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Cite This: ACS Appl. Energy Mater. 2018, 1, 1316−1327

Improved Phase Stability and CO2 Poisoning Robustness of Y‑Doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ SOFC Cathodes at Intermediate Temperatures Laura Almar,*,† Heike Störmer,‡ Matthias Meffert,‡ Julian Szász,† Florian Wankmüller,† Dagmar Gerthsen,‡ and Ellen Ivers-Tiffée† †

Institute for Applied Materials (IAM-WET), Karlsruhe Institute of Technology (KIT), Adenauerring 20 b, D-76131 Karlsruhe, Germany ‡ Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT), Engesserstrasse 7, D-76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: The outstanding oxygen permeability of the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and its applicability as a cathode in solid oxide fuel cells are remarkable, yet have been hindered by the formation of secondary phases at T < 840 °C and the subsequent degradation. The other main drawback to BSCF is related to the formation of carbonates in the presence of CO2. These degrade its excellent oxygen surface-exchange kinetics. In this work, 10% Y-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF10Y) is electrochemically, microstructurally, and chemically characterized in O2- and CO2-containing atmospheres as porous cathodes in symmetrical cells with Gd-doped ceria as electrolyte. Experiments in oxygen/nitrogen gas mixtures (pO2 = 0.02−0.40 atm) at T = 600−900 °C showed high performance with a cathode specific resistance of 49.9 mΩ cm2 at 600 °C in air (pO2 = 0.21 atm), which is very comparable to 47.8 mΩ cm2 for undoped BSCF, but which deteriorates constantly under the same conditions. Moreover, adding significant amounts of CO2 (from 1% to 3% vol) to air at 700 °C shows that BSCF10Y has a remarkably enhanced tolerance toward the adsorption of CO2 molecules (compared to the undoped BSCF cathodes). The electrode microstructure was analyzed by focused ion beam (FIB) combined with scanning electron microscopy (SEM). The chemical composition was analyzed by scanning transmission electron spectroscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDXS). The presence of secondary phases in the BSCF10Y cathodes was strongly reduced or even suppressed compared to undoped BSCF, which explains the superior oxygen reduction reaction kinetics over time. Also, the segregated volume fraction of carbonates is greatly reduced after 50 h in 1% CO2. The improved stability of the cubic phase in BSCF10Y compared to BSCF is linked to the monovalent agent and higher radius of the Y-dopant, while the improved stability in CO2-containing atmospheres is attributed to the higher acidity of Y3+ compared to the A-site cations and the high concentration of the Y-dopant, which is located on both the A- and B-sites of the perovskite. KEYWORDS: BSCF, Y-doped BSCF, SOFC cathode, electrochemical impedance spectroscopy, focused ion beam tomography, CO2 tolerance from a cubic to a hexagonal phase structure.4,5 Consequently, the conductivity and transport properties degrade.6 Detailed electron microscopy studies also show the common presence of • cobalt oxide precipitates • a BCO-type (Ban+1ConO3n+3(Co8O8) (n ≥ 2)) phase • a phase with a plate-like morphology (lamellar phase composed of stackings of BCO, cubic and hexagonal phases)7,8 Several studies have revealed that secondary phase formation in BSCF is induced by oxidation state increase and ionic radius reduction of the Co-ion.9−11

1. INTRODUCTION Mixed ionic−electronic conducting (MIEC) materials combine excellent ionic and electronic transport properties, making them promising candidates in high temperature applications (T = 600−900 °C) such as oxygen transport membranes (OTMs) and solid oxide fuel cells (SOFCs). A key step for achieving high performance in both applications is the oxygen reduction reaction (ORR). This requires both high oxygen surface-exchange (k) and high oxygen ion diffusion (D). The perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) has attracted considerable focus in the past decade due to its outstanding oxygen permeation flux1,2 and extremely low area specific resistance,3 caused by its high oxygen nonstoichiometry. However, at temperatures below T < 840 °C, the material suffers a transformation © 2018 American Chemical Society

Received: January 8, 2018 Accepted: March 5, 2018 Published: March 5, 2018 1316

DOI: 10.1021/acsaem.8b00028 ACS Appl. Energy Mater. 2018, 1, 1316−1327

Article

ACS Applied Energy Materials

based electrolytes at 600 °C in pO2 = 0.21 shows a scatter of 2 orders of magnitude, disclosing the motivation to explore rigorously cubic phase stabilized BSCF as SOFC porous cathode (Figure 1).

