Enhanced ionic conductivity of co-doped ceria solid solutions and applications in IT-SOFCs

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Enhanced ionic conductivity of co-doped ceria solid solutions and applications in IT-SOFCs Monika Srivastava, Kuldeep Kumar, Nandini Jaiswal, Nitish Kumar Singh, Devendra Kumar, Om Parkash

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S0272-8842(14)00427-1 http://dx.doi.org/10.1016/j.ceramint.2014.03.086 CERI8267

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Cite this article as: Monika Srivastava, Kuldeep Kumar, Nandini Jaiswal, Nitish Kumar Singh, Devendra Kumar, Om Parkash, Enhanced ionic conductivity of co-doped ceria solid solutions and applications in IT-SOFCs, Ceramics International, http://dx.doi.org/ 10.1016/j.ceramint.2014.03.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced ionic conductivity of co-doped ceria solid solutions and applications in IT-SOFCs §

§,

§

§

§

Monika Srivastava , Kuldeep Kumar *, Nandini Jaiswal , Nitish Kumar Singh , Devendra Kumar and Om §, 

Parkash * §

Department of Ceramic Engineering, Indian Institute of Technology, Banaras Hindu University, Varanasi (Uttar Pradesh) - 221005, (India)

*Corresponding author(s)

Author 1: Om Parkash

Department of Ceramic Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi (Uttar Pradesh), India – 221005; Tel: +91-542-6701791; Fax: +91-542-2368428; Email address: [email protected]

Author 2: Kuldeep Kumar

Department of Ceramic Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi (Uttar Pradesh), India – 221005; Email address: [email protected]

1   

Abstract Co-doping effect of Sm3+ and Ca2+ ions in ceria has been studied using microwave assisted synthesis (MAS) technique. Thermal response of the prepared compositions was studied using differential thermal and thermogravimetric techniques (DTA/TG). Structural studies were performed using X-Ray diffraction (XRD) and fourier transform infrared spectroscopy (FT-IR). The impedance analyses were performed over a wide range of temperature (20-500ºC) and frequency (100Hz to 1MHz). The conductivity data show that 10% doping of both Sm3+ and Ca2+ in ceria provides a significant rise in the conductivity value of pure ceria.

Introduction Solid oxide fuel cells (SOFCs) have attracted a great attention as promising systems for electrical power generation because of their high efficiency of chemical energy to electric power conversion. Ceria based solid electrolytes (Ce(M)O2-δ , M: rare-earth or alkaline-earth cations) are of considerable interest for potential use in SOFCs due to their higher ionic conductivity and lower cost as compared to stabilized zirconia and lanthanum gallate based phases respectively [1–5]. The main drawback of ceria-based electrolytes, complicating their commercial application is their increased electronic conduction which is caused by the reduction of Ce4+ to Ce3+ [3, 6] under low oxygen partial pressure (below 10−10 atm) at 800 °C. It has been reported that the reduction of ceria is negligible at temperatures around 600–700 °C. However, such low temperatures are not suitable for single ion doped ceria as an electrolyte in SOFC due to its high electrical resistance [7]. Herle, et all [8] found that ceria doped with two and more cations shows significantly higher ionic conductivity than the pure form.

2   

Among ceria based ionic conductors, the highest level of ion conductivity is the characteristic of Sm3+ and Gd3+ doped solid solutions - Ce1-xMxO2-δ, where M = Gd3+ or Sm3+, x = 0.10–0.20 [9-13]. Some ternary systems involving CeO2–Gd2O3 or CeO2–Sm2O3, the third components being Pr2O3 [14], Y2O3 [15], Tb2O3 [16], MgO [17], CaO [18] have been studied from the viewpoint of structure and electrical conductivity. These materials when costabilized with Sm2O3 or Gd2O3 and other trivalent cations, depending on their chemical composition have generally improved ionic conductivities, although in some cases deterioration of the ionic conductivity or increased electronic conductivity was observed [19].

