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Journal of Power Sources 195 (2010) 80718074

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Journal of Power Sources

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r

Short communication

Sr2Fe4/3Mo2/3O6as anodes for solid oxide fuel cells

Guoliang Xiao, Qiang Liu, Xihui Dong, Kevin Huang, Fanglin Chen

Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States

a r t i c l e i n f o

Article history:

Received 2 June 2010Received in revised form 10 July 2010Accepted 12 July 2010Available online 17 July 2010

Keywords:

Solid oxide fuel cellsAnodePerovskiteElectrochemical performance

a b s t r a c t

Sr2Fe4/3Mo2/3O6 has been synthesized by a combustion method in air. It shows a single cubic per-ovskite structure after being reduced in wet H2 at 800 C and demonstrates a metallic conductingbehavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 C in wet H2

(3% H2O) is about 16S cm1

. This material exhibits remarkable electrochemical activity and stability inH2 . Without a ceria interlayer, maximum power density (Pmax) of 547mWcm2 is achieved at 800 Cwith wet H2 (3% H2O) as fuel and ambient air as oxidant in the single cell with the configuration ofSr2Fe4/3Mo2/3O6|La0.8Sr0.2Ga0.83Mg0.17O3(LSGM)| La0.6Sr0.4Co0.2Fe0.8O3(LSCF). The Pmaxeven increases to595mWcm2 when thecell is operated at a constantcurrent load at 800C foradditional 15h. This anodematerial also shows carbon resistance and sulfur tolerance. ThePmax is about 130mWcm2 in wet CH4(3% H2O) and 472mW cm2 in H2with 100ppm H2S. The cell performance can be effectively recoveredafter changing the fuel gas back to H2.

2010 Elsevier B.V. All rights reserved.

1. Introduction

As one of the cleanest and most efficient power generationdevices, solid oxide fuel cell (SOFC) has drawn considerable atten-tions from worldwide researchers in the past few decades. Inparticular, the high operating temperature makes SOFC directlyutilize hydrocarbon fuels possible[15]. This feature makes thesystem design much simplified and reduces the cost. However,the state-of-the-art Ni-containing anodes presently used in SOFCare susceptible to carbon formation and sulfur poisoning, both ofwhichwouldseverely deteriorate the cell performance, making thedirect oxidation of sulfur-containing hydrocarbon fuels impossible[6,7].Therefore, there is a growing need in developing a new classof anode materials with carbon resistance and sulfur tolerance forthe low-cost, direct hydrocarbon fueled SOFCs.

Ceramic oxides have emerged as the potential candidatesfor the direct oxidation anodes in the past few years, amongwhich perovskite oxides have received the most attention due

to the exhibited high mixed conductivity and good electro-chemical activity in reducing atmospheres [8]. For instance,(La1xSrx)0.9Cr0.5Mn0.5O3 was reported to show reasonably goodperformance at 900 C in wet CH4 (3% H2O) without carbonformation when used as an anode material in SOFCs [911].La4Sr8Ti11Mn0.5Ga0.5O37.5 is another example of potential anodeexhibiting good catalytic activity to wet H2 and CH4 at 950 C[12]. At lower SOFC operating temperatures, however, the cat-

Corresponding author. Tel.: +1 803 777 4875; fax: +1 803 777 0106.E-mail address:chenfa@cec.sc.edu(F. Chen).

alytic activity of these materials is insufficient. Recently, Huangand Goodenough have demonstrated a new class of anodematerials Sr2MMoO6 (M = Mn, Ni, Co) with double perovskitestructure[13,14].The SOFCs using such anode compositions haveshowedgood electro-catalyticactivitiesand sulfur tolerance.Whencombined with ceria based interlayer and Pt current collector,impressive performance was obtained at intermediate tempera-tures. Sr2NiMoO6was also reported to have excellent performancewith H2 in absence of ceria interlayer and Pt current collector[15].These results suggest that the double perovskite type oxideshave resistance to carbon formation and tolerance to sulfur, andtherefore have a great potential to be anode materials for directhydrocarbon fueled SOFCs.

