CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 25, NUMBER 5 OCTOBER 27, 2012

ARTICLE

Pervoskite-type Ba0.5Sr0.5Al0.1Fe0.9O3−δ as Intermediate-Temperature SolidOxide Fuel Cell Cathode

Yun Gan, Kui Xie∗

Department of Energy Materials, School of Materials Science and Engineering, Hefei University ofTechnology, Hefei 230009, China

(Dated: Received on April 4, 2012; Accepted on May 3, 2012)

A cobalt-free perovskite-type Ba0.5Sr0.5Al0.1Fe0.9O3−δ (BSAF) is developed and electro-chemically studied as solid oxide fuel cell (SOFC) cathode. The structures, electrical con-ductivity, and electrode polarizations in symmetrical cell based on mixed ion conductingelectrolyte were investigated, respectively. The temperature dependence of conductivity ofBSAF in air shows a typical semiconductor behavior with positive temperature coefficientup to 450 C where the conductivity reaches 14.0 S/cm while above this temperature thenegative temperature coefficient dominates the total conductivity. Electrochemical charac-terizations show desirable polarization resistance of BSAF cathode in a symmetric cell basedon mixed ion conducting electrolyte at 650−700 C. A single SOFC with BSAF cathodeshows OCV of 1.0 V and maximum output of 420 mW/cm2 at 700 C with humidifiedhydrogen fuel and static air oxidant.

Key words: Solid oxide fuel cell, Perovskite, Cathode, Cobalt-free, Mixed ionic conductor

I. INTRODUCTION

Significant efforts are devoted to low temperaturesolid oxide fuel cells (SOFCs) by developing advancedcathodes with sufficient electrocatalytic activity andmixed conductivity [1]. A lot of works are reported[2, 3] among which pervoskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ

(BSCF) has been considered as one of the best candi-dates with regard to the requirements [4]. The long-term structural stability of cobalt-based cathodes atintermediate temperature (∼700 C) however is seri-ously questioned because the ionic radius change ofCo4+/3+ ions results in significant chemical decomposi-tion and adverse thermal expansion that definitely de-grades fuel cell performances [5]. It is therefore stilla continuous incentive to search for alternative SOFCcathode. Martynczuk and his co-workers have recentlyreported a novel stable cobalt-free perovskite-type solidoxide, Ba0.5Sr0.5Al0.1Fe0.9O3−δ (BSAF), which exhibitsthe highest oxygen permeation flux among all thealuminum-based and iron-based materials at interme-diate temperatures [6]. The high oxygen permeationflux indicates the sufficient oxygen surface reaction andbulk diffusion in BSAF oxide. It is therefore reasonableto assume that BSAF oxide might offer possibility towork as an SOFC cathode.

In this work, we develop BSAF as SOFC cathode

∗Author to whom correspondence should be addressed. E-mail:xiekui@hfut.edu.cn

and systematically investigate the electrochemical per-formance. The SOFC is based on a mixed ion conductorBaCe0.5Zr0.3Y0.16Zn0.04O3−δ (BCZY) [7].

II. EXPERIMENTS

BSAF powder was synthesized by combustionmethod at 1050 C for 3 h [8]. The prepared powderswere about 0.5 µm in average size after ball millingin ethanol for 3 h. X-ray diffraction (XRD, Cu Kα1,stone, step=0.02/s, transmission mode) coupled witha general structure analysis system (GSAS, MIT, USA)was performed to analyze the phase formation. Properamount of BSAF powder was pressed into disk followedby sintering at 1150 C for 3 h for conductivity testand the relative density reached 96%. The sintereddisk was polished with sand paper (2×103 mesh) andAu slip was adhered onto disk surface with Au pastefollowed by a heat treatment to assemble current col-lectors. Conductivity test of BSAF disk was carriedout in air using dc four-terminal method from 50 Cto 750 C (van der Pauw configurations) and the con-ductivity was recorded with online system at a stepof 0.01 C. The dependence of conductivity on par-tial oxygen pressure was tested at 700 C with oxygenpartial pressure ranging from 20.2 kPa to 10−19 kPaadjusted by flowing 5%H2/95%Ar. The oxygen par-tial pressure was recorded with an online oxygen sen-sor (ZrO2-based oxygen sensor) with a step of 0.01. Ina typical process, a proton-conducting symmetric cellbased on BCZY electrolyte with BSAF cathode was fab-

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606 Chin. J. Chem. Phys., Vol. 25, No. 5 Yun Gan et al.

