Polarization-Induced Interface and Sr Segregation of in Situ

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Polarization induced interface and Sr segregation of in situ assembled La Sr Co Fe O electrodes on YO-ZrO electrolyte of solid oxide fuel cells 0.6

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Kongfa Chen, Na Li, Na Ai, Yi Cheng, William D.A Rickard, and San Ping Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11665 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 7, 2016

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ACS Applied Materials & Interfaces

Polarization induced interface and Sr segregation of in situ assembled La0.6Sr0.4Co0.2Fe0.8O3-δ electrodes on Y2O3-ZrO2 electrolyte of solid oxide fuel cells Kongfa Chen,a,b Na Li,c Na Ai,b Yi Cheng,b William D.A. Rickard,d San Ping Jiang b,∗ a

College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, China b

Fuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia

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College of Science, Heilongjiang University of Science and Technology, Harbin 150022, China

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John De Laeter Centre & Department of Physics and Astronomy, Curtin University, Perth, WA 6102, Australia

Abstract Application of cobaltite-based electrodes such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) on Y2O3-ZrO2 (YSZ) electrolyte in solid oxide fuel cells (SOFCs) generally requires the use of a doped ceria barrier layer to prevent the interaction between LSCF and YSZ during sintering at high temperatures. In this paper, we report for the first time an in situ assembly approach to directly incorporate LSCF cathode to YSZ electrolyte without the use of a doped ceria barrier layer and without pre-sintering at high temperatures. A Ni-YSZ anode-supported YSZ electrolyte cell with an in situ assembled LSCF electrode exhibits a peak power density of 1.72 W cm-2 at 750 oC. However, the cell performance degrades significantly at 500 mAcm-2 and 750 oC. The results indicate that cathodic polarization not only induces the formation of the interface but also accelerates the Sr segregation. The segregated Sr migrates to the LSCF electrode/YSZ electrolyte surface and forms an SrO layer. Using a Sr-free LaCoO3-δ ∗ Corresponding author. Tel.: +61 8 9266 9804; fax: +61 8 9266 1138. Email address: [email protected] (S.P. Jiang).

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composite cathode overcomes the Sr segregation problem, showing an excellent peak power density of 1.2 Wcm-2 and good stability at 750 oC for over 100 h. The present study shows that cobaltite-based perovskites can be directly used on YSZ based electrolyte via the in situ assembly, providing an effective means to advance the application of highly active mixed ionic/electronic conducting cathodes to YSZ electrolyte based SOFCs. Keywords: Solid oxide fuel cells; Polarization induced electrode/electrolyte interface; In situ assembled cobaltite; LSCF cathodes; Polarization accelerated strontium segregation. 1. Introduction Solid oxide fuel cells (SOFCs) are an energy conversion device to directly convert the chemical energy of fuels such as hydrogen, natural gas and hydrocarbons to electrical energy. SOFCs with combined power and heat have a high efficiency of 70-90% and are environmentally friendly with much less CO2 emission when compared to direct combustion.1 In the case of intermediate temperature SOFCs with operation temperature of 600-800 oC, the performance and power output of the cell depend strongly on the electrocatalytic activity of electrode materials of anode and cathode. La0.8Sr0.2MnO3+δ (LSM) is the most common cathode for high temperature SOFCs but performs poorly at reduced temperatures because it is a predominantly electronic conductor with negligible ionic conductivity and its activation energy for the O2 reduction reaction is high.2-4 Cobaltite perovskite materials such as Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF) and La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF), on the other hand, are mixed ionic/electronic conductors (MIEC) and show a much higher electrocatalytic activity for the O2 reduction reaction as compared to LSM based cathodes.4-8 However, it has been well known that (La,Sr)(Co,M)O3 (where M = Mn, Fe, etc.) cobaltite-based electrode materials have a high tendency to react with yttria-stabilized


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zirconia (YSZ) electrolyte, forming insulation phases such as La2Zr2O7 and/or SrZrO3 and consequently degrading the performance of SOFCs.9-12 To prevent the interaction between cobaltite cathodes such as LSCF and YSZ, the most effective way is to apply an interface barrier layer such as Gd-doped ceria (GDC) via an additional heat treatment at ~1200 oC.13-15 The addition of the ceria barrier layer increases the process steps and cost of cell fabrication as well as risking electrolyte delamination during the long-term operation.16-17 Diffusion and migration of cations of LSCF such as Sr across the porous ceria layer to the YSZ also occurs during the electrode pre-sintering process and under the SOFC operation conditions, forming a resistive SrZrO3 reaction layer at the ceria/YSZ interface.14, 18-19 Most recently, we showed that cathodic polarization can induce an electrode/electrolyte interface for the O2 reduction reaction in situ assembled LSM cathode on YSZ electrolyte and LSCF cathode on GDC electrolytes without the usual high temperature sintering steps.20 The in situ formed electrode/electrolyte interface shows comparable electrochemical performance as compared to the counterparts formed by the conventional high temperature sintering. This implies that the interface between cobaltite-type electrode materials and YSZ electrolyte could be formed under the influence of polarization potential, thus avoiding the detrimental interfacial reaction associated with high temperature sintering steps. To prove the concept, we selected LSCF electrode material as LSCF is the most studied MIEC cathode.14, 21-23 LSCF was in situ assembled on the barrier layer-free YSZ electrolyte by coating the electrode ink without the high temperature sintering step, followed by the electrochemical polarization tests at SOFC operation temperature of 750 oC. The results indicate that cathodic polarization not only induces the electrode/electrolyte interface but also accelerates the Sr surface segregation of in situ assembled LSCF electrodes. 2. Experimental 2.1. Fabrication of anode-supported cells and in situ assembly of electrode 3 ACS Paragon Plus Environment


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NiO-YSZ anode-supported YSZ electrolyte cells were fabricated by slurry spin coating.2425

