Synthesis & characterization of Dy and Ca Co-doped ceria based solid electrolytes for IT-SOFCs

Accepted Manuscript Synthesis & characterization of Dy and Ca Co-doped ceria based solid electrolytes for IT-SOFCs Khagesh Tanwar, Nandini Jaiswal, Devendra Kumar, Om Parkash PII:

S0925-8388(16)31572-9

DOI:

10.1016/j.jallcom.2016.05.223

Reference:

JALCOM 37743

To appear in:

Journal of Alloys and Compounds

Received Date: 7 April 2016 Revised Date:

18 May 2016

Accepted Date: 20 May 2016

Please cite this article as: K. Tanwar, N. Jaiswal, D. Kumar, O. Parkash, Synthesis & characterization of Dy and Ca Co-doped ceria based solid electrolytes for IT-SOFCs, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.223. 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 proof before it is published in its final 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.

ACCEPTED MANUSCRIPT

Synthesis & Characterization of Dy and Ca Co-Doped Ceria Based Solid Electrolytes for IT-SOFCs Khagesh Tanwara, Nandini Jaiswalb, Devendra Kumarc and Om Parkash1 a,b,c,1

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

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Abstract

Numerous compositions in the system, Ce1-x-yDyxCayO2-δ have been synthesized using citratenitrate auto-combustion route. Samples sintered at 1350 °C for 4 hrs have density more than

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93% of the theoretical value. Single phase formation has been confirmed by X-ray diffraction in all the samples. Reitveld refinement has been carried out to confirm the cubic fluorite

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structure with space group Fm m. Surface morphology of the sintered samples show the distinct grains and grain boundaries. Complex plane impedance analysis has been employed to separate the contribution of the grains, grain boundaries and electrode/specimen interface polarizations. A significant improvement in the electrical conductivity has been observed by

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partial replacement of Dy3+ with Ca2+ in Ce0.80Dy0.20O1.90 (keeping total number of oxygen vacancies fixed). Composition, Ce0.83Dy0.14Ca0.03O1.90 shows the highest conductivity, 1.45×10-2 S/cm at 600 ºC. This value is about two orders of magnitude higher than that of

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singly Dy3+ doped ceria (6.01×10-4 S/cm) at the same temperature.

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Keywords: Co-doped ceria; Reitveld refinement; Electrical conductivity; IT-SOFCs.

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Corresponding author Tel:+91-542-6701791; Fax:+91-542-2368428 Email address: [email protected]

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1. Introduction For the last few decades, excessively increasing demand of energy and use of fossil fuels has attracted much attention of researchers towards the efficient energy conversion devices. Solid oxide fuel cells (SOFCs) are one of the highly efficient (>65%) and environment friendly

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energy conversion devices. These convert chemical energy in fuels into electrical energy directly without involving the step of combustion. SOFCs mainly consist of three components viz. anode, cathode and electrolyte. Electrolyte is the key component because

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conduction takes place due to migration of oxide ions through the electrolyte via oxygen vacancies. Therefore, electrolyte should have high oxide ion conductivity, negligible

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electronic conductivity and high density to avoid crossover of the gases. Conventionally, yttria stabilized zirconia (YSZ) has been commercially used as solid electrolyte for SOFCs considering its excellent chemical and mechanical stability. It shows promising oxide ion conductivity at high temperatures (800-1000 °C) [1]. This high operating temperature creates

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material complexing problems such as mechanical stress due to mismatch of thermal expansion co-efficient, long term stability and interfacial diffusion between the electrolyte and the electrodes [2]. Efforts have been made to develop the solid electrolytes with high

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ionic conductivity in the intermediate temperature range (500-700 °C) for e.g., lanthanum

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gallate and ceria based solid electrolytes [3, 4]. Ceria doped with rare earth or alkaline earth ions has been widely investigated as solid electrolyte for intermediate temperature SOFCs (IT-SOFCs) [5]. Ceria doped with trivalent cations such as Y3+, Dy3+, Gd3+ and Sm3+ has been investigated [6]. Maximum ionic conductivity was observed in Sm3+ (SDC) and Gd3+ (GDC) doped ceria [7, 8]. But, samarium and gadolinium are very costly. A co-doping idea was suggested by Herle et al. to further enhance the ionic conductivity of ceria [9]. It has been observed that co-doping either with trivalent (Sm3+, Y3+, La3+) or

ACCEPTED MANUSCRIPT divalent cations (Ca2+, Sr2+) enhances the conductivity of ceria than that of singly doped ceria [9]. Use of alkaline earth ions as co-dopant plays an important role in scavenging the siliceous impurities present at the grain boundaries. Siliceous impurities are present in the chemicals, which tend to segregates at the grain boundaries during sintering and make them

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highly resistive. MgO, SrO and CaO have been proved to be an effective grain boundary scavenger for Gd-doped ceria (GDC) [10-13]. Kim et al. studied the scavenging effect of SrO [14]. They observed that SrO reacts with SiO2 present at the boundaries and the product

boundaries for easy diffusion of the oxide ions [14].

