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Superior Oxide Ion Conductivity of Novel Acceptor Doped Cerium Oxide Electrolytes for IT-SOFC Applications Kanagaraj Amarsingh Bhabu, Jayaraman Theerthagiri, Jagannathan Madhavan, Thangaraj Balu, and Thanjavur Renganathan Rajasekaran J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05873 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 9, 2016

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The Journal of Physical Chemistry

Superior Oxide Ion Conductivity of Novel Acceptor Doped Cerium Oxide Electrolytes for IT-SOFC Applications

Kanagaraj Amarsingh Bhabu 1, Jeyaraman Theerthagiri 2, Jagannathan Madhavan 2, Thangaraj Balu 3 and Thanjavur Renganathan Rajasekaran 1,4,*

1

Department of Physics, Manonmaniam Sundaranar University, Tirunelveli – 627 012, Tamilnadu, India

2

Solar Energy Lab, Department of Chemistry, Thiruvalluvar University, Vellore - 632 115, Tamilnadu, India 3

Department of Physics, Aditanar college of Arts and Science, Tiruchendur – 628 216, Tamilnadu, India 4

Department of Renewable Energy Science, Manonmaniam Sundaranar University, Tirunelveli – 627 012, Tamilnadu, India

*Corresponding Author. Tel.: +91 9442327921, Fax: 91-462-2334363 E-mail address: [email protected] (T R Rajasekaran)

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Abstract Novel compositions of Nd3+ and Dy3+ co-doped cerium oxide according to the system of Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) have been synthesized by a simple sol-gel method and studied as electrolytes for an intermediate temperature solid oxide fuel cells (ITSOFCs). Thermal, microstructural, optical and electrical properties have been enhanced to different extents by the addition of Nd3+ and Dy3+ ions in cerium lattice, in particularly Dy3+ ions because of its low ionic radius mismatch. TG-DSC analysis exhibited a small weight loss and high thermal stability in the intermediate temperature range (400 – 800°C). The addition of Dy3+ ions stabilized the cubic fluorite structure which is confirmed from XRD studies. Lattice parameters expansion and contraction has been observed on account of their ionic radii trend. The formation of cubic fluorite structure has been confirmed by the HRTEM along with XRD studies. Addition of Dy3+ ions acts as an oxygen vacancy generator that increases the oxygen vacancy concentration and efficient conversion of Ce4+ to Ce3+ which are affirmed with optical studies. Complex impedance analysis was performed at the temperature range from 300 to 600 °C in air atmosphere. Compositions of the system Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) offer a competitive oxide ion conductivities in the intermediate temperature range. Ce0.8Nd0.1Dy0.1O1.85 has been found to be an optimum composition with superior oxide ion conductivity of 2.2 X10-2 S/cm at 600 °C and activation energy of 0.83 eV. Oxide ion conductivity is largely enhanced by the introduction of Dy3+ at intermediate temperature due to the low ionic radius mismatch, concentration of surface oxygen vacancies and stabilization of cubic fluorite structure. Hence, these results suggest that the composition of Ce0.8Nd0.1Dy0.1O1.85 can be a potential electrolyte for IT-SOFC applications.


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1. INTRODUCTION Nanostructured materials have attracted growing interest due to their greatest potentials for improving performance and extended capabilities of products in a number of industrial sectors. Their uniqueness is due to the large percentage of atoms at interfaces and partially to quantum confinement effects. The enhancement of ionic conductivity in the nanostructured solid conductors termed as “nanoionics” become one of the intensive research fields of nanoscale phenomena since they can be inevitable in advanced energy conversion and storage applications especially solid oxide fuel cells (SOFCs)1-3. SOFCs have considerable research interest because of their efficient energy conversion, long term stability, fuel flexibility, low emissions and economic running costs4. At present, yttria stabilized zirconia (YSZ) used as a potential electrolyte for most conventional SOFCs which operates at high temperatures around 1000 °C. However, the high operating temperature leads to various drawbacks such as materials degradation, long start-up times, economic obstacles and limits the development of future energy conversion systems5. To overcome these drawbacks is the great challenge to the researchers to develop novel materials for solid oxide fuel cell electrolytes with high electrochemical activities and oxide ion conductivities at intermediate temperature. Moreover, the electrolytes with specific oxide ion conductivity at 10-1 to 10-2 could be most likely in the field of solid oxide fuel cells6,7. Our approach is to deploy a novel ceria based electrolytes having significant oxide ion conductivity at intermediate temperatures. Most of the research efforts in the past have been mainly focused on the development of novel electrolyte materials for intermediate applications8,9. Currently rare earth doped ceria electrolytes depicts high oxide ion conductivity at lower temperatures compared to yttrium stabilized zirconia and they have been widely analyzed as a 3 ACS Paragon Plus Environment


