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Novel way of phase stability of LSGM and itsconductivity enhancement

Ramesh Chandra Biswal*, Koushik Biswas

Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India

a r t i c l e i n f o

Article history:

Received 28 June 2014

Received in revised form

6 September 2014

Accepted 22 October 2014

Available online xxx

Keywords:

SOFC

LSGM

Impedance spectroscopy

XPS

Pechini method

* Corresponding author. Tel.: þ91 3222 28227E-mail address: rchbiswal@gmail.com (R.

Please cite this article in press as: Biswal RInternational Journal of Hydrogen Energy

http://dx.doi.org/10.1016/j.ijhydene.2014.10.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Instead of using various methods to synthesize phase pure Sr- and Mg-doped LaGaO3

(LSGM), it is very interesting to achieve phase purity by changing the stoichiometry and

additional co-doping. In this work, Pechini method is adopted to synthesize LSBGM

(La0.8Sr0.2�xBaxGa0.8Mg0.2O2.8) by co-doping of Ba2þ and Sr2þ at La-site, and Mg2þ at Ga site.

These multi-dopings eliminate the complexities in the synthesis and processing tech-

niques, and desired single phase is achieved more easily. Tolerance factor, which is a

measure of structural symmetry, increases from 0.9622 (in LSGM) to 0.9652 (in LSBGM).

Moreover, an increase in 0.4% free volume occurred due to these multiple dopings at La-

site. Enhancement in free volume and structural symmetry attribute to a higher ionic

conductivity of LSBGM, which is also supported by XPS analysis. Ionic conductivity (0.046 S/

cm at 600 �C) measured by impedance spectroscopic analysis is found to be >44% superior

compared to the LSGM synthesized by similar/different processes. Additionally, a decrease

in activation energy value suggests the lower vacancyedopant interaction in LSBGM.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Fuel cell, an electrochemical energy conversion device, is

getting popularity because of its direct conversion of chemical

energy in the fuel to electricity. As its efficiency is not limited

by the Carnot's cycle, it can be much higher than that of

combustion engine. Among the different types of fuel cell,

solid oxide fuel cell (SOFC) is gettingmore attention because of

its fuel flexibility, higher energy conversion efficiency and

durability. Hence, it is believed to be the most promising de-

vice for stationary electrical power generation. Moreover, in

the SOFC system, electrolyte plays the pilot role in deciding

the operating temperature and compatible electrodes. In that

aspect, over past few decades yttria-stabilized zirconia (YSZ)

9.C. Biswal).

C, Biswas K, Novel way(2014), http://dx.doi.org

99gy Publications, LLC. Publ

is extensively used as an electrolytematerial. However, due to

its high operating temperature, compatible electrodes and

interconnect material is of great concern, and subsequently

durability of the cell is a major issue. Afterward, doped ceria

and LaGaO3 based materials are found to be the alternate

electrolyte materials for intermediate temperature SOFC (IT-

SOFC). But in the reducing atmosphere or at slightly higher

temperatures, electronic conductivity is developed within the

ceria based materials due to the reduction of Ce4þ to Ce3þ,which limits its usage as electrolyte material in those condi-

tions [1].

Doped LaGaO3, on the other hand, found to be a promising

candidate for IT-SOFC electrolyte in which, hopping of the

ionic species via vacant sites is the basis of its transport

mechanism [1]. As shown in Fig. 1(a), when La3þ sits at the

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

ished by Elsevier Ltd. All rights reserved.

Fig. 1 e (a) Cubic perovskite structure of LaGaO3, (b)

calculated pathway of oxygen ion migration between

occupied and vacant O-site (Fig. 1(b) is reproduced with

permission from Ref. [2]).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 02

edge of the LaGaO3 perovskite structure, Ga3þ takes the central

position with O2� in each face, and GaO6 form an octahedron.

Hence, amigrating oxygen ion has to cross the barrierwhich is

a triangular orifice consisting of two large La3þ and one small

Ga3þ cations in the crystal lattice (Fig. 1(a) and (b)) [2,3]. To

improve the conductivity, this barrier created by the neigh-

boring atoms needs to be minimized to allow easier diffusion

of anions. Both, thermally induced atomic vibration and cre-

ation of more vacancies enhance the diffusion. However,

when there is no vacancy or free volume to propagate the

migrating ion, then temperature has little effect. Therefore,

the conductivity, along with the vacancies, free volume need

to be increased by suitable doping with lower valent and

oversized atoms. Moreover, vacancyedopant interaction,

though not proved experimentally for doped LaGaO3, is found

theoretically to be varied from dopant to dopant [4]. And their

binding energy should be minimized to easily activate the

vacancy propagation and to enhance the conductivity. Apart

from this, tolerance factor (t), which is basically a measure of

symmetry and free volume of the crystal structure plays an

important role in affecting the mobility of the ions [1].