In this article, we have reported the synthesis of pure and co-doped ceria solid solutions via a newly developed route of synthesis. The structural and electrical characterizations of Sm3+ and Ca2+ doped ceria solid solutions have been performed. The results obtained make this study crucial for electrolyte applications in intermediate temperature solid oxide fuel cells (IT-SOFCs).

Keywords: CeO2 (B); impedance spectroscopy (C); ionic conductivity (C); fuel cells (E);

3   

Experimental All the required chemicals for Ce0.8Sm0.2-xCaxO2-δ (0≤x≤0.2), (CSC) system were taken in their nitrate forms. To introduce Sm3+ and Ca2+ ions in ceria, the stoichiometric amounts of Sm(NO3)3 (Purity 99%) and Ca(NO3)2 (Purity 99%) were mixed together with Ce(NH4)2(NO3)6 (Purity 99.9% ). All of these components were dissolved in the minimum possible amount of ethylene glycol (C2H4(OH)2) which acts as a reaction medium. The stoichiometric amount of urea was also added during the dissolution which works as a reducing agent as per the proposed reaction below.

Ce(NH4)2(NO3)6 + 4 NH2CONH2 → CeO2 + H2O + Oxides of nitrogen↑

The prepared solution was transferred in a round bottom flask which was placed in a microwave oven and mounted with a vertical water condenser. The solution was subjected to microwave heating for 15 minutes. In the initial 2-3 minutes, yellow turbidity starts to appear in the solution. The solution was allowed to cool after the process, followed by filtration and washing with ethanol for 5 to 6 times to remove the residual surfactants. The obtained powder was dried at 80-90⁰C for 10 minutes and calcined in oxygen atmosphere at 400⁰C for 2 hours. The calcined powder was ground and uniaxially pressed under 10 ton pressure to form green pellets, using a stainless steel die of 13 mm diameter. The green pellets were sintered at 1275⁰C for 4 hours to get high density mass.

Differential thermal and thermo gravimetric analyses (DTA/TG) were carried out on the powder as prepared, by heating up to 600 °C at the rate of 10°C/minute by employing Diamond DTA/TG Perkin Elmer, USA. The crystal structure was determined by the powder XRD using Rigaku X-Ray Diffractometer employing CuKα radiation

4   

with Ni filter. Lattice parameters were determined by using a non-linear least-square fitting program - ‘Unit Cell’. The doping effect on the bonding in ceria was studied by FT-IR spectroscopy. Impedance spectroscopy was carried out over silver coated pellets (where silver acts as an electrode), applying two probe method in the frequency and temperature range of 100Hz - 1MHz and 200°C - 500°C respectively using Novocontrol Impedance Analyzer.

Results and Discussion The microwave assisted synthesis (MAS) adopted here is a very quick and economical approach of preparing electrolytes [20]. Several other methods e.g. homogeneous co-precipitation, reverse microemulsion system, and reverse precipitation require temperature above 400 °C for the synthesis [21-25]. In the MAS, formation of the electrolyte powder occurs at 300 °C (as indicated by the TG below). Comparatively lower sintering temperature (1275 °C, 4hrs) than the previously reported methods [2629] produces more than 95% of theoretical density. The thermal decomposition behavior of all the compositions listed in Table 1, has been shown in Fig. 1. From the TG plots, it is clear that all the members have shown minor and major weight losses at temperatures 100⁰C and in the range of 200-300⁰C, respectively. These losses may be attributed to the evaporation of absorbed water and burning of carbonaceous content, respectively. Correspondingly, one endothermic and one exothermic peak were observed in the DTA plots of these compositions. The presence of two extra peaks without any weight loss/gain in the DTA pattern of CS20 might be due to some interstitial changes, which are yet to be investigated. Thus, we concluded that the prepared compositions can be calcined at 400⁰C to exhibit the complete crystallization.