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potential electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs)10. The high oxide ion conductivity of ceria electrolytes could be due to the results of small association enthalpy between the dopant cation and the associated oxygen vacancy in the fluorite lattice. Partially substituted alkaline earth or rare earth materials with ceria suppress the oxygen vacancy ordering process that lowering the activation energy (Ea) which facilitates the excellent oxide ion conductivity at intermediate temperatures. Oxide ion conductivity can be optimized with minimizing activation energy for oxygen diffusion, which can be attained with the choice of pertinent dopants11. Recently, various acceptor dopants such as Pr3+, Nd3+, Dy3+, La3+, Sm3+, Gd3+, Ca2+ etc. doped with ceria based solid electrolytes have been extensively investigated for SOFC applications12. Among the various dopants, Nd3+ doped ceria (σ = 1.17 S/m at 600° C)13 and Dy3+ doped ceria (σ = 7.42 10-2 S/cm at 550° C)14 have been reported as an electrolyte for the SOFC applications. As the consequence of ceria with two and three dopants, it exhibited higher oxide ion conductivity in air than the single doped ceria materials with same oxygen vacancy concentration. For example, Ramesh et al15 have analyzed Ce1-x(Gd0.5Pr0.5)xO2 and achieved oxide ion conductivity of 1.059 X 10-2 S/cm at 500° C, Rai et al16 have synthesized SmxNd0.15-xCe0.85O2−δ and attained the conductivity of 7.1 - 13.7 mS/cm at 600° C, Yin et al17 have investigated Sm0.2Ce0.8O1.9/Na2CO3 which afforded a conductivity of 4.72 mS/cm at 600° C. As far as development in this field is concerned, the present investigation describes a novel composition and comparison of Nd3+ and Dy3+ co-doped cerium oxide within the system of Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) in rare earth family with modified oxide ion conductivity for intermediate temperature solid oxide fuel cells. For the sake of comparison, the compositions of Ce0.8Nd0.2O1.9 and Ce0.8Dy0.2O1.9 are also investigated.


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In particular, the synthesis route has an considerable deal of influence on the microstructure and stability of electrolytes. Several synthetic routes have been developed in order to produce a hyper-fine cerium oxide based electrolytes with high oxide ion conductivity and good microstructures. The synthetic routes are includes, co-precipitation, reverse micellar, hydrothermal, solvothermal and combustion synthesis etc,. Among the various routes, here a simple and low cost sol-gel via hydrolysis process has been proposed to develop a novel ceria based electrolytes for IT-SOFC applications. It is a popular technique to synthesize the materials with uniform, small particle size and various morphologies. Single phase of the materials can be achieved at low temperatures with high homogeneity18. The main theme of this present investigation is to identify the proper Nd3+ and Dy3+ dopant concentrations in cerium oxide, to exhibits enhanced oxide ion conductivity. Systematic studies of thermal, microstructural, optical and electrical properties are also investigated. 2. EXPERIMENTAL SECTION Cerium (III) nitrate hexahydrate (Ce(NO3)3.6H2O), neodymium (III) chloride hexahydrate (NdCl3.6H2O) and dysprosium (III) chloride hexahydrate (DyCl3.6H2O) were purchased from Sigma Aldrich, India. Ammonium hydroxide (30% pure) and deionized water were obtained from MERCK, India. All reagents were of analytical grade and used without further purification. The 0.1 N solution of Ce(NO3)3.6H2O, NdCl3.6H2O and DyCl3.6H2O were prepared separately by using deionized water for 800 mL, 400 mL and 400 mL respectively. Then, the solutions are subjected to forced hydrolysis and condensation for 60 h. Ammonium hydroxide was slowly added (4 drops/min) in the solution to convert the sol into gel. White and light grey coloured precipitates were formed. These precipitates were washed thoroughly until the traces of NO and Cl were removed by using centrifugation and filtered using high quality Whatmann



