Electrochimica Acta 150 (2014) 151–156

Electrochemical performance of novel cobalt-free perovskiteSrFe0.7Cu0.3O3-d cathode for intermediate temperature solidoxide fuel cells

Qiang Li *, Tian Xia, Liping Sun, Hui Zhao *, Lihua HuoKey Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University,Harbin 150080, PR China

A R T I C L E I N F O

Article history:Received 18 September 2014Received in revised form 24 October 2014Accepted 26 October 2014Available online 30 October 2014

Keywords:Solid oxide fuel cellsCathode materialElectrochemical performanceOxygen reduction reaction

A B S T R A C T

A cubic perovskite oxide SrFe0.7Cu0.3O3-d (SFC) is investigated as a novel cathode material forintermediate temperature solid oxide fuel cells (IT-SOFCs). The average thermal expansion coefficient(TEC) of SFC is close to that of typical Ce0.9Gd0.1O1.95 (CGO) electrolyte. The electrical conductivity of SFCsample reached 25–54 S cm�1 over the temperature range of 500–800 �C. According to analysis results ofAC impedance spectroscopy and DC polarization measurement, the polarization resistance (Rp) of SFC ascathode material on CGO electrolyte was successfully reduced to 0.14 V cm2 at 700 �C in air. Two mainoxygen reduction reaction (ORR) processes are identified in accordance with the oxygen partial pressuredependence of Rp, in which the rate limiting step for the ORR is the oxygen ion transfer within the bulkelectrode and/or from electrode to CGO electrolyte through the triple phase boundary (TPB). Theexcellent electrochemical performance indicated that SFC can be a promising cathode material for IT-SOFCs.

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1. Introduction

In recent years, continuous efforts were paid in reducing theoperation temperature of solid oxide fuel cells (SOFCs) into anintermediate range of 500–800 �C [1,2]. However, in this tempera-ture range, the insufficient catalytic activity of the cathodes duringthe oxygen reduction reaction is always one of the most significantchallenges for practical applications of so-called intermediatetemperature SOFCs (IT-SOFCs). In this sense, the development ofhigh-performance cathode material is critical for the furtherimprovement of cell performance.

Perovskite-type SrFeO3 is an interesting material with thefeature of highly mixed oxide ionic and electronic conductivity(500–700 �C, 95–180 S cm�1), which thereby can be potentiallyused in electrochemical devices, such as oxygen permeationmembranes, oxygen sensors and SOFCs [3–5]. However, thestructure transformation from cubic perovskite into brownmiller-ite, accompanied with serious deterioration in electrochemicalproperties, during device operation limited their practical appli-cations [6]. In this case, developing SrFeO3 system with enhanced

* Corresponding authors. Tel.: +86 45186608426.E-mail addresses: liqiang@hlju.edu.cn (Q. Li), zhaohui98@yahoo.com (H. Zhao).

http://dx.doi.org/10.1016/j.electacta.2014.10.1370013-4686/ã 2014 Elsevier Ltd. All rights reserved.

structural stability becomes crucial for facilitating their deviceapplications.

One of the promising approaches to enhancing the electrodeperformance of SrFeO3 in SOFCs is cation doping [7–10]. Simneret al. [11] reported LaxSr1-xFeO3 ferrites with low thermalexpansion coefficients, which match with Sm-doped CeO2 (SDC)or Y-doped ZrO2 (YSZ) electrolytes. Nevertheless, electrochemicalperformance of LaxSr1-xFeO3 as a cathode in SOFCs is still notsatisfied. Xiao et al. [12] have evaluated a series of SrFe1-xMoxO3-d

(x = 0.05–0.25) oxides as prospective promising cathode for SOFCs,in which SrFe0.8Mo0.2O3-d electrode exhibited the highest perfor-mance with a polarization resistance of 0.074 V cm2 at 800 �C.However, these SrFe1-xMoxO3-d exhibited relatively larger thermalexpansion coefficients (TECs) (20.9–25.7 � 10�6 K�1, at 800 �C).Recently, the relatively high oxygen permeating ability and thestability of SrFe1-xCuxO3-d perovskite-type mixed conductor weredemonstrated [13]. Furthermore, the direct proportionalitybetween oxygen permeability and oxygen vacancy concentrationof SrFe1-xCuxO3-d and the corresponding Cu doping concentrationwas established with the highest oxygen vacancy concentrationachieved by SrFe0.7Cu0.3O3-d [14]. As we know, Fe ion normallytakes a mixed oxidation state between 3+ and 4+ in perovsikiteoxide [15,16]. Therefore, replacing Fe ions with divalent andtrivalent Cu ions in a perovskite lattice can lead to an increase ofoxygen vacancies for improving oxygen conducting behavior [17].