To increase the stability of the cubic regime of BSCF, one reasonable solution would be rational doping on the B-site. Suitable dopant elements should have a fixed valence state cation to reduce the valence change effect. Also, the Goldschmidt r +r tolerance factor (t = 2 A(r +Or ) ) indicates that the dopant on the B

O

B-site must have a larger ionic radius than Co or Fe, in order to shift the structure from hexagonal (t > 1) to cubic (t = 0.9−1). Yttrium (Y3+) has an ionic radius of 90 pm and satisfies both requirements. It was reported that 10% Y-doped BSCF (denominated as BSCF10Y in this paper) increases the stability of the cubic phase at lower temperatures (ΔT ≈ 100 °C)12,13 and that no degradation in conductivity was measured at 800 °C for 850 h in ambient air.14 Recently, Meffert et al. determined the Y-dopant site in BSCF10Y by atom location by channeling enhanced microanalysis (ALCHEMI). Even though we only intended to dope the B-site, we showed that 25% of the yttrium is located in the A-site of the BSCF.15,16 The other main drawback of undoped BSCF is the performance degradation upon exposure to CO2-containing atmospheres. It is well-known that perovskites containing alkalineearth species are susceptible to CO2 poisoning. The susceptibility of a perovskite to forming carbonates in CO2 can be evaluated from a thermodynamic perspective or by the Lewis acid−base concept.17−19 Many groups have reported on the degradation of the oxygen permeation flux of BSCF in the presence of CO2.20−22 Arnold et al. found Ba- and Sr-carbonates at the BSCF surface by SEM, XRD, and TEM investigations with pure CO2 as sweep gas at 875 °C, causing instant cessation of the oxygen permeation flux.23 Yan et al. reported a negative effect of the SOFC performance using BSCF as cathode, but also saw reversible degradation at temperatures higher than 550 °C under 3% CO2 in O2.20 Bucher et al. reported a degradation of the oxygen-exchange kinetics of ∼80% after 7 days under ambient air at T = 600 °C.21 On the basis of the aforementioned studies, undoped BSCF is not suitable for use in a high CO2 atmosphere, e.g., in oxygen transport membranes with flue contact or in a single-chamber solid oxide fuel cell using hydrocarbons as fuel (approximately 10 vol % CO2).22 A few experimental analyses reported enhanced CO2 resistance of the BSCF perovskite by the proper partial B-site cation substitution.24−26 Wang et al. used TG-DTA investigations in 5% CO2 and reported improved tolerance of Nb-doped BSCF. The enhancement is seemingly caused by the high acidity of the Nb-dopant. Bi et al. studied Ti-doped BSCF as a proton-conducting SOFC cathode. They determined an improvement of the chemical stability in pure CO2 at 600 °C (compared to undoped BSCF). La-doped BSCF was also seen to have excellent chemical stability in air and CO2. XRD studies performed by Kim et al. showed no secondary phases formed after 24 h in 10% CO2. Enhancing the CO2 tolerance of BSCF could allow the use of this high performance perovskite in previously impossible applications, e.g., as the oxygen electrodes of SOFCs or solid oxide electrolysis cells (SOECs) operating in ambient air. At present, the concentration of CO2 in ambient air constitutes 0.04% (i.e., 403.53 ppm, November 2016).27 As mentioned above, recent studies on 10% Y-doped BSCF showed a thermal extension of its cubic phase, with no degradation of the long-term conductivity or of the oxygen diffusion coefficient at 800 °C for 200 h. However, insufficient attention has been paid to the electrochemical properties and phase stability of BSCF10Y as a SOFC cathode in O2- and CO2-containing atmospheres. A literature overview on the polarization resistance values of BSCF as electrode in symmetrical cells with ceria

Figure 1. Total polarization resistance (ASRpol) at 600 °C of undoped BSCF cathodes in symmetrical cells with ceria based electrolytes reported in the literature (see also Table 1).3,28−40