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accumulates at the grain boundary triple point junction [14]. This leaves the clean grain

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Ce0.80-xSm0.20SrxO2-δ [15], (1-x)SDC-xSr [16], Ce0.80Y0.20-xSrxO2-δ [17] and Ce1-x-yLaxSryO2-δ [18] have been already investigated. Recently, the effect of co-doping with Dy3+ and Sr2+ on density, structure and electrical conductivity of ceria has been studied by Ramesh et al. [19]. However, there is a lack of study on Dy3+ and Ca2+ doped ceria system.

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In the present study, a series of compositions in the system, Ce1-x-yDyxCayO2-δ {(x=0.20, y=0.00), (x=0.16, y=0.02), (x=0.14, y=0.03), (x=0.13, y=0.035)}, keeping the oxygen vacancies fixed, have been synthesized via citrate nitrate auto-combustion route. The samples

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have been characterized for their crystal structure, microstructure and electrical conductivity.

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2. Experimental

2.1 Sample preparation

Nanocrystalline powders in the system, Ce1-x-yDyxCayO2-δ {(x=0.20, y=0.0), (x=0.16, y=0.02), (x=0.14, y=0.03) and (x=0.13, y=0.035)} were synthesized using citrate-nitrate auto-combustion route. Ammonium ceric nitrate (99% purity, Qualikems, India), dysprosium oxide (99.9% purity, Sigma Aldrich), calcium carbonate (99% purity, Sigma Aldrich), citric acid (99.5% purity, Loba Chemie, India) were used as the starting materials. Dy2O3 was heated at 800 °C for 1 hr to remove the adsorbed moisture and then cooled in the furnace up

ACCEPTED MANUSCRIPT to 500 °C and subsequently stored in a vacuum desiccator. Stoichiometric amount of Dy2O3 and CaCO3 were weighed and dissolved in dilute nitric acid (1:4) to obtain Dy(NO3)3 and Ca(NO3)2 followed by heating at 100 °C till complete dryness. Ceric ammonium nitrate, calcium nitrate, dysprosium nitrate and citric acid were then dissolved in double distilled

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water separately to make aqueous transparent solution. All the prepared aqueous nitrate solutions were then added to the citric acid solution keeping the citrate to nitrate molar ratio (C/N) ~ 0.3 for controlled and smooth combustion [20]. The final mixed solution was then

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heated continuously at 200 °C with continuous stirring. During heating all the excess water evaporated furthermore mass became viscous and turned into a gel. The gel slowly foamed

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followed by ignition and burnt into ash within a very short time period. The ash was collected and grounded in agate mortar. The ground powder was then calcined at 800 °C for 4 hrs in air based on DTA/TGA results. The calcined powders were then mixed with 2% PVA and pressed uniaxially with a load of 50 kN to form cylindrical pellets of diameter ~12 mm and

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thickness ~1.5 mm. The pellets were sintered in five steps using an electrical furnace (Lenton, made in Germany). In the first step, temperature was raised to 500 °C with a heating rate of 2 ºC/min for 1 h to remove the binder and then again raised the temperature up to 1350 ºC

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(heating rate 5 ºC/min) and hold it for 4 hrs and finally cool it to the room temperature.

