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 ) 1 3 0 5 9e1 3 0 6 6

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Cathode materials for La0.995Ca0.005NbO4 proton ceramicelectrolyte

K.V. Kravchyk a,*, E. Quarez a, C. Solıs b, J.M. Serra b, O. Joubert a

a Institut des Mateteriaux Jean Rouxel (IMN), Universite de Nantes, CNRS, 2, rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3, Franceb Instituto de Tecnologıa Quımica (Universidad Politecnica de Valencia - Consejo Superior de Investigaciones Cientıficas),

av. Los Naranjos s/n E-46022, Valencia, Spain

a r t i c l e i n f o

Article history:

Received 14 April 2011

Received in revised form

15 July 2011

Accepted 16 July 2011

Available online 15 August 2011

Keywords:

La0.995Ca0.005NbO4

Solid oxide fuel cells

Proton conductivity

Compatibility

AC impedance

* Corresponding author. Tel./fax: þ330240373E-mail address: Kostiantyn.Kravchyk@cn

0360-3199/$ e see front matter Crown Copyri

doi:10.1016/j.ijhydene.2011.07.069

a b s t r a c t

The study presents the chemical and mechanical compatibility of the proton conducting

electrolyte La0.995Ca0.005NbO4 (LCNO) with the LSM, LSCM and BSCF cathodes and the

electrochemical performance of symmetrical cells based on LCNO. After annealing at high

temperature the electrolyte-cathode mixtures in air and wet air, the obtained products

were analyzed by X-ray powder diffraction (XRPD). The microstructure of the cathode and

electrolyte materials and the interfaces were observed by scanning electron microscopy

(SEM) and energy dispersive spectroscopy (EDX). The results show that LSCM cathode is

chemically and mechanically stable with the LCNO electrolyte although the BSCF cathode

reacts with it. Cation diffusion was observed between LSM cathode and LCNO electrolyte

after the heat treatment of their mixture at T ¼ 1150 �C. The electrochemical study per-

formed on symmetrical cells revealed that the LSCM cathode presents the lowest value of

area specific resistance (ASR) compared to the ones of the LSM and BSCF cathodes:

ASRLSCM ¼ 35 U cm2; ASRLSM ¼ 57 U cm2; ASRBSCF ¼ 416 U cm2 (in humidified air at 750 �C).

Finally, a CEReCER approach was used in order to minimize the polarisation resistance of

the LSM cathode by mixing LSM and LCNO in different volumetric ratios. The lowest value

of ASR for LSM-based composite cathode was obtained by adding 50 vol.% of LCNO to LSM

cathode (ASRLSM/LCNO ¼ 22 U cm2 in humidified air at 750 �C).

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

rights reserved.

1. Introduction dilution as the water production takes place on airside of the

High temperature solid oxide fuel cells (SOFC) are one of the

most important devices for the efficient conversion of chem-

ical energy into electricity. The funding for the SOFC world-

wide development has risen considerably and this trend is

expected to continue for at least the next decade. Among

different kinds of fuel cells, high temperature proton con-

ducting fuel cells (PCFC) become more and more studied year

by year as they have essential advantages: absence of fuel

929.rs-imn.fr (K.V. Kravchyk)ght ª 2011, Hydrogen Ene

cell involving an easier management of the fuel circulation

and higher fuel utilization. Moreover, the lower operating

temperatures in comparison with those used in SOFC would

extend the operating lifetime and improve the thermo-

mechanical materials compatibility [1e4].

Whatever the type of cell, SOFC or PCFC, the actual main

issue to obtain good performances is the chemical compati-

bility between the cathode and the electrolyte materials.

Indeed, the formation of reaction products at the cathode/

.rgy Publications, 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 ) 1 3 0 5 9e1 3 0 6 613060

electrolyte interface during the high temperature sintering

steps could be detrimental to the cell efficiency. The cationic

inter-diffusion has to be present for the good cathode-

electrolyte adhesion but limited in order to prevent the

degradation of the conduction and mechanical properties of

both cathode and electrolyte materials. If interfacial phases

are generated, it is of fundamental importance to know their

electrical nature since their presence traditionally increases

the polarisation resistance of the system. For instance,

a widely studied case of dramatic increase of polarisation

resistance due to the interdiffusion of elements is the

formation of the insulating phase La2Zr2O7 at the

(ZrO2)0.92(Y2O3)0.08/LaMnO3 interface [5]. Finally, the determi-

nation and comparison of thermal expansion coefficients are

needed to select the mechanically compatible cathode/elec-

trolyte materials.