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filter paper. The resultant powder was grounded in an agate mortar to get fine cerium oxide, neodymium oxide and dysprosium oxide. Then the synthesized neodymium oxide and dysprosium oxide were mixed with cerium oxide in the system of Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1). Further, it was grounded for 2-3 h by using an agate mortar to get a homogeneous mixture and prefired at 600 oC for 5 h. Compositions of Ce0.8Nd0.2O1.9 and Ce0.8Dy0.2O1.9 were prepared according to the system of Ce0.8AxO2-x/2 (A = Nd, Dy) and prefired at 600 oC for 5 h. To investigate the electrical properties of the solid electrolytes, samples were made into pellet form with dimensions of ~ 10 mm dia X 1 mm thickness and sintered around 1200 °C for 5 h in a muffle furnace under air atmosphere. Thermal properties of the synthesized electrolyte materials were examined by the TG/DSC analysis using NETZCH simultaneous thermal analyzer from room temperature to 800 °C. The purity and phase formation of the samples were analyzed by using a powder X-ray diffraction (XRD) with PANalytical X’pert pro powder diffractometer. The CuKα radiations (λ = 1.5406 Å) from a copper target were used. The X-ray diffraction patterns were recorded from 10o to 80o (2θ) in steps of 0.02o with four decimal accuracy. Rietveld refinement was performed by using JANA2006 program and this program allowed to refining the lattice constants, atomic positions and occupancy factors. The fit quality was evaluated by the residual (R) – weighted pattern (Rwp) and goodness of fitting (χ2). The morphology and microstructure features of solid electrolytes were observed by HRTEM analysis using a JEOL JEM 2100 High Resolution Transmission Electron Microscope. BET specific surface area was measured at 77.4 K using NOVA-1000 high speed gas sorption analyzer Ver. 3.70. The fourier transform infrared (FT-IR) spectroscopy were recorded using Perkin Elmer spectrum RX I with spectral range of 4000-400 cm-1. The ultraviolet-visible (UV-vis) absorption spectra were recorded in a Perkin Elmer


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Lambda 35 spectrophotometer. Photoluminescence (PL) measurements were performed at room temperature by the Perkin Elmer LS 45 luminescence spectrophotometer. Electrical conductivity of the sintered pellets was measured in a temperature range of 300 to 600 °C with a CHI604E electrochemical workstation. Relative density of the pellets found approximately 95% of the theoretical density was obtained by Archimedes principle. 3. RESULTS AND DISCUSSION 3.1. Thermal, Phase and Microstructure studies Typical TG and DSC curves of Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) solid electrolytes are shown in Fig. 1. The small endothermic peak observed around 90 °C along with a weight loss in the TG curve could be attributed to desorption of water from the surface of solid electrolytes17,19. Broad exothermic peaks followed by endothermic peaks could be correlated to the release of heat during crystallization of the samples20. The absence of thermal effects that are related to phase transition and sample decomposition indicates the relative high thermal stability of the solid electrolytes in the intermediate temperature range from 400 to 800 °C. From the TG curves, the final residual mass of Ce0.8Nd0.16Dy0.04O1.88 (88.9%), Ce0.8Nd0.12Dy0.08O1.86 (96.9%) and Ce0.8Nd0.1Dy0.1O1.85 (98.5%) exhibits small amount of weight loss which is also confirms the high thermal stability at intermediate temperature range of solid electrolytes.



