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Potential low-temperature application and hybrid-ionicconducting property of ceria-carbonate composite electrolytesfor solid oxide fuel cells

Liangdong Fan a,b, Chengyang Wang a,*, Mingming Chen a, Jing Di a, Jiaming Zheng a,Bin Zhu b

aKey Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology,

Tianjin University, Tianjin 300072, PR ChinabDepartment of Energy Technology, Royal Institute of Technology, Stockholm S-100 44, Sweden

a r t i c l e i n f o

Article history:

Received 14 February 2011

Received in revised form

28 April 2011

Accepted 10 May 2011

Available online 12 June 2011

Keywords:

Solid oxide fuel cells

Composite electrolyte

Compatibility

Electrochemical impedance

spectroscopy

Hybrid-ionic conduction

* Corresponding author. Tel./fax: þ86 22 278E-mail address: cywang@tju.edu.cn (C. W

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.05.055

a b s t r a c t

Ceria-carbonate composite materials have been widely investigated as candidate electro-

lytes for solid oxide fuel cells operated at 300e600 �C. However, fundamental studies on the

composite electrolytes are still in the early stages and intensive research is demanded to

advance their applications. In this study, the crystallite structure, microstructure, chemical

activity, thermal expansion behavior and electrochemical properties of the samaria doped

ceria-carbonate (SCC) composite have been investigated. Single cells using the SCC

composite electrolyte and Ni-based electrodes were assembled and their electrochemical

performances were studied. The SCC composite electrolyte exhibits good chemical

compatibility and thermal-matching with Ni-based electrodes. Peak power density up to

916 mW cm�2 was achieved at 550 �C, which was attributed to high electrochemical activity

of both electrolyte and electrode materials. A stable discharge plateau was obtained under

a current density of 1.5 A cm�2 at 550 �C for 120 min. In addition, the ionic conducting

property of the SCC composite electrolyte was investigated using electrochemical imped-

ance spectroscopy technique. It was found that the hybrid-ionic conduction improves

the total ionic conductivity and fuel cell performance. These results highlight potential

low-temperature application of ceria-carbonate composite electrolytes for solid oxide fuel

cells.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction high integration costs, materials degradation and limited

Solid oxide fuel cells (SOFCs) are potential next-generation

energy conversion devices and have reached a proper level of

maturity for their high thermodynamic efficiency and fuel

flexibility [1]. Conventional SOFCs operating at high

temperature (800e1000 �C) enable adequate ionic conductivity

of the electrolyte and superior catalytic activity of electrode

materials; on the other hand it brings many problems, such as

90481.ang).2011, Hydrogen Energy P

applications [2]. Therefore, the development of SOFCs

operating at intermediate temperatures (600e800 �C), or evenin a low-temperature range (300e600 �C), is urgent since they

can effectively overcome these problems.

As the heart of fuel cells, electrolytes play an essential role

as concerns the overall cell performance. In the past decades,

thedevelopmentofadvancedelectrolytematerialshasbeenan

active research topic. Currently, electrolytes in SOFCs usually

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

i n t e rn 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 3 6 ( 2 0 1 1 ) 9 9 8 7e9 9 9 39988

consist of single-phase materials, such as stabilized zirconia,

doped ceria oxides [3] and doped LaGaO3 perovskite [4]. Among

these candidates, doped ceria oxides have been greatly

investigated for their low activation energy for oxygen ion

diffusion and well chemically compatible with electrode

materials. Nevertheless, there are still some inherent

drawbacks for ceria-based single-phase electrolyte, such as

the inadequate ionic conductivity at low temperatures and

electronic conduction under low oxygen partial pressure [5],

which remain as obstacles to their widespread applications.

As a result, developing novel electrolyte materials with high

ionic conductivity and low electronic conductivity for low-

temperature utilization is still a challenging issue.