The purpose of the present work is to study the mechan-

ical, chemical and electrochemical compatibility between

La0.995Ca0.005NbO4 (LCNO) proton conductor electrolyte and

La0.7Sr0.3MnO3 (LSM), La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) and

Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) usual cathode materials. LCNO

has been found to be an promising proton conductor stable in

CO2 atmosphere. The proton conductivity is dominant in wet

atmospheres below roughly 800 �C and the highest proton

conductivity of approximately 10�3 S cm�1 was observed [6].

Acceptor doped orthoniobates are fully stable during opera-

tion, cycling and storage. Moreover, they exhibit very good

mechanical properties due to ferroelasticity of the low-

temperature phase [6].

Among mixed ionic and electronic conductors (MIEC)

proposed as cathodes for PCFC based on LCNO electrolyte,

LSM and LSCM seem to be good candidates due to their close

thermal expansion coefficients with that of LaNbO4

(TECLaNbO4¼ 8.4� 10�6 (530e700 �C) [7]; TECLSM¼ 11.7$10�6 K�1

(25e830 �C) [8]; TECLSCM ¼ 12$10�6 K�1 (25e830 �C) [9]). Theseperovskites type cathode materials are known (i) to have

adequate catalytic activity for oxygen reduction as SOFC

cathodes at temperature above 700 �C; (ii) to retain their

oxygen deficiency and high oxygen-self-diffusion coefficients

even in an oxidizing atmosphere; and (iii) to exhibit high

electronic conductivity [10].

Despite the TEC discrepancy between LCNO and BSCF

(TECBSCF ¼ 20$10�6 K�1 (25e850 �C) [11], this later was also

chosen as an alternative candidate because of its good

performances in intermediate temperature SOFC due the high

oxide ion conductivity [12].

Fig. 1 e XRD patterns of LCNO, LSM, LSCM and BSCF

compounds measured at room temperature used in the

present study.

2. Experimental section

The Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) powder was synthesized by

self-combustion method as prepared in [13]. Ba(NO3)2,

Sr(NO3)2, Co(NO3)2$6H2O, Fe(NO3)3$9H2O, all in analytical

grades, were used as raw materials and glycine was used as

chelating agent and fuel. Stoichiometric amounts of metal

nitrates were dissolved into a water solution. Nitrates act as

oxidant in the combustion. Afterwards the required amount

of glycine was added into the solution under stirring. The

mixed solution was then heated at 80 �C on a hot plate under

constant stirring for 4e5 h until a gel was formed. One hour

later the precursor powder of BSCF was formed by self-

combustion of the gel. The obtained powder was then

annealed at T ¼ 1000 �C for 10 h in order to obtain the BSCF

powder.

The La0.995Ca0.005NbO4 powder was provided by Ceramic

Powder Technology AS (CERPOTECH) (Trondheim, Norway),

LSM (La0.7Sr0.3MnO3) and LSCM (La0.75Sr0.25Cr0.5Mn0.5O3) by

Marion Technology (France).

The XRD patterns of synthesized and provided powders

(LCNO, LSM, LSCM and BSCF) confirmed the absence of any

impurity phases (Fig. 1). LCNO adopts similar cell parameters

and the same space group as LaNbO4 (I2/c) [14].

In order to check chemical compatibility between LCNO

and LSM, LSCM, BSCF, the electrolyte/cathode mixtures with

1:1 weight ratio were ground together, pressed into pellets and

annealed at 1150 �C for 36, 72 and 144 h in air. The XRD

patterns of electrolyte, cathode and electrolyte/cathode

powders were recorded at room temperature using Siemens

D5000 (Cu Ka1: l ¼ 1.54060 A, Cu Ka2: l ¼ 1.54439 A) diffrac-

tometer. The refinements of the cell parameters were carried

out using the program FULLPROF [15] in the full pattern

matching mode with the program WinPLOTR interface [16].

Sintering studies of LSM and LSCM provided powders were

performedwith a dilatometer Netzsch DIL 402C at heating and

cooling rates of 5 �C min�1 in air.

