Cobalt-free perovskite oxide La0.6Sr0.4Fe0.8Ni0 ... - ACS Publications

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Cobalt-free perovskite oxide La Sr Fe Ni O as active and robust oxygen electrode for reversible solid oxide cells Yunfeng Tian, Wenjie Wang, Yun Liu, Lingling Zhang, Lichao Jia, Jun Yang, Bo Chi, Jian Pu, and Jian Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00115 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Energy Materials

Cobalt-free Perovskite Oxide La0.6Sr0.4Fe0.8Ni0.2O3-δ as Active and Robust Oxygen Electrode for Reversible Solid Oxide Cells Yunfeng Tiana, Wenjie Wanga, Yun Liua , Lingling Zhanga, Lichao Jiaa, b, Jun Yangc, Bo Chia, b*, Jian Pua, b, Jian Lia a. Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China b. Key Laboratory of Material Chemistry for Energy Conversion and Storage of Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China c. Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Abstract: Reversible solid oxide cells have received increasingly attention due to high efficiency. The cobalt-free perovskite electrode has compatible thermal expansion coefficient matching with the electrolyte and the reversible operation to inhibit the segregation of Sr. Herein a novel cobalt-free La0.6Sr0.4Fe0.8Ni0.2O3-δ perovskite is developed and investigated as oxygen electrode for reversible solid oxide cells. The electrochemical performance of La0.6Sr0.4Fe0.8Ni0.2O3-δ oxygen electrode in fuel cell mode and electrolysis mode is investigated in detail. The maximum power density of 961 mW cm-2 and polarization resistance of 0.142 Ω cm2 at 800 °C can be achieved in fuel cell mode. While the cell is operated in electrolysis mode, the current density

*

Corresponding author. Email: [email protected] (B. Chi) 1

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ranges from 0.53 A cm-2 at 750 °C to 1.09 A cm-2 at 850 °C with 50 vol% absolute humidity at 1.3 V, and the hydrogen generation rate can reach up to 1348.5 mL (cm2·h)-1 with 90 vol% absolute humidity at 800 °C. The reversible solid oxide cells show excellent reversibility and stability during 144 h medium-term reversible operation. The results indicate that La0.6Sr0.4Fe0.8Ni0.2O3-δ has a bright prospect as the oxygen electrode material for reversible solid oxide cells. Keywords: Oxygen electrode; Nickel-iron perovskites; Cobalt-free; Reversible solid oxide cells; Reversibility; Stability

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1. Introduction Reversible solid oxide electrochemical cells (RSOCs) can work in solid oxide fuel cell (SOFC) mode to generate power 1 or in solid oxide electrolysis cell (SOEC) mode to electrolyze H2O and CO2 to get chemical fuel as shown in Fig.1 2. Therefore, they are expected to have significant application in many fields such as large-scale energy storage due to their high efficiency and have attracted more and more attention 3-4. At present, RSOCs are usually comprised of hydrogen electrode (Ni-YSZ), electrolyte (YSZ) and oxygen electrode (La0.8Sr0.2MnO3-δ (LSM)) 5. Among them, the oxygen electrode requires excellent catalytic activity as well as good stability towards both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) 6. Recently, many researchers have confirmed that RSOCs suffer more severe performance decay in SOEC mode than in SOFC mode 7-8. In particular, the degradation of oxygen electrode in SOEC mode is quite serious, which limits the development and application of RSOCs

9-10.

Microstructure destruction is one of the main degradation

factors because of the high oxygen partial pressure at the electrolyte- oxygen electrode interface resulted from the high anodic over-potential

11-12.

LSM has been widely

investigated as oxygen electrode. But its low ORR and OER catalytic activity at intermediate temperature hinders its application

13-16.

Cobalt-based perovskites have

also been studied as the oxygen electrode for RSOCs because of their good conductivity and excellent catalytic activity, López-Robledo et al. fabricated the microtubular cells using La0.6Sr0.4Co0.2Fe0.8O3-δ electrode and Gd0.1Ce0.9O2-δ (GDC) barrier layers and achieved high current density (0.845 Acm-2 at 800 °C and 1.3 V)

17.

