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Comparative study of the nano-compositeelectrolytes based on samaria-doped ceria for lowtemperature solid oxide fuel cells (LT-SOFCs)

M. Ajmal Khan a,b, Rizwan Raza a,c,*, Raquel. B. Lima a,e,M. Asharf Chaudhry b, E. Ahmed b, Ghazanfar Abbas b,d

aDepartment of Energy Technology, Royal Institute of Technology (KTH), S-10044 Stockholm, SwedenbDepartment of Physics, Bahauddin Zakariya University (BZU), 60800 Multan, PakistancDepartment of Physics, COMSATS Institute of Information Technology, 54000 Lahore, PakistandDepartment of Physics, COMSATS Institute of Information Technology, 44000 Islamabad, PakistaneDepartment of Fiber and Polymer Technology, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden

a r t i c l e i n f o

Article history:

Received 12 December 2012

Received in revised form

8 May 2013

Accepted 12 May 2013

Available online xxx

Keywords:

Electrolytes

Solid oxide fuel cell

Ceria carbonate

Nanocomposites materials

* Corresponding author. Department of Ener87907403.

E-mail address: razahussaini786@gmail.c

Please cite this article in press as: Khandoped ceria for low temperature solid odx.doi.org/10.1016/j.ijhydene.2013.05.060

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.05.0

a b s t r a c t

Ceria-based electrolyte materials have great potential in low and intermediate temperature

solid oxide fuel cell applications. In the present study, three types of ceria-based nano-

composite electrolytes (LNK-SDC, LN-SDC and NK-SDC) were synthesized. One-step co-

precipitation method was adopted and different techniques were applied to characterize

the obtained ceria-based nano-composite electrolyte materials. TGA, XRD and SEM were

used to analyze the thermal effect, crystal structure and morphology of the materials.

Cubic fluorite structures have been observed in all composite electrolytes. Furthermore,

the crystallite sizes of the LN-SDC, NK-SDC, LNK-SDC were calculated by Scherrer formula

and found to be in the range 20 nm, 21 nm and 19 nm, respectively. These values

emphasize a good agreement with the SEM results. The ionic conductivities were measured

using EIS (Electrochemical Impedance Spectroscopy) with two-probe method and the

activation energies were also calculated using Arrhenius plot. The maximum power den-

sity was achieved 484 mW/cm2 of LNK-SDC electrolyte at 570 �C using the LiCuZnNi oxide

electrodes.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction conductivity (typically� 1� 10�1 Scm�1), a thermal expansion

Nanostructured high ionic solid electrolytes play an important

role in the performance of solid oxide fuel cells because of

their fast ionic transportation. An ideal solid oxide fuel cell

electrolyte possesses following properties: minimum elec-

tronic conductivity approaches to zero, maximum ionic

gy Technology, Royal Inst

om (R. Raza).

MA, et al., Comparativexide fuel cells (LT-SOFC

2013, Hydrogen Energy P60

co-efficient that is closely matched between the contacting

components and the electrodes, the thermal and chemical

relationship between the contacting electrodes should be

effectively stable, the entire structure should be dense to block

the electron movement, maximizes the Ions conductivity,

Samarium doped Ceria (SDC) is cubic fluorite structure. The

itute of Technology (KTH), S-10044 Stockholm, Sweden. Tel.: þ46

study of the nano-composite electrolytes based on samaria-s), International Journal of Hydrogen Energy (2013), http://

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 3 ) 1e82

fluorite oxides are classically known as oxygen ions conduc-

tors. Also electrolyte is ionic conductor but electronic insu-

lator, the electronically insulator cannot allow to pass the

electrons from the membrane, and minimize the reactant

cross-over and no complex forming properties. The review

from Kreuer provides information about oxide ion electro-

lytes, the conduction mechanisms and the materials used for

this purpose [1].

