Journal of The Electrochemical Society, 159 (2) B185-B194 (2012) B1850013-4651/2012/159(2)/B185/10/$28.00 © The Electrochemical Society

Effect of Strontium Content on Chromium Depositionand Poisoning in Ba1−xSrxCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7)Cathodes of Solid Oxide Fuel CellsYu-Mi Kim,a Xinbing Chen,b San Ping Jiang,b,z and Joongmyeon Baea,z

aDepartment of Mechanical Engineering, Korea Advanced Institute of Science and Technology,Daejeon 305-701, KoreabFuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth,Western Australia 6102, Australia

The effect of strontium content on Cr deposition and poisoning in barium strontium cobalt ferrite electrodes, Ba1−xSrxCo0.8Fe0.2O3−δ

(BSCF, 0.3 ≤ x ≤ 0.7), was investigated at 900◦C for 20 h with a cathodic polarization of 200 mA cm−2. In contrast to the superb andstable electrochemical performance of the BSCF electrodes without interconnects, electrode polarization resistance and overpotential(RE and η) were found to increase for the O2 reduction reaction on BSCF electrodes in the presence of Fe-Cr metallic interconnects.Most important, the increase in RE and η values, �RE and �η measured after polarized at 200 mA cm−2 for 20 h, increasedsignificantly as the Sr content of the BSCF electrodes was increased, indicating that the Sr content in BSCF electrodes is a governingfactor in Cr poisoning. Scanning electron microscopy (SEM) analysis of the BSCF electrodes after the Cr poisoning test at 900◦Cfor 20 h revealed the presence of Cr deposits on the cathode surface. The magnitude of Cr deposits on the electrode surface underthe rib of the interconnect is significantly higher than that under the channel of the interconnect. The results in this study indicatethat Sr and/or Ba segregation plays an important role in the Cr deposition and poisoning for the O2 reduction on BSCF cathodes,which is closely related to the Ba/Sr ratio of the cathode.© 2011 The Electrochemical Society. [DOI: 10.1149/2.092202jes] All rights reserved.

Manuscript submitted March 2, 2011; revised manuscript received November 16, 2011. Published December 21, 2011.

In recent years, metallic alloys have been studied as interconnectsfor intermediate temperature solid oxide fuel cells (IT-SOFCs).1, 2

In particular, metallic interconnects play important roles in planarSOFCs, providing the electrical connection between single cells andacting as separators between anodic fuels and cathodic oxidants. Thesemetallic interconnects require high corrosion resistance at high tem-perature in both oxidizing and reducing atmospheres. Also, high elec-trical conductivity and suitable thermal expansion coefficients (TECs)compatible with other components of SOFCs are essential require-ments for metallic interconnects. The most common metallic alloysused as interconnects in SOFCs are ferritic steels with chromium con-tents over 20 wt %, referred to as chromia-forming alloys due to theirgood thermal properties, high corrosion resistance at high tempera-ture and conductive chromium oxide, Cr2O3, scales.1 However, Cr2O3

can further react with oxygen and moisture to form various gaseoushexavalent Cr species such as CrO3 and CrO2(OH)2 under the operat-ing temperatures of SOFCs.3, 4 The volatility can be further enhancedif there is presence of any alkali metals in sealing glasses due tothe formation of even more volatile species.5 The gaseous chromiumspecies from the interconnect would migrate through the cathode anddeposit in the form of low valence chromium (III) species either at theelectrode/electrolyte interface, inside the bulk of the electrode and/oron the electrode surface. The accumulation of Cr deposits eventuallyleads to a serious polarization loss of the cathode and performancedegradation of the cell.6–12 The chromium deposition and poisoningis a critical issue for the application of metallic interconnect materialsin IT-SOFCs.10, 13

Numerous studies have shown that the occurrence of Cr deposi-tion and poisoning phenomena depends on the nature of electrodematerials. Badwal et al.12 investigated stacks operated between 900and 1000◦C with air oxidant and 97%H2/3%H2O fuel using highchromium alloy (Plansee) as interconnect. Cell performance declinedrapidly and manganese chromium spinel, (Mn,Cr)3O4, was observedin the (La,Sr)MnO3 (LSM) cathode. The degradation of Cr speciesis considered to be closely related to the oxygen activity at the elec-trode/electrolyte interface.12 In another words, the deposition of Cr isdriven by the electrochemical reduction of gaseous Cr species to solidCr2O3 phase in competition with the O2 reduction reaction.9, 12, 14–16

z E-mail: jmbae@kaist.ac.kr; s.jiang@curtin.edu.au

Hilpert et al.3 discussed the thermodynamics of the volatile chromiumspecies and the results indicate that the electrochemical reduction ofthe high valent chromium species and the O2 reduction reaction arethermodynamic compatible under the SOFC operation conditions.