NiO (J.T. Baker), YSZ (TZ-8Y, Tosoh) and tapioca in a weight ratio of 5:5:2.5 were mixed

in a BMT-30D ball mill (Hsiangtai Machinery Industry Co. Ltd., Taiwan) for 24 h. The mixed powder was compacted in a stainless steel die (ID 18 mm) and pre-sintered at 1000 oC for 2 h to form the anode substrates. The anode functional layer (AFL) with NiO and YSZ in a weight ratio of 5:5 and YSZ thin film were prepared on the anode substrates by using a spin coater (VTC-100, MTI), followed by sintering at 1450 oC for 5 h. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and LaCoO3-δ (LC) powder were prepared by combined citrate and Ethylenediaminetetraacetic acid (EDTA) complexing method. The starting chemicals were La(NO3)3·6H2O (99.9%, Alfa-Aesar), Sr(NO3)2 (99%, Sigma-Aldrich), Co(NO3)2·6H2O (98.0-102.0%, Alfa-Aesar), Fe(NO3)3·9H2O (98%, Chem Supply), citric acid (99.5%, Chem Supply), EDTA (99%, Acros Organics) and ammonia (28%, Ajax Finechem). The molar ratio of metal ions/citric acid/EDTA was 1:1.5:1. The resultant powders were calcined at 900 oC in air for 2 h. The formation of perovskite phases was examined using a Bruker D8 Advance X-ray diffractometer with a Cu Kα x-ray source. The as prepared LSCF and LC powders were dispersed in an ink vehicle (Fuel Cell Materials) at a weight ratio of 5:5. The ink was prepared on the YSZ electrolyte side of the cells by slurry coating and dried at 100 oC for 2 h to form the in situ assembled cathode without a further pre-sintering step at high temperatures. LSCF and LC were also mixed with Gd0.1Ce0.9O1.95 powder (GDC, AGC Seimi Chemical Co Ltd) at a weight ratio of 6:4 to form composite cathodes. LSCF-GDC and LC-GDC composite cathodes were also in situ assembled on YSZ electrolyte cells, similar to the procedures as described above. The effective cathode area was 0.25 cm2. Pt ink (Gwent Electronic Materials Ltd) was painted on the electrode surface as the current collector. 2.2. Electrochemical tests 4 ACS Paragon Plus Environment

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Hydrogen flow rate at the anode was 50 ml min-1 and the cathode was exposed to the surrounding stationary air. The cells were tested by a Gamry Reference 3000 Potentiostat. Electrochemical impedance curves were measured in a frequency range of 100 kHz–0.1 Hz with a signal amplitude of 10 mV under open circuit. The ohmic resistance of cells, RΩ was obtained from the high frequency intercept, and is the sum of the ohmic resistance of the electrolyte, electrode and contact resistance at the electrode/electrolyte. The overall electrode polarization resistance, RE was estimated by the differences between the high and low frequency intercepts. The testing temperature was in the range of 650-750 oC at an interval of 50 oC. To evaluate the cell stability, cell voltage was recorded under a constant current of 500 mA cm-2 at 750 oC and/or of 250 mA cm-2 at 650 oC. 2.3. Microscopic and spectroscopic characterizations The microstructure of the cathodes was examined using scanning electron microscopy (SEM, Zeiss NEON 40EsB). The in situ assembled cathodes were removed by adhesive tape, and the elemental distribution on the YSZ electrolyte surface was analyzed by time of flight secondary ion mass spectroscopy (ToF-SIMS) in conjunction with a Tescan Lyra focused ion beam-SEM (FIB-SEM). ToF-SIMS was conducted over a 5 x 5 µm area using an ion beam accelerating voltage of 30 kV and current of 35 pA. A cross-sectional lamella at the electrode/electrolyte surface was prepared by the FIB-SEM. Microstructural observation and elemental mapping analysis of the lamella were characterized using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM, FEI Titan G2 80-200 TEM/STEM with ChemiSTEM Technology) at 200 kV. In some cases, the cathodes were removed from the YSZ electrolyte surface in 32% HCl solution. The acid cleaned YSZ surface was examined with tapping mode atomic force microscopy (AFM) using a Dimension FastScan AFM (Bruker). X-ray photoelectron spectroscopy (XPS) was carried out on the acid cleaned YSZ electrolyte surface in contact with an in situ assembled LSCF electrode after 5 ACS Paragon Plus Environment


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polarization at 500 mAcm-2 and 750 oC for 100 h using a Kratos AXIS Ultra DLD system, with monochromated Al Kα X-rays (photon energy 1486.7 eV) at a pass energy of 40 eV. 3. Results and discussion 3.1. Initial characterizations To investigate the chemical compatibility between LSCF and YSZ, LSCF-YSZ oxide couples were calcined at 750-900 oC for 4 h in air. XRD data show that LSCF starts to react with YSZ at a temperature of 800 oC, forming a new phase of SrZrO3 (Fig. 1a). Su et al.26 reported the deposition of BSCF cathode on YSZ electrolyte by pre-sintering the BSCF cathode at 800 oC. A high power density of 1.8 W cm-2 was observed, but the cell stability could be an issue as BSCF may react with YSZ at 800 oC.26 At 750 oC, no chemical reaction occurs between LSCF and YSZ. Thus we selected 750 oC as the upper limit of the operation temperature for the in situ assembled LSCF electrode. Fig. 1b shows the SEM image of as prepared LSCF powder. The average particle size of LSCF powder was 110 ± 33 nm. Fig. 1c and d show an anode-supported YSZ electrolyte cell with the as-prepared in situ assembled LSCF electrode. The YSZ electrolyte film is dense with an average grain size of 5.4 ± 1.8 µm, and the thickness of AFL and YSZ is 14 and 8 µm, respectively (Fig. 1c). The LSCF electrode is porous, forming intimate physical contact to the dense YSZ electrolyte (Fig. 1d). The LSCF electrode thickness is 18 µm with an average particle size of 95 ± 27 nm (Fig. 1d), which is close to that of the as-prepared LSCF powder (see Fig. 1b). 3.2. Electrochemical performance Fig. 2 shows the time-dependent electrochemical performance curves of a cell with an in situ assembled LSCF electrode at a constant current of 500 mA cm-2 and 750 oC for 100 h. The initial peak power density (PPD) of the cell is 1.22 W cm-2 and increases to 1.72 W cm-2 after the polarization for 6 h (Fig. 2a). As shown in the impedance curves, the cell ohmic