5   

The X-Ray diffraction spectra of the calcined compositions have been shown in Fig 2. Only the peaks corresponding to the standard spectrum for ceria with fluorite structure (JCPDS file no: 340394) have been observed. The absence of any other peaks, confirms the single phase synthesis of the co-doped ceria solid solutions. Doping of Ca2+ and Sm3+ doesn’t create any noticeable distortion in the CeO2 lattice. The lattice parameter of pure ceria, as shown in Table 1, increases with the co-doping of Ca2+ and Sm3+, which is due to the little higher ionic radius of Sm3+ (1.04Å ) as compared to Ce4+ (1.01Å). It is also observed that the lattice parameter a = 5.398Å, calculated for only Ca2+ doped sample is less as compared to pure CeO2 which can be attributed to the smaller size of Ca2+ (0.99 Å) in comparison to that of Ce4+.

Fig. 3 shows the FT-IR spectra of the calcined powders in the frequency range of 6000-400 cm-1. The vertically

stacked spectra of all the compositions show that the position of the peaks is almost the same in all the samples. This indicates that there is no major change in the bonding due to the doping. In these spectra, a relatively broad band centered on 3445 cm−1 is assigned to the –OH stretching vibration. The presence of –OH group in the calcined samples is possibly due to the moisture absorption. The bands at 1620 and 1531 cm−1 are ascribed to the bending mode of hydroxyl groups of the adsorbed water. The peak appearing at 1384 cm−1 corresponds to the vibration modes of -NO3 group. The absorption bands around 1100 cm−1 are probably due to CH–OH stretching vibration. In the 850–600 cm−1 region of the IR spectra, the observed peak may be attributed to the characteristic Ce–O vibrations [30].

To perform the electrical measurements, all the samples were sintered at 1275ºC for 4 hours in a conventional furnace, leading to attainment of more than 95% of theoretical density for each composition. AC impedance spectroscopy was performed to determine the electrical properties of all the sintered compositions of CSC system. 6   

The contributions of the grains and grain boundaries have been calculated in the temperature and frequency range of 200-500⁰C and 100Hz-1MHz respectively. Fig. 5 shows typical complex plane impedance plots for pure

ceria and all the compositions of the CSC system at 500 °C. Three circular arcs are observed viz. a smallest arc in the highest frequency range passing through the origin, a second arc in the intermediate frequency range and a third arc in the lower frequency range which grows with increasing temperature. The least square fitting of the data points was done at various temperatures and the corresponding grain and grain boundary resistance were determined from the intersection of the circular arcs with the real axis. The grain and bulk conductivities of the samples have been determined using the relation

σg/t

=

1 d . R g/ t A

Where σ, R, d and A stand for conductivity, resistance, pallet thickness and circular area of the pallets, respectively. The letters g and t in the suffix of σ and R stand for grain and total part respectively. The grain boundary conductance of all the samples was calculated using the relation

Ggb =

 

1 Rgb

Where Ggb and Rgb stand for grain boundary conductance and resistance values respectively. The activation energy Ea, for grain, grain boundary and bulk conduction have been calculated using the Arrhenius relation

σ =

σo −E exp ( a ) T kT

Where σo is the pre exponential factor, k is the Boltzmann constant and T is the absolute temperature. Arrhenius plots for total conductivity of all the compositions have been shown in Fig. 5. The slopes of the lines provide the

7   

activation energy values for the compositions. The same calculations were performed on the Arrhenius plots for grain and grain boundary conduction and obtained Ea values have been given in Table 2. The mechanism for electrical conduction in ceria samples changes at higher temperature. At elevated temperatures, the electrical conduction is controlled by the population of charge-carrying defects (oxygen vacancies) created by dopants (Ca2+ or Sm3+). At low temperatures, the population of charge-carrying defects is determined by the thermodynamic equilibrium between the free defects and the associated pairs. But for the pure CeO2 electrolyte, the conductivity is much lower than doped samples and the temperature dependence of the electrical conduction is opposite to that of doped ceria. This is due to a different conduction mechanism in CeO2. The electrical conduction of pure ceria results from impurity and intrinsic factors. At low temperatures, its conduction is dominated by the dissociated electron concentration from the energy band of the impurity whose activation energy of electrical conduction is much lower than that of the intrinsic conduction. At high temperatures, the conductivity increase is predominantly due to the intrinsic factor while electrons from the energy levels of the impurity are all dissociated and activated. The total electrical conductivity values (at 500ºC) of all the prepared compositions have been shown in Table 2.