Dense ceramics (97% of relative density) for symmetrical

cell measurements were obtained by uniaxial compression at

16 MPa (kg cm�2) of LCNO powder, followed by a heat treat-

ment at 1200 �C during 4 h. All pellets were then polishedwith

fine abrasives to reach 0.3e0.4mmof thickness and slurries of

cathode materials were coated by screen printing (DEK-248)

on both sides of electrolyte (B ¼ 6 mm). Afterwards, the

symmetrical cells were annealed at 1150 �C for 2 h with

heating and cooling rates about 150 �C/h. The microstructure

and element analysis of the obtained symmetrical cells were

studied by SEM imaging and EDX analysis using a scanning

electron microscope (JEOL JSM 7600) equipped with a germa-

nium X-ray detector. Porosity of the sintered cathode layers

was estimated by Image Tool program.

The AC impedance spectra were collected in air over the

frequency range 0.1 Hze1 MHz, in the temperature range

Table 1 eUnit cell volumes of LSM, LSCM, BSCF and LCNOphases in cathode/LCNO mixtures and alone at T [ 25 �Cand after heat treatment at T [ 1150 �C for 36 h in air.

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 ) 1 3 0 5 9e1 3 0 6 6 13061

200e850 �C upon cooling and heating, using a Solartron 1260

impedance analyser. The analysis of the impedance spectra

was carried out using the program Zview [17].

V (A3) atT ¼ 25 �C

V(A3) after heat treatmentat T ¼ 1150 �C for 36 h in air

LSM þ LCNO 350.8(9) 354.4(1)

333.1(3) 333.1(2)

LSCM þ LCNO 349.7(8) 350.7(1)

333.1(3) 332.7(1)

BSCF þ LCNO 63.1(2) 63.4 (1)

(solid solution)333.1(3)

LSM 350.8(9) 351.7(4)

LSCM 349.7(8) 349.7(9)

BSCF 63.1(2) 63.4(2)

LCNO 333.1(3) 333.1(2)

3. Results and discussion

Fig. 2 shows powder diffraction (XRPD) patterns of the LCNO,

LSM, LSCM compounds and LCNO e LSM, e LSCM mixtures

recorded at room temperature before and after thermal-

treatment at T ¼ 1150 �C for 36, 72 and 144 h in air. No

secondary phase was observed after the thermal-treatment of

LCNOe LSM,e LSCM powdermixtures. From these data it can

be assumed that no reaction takes place between LCNO and

LSM or LSCM compounds. However, an increase of the unit

cell volume of LSM phase took place after its heat-treatment

with LCNO at T ¼ 1150 �C for 36 h (Table 1) with almost no

change of cells parameters for LCNO phase. In case of LCNO e

LSCM mixture, no significant change of unit cell volume was

observed for both phases.

Studies of LCNO-BSCF thermal-treated mixtures at

T¼ 1150 �C for different times showed the formation of a solid

solution between LCNO and BSCF compounds adopting the

same space group as BSCF (Fig. 3). It is plausible that, by

Fig. 2 e XRD patterns of LCNO e LSM (a), LSCM (b) mixtures

measured at room temperature after heat treatment at

T [ 1150 �C for 36, 72, 108, 144 h in air. XRD patterns of

pure LCNO, LSM and LSCM phases are given for

comparison.

reaction with BSCF, LCNO decomposes into elements to form

a perovskite based on BSCF and in which La and Ca occupy the

A site and Nb occupies the B site. Further investigations con-

cerning this solid solution are currently under progress.

Fig. 4a,b,c show typical cross-sectional SEM images of LSM,

LSCM and BSCF cathodes (on the left side) sintered at 1150 �Cwith LCNO electrolyte (on the right side). The electrolyte layer

is well densified and shows a mean grain size of 1e2 mm

(Fig. 5). From Fig. 4a and b, it appears that LCNO-LSM and

LCNO-LSCM interfaces between cathode and electrolyte have

no sign of crack or delamination although their characteristics

are different. Specifically, the interface transition is smooth

(Fig. 4a) in the case of LSM cathode while interface is abrupt

(Fig. 4b) for LSCM cathode. Moreover, the LSM and LSCM

cathode layers have a different porosity (Table 2). Namely, the

LSCM cathode showed larger particle sizes with a homoge-

neous particle size distribution and larger porosity in

comparison with LSM cathode. It can be related with the

different sintering temperature of LSM and LSCM cathodes

which is much higher in the case of LSCM compound (Fig. 6)

despite quite similar particle size distribution and specific

surface area (Fig. 7). Therefore, LSCM layer is less dense and

exhibits higher porosity than LSM layer. Compositional anal-

ysis carried out by EDX method on the interface area,

Fig. 3 e XRD patterns of LCNO e BSCF mixtures measured

at room temperature after heat treatment at T[ 1150 �C for

36, 72 h in air. XRD patterns of pure LCNO and BSCF phases

are given for comparison.