Yttria Stabilized Zirconia (YSZ) electrolyte material has

been widely used as electrolytes for solid oxide fuel cells. YSZ

provides good chemical and physical stability over a wide

range of operating temperatures without suffering from

electronic conductivity [2]. The oxygen vacancies are created

by doping aliovalent cations such as Y3þ, Eu3þ, Gd3þ, Yb3þ,Er3þ, Dy3þ, Sc3þ, Ca2þ, and Mg2þ in basematerial CeO2 or ZrO2,

this scheme of doping enhances the ionic conductivity

because there is creation of oxygen vacancies in the CeO2 or

ZrO2 [3]. The doped ceria materials are considered to be

auspicious electrolytes for the intermediate temperature solid

oxide fuel cell (IT-SOFCs) [3,4]. The structure of ceria (CeO2)

has fluorite and oxygen vacancies, which are created by

substituting Ce4þ with trivalent rare earth ions or divalent

alkaline earth ions [5].

To decrease the operating temperature of solid oxide fuel

cells (SOFCs), new electrolytematerials are being developed. In

this context, different cationic doped ceria electrolytes, such as

gadolinium-doped ceria (GDC), yttrium-doped ceria (YDC), and

samarium doped-ceria (SDC), have permitted the solid oxide

fuel cell (SOFC) to operate at temperature below 600 �C. Thestrong “side effect” means single-phase ceria-based electro-

lytes have met a number of challenges that limit the applica-

tions due to: (i) low conductivity, 5� 10�3 � 10�2 Scm�1 (600 �C)for GDC and SDC, which is not sufficient for high performance

SOFCs that require 0.1 Scm�1; (ii) partial reduction of Ce4þto

Ce3þ in the operating fuel cell causing e-conduction and a sig-

nificant decrease of the power output [6,7] and (iii) poor me-

chanical properties caused by the reduction process [7] has

been found in single phase electrolyte material which shows

the low ionic conductivity [8]. In order to get rid of this problem,

recently, two-phase nano-composite ceria-based materials

have been discovered which can exhibit improved ionic

transport properties as well as fuel cell performances as

compared to single-phasedopedceria below600 �C.The resultsare comparable to those of YSZ, which is operated at 1000 �C.The two phase nano-composite electrolyte materials are pre-

pared bymixing different cationic-doped ceria (GDC, YDC, and

SDC) with numerous salts, such as chlorides, fluorides, hy-

droxides or carbonates, and these materials have been

demonstrated to be effective for intermediate low temperature

solid oxide fuel cell (ILT-SOFCs) applications [9e15]. The two-

phase ceriaecarbonate composite, which was used as a func-

tional electrolyte in a fuel cell device, exhibited excellent per-

formance over the temperature range of 300e600 �C [13,16e18].

Superionic conductors are a new approach to the design and

development of low temperature solid oxide fuel cells (LT-

SOFCs). These superionic conductors are developed by in-

terfaces and these interfaces have an effective, considerable

role in the performance of two-phase ceria-based carbonate

systems. Furthermore, in two-phase materials systems, these

superionic conductors may act as “superionic highways”.

Please cite this article in press as: Khan MA, et al., Comparativedoped ceria for low temperature solid oxide fuel cells (LT-SOFdx.doi.org/10.1016/j.ijhydene.2013.05.060

These interfaces enhanced the physical, dynamic properties

and the interfacial structures by producing high conductivity

paths for ionic conduction. In this way, high mobile ion con-

centration, long hop aloofness and low activation energies can

be obtained without the structural limitations [22].

The present work is a comparison study of two phase

nano-composite ceria-based electrolytes coated with carbon-

ate. These nano-composite electrolytes were prepared using a

one-step co-precipitation method and characterized by TGA,

XRD, SEM and EIS. These ceria-based nano-composite elec-

trolytes were fabricated in the light of NANOCOFC (Nano-

composites for advanced fuel cell technology) approach

[19,20].