We studied in details the chromium deposition on the LSM cath-ode and found that at the initial stages of the O2 reduction reaction inthe presence of Fe-Cr alloy interconnect, chromium deposition doesnot occur preferentially at the three phase boundaries at the LSMelectrode/YSZ electrolyte interface region, and in fact the chromiumdeposition preferentially occurs on the YSZ electrolyte and fills thespace between the LSM grains with the increase in the cathodic po-larization time.11, 17, 18 The deposition process is most likely limitedby the nucleation reaction between gaseous Cr species and nucle-ation agent, e.g., manganese species (Mn2+) generated under cathodicpolarization or at high temperatures in the LSM electrode/zirconiaelectrolyte system. Wang and Fergus19 studied the chromium depo-sition at a Pt electrode on undoped and Mn-doped YSZ electrolyte.The results show the significantly higher chromium deposition on theMn-doped YSZ electrolyte surface, indicating that the presence ofmanganese species promotes the chromium deposition. Konyshevaet al.8 found that the chromium deposition increases with the polar-ization current and chromium deposition in the LSM electrode alsooccurs in open circuit (i.e., no polarization current). Using high res-olution TEM, Krumpelt reported that the observed microstructrualdegradation and partial decomposition of LSM cathode in the pres-ence of un-coated E-Brite interconnect is most likely related to thehigh cell overpotential and the presence of Cr species.20, 21

Very different from that observed on the LSM electrode, thechromium deposition on (La,Sr)(Co,Fe)O3 (LSCF) electrodes oc-curs predominantly on the surface of the LSCF cathode, formingSrCrO4.8, 22 Matsuzaki et al. studied the performance of an LSCFelectrode/samaria-doped ceria (SDC) electrolyte in the presence of achromia-forming alloy and found no preferential precipitation of Crspecies near the electrode/electrolyte interface.16 Significant Cr depo-sition was found in areas close to the top surface of the LSCF cathode.In the case of direct contact between LSCF electrode and Fe-Cr alloy,Simner et al. also found significant deposition of Cr species on theLSCF surface but no Cr deposition within the electrode coating.23

This was explained that the formation of SrCrO4 phase on the LSCFsurface would prevent the volatile Cr species from reaching the LSCFcathode and the SDC/YSZ electrolyte interface.

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B186 Journal of The Electrochemical Society, 159 (2) B185-B194 (2012)

Barium strontium cobalt ferrite (Ba0.5Sr0.5Co0.8Fe0.2O3−δ, BSCF)is an excellent mixed ionic and electronic conductor (MIEC), andhas been shown to have a high electrocatalytic activity for the O2

reduction reaction of SOFCs at reduced temperatures.24–28 However,the thermal expansion coefficient (TEC) of BSCF is ∼20×10−6 K−1

and the conductivity is low, ∼25 Scm−1 at 800◦C.29 The low con-ductivity of the BSCF is a serious concern in practical applicationin SOFCs as the low conductivity of the electrode materials wouldlead to the significant increase in the contact resistance between thecurrent collector and the electrode, resulting in the increase in theelectrode polarization losses.30 On the other hand, a good candidateIT-SOFC cathode material must also be highly tolerant toward thedeposition and poisoning of chromium species in order to achieve thelong-term stability of the electrodes. Recent study on the chromiumdeposition and poisoning on a Ba0.5Sr0.5Co0.8Fe0.2O3−δ electrode (x= 0.5), a commonly recommended composition for IT-SOFCs is notchromium-tolerant.31 However, it has also been reported that the ef-fects of Cr deposition and poisoning on the electrochemical perfor-mance of the cathode varied with respect to cathode composition.Chen et al. reported that significant differences in Cr poisoning oc-curred with (La0.6Sr0.4−xBax)(Co0.2Fe0.8)O3 electrodes as a functionof Ba content; at high Ba contents, the Cr deposition significantlydecreased.32 Thus it is of scientific significance and importance toinvestigate the effect of the Ba/Sr ratios on the chromium depositionand poisoning of BSCF cathodes under SOFC operation conditions.