Choi et al. 3



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investigated La0.1Sr0.9Co0.8Fe0.2O3-δ as the oxygen electrode of RSOCs and achieved good performance (0.55 Acm-2 at 750 °C and 1.3 V)

18.

Tan et al. impregnated

La0.8Sr0.2Co0.8Ni0.2O3-δ into GDC backbone as the oxygen electrode and obtained better electrochemical performance (0.95 Acm-2 at 750 °C and 1.3 V)

19.

Others materials

such as La0.6Sr0.4Co0.2Fe0.8O3-δ 20, SrCo0.8Fe0.1Ga0.1O3-δ 21, PrBaCo2O5+δ

22

were also

investigated. However, the cobalt-based oxygen electrode has a larger thermal expansion coefficient (TEC) than YSZ electrolyte, which would cause serious thermal mismatching of electrode and electrolyte and then weaken the charge exchange and mass transfer process leading to a bigger ohmic resistance. Most importantly, the deterioration near the oxygen electrode/electrolyte interface is extremely serious when the cell is reversibly operated

23-24.

In addition, Co element is volatile during high

temperature sintering process and the cost of Co is relatively high

25.

Therefore, the

development of cobalt-free oxygen electrode is highly desired. Laguna-Bercero et al. used Pr2NiO4+δ (PNO) as the oxygen electrode and good performance (0.78 Acm-2 at 800 °C and 1.3 V) was achieved

26.

Nickel-iron based

perovskites with mixed ionic electronic conductors (MIECs) have much more reasonable TECs and better ORR catalytic activity compared with cobalt-based perovskites 27-29. In our previous study, La0.6Sr0.4Fe0.8Ni0.2O3-δ (LSFN) was used as the electrode of a symmetrical cell for direct CO2 electrolysis, and achieved good CO2-RR and OER performance30. However, its physicochemical properties have not been described in details, and its electrocatalytic activity and stability of ORR and OER also need further explored. In addition, in some case RSOCs switching modes in a short 4


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period of time (1~4 h) have displayed good stability, but in a longer period of time the unfavorable degradation happened

3-4, 31-32.

Therefore, in this work, LSFN will be

studied in detail about its OER and ORR catalytic activity as oxygen electrode in both modes for RSOCs. The reversibility for the longer period of time (7 h SOEC and 14 h SOFC) and stability of LSFN as oxygen electrode for RSOCs are also investigated.

Fig.1. Schematic diagram of reversible solid oxide electrochemical cells. 2. Experimental 2.1 Sample and cell preparation La0.6Sr0.4Fe0.8Ni0.2O3-δ (LSFN) powders were prepared by sol-gel method as previously reported 30. Three-electrode cell was developed for investigating the anodic and cathodic polarization resistances of LSFN (see supporting information). The detail preparation steps of single cell with hydrogen electrode support and YSZ electrolyte can be found in our previous work 19. The fuel electrode (Ni-YSZ, 60 wt%:40 wt%) and YSZ electrolyte (10 μm in thickness) were fabricated on the Ni-YSZ (57 wt%:43 wt%) tape by screen-printing followed by sintered at 1390 °C for 4 h. Gd0.1Ce0.9O2-δ (GDC) paste was screen-printed onto the surface of the YSZ electrolyte to form the 5