2. Experimental

2.1. Synthesis of nanocomposite ceria based electrolytes

To begin the investigation, nanocomposite (samaria-doped

ceria) SDC:M2CO3 (M ¼ Na, Li, K) electrolytes were prepared

using the solution route with the one-step co-precipitation

method. In the synthesis of the ceriaecarbonate composites,

the following raw chemicals were used: Ce(NO3)3.6H2O cerium

nitrate hexahydrate (SigmaeAldrich, USA), Sm(NO3)3.6H2O

samarium nitrate hexahydrate (SigmaeAldrich, USA), Na2CO3

sodium carbonate (SigmaeAldrich), Li2CO3 lithium carbonate

(SigmaeAldrich, USA) and K2CO3 potassium carbonate (Sig-

maeAldrich,USA). Initially, Ce(NO3)3.6H2OandSm(NO3)3.6H2O,

with the molar ratio of Ce3þ:Sm3þ ¼ 4:1, were dissolved in

1000mlofdeionizedwaterundervigorous stirringat 800 rpmat

80 �C. Alkali carbonates, M2CO3 (M ¼ Na, Li, K), i.e., sodium

carbonate (Na2CO3), lithium carbonate(Li2CO3) and potassium

carbonate (K2CO3) powders, were used as the precipitation

agent in an appropriate amount for the following molar ratios:

LN-SDC (2:1), NK-SDC (2:1) and LNK-SDC (1:1). These precipi-

tation agents were prepared separately in 500 ml deionized

waterat 1000 rpmat100 �C for 30min.Then, thesolutionsof the

precipitationagentswereaddeddropwise intoSDCsolution for

LN-SDC, NK-SDC and LNK-SDC continuously for 1 h at

1100 rpm. Finally, the heating temperature of the stirrer was

controlled at 160 �C. The precipitates were soaked three times

indeionizedwater followedbyvacuumfiltration; agglomerates

of the LN-SDC, NK-SDC, LNK-SDC electrolytes were obtained

and kept in an oven at 120 �C overnight in order to dry. Finally,

these dried powderswere sintered in a digital furnace at 800 �Cfor 5 h to obtain dense nanocomposite electrolytes, and the

sintered powders were ground in a mortar with a pestle to

obtainhomogeneity.Aflowchart for thesolutionroutewith the

one-step co-precipitation method is shown in Fig. 1.

2.2. Preparation of nanocomposite electrode

The slurry method was used to prepare the LiCuZnNi oxide

electrodes. Li2CO3 (Sigma Aldrich, USA), NiCO3$2Ni(OH)2$xH2O

(Sigma Aldrich, USA), CuCO3.Cu(OH)2 (Sigma Aldrich, USA)

and Zn(NO3)2$6H2O. (Sigma Aldrich, USA) were mixed in a

weight ratio of 1: 1.5: 0.3: 0.9 g. Themixture was dissolved in a

1MHNO3 solution at 170 �Cwhile stirring at 1000 rpm for 1 h to

obtain the sol-type slurry. The LiCuZnNi oxide slurry was

study of the nano-composite electrolytes based on samaria-Cs), International Journal of Hydrogen Energy (2013), http://

Fig. 1 e Flow chart for the synthesis of the nanocomposite electrolyte powders using the one-step co-precipitation

technique.

i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e8 3

sintered at 850 �C for 4 h to obtain the ash, and then it was

ground in a mortar with a pestle for homogeneity. Finally, the

nanocomposite electrode of the LiCuZnNi oxides was ac-

quired. Nickel and copper oxides have good catalytic activity;

therefore, they were used as an anode catalyst. The conduc-

tivity of electrode is 4.5 S/cm at 500 �C in H2 atmosphere and in

air is about 2 S/cm. The prepared electrode was then mixed

with the prepared electrolyte at a 50% volumetric ratio.

Consequently, the catalytic property of the anode was

enhanced.