In this paper, we investigated the chromium deposition and poi-soning on Ba1−xSrxCo0.8Fe0.2O3−δ cathode powders (0.3 ≤ x ≤ 0.7)and the effects of Sr content in the cathode composition on the Crpoisoning phenomena. Differences in electrochemical performancewere observed with respect to cathode composition and microstruc-tural changes, and the Cr deposition and poisoning mechanism arediscussed.

Experimental

Ba1−xSrxCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7) cathode powders were pre-pared using the glycine nitrate process (GNP). Stoichiometric amountsof Ba(NO3)2, Sr(NO3)2, Co(NO3)2 · 6H2O and Fe(NO3)3 · 9H2O (all99%, Sigma-Aldrich) were dissolved in distilled water, and glycinewas added as the fuel. The mixtures were heated to 60◦C to evaporatethe water, and combustion was carried out in a beaker on a hot plate.Subsequently, the powders produced by spontaneous combustion werecollected and heat treated at 1000◦C for 2 h. Gadolinium-doped ceria(Gd0.2Ce0.8O1.95; GDC) electrolyte disks were prepared by solid-statereaction from CeO2 (99.9%, Praxair, USA) and Gd2O3 (99.9%, Prax-air, USA). The mixed oxides were milled in isopropanol with zirconiaball for 24 h. After drying, the mixture was uniaxially pressed intodisks and sintered at 1600◦C for 6 h in air to form the electrolytepellets, which were 1 mm thick and 20 mm in diameter.

BSCF cathode materials were applied to the GDC electrolyte byscreen printing and sintered at 1000◦C for 2 h. Pt paste was symmet-rically painted in the center on the other side of the GDC electrolytepellet to serve as the counter electrode. Pt paste was also painted ina ring around the counter electrode to form the reference electrode,with a gap of ∼ 4 mm between the reference and counter electrodes;both electrodes were sintered at 850◦C for 1 h. For convenience,the Ba1−xSrxCo0.8Fe0.2O3−δ electrode with x = 0.3 was designatedBSCF7382, and the electrodes with x = 0.4, 0.5, 0.6 and 0.7 werenamed BSCF6482, BSCF5582, BSCF4682, BSCF3782, respectively.

A Fe-Cr alloy (RA446, with 23–27% Cr, 1.5% Mn, 1% Si, 0.2% Cand the remainder Fe, Rolled Alloy Co., Ontario, Canada) was used asthe interconnect material. The alloys were machined into coupons (12× 12 × 4 mm) with channels (1.2 × 1.2 mm) cut on one side of thecoupon. Air was fed to the channels through an alumina tube. Two Ptwires were spot-welded to the coupon to serve as voltage and currentprobes. The interconnect was then directly placed on the electrodeto complete the setup. In this arrangement, the alloy also acted as acurrent collector. Air was dried through a molecular sieve before useto an H2O content of < 3 ppm. The air flow rate was controlled at

Figure 1. Schematic diagram of experimental setup for Cr deposition andpoisoning tests.

100 mL−min−1. The cell configuration and the arrangement of theFe-Cr alloy interconnect are shown in Fig. 1. For the experimentswithout the alloy interconnect, Pt mesh was used as the current col-lector for the BSCF electrodes, and a separate and Cr-free sampleholder was used to avoid Cr contamination.

The polarization behaviors of BSCF electrodes with various Srcontents were measured at a constant current density of 200 mA cm−2

at 900◦C in air. The polarization potential (Ecathode) was measuredagainst the Pt air reference electrode. The current passage was peri-odically interrupted for the electrochemical impedance spectroscopy(EIS) measurements (Solartron 1260 and 1287, UK). EIS curves weremeasured with an open circuit in the frequency range of 0.1 Hz to100 kHz and a signal amplitude of 10 mV. The ohmic resistances(R�) of the electrode and electrolyte were determined from the high-frequency intercept; the electrode interface (polarization) resistance(RE) was calculated as the difference between the high- and low-frequency intercepts on the impedance spectrum. Thus, the overpo-tential (η) was obtained from Ecathode and R� using the followingequation:

η = Ecathode − j R�

where j is the current density.X-ray diffraction (XRD) patterns of BSCF powders with various