Recently, two-phase composites consisting of doped ceria

oxides and various carbonates have been developed and

extensively studied as functional electrolytes for low-

temperature operating SOFCs because of their impressive

electrical conductivities and unique hybrid-ionic conducting

properties [6e13]. SOFCs using ceria-carbonate composite

electrolytes and Ni-based electrodes (Ni for the anode and

lithiated NiO as cathode) have demonstrated exceptional fuel

cell performances below 600 �C [7,9]. Combined with the low

cost of raw materials and reduced fuel cell manufacturing

processes, ceria-carbonate composites have shown promising

applications in low-temperature SOFCs.

Although the previous performance results are very

encouraging, basic research on composite electrolytes is still

needed. To approach fuel cell performance of commercial

interest, many issues need to be clarified, such as the interac-

tions and thermal expansion properties between cell compo-

nents, fuel cell durability and the possibility for performance

improvement. Besides, numerous results from previous

studies on ionic conducting property of composite electrolytes

in fuel cells are still controversial [7,11,13,14]. For example,

Di et al. [11] had adopted a ceria-carbonate composite as

electrolyte for low-temperature SOFCs and reported that

the composite electrolyte was a mixed proton and oxygen

ion conductor at fuel cell condition and the oxygen ionic

conductivity contributed to the major part of the whole

conductivity. However, Huang et al. [14] found that the ceria-

based composite electrolyte was a proton conductor at the

same operating condition. Therefore, the objective of this

work is to study the chemical and thermal compatibility

between SCC composite electrolytes and Ni-based electrodes

and investigate their operating stability in fuel cells in order to

evaluate their feasibility for low-temperature applications.

The hybrid-ionic conducting property of the composite

electrolyte is also analyzed using the electrochemical

impedance spectroscopy (EIS) technique.

2. Experimental

2.1. Materials preparation

SCC electrolytes with a composition of 80 wt.% Ce0.8S-

m0.2O1.9e20 wt.% (Li2/Na2)CO3 were prepared according to our

previous work [11]. The NiO was synthesized by the citratee

nitrate combustion method. The preparation procedure is

similar to that of composite electrolyte precursor-Ce0.8Sm0.2O1.9

(SDC) [11]. To get a pure nickel oxide, the powders were

calcined at 600 �C for 1 h. The cathode materials-lithiated NiO

powders were prepared by solid-state reaction at 700 �C for 3 h

with an equal molar ratio of LiOH (Guangfu Fine Chemical

Research Institute, China) and as obtained NiO powders.

2.2. Cell construction and performance test

Single cells were fabricated by a one-step dry-pressing and co-

firing technique using the SCC composite as electrolyte. The

electrodes were mixture of catalysts (NiO for anode and

lithiated NiO for cathode) and composite electrolyte at a mass

ratio of 55:45. The anode, electrolyte and cathode powders

were successively loaded in a steel die, and were dry-pressed

in one-step under 300 MPa to form sandwich green pellets.

The green pellet was then sintered in air at 600 �C for 0.5 h.

Finally, both surfaces of the sintered pellet were painted with

silver paste (DAD-87, Shanghai Research Institute of Synthetic

Resins, China) as current collector and sintered in air at 600 �Cfor 0.5 h. The final thicknesses of the anode, electrolyte and

cathode are 500 mm, 250 mm and 200 mm, respectively.

The currentevoltage data were recorded by a fuel cell test

instrument (SM-102, SanMu Corp., China) between 600 �C and

450 �C at 50 �C intervals, using humidified H2 (3 vol.% H2O) as

fuel and flowed air as oxidant. The gas flow rates were

controlled in the range of 80e100 ml min�1. Prior to the elec-

trochemical performance testing, the composite anode was

reduced to NieSCC by flowingH2 at 600 �C for 0.5 h. Thereafter,

air was allowed to pass through the surface of the cathode.

The active area of the single cell is 0.785 cm2. Short-term

stability test was examined under a constant current density

of 1.5 A cm�2 at 550 �C. EIS was recorded under open circuit

voltage (OCV) or discharging conditions during performance

measurements.