In addition to the double perovskite type anode materials men-tioned above, Sr2FeMoO6 is another interesting candidate. It hasbeeninvestigatedasaferromagneticmaterialandisstableinreduc-ing atmospheres due to the protection of the Mo4+/Mo5+ redoxcouple that prevents Fe3+ from being reducedto Fe2+ [16,17]. Since

in air the most preferable oxidation state of Mo is +6, the phase ofSr2FeMoO6would not form in air. Special cares are needed duringSOFC fabrication in order to ensure the formation of the desirablephase. Although Sr2FeMoO6hasrecentlybeenreportedas an anodein SOFCs by firing the anode in a protective atmosphere [18],it isstill needed to explore Sr2Fe1+xMo1xO6 with different Fe and Moratios, sothat itcan besynthesized inair,or atleastit can beformedunder the fuel cell operating condition.

In this paper, Sr2Fe1+xMo1xO6 (x =0, 0.15, 1/3) have beensynthesized by a combustion method. The Sr2Fe4/3Mo2/3O6composition can be easily reduced at 800C to forma single phase double perovskite. The performance of

0378-7753/$ see front matter 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.jpowsour.2010.07.036

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8072 G. Xiao et al. / Journal of Power Sources 195 (2010) 80718074

the single cell Sr2Fe4/3Mo2/3O6|La0.8Sr0.2Ga0.83Mg0.17O3(LSGM)|La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) has been evaluated byusing H2, CH4, or sulfur-containing H2as fuel.

2. Experimental

2.1. Synthesis

Sr2Fe1+xMo1xO6 (x = 0, 0.15, 1/3) were synthesized by aglycinenitrate combustion process (GNP). In a typical process,strontium nitrate (99%, Alfa Aesar), iron nitrate (99%, Alfa Aesar)and ammonium molybdate (99%, Mallinckrodt Baker) were dis-solved in de-ionized water according to the stoichiometry. Citricacid was used to adjust the pH value of the solution and to preventany precipitation, while glycine was used as a fuel for combustion.After being stirred for several hours, the solution was heated in amicrowave oven until self-ignition. The combustion product, ash,was collected and subsequently calcined at 600 C for 2h and at1100 C for 5 h to remove any organic residues. The powders werethen reduced in a flowing wet H2 (3% H2O) at 800 C for 5h forphase and structure analysis.

The La0.8Sr0.2Ga0.83Mg0.17O3 (LSGM) electrolyte andLa

0.6Sr

0.4Co

0.2Fe

0.8O

3 (LSCF) cathode powders were also pre-

pared by the GNP method which was described elsewhere[19,20].

2.2. Characterizations

Thepowder X-ray diffraction (XRD) patterns were recorded on aD/MAX-3C X-ray diffractometer with graphitemonochromatizedCu-K radiation (=1.5418A) at a scanning rate of 5min1 inthe 2range of 1090. MDI-Jade program was used to identifythe phases and to analyze crystal structures of the samples. Themicrostructural morphology was characterized by scanning elec-tron microscopy (SEM, FEI Quanta and XL 30).

The total electrical conductivity of Sr2Fe4/3Mo2/3O6 in a rect-angular bar shape was measured in wet H

2 (3% H

2O) using the

standard four terminal DC method. The pressed green bar was firstsintered at 1400 C for 5 h and thenreducedat 800 CinwetH2(3%H2O) for 10 h before the conductivity measurement.

2.3. Single cell test

The single cell was fabricated on the LSGM electrolyte substratemade bypressing LSGMpelletsand then sinteringat 1400 Cfor5h.The thickness of the LSGM pellets was about 300 m. The inks ofthe Sr2Fe4/3Mo2/3O6anode and the LSCF cathode were then screenprinted on the two sides of the LSGM electrolyte pellet. The anodeand the cathode together with the LSGM electrolyte were then co-fired in air at 1100 C for 2h. The thickness of the electrodes was

about 2030m. Au paste was printed on the anode surface whilePt paste was printed on the cathode surface as the contact layersfor current collection. Au is inert to the fuel oxidation. Ambientair was used as oxidant. The flow rate of the fuel gases was set at40mlmin1. The Sr2Fe4/3Mo2/3O6anode was reduced in situ uponexposure to wet H2(3% H2O) at 800 C for 5 h. Before any electro-chemical measurement, the cell was stabilized at least for 1 h eachtime when the fuel gas was switched.

