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Electrochimica Acta 139 (2014) 13–20

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

ffect of cobalt-substitution Sr2Fe1.5-xCoxMo0.5O6-ı for intermediateemperature symmetrical solid oxide fuel cells fed with H2-H2S

ang Songa, Qin Zhonga,∗, Wenyi Tanb,a,∗, Cai Panb

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR ChinaDepartment of Environment Engineering, Nanjing Institute of Technology, Nanjing 211167, PR China

r t i c l e i n f o

rticle history:eceived 9 May 2014eceived in revised form 26 June 2014ccepted 2 July 2014vailable online 17 July 2014

eywords:T-SOFCs

a b s t r a c t

Double-perovskite Sr2Fe1.5Mo0.5O6-� with high performance has been investigated as potential symmet-rical electrode materials for Solid Oxide Fuel Cells (SOFCs). In order to enhance its catalytic activity andelectrochemical performance, a series of cobalt-substituted Sr2Fe1.5-xCoxMo0.5O6-ı (SFCM, x = 0, 0.5, 0.75and 1) were synthesized in this work. XRD, BET, XPS, PL, O2-TPD (H2-TPR), SEM and electrochemicalperformance tests have been conducted to characterize the improved performances. XRD and BET mea-surements indicate SFCM with the double-perovskite structure, and smaller grain size is 30.51 nm, aswell as the larger BET surface areas 9.51 m2/g for cobalt-rich sample x = 0.75. The redox couples Fe3+/Fe2+,

3+ 2+ 6+ 5+

ouble-perovskiteobalt-substitutionymmetric electrodeydrogen sulfide

Co /Co and Mo /Mo are revealed by XPS, and more oxygen vacancy ı=0.46 is detected by PL andiodometry for sample x = 0.75, which is conducive to the redox catalytic activity and ionic conductivityyielding highest 59.48 S·cm−1 at 800 ◦C. Sample x = 0.5 with the lowest conductance activation energy0.42 eV also exhibits best electrochemical performance with the highest power density of 42.6 mW·cm−2

for the configuration of SFCM/LSGM/SFCM in 0.5% H2S-3% H2 at 800 ◦C.© 2014 Elsevier Ltd. All rights reserved.

. Introduction

Symmetric Solid Oxide Fuel Cells (SSFCs) as a novel conceptf Solid Oxide Fuel Cells (SOFCs) have attracted more and morenterest in recent decades [1]. Compared with conventional SOFCs,SFCs have many merits, such as the simpler fabrication processnd therefore the reduced costs, the easier co-sintering, and so on1–3].

The greatest challenge for SSFCs is the choice of symmetriclectrode materials using as both anode and cathode. The final can-idate first should have all characteristics as a SOFC electrode in

general sense, such as high ionic and electronic conductivity, elec-rocatalytic activity and good chemical and thermal compatibilityith the electrolyte materials, and so on. [4–9]. The state-of-the-artickel anode is readily fouled by carbon deposition and sulphur poi-oning, which not only may block the porosity of the anode but alsoisrupts the cermet microstructure, and finally causes degradation

f cell performance [10–12]. Perovskite La0.75Sr0.25Cr0.5Mn0.5O3-ı

LSCM) as the goal electrode has been investigated sophisticat-dly by multiple groups including ours. LSCM as a mixed ionic and

∗ Corresponding author. Tel.: +862584315517; fax: +862584315517.E-mail addresses: zq304@mail.njust.edu.cn (Q. Zhong), twy1102@gamil.com

W. Tan).

ttp://dx.doi.org/10.1016/j.electacta.2014.07.022013-4686/© 2014 Elsevier Ltd. All rights reserved.

electronic conductor (MIEC) was applied as SOFC anode materialsdue to its excellent resistance to sulfur poisoning, high ther-mochemistry stability and extended triple phase boundary (TPB)[13–17]. Nevertheless, the total conductivity of LSCM especially inreducing conditions is a little low and the majority of sulphur poi-soning studies have been carried at elevated working temperatures(800-900 ◦C) [18–21].