Sr contents were obtained (Rigaku D/MAX IIIC, Japan) using CuKα radiation (λ = 1.5428 Å) at 40 kV and 45 mA. The XRD datawere collected at 0.01◦ with a counting time of 1 s per step in the2θ range from 20 to 80◦. The chemical compatibility between Cr2O3

and GDC electrolyte was also investigated by XRD analysis. For thisexperiment, Cr2O3 and GDC powders were mixed and formed intopellets, followed by sintering at 900◦C for 20 and 100 h in air. Thescanning electron microscopy (SEM, Hitachi FE-SEM S-4300, Japanand FEI Nova230, USA) images of the BSCF electrodes were takenat 15–30 kV under a vacuum of 1.5 × 10−5 Torr.

As shown before,11 chromium deposition is a strong function oftemperature, for example, the amount of the chromium depositedon the LSM electrode at 700◦C is significantly lower than that ob-served on the electrode at 900◦C. However, the pattern or the processof the chromium deposition does not change with the decrease in

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Journal of The Electrochemical Society, 159 (2) B185-B194 (2012) B187

Figure 2. XRD patterns of a mixture of Cr2O3 and GDC powders after sin-tering at 900◦C for 20 h and 100 h in air.

temperature. The benefit of the studying the chromium deposition at900◦C is the clear visibility of the chromium deposition phenomenawithin relatively short experimental time, e.g., 24 h.17, 33, 34

Results and Discussion

XRD analyzes.— Synthesized BSCF powders with various Sr con-tents at the B-site of BSCF were characterized by XRD. The XRDpeaks for cubic (pm3m) perovskite structure were detected in all com-positions, and no secondary phases were found within the detectionrange. From these XRD patterns, it is observed that the main peaknear 32◦ gradually shifted to lower angles with decreasing Sr content.It can be explained by Bragg’s law that the lattice parameters of theBSCF increased as Sr content decreased. Because both Ba and Sr aredivalent cations, it is assumed that the increase of Sr content at theA-site would have no effect on the formation of oxygen vacancies.Therefore, it can be concluded that the changes in lattice parameter asa function of Sr content were due to the different ionic radii of Ba2+

and Sr2+ (Ba2+ = 1.34 Å and Sr2+ = 1.18 Å). Similar results werepreviously reported by Wei et al.35

Figure 2 shows XRD patterns of the mixture of Cr2O3 and GDCpowders after heat-treatment at 900◦C for 20 and 100 h in air. It can befound that all peaks indicated either Cr2O3 or GDC; in other words, noadditional peaks were found in the XRD patterns. Chemical reactionsbetween Cr2O3 and GDC electrolyte powder thus did not occur underconditions of the present study. Therefore, this observation suggeststhat the reactivity between gaseous Cr species and the GDC is verylow and negligible, and that the presence of gaseous Cr species wouldnot affect the electrolyte activity.

Electrochemical behavior.— Figure 3 shows the impedance curvesof the BSCF electrodes with various Sr contents in the absence of in-terconnects with a cathodic current of 200 mA cm−2 at 900◦C. Theimpedance responses were characterized by a significant high fre-quency inductance and a small low frequency loop for the O2 reduc-tion reaction in some compositions. However, the overall impedancesmeasured in five BSCF electrodes are very small regardless of Srcontent; this result indicates that the BSCF cathodes have high elec-trochemical activities for the oxygen reduction reaction. Additionally,the R� values of the five electrodes gradually decreased as polar-ization time increased, which could be due to an improved contactbetween the cathode and the Pt current collector. Such sensitivity ofthe contact resistance between BSCF electrode and Pt mesh currentcollector may be related to the low electronic conductivity of thebarium-based cobalt ferrite oxides, ∼30 Scm−1 at 800◦C.36 The elec-tronic conductivity of BSCF electrodes is significantly lower than thatof the LSM-based perovskite oxides.37, 38