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buffer layer followed by sintered at 1300 °C for 5 h. The cell was fabricated by screenprinting LSFN-GDC composite (60 wt%:40 wt%) paste on the YSZ electrolyte surface with GDC as the buffer layer. Then the cell was calcined at 1000 °C for 2 h. The active area of cell is 0.5 cm2 and Pt is used as the current collector. 2.2 Characterization and test X-ray diffraction (Cu Kα, 40 KV, 40 mA, Shimadzu XRD-7000S) was used to characterize the phase of pristine LSFN. More structural analysis was performed by the Rietveld method with the GSAS/EXPGUI software. Morphology and structure were characterized by scanning electron microscope (SEM, Sirion 200) and transmission electron microscopy (JEOL JEM-2011). TG-DSC was measured by thermogravimetric analysis (TGA, STA449F5, NETZSCH) from 20 °C to 1000 °C with a heating rate of 10 °C min-1 under air. The characterization of LSFN was also presented by hydrogen temperature-programmed reduction (H2-TPR). Powders were pretreated at 350 °C in Ar for 1 h and then H2-TPR test was conducted from 20 to 950 °C at 10 °C min-1 in 5% H2/N2 flow of 30 mL min-1. LSFN bars (24 mm×6 mm×2 mm) were obtained by pressing and sintering LSFN powders at 1350 °C for 5 h. Conductivity test was carried out in air by four-probe method using an Agilent B2901A Precision Source/Measure Unit. TEC of LSFN was measured using Thermo Dilation (NETZSCH DIL402C). Single cell was tested on a platform by sealing on an Al2O3 tube using ceramic adhesive (552-VFG, Aremco). In SOFC mode, wet hydrogen was fed into the hydrogen electrode and air was fed into the oxygen electrode. For SOEC test, thermostatic water bath was used to control the saturated steam concentration of hydrogen. The 6


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electrochemical performance of RSOCs was recorded by the electrochemical workstation (Zennium IM6 station), including current density-voltage curves and electrochemical impedance spectra in frequency range of 0.1~100 kHz with an amplitude of 10 mV both in SOFC and SOEC mode. Besides, the reversibility and stability of RSOCs in both SOFC and SOEC modes were also tested. The surface element distribution of LSFN electrode before and after test was evaluated by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) using Al Kα radiation (hν = 1486.6 eV) and the C1s peak 284.8 eV was used to calibrate the binding energies of XPS spectra. The phase structure of LSFN electrode after test was also characterized by XRD. The microstructure of the cell was observed by environment scanning electron microscope (ESEM, Quanta 200) and electron probe micro analyzer (EPMA-8050G, SHIMADZU). 3．Results and discussion 3.1 Materials characterization The X-ray diffraction patterns and Rietveld refinement of the as-prepared LSFN powders are given in Fig. 2 (a), which present cubic symmetrical perovskite structure (space group Pm-3m) without any detectable impurity peaks. The diffraction peaks at 22.8°, 32.5°, 40°, 46.7°, 52.6°, 58°, 68.1°, 73.1° and 77.3° can be corresponded to (100), (110), (111), (200), (210), (211), (220), (221) and (310) planes of LaFeO3 (PDF # 750439), respectively. The lattice constant of a = 3.894 Å is a little bit larger than LaFeO3 (a = 3.89Å) since Sr2+ (1.18 Å) and Ni2+ (0.69 Å) are doped to the smaller La3+(1.03 Å) and Fe3+(0.645 Å) lattice site. Lower reliability factor (Rp=2.18%, wRp=2.95%, 7



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chi2=1.38%) indicates the goodness of fit. Fig. 2 (b) confirms that the sample has a uniform and fine particle distribution with size of about 200 nm. TEM images of LSFN powder are presented in Fig. 2 (c, d). The lattice fringe spacing of the powder is measured as 0.3827 nm, corresponding to (100) plane of the cubic perovskite LSFN. The lattice space of 0.2629 nm can be confirmed by (110) crystal plane of the cubic perovskite LSFN.

Fig. 2. XRD patterns and refined results (a), SEM image (b), TEM image (c) and high-resolution TEM image (d) of LSFN powders. Fig. 3 (a) shows the conductivity of LSFN in air from 100 °C to 900 °C. The conductivity increases with temperature to a maximum value of 130 S cm-1 at 500 °C. It shows the semiconducting behavior below 500 °C, while a metallic conducting behavior over 500 °C. This phenomenon reflects the small polaron mechanism and 8


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objects to the Zener double exchange process33 . Thermal expansion behavior of LSFN is studied as shown in Fig. 3 (b). The average TEC of LSFN between room temperature and 1000 °C is 11.9 × 10-6 K-1, similar to the results reported for cobalt-free perovskites in reference