Recently, double perovskites have been extensively investi-gated as alternative SOFC anode materials [22–38]. Sr2 (or Ca,Ba) MMoO6 (M = Ni, Co, Fe, Mg, etc) oxides accommodating largeamounts oxygen non-stroichiometry have attracted the interestof many researchers, particularly in relation to intermediate tem-perature (IT, 600-800 ◦C) SOFCs [22]. In 2006, Huang et al. [23,24]studied Sr2MgMoO6-ı (or/and Sr2Mg1-xMnxMoO6-ı) with toler-ance to sulphur and superior single-cell performance in hydrogenand methane. Sr2NiMoO6 in H2 and dry CH4 [25–27], Ba2MMoO6(M = Fe, Co, Mn, Ni) [28] and their modified materials with alkaliearth and transition-metal elements substituting in A and B sites[29–33] were reported in sequence to obtain better performance.Recently, A series of double perovskites A2FeMoO6-� (A = Ca, Sr, Ba)were synthesized by He et al. [34] and Sr2FeMoO6-ı exhibited the

best power output of 831 mW·cm−2 in dry H2 and 735 mW·cm−2 incommercial city gas at 850 ◦C, respectively. In 2013, SSFCs with theconfiguration of Sr2Fe1.5Mo0.5O6-ı (SFM)/LSGM/SFM, in which SFMwas proposed as a promising electrode by Chen et al. [35–38], reach

1 imica Acta 139 (2014) 13–20

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4 Y. Song et al. / Electroch

35 and 230 mW·cm−2 in wet H2 and CH4, respectively. In order tomprove the further catalytic activity and electrochemical perform-nces of IT-SSFCs, we modified SFM by cobalt-substitution due tohe fact that the presence of cobalt is very beneficial to the oxygeneduction reaction (ORR) and contributes to very low polarizationoss verified by some researchers [39–41]. In this paper, four dif-erent ratios of cobalt-substitution Sr2Fe1.5-xCoxMo0.5O6-ı (SFCM,

= 0, 0.5, 0.75 and 1) were selected as pioneer to use in evaluation ofhe SSFCs performance in H2-H2S. The effects of cobalt-substitutionn crystallite structure, catalytic activity and electrochemical per-ormances were investigated, and the structure defects of SFCMaused by doping cobalt were expounded and explain further.

. Experimental

.1. Sample preparation

A series of Co-doping double-perovskite materialsr2Fe1.5-xCoxMo0.5O6-ı (SFCM, x = 0, 0.5, 0.75 and 1) powdersere synthesized via a combined ethylenediaminetetraacetic acid

EDTA) and citric-acid complexing method. Briefly, the weighedr(NO3)2, Fe(NO3)3·6H2O, Co(NO3)3·6H2O, (NH4)6Mo7O24·4H2Oall A.R. grade) as the starting materials were dissolved in aqueousolutions, and then EDTA and citric-acid were added in sequenceo serve as complexing agents at a mole ratio of 1: 1: 1.5 for total

etal ions: EDTA: citric-acid. Ammonia is used to adjust the pHalue of the aqueous solution around 7-8. The resulting solutionsere stirred further to form gels and then were carried out

ombustion on hot plates in the fume hood. After burning out therganic components, the precursors were pelletized and calcinedt 1100 ◦C in 5% H2/N2 for 18 h to obtain a pure double-perovskitetructure without secondary phases. The pellets were ground andall milled into rather fine SFM powders. Electrolyte powdersa0.8Sr0.2Ga0.83Mg0.17O3-� (LSGM) was synthesized by ball millingnd calcined in air at 1450 ◦C for 10 h.

The well-calcinated double-perovskite electrode SFCM and elec-rolyte powders LSGM were recorded by X-ray diffraction (XRD)sing Bruker D8 Bruker Advance diffractometer equipped with Cu

radiation at 35 kV and 20 mA (�=0.l5406 nm) at room tempera-ure. Intensity data were collected over a 2 h range of 20-80 degreeith intervals of 0.02 degree.