Figure 4 depicts the polarization curves of the BSCF electrodeswith various Sr contents, as a function of cathodic polarization timeat 900◦C in the absence of Fe-Cr alloy interconnects. As shown inFig. 4a, the RE value for the BSCF electrode with x = 0.7, BSCF3782,is 0.0076 � cm2 and is almost constant during the entire cathodicpolarization period. Similar electrochemical behaviors were also ob-served for the oxygen reduction reactions on BSCF electrodes with x= 0.6, 0.5, 0.4 and 0.3. The cathode showed very similar polarizationresistance below 0.08 � cm2 and the polarization resistance main-tained constant during current passage. It was also observed that thechange in η is negligible with the passage of cathodic current, and theη value was very small. The stable RE and η values indicate that in theSr contents studied, the BSCF electrodes show very low polarizationresistance and are very stable under the passage of a cathodic currentof 200 mAcm−2. This electrochemical behavior is consistent with thegenerally known properties of mixed ionic and electronic conductingcathodes.39

The impedance characteristics of the BSCF electrodes at variousSr contents measured at 900◦C in the presence of metallic intercon-nects is shown in Fig. 5. BSCF cathodes in the presence of metallicinterconnect behave very differently as compared to that in the ab-sence of metallic interconnect. The size of the impedance arc for theO2 reduction on BSCF cathode in the presence of the interconnect issignificantly larger than that in the absence of the interconnect, andit in general increases with the current passage time. For example, inthe case of BSCF with Sr content of 0.7, BSCF3782, RE is 0.35 �cm2

at the beginning of the current passage (t = 0 min) and increased to2.15 �cm2 after current passage for 20 h (Fig. 5a). The significantincrease in the electrode polarization resistance indicates the poison-ing effect of gaseous chromium species on the O2 reduction reactionon BSCF cathode, consistent with previous results on the chromiumpoisoning effect on MIEC electrodes such as LSCF.22 However, thechange in the RE for the O2 reduction on BSCF cathodes appear todepend strongly on the Sr contents at the A-site of the cathode. Inthe case of BSCF with Sr content of 0.3, BSCF7382, RE is 0.036�cm2 at the beginning of the current passage and 0.038 �cm2 aftercurrent passage for 20 h (Fig. 5e). The increase in RE is negligible.The R� values of all BSCF electrodes are higher in the presence ofthe interconnects than that in their absence and gradually increase aspolarization time increased. This indicates the increased contact resis-tance between the cathode and metallic interconnects with increasingpolarization time. It has previously been reported that the chromiumoxides such as Cr2O3 are formed in Fe-Cr alloy metallic interconnectsat high temperature, and this oxide formation can increase the electri-cal resistance of the interconnect and reduce cell performance.40 Thedeposition of Cr species particularly under the rib of the interconnectcould also increase the R� of the BSCF electrode similar to that ob-served on LSCF and (La,Sr,Ba)(Co,Fe)O3 cathodes.22, 32 This appearsto be supported that in the case of LSM cathodes, the electrode ohmicresistance in the presence of Fe-Cr interconnect is generally stablemost likely due to that fact Cr deposition occurs preferentially at theLSM/YSZ interface region.11, 41

Figure 6 is the polarization curves of the BSCF cathodes in thepresence of the metallic interconnect operated at 200 mAcm−2 and900◦C for 20 h. In contrast to the values in the absence of inter-connects (shown in Fig. 4), here it can be observed that RE and ηincreased sharply during cathodic polarization, and the changes inRE and η varied with respect to the Sr content. Serious performancedegradation was expected to result from chromium poisoning of thecathode, especially on the oxygen reduction reaction. As polarizationtime increased, the RE and η values measured in BSCF3782 in thepresence of interconnects at 900◦C increased from 0.1 � cm2 and80 mV to 2.1 � cm2 and 500 mV, respectively (Fig. 6a). Therefore,the corresponding increase in RE and η, �RE and �η values are2.0 � cm2 and 420 mV, respectively for the oxygen reduction on theBSCF3782 electrode. Here, � means the changes in values between0 h and 20 h of measurement time. Similarly, the �RE values weredetermined to be 1.2, 0.7, 0.3 and 0.05 � cm2 for the BSCF elec-trodes with Sr contents of 0.6, 0.5, 0.4 and 0.3, respectively, and the

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B188 Journal of The Electrochemical Society, 159 (2) B185-B194 (2012)

Figure 3. Impedance curves of (a) BSCF3782, (b) BSCF4682, (c) BSCF5582, (d) BSCF6482 and (e) BSCF7382 electrodes in the absence of interconnects,measured at 200 mAcm−2 and 900◦C as a function of polarization time. Impedance curves were measured under open circuit.