A close study of the obtained results for total activation energy shows that the 20% molar doping of Samarium in ceria results in the lowest activation energy value. On replacing the Sm3+ sites in CS20 composition with Ca2+, this energy value increases, as shown for CS15C5, CS10C10 and CS5C15. Comparing the single doping effect of Sm3+ and Ca2+ in the ceria matrix, we can conclude that both of the elements lower the activation energy of ceria, but the effect of Sm3+ shows a better result which is in agreement with the previous studies [9-13]. Single doping of Sm3+ and Ca2+ in the ceria matrix increases the activation energy for the grain conduction but lowers it for the

8   

grain boundary conduction. The substitution of Sm3+ sites in CS20 by Ca2+ increases the grain and grain boundary activation energies in a non-systematic manner which is not clear at the present.

Total conductivity data recorded at 500oC shows that the single doping of Sm3+ and Ca2+ in the ceria matrix increases the conductivity values. As shown in Table 1, the 20% molar inclusion of Sm3+ in the ceria matrix enhances the conductivity more than 20 times. The substitution of Sm3+ in CS20 by Ca2+ , further increases this value for CS15C5 and CS10C10, respectively and then decreases it for CS5C15.

Conclusion Microwave assisted synthesis opted

to prepare Sm3+ and Ca2+ doped ceria compositions is a quick and

economical approach of synthesis. Sintering temperature of 1275oC with a dwelling time of 4 hours provides more than 95% of the theoretical density. Structural studies performed on Ce0.80Sm0.20-xCaxO2-δ system doesn’t show any change in the bonding among atoms except the variation in lattice parameter which is because of the odd ionic radii of the dopants .Impedance Spectroscopy analysis has shown that at 500oC, Ce0.8Sm0.10Ca0.10O2-δ has the highest conductivity value while Ce0.8Sm0.2O2-δ has the least activation energy for total conduction. As the difference in activation energy values of these two compositions is negligible, Ce0.8Sm0.10Ca0.10O2-δ seems to be a

potential candidate for IT-SOFC applications

9   

Acknowledgement Authors are thankful to Department of Science and Technology, New Delhi for supporting this research. Two of the authors (Monika Srivastava and Kuldeep Kumar) are grateful to University Grant Commission, New Delhi for award of fellowship during this work. The authors thank Dr. Ashutosh Dubey and Ankit Mehta for proof reading the manuscript.

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Fig.1: 1: DTA/TG plots of as prepared (a) C, (b) CS20, (c) CS15C5, (d) CS10C10, (e) CS5C15 and (f) CC20 compositions

Fig.2: : XRD Spectra of (a) C, (b) CS20, (c) CS15C5, (d) CS10C10, (e) CS5C15 and (f) CC20 compositions Fig.3: FT-IR Spectra of (a) C, (b) CS20, (c) CS15C5, (d) CS10C10, (e) CS5C15 and (f) CC20 compositions

Fig.4: Complex plane impedance plots for undoped ceria and compositions of system Ce0.80Sm0.20-xCaxO2-δ at 500 °C. Fig.5: Arrhenius plots of total conductivity for undoped ceria and compositions of system

Ce0.80Sm0.20-xCaxO2-δ.

14   

Fig. 1

60 0

60 0

o

297 C

(a)

50

(b)

o

270 C

50

-2

-2

30

-6

20

Mass Loss(mg)

Mass Loss(mg)

Heat Flow(μV)

-4

40 -4 30 -6 20

10

-8

-8

10

0 -10

-10 100

200

300

400

500

600

-10 700

100

200

300

500

600

0 700

Tem perature ( C)

Temperature ( C)

60

60

o

260 C

o

264 C

0

400 o

o

0

(c)

(d)

50

50

-2

30 -6 20 -8

40

Heat Flow(μV)

Heat Flow(μV)

40 -4

Mass Loss(mg)

-2

Mass Loss(mg)