.2. Characterization

Specific surface areas of SFCM (x = 0, 0.5, 0.75 and 1) were deter-ined by N2 adsorption-desorption measurements at -196 ◦C by

mploying the Brunauer-Emmet-Teller (BET) method (Gold App V-orb 2008), and the pore volume and pore size of the samples werealculated by Barrett-Joyner-Halenda (BJH) method. Prior to N2dsorption, the sample was outgassed at 200 ◦C for 12 h to desorboisture adsorbed on the surface and inside the porous network.The elements in powders were investigated by X-ray photo-

lectron spectroscopy (XPS) using a monochromatized X-rayource operating at 150 W (Thermo ESCALAB 250, US) at roomemperature equipped with a monochromatic Al Ka radiationh�=1486.6 eV). Samples SFCM were compensated for chargingith low-energy electron beam, and the binding energies were cal-

brated with respect to setting the measured binding energy of C1st 284.8 eV.

Oxygen vacancies were characterized by photoluminescencePL) spectra at room temperature using a Labram-HR800-type

pectro-photometer (Jobin Yvon Co., France) with a He-Cd laser�=325 nm) as the light source.

Accurate weighed amount of the samples (m, 50-80 mg)n 250 mL conical flask were dissolved in aqueous solution

Fig. 1. XRD patterns of (a)Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1) in 5% H2 and(b) La0.8Sr0.2Ga0.83Mg0.17O3-� in air.

containing 1 g KI (A.R. grade), and N2 as shielding gas at the sametime. After adding 5 mL HCl solution (5.0 mol/L) and 10 mL buffersolution with pH 6.0, water was added to dilute solution. The starchindicator and calibration concentrations of Na2S2O3 (C) solutionwere used to direct titration the solution mentioned above. Theconsumption volume of Na2S2O3 (V) was written down.

The O2 temperature-programmed desorption (O2-TPD) experi-ments were performed on Chembet PULSAR TPR/TPO (p/n 02139-1)using about each weighed amount of the sample (100 mg). After O2saturation in 2 h, the gas was switched to He for 1 h. Then, the TPDexperiments were performed, and the flow rate of the system waskept at 60 ml/min with the heating rate of 10 ◦C·min−1 from 50 upto 800 ◦C.

The H2 temperature-programmed reduction (H2-TPR) experi-ments were carried out on ChemBET TPR/TPO (Quantachrome, US)equipped with TCO analyzer using a 10%.vol% H2/Ar gas flown of60 ml/min and passed approximately over 100 mg samples. Thereduction temperature was linearly raised at 10 ◦C·min−1 from 200to 800 ◦C.

SFCM (x = 0, 0.5, 0.75 and 1) powders were pressed into densebar samples of single phase materials, and then sintered at 1400 ◦C.The evolution of the electrical conductivity as a function of tem-perature was determined in air and 5% H2 up to 900 ◦C during onethermal cycle by using a four probe dc technique.

Single cell tests were performed using LSGM with 200 �m-thickas electrolyte, and SFCM with 80 �m-thick as electrodes, respec-tively. The cell pellets of SFCM/LSGM/SFCM (10 mm in diameter)were formed by co-pressing the three layers powers uniaxiallyunder 10 Mpa. The electrochemical characterizations were mea-sured via electrochemical impedance spectrum measurement byan electrochemical work station (ParSTAT4000, US).

The electrode surface morphology and the electrode/electrolyteinterface of one sample SFCM before and after cell testing wereinspected via a scanning electron microscope (SEM, S-4800 IIFESEM, Japan).

