Sintering and electrical properties of Ce0.75Sm0.2Li0.05O1.95

Sintering and electrical properties of Ce0.75Sm0.2Li0.05O1.95

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Sintering and electrical properties of Ce0.75Sm0.2Li0.05O1.95 Santanu Basu*, Subhasis Khamrui, N.R. Bandyopadhyay School of Material Science & Engineering, IIEST(formerly BESU), Shibpur, India

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Ceria-based electrolytes have been widely investigated in intermediate-temperature solid

Received 15 May 2014

oxide fuel cell (SOFC), which might be operated at 500e600  C. Samarium doped (20 mol%)

Received in revised form

ceria (20SDC) one of the most promising material in this class of compounds. In this work

1 August 2014

we report effect of lattice substitution of 5 mol % Li on Sm in (20SDC). It was prepared by

Accepted 10 August 2014

citrateenitrate auto combustion synthesis having a powder of average particle size ~50 nm.

Available online xxx

The sintered density of more than 98% of the theoretical density at 950  C has been ach-


Ce0.75Sm0.2Li0.05O1.95 compare to that of Ce0.8Sm0.2O1.95. Corresponding activation energy of

Samarium doped ceria

conduction ~0.7 eV has been calculated in the temperature range of 200e600  C. In


reducing atmosphere the electrical conductivity has not been altered much. Thus

ieved. Increased ionic conductivity (lattice) at 500

C has also been achieved in

Ionic conduction

Ce0.75Sm0.2Li0.05O1.95 has been found to be quite promising in terms of reducing the pro-

Redox stability

cessing temperature as well as operating temperature of SOFC. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction CeO2-based oxides are well known for their high ionic conductivity, which is reflected in their application as electrolyte for low temperature solid oxide fuel cell (LT-SOFC) operating at 500e600  C [1e3]. Choices of rare earth cation substitutions are numerous due to relatively large host lattice size of cerium oxide [4]. 20 mol% samarium doped cerium oxide (20SDC) is one of the highest ionic conducting material among the doped compositions [5]. CeO2 substituted with samarium are claimed to have low redox stability than the conventional yttria stabilized zirconia (YSZ) because of the variable

oxidation states of Ce at higher temperature in reducing atmosphere, significant electronic conductivity is observed as the electronic conductivity in CeO2 increases by hopping of polaron between Ce3þ and Ce4þ charge states in the reducing environment [6,7]. Again densifications of ceria based materials are difficult to obtain below 1500  C [8]. Li2O was evaluated as a promising sintering aid for Gd0.1Ce0.9O2d (GDC) [9]. Conductivity of GDC was found to be increased by addition of Li or Co [10]. The enhancement of conductivity was recognized to the electronic conduction due to segregation or dissolution of heavy metal oxides on the grain boundaries [11,12]. It has also been proposed that LiNO3

* Corresponding author. School of Material Science and Engineering (SMSE), IIEST (Formerly BESU), Shibpur, Howrah e 711103, West Bengal, India. Tel.: þ91 33 2555 3402. E-mail address: [email protected] (S. Basu). 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Basu S, et al., Sintering and electrical properties of Ce0.75Sm0.2Li0.05O1.95, International Journal of Hydrogen Energy (2014),


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melted and evaporated in large part during sintering, and thus, enhanced the ionic conductivity especially at the grain boundaries [13]. Li et al. [14] reported that bulk (lattice) conductivity of the 20SDC remained unaffected with addition of Liþ, however electrical conductivity at grain boundaries of low-temperature sintered SDC was increased. Considering scarcity of information in literature on the lattice substitution of Liþ1 in SDC we employ Liþ1 as a lattice substitute to 20SDC. The aim of the present work is to investigate whether minute (5 mol%) concentration of Liþ1 can enter the lattice of 20SDC and its effect on sintering, conductivity and redox stability of 20SDC.

Experimental procedure Ce0.75Sm0.2Li0.05O1.95 powder was prepared by citrateenitrate route of auto combustion technique [15]. Calculated proportions of ceric ammonium nitrate, samarium nitrate hexahydrate and lithium nitrate were mixed in deionised water with citric acid maintaining citrate to nitrate ratio of 0.3. The as-synthesized powder was calcined at 400  C for 6 h to remove carbonaceous residues. X-ray diffractograms of the powders were recorded by Philips X'pert X-ray diffractometer (Cu-Ka radiation, 40 kV, 40 mA). The structural parameters were refined employing a least-square fitting program FullProf. The powder morphology and grain microstructures were examined by SEM (Hitachi High-TechScienceSystemCorporation,Model Se3400N). The calcined powders were pressed uniaxially with a specific pressure of 170 MPa to prepare green pellets (dia.~10 mm) and subsequently sintered at 950  C for 2 h in air. The densities of the pellets were determined by standard Archimedes principle. Two probe alternating current (ac) impedance measurements were conducted on sintered, electroded samples in the frequency range 0.1 Hze10 MHz with an applied voltage of 0.1 V using a Solartron frequency response analyzer (FRA 1260) in the temperature range 200e600  C. The impedance data were analyzed using Z-VIEW software. For the electrical and electrochemical measurement platinum paste (metalor) was used as electrode on both sides of the sintered disk and the contacts were cured at 900  C for 2 h. AC impedance of the samples were performed in the heating cycles and sufficient time was allowed for equilibration of both oxygen partial pressure (atmosphere) and temperature. Few low temperature impedance data were discarded due high presence of stray error.