Fig. 4 e Typical fracture cross-section SEM images and backscattered electron micrographs of LSM (a), LSCM (b) and BSCF (c)

diffusion couples screen-printed on the LCNO surface and heat-treated at T [ 1150 �C for 36 h.

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 ) 1 3 0 5 9e1 3 0 6 613062

confirmed a clear phase boundary and the absence of any

cation diffusion between electrolyte and cathodes in the

LCNO-LSCM symmetrical cells (Fig. 4b). However, La and Nb

cation diffusion was observed at the LSM-LCNO interface

(Fig. 4a, see curves for La and Nb). It is likely that the unit cell

volume evolution (see Table 1) is attributed to the diffusion of

La and Nb cations from LCNO to LSM materials.

It is worth noting that the several attempts to prepare

symmetrical cells based on BSCF cathode resulted always in

partial cathode delamination (Fig. 4c). This is probably related

to the large TEC mismatch between LCNO and BSCF

(TECLaNbO4 ¼ 8.4 � 10�6 K�1 (530e700 �C) [7],

TECBSCF ¼ 20$10�6 K�1 (25e850 �C) [18], TECLSM ¼ 11.7$10�6 K�1

(25e830 �C) [8]; TECLSCM ¼ 12$10�6 K�1 (25e830 �C) [9]). More-

over, EDX analysis on the cathode-electrolyte interface

Fig. 5 e SEM image of ceramic LCNO electrolyte sintered at

1200 �C for 4 h.

confirmed the absence of any cation concentration change

despite the solid solution formation between LCNO and BSCF

compounds.

Fig. 8a presents typical impedance diagrams of symmet-

rical cells based on LCNO electrolyte with LSM, LSCM and

BSCF cathodes, plotted in the Nyquist plane, and recorded at

T ¼ 700 �C under wet air. For BSCF symmetrical cells, the

delamination of the cathode being partial, the measurements

were nevertheless carried out. The impedance datawere fitted

using an electrical equivalent circuitmade of R//CPE (Constant

Phase Element) elements. As the diagrams display three semi-

circles at T ¼ 700 �C, three serial (R//CPE) elements were used

to fit the impedance diagrams at high temperature. The high

frequency semi-circles are attributed to the motion of the

charged species in the grain boundaries of the ceramic

(capacitance CLSM z 10�10 F, CLSCM z 10�9 F, CBSCF z 10�9 F)

plus the contributions due to Au current collector contact and

the medium frequency ones - to polarisation resistance at the

cathode-electrolyte interface Rpi (CLSM z 10�7 F,

CLSCM z 10�6 F, CBSCF z 10�8 F). Finally, the semi-circles at low

frequency are related to the bulk polarisation Rpb of the LSM,

LSCM and BSCF cathodes (CLSM z 10�4 F, CLSCM z 10�4 F,

CBSCF z 10�3 F). Bulk polarisation resistance in this article

means polarisation resistance of the cathode part not located

at the electrolyte-cathode interface. At lower temperatures

(<400 �C) interfacial polarisation resistance disappears and

Table 2 e Porosity of LSM, LSCM, and composite LSM/LCNO (50 vol. % of LCNO) cathode layers.

Cathode Porosity, %

LSM 30

LSCM 41

LSM/LCNO 20

Fig. 6 e Dilatometry curves as a function of temperature

and time of LSM and LSCM powders.

Fig. 8 e Impedance spectra of LSM/LCNO/LSM, LSCM/LCNO/

LSCM and BSCF/LCNO/BSCF symmetrical cells measured at

T [ 700 �C (a) and 350 �C (b) under wet air.

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 ) 1 3 0 5 9e1 3 0 6 6 13063

semicircle corresponding to bulk resistance of electrolyte

appears (Fig. 8b). Assuming that current collection resistances

are negligible, the total conductivity of the LCNO was deter-

mined from the values of bulk and grain boundary resistances

of the electrolyte. As it can be seen from the Fig. 8a,b, elec-

trolyte response (grain and grain boundary resistances) is

similar for all measured symmetrical cells and close to that

found in literature [6]. However the polarisation resistance

differs strongly depending on the cathode material. The

highest value of interfacial polarisation resistance (medium

frequency semi-circles) was observed for LSM cathode

(Rpi z 86 U), lower for BSCF (Rpi z 56 U) and the lowest for

LSCM (Rpi z 44 U) at T ¼ 700 �C. The bulk polarisation resis-

tances of cathode are at 700 �C: RpbLSM ¼ 439 U;