2.3. Other characterizations

Crystal structures of the prepared powders were identified by

X-ray diffraction (XRD) at room temperaturewith aD/max 2500

v/pc instrument (Rigaku Corp., Japan) using Cu Ka radiation

(l ¼ 1.5406 A). The powder morphologies and single cell

microstructures were characterized by scanning electron

microscopy (SEM, JSM-6700F, JEOL Ltd., Japan). For electrical

conductivity measurements, SDC and SCC composite pellets

with 13mmindiameterwerepreparedbyuniaxiallypressingof

the powders under 300 MPa and sintering at 1350 �C for 2 h and

650 �Cfor0.5h, respectively.Then,bothsidesofpreparedpellets

were covered with silver electrodes to form symmetrical cells.

Thermal expansion characteristics of the cylindrical cell

components (all sintered at 680 �C for 2 h) weremeasured from

100 �C to 650 �C using a dilatometer (402C, NETZSCH DIL,

Germany) at a ramp rate of 5 �Cmin�1 in air. EISwas performed

on an electrochemical workstation (PARSTAT 2273, Princeton

Applied Research, USA) over a frequency range from 100 kHz to

0.01 Hz with excitation potentials of 10 mV. The inductance

contributions, caused by the stainless steel clamp and the

instrument leads, to ohmic resistances were subtracted by

a blank cell EIS test atdifferent temperatures before theEISdata

were normalized by the thickness and active area of the pellets.

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 3 6 ( 2 0 1 1 ) 9 9 8 7e9 9 9 3 9989

3. Results and discussion

3.1. Powder morphologies and electrolyte microstructurebefore test

SEM images of SDCandSCCpowders are shown inFig. 1a andb.

It can be seen that SDC powder before pulverization presents

a highly foam-like microstructure with severe agglomeration,

which is the typical structure for materials prepared by the

combustion synthesis method. From Fig. 1b, we can see that

SDC and carbonate are well dispersed and the SDC is

homogeneously covered by carbonate. As shown in Fig. 1c,

sub-micrometer particles with good size distribution are

observed for lithiated NiO prepared by solid-state reaction.

The microstructure of the compacted composite electrolyte

before test is also displayed in Fig. 1d. Plenty of micro-pores

still exist after sintering at 600 �C, which is visibly different

from that of SDC-based SOFCs, in which 95% of relative

density is required to reduce gas leakage and ensure ion

conductivity. SCC composite electrolytes cannot be fully

dense because of the solidifying of carbonate around melting

point during the cooling cycle and the relatively lower

sintering temperature. However, they still can be used as gas-

tight electrolytes for SOFCs when the operating temperature

reaches a critical point.

Fig. 1 e SEM images of the powders: (a) SDC without pulverizatio

the compacted SCC electrolyte layer before the stability test.

3.2. Phase structures and chemical compatibility

As shown in Fig. 2, the XRD pattern of the SCC powder exhibits

only the peaks of SDC phase (JCPDS No. 75-0158) and no other

peaks related to carbonate can be identified, indicating that

the carbonate in the SCC composite is amorphous. To study

the chemical compatibility between lithiated NiO and SCC, the

mixed powders were sintered at 650 �C for 10 h. All peaks in

the mixture XRD pattern correspond to lithiated NiO and SCC,

implying there are no reactions and/or inter-diffusion of

elements between thesematerials. It reveals that lithiated NiO

is chemically compatible with the SCC composite electrolyte.

3.3. Fuel cell performances

Fig. 3 shows the electrochemical performance of SCC

composite electrolyte-based SOFCs at different temperatures.

As can be seen from Fig. 3a, The OCVs are 0.97, 1.01, 1.03 and

1.04 V at 600, 550, 500 and 450 �C, respectively. The high OCV

values suggest that SCC composite electrolyte layer is

acceptable gas-tightness and exhibits negligible electronic

conduction. A maximum power density of 916 mW cm�2 is

achieved at 550 �C. This desirable performance highlights the

capability of SCC composite as electrolyte for low-

temperature SOFCs. Considering the thickness of the

n; (b) SCC; (c) lithiated NiO and (d) the cross-section view of

Fig. 2 e XRD patterns of lithiated NiO, SCC and composite

cathode (mixture of lithiated NiO and SCC) sintered at

650 �C for 10 h.