3. Results and discussion

3.1. Effect of cobalt-substitution in A-site on crystal structure

Fig. 1 displays the X-ray diffraction (XRD) patterns of SFCM (x = 0,0.5, 0.75 and 1) calcined at 1100 ◦C in reducing environment andelectrolyte LSGM calcined at 1450 ◦C in air. Curvs a1-a4 exhibit nicedouble-perovskite structures, and no other new peak is observed

Y. Song et al. / Electrochimica Acta 139 (2014) 13–20 15

Table 1Crystallite sizes and Lattice parameters of Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and1).

Sample Crystallite sizes Lattice parameters

� (degree) � (degree) D (nm) a (Å) V RWP (%) RP (%)

x = 0 16.05 0.256 31.93 7.8307 480.17 5.92 4.92x = 0.5 16.11 0.264 30.97 7.8119 476.74 5.22 4.10x = 0.75 16.13 0.268 30.51 7.7873 472.24 5.61 4.30x = 1 16.14 0.280 29.21 7.7726 470.48 7.34 5.34

Table 2BET surface area and pore structure results of Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75and 1).

Sample BET surface area and pore structure results

SBET (m2/g) Pore volume(mm3/g) Average pore diameter(nm)

x = 0 4.57 19.92 5.92x = 0.5 7.02 25.14 8.02

aiptwue

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Table 3XPS results for Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1).

Sample The relative cation and oxygen ratios (%)

Fe2p3/2 Co 2p3/2 Mo 3d O 1s

Fe2+ Fe3+ Co2+ Co3+ Mo5+ Mo6+ O� O�

x = 0 22.5 77.5 - - 46.8 53.2 50.9 49.1

activity of electrode materials. PL spectra have been widely used

x = 0.75 9.51 33.77 8.67x = 1 8.73 31.16 7.19

s cobalt doping, which nearly suggests the synthesis procedures successful in introducing cobalt without impacting the double-erovskite structure. However, the diffraction peaks position shiftso the higher angle by increasing cobalt-doping concentration,hich indicates the samples interplanar spacing is decreasing grad-ally with more and more Co instead of Fe. According to the Schererquation:

= K�

cos �(3.1)

Where D is the diameter of particle; K is the Scherer constant.89; � is the wavelength of Cu-K� radiation 0.15406; � is theragg’s diffraction angle and is the full width at half maximaFWHM) of the diffraction peak.

The calculated crystallite sizes of SFCM (x = 0, 0.5, 0.75 and 1)re summarized in Table 1. It is obvious that and � for samplesncrease with dopant cobalt amount, as well as the particle sizesields minimum for x = 1. Additionally, to get more structural infor-ation, the lattice structures of SFCM (x = 0, 0.5, 0.75 and 1) were

urther analyzed by Rietveld refinement. From Table 1, the calcu-ated lattice parameters are respectively 7.8307, 7.8119, 7.7873 and.7726 A for SFCM (x = 0, 0.5, 0.75 and 1) as a cubic double perov-kite in space group Fm3 m, and the low reliability factors (RWP andP) indicate good fitting between the experimental and calculatedRD patterns. The above results are due to the smaller radius of Co

han Fe as well as the interactions between Co and Fe causing therystal defects so as to produce a certain degree of shrinkage [42].

Table 2 shows the BET surface areas (SBET), total pore volumesnd average pore diameters of SFCM (x = 0, 0.5, 0.75 and 1). Becausehe samples form at temperature as high as 1100 ◦C, the SBET valuess generally small, usually less than 10 m2/g. With the increasingf Co/Fe up to x = 0.75, SBET and pore volume of samples increase.he sample x = 0.75 shows a higher SBET (9.51 m2/g) than that ofther samples. These samples with average pore diameter between

to 50 nm belong to the mesoporous materials. There is a simpleelationship between the BET surface areas (S) and the diameter ofarticle (D) [43]:

= 6(�D)

(3.2)

Where � is the density of samples. Obviously, samples withmaller the grain size own greater SBET without consideringhe effect of density.

x = 0.5 20.8 79.2 47.0 53.0 53.9 46.1 51.2 48.8x = 0.75 18.9 81.1 48.6 51.4 59.9 40.1 58.6 41.4x = 1 19.8 80.2 53.5 46.5 45.9 54.1 43.3 56.7

3.2. Effect of cobalt-substitution in A-site on oxygen vacancyconcentration

The electrode of SOFC has three functions [44]: in addition toallowing access to reacting gases and allowing transport of elec-trons as well as ions, electrodes must provide active catalytic sites.