RpbLSCM ¼ 343 U and RpbBSCF ¼ 4095 U (Fig. 8a). Area specific

resistance was calculated from the total polarisation resis-

tances (R ¼ Rpi þ Rpb) of cathodes using the following

equation:

ASR ¼ R$S=2 (1)

Fig. 7 e Particle size distribution of the LSM and LSCM

powders as determined by analysis of SEM images.

where S is the surface area of the electrolyte covered by

cathode on one side.

Under wet air below T ¼ 700 �C, electrolyte conductivity

(Fig. 9) shows an increase in comparison with measurements

in dry atmosphere. This is related to the hydration behavior

and the addition of the proton conductivity to the total

conductivity (p-type electronic and oxygen ionic) [19,20]. Only

a slight increase of the total conductivity of LCNO was

observed with Au cathodes and this ascribed to the fact that

the resistances of electrolyte are similar and there is no big

alteration due to the cation interdifusion or formation of new

phases in the interface.

Temperature dependence of area specific resistance (ASR)

of LSM, LSCM and BSCF cathodes are presented in the Fig. 10.

Whatever the cathode material, an Arrhenius behavior is

observed with an activation energy ranging from 1.44 to

1.61 eV, from BSCF to LSCM respectively. The electrochemical

Fig. 9 e Temperature dependences of total conductivity of

LCNO measured using Au, LSM, LSCM and BSCF electrodes

in wet air and with Au electrode in dry air.

Fig. 10 e Temperature dependence of area specific

resistance for LSM, LSCM and BSCF cathodes measured in

symmetrical cell configuration with LCNO electrolyte.

Fig. 11 e Schematic diagrams of cathode-reaction mechanisms

oxygen-proton-electronic conductor (a) and mixed oxygen-elec

materials.

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behaviors of symmetrical cells based on LCNO tested with

different LSM, LSCM, BSCF cathode materials show different

results. The lowest ASR value was obtained for LSCM

(ASR z 35 Ohm$cm2 at 750 �C). For LSM and BSCF cathodes,

ASR values were equal to 57 and 416 Ohm$cm2 respectively at

T ¼ 750 �C under humidified air.

Considering possible limiting steps determining polar-

isation resistance for each cathode, the following aspects

should be taken into account: level of electronic, oxygen and

proton conductivity, electrocatalytic activity for oxygen acti-

vation, porosity (bulk Rpb) and presence of extra phase due to

chemical reaction or cation diffusion between electrolyte and

cathode (interfacial Rpi). Higher interfacial Rpi values of LSM

and BSCF cathodes than that of LSCM one (RpiLSM z 86 U;

RpiBSCF z 56 U; RpiLSCM z 44 U) can be related to cation

diffusion and solid solution formation for LSM and BSCF

cathodes respectively (see Table 1 and Fig. 3). The new La and

Nb-rich LSM phase and LCNO-BSCF solid solution formed at

the cathode-electrolyte interface probably are not as good

conducting compounds as the materials alone and therefore

increase the interfacial polarisation resistance.

High values of bulk polarisation resistance of BSCF cathode

compared to that of LSM and LSCM ones (RpbLSM ¼ 439 U;

RpbLSCM ¼ 343 U; RpbBSCF ¼ 4095 U) can be attributed to the

partial delamination of BSCF cathode from LCNO electrolyte

surface. Difference between bulk Rpb of LSM and LSCM cath-

odes can be explained by higher electronic conductivity level

of LSCM cathode (sLSM w 4 S,cm�1 [21]; sLSCM w 60 S,cm�1

[22]) and the higher porosity of the LSCM cathode layer (Table

2). Oxygen conductivity level (sLSCM w 3$10�5 S cm�1 [23];

(sLSMw 2$10�3 S cm�1 [24] at T¼ 950 �C) is not a limiting step in

these systems even if LSM has higher oxygen conductivity

than LSCM one. Another important parameter is the level of

proton conductivity for above-investigated cathode materials

since the use of a protonic mixed conducting oxide would

allow the production of water not only at the TPB sites, but

also over the whole cathode surface [25e27] (Fig. 11a). In this

case, symmetrical cell system would obviously exhibit faster

electrode processes. According to the structure and compo-

sition of LSM and LSCM cathodes, they may react with water

vapor to form compounds with hydroxyl groups and exhibit

some proton transport properties. However, in the literature,

there is no data highlighting proton conductivity in these

in the case of proton conducting electrolyte with: mixed

tronic D proton conductors (CER-CER) (b) as cathode

Fig. 12 e Typical fracture cross-section SEM image of LCNO/LSM composite cathode screen-printed on the LCNO surface and

heat-treated at T [ 1150 �C for 36 h.