2.3. Fuel cell fabrication

The dry pressing technique was used to prepare the solid

oxide fuel cell three layer pellets. The ceria based nano-

composite electrolytes (LN-SDC, NK-SDC, and LNK-SDC) were

pressed between layers of the composite anode (LiNiZnCu

oxide: electrolytes) and the composite cathode (LiNiZnCu

oxide: electrolytes) using a hydraulic press with a load of

280 kg.cm�2. For a single cell test, a small pellet with a diam-

eter of 13 mm was created, which had an active area of

0.64 cm2. The cell thickness was measured at 1 mm, which

consisted of the composite anode layer (0.57 mm), the elec-

trolyte layer (0.23 mm), and the composite cathode layer

(0.20 mm). The pressed pellets were sintered at 670 �C for

Please cite this article in press as: Khan MA, et al., Comparativedoped ceria for low temperature solid oxide fuel cells (LT-SOFCdx.doi.org/10.1016/j.ijhydene.2013.05.060

50 min. The both external surfaces of (anode and the cathode)

of the pellet were painted with a silver paste to improve the

electrical contacts. Tomeasure the IeV and IeP characteristics

of the fuel cell, a stainless steel sample holder was employed,

as shown in Fig. 2.

2.4. Ionic conductivity measurement

For the ionic conductivity measurements of the ceria-based

nanocomposite electrolytes, pellets that were 13 mm in

diameter with a thickness of 2.8mmwere fabricated using the

dry pressing technique and sintered at 690 �C for 40 min. To

achieve a better electrical contact, silver paste was painted

onto both sides of the pellets. The EIS (electrochemical

impedance spectroscopy) method was used to measure the

ionic conductivity in both air and hydrogen atmospheres by

implementing Auto VERSA STAT 2273 (Princeton Applied

Research, Oak Ridge, TN). The frequency was varied from

0.1 Hz to 1 MHz. The conductivity was measured using the

following formula

s ¼ L=AR (1)

where s is the ionic conductivity, R is the internal resistance, L

is the thickness of the pallets and A is the active area of the

pellets.

study of the nano-composite electrolytes based on samaria-s), International Journal of Hydrogen Energy (2013), http://

Fig. 2 e Testing holder for the full cell measurements.

Fig. 3 e XRD patterns of the LN-SDC, NK-SDC, LNK-SDC

electrolyte.

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 3 ) 1e84

2.5. Fuel cell performance

The fuel cell performance was measured using the electronic

load (PLZ664WA, Kikusui) by providing hydrogen as a fuel at

the anode surface and air at the cathode surface. The open

circuit voltage (OCV) and current data were recorded over the

temperature range 300e650 �C and IeV curves were drawn.

From these results, the power density also was calculated and

current versus power densities (IeP curves) were also drawn.

The H2 gas flow rate was controlled 120 ml/min at 1 atm

pressure.

2.6. Characterizations

Thermogravimetric analysis (TGA) was conducted on a Met-

tler Toledo TGE/SDTA 851e (Greifense, Switzerland), and the

sample was heated from 25 �C to 1000 �C at a rate of

10 �C min�1 in a 70 mL alumina pan. A constant flow of ni-

trogen (50 ml min�1) was used to provide an inert atmosphere

during the pyrolysis. Powder X-ray diffraction (XRD) patterns

were recorded (PANalytical X’Pert Pro MPD, Netherlands)

using CuKa radiation (l ¼ 1.5418 �A). SEM analysis was per-

formed (Hitachi High-Tech, S-3400 energy used between 5 ev

and 15 kV) to observe the morphology.

3. Results and discussion

Fig. 3 the X-rays diffraction patterns of the SDC: M2CO3

(M ¼ Na, Li, K) nanocomposite electrolytes sintered at 800 �Cfor 5 h. The indexing of the pattern indicates that the

Please cite this article in press as: Khan MA, et al., Comparativedoped ceria for low temperature solid oxide fuel cells (LT-SOFdx.doi.org/10.1016/j.ijhydene.2013.05.060

samarium atomswere completely doped into the lattice of the

CeO2 crystals. However, there are no reflections were detected

for carbonates in XRD patterns [19,21e23]. The nano-

composite electrolytes SDC:M2CO3 (M ¼ Na, Li, K) prepared

using the one-step co-precipitation [24] procedure contains

only CeO2 like cubic fluorite structure -(JCPDS 34-0394) [23].