20 30 40 50 60 70 80 902θ / ( )o

FIG. 1 XRD rietveld refinement of as prepared BSAF cath-ode.

ricated with screen-printing method followed by a heattreatment at 1050 C for 3 h [18, 19]. In this process,BCZY powder prepared with combustion method waspressed into disk (18 mm in diameter and 3 mm in thick-ness) under 200 MPa followed by a subsequent firing at1350 C for 5 h. The sintered disk was polished withsand paper and stored for future use. BSAF was thenball-milled in ethanol with proper amount of organicadditives to form cathode paste which was then printedonto BCZY electrolyte and then fired at 1050 C for3 h. Silver paste was applied onto electrodes and firedat 550 C for 30 min to make current collectors. Sym-metric cell was tested in 3%H2O/Air atmosphere from650 C to 700 C using electrochemical impedance sys-tem (EIS, Solartron 155+1287). A single SOFC withBSAF cathode based on BCZY electrolyte was fabri-cated by dry-pressing method and tested from 650 C to700 C with ∼3%H2O/H2 (40 mL/min) fuel and staticair oxidizer. The interfacial polarization were recordedwith EIS (0.01 kHz to 100 kHz) and the impedance datawas analyzed using Zview software. The microstructureof cell was observed with scanning electron microscope(JEOL 5600).

III. RESULTS AND DISCUSSION

As shown in Fig.1, the Rietveld refinement (goodnessof fit is 1.1, factor of XRD refinement is 0.115, elec-trode polarization is 0.105) revealed that the obtainedpowder is composed of a predominant perovskite phase(97%), which means 10% Al may be not completelyincorporated into the perovskite lattice. BSAF is incubic phase with space group Fm-3m and cell param-eter of 396.69 µm. The second and third phases couldbe BaAl2O4 (2.5%) [9] and Al3Ba2O6 [10]. As shownin Fig.2(a), the temperature dependence of conductiv-ity shows a semiconducting behavior with a positivetemperature coefficient up to 450 C, corresponding toactivation energy ∼32.54 kJ/mol. The conductivity in-creases to 14.0 S/cm at 450 C, which is much higher

FIG. 2 Conductivity of Ba0.5Sr0.5Al0.1Fe0.9O3−δ versus (a)temperature from 150 C to 750 C in air and (b) oxygenpartial pressure.

than that of reported Ba0.5Sr0.5Co0.8Fe0.2O3−δ cath-ode in similar temperature region. However, the nega-tive temperature coefficient was observed with activa-tion energy of −9.48 kJ/mol between 450 and 750 Cwhere the conductivity finally decreases to 6 S/cm. Thedecrease of conductivity may be attributed to the de-crease of p-type conduction caused by the temperature-induced oxygen loss at higher temperatures as demon-strated by Ullmanna et al. [11]. Figure 2(b) shows thechange of dc conductivity as a function of oxygen par-tial pressure. It was observed that the sharp decreaseof conductivity at lower PO2 further indicates p-typeconduction, which is in agreement with the oxygen lossof the ferrite-based materials that causes the degrada-tion of mixed conductivity of solid oxide materials. Theslight increase of conductivity in extremely low oxygenpartial pressure may come from the n-type conductionof Fe2+/Fe3+. In total, it may be reasonable to concludethe conductivity of BSAF is more promising than thatof Ba0.5Sr0.5Co0.8Fe0.2O3−δ at low temperature [12].

Figure 3 shows the cathode polarization in a symmet-ric cell based on BCZY electrolyte at 650 and 700 Crecorded with AC impedance. The series resistance (Rs)of the symmetric cell is determined with the theoreticalintercept of impedance plot with real axis at high fre-quency while the intercept at low frequency representsthe total resistance (Rt). The difference between Rt andRs is corresponding to the polarization resistance (Rp)

DOI:10.1088/1674-0068/25/05/605-608 c©2012 Chinese Physical Society

Chin. J. Chem. Phys., Vol. 25, No. 5 Ba0.5Sr0.5Al0.1Fe0.9O3−δ as Solid Oxide Fuel Cell Cathode 607

FIG. 3 (a) AC impedance spectra of BSAF cathode basedon BCZY electrolyte at 650 and 700 C. Z′ and Z” representthe real part and the imaginary part, respectively. (b) Thecircuit diagram.

10 µm

50 µm

(a)

(b)

FIG. 4 Microstructures of BSAF cathode in (a) symmetriccell and (b) single SOFC based on BCZY electrolyte.

of BSAF cathode. As shown in Fig.3, Rp at 700 Cis 0.1 Ω·cm2 which is comparable to BSCF cathodesunder similar conditions [6]. However, Rp increases to0.2 Ω·cm2 at 650 C, which may be due to the loss ofionic conductivity and electrocatalytic activity thoughthe total mixed conductivity is higher at lower temper-atures. The porous and uniform BSAF cathode layerwith thickness of 15 µm adheres to electrolyte very wellas shown in Fig.4(a) indicating better thermal compat-ibility. Figure 4(b) shows the microstructure of a single

FIG. 5 Performance of SOFC with Ba0.5Sr0.5Al0.1Fe0.9O3−δ

at 650 C (square) and 700 C (circle).