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resistance, RΩ decreased from 0.21 Ω cm2 before the polarization to 0.14 Ω cm2 after the polarization for 6 h (Fig. 2b). In the case of pre-sintered LSM cathodes on NI-YSZ supported YSZ thin electrolyte cells at 1150 oC, the cell ohmic resistance remains the same before and after polarization at 500 mAcm-2 and 800 oC for 40 h.27 This indicates that Ni-YSZ cermet is fully reduced under the conditions of the present study and thus the change in RΩ would not be related to the Ni-YSZ cermet anode. As RΩ mainly consists of the contributions from the electrolyte film and contact at the electrolyte/electrode interfaces, the decrease of RΩ indicates the establishment of an interfacial contact between the LSCF electrode and YSZ electrolyte as the resistance of the YSZ electrolyte is not expected to change. The decrease in the cell ohmic resistance is consistent with that observed for the LSM/YSZ interface induced by the cathodic polarization.20 In addition to the decrease in RΩ, the electrode polarization resistance, RE also decreases from 0.27 Ω cm2 to 0.21 Ω cm2 after polarization for 6 h. The results indicate that polarization is effective to decrease the ohmic resistance as well as polarization resistance of the cell with in situ assembled LSCF cathode, consistent with the increase of the power density of the cell. However, the cell polarization performance decreases as the polarization time increases. The cell voltage is 0.96 V at 6 h and degrades to 0.84 V after 100 h under a constant polarization current of 500 mA cm-2 (Fig. 2c). After polarization for 100 h, the PPD drops to 0.8 W cm-2, even lower than the initial value of 1.22 W cm-2 (Fig. 2a). RΩ became constant after polarization for 6 h (indicated by dashed line, Fig.2b) but RE increased significantly with the polarization time. After polarization for 100 h, RE is 0.45 Ω cm2, substantially higher than 0.21 Ω cm2 after polarization for 6 h (Fig. 2b). This indicates that the deterioration in the performance of the cell with in situ assembled LSCF cathodes is mainly due to the increase of the electrode polarization resistance. The increase in the cell impedance is primarily on the



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high frequency region, indicating that cell polarization increase most likely originates from the in situ assembled LSCF cathode.28-30 The polarization performance of a cell with an in situ assembled LSCF-GDC composite cathode was also studied at 500 mA cm-2 and 750 oC and the results are shown in Fig. 3. Similar to the in situ assembled pristine LSCF cathode, RΩ decreased from 0.16 to 0.076 Ω cm2 after polarization for 40 h, but RE increased from 0.38 to 0.47 Ω cm2 (Fig. 3c). The initial PPD was measured to be 1.26 W cm-2 and decreased to 0.90 W cm-2 after polarization for 40 h (Fig. 3a). The cell voltage also reduced from 0.91 V to 0.84 V after polarization for 40 h (Fig. 3b). The increase of the cell impedance is mainly related to the increase of the electrode polarization resistance associated with the high frequency region (Fig. 3c), indicating that cell degradation is also primarily related to the in situ assembled LSCF-GDC composite cathodes. This is similar to the case of the pristine LSCF as shown above. To study the effect of the testing temperature on the cell stability, a cell with an in situ assembled LSCF electrode was evaluated at 250 mA cm-2 and 650 oC (see Fig.4). Prior to the test at 650 oC, the cell was polarized at 500 mA cm-2 and 750 oC for 6 h to reach the stable power output. As shown in Fig.4, cell degradation also occurs at 650 oC. The initial PPD at 650 oC was 0.47 W cm-2 and reduced to 0.32 W cm-2 after polarization for 100 h (Fig. 4a). The loss of cell performance is also evident by the continuous decrease of cell voltage (Fig. 4b). The initial voltage at 250 mA cm-2 was 0.87 V and dropped to 0.75 V after polarization for 100 h. RE also increased from 1.04 to 1.66 Ω cm2 after polarization for 100 h, and the increase of RE is mainly on the high frequencies (Fig. 4c), similar to the case of the test at 750 o

C.

3.3 Microstructure and composition of the polarization induced interface Fig. 5a shows the SEM micrograph of the cross-section of the LSCF/YSZ interface after polarization at 750 oC for 100 h. Good contacts were formed at the electrode/electrolyte 8 ACS Paragon Plus Environment

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interface after the polarization (indicated by the circles, Fig. 5a). This is consistent with the reduction of the RΩ of the cell with the polarization treatment (see Fig.2). After the removal of LSCF electrode coating by adhesive tape, a distinctive thin layer was observed on the YSZ electrolyte surface and there is also the presence of LSCF particles on the YSZ surface (Fig. 5b). The reaction layer appears uniform and covers the whole YSZ electrolyte surface. The layer has plate-like structure, very different from the LSCF particles. However, the formation of such a thin layer appears gradual. After polarization for 14 h, the reaction layer mainly forms around the LSCF particles (Fig. 5c). This indicates the reaction layer initially starts from the LSCF particles/YSZ electrolyte contact regions and propagates to the entire YSZ surface. On the other hand, the YSZ electrolyte surface between the LSCF particles is generally clean at 750 oC for 100 h under open circuit conditions (Fig. 5d). This suggests that the formation of such a reaction layer on the YSZ electrolyte surface is related to the electrochemical polarization at 750 oC. To investigate the composition of the reaction layer, STEM and ToF-SIMS were conducted at the electrode/electrolyte interface after polarization at 750 oC for 100 h. A lamella from the sample at the electrode/electrolyte interface was prepared by SEM-FIB and the STEM-EDS elemental mapping of the interface is shown in Fig. 6a. A Sr-rich layer is visible along the electrode/electrolyte interface, while there is no clear accumulation of La, Co or Fe on YSZ electrolyte surface. The thickness of the Sr-rich layer is ~30 nm. The accumulated Sr layer has no noticeable overlap with Zr, indicating that the accumulated Sr most likely exists as SrO. The LSCF electrode was also peeled off from the YSZ electrolyte and SIMS depth profiles clearly indicate the significant accumulation of Sr species on the electrolyte surface (Fig. 6b). The thickness of the SrO layer is in the range of 20-30 nm. In the top ~10 nm of the sample, Sr distribution is still clearly visible (Fig. 6c). Thus the surface reaction layer would be mainly attributed to the propagation of the segregated Sr species on