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2.2 Characterizations

To confirm the phase formation, powder X-ray diffraction (XRD) patterns of the calcined and sintered powders were recorded using a Rigaku (smart lab) X-ray diffractometer employing Johanson monochromator in the incident beam. Data were collected in the diffraction angle (2θ) range 20º- 90º with a very slow scan rate. Crystal structure and lattice parameter were determined using Fullprof Reitveld refinement. Average crystallite size, D of the calcined powders was determined using Scherrer’s formula: (1)

ACCEPTED MANUSCRIPT where, β is the full width at half maxima (FWHM) excluding instrumental broadening, λ is the wave length of X-rays and θ is Bragg angle. β is taken for the strongest Bragg’s peak corresponding to (111) reflection for all the samples. Density of the sintered pellets was determined using Archimedes principle. For microstructural analysis, sintered pellets were

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polished using emery papers of grade 1/0, 2/0, 3/0, and 4/0 (Sia, Switzerland) followed by polishing on a velvet cloth using diamond paste of grade 1/4-OS-475 (HIFIN). Then these were etched thermally at 1250 °C for 15 min. Micrographs of the thermally etched samples

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were taken using a scanning electron microscope (ZEISS). 2.3 Impedance measurement

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For impedance measurements, both the surfaces of the sintered pellets were polished using emery papers of different grades. A high temperature silver paste was applied on both the surfaces of the polished pellets. The paste was matured by heating at 700 οC for 15 min. Impedance measurements were made in the temperature range 200–625 οC at an interval of

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25 ºC using a Novocontrol Alpha-A High Performance Frequency Analyzer in the frequency range 1 Hz-1 MHz by applying 20 mV ac signal. Data were collected using ‘Win data’ program and fitted via ZView software.

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3. Result and discussion

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3.1 Phase analysis

Powder XRD patterns for all the sintered samples are shown in Fig. 1. All the diffraction peaks have been indexed on the basis of fluorite structure similar to CeO2 using JCPDS file no. 43-1002 with space group Fm m. It is observed from Fig. 1 that the diffraction peaks shifts slightly towards lower angle. This is ascribed to the introduction Dy+3 and Ca+2 ions into ceria lattice. Reitveld refinement of the composition, Ce0.83Dy0.14Ca0.03O1.90 is shown in Fig 2. The structural refinement has been carried out using FULLPROF software [21]. Reitveld parameters are given in Table 1. The background variation is described by linear

ACCEPTED MANUSCRIPT interpolation between a set of background points with refinable heights. All the atomic positions have been fixed by the symmetry of Fm m space group. Rare earth and alkaline earth cations are situated at 4a site with the atomic coordinate (0 0 0). Oxygen atoms are situated at 8c site corresponding to (1/4 1/4 1/4) position. Lattice parameters for all the

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samples have been determined using Reitveld refinement. Lattice parameter increased from 5.4019 0.0016 Å [22] to 5.4157 Å on addition of 20 mol% Dy+3 to CeO2 lattice. This is because ionic radius of Dy+3 (1.027 Å) is larger than that of Ce+4 (0.97 Å) [23]. The lattice

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parameter decreases with increasing the Ca content (as given in Table.1). This is because the

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total dopant content decreases to maintain the molar concentration of oxygen vacancies fixed. Average crystal size, D, of the calcined powders determined using Scherrer’s formula is in the range 15-17 nm (Table 2). All the samples sintered at 1350 °C have density more than 93% of the theoretical density (Table 2). 3.2 Microstructure

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Figs.3 (a)-(d) show the micrographs of the sintered samples thermally etched at 1250 °C. SEM micrographs show the dense morphology and well defined grains separated by the grain boundaries. Some pores are also seen in the micrographs. This is in conformity with

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93% of the theoretical density. Average grain size (AGS) has been determined by linear

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intercept method. All the samples show grains of varying size. AGS for the samples with y = 0.00, 0.02, 0.03 and 0.035 is found to be <1.0 µm, 2.45 µm, 1.55 µm and 2.00 µm respectively. Image of all the samples except for the composition with y = 0.00 indicates the presence of faceted grains. It is interesting to notice that average grain size of the Ca2+ codoped samples is higher than that of the singly Dy3+ doped ceria. 3.3 Electrical conductivity It has been reported that doped ceria in air shows the conductivity purely ionic in nature [5]. In the present investigation, electrical conductivity is measured in air and assumed as oxide

ACCEPTED MANUSCRIPT ion conduction. Electrical conductivity of the prepared samples has been measured using complex plane impedance analysis. Complex plane impedance plots for all the compositions measured at 200 °C are shown in Fig.4. The impedance plot for the composition, Ce0.80Dy0.20O1.90 shows two depressed circular arcs in the high and intermediate frequency

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range corresponding to the contribution of the grains [plot in the inset Fig. 4(a)] and the grain boundaries respectively. The compositions with y= 0.02, 0.03 and 0.035 show two depressed circular arcs (corresponds to the contribution of the grains and the grain