On the other hand, the perovskite materials accommodate

a number of dopants in its structure and show various prop-

erties accordingly. However, in LaGaO3, as studied by Islam

et al. [3,4], Sr2þ at La-site and Mg2þ at Ga-site require less so-

lution energy favouring optimum oxygen vacancy formation

and higher conductivity [1]. Most importantly, Sr2þ and Mg2þ

doped LaGaO3 (LSGM) is stable over its operating temperature

(600e1000 �C) and even at low oxygen partial pressure

(~10�20 atm). Comparing the results of oxygen tracer diffusion

data obtained by SIMS (secondarymass ion spectroscopy), and

electrical conductivity obtained from the NernsteEinstein

relationship, Ishihara et al. [5] observed the strong ionic con-

ductivity (transference number z1) for LSGM electrolyte ma-

terials. However, phase pure LSGM synthesis is a great

challenge and thus over a decade many attempts have been

made to synthesize it, viz. solid state route [6e8], polyol

method [9], glycine-nitrate combustion method [6,10,11], hy-

drothermal precipitation process [12,13], carbonate copreci-

pitation [14], Pechini method [6,15e17], organic precursor

method [18], regenerative solegel method [19], steric entrap-

ment synthesis method [20], etc. But, the researchers admit

the difficulties in synthesizing pure phase LSGM. Because,

increasing the amount of Sr2þ, though increases the number

Please cite this article in press as: Biswal RC, Biswas K, Novel wayInternational Journal of Hydrogen Energy (2014), http://dx.doi.org

of oxygen vacancies, the solid solubility of Sr2þ into La-sites of

LaGaO3 is limited [1,21]. And beyond certain limit, some sec-

ondary phases like LaSrGaO4, La4Ga2O9, SrGaO3 or La4SrO7,

etc., are formed [1]. However, adding Mg2þ in Ga-site further

increases the Sr2þ solubility and reduces the secondary pha-

ses [1,4,21]. But, the balanced amount of Sr2þ and Mg2þ doping

and a proper synthesis-cum- post processing technique in-

creases its phase stability, as well as its ionic conductivity

[21,22].

Again, in spite of higher solution energy of Ba2þ at La-site in

LaGaO3 as stated by Islam et al. [4], Ba2þ and Mg2þ doped

LaGaO3 (LBGM), has been studied by some researchers [23,24].

Observed that, similar to LSGM, some secondary phases like

BaLaGa3O7, BaLaGaO4, etc. are formed for LBGMmaterials and

more than 10% of Ba at La-site intensifies the chance of more

secondary phase formation. Having looked at LSGM and

LBGM, it is observed that in addition to the synthesis tech-

niques, stoichiometries of the materials play a significant role

in pure phase formation. Considering the tolerance factor,

ionic radii and free volume, it is a great idea to modify the

LSGM stoichiometry by partial substitution of Ba2þ (Ba2þ(XII):0.161 nm) in La-site (La3þ(XII): 0.136 nm) along with the Sr2þ

(Sr2þ(XII): 0.144 nm) to maximize the conductivity. Recently,

Raghvendra et al. [25,26] has though carried out this type of

work, it is not encouraging because of the synthesis method

they have followed and the stoichiometry. Therefore, they

found some secondary phases, which enhanced the activation

energy lowering the conductivity. However, in this work, we

have followed the Pechinimethod to synthesize thematerials,

and stoichiometry has been selected to keep the number of

vacancies constant as LSGM. Again the Sr2þ and Ba2þ contents

have been optimized for best output.