The crystallite sizes D(b) of the LN-SDC, NK-SDC, and LNK-SDC

nanocomposite electrolytes were determined to be 18 nm,

21 nm, and 15 nm, respectively, based on the application of

Scherrer’s equation to the (111) reflection.

Dß ¼ 0:89l=ßCos q (2)

Here, l is wavelength and b is the full width at half

maximum.

SDC:M2CO3 (M¼Na, Li, K) nanocomposite electrolytesmay

be present in the two distinct phases; here, SDC is in the

crystalline phase, but the carbonates are in an amorphous

phase. Based on the XRD results, it may be concluded that

there was no chemical reaction and no new compound

formed between the SDC and the carbonate phases [25].

The structural morphologies of the nanocomposite elec-

trolytes (LN-SDC, NK-SDC, LNK-SDC) were characterized

using SEM, as shown in Fig. 4. The particle sizes were also

measured in order to compare the XRD calculated size. It can

be seen that the powders of a) LN-SDC, b) NK-SDC, c) LNK-

SDC, consisted of agglomerated nanocrystallites and do not

have the same morphologies at the low resolution. The

morphology of the LN-SDC has a tight structure, whereas NK-

SDC and LNK-SDC have loose structures. During the compo-

sitional process, it is assumed that the molten carbonates,

which might cover the surface of the SDC particles, could

prevent the successful agglomeration of the SDC particles.

When these composites samples were directly removed from

the oven, there were a fast freezing process that occurred in

the carbonates, and these frozen carbonates could create a

coreeshell on the nano-SDC particles [26]. This coreeshell

enhanced the properties of the electrolyte. Therefore, high-

quality, dispersed composite powders were obtained. The

study of the nano-composite electrolytes based on samaria-Cs), International Journal of Hydrogen Energy (2013), http://

Fig. 4 e SEM analyses of the microstructures of the ceria carbonate electrolytes. I. (a) LN-SDC, (b) NK-SDC, (c) LNK-SDC at low

resolutions. II. (d) LN-SDC, (e) NK-SDC, (f) LNK-SDC at high resolutions.

i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e8 5

images of the nanocomposites at the high resolution exposed

in the figures LN-SDC (d), NK-SDC (e), LNK-SDC (f) have

average particle sizes of 47 nm, 51 nm, and 42 nm, respec-

tively. The prepared electrolytes materials are 90% dense

which was measured with He-pycnometry (AccuPyc 1340

Micromeritics, USA).

The TGA curves of the SDC: M2CO3 (M ¼ Na, Li, K) nano-

composite electrolytes are presented in Fig. 5(a). The weight

losses are categorized into two regions: (I) room temperature

to 290 �C and (II) 290 �Ce720 �C. Initially, there are slight

weight losses observed from the starting temperature to

290 �C, which is due to the evaporation of absorbed water. At

temperatures greater than 290 �C, there are no weight losses

observed during heating until 720 �C, which suggests that

there is no decomposition reaction occurring from 290 �C to

720 �C. At temperatures greater than 720 �C, the TGA curves

reveal that there are weight losses occurring, which indicates

that the carbonate salts (M2CO3 (M ¼ Na, Li, K)) begin to

volatilize at greater than 720 �C [17,20,27]. The TGA results

confirm that the carbonate salts (M2CO3 (M¼Na, Li, K)) exist in

the SDC: M2CO3 (M ¼ Na, Li, K) nanocomposite electrolytes.