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Fig. 1 Typical TG and DSC curves of Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes. The phase formation and purity of the synthesized solid electrolytes were ascertained by powder X-ray diffraction studies. XRD pattern for Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9 and Ce0.8NdxyDyyO2-{x-y/2+y}

(x = 0.2; y = 0.04, 0.08, 0.1) were shown in Fig. 2. XRD pattern depicts the main

reflections represents the cubic fluorite structure of pure cerium oxide (JCPDS card no. 34-0394) with space group of Fm3m12,21,22, but a minor amount of hexagonal and cubic phases of Nd3+ and Dy3+ (indicated as circles in XRD pattern) were observed. Percentage of hexagonal and cubic phases becomes negligible while increase the amount of Dy3+ ions in the system. The XRD pattern of Ce0.8Nd0.1Dy0.1O1.85 shows that there is no other residual impurities which indicates the Nd3+ and Dy3+ ions has entered completely in the cerium host lattice and stabilized the cubic fluorite structure i.e., a perfect solid solution have formed. Further to confirm its phase purity, 8 ACS Paragon Plus Environment

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diffractograms were fitted by Rietveld refinement method and it was shown in Fig. 3 (only Ce0.8Nd0.1Dy0.1O1.85 fitting was shown). It confirms the occurrence of CeO2 with cubic structure as the major phase, which indicates the Nd3+ and Dy3+ ions are uniformly distributed into cerium lattice to forming a solid solution. Reasonable values for the indices of refinement point out that the refinement was successful because of small distortion between experimental and theoretical curve and its high goodness of fitting23. Two types of trends have been observed in the XRD pattern due to the doping of Nd3+ and Dy3+ in the system. First trend is, while compared the Ce0.8Dy0.2O1.9 with Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1), it shows a lower angle shift of 2θ (shift – I in the XRD pattern) due to the larger ionic radius of Nd3+ (1.12 Å) ions took place in the sites of smaller Ce4+ (0.97 Å) ions. This demeanor has been adapted to the disturbance of host structure when dopants are frequently introduced24. Second trend is, increase in Dy3+ addition on the system Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) favors the peaks shifted to higher diffraction angle (shift – II in the XRD pattern) and it indicates the contraction of lattice parameters. This behavior could be due to the incorporation of smaller Dy3+ (0.912 Å) ions in the larger Ce4+ lattice. This contraction of lattice parameter must effect in a structure deviation that strongly favors the formation of defects and tends to the increase in oxygen ion mobility by acquitting the internal stress or microstrain caused by the contraction25. Low ionic radius mismatch caused by the addition of Dy3+ ions will be an impact for enhancing the properties. Inset of Fig. 3 exhibited the structural 3D image obtained using Vesta 3D visualization

program

to

affirm

the

atomic

positions

and

occupancy

factors

of

Ce0.8Nd0.1Dy0.1O1.85 system. It also confirms the cubic structure composed of cerium, neodymium, dysprosium and oxygen ions. Average crystallite sizes were calculated by using Debye-Scherre’s equation varies from 18 to 21 nm and the calculated average crystalline size for



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the synthesized solid electrolyte samples are presented in Table 1. Lattice parameters and BET surface area for the solid electrolytes are also shown in the same Table 1. The introduction of Dy3+ ions in the system assist the decreasing of particle size, increasing of surface area and expose the feasibility to obtain high oxide ion conductivity, because in such cases conductivities are greatly affected by the particle sizes of the annealing materials.

Fig. 2 XRD patterns of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes.