In this work, porous BSCF and BSCF10Y electrodes were sintered in situ and immediately characterized afterward to avoid any environmental degradation. The stability of 10% Y-doped BSCF was electrochemically characterized on porous cathodes under varying temperature (T = 600−900 °C) and mixtures of N2 and O2 (pO2 = 0.02−0.4 atm) at 700 °C by impedance spectroscopy (EIS). The different physical processes contributing to the total area specific resistance were quantified by distribution of relaxation times (DRTs). A 3D-reconstructed volume and the microstructure parameters were obtained by FIB-SEM tomography, and the oxygen-exchange coefficient (k*) was determined under the different measured conditions. The effects of CO2containing atmospheres were electrochemically studied for both BSCF and BSCF10Y cathodes. Moreover, detailed microstructural characterization was performed by (scanning) transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy (STEM-EDXS). High concentrations of CO2 (1 and 3% vol CO2 in synthetic air, which is 25−75 times higher than the CO2 content in ambient air) were chosen to accelerate the performance degradation of the cells. Both the cell performance and the STEM-EDXS characterization show that the Y-doped BSCF cathodes present less degradation which is attributed to the larger ionic radius and the higher acidity of the Y3+-dopant.

2. EXPERIMENTAL PROCEDURE The Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and (Ba0.5Sr0.5)(Co0.8Fe0.2)0.9Y0.1O3−δ (BSCF10Y) powders used to fabricate the electrodes for the symmetrical cells were synthesized by a solid-state reaction. First, the precursors BaCO3 (99.5%, Merck Technipur), SrCO3 (99.7%, Merck Technipur), Co3O4 (>98%, ChemPur), Fe2O3 (99.5%, Alfa Aesar), and Y2O3 (99.99%, Alfa Aesar) were separately ball-milled in isopropanol with ZrO2 balls to a mean particle size of d50 = 0.5 μm (CILAS 1064L). A stoichiometric amount of the precursors was further mixed in distilled water and ball-milled to a mean particle size of d50 = 2 μm, with a final calcination step at 900 °C for 4 h for the BSCF and at 970 °C for 4 h for the BSCF10Y. The stoichiometry of BSCF10Y was chosen to substitute yttrium on the B-cation site (Ba0.5Sr0.5)(Co0.8Fe0.2)1−xYxO3−δ with x = 0.1. The pastes were prepared by mixing and calendering respective powders with 60 wt % solid concentration and an organic binder. Symmetrical cells were fabricated by screen-printing the BSCF or BSCF10Y pastes onto Ce0.9Gd0.1O2−δ (Daiichi Kigenso Kagaku Kogyo Co. Ltd., Japan) (GDC) electrolyte pellets with a thickness of 700−800 μm. After drying, the electrodes of the BSCF/GDC/BSCF and BSCF10Y/GDC/BSCF10Y symmetrical cells were 30−35 μm thick, and the electrode active area was 1 cm2. 1317


