Microwave Sintered Pr 3+ and Y 3+ Co-Doped Ceria Ceramics as Solid Electrolytes for IT-SOFCs

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 7, Issue 11, November 2017)

78

Microwave Sintered Pr

3+

and Y

3+

Co-Doped Ceria Ceramics as

Solid Electrolytes for IT-SOFCs

Ch. Madhusudan

1

, Kasarapu Venkataramana

2

, C. Vishnuvardhan Reddy

3

1,2,3_{Department of Physics, Osmania University, Hyderabad-500007, India}

Abstract— Nanopowdered Y and Pr co-doped solid solutions Ce0.8Y0.2-xPrxO2–δ [CYP0 (x=0.00), CYP5 (x=0.05),

CYP10 (x=0.10)] were synthesized by sol-gel auto-combustion method. Dense pellets were prepared by sintering at 13000C by using microwave radiation. The relative densities of all the samples were noticed above 95%. The morphological structure and electrical properties were investigated for

application as electrolyte materials for intermediate

temperature solid oxide fuel cells. XRD measurements ascertained the sample formation with a single-phase cubic fluorite structure. SEM micrographs showed spherical faceted near- uniform grains with distinct grain boundaries and of negligible porosity. Total ionic conductivity values and activation energies were measured depending on the Pr content. These measurements gave rise to the firm conclusion that, the co-doped ceria has a potential to be utilized as the electrolyte for intermediate temperature solid oxide fuel cell applications.

Keywords— co-doped ceria, X-Ray Diffraction, Scanning Electron Microscopy, Microwave Sintering, Electrochemical Impedance Spectroscopy, Total Ionic Conductivity.

I. INTRODUCTION

Rising energy costs and the growing demand continue to urge the people advance in new technology designed to cater to the needs of energy efficiency across the globe. One such technology is Fuel cell. In the existing varied fuel cell technologies, Solid Oxide Fuel Cells (SOFC) has the ability to convert the chemical energy into electrical energy with certain attractive features including high conversion efficiency, fuel flexibility and portability. They have significantly lower emissions of COx, NOX and SOx compared to conventional power generation devices [1] [2].

State-of-the-art SOFC electrolyte Yttria Stabilized Zirconia (YSZ), requires higher operating temperatures (>1000°C) to exhibit high ionic conductivity. The high operating temperature is the major limiting factor to using SOFCs and their commercialization. Solid electrolyte, as the major component in SOFC devices, determines the operating temperature and related systems [18]. Over the past several years efforts are geared towards lowering the operating temperature of SOFCs to the intermediate temperature range (500-800C).

Rare earth (RE) doped ceria is one of the candidates for the electrolyte of intermediate temperature solid oxide fuel cell (ITSOFC) because of its ability to store and release oxygen ion vacancies due to its easily accessible oxidation states from Ce3 Ce4 and hence has high ionic conductivities at lower temperatures compared to that of YSZ [3]. The doping with lower valent RE cations into ceria creates a number of anion vacancies namely oxygen vacancies for charge compensation. Enhanced Conductivity results via oxygen diffusion through the oxygen vacancies introduced. In addition, doping of RE ions in ceria lattice leads to many industrial & technical applications such as catalysis, luminescent materials, [4] sensors and magnetic applications [5].

Single doping of ceria with RE ions is a way to create vacancy defects (oxygen vacancies) for improved optical, electrical and catalytic properties. An enhancement of these properties have been observed following the co-doping of ceria to produce materials such as Ce1-x-ySmxSryO2 [5], Ce

1-xSmx-yMgyO2−δ [6], Ce1-x(Gd0.5Pr0.5)O2 [7], Ce0.8Sm0.12Pr0.08O2−δ [8], Ce1-x(Pr1/3Sm1/3Gd1/3)xO2−δ [9].

Densification of the electrolyte material at lower sintering temperature/time still remains as one of the major technical challenges which can save energy and reduce the fabrication costs of SOFCs. Of late, microwave sintering (MS) of ceramics, has emerged as an alternative approach for densification of ceramics and has become a most sought-after process due to its rapid heating, enhanced densification rate yielding in uniform, finer microstructure with much higher densities at lower temperature/time when compared to conventional sintering. [10] [19].