The binary carbonates M2CO3 (M ¼ Na, Li, K) can be attributed

to start the melting at temperature 720 �C which is related to

melt of binary carbonates M2CO3 (M ¼ Na, Li, K) eutectic [17],

there is no partial decomposition of the carbonates in the

composite electrolytes. The lower binary carbonates M2CO3

(M ¼ Na, Li, K) contents were dispersed among the SDC par-

ticles and no eutectic was formed after heat-treated at 800 �Cfor 5 h, but at higher carbonate content, the binary carbonates

Please cite this article in press as: Khan MA, et al., Comparativedoped ceria for low temperature solid oxide fuel cells (LT-SOFCdx.doi.org/10.1016/j.ijhydene.2013.05.060

trended to aggregate and form eutectic composition. The TGA

curves indicate that there is neither a chemical reaction nor

any intermediate compound between SDC and binary car-

bonates M2CO3 (M ¼ Na, Li, K). The existence of carbonates at

elevated temperature of 720 �C emphasizes the presence of

carbonate shell on SDC core during the operating temperature

of 300 �Ce650 �C.In the field of fuel cell, electrochemical impedance spec-

troscopy (EIS) is an ideal tool to determine the conductivities

of all type of solid electrolytes and widely used in fuel cell

community. Fig. 5(b) shows the ionic conductivities of SDC:

M2CO3 (M ¼ Na, Li, K) nanocomposite electrolytes were

measured at both air and hydrogen atmosphere. The result of

measurements reveals that conductivities in air atmosphere

are greater than that of hydrogen atmosphere [28]. It can be

seen that there is a sharply jump around 300 �C, where it could

be related to glass transition temperature [22]. The behaviors

of conductivities in the (air and hydrogen) atmospheres were

increased with increased the temperature. At higher temper-

atures, its conductivities increased due to all ions from

the constituent phases (Liþ, Naþ, Kþ, CO32� and O2� ions)

contribute to the conductivities. At lower temperatures, the

defects in the interface phases are not highlymobility because

of the activation barrier. The ionic conduction in composite

electrolytes is considered to be responsible for the enhance-

ment of the conductivity. The ions mechanism of the com-

posite electrolyte is depicted in a schematic Fig. 6. In the

ceriaecarbonate two-phase systems the superionic conduc-

tion occurs at the interfacial regions between the two

study of the nano-composite electrolytes based on samaria-s), International Journal of Hydrogen Energy (2013), http://

0 200 400 600 800 100090

92

94

96

98

100 region(II)

LNK-SDC

LN-SDC

Temperature

Weig

ht(%

)

NK-SDC

region(I)

100 200 300 400 500 600 7000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10 LNK-SDC in air atmosphere LN-SDC in air atmosphere NK-SDC in air atmosphere LNK-SDC in H atmosphere LN-SDC in H atmosphere NK-SDC in H atmosphere

Co

nd

uctivity (S

cm

-1

)

Temperatureo

C

(a)

(b)

Fig. 5 e (a) TGA analysis of ceria carbonate nanocomposites

electrolytes (b) Temperature dependence conductivity of

ceria carbonates nanocomposites electrolytes in air and

hydrogen atmospheres.

Fig. 6 e Schematic diagram for ions transportation in nano-

composite electrolyte.

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 3 ) 1e86

constituent phases. The conductivity is then strongly subju-

gated by the interactions effect between the constituent

phases. Therefore, the two-phase co-existence can create the

interfacial effects, which cause the superionic conduction.

The protons ions or positive ions (Hþ) passes through the in-

terfaces of the particles and oxygen ions (O2�) are passing

through the host particles (doped ceria). The second phases of

alkali elements (Li, Na, K) are coated on the SDC particles

create the core shell, these core shells are connected each

other which provides fast highways for the transportation of

ions (Liþ, Naþ, Kþ, CO32�, Hþ and O2�)from the cathode to the

anode [28]. Thus, the conductivity results demonstrate that

the SDC: M2CO3 (M ¼Na, Li, K) nanocomposite electrolytes are

dual-phase proton/oxygen conductors [28]. In the present

work, the ionic conductivities of the LNK-SDC, LN-SDC, and

NK-SDC nanocomposite electrolytes in an air atmosphere are

0.098 Scm�1, 0.093 Scm�1, 0.090 Scm�1 and at hydrogen at-

mosphere are 0.033 Scm�1, 0.027 Scm�1, 0.024 Scm�1 respec-

tively. These ionic conductivities (s) of the SDC: M2CO3

Please cite this article in press as: Khan MA, et al., Comparativedoped ceria for low temperature solid oxide fuel cells (LT-SOFdx.doi.org/10.1016/j.ijhydene.2013.05.060