152 Q. Li et al. / Electrochimica Acta 150 (2014) 151–156

It was generally believed that these oxygen defects offered thepossibility of rapid oxygen transport through the ceramic material.These results revealed that SrFe1-xCuxO3-d oxides would bepromising as cathode materials for SOFC.

In this preliminary work, the electrochemical properties ofSrFe0.7Cu0.3O3-d (SFC) material supported on Ce0.9Gd0.1O1.95 (CGO)electrolyte and the kinetics of oxygen reduction reaction (ORR) onthese electrodes are studied.

2. Experimental

The SrFe0.7Cu0.3O3-d (SFC) powder was prepared by the solidstate reaction. Stoichiometric amount of SrCO3 (99.99%), Fe2O3

(99.99%) and CuO (99.99%) were mixed and grounded in an agatemortar, then calcined in air at 1000 �C for 24 h. Ce0.9Gd0.1O1.95

(CGO) powder was prepared according to ref. [18]. The CGOpowders were uniaxially pressed at 220 MPa and then sintered at1400 �C for 10 h to form a dense electrolyte pellet. The pellet wasabout 1 mm in thickness and 15 mm in diameter.

The SFC powder was dispersed in terpineol as ink, which wassubsequently painted on one side of the CGO pellet to formworking electrode (WE) with area of 0.5 cm2. Platinum paste waspainted on the other side of the pellet in symmetric configuration,as the counter electrode (CE). A Pt wire was used as referenceelectrode (RE) and put on the same side of the working electrode.Pt gauze attached to a Pt wire, was then press-contacted to the SFCand Pt paste electrodes with the aid of a spring loaded ceramic cap,thus serving as the final current collector. Fig. 1 shows theschematic illustration of the configuration of the cell. The cell wasfirst heated up to 400 �C to eliminate organic binders, followed bysintering at 800–1000 �C for 4 h in air.

The structure and phase stability of the material wereinvestigated by X-rays powder diffraction with a Bruker D8-Advance diffractometer (operating at 20 kV and 40 mA; Cu Karadiation with l=0.15418 nm). The diffraction angle was variedfrom 20–80� with a step size of 0.02�. The morphology andmicrostructure of the sintered electrodes were examined by fieldemission scanning electron microscope (FEG-SEM, Hitachi S-4700)at an accelerating potential of 20 kV. The conductivity of SFC wasmeasured with the four-terminal DC method using Keithley2400 digital source meter and AI 808P program temperaturecontroller controlled by computer, from room temperature to800 �C in air. Thermal expansion behavior was measured withSetsys Evolution TMA system (SETARAM, France). The measure-ments were carried out in air with a heating rate of 5 �C min�1 fromroom temperature to 800 �C. The impedance spectra were recordedover the frequency range 1 MHz to 0.1 Hz using AutolabPGStat30 potentiostat. The measurements were performed atequilibrium potential (OCV) as a function of temperature (500–700 �C) and oxygen partial pressure (in an N2/O2 mixed atmo-sphere). The EIS fitting analysis was performed with the Zviewsoftware. The dc polarization experiments were performed by the

Fig. 1. Schematic representation of the cell.

chronoamperometry method [19], which involved a potential stepfollowed by recording the current density as a function of time. Thecathode overpotential was calculated according to the followingrelation:

hWE ¼ DUWR � iRel (1)

Where hWE represents the cathode overpotential, DUWR is theapplied voltage between working electrode and reference elec-trode, i is the current flowing through the test cell and Rel is theresistance of the electrolyte obtained from the impedancespectrum.

3. Results and discussions

3.1. Chemical stability of the cathode material

XRD experiments were first performed to check the phasepurity of the materials. Fig. 2a shows the XRD pattern of preparedSFC powder. It indicated that SFC crystallized in a single phaseconsistent with perovskite-type structure [14] without anyimpurities observed. As we know that in some cases, the interfacialchemical reaction between cathode and electrolyte is undesirablefor long term stability of SOFCs, which increases the interfacialpolarization resistance and thus results in the degradation of cellperformance [20]. Therefore, it is crucial to investigate the possiblechemical reaction between SFC and CGO. With this concern, thereactivity of SFC with the CGO electrolyte was studied by mixingSFC with CGO powders in 1:1 weight ratio, and then heat treatingat 1100 �C for 24 h in air. Fig. 2c shows the XRD patterns of theresulted SFC-CGO mixture, in which no new diffraction patternswere detected except for those from SFC and CGO (Fig. 2c). Thepreserved SFC and CGO structures verified that no reaction and/orinter-diffusion of elements occurred between SFC and CGO attemperature up to 1100 �C.