To elucidate the oxygen vacancies, created by the doping of

SrO, MgO and partial substitution of SrO by BaO in LaGaO3,

Kr€ogereVink notations are used to write down the equations

as follows;

SrOþMgO�������!LaGaO3 Sr0La þMg0Ga þ 2O�

O þ V$$O (1)

12SrOþ 1

2BaOþMgO�������!LaGaO3 1

2Sr0La þ

12Ba0

La þMg0Ga þ 2O�

O þ V$$O

(2)

Experimental technique

Synthesis

La0.8Sr0.2�xBaxGa0.8Mg0.2O2.8 (x ¼ 0, 0.03, 0.05 and 0.08) were

synthesized by well known Pechini method. Calculated

amount of La(NO3)3$6H2O, Sr(NO3)2, Ga(NO3)3$xH2O,

Mg(NO3)2$6H2O and Ba(NO3)2 (Alfa Aesar, USA) were used as

precursor materials. The detailed compositions are

La0.8Sr0.17Ba0.03Ga0.8Mg0.2O3�d (LSBGM- 3Ba), La0.8Sr0.15Ba0.05-Ga0.8Mg0.2 O3�d (5Ba), La0.8Sr0.12Ba0.08Ga0.8Mg0.2O3�d (8Ba) and

La0.8Sr0.2 Ga0.8Mg0.2 O3�d (LSGM or 0Ba). From thermogravi-

metric analysis (TGA) (Perkin Elmer, USA), the value of “x” in

the above given formula of gallium nitrate (as assigned by the

manufacturer) was determined to be 4. At room temperature,

these precursor materials were dissolved in distilled water

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 3

separately, followed by mixing in a single beaker and contin-

uous stirring by a magnetic stirrer. To form metal citrate

complex, nearly one mol of citric acid per mol of nitrate of the

precursor material and ethylene glycol (one third mole of

citric acid) were added to the solution with constant stirring.

Solution was then heated for 1 h at 100 �C to reduce the water

content and initiate the polymerization. Gel was formed

slowly with escape of some reddish yellow gases, when the

temperature was raised to ~200 �C. Then the beaker contain-

ing the gel was transferred to a furnace, and thermally treated

at 400 �C for 1 h. It leads to the combustion of the gel, which

ultimately produces the amorphous ash. The resultant ash

was then collected and calcined at 1000 �C for 5 h to remove

the residual carbons. This powderwas finally ground and used

for further processing and characterizations.

Characterizations

The calcined powder was again ground and heat treated for

1 h at 1400 �C in air and was characterized by XRD (Bruker D8

Advance, Germany) using CuKa radiation (l ¼ 0.1541 nm), in

the Bragg angle range 20� � 2q � 100�. Rietveld analysis of the

XRD pattern was done to quantify the phase using X'pert High

Score Plus software.

The calcined powder (at 1000 �C for 5 h) was then pressed

uniaxially at a pressure of 500 Mpa, and these green pellets

were subsequently sintered at 1400 �C for 8 h. Densities of the

sintered pellets were measured by Archimedes principle.

Microstructural analyses of the sintered pellets were carried

out by field emission-scanning electron microscope (FESEM)

(FEI Quanta 250, USA). To analyze the particle morphology,

high-resolution transmission electron microscope (HRTEM)

(JEOL, Japan) was used. In this case, powder was ultrasonically

dispersed in ethanol for 15 min using an ultrasonic probe and

then a drop of dispersed powder spreads over a carbon coated

copper grid. The specific surface area of the calcined powders

were measured after thoroughly degassing under vacuum for

3 h at 300 �C following BET method using NOVA surface area

analyzer (Quanta chrome, USA). The X-ray photoelectron

spectroscopy (XPS) spectra of the samples were carried out

using PHI5000 Versa Probe II (ULV AC - PHI, INC, Japan) system

using the Al Ka (hn¼ 1486.6 eV) as the excitation source with a

hemispherical analyser and a multichannel detector. Carbon

Fig. 2 e XRD plots: (a) La0.8Sr0.2¡xBaxGa0.8Mg0.2O3¡d

Please cite this article in press as: Biswal RC, Biswas K, Novel wayInternational Journal of Hydrogen Energy (2014), http://dx.doi.org

(C1s, hn ¼ 284.6 eV) was taken as a reference, and all of the

samples were well polished and cleaned with alcohol prior to

the measurement.

Thermal expansion co-efficient of the materials was

determined by heating the specimen up to 1000 �C in air

keeping heating rate of 5 �C/min and applying a mechanical

force of 0.1 N in thermo mechanical analyzer (TMA) (Perkin

Elmer, USA).