(M ¼ Na, Li, K) nanocomposite electrolytes were calculated at

different temperatures using AC impedance spectroscopy

measurements. The Arrhenius equation was used to analyze

the measured conductivities,

s ¼ s+expð � Ea=RTÞ (3)

Where s� is a pre-exponential factor and is constant for charge

density carriers, R is the ideal gas constant, T is the absolute

temperature, and Ea is the activation energy for the ionic

migration. These conductivities depend on the mixing of the

particles of carbonates, M2CO3 (M ¼ Na, Li, K) coated on the

SDC.

Arrhenius plots were drawn from the conductivity data by

linear fitting technique to calculate the activation energies (Ea)

of the LN-SDC, NK-SDC, and LNK-SDC nanocomposite elec-

trolytes at air and hydrogen in the temperature range

150 �Ce700 �C and results are shown in Fig. 7(a, f).

The activation energies (Ea) of the nanocomposite electro-

lyte materials were calculated using the following equation,

s ¼ A=T expð � Ea=kTÞ (4)

where s is related to the ionic conductivity of the nano-

composite electrolytematerials,A is a pre-exponential factor, k

is Boltzmann’s constant, T is the absolute temperature in

Kelvin and Ea is the activation energy of the nanocomposite

electrolyte materials. However the linear fitting curves are

shown in Fig. 7(aef). A chemical reaction starts on the basis of

the activation energy. The calculated values of the activation

energies for the LNK-SDC, LN-SDC, and NK-SDC nano-

composite electrolytes in the air atmosphere are 0.59 eV,

0.48 eV and 0.32 eV, and in the hydrogen atmosphere are

0.17 eV, 0.16 eV and 0.14 eV, respectively. The LNK-SDC, LN-

SDC, and NK-SDC nanocomposite electrolytes conduct the

electricity in both air and hydrogen atmospheres. The above

activation energy results predict that the LNK-SDC, LN-SDC

and NK-SDC nanocomposite electrolytes have dual character-

istics, and they conduct electricity through the positive ions.

study of the nano-composite electrolytes based on samaria-Cs), International Journal of Hydrogen Energy (2013), http://

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

1

2

3

4

5

6

1000/T(K )

ln

T

(S

/c

m. K

)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1000/T(K )

ln

T (

S/c

m. K

)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

1

2

3

4

5

6

7

1000/T(K )

ln

T

(S

/cm

. K

)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1000/T(K )

ln

T (S

/c

m. K

)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

0

1

2

3

4

5

ln

T

(S

/c

m. K

)

1000/T(K )

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

ln

T (S

/cm

. K

)

1000/T(K )

σ

σ

σσ

σσ

a

d e

b c

f

Fig. 7 e Arrhenius plot and activation energies of conductivities under air and hydrogen atmosphere.

i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e8 7

The performances of the fuel cells were measured and are

shown in Fig. 8. Power densities of 286mW/cm2, 337mW/cm2,

and 484 mW/cm2 and open circuit voltages (OCV) of 0.97 V,

0.99 V, and 1.02 Vweremeasured at 570BC for the NK-SDC, LN-

SDC and LNK-SDC electrolytes, respectively. The fuel cell

using the LNK-SDC nanocomposite electrolyte exhibited the

maximum power density of 484 mW/cm2 at 570 �C. In order to

find the stability of the fuel cell, the performance (power

density) based on LNK-SDC nanocomposite electrolyte was

recorded for 14 hwith a regular interval of an hour in two days

(8 h/day). The results of measurement were shown in

Fig. 9.This performancemay be achieved due to an inter-facial

0 100 200 300 400 500 600 700 800 900 1000 1100

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0

100

200

300

400

500

600

Vo

ltag

e (V

)

Current Density (mA/cm2

)

Po

wer d

en

sity

( m

W/cm

2

)

NK-SDC LN-SDC LNK-SDC

Fig. 8 e Fuel cell performance at 570 �C with different

electrolytes.