Results and discussion The X-ray patterns of as-synthesized and 400  C calcined Ce0.75Sm0.2Li0.05O1.95 powders are shown in Fig. 1. The formation of completely phase-pure CeO2 with cubic fluorite structure is observed in both cases which indicate that Ce0.75Sm0.2Li0.05O1.95 does not show any additional peak for lithium oxide or other complexes. The crystallite size, calculated from Scherrer's formula, is found to be ~40 and 45 nm for as-synthesized and calcined Ce0.75Sm0.2Li0.05O1.95 powders, respectively. The powder morphology of as-synthesized

Fig. 1 e X-ray patterns of as-synthesized and 400  C calcined Ce0.75Sm0.2Li0.05O1.95.

powder indicates particle agglomeration with estimated size of ~50 nm (inset Fig. 2). It has been found that lattice parameter decreases minutely (5.4092 Å) for Ce0.75Sm0.2Li0.05O1.95 compare to 20SDC (Ce0.8Sm0.2O1.95) (5.4132 Å) which is evitable from the fact that rþ1 Li (viii) (0.92 Å) is slightly lower than that of rþ4 Ce (Viii) (0.97Å). This strongly supports lattice substitution of Liþ1. Samples sintered at 950  C for 2hr reaches >98% relative density which can be achieved by 20SDC only at around 1250  C. The corresponding dense microstructure having average grain size in the range 200e500 nm is shown in Fig. 2. No secondary phase accumulation at grain boundary is evident which is further manifested from the XRD of 950  C sintered sample (inset Fig. 2). In order to confirm that there is no secondary phase present in the sintered samples, slow scanned XRD patterns were recorded. Fig. 2 inset shows a slow scanned XRD patterns of Ce0.75Sm0.2Li0.05O1.95 sintered at 950  C for a selected 2q region (32e34 ) corresponding to the most intense diffraction peak which does not show any presence of secondary phases like Li2O or LiOH (reported to show intense at 33 peak and small bulge near 32 for Li2O and LiOH respectively) [16]. In fact formation of LiOH or Li carbonates are very negligible as in this work combustion synthesis have been used in this work which being a highly exothermic process restrict formation of such compounds. Even if such compound formed it will definitely decompose during calcinations and initial stage of sintering. Densification of 20SDC is enhanced by addition of Liþ1 by means of lattice diffusion of host atoms [17] which is contradictory with the decrease of lattice parameter hence the free cell volume. Zhu et al. [8] has explained similar enhancement of sintering of 20GDC (Ce0.8Gd0.2O2-d) with addition of Liþ1 might be due to very high vapor pressure of Li2O (at 1000  C, the vapor pressure reaches1.09 E8 bar for Li2O). They have also postulated that Li diffusion into the GDC lattice and Li2O vaporization from the GDC lattice may take place consecutively at elevated temperature. In this work we differ with their view and postulate that substitution of Ceþ4 by Liþ1 can increase the oxygen vacancy by following Kroger e Vink equation. 2CeO2


x Li2 Oƒƒƒƒƒƒ!2LiCe þ 3V O þ OO


Thus incorporation of each Liþ1 is compensated by three oxygen vacancy. Increase of V O number enhance the vacancy diffusion. Enhanced vacancy diffusion results in quicker mass transport to the opposite direction results in densification in spite of small decrease of lattice parameters. Impedance spectra at 300  C for Ce0.75Sm0.2Li0.05O1.95 sample, sintered at 950  C in air and hydrogen atmosphere is

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Fig. 2 e Microstructure of samples sintered at 950  C for 2 h. (Inset: powder morphology of as-synthesized Ce0.75Sm0.2Li0.05O1.95, XRD of 950  C sintered sample and slow scanning of 950  C sintered sample).

shown in Fig. 3. By fitting and simulation of the depressed semicircular plot, equivalent circuits with different circuit models for air and hydrogen atmosphere are obtained in combination of R-CPE and Warburg element (as shown in Fig. 3). It is evident that the grain semicircles are incomplete (rather invisible) in the experimental frequency range may be the time constant of the grain response is too short for these materials at the experimental conditions, at higher frequency the impedance plots make a cut on real axis (which has been