Dense pellets were characterized by impedance spectro-

scopic study in air, to determine the conductivity of the syn-

thesizedmaterials. It was carried out in the temperature range

from 300 to 600 �C using a computer-controlled impedance

analyzer (Hioki LCR Hi Tester 3532-50). During this analysis, a

trigger voltage of 70 mV was applied in the frequency range

from 50 Hz to 5 MHz. Prior to the measurement, both the flat

surfaces of the pellets were polished followed by ultrasonic

cleaning in acetone for 15 min. After a complete dry,

conductive silver paste was coated carefully on both the flat

surfaces avoiding short-circuiting between the two surfaces.

Then, the coated pellets were fired at 300 �C for 4 h, to remove

the organic binder. A blank cell was tested at all the temper-

ature, and to eliminate the lead resistance, this resistance

value was subtracted from themeasured impedance values of

the specimen. Using the free software ZSimpDemo 3.22d, the

equivalent electrical circuit was modeled for the tested

materials.

Results and discussion

Structural analysis

Single doping (either Sr2þ or Ba2þ) at La-site (i.e. LSGM or

LBGM), creates more secondary phases as reported by various

groups [1,21,23,24]. In our previous work [22], single phase

LSGM has been obtained following a particular heat treatment

schedule (15 �C/min up to 700 �C, 10 �C/min up to 1000 �C for

5 h, 5 �C/min till 1300 �C and then 2 �C/min up to 1400 �C and

holding for 8 h) and with repeated grinding. However, in case

of Ba-doped samples (3Ba and 5Ba), single phase has been

obtained at 1400 �C for 8 h without any difficulties or any

special heat-treatment schedule. That means; co-doping of

Ba2þ with Sr2þ at La-site makes the LSGM structure more

(x ¼ 0, 0.3, 0.5, 0.8), (b) showing the peak shift.

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

Fig. 3 e Rietveld analysis of La0.8Sr0.2¡xBaxGa0.8Mg0.2O3¡d: (a) 0Ba, (b) 5Ba.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 04

stable and eliminates the complexity in the synthesis route or

processing techniques to get the desired single phase. But

increasing the Ba content (as in the case of 8Ba), retain some

undissolved BaO as a secondary phase as detected from XRD

analysis (Fig. 2(a)). However, Raghvendra et al. [25,26], has not

achieved the pure phase of LSGM or LSBGM, either due to their

synthesis method and/or their chosen stoichiometry. But in

this work, stoichiometry of LSBGM

(La0.8Sr0.2�xBaxGa0.8Mg0.2O3�d) has been chosen to keep the

concentration of oxygen vacancies (d ¼ 0.2) constant as in

LSGM (La0.8Sr0.2Ga0.8Mg0.2O2.8). Rietveld analysis (Fig. 3) re-

veals that increasing the Ba content, lattice parameters also

increase causing an overall expansion of unit cell volume as

given in Table 1. It is also clarified from the XRD peak shift

towards the lower angle (Fig. 2(b)). Moreover, the free volume

(Vf) is calculated by subtracting the total volume of constituent

ions from the total volume of the unit cell, eg.,

Vf ¼ Vcðunit cell volumeÞ �X

Vions (3)

And the Vf (free volume) is found to be almost 0.4% higher

for 5Ba than 0Ba. Therefore, it was expected that these more

vacant spaceswithin the unit cell would favour themigration/

diffusion of oxygen ion [27,28]. Microstrain calculated from

Rietveld analysis (Table 1) shows higher structural distortion

Table 1 e Structural parameters found from Rietveldanalysis.

Parameters aLaGaO3bLSGM bLSBGM

a (A) 5.5267 5.5371 5.5491

b (A) 7.7792 7.8132 7.8543

x (A) 5.4947 5.5818 5.5680

a ¼ b ¼ g 90 90 90

Unit cell volume (A3) 236.24 241.48 242.677

Lattice strain (%) e 0.118 0.029

Crystallite size (nm) e 27 44

c2 e 1.63 1.8

Rp e 8.6 9.3

Rwp e 9.7 11.3

a Taken from the PDF no: 04-013-6815.b La/Sr/Ba is located at 4e site, Ga/Mg at 4a, O1 at 4e and O2 at 8 g

sites with reference to PDF No. 04-013-6833.