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Journal of The Electrochemical Society, 159 (2) B185-B194 (2012) B189

Figure 4. Polarization curves of (a) BSCF3782, (b) BSCF4682, (c) BSCF5582, (d) BSCF6482 and (e) BSCF7382 electrodes in the absence of interconnects,measured at 200 mAcm−2 and 900◦C for 20 h.

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B190 Journal of The Electrochemical Society, 159 (2) B185-B194 (2012)

Figure 5. Impedance curves of (a) BSCF3782, (b) BSCF4682, (c) BSCF5582, (d) BSCF6482 and (e) BSCF7382 electrodes in the presence of interconnects,measured at 200 mAcm−2 and 900◦C as a function of polarization time. Impedance curves were measured under open circuit.

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Journal of The Electrochemical Society, 159 (2) B185-B194 (2012) B191

Figure 6. Polarization curves of (a) BSCF3782, (b) BSCF4682, (c) BSCF5582, (d) BSCF6482 and (e) BSCF7382 electrodes in the presence of interconnects,measured under 200 mAcm−2 at 900◦C for 20 h.

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B192 Journal of The Electrochemical Society, 159 (2) B185-B194 (2012)

Figure 7. Plots of �RE and �η as functions of the Sr/Ba ratios (i.e, the Srcontents) of the BSCF cathode. The �RE and �η values were taken fromFig. 7.

corresponding �η values were 350, 185, 65 and 5 mV. It is evidentthat the electrochemical performance of BSCF cathodes decreases inthe presence of Fe-Cr interconnects, and the performance degradation(i.e., the Cr poisoning effect) is more severe as the Sr content in BSCFis increased; the performance degradation increases with cathodic po-larization time. The results clearly show that the Cr poisoning effecton the electrochemical activities of BSCF cathodes is strongly relatedto the Sr content in the BSCF electrodes.

The increase in the polarization resistance and/or overpotential,�RE and �η, is an indication of the effect of the chromium poi-soning on the electrochemical activities of the cathodes for the O2

reduction reaction.42 Figure 7 summarizes the relations between theelectrochemical performance degradation, �RE and �η of the BSCFelectrodes for the O2 reduction reaction in the presence of metallicinterconnect and the Sr content of BSCF electrodes measured at 200mAcm−2 and 900◦C for 20 h (see Fig. 6). The �RE and �η bothsignificantly increased as the Sr/Ba ratio (i.e., the Sr content) of theBSCF electrode was increased. Additionally, it is observed that theelectrode with the lowest Sr content (BSCF7382) is rarely influencedby the Cr poisoning from the metallic interconnects, indicating thatthe strontium content in the BSCF electrode plays a significant rolein the Cr-tolerance of BSCF cathodes in the presence of metallicinterconnect.

SEM results.— Figure 8 is the SEM images of the surface of theBSCF cathodes with various Sr contents after cathodic polarization at200 mA cm−2 and 900◦C for 20 h. The SEM images are of cathodesurface areas under the rib and channel areas of the interconnect.All SEM images show that the cathode particles were covered by Crdeposits. As Sr content of the BSCF cathodes was increased, moreCr deposits were found on the cathode, and the pores were closedby these deposits. In a previous study, the main Cr deposits wereidentified as BaCrO4 and SrCrO4 from an XRD study.31 Therefore,the SEM results in this study suggest that the Cr deposits on theBSCF electrodes could be mainly comprised of BaCrO4 and SrCrO4.Moreover, larger cathode particles covered with Cr deposits and fewerpores were observed on the BSCF electrodes in the area under the ribof the cathode than in the channel regardless of Sr content, indicatingthat Cr deposition is more significant in the rib area. The cathodehas physical contact with the metallic interconnect at the rib, thusCr species can migrate to the BSCF electrode surface via solid statediffusion, while in the case of the electrode surface under channelsof the interconnect, Cr species can only reach the surface by gaseousdiffusion. The significantly more Cr deposits on the surface underrib as compared to that under channels indicate the fast solid statediffusion of Cr species. This is consistent with the results reported byTucker et al.43

Figure 8. SEM images of the surface of (a) BSCF3782, (b) BSCF4682, (c)BSCF5582, (d) BSCF6482 and (e) BSCF7382 cathodes under the rib andchannel of the interconnect after operating at 200 mAcm−2 and 900◦C for20 h.