34,

and apparently it is lower than those for cobalt-based perovskite as

La0.4Sr0.6Co0.2Fe0.8O3-δ (19.95 × 10-6 K-1 at 100~800 °C) 35, PrBa0.5Sr0.5Co2O5+δ (20.46 × 10-6 K-1 at 30~850 °C) 36, SrCo0.9Nb0.1O3-δ (24.2 × 10-6 K-1 at 30~1000 °C) 37. And TEC value of LSFN matches perfectly with the GDC electrolyte (12 × 10-6 K-1) 38 . This thermal compatibility can achieve a better adhesion and interface stability with GDC buffer layer, which may be beneficial to the long-term stability of RSOCs. H2-TPR curve of LSFN is shown in Fig. 3 (c), exhibiting two main reduction peaks in 100~900 °C. The first peak α at around 400 °C indicates the reduction of Ni3+ to Ni2+, whereas the second peak β represents Ni2+ is further reduced to Ni accompanied by a little reduction of Fe3+ to Fe2+ 39-40. Compared with undoped LaFeO3, the reducibility of the oxygen carrier can be enhanced

by doping Sr2+ and Ni2+ 41. The TG-DSC

curve of LSFN under air is recorded in Fig. 3 (d). The loss of weight corresponds to the generation of oxygen vacancies at the higher temperature. It also can be identified by the peak of the DSC curve. It is worth noting that the mass of LSFN just has a little change at high temperatures (>800 °C), indicating that it possesses good stability. As known, oxygen vacancies not only contribute to the improvement of oxygen ion conductance, but also contribute to the enhancement of ORR and OER performance.

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Fig. 3. The conductivity (a), thermal expansion behavior (b), H2-TPR profile (c) and TG-DSC curve (d) of LSFN. 3.2 Electrochemical performance of RSOCs The current density-voltage-power density (I-V-P) curves for RSOCs under SOFC mode are shown in Fig. 4 (a). The OCV is very close to the Nernst potential 42, and the maximum power density changes from 729 mW cm-2 to 1159 mW cm-2 at 750 °C to 850 °C. Nyquist plots of RSOCs at various temperatures are presented in Fig. 4 (b). The polarization resistance (Rp, estimated from the intercept difference between two intersections with real axis) of RSOCs is only 0.142 Ω cm2 at 800 °C, which is relatively lower than those of the cells with cobalt-based or cobalt-free perovskite oxygen electrodes shown in Fig.4 (e)43-45 . Moreover, LSFN oxygen electrode has better ORR catalytic performance as shown in Fig. S1 (a, b). These results demonstrate that LSFN displays a high ORR activity and RSOCs based on LSFN oxygen electrode in SOFC 10


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mode could achieve superior performance. The electrochemical performance of RSOCs in SOEC mode is characterized by electrolyzing H2O. Fig. 4 (c) presents the I-V curves of RSOCs tested at different temperatures with 50 vol% AH. Obviously, the current density value near the thermoneutral voltage (1.3 V) ranges from 0.53 Acm-2 at 750 °C to 1.09 Acm-2 at 850 °C. However, when the temperature and voltage are quite high, the slope of curves changes slightly due to the concentration polarization 17, 46. Nyquist plots of RSOCs under SOEC mode are presented in Fig. 4 (d). The value of RP for LSFN oxygen electrode is only 0.081 Ω cm2 at 800 °C, which is relatively lower than those of the cells shown in Fig.4 (f)

43, 47.

The I-V curves and Nyquist plots for the

RSOCs under SOEC mode at various humidity are shown in Fig. S2. Moreover, the hydrogen production rate of RSOCs under SOEC mode is up to 1348.5 mL (cm2 h)-1 (Fig. S3). It is worth noticing that the smooth transition around OCV in the IV curve means that RSOCs with LSFN oxygen electrode have favorable reversibility.

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Fig. 4. The I-V-P curves (a) and Nyquist plots (b) for RSOCs under SOFC mode, the I-V curves (c) and Nyquist plots (d) for the RSOCs under SOEC mode at various temperatures; the comparison among various oxygen electrodes for SOCs in SOFC mode (e) and SOEC mode (f).