The surface chemical species of fresh samples were character-ized by XPS. Fig. 2 depicts the XPS spectra over the spectral regionsof Fe 2p, Co 2p, Mo 3d and O 1s, respectively. A detailed summaryof the analysis of the relative content of different valence elementsis presented in Table 2.

Fe 2p3/2 XPS spectra shown in Fig. 2a for the series of SFCM (x = 0,0.5, 0.75 and 1) are fitted by three sub spectra. In the cobalt-freesample (x = 0), it could be seen that three peaks at 709.3, 710.6 and712.3 eV are attributed to Fe2+ 2p3/2, Fe3+ 2p3/2 in the perovskite-type compound structure and Fe3+ 2p3/2 bonded with hydroxylgroup [44–46]. From Table 2, the relative content of Fe3+ for SFCM(x = 0, 0.5, 0.75 and 1) is calculated to be 77.5%, 81.1%, 79.2% and80.2%, respectively. Furthermore, as the cobalt content increases,these peaks progressively shifted to higher binding energy (BE)values up to 709.7, 711.0 and 712.7 eV for sample x = 1, which inten-sity decreases as a function of growing number of Co instead of Fe.Fig. 2b reveals the XPS of Co 2p3/2 in SFCM (x = 0.5, 0.75 and 1),including two peaks for Co3+ 2p3/2 (779.8-780.2 eV), Co2+ 2p3/2(781.3-781.7 eV), which are also clearly shifted towards lower BEand the intensity significantly enhanced. From Table 3, the relativecontent of Co2+ increases gradually. Mo 3d spectra (Fig. 2c) havefour peaks at about 231.8, 232.2, 235.0 and 235.4 eV, which canbe attributed to Mo in Mo5+ 3d5/2, Mo6+ 3d5/2, Mo5+ 3d3/2 andMo6+ 3d3/2 peaks, respectively. More and more relative content ofMo5+ is observed up to x = 0.75, but the content of Mo5+ for x = 1declines sharply. O 1s XPS spectra for SFCM are shown in Fig. 2d,there is a twin peaks with the BE at around 529.5 and 530.1 eV,which can be attributed to lattice oxygen (O�) associated with theredox properties of metal ions and O� might be composed of var-ious oxides, such as CO3

2−, OH−, O−, O22−, O2− etc., respectively

[47]. As shown in Table 3, the change trend of Mo5+ is consistentwith that of O�, which is because that chemical adsorption oxygenis produced between the surface Mo5+ with higher electron clouddensity and oxygen in the gas phase, causing the surface Mo5+ loseelectrons. Simultaneously, O2 changes into the adsorption oxygenwith negative charges.

By substitution of Fe3+/Fe2+ with similar-sized Co3+/Co2+,defects can be introduced into the structure. Specifically, oxy-gen vacancy can be introduced by substitution of Fe3+ with lowervalence Co2+, which can be expressed as Eq. (3.3):

2OxO + 2Fex

Fe + CoO ↔ Co"Fe + V ••

O + Fe2O3 (3.3)

Where OxO is lattice oxygen, Fex

Fe is trivalent B-site ion, Co"Fe is a

divalent ion and V ••O is oxygen vacancy concentration.