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Fig. 3 Rietveld refinement of Ce0.8Nd0.1Dy0.1O1.85 and inset shows its structural 3D image confirmed the cubic structure composed of cerium, neodymium, dysprosium and oxygen ions. Table 1. Average crystallite size, lattice parameters and BET surface area of the solid electrolytes BET surface Compositions

Average crystallite size (nm)

Lattice parameter (Å) area (m2/g)

Ce0.8Nd0.2O1.9

21.24

5.4488

21.80

Ce0.8Dy0.2O1.9

19.78

5.4009

22.59

Ce0.8Nd0.16Dy0.04O1.88

19.10

5.4323

19.03

Ce0.8Nd0.12Dy0.08O1.86

18.84

5.4302

24.15

Ce0.8Nd0.1Dy0.1O1.85

17.92

5.4252

28.02



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Fig. 4 Typical HRTEM photographs and its inset shows the selected area electron diffraction (SAED) patterns for (a) Ce0.8Dy0.2O1.9, (b) Ce0.8Nd0.16Dy0.04O1.88, (c) Ce0.8Nd0.12Dy0.08O1.86 and (d) Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes. Fig. 4 represents the typical HRTEM photographs and its inset shows the selected area electron diffraction (SAED) patterns for Ce0.8Dy0.2O1.9 and Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) solid electrolytes. HRTEM photographs clearly exhibited the formation of well separated uniform nanoparticles and some of them partially aggregated. The observed particles 12 ACS Paragon Plus Environment

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are of roughly cubic in shape. SAED patterns are well accordance with cubic fluorite structure of cerium oxide with strong diffraction rings and relatively clean background due to (111), (200), (220), (311), (222), (400) and (331) planes, which are in good agreement with XRD studies. HRTEM photographs revealed that the lattice fringes corresponding to d spacings of 0.33, 0.33, 0.32 and 0.33 nm are in comparable with the d111 spacing in the XRD pattern of Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 respectively.

Fig. 5 FT-IR spectra of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes. In order to confirm the presence of functional groups and the formation of Nd3+ and Dy3+ ions in the cerium lattice, FT-IR spectra for solid electrolytes were recorded in the range 4000 – 400 cm-1 and presented in Fig. 5. Absorption bands were found around 3440 cm-1 in all the solid electrolytes could be due to the stretching vibrations of O-H associated with the hydroxyl group26. A sharp band noticed around 1473 cm-1 corresponds to the stretching vibrations of COO- or skeletal C = C. Strong bands around 1084 cm-1 and 860 cm-1 are indicates the stretching 13 ACS Paragon Plus Environment


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mode vibrations of Ce-O27. In addition, the observed bands at 675 and 567 cm-1 are due to the frequency modes of Nd-O and Dy-O, respectively28,29. The FT-IR results evidently indicates the incorporation of Nd3+ and Dy3+ ions in the cerium host lattice and formation of an exact solid solution. 3.2. Optical and luminescence studies UV-vis absorption spectroscopy is an powerful nondestructive technique for obtaining the information about optical properties of synthesized solid electrolytes. UV-vis absorption spectra of the synthesized solid electrolytes are shown in Fig. 6.

Fig. 6 UV-vis absorption spectra of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes. All the solid electrolytes were exhibit a strong absorption below 400 nm with a well defined absorbance edge at approximately 320 nm. It can be noticed that the increases in absorption intensity by increase of Dy3+ amount and high absorption intensity was observed in


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the composition Ce0.8Nd0.1Dy0.1O1.85. Moreover the absorption edge shifted towards longer wavelength indicates the red shift related with the charge – transfer transition from O2- (2p) to Ce4+ (4f)26 orbitals in cerium oxide and it influence the decreasing of band gap. In this connection, the observed red shift behavior could be due to the interface polaron effect arising from electron phonon coupling. Band gap values were calculated using Tauc’s relation αhν = A(hν- Eg)n are shown in Fig. 7, which are found in the range of 3.37, 3.3, 3.36, 3.34 and 3.26 eV for