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boundaries) associated with a low frequency tail represents the electrode/electrolyte interface polarization. It is noted from Fig.4 that the intercept made by grains and grain

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boundaries arcs reduces in the case of calcium co-doped samples as compared to the singly dysprosium doped ceria. It is therefore concluded that the partial replacement of Dy+3 with Ca+2 in Ce0.80Dy0.20O1.90 has a significant effect on the contribution of the grains and grain boundaries to the total resistance. The capacitance of the grains (10-10 to 10-12 pF) and the

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grain boundaries (10-7 to 10-9 nF) have been determined using the relation ωRC=1, where ω, R, C represents the angular frequency at the maxima of the circular arc, resistance and capacitance respectively [24].

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Complex plane impedance plots for a typical composition, Ce0.83Dy0.14Ca0.03O1.90 at various

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temperatures are shown in Fig.5. It is observed that only two depressed arcs have been observed at 200 °C ascribed to the contribution of the grains and the grain boundaries. At 350 °C, grains arc disappears and only grain boundaries arc and a low frequency tail are present. At higher temperatures >475 °C, only electrode arc is present. This is because the time constant, τ, for a particular relaxation process decreases as the temperature increases. Therefore, arcs shift towards higher frequency with increasing the temperature. The impedance data were fitted using ZView software to determine the contribution of the grains and the grain boundaries. At higher temperatures (> 475 °C), the intercept on the Z’

ACCEPTED MANUSCRIPT axis on higher frequency side of the electrode arc has been taken as the total resistance of the electrolyte. The conductivity, σ, at different temperatures is determined using the formula: (2)

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where L, S and R represents the thickness, area and resistance of the sample respectively. Arrhenius plots of the bulk conductivity (σg), specific grain boundaries conductivity (σ*gb) and the total conductivity (σt), for all the composition are shown in the Figs. 6-8. It is observed from Fig. 6 that the bulk conductivity increases with increasing the concentration

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of Ca2+ up to 3 mol% [plot in the inset for clear visual]. The bulk conductivity of the co-

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doped samples is more than that of the singly Dy3+ doped ceria. This is due to increase in the configurational entropy, S of the co-doped system [25]. The values of configurational entropy of the system, Ce1-x-yDyxCayO2-δ have been calculated using the Eq.(3): (3)

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where, R is the gas constant. Co-doping suppresses the ordering of oxygen vacancies and increases the configurational entropy [25]. This leads to decrease in the activation energy and hence increases the conductivity [25]. Values of S, for all the samples are given in Table

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3. It is noted from Table 3 that all the co-doped samples have slightly higher values of S than that of the singly Dy3+ doped ceria. The composition with y=0.03 has the highest

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configurational entropy among the series and hence exhibit the maximum conductivity with minimum activation energy. The number of associated defect pairs, [

]• decreases

as the concentration of Ca2+ ion increases. This decreases the association enthalpy and hence the total activation energy. This subsequently causes an increase in the conductivity up to y=0.03. Bulk conductivity decreases for the composition with y=0.035. This is due to decrease in the configurational entropy (Table 3). The ionic radius of Ca+2 (1.12Å) is large as compared to that of Dy+3 (1.027 Å) and Ce+4 (0.97Å) [23]. A large lattice mismatch between the host and

ACCEPTED MANUSCRIPT dopant cations produces elastic strain in the lattice leads to increase in the activation energy which decrease the conductivity for the composition with y>0.03 [14]. This may be due to increase in the number of [

]˟ defect pairs and hence increases the association

enthalpy for y>0.03.

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Fig.7 shows that the grain boundaries conductivity is higher for Ca co-doped samples. The composition, Ce0.83Dy0.14Ca0.03O1.90 shows the maximum conductivity. This is due to scavenging effect of Ca2+. To confirm the scavenging effect of Ca, the grain boundaries

(4)

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blocking factor, αgb has been determined using Eq. (4) [26, 27].

where, Rg and Rgb represents the resistance of the grains and the grain boundaries respectively. The values of αgb are given in Table.3. The blocking factor for the composition Ce0.83Dy0.14Ca0.03O1.90 has been determined to be 0.41 which is 58% less than that for the composition, Ce0.80Dy0.20O1.90 (0.98). The value of αgb in the case of Ca2+ co-doped samples

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are less as compared to the singly Dy3+ doped samples subsequently by considering the previously reported results in zirconia and ceria ceramics [28]. It is, therefore concluded that

value of 3 mol%.