In a previous work, S.K. Tadokoro et. al synthesized Ce0.85Y0.13Pr0.02O2–δ sample by both co-precipitation and mixing of powder techniques, sintered at 14500C for 4h with conventional sintering approach. They reported structural and electrical properties of the prepared sample [11].

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International Journal of Emerging Technology and Advanced Engineering

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We used the energy efficient frittage -- the microwave sintering (at 13000C for 30min) which has several advantages over that of the conventional sintering and developed a co-doped solid ceria electrolyte, which may be a potential electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs).

II. EXPERIMENTAL

2.1. Powder and Pellets Preparation:

The Y3+ and Pr3+ co-doped ceria Ce0.8Y0.2-xPrxO2–δ (x = 0.00, 0.05, 0.10) electrolyte materials were synthesized via sol-gel auto-combustion synthesis. Stoichiometric proportions of ammonium ceric nitrate, yttrium nitrate hex hydrate, Praseodymium nitrate hex hydrate, citric acid and ethylene glycol were weighed and were dissolved in deionized water under magnetic stirring. Citric acid (as a chelating agent) and ethylene glycol (as a gel formation agent) were added to the resultant solution, with metal to citric acid ratio as 1:1.5 and citric acid to ethylene glycol ratio as 1:1.2. The pH of the solution was adjusted near around 7 (pH=7) by adding aqueous Ammonia. The final homogeneous solution was heated to nearly 90°C to evaporate the water and excess residues and to form a viscous gel like substance. The obtained gel was further heated to 150°C for reaching the combustion point, yielding solid particles. These particles were ground thoroughly in the agate mortar. The powder thus ground was calcined for 2 h at 800°C to eliminate unwanted carbonaceous materials and also to get crystallized formation of samples. The calcined powders were further ground and were compressed with a pellet die at a pressure of 5 MPa to fabricate pellets (10 mm in diameter and 2 mm in thickness), and subsequently sintered at 1300°C for 30 minutes using a programmable microwave furnace.

2.2. Characterization:

The crystalline nature and phase purity of the synthesized samples were characterized by powder X-ray diffraction using Philips Xpert PRO ALPHA1 Panalytical diffractometer with cu Kα1 monochromatic radiation (λ= 0.15406 nm) in phase-wise analysis. Data were collected by step scanning over a 2θ range from 20 to 80° with a step size of 0.02° at the scanning rate of 2° min–1. The microstructure of sintered samples was executed by scanning electron microscope (SEM) using ZEISS EVO 18 special edition. To enable SEM characterization, the prepared pellets were sputtered with Au coating.

Impedance measurements of all the samples were taken using the Wayne Kerr impedance analyzer 6500B with temperature ranging from 250°C to 500°C. Netzsch DIL 402 PC dilatometer was used to investigate the thermal expansion studies in the temperature ranging from room temperature to 800°C at a heating rate of 3° min–1.

III. RESULTS AND DISCUSSION

3.1 X-Ray Powder Diffraction analysis (XPRD)

Fig.1 (a) illustrates the X-Ray Powder Diffraction patterns of all the Ce0.8Y0.2–xPrxO2–δ powder samples recorded at room temperature. The diffraction peaks are indexed to (111), (200), (220), (311), (222), (400), (331) and (420). For all the samples, only cubic fluorite phase

with Fm m3 space group was detected. The diffraction peaks in each XPRD pattern coincided well with the standard data of CeO2 [JCPDS PDF no: 34–0394].

Fig. 1(a). XRD patterns of sintered CYP system

It can also be observed from the XPRD patterns that the main diffraction peaks shift towards smaller 2θ with the increase in the dopant concentration, as can be seen in Fig. 1(b) with the shifting of (111) diffraction peaks to the left or smaller 2θ. This shift is indicative of change of lattice parameter, which confirms that the dopants are dissolved into the Ce sites, and the subsequent creation of oxygen vacancies [14]. It can be seen from Table 1 that the lattice parameter increases linearly with dopant concentration in agreement with Vegard‘s law as ionic radius of Pr3+

[image:2.612.336.548.349.517.2]

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International Journal of Emerging Technology and Advanced Engineering

80 Fig. 1(b). Peak shift in XRD patterns of CYP system

Table I showsvarious Crystallographic parameters such as crystal structure, unit cell volume and relative density of the obtained samples. The relative densities of the obtained samples were calculated as the ratio between bulk density and theoretical density.