3.3 Reversibility and stability of RSOCs To investigate the reversibility of RSOCs with LSFN oxygen electrode, Nyquist plots 12


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of RSOCs in both modes are measured as shown in Fig. 5 (a). Rp of the cell under OCV, SOEC (0.4 A cm-2) and SOFC modes (-0.4 A cm-2) is 0.086 Ω cm2, 0.096 Ω cm2 and 0.078 Ω cm2, respectively. Rp in SOFC mode is inferior to that in SOEC mode, which is accordant with the results of 3-electrode cell tests (Fig S2 (d, e, f)). Overall, the value of Rp is very close in both modes, indicating that the cell has perfect reversibility. Nyquist plots of RSOCs under different working conditions (0.2, 0.4, and 0.6 A cm-2) in SOEC mode are shown in Fig. 5 (b). No significant change in resistance can be found, regardless of the current density, suggesting that the cell has excellent stability in SOEC mode. Distribution of relaxation time (DRT) is a very useful tool for analyzing electrochemical processes48. It can be seen in Fig.5 (c) that five peaks are identified in the frequency range. Each peak represents a rate-determining electrode process and its area is equivalent to the impedance of each process. These five processes respectively represent the ion transport (P1) and the electron transfer (P2) of the hydrogen electrode, the oxygen surface exchange of the oxygen electrode (P3), the gas diffusion of the hydrogen electrode (P4) and the gas diffusion of the oxygen electrode (P5) from high frequency to low frequency49. It can be seen that at OCV, P1, P2, and P3 dominate the main process. In SOFC mode, P2 and P3 are significantly reduced because of the ORR process. In SOEC mode, P1 and P5 increase remarkably because H2O electrolysis reaction occurs, and the OER of the oxygen electrode is relatively inactive, resulting in a stagnant electrochemical reaction process. Comparing the P5 peaks in SOFC and SOEC modes, the ORR performance is preferable to the OER performance, which is consistent with previous three-electrode results. The DRT curves in SOEC mode with 13



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different current densities are shown in Fig.5 (d), slight variation in each process proves the stability of the cell.

Fig. 5. Nyquist plots of RSOCs in both modes (a), various applied current density in SOEC mode (b). DRT spectra for RSOCs in both modes (c), various applied current density in SOEC mode (d). The reversibility of cell with the short time interval is displayed in Fig. 6 (a). The cell is operated in SOFC mode (0.7V) for 30 min then in SOEC mode (1.3V) for another 30 min alternatively to finish one reversible cycle. After 5 cycles, RSOC exhibits excellent stability without obvious degradation. When the cell works continuously in independent SOFC or SOEC mode for 24 h, the cell still performs stably without obvious performance degradation (Fig. 6 (b)). A longer test duration is conducted up to 144 h with continuous switching between SOFC and SOEC modes to evaluate the reversibility of the cell 50, as shown in Fig. 6 (c). It is first operated in SOEC mode for 14


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7 h and then operated in SOFC mode for another 14 h. It could be found that no matter what kind of test is applied, RSOC with LSFN oxygen electrode always exhibits excellent reversibility and stability. Unlike RSOCs based on LSM and LSCF oxygen electrode showing degradation in a longer period time switch3,24, this cell has excellent reversibility and stability. After test, the ohmic impedance of the cell is almost constant, and the polarization resistance is only increased a little as shown in Fig.S4 (a). DRT spectra (Fig.S4 (b)) shows the peak intensity at low frequencies increasing, suggesting that gas diffusion becomes difficult.

Fig. 6. Reversible operation of cell in both modes alternative at 800 ºC in a short time intervals (a), 24 h test in both independent modes (b), the medium-term reversible operation at 800 ºC (c). The microstructure of the cell before and after the medium-term operation is illustrated. In Fig. 7 (a), the YSZ electrolyte is completely dense with a thickness of 15



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about 20 μm. The porous oxygen electrode is conducive to the adsorption and dissociation of gas with a thickness of about 20 μm. After medium-term reversible test, there is no discernible delamination layer as shown in Fig. 7 (c). The interfaces of the GDC buffer layer and the electrolyte, the oxygen electrode and the buffer layer are all in good contact, which indicates the stability of cell structure. In addition, there is no elemental diffusion and delamination after medium-term test (Fig. S5). Unfortunately, a slight aggregation phenomenon has happened after the medium-term test as shown in Fig. 7 (d). This may cause difficulty in gas transport and increase the polarization resistance, which is consistent with the DRT analysis of cells as shown in Fig. S4.