Please cite this article in press as: Khan MA, et al., Comparativedoped ceria for low temperature solid oxide fuel cells (LT-SOFCdx.doi.org/10.1016/j.ijhydene.2013.05.060

superionic conduction mechanism in the two-phase com-

posites, which provides the path for the transportation of

ions, protons and carbonates. There is very small degradation

in performance was observed in first 2 h, may be due to

change in morphology at cathode/electrolyte interface. It was

also previously reported by Huang and Fan et al. [28,29] that

the addition of ZnO has a strong positive effect on the

reduction of NiO solubility in carbonate melts. Due to lack of

facilities, we could not measure for long term stability but still

for short term results are encouraging. In future, long term

stability will be performed.

0 2 4 6 8 10 12 14 16

0

200

400

600

800

Po

wer d

en

sity ( m

W/cm

2

)

Time (hrs)

Power density @ 570 oC

Fig. 9 e Short term stability of the fuel cell with LNK-SDC

nanocomposite electrolyte.

study of the nano-composite electrolytes based on samaria-s), International Journal of Hydrogen Energy (2013), http://

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 3 ) 1e88

4. Conclusions

The one-step co-precipitation technique has a number of sci-

entific advantages, such as simple preparationmodus operandi

for enhanced quality control; better homogeneity at the

nanoscale; improve and enhance the ionic conductive proper-

ties of ceriaecarbonate electrolyte and cause superionic con-

duction at low temperatures. The as-prepared electrolytes

exhibited a glass transition 300 �C. The XRD indexing empha-

sizes that all electrolytes execute cubic fluorite structure. Since

the as prepared ceria based nanocomposite electrolytes are

two-phase materials. The first phase is cubic crystallite phase

and second phase of alkali elements (Li, Na, K)were found to be

amorphous. The Arrhenius plot was obtained using linear

fitting technique from the electrochemical impedance spec-

troscopy data. The LNK-SDC nanocomposite electrolyte ex-

hibits 0.098 Scm�1 ionic conductivity in air atmosphere, which

is greater than that of others LN-SDC and NK-SDC electrolytes.

The low activation energies of the nanocomposite electrolytes

(LNK-SDC, LN-SDC, NK-SDC) in the air atmosphere were found

to be 0.59 eV, 0.48 eV and 0.32 eV respectively, which indicates

the fast chemical reaction occurs after supplying the fuel.

Power peak densities of 286 mW/cm2, 337 mW/cm2, and

484 mW/cm2 were achieved at 570 BC for a single cell based

electrolyte (NK-SDC, LN-SDC and LNK-SDC). It has been found

that the contribution of ternary carbonated electrolyte LNK-

SDC is a good electrolyte that has acquired the high power

density of 484mW/cm2 at 570 �C than that of YSZ electrolyte at

1000BC this all has been achieved by applying the NANOCOFC

approach and it may also be concluded that the NANOCOFC

approach provides a potential electrolytematerial for LTSOFCs.

Acknowledgments

Higher Education Commission, Pakistan (HEC) is highly

acknowledged for financially support under International

Research Support Initiative Program (ISRIP) to complete this

work and theDepartment of EnergyTechnology, Royal Institute

of Technology,KTH,Sweden isalso acknowledged to provideall

facilitations to achieve the results for the completion of work.

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study of the nano-composite electrolytes based on samaria-Cs), International Journal of Hydrogen Energy (2013), http://