(%) 100

rel

th D D

D

  ---(1)

The bulk density (D) of the pellets was measured using the Archimedes principle using xylene as the liquid and theoretical density (Dth) was calculated from the XRD data. The relative densities of all the samples were found to be above 95%. This indicates the low porosity in the co-doped ceria samples.

TABLE I

CRYSTALLOGRAPHIC PARAMETERS OF CYP SYSTEM

Sample Id

Crystal structure

Lattice constant

(a)

Volume of unit cell (V)

Relative Density (%) CYP0 Cubic 5.4010 157.55 95.4

CYP5 Cubic 5.4046 157.87 95.7

CYP10 Cubic 5.4082 158.18 96.8

3.2. Micro Structure(SEM analysis)

Fig. 2 depicts the SEM micrographs of the synthesized samples of ceria co-doped with Y3+ and Pr3+ microwave sintered at 13000C for 30minutes. The co-doped ceria samples Ce0.8Y0.2–xPrxO2–δ show very dense structure in accordance with the relative density of the sintered pellets.

[image:3.612.67.270.128.300.2]

Microwave Sintering at 1300°C yielded in a near uniform sub-micrometer grain size with negligible porosity. The spherically shaped smaller particle size sample is very much desired to obtain dense ceramics. [6] [10].

Fig. 2. SEM micrographs of CYP system

3.3. Electrochemical Impedance Spectroscopy (EIS) results

Fig. 3 shows the AC impedance spectra of the obtained co-doped ceria samples at 250°C. The impedance analysis of all the specimens was performed in the temperature ranging from 250°C to 500°C with the frequency range of 50Hz and 5MHz. Impedance data collected for the obtained samples are graphed as Nyquist plots. A Nyquist plot is graphed with real part of the impedance (Z/) on X-axis and negative imaginary part of impedance (–Z//) on Y-axis. Nyquist plot of an electrolyte material is usually consisted of three successive semicircular arcs; High frequency semi-circle corresponds to grain resistance (Rg), intermediate frequency semi-circle corresponds to grain boundary resistance (Rgb) and low frequency incomplete arc corresponds to electrode resistance (Re) [7]. The complex impedance plots of all the compositions were fitted by Z-View software. It can be observed that the radius of the semicircle decreases with the increase in the composition.

[image:3.612.384.505.189.379.2]

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International Journal of Emerging Technology and Advanced Engineering

[image:4.612.337.545.114.295.2]

81 Fig. 3. Complex impedance plots of CYP system at 250°C

3.4. Total Ionic Conductivity (TIC)

Fig. 4 is the typical Arrhenius plot of the dependence of Total conductivity with temperature. The total ionic-conductivity ‗σt‘ is calculated as

t t

t R A

  --- (2)

Where ‗t‘ is the specimen thickness, ‗A‘ is cross sectional area of the specimen. ‗Rt‘is the total resistance of the specimen. Rt includes the grain resistance (Rg) and grain boundary resistance (Rgb) from the complex impedance plot.

These temperature dependent conductivity plots may be linearized by plotting the logarithmic relation between log (σT) and 1000/T [12].The activation energy for conduction of all samples was calculated from the conductivity plots by using Arrhenius relation

0exp( )

E T

kT

  

---(3)

Where, ‗E‘ is the activation energy for conduction (ion migration), ‗T‘ indicates the absolute temperature, ‗k‘ is the

Boltzmann constant and ‗σ0‘ is the pre–exponential factor.