Fig. 7. Cross-section view of the cell before (a, b) and after (c, d) medium-term reversible test. In working condition, Sr-containing perovskite materials are proved to cause Sr segregation easily, which will migrate to the surface of the YSZ electrolyte and react 16


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with YSZ to form a high-resistance phase of SrZrO3 51. But the EPMA test shows that Sr element did not diffuse and no SrZrO3 is formed in the interface (Fig. S5). The surface properties of LSFN electrode before and after test were evaluated by XPS, as shown in Fig.8. Before the test, the ratio of surface Sr and lattice Sr is 0.62. After the reversible operation, the ratio is reduced to 0.59. The segregation of Sr on the surface is suppressed and even it is alleviated during the working condition. This is consistent with the Yu’s and Jiang’s research results5, 52, suggesting that anodic polarization can suppress the segregation of Sr. In addition, the phase of oxygen electrode after test was performed shown in Fig. S6, presenting no detectable impurity peaks other than LSFN and GDC phase, which indicates the stability of oxygen electrode.

Fig.8. XPS spectra of LSFN oxygen electrode before (a) and after medium-term reversible test (b). The stabilization mechanism of RSOC with LSFN oxygen electrode is proposed in this study. TEC value of LSFN matches perfectly with GDC, which achieves a better adhesion and interface stability with GDC buffer layer. When RSOC is operated in SOFC mode, O2 molecules are reduced into oxygen ions at the three-phase boundary 17



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(TPB). LSFN and electrolyte can be further combined with the transport of oxygen ions as shown in Fig. 9. While RSOC is operated in SOEC mode, OER reaction causes high oxygen partial pressure at the TPB, and it may cause a slight separation of LSFN from the electrolyte. Then in SOFC mode again, it can repair the separation. So RSOC with LSFN oxygen electrode has excellent stability. Furthermore, the reversible operation can also suppress Sr segregation on the surface, which is beneficial to the stabilization of the electrode performance.

Fig. 9. Scheme of the reversible operation process.

4. Conclusion Cobalt-free perovskite-type oxide LSFN is investigated as a kind of oxygen electrode candidate for RSOC. It exhibits outstanding performance in SOFC as well as SOEC mode. In SOFC mode, the maximum power density of 961 mW cm-2 and Rp of 0.142 Ω cm2 at 800 °C can be achieved. The hydrogen production rate of RSOC is up 18


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to 1348.5 mL (cm2•h)-1 in SOEC mode. Most importantly, the cell exhibits excellent reversibility and stability. After 144 h medium-term alternative cycle test, the performance and structure of RSOCs remain stable. The reason can be concluded as the low TEC of the cobalt-free material and the reversible operation inhibit the segregation of Sr. The studies find out that Fe-based perovskite has a bright prospect as an oxygen electrode for RSOCs. ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at ** Experimental details and additional results, e.g., the results of three electrode test, cell performance under SOEC mode at various humidity, hydrogen production rates, EPMA images and XRD of cell after test (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. ORCID Bo Chi: 0000-0002-1984-2289 Notes The authors declare no competing financial interest Acknowledgments

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We gratefully appreciate for financial support by National Key Research & Development

Project-International

Cooperation

Program

(2016YFE0126900),

National Natural Science Foundation of China (51672095, 51702108, 51502103), and Hubei Province (2018AAA057). We also gratefully thank Analytical and Testing Center of Huazhong University of Science and Technology for sample characterizations assistance.

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Table of contents

Cobalt-free Perovskite Oxide La0.6Sr0.4Fe0.8Ni0.2O3-δ as Active and Robust Oxygen Electrode for Reversible Solid Oxide Cells

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Cobalt-free perovskite oxide La0.6Sr0.4Fe0.8Ni0 ... - ACS Publications

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