The existence of oxygen vacancy could improve the catalytic

in the study of electronic capture and surface defects. It playsan important role in the study of seperation and composite ofelectrons and holes on semiconductor materials [48]. PL spectra

16 Y. Song et al. / Electrochimica Acta 139 (2014) 13–20

O 1s

svcoiot

Fig. 2. XPS spectra of (a) Fe 2p (b) Co 2p (c) Mo 3d (d)

hown in Fig. 3, the peak at 525 nm is assigned to the oxygenacancies with one trapped electron [49]. In general, the higher theontent of oxygen vacancy, the greater the probability of formationf excitons, the stronger the exciton emitting signal, correspond-

ng to the stronger the peak forms [50]. From Fig. 3, the intensityf the PL peak for sample x = 0.75 is much stronger than that ofhe other doping ratio ones, indicating that cobalt-doping trend to

Fig. 3. PL spectra of Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1).

levels in Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1).

the Eq. (3.3) results in more oxygen vacancies. However, the oxy-gen vacancy concentration decreases obviously with the excessiveamounts of Co doping up to x = 1, which indicates that only for alow concentration of cobalt-dopant, the oxygen vacancy concen-tration monotonically increases with dopant amount. In order tofurther quantitative study the content of oxygen vacancy, the oxy-gen nonstoichiometry at room temperature was measured by theiodometry. The oxygen vacancy ı for Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0,0.5, 0.75 and 1) are shown in Table 4, which is in accordance withPL results. Generally, the higher the cobalt content, the higher theoxygen vacancy ı for Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75).

3.3. Effect of cobalt-substitution in A-site on redox catalyticactivity

Redox reaction of symmetrical electrode materials linked withthe oxygen reduction reaction ORR and hydrogen oxidation reac-tion (HOR) is an indispensable factor for SSFCs. In order to furthercompare the redox catalytic activity of SFCM (x = 0, 0.5, 0.75 and

Table 4The oxygen vacancy ı for Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1).

Sample m (g) C (mol/L) V (mL) the oxygen vacancy ı

x = 0 0.0517 0.12 1.71 0.29x = 0.5 0.0587 0.12 2.02 0.32x = 0.75 0.0513 0.12 2.06 0.46x = 1 0.0561 0.12 2.09 0.39

Y. Song et al. / Electrochimica Acta 139 (2014) 13–20 17

1.5-xC

1d2sGtcxialgwFaPTbCpootFs

artf

3c

tipr

resulting in the decreased oxide ion conductivity as well as acti-vation energy for conduction. ln(T) is almost linearly dependentupon 1/T, indicating that the conductivity behavior is a thermallyactive process associated with the a temperature-independent

Fig. 4. O2-TPD and H2-TPR of Sr2Fe

), O2-TPD and H2-TPR curves (Fig. 4) of SFCM with different Cooping-ratios were implemented within the temperature range00-800 ◦C. O2-TPD characterizing the bonding strength of oxygenurface forms and oxygen mobility is prepared as Fig. 4a shows.enerally, there are three kinds of active oxygen species: �, � and

on the samples surface. � oxygen species has a lower desorptionemperature signal (< 350 ◦C), which is assigned to the chemi-al adsorption oxygen. Herein, � oxygen species peak areas for

= 0-1 samples ranked as: x = 0.75 > x = 0.5 > x = 0 > x = 1, which isn accord with XPS results for O� summarized in Table 2. Signalt higher temperature (> 750 ◦C) corresponds to oxygen in crystalattice (called oxygen species). Desorption temperature of � oxy-en species between the two desorption temperatures is relatedith oxygen defect and is considered as partial crystal oxygen [51].

ig. 4a shows the highest content of chemical adsorption oxygens well as � oxygen species when x = 0.75, which is identified withL results as mentioned above. Fig. 4b presents the recorded H2-PR curves. The hydrogen reduction peaks at around temperatureetween 300 to 600 ◦C are related to the redox couples Fe3+/Fe2+,o3+/Co2+ and Mo6+/Mo5+ in the four samples. The H2 consumptioneak of cobalt-free sample appeared at lower temperature than thatf other samples. According to Huang et al. [28], Mo6+ to Mo5+ onlyccurs at temperatures higher than 500 ◦C. Hence, the peak at loweremperature (< 500 ◦C) is assigned to H2 consumption peaks aboute3+/Fe2+ and Co3+/Co2+. Evidently, the highest activity of HOR forample x = 0.75 is attained by cobalt substitution.