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the surface of LSCF electrodes under the influence of electrochemical polarization. The more or less constant RΩ after polarization for 6 h (Fig. 2b) indicates that the interfacial contact between LSCF and YSZ is not affected by the formation of SrO surface layer. The cell with an in situ assembled LSCF electrode tested at 650 oC for 100 h (see Fig. 4) was also examined by SEM and ToF-SIMS and the results are shown in Fig. 7. The in situ assembled LSCF electrode was removed by adhesive tape. Similar to that tested at 750 oC, a thin reaction layer was also formed on the YSZ surface (Fig. 7a). SIMS depth profiles indicate that the surface layer mainly consists of Sr species of ~40 nm thickness (Fig. 7b-d), which is greater than ~30 nm in the case of test at 750 oC. The formation of the thicker Sr rich layer at 650 oC indicates that the effect of polarization on forming the Sr surface layer is more pronounced at reduced temperatures. It is not clear at the moment for the formation of thicker Sr-rich layer at lower temperatures. The in situ assembled LSCF electrode/YSZ electrolyte interface after polarization at 750 o

C for 100 h was examined by AFM and the results are shown in Fig. 8. In this case, the in

situ assembled LSCF was removed by HCl treatment.32 The YSZ surface is characterized by formation of contact marks or strips (Fig. 8a and b), which is different to the clean surface of as prepared YSZ electrolyte (Fig. 8f). The edge of the contact strips has a height of 2-3 nm (Fig. 8c). The formation of contact marks was also observed during the early stage of polarization for 1 h and 14 h (Fig. 8d and e). The observation of contact marks on YSZ surface is an indication of interface formation by the cathodic polarization, which is consistent with the substantial decrease of cell ohmic resistance, e.g., RΩ of the in situ assembled LSCF cell reduced from the initial 0.21 Ω cm2 to 0.14 Ω cm2 after polarization for 6 h (Fig. 2b). Figure 9 shows the XPS core levels of the acid cleaned YSZ surface after polarization at 750 oC for 100 h. Distinct Zr and Y peaks from YSZ films were observed (Fig. 9a). There is


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also detection of Ni 2p peaks at 853 eV, which is assigned to Ni2+.33 The Ni most likely comes from the residual nickel deposited on the YSZ electrolyte surface during HCl treatment of the cell. Nevertheless, no La 3d or Sr 3d peaks are detected on the YSZ electrolyte surface. The absence of the Sr peaks indicates the Sr rich surface layer is completely dissolved in the acid treatment. The XPS results confirm that the presence of contact strips or marks (Fig. 8) is related to the formation of electrode/electrolyte interface rather than the reaction product between LSCF and YSZ under the influence of the fuel cell polarization conditions. 3.4. Activity and stability of Sr-free LC electrode The above results clearly demonstrate that the Sr segregation and accumulation at the electrode/electrolyte interface under the influence of cathodic polarization contribute primarily to the performance degradation. Thus the use of Sr-free cathodes would alleviate such Sr segregation problem. To demonstrate this concept, a Sr-free LaCoO3 (LC) was used. As LC is a predominantly electronic conductor with negligible ionic conductivity,34-35 GDC was added to form the in situ assembled LC-GDC composite electrode with increased ionic conductivity. Fig. 10 shows the polarization performance of a cell with an in situ assembled LC-GDC composite cathode at 500 mA cm-2, 750 oC. The cell shows good performance and excellent stability over polarization for 100 h. The initial PPD is 0.45 W cm-2 and increases to 0.62, 1.05 and 1.19 W cm-2 after polarization for 4, 20 and 100 h, respectively (Fig. 10a). The cell voltage at 500 mA cm-2 increases rapidly in the first 20 h and becomes stable with the further polarization (Fig. 10b). The initial cell voltage is 0.53 V and increases to 0.86 V after polarization for 100 h. There is a significant decrease in the cell ohmic resistance. The initial RΩ was 0.26 Ω cm2 and decreased substantially to 0.05 Ω cm2 after polarization for 100 h (Fig. 10c). The significant decrease in cell ohmic resistance is a clear indication of the establishment of the electrode/electrolyte interface on the cathode side under the SOFC 11 ACS Paragon Plus Environment


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operation conditions. The cell polarization resistance also decreases significantly with the polarization. The initial RE was 0.82 Ω cm2 and decreased to 0.62 Ω cm2 after polarization for 100 h. The decrease of cell impedance is mainly at the high frequencies, indicating enhanced electrocatalytic activity of the cathode by the polarization. The increase of the cell performance corresponds to the substantial decrease in both RΩ and RE. Fig. 11 show the SEM and AFM images of the YSZ electrolyte surface in contact with an in situ assembled LC-GDC electrode after polarization at 500 mA cm-2, 750 oC for 40 h. On the YSZ surface there are GDC and LC particles and contact marks (indicated by the arrows, Fig. 11a). AFM image shows that the contact marks are ring shaped (Fig. 11a and b). The contact rings are 88 ± 27 nm in dimeter and 8.5 ± 3.0 nm in height (Fig. 11c). The morphology of the contact marks looks very similar to that of the contact rings on YSZ electrolyte for pre-sintered LSM electrode.20, 36-38 Therefore, the presence of contact rings indicates the formation of electrode/electrolyte interface under the influence of cathodic polarization, which is consistent with the substantial decrease of cell ohmic resistance and increase of cell power density under SOFC operation conditions (Fig. 10). In contrast to the case of the in situ assembled LSCF electrodes, there is no formation of a uniform and dense Sr-rich layer on the YSZ electrolyte surface, indicating the effectiveness of the Sr-free LC cathodes in alleviating the Sr segregation problem. 3.5. Polarization induced interface and Sr surface segregation of LSCF The formation of thin SrO reaction layer on YSZ surface on the in situ assembled LSCF electrodes under the influence of cathodic polarization conditions can be explained by the well known Sr surface segregation reported on the Sr-containing cathode materials.39-43 For example, Oh et al.41 carried out detailed study on Sr surface segregation on LSCF bar samples in the temperature range of 600-900 oC, and found that the SrO surface segregation increases with the increase of temperature and oxygen partial pressure. Our study of LSCF 12 ACS Paragon Plus Environment