Fig. 3 e Impedance spectra at 300  C for Ce0.75Sm0.2Li0.05O1.95 sample, sintered at 950  C in air and hydrogen atmosphere (dotted lines are ZView simulated cures) along with sintered Ce0.8Sm0.2O1.95 at 300  C in air.

derived from Z-view simulation). Conductivity value for bulk is calculated from the resistance of the simulated 1st semicircle. A middle semicircle is manifested at medium frequency which stands for grain boundary phenomena. Depression by an angle of 15 of the grain e boundary semicircle in air at 300  C is typical of ceria based systems. Both the grain boundary semicircles are almost similar in appearance for 20SDC as well as Liþ1 substituted compound. It implies grain boundary do not electrically differ much with addition of lithium, eliminating segregation at grain boundaries. In hydrogen atmosphere it does not change nature of the Nyquest plots only the electrode contribution of resistance are found to have some changes in nature. At higher temperature (600  C), irrespective of partial pressure of oxygen significant change in grain boundary contribution of resistance does not evident which signifies the fact that even at high temperature Ce4þ/Ce3þ reduction not significant [18]. Temperature dependence of grain (lattice) conductivity at air and hydrogen atmospheres are compared in the Fig. 4 along with the 20SDC (in air) obtained from literature [19]. It is clear that the Liþ1 enhanced grain conductivity of 20SDC which is a definitive proof for lattice substitution of Liþ1. Increase of grain conductivity can be explained by the increase of oxygen vacancy as already discussed during explanation of sintering enhancement. At higher temperature deviation of straight line slope is not observed for Liþ1 substituted compositions (evident for 20SDC) which indicate vacancy ordering probably not takes place for Ce0.75Sm0.2Li0.05O1.95. Highly charged substituted constituents may repel themselves and prevent their clustering with oxygen vacancy. This also contributes to the enhancement of grain conductivity at elevated temperature for Ce0.75Sm0.2Li0.05O1.95 compare to 20SDC.

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activation enthalpy calculated from the Arrhenius curve is 0.72 eV for air atmosphere which is lower than the 20SDC (0.89 eV upto 600  C in air) which validates ease of oxide ion conduction in Ce0.75Sm0.2Li0.05O1.95 compositions compare to SDC. This may be due to the increase of unit cell volume with substitutions. The value of activation enthalpy has found to be 0.74 eV very similar to the value obtained in air corroborating to the fact that the substitution of Liþ1 in 20SDC enhances redox stability. Fig. 5 shows the XRD of the Ce0.75Sm0.2Li0.05O1.95 after the measurement in hydrogen atmosphere, formation of LiCeO2 is evident. At elevated higher temperature part of Liþ1 may form LiCeO2in reducing atmosphere which may prevent substituted 20SDC from further reduction in reducing atmosphere thus conductivity is not altered much from that of air atmosphere.

Fig. 4 e Temperature dependence of conductivity (grain) at air and H2 atmosphere (along with 20SDC in air and hydrogen).

Although conductivity of Ce0.75Sm0.2Li0.05O1.95 shows higher conductivity than that of 20SDC at 500  C [20], it shows little lower conductivity than the earlier report of Li added SDC (0.05 Scm1 at 500  C) [14]. This lower value may be due to different amount and form of substitution. In our work only 5 mol% Liþ1 is added in solid solution with SDC (replacing Ceþ4 by Liþ1) while in the earlier work 5 mol% Liþ1 was added as a liquid phase former (molar ratio of 1:4 for Sm3þ:Ce3þ), the Liþ1 bearing phase may enhance conductivity, but that lack of reducing condition stability report [14]. From Fig. 4 it is also evident that even at a temperature ~500  C conductivity does not change much even in reducing atmosphere which corroborates with the results of grain boundary conductivity analysis in impedance data whereas for 20SDC a huge increase in conductivity is evident. This sudden increase of conductivity of 20SDC in hydrogen atmosphere is a definite proof of reduction of Ceþ4 to Ceþ3 at reducing atmosphere which develop electronic conduction, in other word thermodynamic instability of 20SDC in reducing condition. From the straight line fitting of these Arrhenius plot at H2 atmosphere, energy of activation is obtained. In this work conductivity values in air and reducing atmosphere are comparable in 300  C as well as 500  C indicating Li substituted SDC's stability in reducing atmosphere. In hydrogen atmosphere conductivity remains almost same as that of air. The

Fig. 5 e X-ray patterns of Ce0.75Sm0.2Li0.05O1.95 after exposure to hydrogen atmosphere.

Conclusions Using nanocrystalline powder (~50 nm) of composition Ce0.75Sm0.2Li0.05O1.95 fully densed (>98% of the theoretical density) material is obtained at sintering temperature as low as 950  C. Ionic conductivity (lattice) at 500  C in air is increased compare to that of 20SDC. The conductivity values do not altered in reducing atmosphere. Prima facie evidence confirm that Ce0.75Sm0.2Li0.05O1.95 could be a potential candidate as an electrolyte for application in LT-SOFCs.


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