3. Results and discussion

3.1. Phase, structure and conductivity

Fig. 1shows the XRD patterns of Sr2Fe1+xMo1xO6 (x = 0, 0.15,

1/3) before and after reduction at 800

C in wet H2 (3% H2O). It is

Fig. 1. XRD patterns of Sr2Fe1+xMo1xO6 (x = 0, 0.15, 1/3) synthesized in air andreduced at 800 C in wet H2(3% H2O).

evident that the oxidized samples (calcined at 1100C for 5h in

air) contain multiple phases. The major impurity can be indexedas SrMoO4 (JCPDS 08-0482). The impurity peaks become weakerwith the decrease of the Mo concentration in the compound. Uponreductionat 800 CinH2, Sr2Fe4/3Mo2/3O6 showsa cubicperovskitestructure with no impurity peak detectable in the XRD pattern. ThecalculatedlatticeparameterofSr2Fe4/3Mo2/3O6 is7.878(3),whichis consistent with the reported value[21].

The conductivity of Sr2Fe4/3Mo2/3O6as a function of tempera-tureisshownin Fig.2. Inthetemperaturerange370830 Cstudied,the electrical conduction follows the metallic behavior that thevalue decreased while the temperature increased. At 800C,it wasabout 16 S cm1.

3.2. Single cell performance

The LSGM electrolyte supported fuel cell with Sr2Fe4/3Mo2/3O6as the anode was tested with wet H2(3% H2O) as the fuel and airas the oxidant. Before testing, the cell was held at 800 C for 5h inorder to reduce the anode.Fig. 3shows the cell voltage and powerdensity as a function of current density at different temperatures.The maximum power density (PMax) of the cell reached 268, 392,547mWcm2 at 700, 750 and 800 C, respectively.

Fig. 2. Conductivity measured in wet H2(3% H2O) on the Sr2Fe4/3Mo2/3O6bar sin-tered in air at 1400 C. The bar was reduced at 800 C in the testing atmosphere for

10 h before test.

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Fig. 3. Cell voltage and power density as a function of current density in wet H2(3% H2O) of the single cell with Sr 2Fe4/3Mo2/3O6anode, LSGM electrolyte and LSCFcathode at 700, 750 and 800 C.

The electrochemical impedance spectra at different tempera-tures were also measured under open-circuit conditions and theresults are shown inFig. 4a. The high frequency intercept on the

real axis represents the total ohmic resistance of the cell (Rohm).Thelowfrequencyinterceptontherealaxisrepresentsthetotalcellresistance (Rtotal). The difference between the two is the electrodepolarization resistance (Rp), including the contribution from boththe cathode and anode. The values ofRp,Rohmand Rtotalof the cellat 700, 750 and 800 C are plotted inFig. 4b.It can be seen that thetotal resistance was mainly dominated byRohm. The value ofRohmat800 C was about 0.4cm2. It is higherthanthat reportedwitha300-m thick LSGM electrolyte supported cell using a Sr2FeMoO6anode due to the lower electrical conductivity of Sr2Fe4/3Mo2/3O6compared with that of Sr2FeMoO6, which is about 200Scm1 at800 C in H2 [18].However, the Rp 0.2cm2 at 800 C is lowerthan that of Sr2FeMoO6, which is about 0.3 cm2 at 800 C[18].

Fig. 4. Impedance spectra and resistances of a single cell measured at 700, 750 and800 C underopen-circuitconduction: (a)impedancespectraof thesingle cellat dif-ferent temperature and (b) the ohmic resistance (Rohm), the polarization resistance

(Rp) and the total resistance (Rtotal) of the single fuel cell.

Fig.5. Electrochemcialstabilityof a singlecell withSr2Fe4/3Mo2/3O6anode at800 Cin wet H2(3% H2O).

The lower Rp is probably due to the increased concentration ofoxygenvacanciesby increasing theFe/Mo ratio in Sr2Fe4/3Mo2/3O6,resulting in a higher catalytic activity[22].

3.3. Stability, carbon resistance and sulfur tolerance

The stability of the Sr2Fe4/3Mo2/3O6anode was investigated byoperating the single cell at Pmax (the current density was set at1020mAcm2 according to the voltagecurrent density curve) inwet H2 for 16h at 800 C. As shown inFig. 5, a slight improve-ment in the cell performance was observed and the Pmaxreached595mWcm2 at 800 C at the end of the tested period. This resultdemonstrates that Sr2Fe4/3Mo2/3O6 is a very promising anodematerial with very good performance stability under the fuel celloperating conditions. It further indicates that there exists no com-patibility issues between the Sr2Fe4/3Mo2/3O6 anode and LSGMelectrolyte and a ceria interlayer is not needed.