Ce0.8Nd0.2O1.9,

Ce0.8Dy0.2O1.9,

Ce0.8Nd0.16Dy0.04O1.88,

Ce0.8Nd0.12Dy0.08O1.86

and

Ce0.8Nd0.1Dy0.1O1.85 respectively. Due to the substitutional nature, the addition of Dy3+ form donor levels in the band gap which is responsible for the effective reduction in the band gap. Dy3+ ions act as an oxygen vacancy generator and stabilize the charge compensation due to Dy3+ → Ce4+ substitution that facilitates the formation oxygen vacancy concentration and conversion of Ce4+ to Ce3+ associated with positive vacancy-dopant association energy30,31. Higher oxygen vacancy concentration increased the oxygen ion conductivity and determines the efficiency of the solid electrolytes. Composition of Ce0.8Nd0.1Dy0.1O1.85 offered a low band gap value of 3.26 eV is a probability for obtaining high oxide ion conductivity. Higher concentrations of oxygen vacancies and efficient conversion of Ce3+ ions resulted in band gap narrowing and a well separated efficiency of electron – hole pair interactions31. Our results are in well agreement with the previous reported band gap (Eg) values26,32.



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Fig. 7 The variation of (αhν)2 versus hν curve on Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes. In order to understand the modification of electron-hole pair interactions, charge carrier trapping, immigration and transfer upon doping, room temperature PL emission spectra for the synthesized solid electrolytes were recorded in the wavelength range of 300-600 nm. The excitation wavelength of 320 nm was used to perform the emission spectrum for solid electrolytes and are shown in Fig. 8. The main emission band sited at 388 nm is consistent with their attribution to O2- to Ce4+ charge transfer transition. All the peak positions at 388 nm are coherent, but the intensity of emission peaks are remarkably changed. This change could be due to the presence of defects related to the Nd3+ and Dy3+ content which involves to influences the particle sizes and level of defects but the PL emission is mainly based on the Dy3+ content, because its addition induces the changes in defects or disorder of materials21. As Dy3+ content


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increases more oxygen vacancies are produced which affects the properties of solid electrolytes and this phenomena has been noticed previously in the XRD, UV-vis absorption studies. Intensity of PL emission changes with the increase of Dy3+ content is correlated with the structural deorganization level and the charge transfer occurring between the dopant and host ions21. In addition, the increase in Dy3+ content results the decrease in particle size, which tends to slower the recombination rate of electrons in Ce 4f, Dy 3d conduction bands with holes in O 2p valence band. Generally the lower PL intensity corresponds to higher conductivity and higher concentration of oxygen vacancy because of its efficient charge carrier separation and lower recombination rate. Another reason for lowering the PL intensity is due to the characteristic of self trapped exciton defects33. This defects influence the competitive energy transfer process from the Ce4+ conduction band to self trapped exciton defects and the energy states of Dy3+ may appear which reduces the recombination rate. Thus the lower recombination rate was found in the composition Ce0.8Nd0.1Dy0.1O1.85 confirmed its role as a potential electrolyte in the field of IT-SOFC applications.



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Fig. 8 PL emission spectra of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes. The weak emission band observed at 521 nm could be due to the f-f transition i.e., 4

G5/2→6Hn/2, which is significantly enhanced by the addition of dopant ions in the cerium host

lattice. This emission band is not due to the characteristic emission of the doping ions but it may be originated from the oxygen vacancy introduced in the cerium lattice to compensate the negative charges linked with the doping ions34,35. The high PL intensity implies the high crystalline nature of the solid electrolytes with defects.


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3.3. AC impedance studies



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Fig. 9 Typical ac impedance spectra of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes sintered at 1200 °C.

Typical ac impedance spectra for the solid electrolytes Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9 and Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) were measured in air at the temperature range of 300 – 600 °C (300 – 400 °C plot was not shown) shown in Fig. 9. Influence of proton transport may not involved in the conduction of carrier carriers17. Real axis intercept of high frequency depressed arcs corresponding to the contribution of grain resistance (Rg) and low frequency depressed arcs associated with grain boundary resistance (Rgb). Here, the depressed arcs that correspond to the grain and grain boundary conductivities are highly overlapped due to the similar relaxation time of the charge carriers inside grain as well as the grain boundaries23. Total resistance (Rt) is the sum of these two resistances. Impedance spectra clearly exhibit both the grain and grain boundary resistance decreases with increasing temperature and the composition Ce0.8Nd0.1Dy0.1O1.85 showing the lowest resistance value. This phenomenon can be due to the increase in mobility of charge carriers that contributes the conduction process caused by low barrier effect36. On the basis of above analysis, an equivalent circuit model has been used 20 ACS Paragon Plus Environment

