

að5Þ

where σg is the conductivity of grains (bulk), Rg is theresistance of grains, l is the thickness, and ‘a’ is the areaof pellet. Figure 4 shows Arrhenius plots of conductivity ofgrains for all the compositions. These plots are linear with asingle slope in the entire temperature range of measurement.Figure 4 shows that the conductivity of grains increases withincreasing Gd doping from 5% to 10% and multiple-dopedcompositions CS10G5A5 and CG10S5A5 showed relative-ly higher grain conductivity than co-doped compositionCL15A5. Values of activation energy for ionic conductivityof grains, Eg for all the compositions, determined from the

plots by least square fitting of the data in Fig. 4 are given inTable 3.

The conductivity of grain boundaries can be determinedusing the relation

σgb ¼ 1

Rgb� D

d� l

að6Þ

where Rgb, D, d, l, and ‘a’ are resistance of grain boundaries,average thickness of the grain boundary, average grain size,thickness of the pellet, and area of the pellet, respectively.The ratio D/d can be determined from the ratio Cg/Cgb

where Cg and Cgb are the capacitance of grains and grainboundaries determined from the peak point of the circulararcs in Z′ and Z″ plots [29]. However, in the present case, itis not possible to determine Cg because circular arccorresponding to contribution of grains disappears after aparticular temperature for all the compositions. Therefore,σgb cannot be determined from the above relation. Hencegrain boundaries conductance, Ggb (1/Rgb) is determined.Figure 5 depicts plots of log Ggb.T vs. T of grain boundariesfor all the compositions which show a linear variation withsingle slope in the entire temperature range of measurement.The conductance due to grain boundaries increases withincreasing Gd doping from 5% to 10% in multiple-dopedcompositions CS10G5A5 and CG10S5A5, which is higherthan that of CL15A5. Values of activation energy of

Fig. 5 Log Ggb.T vs. 1,000/T plots of compositions CS10G5A5,CG10S5A5, and CL15A5

Fig. 6 Log σt.T vs. 1,000/T plots of compositions CS10G5A5,CG10S5A5, and CL15A5

Table 3 Activation energy of grains Eg, grain boundaries Egb, andtotal activation energy Et for all the compositions

Composition Activationenergy ofgrains, Eg (eV)

Activationenergy of grainboundaries, Egb (eV)

Totalactivationenergy, Et (eV)

CS10G5A5 1.09 1.20 1.20

CG10S5A5 1.15 1.40 1.40

CL15A5 1.04 1.20 1.20

Table 4 Total conductivity σt at 500 °C, 700 °C, and value of pre-exponential factor σ0 for all compositions

Composition Total conductivityσt at 500 °C(S cm−1)

Total conductivityσt at 700 °C(S cm−1)

σ0

CS10G5A5 3.41×10−4 2.00×10−2 1.51×107

CG10S5A5 4.27×10−4 1.38×10−2 3.31×108

CL15A5 2.17×10−4 6.68×10−3 1.12×107

Ionics (2012) 18:473–478 477

conduction for grain boundaries conductance are given inTable 3. These values of Egb are more than the correspondingEg values. This is due to more disordered nature of the grainboundaries.

The total resistance of the sample is given by Rt0Rg+Rgb

[28]. The total conductivity has been determined using theformula

σ ¼ 1

R� l

að7Þ

where l is the thickness and ‘a’ is the area of the sample.Plots of log (σ.T ) vs 1,000/T are shown in Fig. 6. A linearbehavior is observed showing that conductivity data followArrhenius behavior

σ ¼ σ0

T� exp � Ea

kBT

� �ð8Þ

where T stands for absolute temperature, kB for the Boltzmanconstant, and σ0 is the pre-exponential factor. Its value fordifferent samples is given in Table 4. Figure 6 shows that thetotal conductivity increases with increasing Gd doping from5% to 10% and multiple-doped compositions CS10G5A5and CG10S5A5 showed higher conductivity than co-dopedCL15A5. Values of activation energy of total ionic conduc-tivity Et for all compositions are given in Table 3. It is notedfrom Table 3 that Egb and Et are almost equal whereas Eg isless than these (Egb and Et). This is because grain boundariesform continuous path while grains are enveloped in the grainboundaries. Therefore, the overall behavior will resemble thatof grain boundaries. These values are higher than the valuesreported for oxygen migration in ceria doped singly or co-doped with Sm and Gd. Values of conductivities at 500 °C arealso lower than that for Sm or Gd doped ceria. It is concludedthat these are not suitable as solid electrolyte for IT-SOFCs.

Conclusions

Compositions Ce0.80La0.15Al0.05O1.90, Ce0.80Sm0.10

Gd0.05Al0.05O1.90, and Ce0.80Gd0.10Sm0.05Al0.05O1.90 havebeen prepared by auto-combustion method successfully.All the compositions have fluorite-type crystal structuresimilar to pure ceria. The lattice parameter of the composi-tions increases with the increase in Sm content from 5% to10% and CL15A5 showed relatively lower value of latticeparameter. The maximum increase of lattice parameter hasbeen found for the CS10G5A5. Density of the compositionsincreases with increase in Sm content. The morphology of

the samples shows homogeneous and dense structures. Theconductivity of the compositions increases with increase inGd content from 5% to 10%. The highest conductivity wasfound for the triply co-doped CG10S5A5 (σ50004.27×10−4 S cm−1; Ea01.37 eV). These materials are not suitableas solid electrolyte for IT-SOFCs applications.

Acknowledgment Thanks are due to Department of Science and Tech-nology, New Delhi for financial support. One of the authors, NamrataSingh is thankful to University Grant Commission, New Delhi for provid-ing a fellowship during the course of these investigations.

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