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bar samples also showed the Sr segregation is accelerated by the surface reactions, e.g., in the presence of gaseous chromium species.44 The surface segregated Sr species on LSCF are highly mobile and can migrate across the entire porous LSCF electrode.45 The high mobility of surface segregated Sr species on the YSZ electrolyte on LSM and La0.75Sr0.25Cr0.5Mn0.5O3 films has also been confirmed by Huber et al.’s using in situ SIMS analysis.46-47 The Sr surface segregation and its mobility can also be significantly affected by the electrochemical polarization. Vovk et al.48 studied the La0.5Sr0.5CoO3 perovskite oxide surfaces using in situ XPS and observed the increase of Sr/(La+Co) ratio at the oxide surface by 5% after cathodic polarization. Mutoro et al.49 observed the reversible surface composition changes of (La,Sr)CoO3 thin film electrodes under polarization by in situ synchrotron based XPS and found that the increase of Sr and decrease of Co occurred under cathodic polarization. The loss of Sr from the LSCF lattice will substantially reduce the electrocatalytic activity and electrical conductivity of the cathode,50-52 consistent with the performance degradation of the cells with in situ assembled LSCF cathodes as observed in this study. The results of this study indicate that the Sr surface segregation and its migration to the LSCF electrode/YSZ electrolyte interface are responsible for the performance degradation of cells with in situ assembled LSCF cathodes. One effective way to alleviate such Sr segregation problem is to adopt Sr-free cobaltite perovskites such as the LC as the in situ assembled cathodes as demonstrated in this study (see Fig. 10). On the other hand, Sr in the A-site of cobaltite based perovskite can also be stabilized through doping in the B site.53-58 For example, doing Nb at the B-site can substantially increase the stability of Sr at the A-site, thus reducing the surface segregation of Sr.53, 59 4. Conclusions The polarization performance and microstructure of electrode/electrolyte interface of anode-supported YSZ electrolyte cells with in situ assembled LSCF electrodes without GDC 13 ACS Paragon Plus Environment


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barrier layer was investigated at 750 oC under SOFC operation conditions. The cells with in situ assembled LSCF electrodes showed a PPD as high as 1.7 W cm-2 at 750 oC. A decrease of the cell’s ohmic resistance and the establishment of contact marks on the YSZ electrolyte surface indicated the formation of the electrode/electrolyte interface of in situ assembled LSCF electrodes under the influence of cathodic polarization conditions. This is also supported by the decrease in cell ohmic and polarization resistances. The initial RΩ and RE were 0.21 and 0.27 Ω cm2, respectively, and reduced to 0.14 and 0.21 Ω cm2 after polarization for 6 h. However, the cell with in situ assembled LSCF electrodes is not stable. Detailed FIB-STEM and SIME analysis of the electrode/electrolyte interface indicates that cathodic polarization accelerates the Sr surface segregation and migration to the YSZ electrolyte surface, forming a thin and uniform SrO layer at the interface and thus degrading the cell performance. The use of a Sr-free LC-GDC electrode was shown to be effective in alleviating the Sr segregation problem. In conclusion, the in situ assembly approach provides a feasible pathway to directly apply MIEC cobaltite based cathodes to the YSZ electrolyte. However, more detailed and extensive work is required to fundamentally understand the electrode/electrolyte interface formation and Sr segregation under the SOFC operation conditions for in situ assembled Sr-containing cathodes. Acknowledgements The project is supported by Curtin University Research Fellow Program, Australian Research Council under the Discovery Project Scheme (Project number: DP150102044 and DP150102025), and the Science and Industry Endowment Fund. The authors acknowledge the facilities, scientific and technical assistance of the Curtin University Microscopy & Microanalysis Facility and the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia,


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both of which are partially funded by the University, State and Commonwealth Governments, the WA X-Ray Surface Analysis Facility, funded by the Australian Research Council LIEF grant (LE120100026), and the Department of Chemistry/Nanochemistry Research Institute, Curtin University. References 1. Jiang, S. P.; Wang, X., Chapter 5. Fuel Cells: Advances and Challenges. In Handbook of Solid State electrochemistry, Kharton, V. V., Ed. Wiley-VCH: 2011, pp 179-264. 2. Jiang, S. P., Development of Lanthanum Strontium Manganite Perovskite Cathode Materials of Solid Oxide Fuel Cells: A Review. J. Mater. Sci. 2008, 43 (21), 6799-6833. 3. Tsai, T.; Barnett, S. A., Effect of LSM-YSZ Cathode on Thin-Electrolyte Solid Oxide Fuel Cell Performance. Solid State Ionics 1997, 93 (3-4), 207-217. 4. Sun, C. W.; Hui, R.; Roller, J., Cathode Materials for Solid Oxide Fuel Cells: A Review. J. Solid State Electrochem. 2010, 14 (7), 1125-1144. 5. Park, Y. M.; Kim, J. H.; Kim, H., High-Performance Composite Cathodes for Solid Oxide Fuel Cells. Int. J. Hydrog. Energy 2011, 36 (15), 9169-9179. 6. Comminges, C.; Fu, Q. X.; Zahid, M.; Steiner, N. Y.; Bucheli, O., Monitoring the Degradation of a Solid Oxide Fuel Cell Stack During 10,000 h via Electrochemical Impedance Spectroscopy. Electrochim. Acta 2012, 59 (0), 367-375. 7. Shao, Z. P.; Haile, S. M., A High-Performance Cathode for the Next Generation of Solid-Oxide Fuel Cells. Nature 2004, 431 (7005), 170-173. 8. Kim, J.-S.; Yeon, D.-H.; Jung, D. W.; Kwak, C., A Highly Active and Long-Term Stable La-doped BaxSr1−xCo1−yFeyO3−δ Cathode for Solid-Oxide Fuel Cells. J. Power Sources 2014, 249 (0), 66-71. 9. Yokokawa, H., Understanding Materials Compatibility. Ann. Rev. Mater. Res. 2003, 33, 581610. 10. Yokokawa, H.; Sakai, N.; Kawada, T.; Dokiya, M., Thermodynamic Analysis of Reaction Profiles between Lamo3 (M=Ni,Co,Mn) and Zro2. J. Electrochem. Soc. 1991, 138 (9), 2719-2727. 11. Yamamoto, O.; Takeda, Y.; Kanno, R.; Noda, M., Perovskite-Type Oxides as Oxygen Electrodes for High-Temperature Oxide Fuel-Cells. Solid State Ionics 1987, 22 (2-3), 241-246. 12. Petric, A.; Huang, P.; Tietz, F., Evaluation of La-Sr-Co-Fe-O perovskites for Solid Oxide Fuel Cells and Gas Separation Membranes. Solid State Ionics 2000, 135 (1-4), 719-725. 13. Horita, T.; Nishi, M.; Shimonosono, T.; Kishimoto, H.; Yamaji, K.; Brito, M. E.; Yokokawa, H., Visualization of Oxide Ionic Diffusion at SOFC Cathode/Electrolyte Interfaces by Isotope Labeling Techniques. Solid State Ionics 2014, 262, 398-402. 14. Kiebach, R.; Zhang, W.-W.; Zhang, W.; Chen, M.; Norrman, K.; Wang, H.-J.; Bowen, J. R.; Barfod, R.; Hendriksen, P. V., Stability of La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 Cathodes During Sintering and Solid Oxide Fuel Cell Operation. J. Power Sources 2015, 283 (0), 151-161. 15. Yokokawa, H.; Sakai, N.; Horita, T.; Yamaji, K.; Brito, M. E.; Kishimoto, H., Thermodynamic and Kinetic Considerations on Degradations in Solid Oxide Fuel Cell Cathodes. J. Alloys and Compounds 2008, 452 (1), 41-47. 16. Fan, H.; Keane, M.; Singh, P.; Han, M., Electrochemical Performance and Stability of Lanthanum Strontium Cobalt Ferrite Oxygen Electrode with Gadolinia Doped Ceria Barrier Layer for Reversible Solid Oxide Fuel Cell. J. Power Sources 2014, 268 (0), 634-639. 17. Kim, S. J.; Choi, G. M., Stability of LSCF Electrode with GDC Interlayer in YSZ-Based Solid Oxide Electrolysis Cell. Solid State Ionics 2014, 262, 303-306.