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to fit the impedance data. An equivalent circuit model LR(CR(QR)) was used to fit the impedance data to extract values of grain and grain boundary response. Fig. 10 exhibits the fitting parameters of Ce0.8Nd0.1Dy0.1O1.85 at 600 °C with fitting error less than 10% for better clarity. A single capacitor was not adequate to model the equivalent circuit for depressed arcs so a constant phase element (CPE) was used to fit the data. A constant phase element is equivalent to distribution of capacitor in parallel37. The total resistance (Rt) was converted into the corresponding electrical conductivity by using the expression σ =

, where L and A are the

thickness and cross sectional area of the solid electrolytes, respectively. Resistance and its conductivity values for the solid electrolytes measured at 550 and 600 °C are given in Table 2. A significant increase in conductivity with temperature points out a working mechanism based on thermal activation. In addition, the conductivities of Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86, Ce0.8Nd0.1Dy0.1O1.85 are dramatically enhanced to different extents when compared with Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9 and attained maximum enhancement for the composition of Ce0.8Nd0.1Dy0.1O1.85. Moreover the smaller crystallite size, band gap narrowing and lower PL intensity are the probable reasons for significant increase in oxide ion conductivity of Ce0.8Nd0.1Dy0.1O1.85 composition. The oxide ion conductivity of Ce0.8Nd0.1Dy0.1O1.85 (σt = 2.2 X 10-2 S/cm at 600 °C) are much higher than that of recently reported La2Mo1-xWxO9 (σt = 5.6 X 10-3 S/cm at 600 °C)38, BeCe0.8SmxY0.2-xO3-δ (σt = 3.92 X 10-2 S/cm at 800 °C)39, Ce0.9Gd0.1xNdxO1.95

(σt = 6.82 X 10-5 S/cm at 550 °C)40 and La0.8Sr0.2Ga0.8Mg0.2O3 (σt = 7.5 X 10-2 S/cm

at 800 °C)41 values.



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Fig. 10 Typical equivalent electrical circuit for impedance plot of Ce0.8Nd0.1Dy0.1O1.85 at 600 °C. Fig. 11 shows the conductivity and Arrhenius plots of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9 and Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) solid electrolytes over the temperature range from 300 – 600 °C. The activation energy for conduction is obtained by plotting the conductivity data in the Arrhenius equation σ = σ0e(-Ea/KT) for thermally activated conduction. In addition to this, conductivity (σ) also depends upon the values of pre-exponential factor (σ0) and activation energy for oxygen diffusion (Ea). In this relation, materials which have maximum preexponential factor (σ0) and minimum activation energy (Ea) values will exhibit the highest oxide ion conductivity. The activation energy calculated from the slope of Ce0.8Nd0.1Dy0.1O1.85 (Ea = 0.83 eV) is remarkably lower when compared to the other solid electrolytes (Table 2). The increase in Dy3+ concentration is considered responsible for the increase in oxide ion conductivity and decrease in activation energy. Possible reasons for high oxide ion conductivities and low activation energies are the substitution of Nd3+ ions (1.12 Å) in cerium lattice does not


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account for an increase in ionic defects due to its high ionic radius mismatch so the concentration of oxygen vacancies is determined by the concentration of Dy3+ ions (0.912 Å) because of its low ionic radius mismatch. Dy3+ ions in ceria lattice prevent the oxygen vacancy ordering process which creates the oxygen vacancies and minimizes the vacancy-dopant interactions. It favors the significant increase in oxide ion conductivity with minimum activation energy. Kilner et al42,43 also explained this effect as, dissociation energy of defect pairs decreases linearly with the decreasing of ionic radius mismatch between the host ions and dopant ions. Therefore, oxide ion conductivity in fluorite type systems can be enhanced by lowering the ionic radius mismatch. Furthermore, the increase in Dy3+ concentration leads the decrease in particle size because smaller particle size reduces the hopping distance of ionic motion and thereby has a great influence on oxide ion conductivity23. Rupp et al44 have observed the similar trend as the activation energy decreases from 1.04 to 0.77 eV while the particle size decreased from 76 to 29 nm. High oxide ion conductivity and low activation energy indicated that the composition Ce0.8Nd0.1Dy0.1O1.85 could be a potential electrolyte material for intermediate temperature solid oxide fuel cells.