The electrochemical performance of the Sr2Fe4/3Mo2/3O6anodefor direct utilization of hydrocarbon fuel and for sulfur tolerancewas also investigated by testing the cell in wet CH 4 (3% H2O)and H2 containing 100 ppm H2S. As shown in Fig. 6, the maxi-mum power density, Pmax reached 130mWcm2 in wet CH4 at800 C. Further, post-test analysis showed no carbon depositionon the Sr2Fe4/3Mo2/3O6 anode, indicating that Sr2Fe4/3Mo2/3O6 isa promising anode for direct hydrocarbon utilization. On the otherhand, the maximum power density, Pmaxwas 472mW cm2 whenthe fuel was switched from wet H2to H2containing 100 ppm H2S,

Fig. 6. Cell voltage and power density as a function of current density in wet CH 4

(3% H2O) and H2with 100ppm H2S at 800

C.

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8074 G. Xiao et al. / Journal of Power Sources 195 (2010) 80718074

Fig.7. Voltage changeof thesinglecellin differentfuelgas at800 Cat240mAcm2

current density when switchingthe fuel gasbetweenwet H2(3% H2O), wet CH4(3%H2O) and H2with 100ppm H2S.

Fig. 8. EDX line scan of La. La signal was not found in the anode after the test.

showingverylittlecellperformancedropinsulfur-containingfuels.Inordertoinvestigatetheperformancestabilityoffuelcellswith

the Sr2Fe4/3Mo2/3O6 anode in different fuels, a constant currentdensity of 240mA cm2 was applied to the cell at 800 C and the

cell voltage was monitored while the cell was exposed to differ-ent fuels. As shown inFig. 7,a sharp decrease in cell voltage from0.96V to around 0.5V was observed after the fuel was switchedfrom wet H2to wet CH4(3% H2O) due to the low catalytic activityof Sr2Fe4/3Mo2/3O6 anode in CH4 compared with that in H2. Thecell voltage gradually became stable and it was about 0.45 V afterbeing operating for 1 h in CH4. However, the cell voltage immedi-ately recovered back to 0.94V after the fuel gas was switched fromCH4back to wet H2. Similar behavior was also observed when thefuel gas was switched between wet H2and H2with 100ppm H2S.The cell voltage was 0.91V in H2with 100ppm H2S and recoveredto0.93V after the fuel gas was switched from H2with100ppmH2Sback to wet H2 again. These results suggest that Sr2Fe4/3Mo2/3O6is a very promising anode material to directly utilize hydrocarbon

and sulfur contained fuels.The EDX line scan result of the interface between the

Sr2Fe4/3Mo2/3O6anode and LSGM electrolyte after the cell perfor-mance testing is shown inFig. 8.No La signal has been observed inthe anode region, indicating that there are no compatibility issueswhen using the Sr2Fe4/3Mo2/3O6anode andLSGM electrolyte with-out the ceria interlayer.

As it is reported on Sr2MgMoO6, the properties of these dou-ble perovskite oxides are strongly affected by the states of B-sitecations, such as the ratio of the cations and the degree of thecation ordering[21,23].By comparing the single cell performance

of Sr2Fe1+xMo1xO6 anode with different Fe/Mo ratios reportedrecently [18,24], it can be found that both the Mo content andthe intrinsic oxygen vacancies determined by the Fe/Mo ratio playimportant roles in the anode catalytic activity. However, furtherfundamental studies are needed in order to elucidate the mecha-nism of these influencing factors.

4. Conclusions

Sr2Fe4/3Mo2/3O6 has been synthesized in air by theglycinenitrate combustion method. A pure perovskite phasewas obtained by reducing the synthesized powder at 800Cin wet H2 (3% H2O). The electrical conductivity of the reducedSr2Fe4/3Mo2/3O6 shows a metallic behavior from 370 to 830

C inwet H2 (3% H2O). The single cell performance evaluation usingSr2Fe4/3Mo2/3O6as the anode indicates good catalytic activity andstability in H2and also shows the potential to utilize hydrocarbonand sulfur-containing fuels directly. Although long term stabilitytest of Sr2Fe4/3Mo2/3O6 is still needed, this material shows thepotential to be a promising candidate anode material for SOFCs.

Acknowledgements

We gratefully acknowledge the financial support of Heteroge-neous Functional Materials for Energy Systems, an EFRC fundedby DoE Office of Basic Energy Sciences under Award Number DE-SC0001061, the South Carolina Space Grant Consortium, the USCFuture Fuels Initiative, the USC NanoCenter, and the USC PromisingInvestigator Research Award.

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