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of 0Ba compared to 5Ba. The tolerance factor (t), which tells

theoretically about the structural symmetry of the material,

was calculated from Equation (4) as given below:

t ¼ 0:8RLa3þ þ ð0:2� xÞRSr2þ þ xRBa2þ þ RO2�ffiffiffi2

p �0:8RGa3þ þ 0:2RMg2þ þ RO2�

� ; (4)

where, R is the ionic radius of the respective element with

desired co-ordination number, x ¼ 0 or 0.05 for 0Ba and 5Ba

respectively. The calculated values show that undoped

LaGaO3 has the most symmetric structure with higher toler-

ance factor (t ¼ 0.9661). Whereas among the doped LaGaO3,

5Ba has higher t value (0.9652) than 0Ba (0.9622), implying that

5Ba is more structurally symmetric than 0Ba. That means;

incorporation of Ba2þ in LSGM is creating more symmetric

structure compared to LSGM. The symmetry or distortion of

the structure from the ideal perovskite is directly correlated

with the tilting of GaO6 octahedra. Moreover, the degree of

tilting in GaO6 octahedra is strongly affected by the doping,

and these octahedra are more distorted in doped LaGaO3

compared to undoped, which is well supported by Slater et al.

[29] from the neutron diffraction data. However, in spite of the

orthorhombic structure of both LSGM and LSBGM, structural

distortion in GaO6-octahedron is less in LSBGM than LSGM, as

found from tolerance factor.

XPS analysis

With the XPS analysis, core level spectra of the elements of

both 0Ba and 5Ba samples were carried out as shown in Fig. 4.

The XPS spectrum of La 3d states is splitted due to the

spineorbit interaction into a doublet structure, 3d3/2 and 3d5/2

having approximately 17 eV of an energy difference. Addi-

tionally, each line is splitted due to the transfer of an electron

from oxygen ligands to the La 4f (3d94f0 to 3d94f1) [30,31]. The

binding energies and splitting are well agreed with reported

values for La3þ compounds [30,32]. A careful fitting of the Sr3d

spectra gives a doublet, whose binding energies are separated

approximately 2 eV of energy as shown in Table 2a, which can

be assigned as Sr3d5/2 and Sr3d3/2 lines [32]. XPS spectra of

Ba3d also show a doublet having an energy separation nearly

15 eV. Hence, these peaks can be assigned as Ba3d5/2 and

Ba3d3/2, which agree with the literature and implies the

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

Fig. 4 e XPS analysis of 0Ba and 5Ba samples showing the spectra of different elements.

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 5

existence of Ba2þ state in the material [26]. The XPS spectrum

of Ga2p3/2 (Fig. 4 and Table 2) clearly suggests the presence of

Ga3þ state of Ga in both the materials [31]. Also, the binding

energy of Mg1s core level peak suggests the Mg2þ state which

is also observed earlier for LSGM materials [33].

The O 1s spectrum shows a doublet feature with the peak

energies at 528.8 and 531.2 eV as shown in Fig. 4. Some authors

[31e33] ascribed the main energy peak as the lattice oxygen

species, while the broad shoulder peak as molecularly adsor-

bed O2 species. However, as explained by Pawlak et al. [34,35],

the O1s signals of the mixed perovskites have similar binding

energies to XPS O(1s) signals of simple oxides. So one can

deconvolute these two broad singular peaks into the signals of

simple oxides as shown in Fig. 5(a). The deconvoluted peaks

are nearer to the individual oxides as given in Table 2b. That

means, considering this argument, two O1s signals observed

in these cases come out from the polarization of valence shell

Table 2a e Binding energies of each element of 0Ba and 5Ba fo

Elements/ La Sr

States/ 3d5/2 3d3/2 3d5/2 3d3/2

LSGM

(0Ba)

836.3, 832.9 853.2,

849.8

133.0 135.1

LSBGM

(5Ba)

836.1,

832.8

853.1

849.8

133.2 134.9

Please cite this article in press as: Biswal RC, Biswas K, Novel wayInternational Journal of Hydrogen Energy (2014), http://dx.doi.org

electrons of oxygen ions. Due to this effect the 1s electrons

escaping from oxygen atoms in different directions interact

with polarized valence shells, thus gaining different final en-

ergies [34]. Moreover, full width at half maxima (FWHM) of

deconvoluted O1s singular peak (Table 2b), is decreasing in

5Ba compared to 0Ba. It suggests that, LSBGM is more ionic

than LSGM as per the literature [34,35].