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18. Simner, S. P.; Anderson, M. D.; Engelhard, M. H.; Stevenson, J. W., Degradation Mechanisms of La-Sr-Co-Fe-O3SOFC Cathodes. Electrochem. Solid State Lett. 2006, 9 (10), A478-A481. 19. Wang, F.; Nishi, M.; Brito, M. E.; Kishimoto, H.; Yamaji, K.; Yokokawa, H.; Horita, T., Sr and Zr Diffusion in LSCF/10GDC/8YSZ Triplets for Solid Oxide Fuel Cells (SOFCs). J. Power Sources 2014, 258, 281-289. 20. Jiang, S. P., Thermally and Electrochemically Induced Electrode/Electrolyte Interfaces in Solid Oxide Fuel Cells: An AFM and EIS Study. J. Electrochem. Soc. 2015, 162 (10), F1119-F1128. 21. Esquirol, A.; Brandon, N. P.; Kilner, J. A.; Mogensen, M., Electrochemical Characterization of La0.6Sr0.4Co0.2Fe0.8O3 Cathodes for Intermediate-Temperature SOFCs. J. Electrochem. Soc. 2004, 151 (11), A1847-A1855. 22. Shi, M.; Xu, Y. D.; Liu, N.; Wang, C.; Majewski, P., Ionic Conductivity and Microstructure of Srand Mg-doped LaGaO3. J. Inorg. Mater. 2006, 21 (3), 605-611. 23. Marinha, D.; Dessemond, L.; Cronin, J. S.; Wilson, J. R.; Barnett, S. A.; Djurado, E., Microstructural 3D Reconstruction and Performance Evaluation of LSCF Cathodes Obtained by Electrostatic Spray Deposition. Chem. Mater. 2011, 23 (24), 5340-5348. 24. Chen, K. F.; Lu, Z.; Ai, N.; Huang, X. Q.; Zhang, Y. H.; Ge, X. D.; Xin, X. S.; Chen, X. J.; Su, W. H., Fabrication and Performance of Anode-Supported YSZ films by Slurry Spin Coating. Solid State Ionics 2007, 177 (39-40), 3455-3460. 25. Chen, K. F.; Chen, X. J.; Lu, Z.; Ai, N.; Huang, X. Q.; Su, W. H., Performance of an AnodeSupported SOFC with Anode Functional Layers. Electrochim. Acta 2008, 53 (27), 7825-7830. 26. Su, C.; Xu, X. M.; Chen, Y. B.; Liu, Y.; Tade, M. O.; Shao, Z. P., A Top-down Strategy for the Synthesis of Mesoporous Ba05Sr0.5Co0.8Fe0.2O3-delta as a Cathode Precursor for Buffer Layer-free Deposition on Stabilized Zirconia Electrolyte with a Superior Electrochemical Performance. J. Power Sources 2015, 274, 1024-1033. 27. Li, N.; Ai, N.; Chen, K. F.; Cheng, Y.; He, S.; Saunders, M.; Dodd, A.; Suvorona, A.; Jiang, S. P., In situ Assembled La0.8Sr0.2MnO3 Cathode on a Y2O3-ZrO2 Electrolyte of Solid Oxide Fuel Cells Interface and Electrochemical Activity. RSC Advances 2016, 6, 99211-99219. 28. McIntosh, S.; Vohs, J. M.; Gorte, R. J., Impedance Spectroscopy for the Characterization of Cu-Ceria-YSZ Anodes for SOFCs. J. Electrochem. Soc. 2003, 150 (10), A1305-A1312. 29. Ai, N.; Chen, K.; Jiang, S. P.; Lü, Z.; Su, W., Vacuum-assisted Electroless Copper Plating on Ni/(Sm,Ce)O2 Anodes for Intermediate Temperature Solid Oxide Fuel Cells. Intern. J. Hydrogen Energy 2011, 36 (13), 7661-7669. 30. Chen, K.; Zhang, L.; Ai, N.; Zhang, S.; Song, Y.; Song, Y.; Yi, Q.; Li, C.-Z.; Jiang, S. P., Feasibility of Direct Utilization of Biomass Gasification Product Gas Fuels in Tubular Solid Oxide Fuel Cells for On-Site Electricity Generation. Energy & Fuels 2016, 30 (3), 1849-1857. 31. Wang, F.; Kishimoto, H.; Develos-Bagarinao, K.; Yamaji, K.; Horita, T.; Yokokawa, H., Interrelation between Sulfur Poisoning and Performance Degradation of LSCF Cathode for SOFCs. J. Electrochem. Soc. 2016, 163 (8), F899-F904. 32. Jiang, S. P.; Zhang, S.; Zhen, Y. D., Deposition of Cr species at (La,Sr)(Co,Fe)O-3 Cathodes of Solid Oxide Fuel Cells. J. Electrochem. Soc. 2006, 153 (1), A127-A134. 33. Biesinger, M. C.; Payne, B. P.; Lau, L. W.; Gerson, A.; Smart, R. S. C., X-ray Photoelectron Spectroscopic Chemical State Quantification of Mixed Nickel Metal, Oxide and Hydroxide Systems. Surf. Interface Anal. 2009, 41 (4), 324-332. 34. Chen, C. H.; Kruidhof, H.; Bouwmeester, H. J. M.; Burggraaf, A. J., Ionic Conductivity of Perovskite LaCoO3 Measured by Oxygen Permeation Technique. J. Appl. Electrochem. 1997, 27 (1), 71-75. 35. Zhou, W.; Shao, Z.; Ran, R.; Zeng, P.; Gu, H.; Jin, W.; Xu, N., Ba0.5Sr0.5Co0.8Fe0.2O3−δ + LaCoO3 Composite Cathode for Sm0.2Ce0.8O1.9-Electrolyte Based Intermediate-Temperature SolidOxide Fuel Cells. J. Power Sources 2007, 168 (2), 330-337. 36. Mitterdorfer, A.; Gauckler, L. J., La2Zr2O7 Formation and Oxygen Reduction Kinetics of the La0.85Sr0.15MnyO3, O2(g)|YSZ System. Solid State Ionics 1998, 111 (3–4), 185-218. 16 ACS Paragon Plus Environment