Fig. 3 e Temperature dependence of the fuel cell

electrochemical performance: (a) IeV, IeP characteristics

and (b) electrochemical impedance spectra under OCV

conditions.

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electrolyte, w250 mm, one would expect higher power density

when a thin-film electrolyte layer is used.

Apart from showing OCVs and power densities, electro-

chemical impedance spectroscopy analysis is also very useful

for the understanding of the fuel cell performance. More

importantly, it enables us to investigate different contribu-

tions of each cell component to the total impedance. EIS

obtained under OCV condition at different temperatures are

presented in Fig. 3b. All spectra contain inductance effects,

a depressed arc and a tail. The inductance effect is assigned

to the stainless steel clamp and the instrument leads [15].

The high-frequency intercept on the real axis is estimated as

the ohmic resistance (Ro) and the low-frequency intercept

corresponds to the total cell resistance (Rt), while the

difference between the two values is the electrode

polarization resistance (Rp) [15]. According to Steele [16], the

target values of Ro and Rt for high cell performances are

0.15 U cm2 and 0.5 U cm2, respectively. As shown in Fig. 3b,

Ro is 0.15 U cm2 and Rt is 0.34 U cm2 at 450 �C, which are

lower than those recommended values. In addition, both Ro

and Rt decrease with the increase of operating temperature.

Especially, Rp values increase slightly as the operating

temperatures decrease, indicating that Ni-based electrodes

show adequate electro-catalytic activities for hydrogen

oxidation and oxygen reduction reactions between 450 �Cand 600 �C. Consequently, the high cell performance is also

attributed to the well-matched Ni-based electrodes.

3.4. Stability test

The cell operating stability test was investigated under

a constant current density of 1.5 A cm�2 at 550 �C. The time

dependence of the cell voltage curve is plotted in Fig. 4. The

voltage decreases at the initial stage and increases gradually

to reach a plateau of 0.5 V, and then discharges continuously

for a period up to total 120 min. The initial voltage

diminishment is caused by the electrode polarization, while

the improvement afterward should be the result of the

activation of the electrodes and the electrodes/electrolyte

interfaces [17]. The stable performance at a high current

density condition suggests that there are reliable bonding of

electrode and electrolyte interfaces and high electrochemical

activities of cell components. Considering the possibly

carbonate volatilization problem, long-term stability tests will

be carried out in our continued work.

3.5. Microstructure features of single cell after stabilitytest

Fig. 5aec shows the fractured cross-section images of the single

cell after stability test. The electrolyte bonds well to the Ni-

based electrodes and their interfaces are not clear. The

activated electrolyte/electrode interfaces after the stability

test significantly reduce the electrode polarization resistance,

as demonstrated in Fig. 3b. In addition, the fractured cross-

section of SCC composite electrolyte after the stability test is

also shown in the Fig. 5d, which is visibly different from that

before the test (see Fig. 1d); only few isolated pinholes can be

observed. This specific microstructure effectively reduces the

gas leakage, which could give rise to high OCVs. Furthermore,

the activated microstructure of the SCC composite electrolyte

may create more consecutive paths for ionic conduction,

Fig. 4 e Time dependence of the fuel cell voltage at

a constant current density of 1.5 A cmL2 and 550 �C.

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 3 6 ( 2 0 1 1 ) 9 9 8 7e9 9 9 3 9991

which can result in a reduction of ohmic loss and improvement

of current (power) output.