Again in the orthorhombic structure of LSGM, there is one

independent La site, one independent Ga site and two inde-

pendent oxygen sites (O1 and O2). Moreover, vacancy is easier

to form at one of the oxygen sites [29,36]. As studied in some

literature [33,34], because of these inequivalent O-site in

orthorhombic perovskite structure, the binding energy of O1s

might be splitted. Since there are two oxygen atoms in O1-site

and four oxygen atoms in O2-site, the intensity ratio of both

the peaks should be 1:2, which is observed elsewhere [33] for

doped and undoped orthorhombic LaGaO3. Hence, the doublet

und from the peak fitting.

Ba Ga Mg O

3d5/2 3d3/2 2p 1s 1s

e e 1117.3 1304.6 528.8,

531.2

778.9 794.2 1117.2 1304.2 528.8,

531.2

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

Table 2b e Binding energy and FWHM of O1s peak of the material and individual oxides.

Binding energy/FWHM (eV)

La2O3 SrO BaO Ga2O3 MgO

Single oxides/

(from literature)

528.8 [31,35] 530 [29],530.2 [35] 528.4 [37] 530.9 [29], 531.4 [35] 530.7 [31]

LSGM (0Ba) / 528.49/0.92 529.39/1.15 e 531.46/1.58 530.69/1.55

LSBGM(5Ba)/ 528.68/0.58 529.17/0.89 528.29/0.54 531.6/1.54 530.89/1.50

Fig. 5 e XPS O1s peaks of the materials: (a) deconvoluted in to their simple oxides, (b) deconvoluted in to two peaks based

upon the orthorhombic O-sites.

Fig. 6 e TEM image of powder sample calcined at 1400 �C: (a) 0Ba (b) 5Ba.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 06

Please cite this article in press as: Biswal RC, Biswas K, Novel way of phase stability of LSGM and its conductivity enhancement,International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.10.099

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 7

peak of O1s can easily be deconvoluted as shown in Fig. 5(b)

and is clearly observed that there is a significant difference

in the intensities of the two peaks, and bigger one is nearly

twice compared to the small peak in both the cases. Moreover,

there are some differences in the structure of those peaks. The

shoulder peak in the case of 5Ba is getting enhanced, which

might suggest that the vacancies in this material are more

disordered as in the case of more symmetric structure, sug-

gesting higher ionic conductivity compare to 0Ba [29,36].

Microstructural analysis

From the HRTEM micrographs as shown in Fig. 6, nearly

spherical particles are observed with size 80 ± 18 nm for 0Ba

and 80 ± 7 nm for 5Ba after calcining the powders at 1400 �C.Specific surface areas of the powders are found to be 14.4 and

Fig. 7 e SEM image of the sintered pellets:

Please cite this article in press as: Biswal RC, Biswas K, Novel wayInternational Journal of Hydrogen Energy (2014), http://dx.doi.org

12.1 m2/g for 0Ba and 5Ba, respectively. In addition, the par-

ticle sizes found from the BET surface area are calculated to be

64 and 76 nm, respectively for 0Ba and 5Ba, which are close to

the size observed under TEM. From the Archimedes' principle,all the sintered pellets except 8Ba are found to be highly dense

(>98% theoretical density), and was further confirmed by SEM

micrographs as shown in Fig. 7. 8Ba sample is ~95% dense,

which might be due to the unreacted BaO phase.

Thermal analysis

Disparity in thermal expansion coefficients (TEC) of individual

components creates cracking of the cell leads, thus causing

mixing of gas and fuel or breaking continuity of the cell.

Therefore, as a useful parameter, TEC of the materials were

found to be 11.53 � 10�6 and 12.9 � 10�6/�C for 0Ba and 5Ba

(a-b) 0Ba, (c-d) 3Ba, (e-f) 5Ba, (g-h) 8Ba.

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

Fig. 8 e Thermal expansion co-efficient of 0Ba and 5Ba.

Fig. 10 e Arrhenius plots of Ba2þ doped LSGM samples and

temperature vs conductivity plots (inset).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 08

samples respectively as shown in Fig. 8. Since this value for

5Ba is close to the LSGM (0Ba), the established electrode ma-

terials used for LSGM can also be used for LSBGM materials.