The dopant not only affects samples catalytic activity, but alsoffects oxygen ionic conductivity. In the literature, there are manyeports suggesting that the ionic size of the dopant is highly impor-ant in determining the oxide ion conductivity [23,34]. The effector oxygen ionic conductivity is discussed next.

.4. Effect of cobalt-substitution in A-site on oxide iononductivity

The conductivity tests of SFCM (x = 0, 0.5, 0.75 and 1) were inves-

igated in air, and the temperature dependence of the conductivitys shown in Fig. 5. The conductivity increases with the elevated tem-erature up to 750 (or 800) ◦C due to the increase carrier migrationate with temperature. Double perovskite electrode materials SFCM

oxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1).

all exhibit significantly conductivity in air, but the conductivity ofcobalt-doped samples is much higher. The addition of cobalt yieldsincrease in total conductivity in air from 2.51 S·cm−1 at 300 ◦C to54.00 S·cm−1 at 750 ◦C for sample x = 0.5. For x = 0.75 (x = 1), the con-ductivity from 1.63 (1.32) S·cm−1 at 300 ◦C to 59.48 (51.96) S·cm−1

at 800 ◦C in air. However, the conductivity of cobalt-free sampleis 0.55 and 38.40 S·cm−1 at 300 and 750 ◦C, respectively. It is wellknown that the ionic conductivity can be expressed by Eq. (3.4):

� = en� (3.4)

Where is ionic conductivity, e is the charge, n is the num-ber of mobile oxide ion vacancies and � is the mobility. Thus, fora low concentration of cobalt-dopant, the conductivity monoton-ically increases with dopant amount. However, the higher dopingconcentrations lead to the formation of vacancy and dopant cluster

Fig. 5. Electrical conductivities and conductance activation energies ofSr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1) samples.

18 Y. Song et al. / Electrochimica Acta 139 (2014) 13–20

M/Sr2

cm

l

FtSs

Fig. 6. Impedance spectra of a series of cells with Sr2Fe1.5-xCoxMo0.5O6-ı/LSG

arrier concentration and obeys the small polaron conductivity

echanism expressed in the logarithm form of Arrhenius Eq. (3.5):

n(�T) = LnA − Ea

KbT(3.5)

ig. 7. The ohmic resistance (Rohm), the interfacial polarization resis-ances (Rp), and the total resistance (Rtotal) of the single fuel cellr2FeCo0.5Mo0.5O6-ı/LSGM/Sr2FeCo0.5Mo0.5O6-ı determined from the impedancepectra as functions of temperatures.

Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1) in H2/H2S at different temperature.

Where A is pre-exponential factor, Kb is Boltzman constant andEa is conductance activation energy.

As shown in Fig. 5, the Ea values of sample x = 0.5 (0.42 eV) is

a little lower than the one of x = 0.75 (0.48 eV), x = 1 (0.47 eV) andx = 0 (0.53 eV) at 300-750 (or 800) ◦C in air, which makes the oxygendiffusion and surface exchange easier.

Fig. 8. I-V and I-P curves of a series of cells withSr2Fe1.5-xCoxMo0.5O6-ı/LSGM/Sr2Fe1.5-xCoxMo0.5O6-ı (x = 0, 0.5, 0.75 and 1) inH2-H2S at 800 ◦C.

Y. Song et al. / Electrochimica Acta 139 (2014) 13–20 19

F rode ap

3p

CseacgactrFitrtR(nSrb

sabtt

ig. 9. SEM images of cross-sectional view of the cell (a), Sr2FeCo0.5Mo0.5O6-ı electore (d) exposed on H2-H2S.