The mismatch of the TEC between the cathode and electrolytemay result in delamination at the cathode/electrolyte interface, orcracking of the electrolyte because of the stress development uponheating and cooling cycle [21]. Fig. 3 shows the thermal expansioncurve of SFC sample over a temperature range of 50–800 �C in air. Itexhibits an average TEC of �13.8 � 10�6 K�1 (50–800 �C), which isalmost equivalent with that of CGO electrolyte (average TEC13.5 �10�6 K�1) [22] and remarkably lower than those of previousreported cobalt-free perovskites, such as Ba0.65La0.05FeO3 of

Fig. 2. XRD patterns of SFC (a), CGO (b) and SFC-CGO (c) mixtures after sintering at1100 �C for 24 h in air.

Fig. 3. Thermal expansion curve of SFC sample in a temperature range of 50–800 �Cin air.

(a)

(b)

Fig. 5. (a) Impedance spectra of SFC cathode measured in air at varioustemperatures. (b) Arrhenius plots of resistances RH and RL with the correspondingactivation energy for SFC cathode.

Q. Li et al. / Electrochimica Acta 150 (2014) 151–156 153

�24.5 �10�6 K�1 [23], Ba0.5Sr0.5Co0.2Fe0.8O3 of �24.0 � 10�6 K�1

[24] and Sm0.5Sr0.5Fe0.8Cu0.2O3 of �15.9 � 10�6 K�1 [25] in therange temperature range of 30–900 �C. The quite similar TEC canimprove the thermal expansion compatibility between SFCcathode and CGO electrolyte, resulting in substantial improve-ments in both cell performance and operating stability.

3.2. Electrochemical measurements of the cathode material

The electrical conductivity of SCF was measured within thetemperature range of 300–800 �C using the four-terminal DCtechnique with the results shown in Fig. 4. The conductivity of SFCwas in the range of 18–54 S cm�1 at temperature from 300 to800 �C in air, which is smaller than that of un-doped SrFeO3 [12].For the perovskite oxide, the electronic conduction is afforded bypolaron hopping between the B-site cations and the oxygen ionsthrough a mechanism similar to the Zerner double exchangeprocess [26], therefore the remarkably increased concentration ofoxygen vacancies in sub-lattices of SFC would block electronhopping channels between the B-site cations and oxygen ions [27].

Fig. 5(a) shows typical EIS diagrams of the SFC cathodemeasured at 600–700 �C in air. They are composed of a large arclocated in between the high frequency and low frequency zones,

Fig. 4. The electrical conductivity vs. temperature in air.

respectively. This indicated that at least two different processesinvolved in the oxygen reduction reaction (ORR) over the electrode.These Nyquist plots have been fitted by means of the ZviewTM

software (Scribner Associated, Inc.), using an equivalent circuit(inset in Fig. 5a) constituted of one resistance Rel and two R-CPEelements in parallel associated in series. In this equivalent circuit,Rel represents the intercept value in the impedance diagram at highfrequency side with the real axis: it corresponds to the electrolyteresistance, electrode ohmic resistance and lead resistance. RH andRL are the resistances corresponding to the high frequency and lowfrequency arcs, respectively. The cathode polarization resistance(Rp) of the SFC cathode is the sum of RH and RL. CPE is constantphase element whose value reflects the reaction mechanism ofdifferent electrode processes. Its impedance can be expressed as:ZCPE = 1/Yo(jv)p, where Y0 is the module of the CPE element, vangular frequency and p is a fitting parameter between 0 and1 related to the depression angle of the semi-circle(p = 1 corresponds to a pure capacitance). The value of theequivalent capacitance, Ceq, is deduced from the relationship:Ceq ¼ R

1�ppð Þ � Y0

1p

Form the impedance spectra (Fig. 5a), it can be observed thatthe value of the polarization resistance (Rp) decreases withtemperature. The values are: 0.14, 0.34 and 0.85 V cm2 at 700,650 and 600 �C in air, respectively. It was noteworthy that Rp of SFC

Table 1Polarization resistances for different iron-based cathodes measured at 700 �C in air.