Heat Flow(μV)

40

-4

30 -6

20 -8

10

10

-10

-10 100

200

300

400

500

600

0 700

100

o

200

300

400

o

500

600

0 700

Temperature ( C)

Temperature( C) 60

70 0

0

o

215 C

o

262 C

(e)

(f)

50

30 -6

Mass Loss(mg)

-4

50 -4 40

-6

30

20

20

-8

-8 10

10 -10

-10 100

200

300

400

o

500

600

0 700

100

200

300

400

o

Temerature ( C)

Temperature( C)

15 

500

600

0 700

Heat Flow(μV)

40

Heat Flow(μV)

Mass Loss(mg)

-2

 

60

-2

(311)

(420)

(311)

(400)

(f) (222)

(200)

(220)

(111)

Fig. 2

Relative Intensity

(e )

(d )

(c )

(b )

(a ) 2 0

4 0

A n g le (2 θ )

16   

6 0

8 0

Fig. 3

250

Transmission (Arb Unit)

200

(c) (d)

150

(b) 100

(f) 50

(e) (a) 0 6000

5000

4000

3000

2000 -1

W avenumber (cm )

17   

1000

Fig. 4

14.0

30.0

C

7.0 3.5 0.0 0.0

3.5

7.0

4

Z' (10 Ω)

10.5

0.0 0.0

14.0

7.5

CS10C10

400

Z'' (Ω)

15.0

300 200 100

7.5

15.0

22.5

0 0

30.0

100

2

Z' (10 Ω) 14.0

200

300

Z' (Ω)

400

500

20

CC20

CS5C15

15

Z'' (104Ω)

10.5

Z'' (103Ω)

30.0

500

7.5

7.0 3.5

3.5

7.0

3

Z' (10 Ω)

10.5

14.0

18   

22.5

2

22.5

0.0 0.0

15.0

Z' (10 Ω) CS15C5

Z'' (102Ω)

15.0 7.5

30.0

0.0 0.0

CS20

22.5

Z'' (102Ω)

Z'' (104Ω)

10.5

10 5 0 0

5

10

4

Z' (10 Ω)

15

20

Fig. 5:

E T = 1 .3 e V

2

E T = 1 .1 1 e V E T = 1 .1 3 e V E T = 1 .2 6 e V E T = 1 .2 9 e V E T = 1 .2 2 e V

-1

Log σ T(Scm K)

0

T

-2

CS10C10 CS15C5

-4

CS20 C S5C 15 CC20

-6

C 1 .2

1 .4

1 .6

1 .8

-1

1 0 0 0 /T ( K )

19   

2 .0

2 .2

Table 1: Abbreviation and lattice parameters of CeO2 and compositions of system Ce0.80Sm0.20-xCaxO2-δ. Composition

Abbreviation

Lattice Parameter

CeO2

C

5.400

Ce0.8Sm0.2O2-δ

CS20

5.415

Ce0.8Sm0.15Ca0.05O2-δ

CS15C5

5.416

Ce0.8Sm0.10Ca0.10O2-δ

CS10C10

5.403

Ce0.8Sm0.05Ca0.15O2-δ

CS15C15

5.412

Ce0.8Ca0.20O2-δ

CC20

5.398

20   

Table 2: Total conductivity at 500 ºC, Activation energy of conduction Et for CeO2 and system

Ce0.80Sm0.20-xCaxO2-δ

Activation

Activation Activation

Serial

Total conductivity

Energy for

Energy for

Composition

Energy for Total o

-1

at 500 C (Scm )

No

Grain

Grain Boundary Conductivity(eV)

Conductivity(eV) Conductivity(eV) 1

C

2.41*10-5

0.71

1.41

1.30

2

CS20

5.35*10-4

0.73

1.11

1.11

3

CS15C5

6.48*10-4

1.00

1.37

1.13

4

CS10C10

3.55*10-3

0.85

1.29

1.26

5

CS5C15

1.10*10 -4

0.73

1.29

1.29

6

CC20

2.68*10-5

0.78

1.03

1.22

21