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37. Horita, T.; Tsunoda, T.; Yamaji, K.; Sakai, N.; Kato, T.; Yokokawa, H., Microstructures and Oxygen Diffusion at the LaMnO3 Film/Yttria-stabilized Zirconia Interface. Solid State Ionics 2002, 152–153 (0), 439-446. 38. Chen, K.; Liu, S.-S.; Ai, N.; Koyama, M.; Jiang, S. P., Why Solid Oxide Cells Can Be Reversibly Operated in Solid Oxide Electrolysis Cell and Fuel Cell Modes? Phys. Chem. Chem. Phys. 2015, 17 (46), 31308-31315. 39. Kubicek, M.; Limbeck, A.; Frömling, T.; Hutter, H.; Fleig, J., Relationship Between Cation Segregation and the Electrochemical Oxygen Reduction Kinetics of La0.6Sr0.4CoO3−δ Thin Film Electrodes. J. Electrochem. Soc. 2011, 158 (6), B727-B734. 40. Bucher, E.; Sitte, W.; Klauser, F.; Bertel, E., Oxygen Exchange Kinetics of La0.58Sr0.4Co0.2Fe0.8O3 at 600°C in Dry and Humid Atmospheres. Solid State Ionics 2011, 191 (1), 61-67. 41. Oh, D.; Gostovic, D.; Wachsman, E. D., Mechanism of La0.6Sr0.4Co0.2Fe0.8O3 Cathode Degradation. J. Mater. Res. 2012, 27 (15), 1992-1999. 42. Hardy, J. S.; Templeton, J. W.; Edwards, D. J.; Lu, Z.; Stevenson, J. W., Lattice Expansion of LSCF-6428 Cathodes Measured by in situ XRD during SOFC Operation. J. Power Sources 2012, 198 (0), 76-82. 43. Ding, H.; Virkar, A. V.; Liu, M.; Liu, F., Suppression of Sr Surface Segregation in La1-xSrxCo1yFeyO3-δ: A First Principles Study. Phys. Chem. Chem. Phys. 2013, 15 (2), 489-496. 44. Zhao, L.; Drennan, J.; Kong, C.; Amarasinghe, S.; Jiang, S. P., Insight into Surface Segregation and Chromium Deposition on La0.6Sr0.4Co0.2Fe0.8O3-delta Cathodes of Solid Oxide Fuel Cells. J. Mater. Chem. A 2014, 2 (29), 11114-11123. 45. Wei, B.; Chen, K. F.; Zhao, L.; Lu, Z.; Jiang, S. P., Chromium Deposition and Poisoning at La0.6Sr0.4Co0.2Fe0.8O3-delta Oxygen Electrodes of Solid Oxide Electrolysis cells. Phys. Chem. Chem. Phys. 2015, 17 (3), 1601-1609. 46. Huber, A.-K.; Falk, M.; Rohnke, M.; Luer; Gregoratti, L.; Amati, M.; Janek, J., In situ Study of Electrochemical Activation and Surface Segregation of the SOFC Electrode Material La0.75Sr0.25Cr0.5Mn0.5O3+/-[small delta]. Phys. Chem. Chem. Phys. 2012, 14 (2), 751-758. 47. Huber, A.-K.; Falk, M.; Rohnke, M.; Luerssen, B.; Amati, M.; Gregoratti, L.; Hesse, D.; Janek, J., In situ Study of Activation and De-activation of LSM Fuel Cell Cathodes–Electrochemistry and Surface Analysis of Thin-film Electrodes. J. Catalysis 2012, 294, 79-88. 48. Vovk, G.; Chen, X.; Mims, C. A., In Situ XPS Studies of Perovskite Oxide Surfaces under Electrochemical Polarization. J. Phys. Chem. B 2004, 109 (6), 2445-2454. 49. Mutoro, E.; Crumlin, E. J.; Pöpke, H.; Luerssen, B.; Amati, M.; Abyaneh, M. K.; Biegalski, M. D.; Christen, H. M.; Gregoratti, L.; Janek, J.; Shao-Horn, Y., Reversible Compositional Control of Oxide Surfaces by Electrochemical Potentials. J. Phys. Chem. Lett. 2011, 3 (1), 40-44. 50. Tai, L. W.; Nasrallah, M. M.; Anderson, H. U.; Sparlin, D. M.; Sehlin, S. R., Structure and Electrical Properties of La1 − xSrxCo1 − yFeyO3. Part 2. The System La1 − xSrxCo0.2Fe0.8O3. Solid State Ionics 1995, 76 (3–4), 273-283. 51. Huang, K.; Lee, H. Y.; Goodenough, J. B., Sr- and Ni-Doped LaCoO3 and LaFeO3 Perovskites: New Cathode Materials for Solid-Oxide Fuel Cells. Journal of The Electrochemical Society 1998, 145 (9), 3220-3227. 52. Chen, K.; Ai, N.; O'Donnell, K. M.; Jiang, S. P., Highly Chromium Contaminant Tolerant BaO Infiltrated La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes for Solid Oxide Fuel Cells. Phys. Chem. Chem. Phys. 2015, 17 (7), 4870-4874. 53. Chen, X. B.; Jiang, S. P., Highly Active and Stable (La0.24Sr0.16Ba0.6)(Co0.5Fe0.44Nb0.06)O3-d (LSBCFN) Cathodes for Solid Oxide Fuel Cells Prepared by a Novel Mixing Synthesis Method. J. Mater. Chem. A 2013, 1 (15), 4871-4878. 54. Zhou, W.; Shao, Z.; Ran, R.; Cai, R., Novel SrSc0.2Co0.8O3−δ as a Cathode Material for Low Temperature Solid-Oxide Fuel Cell. Electrochem. Commun. 2008, 10 (10), 1647-1651.