Cathode Electrolyte Temperature (�C) Polarizationresistance (V cm2) Ref.

SrFe0.7Cu0.3O3-d CGO 700 0.14 This workPr0.8Sr0.2FeO3-d YSZ 700 1.96 [28]SrFe0.9Si0.1O3-d CGO 700 0.90 [29]La0.5Sr0.5FeO3-d SDC 700 0.79 [30]Bi0.2Sr0.8FeO3-d SDC 700 0.50 [31]La0.6Sr0.4Fe0.8Cu0.2O3-d SDC 700 0.31 [32]Ba0.5Sr0.5Fe0.8Zn0.2O3-d SDC 700 0.23 [33]La0.8Sr0.2Fe0.8Co0.2O3-d SDC 700 1.50 [34]

154 Q. Li et al. / Electrochimica Acta 150 (2014) 151–156

was rather low at 500–700 �C in air in comparison to those ofreported analogous cathodes with different iron-based (Table 1).Therefore, SFC electrode is among the favorable active electro-catalysts for ORR, likely owing to the promising oxygen perme-ability of SFC material [14]. The improved oxygen diffusionproperties of SFC would support rapid oxygen transfer insidethe ceramic material, thereby enhance the electrocatalytic activityof the electrode.

Fig. 5(b) shows the Arrhenius plots of RH and RL of SFC cathode.According to the fitting curves, it was obvious that at the sametemperature the value of RH was larger than that of RL. Thecorresponding activation energy (Ea) estimated according to thecurve slopes was 1.34 eV and 0.93 eV for RH and RL, respectively.Compared to the reported results, Ea value of 0.93 eV wasapproximate to activation energy for oxygen diffusion withinthe bulk material, while Ea value of 1.34 eV corresponded to theinterface charge transfer process, which was involved in the ORRprocess [35].

In order to investigate the effect of sintering temperature on thecathode performance, different sintering conditions were per-formed. Fig. 6 shows the impedance spectra of the SFC cathodeafter sintered at different temperatures for 4 h and then measuredat 700 �C in air. The fitting results once more indicated thepredominance of the high frequency arc in spectra and the largervalues of RH compared to RL. Further analysis of the fitting datafound that the sintering temperature had dramatic effect on RHvalue. RH was relatively large when the sintering temperature waslow (800 �C in this case). When the sintering temperature was900 �C, RH reduced to the lowest value. RH increased again whenthe sintering temperature was 1000 �C. On the contrary RLexhibited almost no dependency on sintering temperature.

Fig. 6. Impedance spectra of SFC cathode sintered at different temperatures for 4 hand measured at 700 �C in air.

The electrode microstructure is related to the surface character-istics and the carrier transport. These properties present thesignificant effect on the fuel cell performance through the reactionkinetics, charge transport and mass transport processes. Therefore,the microstructure evolution of SFC cathode was further studied asa function of sintering temperature. Fig. 7(a) and (b) shows theimages of the surface and cross-section microstructures of SFCcathode sintered on CGO electrolyte at 900 �C for 4 h. Themicrostructure of SFC cathode exhibits moderate porosity andwell-necked particles, in which the average particle size was about1 mm.

Furthermore, the average thickness of the painted cathode layerwas about 25 mm, and the deviation of different electrodes did notexceed 2 mm, respectively. This kind of microstructure wasbeneficial to the improved cathode property as evidenced byimpedance measurements. Therefore, all the cathodes utilizedbelow were prepared with the optimized sintering condition at900 �C for 4 h, to obtain the best performance.

In order to determine the reaction rate limiting step, the effectof oxygen partial pressure on the polarization resistance of SFC

Fig. 7. SEM images of SFC electrode sintered at 900 �C (a) and the cross-sectionimage (b).

Fig. 8. Impedance spectra of SCF cathode measured at 700 �C under various oxygenpartial pressures.

Fig. 9. (a) The relations of polarization resistance to pO2, and schematics of theircorresponding ORR processes on the cathode. (b) Oxygen partial pressuredependence of RH and RL of SFC cathode at 700 �C.