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Ca acts as a grain boundary scavenger and enhances the conductivity up to an optimized

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It was previously reported that the silicon present in the sample, introduced as an impurity in the raw materials. It forms a siliceous phase at the sintering temperature and covers the grains fully or partially results in constriction of the channel for conduction [28]. Gerhardt et al. has been studied the scavenging effect in yttrium doped ceria system [26]. They observed the formation of some yttrium silicate phases on the basis of scanning transmission microscopy (STEM) combined with energy dispersive X-ray microanalysis (EDXM) and electron loss spectroscopy (EELS) [26]. They suggested that the scavenging effect becomes more effective when the larger size cations were used as dopants [26].

ACCEPTED MANUSCRIPT In the present study, the formation of similar silicate phases may be expected. The partial amount of Ca added, reacts with Si present at the grain boundaries to produce a different phase that accumulates at the triple points. This leaves the clean grain boundaries and provides large grain to grain contact area or wide channels for oxygen ion conduction. The

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exact composition, morphology and distribution of these phases need a detailed study as mentioned above.

It is also observed from Fig.7 that specific grain boundaries conductivity,

of the Ca2+ co-doped samples increases with decrease in the

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trend of average grain size.

follows the

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average grain size. This is because samples with smaller grain size have large grain boundaries area. The finite amount of impurity contained in these samples is not sufficient to form a continuous and uniform glassy phase layer along grain boundaries [29]. This causes the remaining grain boundary areas with clean grain to grain contact for diffusion of O2- ions. Activation energy of conduction for the bulk (Eg), grain boundaries (Egb) and total

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conductivity has been determined by Arrhenius equation for thermally activated conduction: (5)

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where, Ea is the activation energy for migration of O-2 ions, k is Boltzmann’s constant, T is

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the temperature in K and σo is pre-exponential factor. Fig.8 shows the 1000/T vs. Logσt plots for the system, Ce1-x-yDyxCayO2-δ. It is clearly seen that the conductivity of Ca2+ co-doped samples is one order of magnitude is higher than that of the singly Dy3+ doped sample. The change in the values of Et (Table.4) with the composition seems to be correlate with change in the conductivity. The maximum value of total ionic conductivity corresponds to the minimum activation energy which is in agreement with the Meyer-Neldel compensation rule [30]. The values of total conductivity at 600 °C for all the compositions are given in Table. 3. The composition, Ce0.83Dy0.14Ca0.03O1.90 shows the

ACCEPTED MANUSCRIPT maximum conductivity of 1.44 10-2 S/cm at 600 ºC which is slightly higher than the values reported for the compositions, Ce0.8Sm0.2O1.90 (1.20×10-2 S/cm) [31] and Ce0.8Gd0.2O1.90 (1.29×10-2 S/cm) [32] at the same temperature. Therefore, use of the composition, Ce0.83Dy0.14Ca0.03O1.90 as a solid electrolyte for IT-SOFCs will reduce the cost of the

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electrolyte as compared to using only rare earth cations doped ceria.

4. Conclusions

A few compositions in the system, Ce1-x-yDyx-yCayO2-δ have been synthesized successfully

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using citrate-nitrate auto-combustion route. Single phase solid solution with the fluorite

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structure (space group Fm m) has formed in all the samples. More than 93% of the theoretical density has been obtained by sintering at 1350 °C. The bulk conductivity as well as the total conductivity increases because of the suppression in the ordering of the oxygen vacancies due to co-doping. Composition, Ce0.83Dy0.14Ca0.03O1.90 shows the highest ionic conductivity among all the compositions studied. This makes it a promising candidate as a

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solid electrolyte for IT-SOFCs being much cheaper than singly Dy3+ doped ceria.

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List of Figure Caption Fig.1 Powder X-ray diffraction patterns of the compositions in the system, Ce1-x-yDyxCayO2-δ (a) x = 0.20, y = 0.00 (b) x = 0.16, y = 0.02 (c) x = 0.14, y = 0.03 and (d) x = 0.13, y = 0.035 sintered at 1350 °C.

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Fig.2 Reitveld refinement of the composition, Ce0.83Dy0.14Ca0.03O1.90 with space group Fm m. Fig.3 Scanning electron micrographs of the sintered samples in the system, Ce1-x-yDyxCayO2-δ (a) x = 0.20, y = 0.00 (b) x = 0.16, y = 0.02 (c) x = 0.14, y = 0.03 and (d) x = 0.13, y = 0.035.