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(a)

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LSCF

(b)

SrZrO3

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

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Figure 1. (a) XRD pattern of LSCF-YSZ mixed powder (5/5, w/w) calcined at 750-900 oC. SEM images of (b) as-prepared LSCF powder, (c) overview of cross section of an anodesupported YSZ film with an in situ assembled LSCF electrode and the inset is the surface of YSZ film, and (d) high resolution image of the YSZ/LSCF interface.



1.2 100 h

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Figure 2. Time dependent polarization performance of a cell with the in situ assembled LSCF cathode at 500 mA cm-2, 750 oC: (a) polarization curves, (b) impedance curves measured at open circuit, and (c) stability curve. Dashed line in (b) indicates the constant RΩ after polarization for 6 h. Numbers in (b) are frequencies in Hz.


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Figure 3. Polarization performance of a cell with the in situ assembled LSCF-GDC composite cathode as a function of polarization time at 500 mA cm-2, 750 oC: (a) polarization curves, (b) stability curve, and (c) impedance curves measured at open circuit. Numbers in (c) are frequencies in Hz.



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Figure 4. Polarization performance of a cell with the in situ assembled LSCF cathode as a function of polarization time at 250 mA cm-2, 650 oC for 100 h. (a) polarization curves, (b) stability curve, and (c) impedance curves measured at open circuit. Numbers in (c) are frequencies in Hz.


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(a)

(b)

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YSZ (c)

LSCF

(d)

Figure 5. SEM images of the cell with in situ assembled LSCF electrode after polarization at 500 mA cm-2, 750 oC for 100 h: (a) cross section of LSCF/YSZ interface and (b) YSZ electrolyte surface after removal of the LSCF electrode by adhesive tape, (c) YSZ electrolyte surface in contact with in situ assembled LSCF after polarization at 500 mA cm-2, 750 oC for 14 h and (d) YSZ electrolyte surface in contact with in situ assembled LSCF under open circuit at 750 oC for 100 h.



(a) Sr, Zr

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Figure 6. (a) STEM image and elemental maps at the electrode/electrolyte interface after the in situ assembled LSCF was polarized at 750 oC for 100 h. (b) SIMS depth profiles for La, Sr and Zr. (c) Near surface 88Sr SIMS map (top ~10 nm).


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Counts / ToF Extraction

Page 25 of 30

1 µm

Figure 7. (a) SEM image of YSZ surface, (b) SIMS depth profiles for La, Sr and Zr, and near surface

88

Sr SIMS maps of the YSZ surface after the in situ assembled LSCF cathode was

polarized at 650 oC for 100 h: (c) top ~10 nm and (d) top ~40 nm.



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Amplitude

30.0 nm Amplitude

Figure 8. AFM images of YSZ film surface in contact with the in situ assembled LSCF electrode after polarization at 500 mA cm-2, 750 oC for different periods. (a,b) 100 h, (d) 1 h and (e) 14 h. As-prepared clean YSZ film is shown in (f). Height profiles of the YSZ surface in (b) are shown in (c).


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Survey

La 3d

(b)

(a)

O 1s Zr 3p

Ni 2p

C1s

Zr 3d

Ni 2p

1200 1000

800

600

400

200

Zr 4s

Y 3d

Y3p

Zr 3s

Ni 2p

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


860

0

Binding energy / eV

850

840

830

Binding energy / eV

Sr 3d

(c)

140

135

130

125

Binding energy / eV

Figure 9. (a) XPS survey scan and high resolution scans of (b) La 3d and (c) Sr 3d core levels of the acid cleaned YSZ electrolyte surface in contact with the in situ assembled LSCF electrode after polarization at 500 mA cm-2, 750 oC for 100 h. Ni 2p signal may be due to contamination during the acid treatment of the cell.



(a) 1.2

1.5 100 h

Voltage / V

1.2

0.8

0.9

0.6 0.6

0.4

0.3

0.2 0.0

0


(b)

1.0

4

0.9 Voltage / V

20 h

-2

4h


0h

1.0

0.8 0.7 0.6 0.5

0.0

0.4

0

20

40 60 Time / h

80

100

(c) 0.8 0h

4h

100 h

2

0.6 -Z" / Ω cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.2

1k

10k 1k

10k

100 10

0.0 -0.2 0.0

100 10

0.2

0.4

0.6 0.8 2 Z' / Ω cm

1.0

1.2

Figure 10. Polarization performance of a cell with the in situ assembled LC-GDC cathode under a constant current of 500 mA cm-2 at 750 oC for 100 h. (a) polarization curves, (b) stability curve, and (c) impedance curves. Numbers in (c) are frequencies in Hz.


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(a)

(b) (c) 25

Profile 1

20

1 2

Height / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 15 Profile 2 10 5 0 0

50

100

150

200

Distance / nm

Figure 11. (a) SEM image, (b) AFM height image and (c) height profiles of YSZ electrolyte surface after the in situ assembled LC-GDC electrode was polarized at 500 mA cm-2, 750 oC for 40 h.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cathodic polarization not only induces the formation of cobaltite based perovskite electrode/YSZ electrolyte but also accelerates the surface segregation of Sr 254x190mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Polarization-Induced Interface and Sr Segregation of in Situ

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