Impedance analysis

The Nyquist plots, as shown in Fig. 9 shows diffused semi-

circular arcs with non-zero high frequency intercept at all

operating temperatures. The ionic transport behaviour of the

sample is related to the semicircular arc patterns and the

extent of their intercept on the Z0-axis [38]. Simulating the

semicircular pattern of Nyquist plot with equivalent electrical

circuit, transport phenomena occurring within the sample

and their correlation with the structural change are deter-

mined. Hence, the impedance data were fitted with different

equivalent circuits by Zsimp win software, and found to be

best matched with (RQ) (RQ) circuit as shown in Fig. 9. So each

semicircular arc in the impedance pattern can be represented

as a parallel combination of constant phase element (CPE) and

a resistor. CPE, represented by the symbol Q, basically

Fig. 9 e Nyquist plots of LSGM and LSBGM sample

Please cite this article in press as: Biswal RC, Biswas K, Novel wayInternational Journal of Hydrogen Energy (2014), http://dx.doi.org

depends on the surface roughness, thickness, composition

and non-uniform current distribution [38]. From the fitted

equivalent circuit, grain and grain boundary responses were

separated as a function of applied frequencies and their re-

sistances were found out. It was observed that, the grain

boundary resistance is lower than the grain resistance in both

LSGM and LSBGM. The grain and grain boundary conductiv-

ities were calculated using the Equation (5).

sg=gb ¼ L�ARg=gb (5)

where, sg/gb is the grain or grain boundary conductivity, L is

the thickness and A is the effective area of the sample and Rg/

gb is the grain or grain boundary resistance of the sample. It

was found that 5Ba sample has the highest conductivity

among the group of synthesized materials. Moreover, its

overall ionic conductivity is 0.046 S/cm at 600 �C, which is

s at 400 �C and fitting with equivalent circuit.

of phase stability of LSGM and its conductivity enhancement,/10.1016/j.ijhydene.2014.10.099

Table 3 e Conductivity and activation energy of thesamples.

Samples s (S/cm) at 600 �C Ea (eV)

0Ba 0.032 1.11

3Ba 0.036 1.06

5Ba 0.046 1.02

8Ba 0.028 1.06

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 9

nearly 44% higher than LSGM (0Ba). This conductivity value is

significantly higher than LSGM synthesized by different

techniques [6,9e17,19,20,22,39,40]. This increase in conduc-

tivity might be due to the overall reduction in O2� ion migra-

tion energy due to an increase in free volume and/or increase

in structural symmetry as found from the structural analysis.

From the Arrhenius plots (Fig. 10), the activation energy for

ion transport through the grains and grain boundaries are

calculated by taking the slope of the 1000/T vs. ln(sT) plot and

placed in Table 3. Activation energy for the grain and grain

boundary of 5Ba sample found to be 0.98 and 1.06 eV respec-

tively, and total activation energy is 1.02 eV, which is less

among the group of materials (Table 3). This decrease in

activation energy compared to LSGM is further supporting the

concept of free volume increment in LSBGM. Moreover, this

decreasing activation energy might suggest that, in case of

LSBGM vacancyedopant interaction is reduced compared to

LSGM.

Conclusions

Co-doping of Ba2þ with Sr2þ at La-site makes the LSGM

structure more stable and eliminates complexity in the syn-

thesis route or processing techniques and desired single

phase is achieved more easily. Some basic calculation shows

~0.4% free volume increases per unit cell of LSBGM compared

to LSGM, and as found from the tolerance factor, structural

distortion in GaO6-octahedron is less in LSBGM than LSGM, in

spite of both are having orthorhombic structure. Optimized

amount of Ba-addition in LSGM results in high conductivity

(0.046 S/cm at 600 �C), which is >44% higher compared to the

LSGM synthesized by similar/different processes. Moreover, a

decrease in activation energy value might suggest the lower

vacancyedopant interaction in LSBGM. Hence, this increase in

conductivity must be due to an overall reduction in O2� ion

migration energy, which might be due to an increase in free

volume and/or increase in structural symmetry, and/or a

lower vacancyedopant interaction.

Acknowledgement

Authors gratefully acknowledge the Council of Scientific and

Industrial Research (CSIR), New Delhi for financial support

(Sanction No:22(0598)/12/EMR-II , Dt. 02-04). Technical staffs

of Central Research Facility (CRF) and Department of Metal-

lurgical and Materials Engineering, Indian Institute of Tech-

nology, Kharagpur are gratefully acknowledged for their

support during characterizations of this material. We would

Please cite this article in press as: Biswal RC, Biswas K, Novel wayInternational Journal of Hydrogen Energy (2014), http://dx.doi.org

also like to acknowledge the DST-FIST funded XPS facility at

the Department of Physics andMeteorology, IIT Kharagpur for

XPS analysis.

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