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compounds. Therefore, based on available data, the ASR

decrease from LSM to LSCM cathodes in symmetrical cell

systems can be explained by a better porosity of LSCM cathode

layer which facilitates the oxygen penetration through the

cathode itself and therefore multiply the triple phase

boundary TPB sites.

One way to decrease the cathode ASR is to prepare

a composite cathode consisting of a mixture of a proton

conductor electrolyte and a mixed O2�/e� conductor cathode

materials. Composite cathodes provide a high density of TPBs

due to the proton conducting phase in the cathode layer

[28e30]. Indeed, the TPBs are spread into the bulk of cathode

layers which improve the diffusion and reaction of protons

and oxygen species leading to ASR decrease (Fig. 11b). Thus,

composite cathodes with 40, 50 and 60 vol. % of electrolyte

were prepared aiming to improve the ASR of LSM cathode in

symmetrical cells based on LCNO electrolyte. Fig. 12 shows

typical cross-sectional SEM image of LSM-LCNO cathode with

50 vol. % of LCNO, sintered at 1150 �C for 2 h. Generally, the

LSM and LCNO particles appear to be more sintered and larger

and therefore a lower porosity is observed for LSM and LCNO

composite cathode compared to pure LSM (Table 2). Despite

the porosity decrease and the particle sizes increase of the

LSM-composite cathodes, the improvement of their electro-

chemical behavior was established. As shown in Fig. 13, ASR

Fig. 13 e Dependence of area specific resistance of LCNO/

LSM composite cathode measured in symmetrical cell

configuration based on LCNO at T [ 750 �C with the

content of LCNO electrolyte in the composite.

of cathode materials decreases with increasing vol.% of LCNO

electrolyte in the composite until reaching an optimum value

at around 50 vol.%. The minimum ASR value is obtained for

LSM-50LCNO and is equal to 22 Ohm$cm2 at T ¼ 750 �C which

is 2.6 times lower than the one obtained for pure LSM. This

confirms the positive role in the polarisation resistance by the

incorporation of the proton conducting material in

a composite cathode. CEReCER composite cathodes based on

LSCM are currently under investigation.

4. Conclusions

LSM, LSCM and BSCF compounds were investigated for

potential application as cathode materials for PCFC based on

LCNO electrolyte. It was shown that LSCM cathode is chemi-

cally and mechanically stable with the LCNO and that BSCF

reacts with it. La and Nb cation diffusion was observed

between LSM and LCNO after the thermal treatment of their

mixture at T ¼ 1150 �C.Conductivity measurements performed on symmetrical

cells revealed that the LSCMcathode presents the lowest value

of area specific resistance (ASR) compared with those of the

LSMandBSCFcathode:ASRLSCM¼ 35U cm2;ASRLSM¼ 57U cm2;

ASRBSCF ¼ 416 U cm2 in humidified air at 750 �C. This can be

partly explained by lower sinterability of LSCM cathode

compared toLSM,asotherparameters like levelsofoxygenand

electronic conductivities are not limiting in present system.

High values of ASR for BSCF cathode were related to its partial

delamination at the electrolyte/cathode interface due to high

TEC difference between LCNO and BSCF compounds

(TECLaNbO4 ¼ 14 � 10�6 (200e500 �C); TECLaNbO4 ¼ 8.4 � 10�6

(530e700 �C); TECBSCF ¼ 20$10�6 K�1 (25e850 �C)).LSM cathode resistance was reduced by the preparation of

CEReCER composite cathodes with different cathode/elec-

trolyte vol. ratios. The lowest value of ASR for LSM-based

cathode was obtained when adding 50 vol.% of LCNO to LSM

cathode: ASRLSM ¼ 22 U cm2 in humidified air at 750 �C.

Acknowledgments

This work has been performed in the frame of the FP7 Project

EFFIPRO “Efficient and robust fuel cell with novel ceramic

proton conducting electrolyte” (Grant Agreement 227560).

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 ) 1 3 0 5 9e1 3 0 6 613066

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