3.6. Thermal expansion characteristics

It is generally recognized that having a comparable thermal

expansion coefficient (TEC) between electrodes and electrolyte

is an extremely critical factor for reliable SOFCs. Dilatometric

curves for the three components (anode, SCC composite elec-

trolyte and cathode) from100 �C to 650 �Care plotted in Fig. 6. It

can be seen that the thermal expansion behaviors of all

samples are relatively abnormal. All of them are nonlinear

with an inflection around 450 and 600 �C. The appearance of

these inflection points, occurred around 450 �C, especially for

Fig. 5 e SEM images of fuel cell after the stability test: (a) cross-

electrodes interfaces and (d) the fractured cross-section of SCC

the SCC composite electrolyte, can be attributed to the phase

transition of carbonate from solid state to softening phase.

The shrinking above 600 �C is the result of the melting of the

remaining Li2CO3 [18] and the related powder sintering

behaviors at high temperature. In general, Ni is the “state-of-

the-art” materials for SOFC anode catalysts; a composite

anode containing Ni and amount of electrolyte material is

universally employed to balance the ionic and electronic

conductivity and adjust the TEC difference between anode

and electrolyte. Much attention should therefore be focused

on the electrolyte/cathode interfacial thermal expansion

difference. The composite cathode shows nearly the same

thermal expansion behavior with the SCC composite

electrolyte below 450 �C. However, the different thermal

shrinkage around 450 �C possibly becomes a factor affecting

the cell stability. Certainly, it is not expected to lead to any

cracking or delamination problems. On the contrary, the cell

components bond well to each other, as shown in Fig. 5aec.

There may be two reasons. One is their small TEC difference,

which is much less than that between Ba0.4Sr0.6Co0.8Fe0.2O3�d

(BSCF) cathode and SDC electrolyte for intermediate-

temperature SOFCs, where the TEC of the BSCF cathode is

almost twice of the SDC electrolyte [19]. The other is that the

existence of molten carbonate at interface acts as a lubricant

to minimize the interfacial thermal stress.

3.7. Hybrid-ionic conducting property

Fig. 7a shows electrical conductivity of the SCC composite

electrolyte at different temperatures, and the conductivity of

SDC is also included for comparison. AC conductivities of the

SCC composite electrolyte are much higher than those of SDC

section view of the whole fuel cell, (b) and (c) electrolyte/

composite electrolyte.

Fig. 6 e Dilatometric curves of the anode, SCC and cathode

from 100 �C to 650 �C in air.

Fig. 7 e (a) Electrical conductivities of SCC composites and

SDC at various temperatures; and (b) Nyquist plots of

single cell with Ni-based electrodes under OCV and

discharging modes (0.61 V/1.0 A) in a H2/air atmosphere at

500 �C. The inset: Nyquist plot of single cell with

symmetrical Ag electrodes in air at 500 �C.

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over the operating temperature range of 450e600 �C. Since the

carbonate softens or becomes molten at this temperature

range, high AC conductivities of the SCC composite electrolyte

are largely attributed to molten cations (Liþ, Naþ) and anion

(CO32�) mobility. However, only proton and oxygen ionic

transport in composite electrolyte can contribute to the cell

performance in a H2/air atmosphere [20]. On the other hand,

the direct current (DC) conductivity, derived from the slope of

current densityevoltage characteristics, can in situ reflect the

sources ionic contribution to the fuel cell performance [20]. As

shown in Fig. 7a, the DC conductivities of the SCC composite

electrolyte are about one order of magnitude higher than

those of SDC between 450 and 600 �C. Therefore, the excellent

cell performance is attributed to the high ionic conductivity of

the SCC composite electrolyte. Besides, DC conductivities are

slightly lower than AC conductivities at 550 �C and 600 �C, butthey are higher at 450 �C and 500 �C. In previous work, many

studies have proved that interfacial conduction in a composite

plays an important role on the ionic conduction [21,22].

Oxygen ions can transfer through SDC grain bulk and the

interface between SDC and carbonate, and protons can

transfer along the interface and by a intermediate proton

carrier-HCO3� in molten carbonate formed by coupling Hþ

with CO32� [23]. At high temperature, the oxygen ion transport

will contribute the majority of the total ion conductivity.