Q. Li et al. / Electrochimica Acta 150 (2014) 151–156 155

cathode was further studied. Fig. 8 shows typical impedancespectra of SFC cathode measured at 700 �C under different oxygenpartial pressures (pO2). Two characteristics of these EIS diagramsare readily noticeable. First, the polarization resistance increases(Rp) dramatically with deceasing oxygen partial pressures. As thepO2 decreases from 0.150 to 0.020 atm, the Rp value increases from0.58 to 0.91 V cm2, indicating an influence of the oxygenconcentration on the ORR process. Secondly, all the impedancediagrams are fitted using two arcs located at high frequency andlow frequency zones, implying the polarization resistance mainlycomes from two different electrode processes. Generally, Rp varieswith the oxygen partial pressure according to the followingequation [36],RP ¼ R0

P PO2ð Þ�n (2), where R0P is a constant. The value

of n may give useful information about the type of species involvedin the ORR [37,38]. The overall ORR can be described as1=2O2ðgÞþ 2e= ¼ O2�, while it is a complex multi-process whichbasically involves the gas diffusion, surface adsorption/dissocia-tion, charge transfer reaction etc. For metal oxide electrodes onsolid electrolytes, n = 1 can been attributed to the gaseousdiffusions and adsorptions of oxygen molecules; n = 1/2 corre-sponded to the oxygen adsorption-desorption process, involvingoxygen diffusion at the interface of gas/cathode and surfacediffusion of related intermediate oxygen species; and n = 1/4 related to the charge transfer process on the electrode, occurringat the interfaces of current collector/electrode and the electrode/electrolyte, respectively; n = 0 can been attributed to the oxygenion diffusion from the triple phase boundary (TPB) to theelectrolyte, as schematically illustrated in Fig. 9(a).

The oxygen partial pressure dependence of Rp for SFC cathode at700 �C is shown in Fig. 9(b); from the linear variation of the pO2

dependence, the slope can be calculated. As seen from Fig. 9(b), a0.51 pO2 dependency of RL was observed. We propose that the lowfrequency arc be assigned to the oxygen adsorption-desorptionprocess. The characteristic of RH showed quite weak pO2

dependency (n = 0.09), which could be related to the oxygen iontransfer within the bulk electrode and/or from electrode to CGOelectrolyte through the TPB. With regard to the results of Fig. 9(b),it is also observed that RH is always larger than RL in the wholerange of oxygen partial pressures at 700 �C. Therefore, it isconcluded that the oxygen ion transfer within the bulk electrodeand/or from electrode to CGO electrolyte through the TPB is themajor rate limiting step for SFC cathode.

Cathode overpotential is an important factor when evaluatingthe electrode performance. Typical overpotential curves for SFC

cathode as a function of current density shows in Fig. 10. Asexpected, the cathode overpotential decreased with temperatureincreasing, indicating the improved catalysis property under hightemperature. The lowest polarization overpotential, 90 mV, wasobtained for SFC cathode with a current density of 103 mA cm�2 at700 �C in air, which was rational with the formation of largeamount of oxygen vacancies through the doping of copper in thelattice. Oxygen vacancies can promote the mobility of the oxygenions to support the amount of absorbed oxygen on the electrodesurface and the enhanced migration of oxygen to the TPB, as well asfacilitated oxygen diffusion to the electrode/electrolyte interface.The SFC material can be considered as a promising cathodecandidate for IT-SOFCs, given that the electrode performance canbe further improved by optimizing the electrode microstructureand constructing composite structures.

Fig. 10. The overpotential-current density curves for SFC cathode measured in air atvarious temperatures.

156 Q. Li et al. / Electrochimica Acta 150 (2014) 151–156

4. Conclusions

A copper-doped perovskite oxide SrFe0.7Cu0.3O3-d (SFC) has beenprepared by solid state reaction and consequently assessed forpotential application in SOFCs. SFC cathode formed good contactwith CGO electrolyte after sintering at 900 �C for 4 h. No chemicalreaction was occurred between SFC electrode and CGO electrolyte.The average TEC of SFC was 13.8 � 10�6 K�1 at 50–800 �C, which wasapproximate with that of CGO electrolyte. The rate limiting step forthe ORR on SFC electrode is the oxygen ion transfer within the bulkelectrode and/or from electrode to CGO electrolyte through the TPB.The lowest polarization resistance obtained at 700 �C in air wasabout 0.14 V cm2. The polarization curve of SFC reveals favorableelectrochemical behavior as a promising IT-SOFC cathode.

Acknowledgements

The Project was supported by National Natural ScienceFoundation of China (51102083, 51302069, 51472077), Programfor New Century Excellent Talents in University (NCET-13-0779),Program for New Century Excellent Talents in HeilongjiangProvincial University (1254-NCET-016), Youth Scholar BackboneSupporting Plan Project of Heilongjiang General Colleges andUniversities (1253G046).

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