FIG. 6 (a) AC impedance spectra of SOFC under OCV at650 and 700 C. (b) The open equivalent circuit diagram.

cell with BSAF cathode based on BCZY electrolyte andNi anode. The 40-µm-thick electrolyte is very dense anduniform and adheres very well to the porous anode andcathode further implying better thermal compatibility.

As shown in Fig.5, the OCVs reach 1.0 and 1.04 V at700 and 650 C with corresponding maximum outputsof 420 and 300 mW/cm2, respectively. However, thetested open circuit voltages (OCVs) are still lower thantheoretical ones (1.17 V at ∼700 C), which may be dueto the electronic conduction of Ce4+/Ce3+ at highertemperatures and reducing conditions.

Figure 6 shows the AC impedance under open circuitconditions at two different temperatures. In the equiv-alent model circuit, Rp and Rc correspond to electrodepolarization resistance and contacting polarization re-sistance of electrical wire, respectively. Impedance arcsbelow 10 kHz are related to the electrode processes [13].As shown in the Fig.6, Rp increases from 0.22 Ω·cm2 to0.41 Ω·cm2 with the temperature dropping from 700 C

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608 Chin. J. Chem. Phys., Vol. 25, No. 5 Yun Gan et al.

to 650 C, which could be due to the loss of electrodeactivity at lower temperature. For this cell, two limitingprocess affected the impedance and the low frequencyone is dominating. The high frequency process com-ing from the limitation of charge transfer demonstratesthat electrical conduction of one electrode could be alimiting step in this process. The summit frequency ofthe low frequency arc is in the range of 1.0−1.25 Hz,which is the rate limiting process caused by gas diffu-sion limitation at electrodes [13].

IV. CONCLUSION

In this work, high performance oxygen permeationmaterial Ba0.5Sr0.5Al0.1Fe0.9O3−δ (BSAF) was pre-pared by a combustion method and electrochemicallycharacterized as a solid oxide fuel cell (SOFC) cathode.The temperature dependence of conductivity showssemiconducting behavior and negative temperature co-efficient below and above 450 C, respectively. The asprepared BSAF cathode demonstrates desirable perfor-mance (0.1 Ω·cm2 at 700 C) in a symmetric cell basedon BaCe0.5Zr0.3Y0.16Zn0.04O3−δ (BCZY) electrolyte. Asingle SOFC with BSAF cathode and BCZY electrolyteshows 420 mW/cm2 at 700 C indicting BSAF could bea proper SOFC cathode candidate.

[1] J. M. Ralph, A. C. Schoeler, and M. Krumpelt, J.Mater. Sci. 36, 1161 (2001).

[2] C. R. Xia, W. Rauch, F. L. Chen, and M. L. Liu, SolidState Ionics 149, 11 (2002).

[3] Y. Lin, R. Ran, D. J. Chen, and Z. P. Shao, J. PowerSources 15, 4700 (2010).

[4] Z. P. Shao and S. M. Haile Nature 431, 170 (2004).[5] M. Arnold, T. M. Gesing, J. Martynczuk, and A. Feld-

hoff, Chem. Mater. 20, 5851 (2008).[6] J. Martynczuk, F. Liang, M. Arnold, V. S. Epelak, and

A. Feldhoff, Chem. Mater. 21, 1586 (2009).[7] L. Yang, S. Z. Wang, K. Blinn, M. F. Liu, Z. Liu, Z.

Cheng, and M. L. Liu, Science 326, 126 (2009).[8] K. Xie, Q. L. Ma, B. Lin, Y. Z. Jiang, J. F. Gao, X. Q.

Liu, and G. Y. Meng, J. Power Sources 170, 38 (2007).[9] R. H. Arlett, J. G. White, and M. Robbins, Acta Crys-

tallography 22, 315 (1967).[10] K. Hilpert, H. Beske, and A. Naoumidis, High Temp.

Sci. 7, 159 (1975).[11] H. Ullmann, N. Trofimenko, F. Tietz, D. Stover, and

A. Ahmad-Khanlou, Solid State Ionics 138, 79 (2000).[12] C. W. Sun, R. Hui, and J. Roller, J. Solid State Elec-

trochem. 14, 1125 (2010).[13] S. Primdahl and M. Mogensen, J. Electrochem. Soc.

146, 2827 (1999).

DOI:10.1088/1674-0068/25/05/605-608 c©2012 Chinese Physical Society