.5. Effect of cobalt-substitution in A-site on electrochemicalerformances

In order to get insight into the effect of different ratios ofo-substitution on electrochemical performance, the electrolyte-upported fuel cells with different Co-doping ratios of SFCMlectrodes were tested with 0.5% H2S-3% H2 as the fuel and airs the oxidant. Fig. 6 shows the impedance spectra of the singleells measured under open-circuit voltage (OCV) conditions ran-ing between 600 and 800 ◦C. The high frequency intercept of therc with the real axis represents the ohmic resistance of the totalell (Rohm). And the low frequency intercept on the real axis is theotal cell resistance (Rtotal). Naturally, the electrode polarizationesistance (Rp) is determined by subtracting Rohm from Rtotal [52].rom Fig. 6a to 6d, the resistance decreases with the temperaturencreasing, which indicates the higher temperature can improvehe performance of MIECs through increasing carrier migrationate. It is worth noting that the Rp (and/or Rtotal) decreases withhe Co-doping in the order of x = 1 > x = 0.75 > x = 0.5. The calculatedp at 800 ◦C in H2S are 4.35, 0.92, 1.86, and 3.34 �·cm2 for SFCMx = 0, 0.5, 0.75 and 1) respectively, and the Rp for x = 0.5 (Fig. 7) sig-ificantly decreased. In addition, the maximum power densities ofFCM (x = 0, 0.5, 0.75 and 1) are 27.3, 42.6, 36.2 and 30.4 mW·cm−2,espectively, as shown in Fig. 8. The cell performance has improvedy almost two times after doping 50% Co.

Fig. 9a shows SEM images for the optimal single cell withample x = 0.5 as symmetrical electrodes and LSGM as electrolyte

fter operating in 0.5% H2S-3% H2. The electrolyte layer excellentlyonded to the both two symmetric electrode layers after test andhe LSGM/SCFM interfaces are distinguished. From the SEM images,he thicknesses of the electrolyte LSGM and electrodes SCFM are

nd LSGM electrolyte interface before (b) and after (c) cell test, Sr2FeCo0.5Mo0.5O6-ı

estimated to be 200 and 80 �m, respectively. Fig. 9b and 9c showthe LSGM/SCFM interfaces before and after cell test. In each pic-ture, the upper part is the dense electrolyte LSGM avoiding the fuelgas (H2S) and the oxidizing gas (air) mixing mechanically, and thebottom portion shows the porous electrode materials SCFM. Theobserved interlayers imply that SFCM has good compatibility withLSGM. From the surface of SCFM uniform particle before and aftercell test, electrode materials SCFM are with approximated porosi-ties of 40% and 30% before and after cell test, which benefits catalyticactivity as a better candidate of MIECs. Fig. 9d additional showsSFCM exposed on H2S at higher magnification, and its average porediameter is estimated 0.4 �m.

4. Conclusions

A series of cobalt-substituted SFCM (x = 0, 0.5, 0.75 and 1) weresynthesized in reducing environment, and their double-perovskitestructures were proved. The effect of different cobalt-substitutionmolar ratio on structural, catalytic activity, oxide ion conductiv-ity and electrochemical properties was systematical investigated,respectively. It is found that cobalt-rich sample for x = 0.75 is withsmaller grain size and lattice parameters, larger BET surface areas,higher HOR catalytic activity as well as more oxygen vacancy,and there is conductance activation energy minimum for samplex = 0.5 caused by the redox couples Fe3+/Fe2+, Co3+/Co2+, Mo6+/Mo5+

and oxygen vacancy. In addition, the impedance of cells varies inthe order of x = 0 > x = 1 > x = 0.75 > x = 0.5 for different Fe/Co molar

ratios, and the single cell with LSGM as supported electrolyte andSFCM (x = 0.5) as symmetrical electrodes exhibits the highest poweroutput with a maximum power density of 42.6 mW·cm−2 fueled by0.5% H2S-3% H2.

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cknowledgements

The authors would like to thank the financial support from Chi-ese Natural Science Foundation on contract Nos. 21207064.

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