However, the lower activation energy of proton transfer may

enhance total conductivity at low temperature. Besides, the

proton conduction facilitates electrode reactions as well as the

interfacial kinetics [24]. Thus, the higher DC conductivity of

SCC composite electrolyte compared to AC conductivity at

450 �C may be attributed to the hybrid-ionic conduction in H2/

air atmosphere.

Although the hybrid-ionic conduction in SCC composite

electrolytes is considered to be responsible for the enhance-

ment of the total conductivity, especially at low temperatures,

experimental evidences are still scarce [11,14]. In this report,we

used the EIS technique to investigate this novel function of SCC

composite electrolytes. Typical nyquist plots of fuel cell recor-

dedat500 �Care shown inFig. 7b. For comparison, EISof theSCC

composite electrolyte in air with symmetrical Ag electrodes

under OCV condition and 500 �C is also given in the inserted

figure. It can be seen from Fig. 7b that the specific ohmic

resistance (Ro) of SCC composite electrolyte in a H2/air

atmosphere (0.11 U cm2) is much lower than that in air

(0.22 U cm2), which demonstrates that the introduction of

proton conduction significantly reduces the ionic transport

resistance, hence enhances the total ionic conductivity.

Furthermore, the Ro value obtained under OCV condition is

larger than that under discharging mode (0.07 U cm2). In fact,

for a pure single ionic conductor, e.g. YSZ, the ohmic

resistances are independent with the applied current densities

when the operating temperature remains constant [25,26].

While the Ro values in the SCC composite electrolyte change

with the applied current densities. The difference becomes

more significant at 450 �C, 0.15 U cm2 under OCV condition

and 0.09 U cm2 at discharging mode (0.48 V/1.0 A). As

mentioned above, the Ro value mainly corresponds to the

electrolytic resistance (ionic transfer resistance). In view of the

different activation energies and ionic conduction behaviors

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 3 6 ( 2 0 1 1 ) 9 9 8 7e9 9 9 3 9993

(interfaceorbulkgrain)betweenoxygen ionandproton transfer

in composite electrolyte, one may extrapolate that the hybrid-

ionic conduction can alter the ohmic resistances when

a single cell works under different conditions at a given

temperature. The EIS measurement and analysis may provide

a new method to identify the hybrid-ionic conduction, i.e. Hþ

and O2� co-conduction, of the SCC composite electrolyte in

fuel cell condition. The hybrid-ionic conduction in the

composite electrolyte system is an intriguing and important

topic for further study; and more efforts are needed to study

the detailed ionic conduction process and the transfer number

of each source ion (Hþ/O2�) under a H2/air atmosphere.

4. Conclusions

SCC composite electrolyte shows chemical compatibility and

thermal-matching with Ni-based electrodes. A maximum

power density of 916 mW cm�2 was achieved at 550 �C. Bothlow ohmic resistances and small electrode polarization

resistances account for the excellent fuel cell performances.

Single cell kept stable at a constant discharging current

density of 1.5 A cm�2 for 120 min. Besides, electrochemical

impedance spectroscopy measurements show that the SCC

composite exhibits mixed Hþ/O2� conduction under a H2/air

atmosphere and the hybrid-ionic conduction improves the

effective ionic conductivity of composite electrolyte. From

these results, it can be concluded that the NieSCC/SCC/lithi-

ated NiOeSCC is a promising functional system for low-

temperature SOFCs applications. Future works will concen-

trate on investigation of long-term stability, interfacial ionic

conducting properties and the Hþ/O2� ion transfer number of

composite electrolyte under fuel cell conditions.

Acknowledgments

Funding from VINNOVA (Swedish Agency for Innovation

Systems) is highly acknowledged. The work has been also

partial supported by the Program of Introducing Talents to the

University Disciplines under file number B06006. Fan appre-

ciates CSC (China Scholarship Council) for financial support.

We also thank prof. Lars Kristoferson and Mr. Richard Ljung-

berg for editing the English.

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