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Fig. 11 (a) Conductivity and (b) Arrhenius plots of Ce0.8Nd0.2O1.9, Ce0.8Dy0.2O1.9, Ce0.8Nd0.16Dy0.04O1.88, Ce0.8Nd0.12Dy0.08O1.86 and Ce0.8Nd0.1Dy0.1O1.85 solid electrolytes from 300 to 600 °C.

Table 2. Electrical properties of the solid electrolytes

Activation S. No.

Compositions

Resistance (Ω)

Conductivity (S/cm) Energy (eV)

550 °C

600 °C

550 °C

600 °C

1

Ce0.8Nd0.2O1.9

139

40

9.6 X 10-4

3 X 10-3

1.19

2

Ce0.8Dy0.2O1.9

42

21

3.2 X 10-3

6.8 X 10-3

1.19

3

Ce0.8Nd0.16Dy0.04O1.88

35

24

4.3 X 10-3

6.1 X 10-3

1.18

4

Ce0.8Nd0.12Dy0.08O1.86

14

11

9 X 10-3

1.2 X 10-2

1.13

5

Ce0.8Nd0.1Dy0.1O1.85

13

6.6

1.1 X 10-2

2.2 X 10-2

0.83


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4. CONCLUSIONS In summary, novel compositions of Nd3+ and Dy3+ co-doped cerium oxide within the system of Ce0.8Ndx-yDyyO2-{x-y/2+y} (x = 0.2; y = 0.04, 0.08, 0.1) have been synthesized by a simple sol-gel method and investigated as electrolytes for (IT-SOFC) applications. TG-DSC studies confirmed the high thermal stability in the intermediate temperature range without any phase transitions and sample decompositions. Single phase of cubic fluorite structure was affirmed by XRD and Rietveld refinement studies with the average crystallite sizes of 18 – 21nm. FT-IR spectra were confirmed the presence of functional groups and chemical bondings. UV-vis absorption spectra indicated that the red shift related with charge – transfer transition from O2- (2p) to Ce4+ (4f) and decreasing of band gap value by the addition of Dy3+. PL emission spectra clearly exhibited the increase in oxygen vacancies due to the increase of Dy3+ ions which results the lower in PL intensity. Oxide ion conductivity enhancement can be attained in the composition Ce0.8Nd0.1Dy0.1O1.85 on account of the band gap narrowing and lower PL intensity. Oxide ion conductivity improved significantly with the addition of Dy3+ ions at the intermediate temperature range. Ce0.8Nd0.1Dy0.1O1.85 composition showed the highest oxide ion conductivity of 2.2 X 10-2 S/cm at 600 °C with lowest activation energy 0.83 eV. Moreover, the addition of Dy3+ would trap the oxygen vacancy ordering process and created the oxygen vacancies resulted the overall increase of oxide ion conductivity and decrease of activation energy. All these results evidenced that the composition Ce0.8Nd0.1Dy0.1O1.85 can be a promising electrolyte for IT-SOFC applications. To further understand the single cell characteristics of the compositions, fabrication works are now in progress.



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ACKNOWLEDGMENTS One of the authors (Kanagaraj Amarsingh Bhabu) is grateful to the University Grants Commission (UGC), Government of India for the award of research fellowship under the UGCSAP-Basic Science Research Program. Authors also thank Mr. M. Veera Gajendra Babu, Research Scholar, Department of Physics, Manonmaniam Sundaranar University for his valid discussions regarding Rietveld Refinement Studies. REFERENCES (1)

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Superior Oxide Ion Conductivity of Novel Acceptor ... - ACS Publications

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