(a) x = 0.20, y = 0.00 (b) x = 0.16, y = 0.02 (c) x = 0.14, y = 0.03 (d) x = 0.13, y

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yDyxCayO2-δ

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Fig.4 Complex plane impedance plots at 200 °C for the compositions in the system, Ce1-x-

= 0.035.

Fig.5 Complex plane impedance plots for the composition, Ce0.83Dy0.14Ca0.03O1.90 at different temperatures.

yDyxCayO2-δ.

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Fig.6 Arrhenius plots for grains ionic conductivity of all the compositions in the system Ce1-x-

Fig.7 Arrhenius plots for specific grain boundaries conductivity of all the compositions in the

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system Ce1-x-yDyxCayO2-δ.

Fig.8 Arrhenius plots for total ionic conductivity of all the compositions in the system Ce1-x-

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yDyxCayO2-δ.

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List of Table Caption Table.1 Refined structural parameters and agreement factors for all the composition in the system, Ce1-x-yDyxCayO2-δ using cubic fluorite structure (#225 space group Fm m).

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Table.2 Crystallite size, AGS and % theoretical density of all the compositions in the system, Ce1-x-yDyxCayO2-δ.

Table.3 Total ionic conductivity (σt), grain boundaries blocking factor (αgb) and

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configurational entropy (S) for all the composition in system Ce1-x-yDyxCayO2-δ.

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Table.4 Activation energy for grains (Eg), grain boundaries (Egb) and total (Et) conductivity

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for all the compositions in the system, Ce1-x-yDyxCayO2-δ.

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Table 1 Compositions

Bragg Rfactor

Rf-factor

Rexp

χ2

V/Å3

Lattice Parameter (Å)

1.

Ce0.80Dy0.20O1.90

5.01

3.15

7.22

6.93

158.841

5.4157(3)

2.

Ce0.82Dy0.16Ca0.02O1.90

6.13

5.74

7.12

6.74

158.771

5.4149(6)

3.

Ce0.83Dy0.14Ca0.03O1.90

4.71

3.11

6.66

5.91

158.709

5.4142(2)

4.

Ce0.835Dy0.13Ca0.035O1.90

10.5

8.40

9.44

8.70

158.674

5.4138(3)

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S. No.

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Table 2 Compositions

Crystallite Size (nm)

AGS (µm)

1.

Ce0.80Dy0.20O1.90

15

<1.00

% Theoretical density 95

2.

Ce0.82Dy0.16Ca0.02O1.90

16

2.45

95

3.

Ce0.83Dy0.14Ca0.03O1.90

15

1.55

96

4.

Ce0.835Dy0.13Ca0.035O1.90

17

2.00

93

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S. No.

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*AGS- average grain size of sintered samples.

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Table 3

S. No.

Compositions

σt at 600 οC (S cm-1)

αgb

1.

Ce0.80Dy0.20O1.90

1.04 10-3

0.97

2.

Ce0.82Dy0.16Ca0.02O1.90

1.26 10-2

0.51

4.44

3.

Ce0.83Dy0.14Ca0.03O1.90

1.44 10-2

0.41

4.45

4.

Ce0.835Dy0.13Ca0.035O1.90

1.12 10-2

0.44

4.43

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SC M AN U TE D EP AC C

S (J/mol-K) 4.16

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Table 4 Egb (eV) 1.15

Et (eV) 1.10

2.

Ce0.82Dy0.16Ca0.02O1.90

0.93

1.00

0.98

3.

Ce0.83Dy0.14Ca0.03O1.90

0.90

1.03

0.94

4.

Ce0.835Dy0.13Ca0.035O1.90

0.92

0.86

0.96

1.

AC C

EP

TE D

M AN U

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Compositions

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Ce0.80Dy0.20O1.90

Eg (eV) 1.02

S.No.

AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

SC

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AC C

EP

TE D

M AN U

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Highlights 1- The system Ce1-x-yDyxCayO2-δ has been investigated for the first time. 2- In present study, number of oxygen vacancies is kept constant in all composition.

4- It is much less costly than that of SDC and GDC.

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3- Composition Ce0.83Dy0.14Ca0.30O1.90 has maximum conductivity at 600 οC.

AC C

EP

TE D

M AN U

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5- This makes it a promising candidate as a solid electrolyte for IT-SOFCs.