Fig. 4. Total ionic-conductivity plots of CYP system

The substitution of cerium cations by lesser valent Y3+ and Pr3+ cations improves total conductivity due to formation of oxygen vacancies. The presence of mobile oxygen vacancies (defects) causes the interaction between the dopant cations and results in smaller calculated activation energies [12]. The increase in the concentration of mobile ions (oxygen vacancies) leads to the enhanced ionic conductivity of the co-doped ceria. [7] [12]. It is clear from Fig. 4 that the total ionic conductivity of each sample increases with the rise of temperature as thermal excitation can boost the kinetic energy of carriers and force oxygen ions to pass through the oxygen vacancies with higher speed, which ultimately lead to the augmentation of conductivity.

Table II shows the average grain sizes calculated from SEM data and the conduction activation energies obtained by slope fitting of Arrhenius plots. It is observed that the co-doped sample CYP5 possess high conductivity with least activation energy. The decrease in lattice binding energy results increase in the number of oxygen vacancies. The number of oxygen vacancies is directly related to the conductivity of the material. [13]

TABLE III

MICRO STRUCTURAL AND ELECTRICAL PARAMETERS OF CYP SYSTEM

Sample Id

Grain Size (nm)

Total

Ionic-Conductivity (S/cm)

Activation Energy (eV)

CYP0 280 0.0030 0.97

CYP5 238 0.0058 0.86

[image:4.612.51.290.129.327.2]

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International Journal of Emerging Technology and Advanced Engineering

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The enhanced ionic conductivity of co-doped ceria due to Y and Pr co-doping can be attributed to; (i) maximized non interfering oxygen vacancies due to doping of lesser valent cations into Ce lattice [14] (ii) the average ionic radii of co-doped cations Pr3+ (1.126 Å) and Y3+ (1.019 Å) is close to that of Ce4+ (0.970 Å) [8] [14] [16] (iii) smaller activation energy [13] and (iv) Increment in the grain-boundary and total conductivities with decreasing grain size for sintered densities of 95% or greater [8].

3.5. Thermal Expansion Studies:

Fig. 5 exhibits thermal expansion curve (using a dilatometer from room temperature to 800 ºC) of the composite CYP5, which is observed to be having least activation energy and highest ionic conductivity. In order to be an efficient electrolyte, electrolyte must have a matching thermal expansion coefficient with the electrode materials (anode & cathode). Mismatching of thermal expansion coefficient might result in undesirable stress during the operation of SOFC and may develop micro cracks in ceramic structure [17].

The linear thermal expansion (dL/Lo) was calculated in the temperature range of 30°C - 800°C. The thermal expansion coefficient (TEC) was calculated as

/ _o

dL L TEC

dT

 ---(4)

Where dL L L_ois the change in length;

o

dT  T T is the temperature change; L0 is the length at initial temperature T0 and L is the length at terminal temperature T of the experiment. The TEC of CYP5 is found to increase linearly with temperature and TEC value is compatible with the thermal expansion coefficients of electrode materials [15].

[image:5.612.61.283.545.714.2]

Fig. 5. Thermal expansion of CYP5

IV. CONCLUSIONS

The Y and Pr co-doped ceria Ce0.8Y0.2–xPrxO2–δ electrolyte materials were successfully synthesized by sol-gel auto-combustion method. The energy efficient microwave sintering resulted in high relative densities (>95%) of the obtained pellets. XRD patterns of all the samples ascertained the formation of single phase cubic fluorite structure. The morphology was analyzed by SEM. The calculated linear thermal expansion coefficient was in good agreement with the commonly used electrode materials for IT-SOFCs.

The results presented in this article show the significant enhancement in the ionic conductivity of the electrolyte which can improve fuel cell performance at intermediate temperatures. The ionic conductivity of the best composition of co-doped composite electrolyte was

3

5.8 10  S/cm at 5000C. The enhanced conductivity is likely to be attributed to the effect of co-doping. This research may lead to lower the operating temperature of SOFC. The results using these materials can strongly support the development of the low temperature SOFC for commercialization with an ultra-low fabrication cost and reliable performance.

Acknowledgement

One of the authors, Ch Madusudhan, extends his heartfelt thanks the University Grants Commission (UGC), New Delhi, India for its assistance under the scheme of UGC-Faculty Development program (12th plan).

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