Figure 9 shows SEM images of the BSCF6482 electrode aftera polarization test at 200 mAcm−2 and 900◦C for 20 h. Evidently,more Cr deposits were formed on the BSCF electrode surface underthe rib (Fig. 9b) than that under the channel (Fig. 9a). The BSCFparticles are well connected, and the amounts of Cr deposits on thecathode surface under the channel is relatively small (Fig. 9a). In therib areas, however, as shown in Fig. 9b, an enrichment of chromiaparticles in the Sr bottom layer was observed, and rectangular Crdeposits were formed alongside the cathode particles. The BSCF6482cathodes maintained a good contact with GDC electrolyte after a Crpoisoning test at 900◦C for 20 h (Fig. 9c). Cr deposits were againfound in the cathode, and it appears that the Cr deposits occur moreheavily on the top portion of the electrode than on the bottom portionof the electrode close to the interface. Energy dispersive spectrometer(EDS) analyzes were conducted on the electrode surface under the

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Journal of The Electrochemical Society, 159 (2) B185-B194 (2012) B193

Figure 9. Enlarged SEM images of aBSCF6482 cathode under the (a) channeland (b) rib of the interconnect after tested at200 mAcm−2 and 900◦C for 20 h; (c) cross-sectional image of the cathode; (d) EDS re-sults taken on the BSCF electrode surface un-der rib and channel areas.

channel and rib areas, and the results are shown in Fig. 9d. Similarpeaks were detected from the electrode surface under the channel andrib areas of the interconnect. Both Cr and Mn were detected from theBSCF electrode surface. The source of Mn on the electrode surfaceis most likely from the interconnect as Fe-Cr alloy used in this studycontains Cr, Mn, Si, C and Fe. Volatile Cr species are well known tobe primary seeds in Cr poisoning; however, it has been suggested thatmanganese ions (Mn2+), having a high ionic mobility, seem to movetoward the cathode through the contact points between the cathodeand the interconnect.

Cr deposition process on BSCF cathodes.— Surface segregationis a well known phenomenon in the perovskite oxides-based cathodesof SOFCs and plays an important role in the electrocatalytic activ-ity for oxygen reduction.44–47 Prior research has revealed that surfacesegregation such as Sr, Ba and Mn plays an important role in themechanism and kinetics of the Cr deposition and poisoning.17, 22, 31, 48

Using angle-resolved X-ray photoelectron spectroscopy, electron en-ergy loss spectroscopy and Auger electron spectroscopy, several in-dependent studies have reported the evidences for Sr segregation onthe surface of LSM perovskites.47, 49–51 Strontium segregation is alsoreported for lanthanum cobatite perovskites. Vovk et al.52 studied theLa0.5Sr0.5CoO3 pervoskite oxide surfaces under electrochemical po-larization using ex situ XPS technique. Under cathodic polarization,the Sr/(La+Co) ratio at the oxide surface increased irreversibly by5% while the La/Co ratio remained constant, indicating the surfaceenrichment of strontium. Tabata et al.53 also studied the surface chem-istry of Lal−xSrxCoO3 and observed a considerably higher Sr/Co ratioon the surface than in the bulk. Recent study showed that segregatedSr and Ba could react with Cr species, forming SrCrO4 and BaCrO4.31

Based on the impedance, polarization and SEM results, the presentstudy clearly indicates that Cr poisoning and deposition take placesignificantly on the BSCF cathodes and the magnitude of the Crpoisoning and deposition depends on the Sr content of the cath-odes (Fig. 6 and 7). A recent systematic study on the Cr deposi-tion on (La0.8Sr0.2)0.95(Mn1−xCox)O3±δ (LSMC, 0.0 ≤ x ≤ 1.0) cath-

odes shows chromium deposition on the electrolyte surface in contactwith the LSMC electrode decreases while on the electrode surface it in-creases as Co content in LSMC increases from 0.0 to 1.0.48 Chromiumdeposition at LSMC cathode is closely related to the chemical affinityand stability of the Mn and Sr species, which depends significantlyon the Co/Mn ratio at the B-site of the LSMC perovskite. In the caseof Ba1−xSrxCo0.8Fe0.2O3−δ (BSCF, 0.3 ≤ x ≤ 0.7), increase in the Srcontent would increase the tendency of Sr segregation on the surface.High Sr surface segregation would lead to the high Cr depositionand the formation of SrCrO4, and Cr deposition on the BSCF inturn could block the active sites and result in the high resistance ofthe electrodes for the O2 reduction. Similar deposition and poison-ing could occur on the BSCF electrode due to the Ba segregation.Thus, based on the previous studies of Cr deposition on LSCF and(La0.6Sr0.4−xBax)(Co0.2Fe0.8)O3 electrodes,32, 33 the deposition processin the BSCF/GDC system can be written as follows:

In the case of segregated Sr:

CrO3 + SrO → Cr-Sr-O (nuclei) [1]

Cr-Sr-O (nuclei) + CrO3 → Cr2O3 [2]

Cr2O3 + CrO3 + SrO → SrCrO4 [3]

In the case of segregated Ba:

CrO3 + BaO → Cr-Ba-O (nuclei) [4]

(Cr-Ba-O (nuclei) + CrO3 → Cr2O3 [5]

Cr2O3 + CrO3 + BaO → BaCrO4 [6]

The volatile Cr gaseous species would react with segregated Srand/or Ba, forming Cr-Sr-O and/or Cr-Ba-O nuclei and subsequentcrystallization and grain growth of Cr2O3 and BaCrO4/SrCrO4 solidphases. The nuclei-induced crystallization and grain growth of Crdeposits is also supported by the direct observation of Mn-rich nuclei

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B194 Journal of The Electrochemical Society, 159 (2) B185-B194 (2012)

within the Cr particles.20 Nevertheless, our early study showed thatrate of the deposition and formation of BaCrO4 between the gaseouschromium species and segregated BaO is much lower as compared toSrCrO4.32 This may be related to the relatively low stability of BaCrO4

in comparison with that of SrCrO4.54 On the other hand, in the case ofthe BSCF electrode/GDC electrolyte system, the chemical interactionbetween Cr species and GDC electrolyte can be neglected based onthe XRD results shown in Fig. 2.

Based on experimental results, it can be offered some explana-tion for the observation of increased Cr deposition under the ribs bySEM. Due to the physical contact between the cathode and the metal-lic interconnect, Cr species can migrate to the cathode surface by afast solid state diffusion and there might be more manganese speciesdiffusion from metallic interconnect to the electrode surface (seeFig. 9d). The high chromium and manganese concentration on theBSCF surface would results in the high reaction rate for the Cr de-position on the surface areas under rib. On the electrode areas underchannels, Cr species can only migrate to the electrode via gaseousdiffusion route. The much slower gaseous diffusion explains the rela-tively low Cr deposition on the electrode surface under the channelsof the interconnect.

Conclusions

The effect of Sr content on the Cr deposition and poisoning inBSCF electrodes was investigated. The results are summarized asfollows:

1. In the absence of metallic interconnect, BSCF showed a superbelectrochemical activity and stability for the O2 reduction reac-tion and its activity was independent of the Sr contents in therange studied. In contrast, in the presence of metallic intercon-nect, the electrode polarization resistance and overpotential forthe reaction on BSCF cathodes increased with the polarizationtime. The results clearly indicate the significant Cr poisoningeffect on the BSCF electrodes.

2. From the impedance and polarization curves in the presence ofinterconnects, it is determined that the RE and η values increasedas the Sr content of the BSCF electrode was increased. TheBSCF electrode with the lowest Sr content (BSCF7382) is rarelyinfluenced by Cr poisoning from the metallic interconnect. Thisresult indicates that the Sr content is a governing factor for Crpoisoning in BSCF electrodes.

3. Cr deposits were observed on the cathode surfaces of the BSCFelectrodes after Cr poisoning tests at 900◦C for 20 h, and otherreactants such as BaCrO4 and SrCrO4 were found as the Srcontent was increased. It is also observed that there are more Crdeposits on the electrode surface under the ribs than that underthe channels. This trend applied to cathodes regardless of theSr contents. Cr and Mn peaks were detected on the Cr-poisonedsurfaces under the ribs and channels that were attributed to theformation of Cr deposits on the cathode.

Acknowledgments

This research was supported by a grant from the Fundamental R&DProgram for Core Technology of Materials and New & RenewableEnergy of the Korea Institute of Energy Technology Evaluation andPlanning (KETEP) funded by the Ministry of Knowledge Economy,Republic of Korea. Furthermore, this work was supported by the BrainKorea 21 (BK21) program that is funded by the Ministry of Education,Science and Technology (MEST), and the Australia Research Council(LP110200281), Australia.

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