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Figure 2. Electrochemical response of a BSCF10Y/GDC/BSCF10Y cell with in situ sintered electrodes with temperature and oxygen partial pressure variation: (a) DRT curves from 700 to 900 °C in synthetic air, (b) DRT curves at pO2 = 0.02−0.4 atm at 700 °C, and (c, d) total area specific resistance and the individual process resistance obtained in the fitting (note that the figure shows the contribution of a one cathode, half symmetrical cell). The electrochemical performance and stability under CO2-containing atmospheres of the fabricated symmetrical cells were evaluated by electrochemical impedance spectroscopy (EIS) with a Solartron 1260 frequency response analyzer in the 0.05−106 Hz frequency range and an amplitude current stimulus chosen to have a voltage response lower than 30 mV. The cells were placed in a test bench between two gold meshes (>99.99% Au, 1024 meshes cm−2, 0.06 mm wires) and then sintered in situ with a final sintering temperature step at 900 °C.41−43 The in situ sintering, the temperature variation (T = 600−900 °C), and the pO2 variation studies (pO2 = 0.02−0.40 atm) at T = 700 °C were in a controlled atmosphere mixture of N2 and O2 with a total flow rate of 500 mL min−1. The degradation under CO2 was studied in 1 and 3% vol of CO2 to air at T = 700 °C. The impedance data was analyzed by simultaneously fitting the impedance spectra and the distribution of relaxation time (DRT) curves by complex nonlinear least-squares (CNLS), using a self-written MATLAB code based on the Tikhonov regularization.44 The microstructure of the symmetrical cells was investigated by scanning electron microscopy (SEM) (Zeiss 1540XB, Carl Zeiss NTS GmbH, Oberkochen, Germany). A 3D-reconstructed volume (V = 9606 μm3) of the BSCF10Y cathode was obtained by combining focused ion beam (FIB), SEM, and data processing using a voxel size of 30 nm (volumetric pixel). All relevant details on sample preparation, FIB-SEM microstructure reconstruction procedure, processing of the images, and the calculation of the parameters can be found in our previous studies.45−47 X-ray diffraction (XRD) patterns were obtained with a Bruker D8 Advance diffractometer with a Ni filter and the Lynx Eye detector in the 2θ range from 20° to 80°. To investigate chemical composition and homogeneity we used scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDXS) analysis using an FEI Osiris (FEI, Hillsboro, OR) ChemiSTEM at 200 keV equipped with 4 Bruker silicon drift detectors. Composition quantification of the BSCF and BSCF10Y samples was performed with the Esprit Software (Bruker). Sample preparation for TEM was either done by scratching the screen-printed BSCF and BSCF10Y films and dispersing the powder onto carbon-films suspended on TEM copper grids or by conventional TEM sample preparation involving grinding, dimpling, and Ar+-ion etching.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Characterization in O2-Containing Atmospheres. Electrochemical impedance spectroscopy of a BSCF10Y/GDC/BSCF10Y symmetrical cell was performed after in situ sintering under synthetic air at 900 °C, with variation of the temperature and the oxygen partial pressure. Please note that the impedances and DRTs always show the contribution of half of the symmetrical cell (one cathode). Figure 2a shows the distribution of relaxation times corresponding to the impedance spectra of the cell as a function of the temperature from 900 to 700 °C with pO2 = 0.21 atm. Four peaks can be identified. A decrease in temperature leads to a higher magnitude of the peaks labeled as P2, P3, and P4, shifting to lower frequencies (slower processes). Peak P1, in contrast, decreases slightly in magnitude. On the basis of the preidentification of the different peaks, the EIS spectra (shown in Figure S1a) and the DRT curves were simultaneously fitted by CNLS to the corresponding meaningful electrical equivalent circuit (EC) (Figure S1b). In the proposed EC, the resistance Rohm corresponds to all ohmic losses (which are mainly associated with the GDC electrolyte) and four RQ-elements representing the four processes identified in the DRT curves.48 Figure 2c shows the area specific resistance (ASRcat) of the cathode and each process contribution obtained by the fittings as a function of the temperature under synthetic air (pO2 = 0.21 atm). The ASRcat was calculated by the sum of the processes P2, P3, and P4 since the gas diffusion losses (P1) are highly dependent on the gas supply and the geometry of the setup.41−43 The ASRcat shows a low value of 49.9 mΩ cm2 at T = 600 °C and pO2 = 0.21 atm, very comparable to the ASRcat of 47.8 mΩ cm2 obtained for BSCF/GDC symmetrical cells measured under the same conditions (depicted in Figure 3).28 In order to give a physical explanation for the processes involved within the MIEC cathode, the symmetrical cell was 1318


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Figure 3. (a) Cathodic area specific resistance of different symmetrical cells with BSCF10Y and BSCF28 electrodes, calculated as ASRcat = RP2 + RP3 + RP4 from 600 to 800 °C under synthetic air (pO2 = 0.21 atm). SEM cross-section images of the (b, d) BSCF10Y/CGO interface and the (c, e) BSCF/CGO interface after the electrochemical measurements.

more extensively characterized as a function of pO2 from 0.02 to 0.40 atm at 700 °C. Figure 2b shows the corresponding DRT curves. Figure 2d shows the ASRcat and the contribution of the individual processes obtained by fitting the data to the proposed EC. Peak P1 shows the strongest pO2 dependency, followed by peak P2, while P3 and P4 remain almost invariable. We recently published a detailed electrochemical characterization of BSCF and LSCF symmetrical cells.28 Special care was taken to avoid degradation regarding time and thermal history of the cathodes by in situ sintering in a controlled oxygen/nitrogen gas atmosphere. The DRT curves revealed four processes. After discussion about the limits of the Adler−Lane−Steele analytical model,49 we concluded that the characterized cathodes were solely surface-exchange controlled, with a reaction zone similar to the particle size. In the present work, the BSCF10Y cathodes were also sintered in situ under synthetic air at 900 °C. The electrochemical performance and process dependency observed in the DRT curves consistently show the same behavior as those for the BSCF and LSCF cathodes characterized in our previous work.28 The same plausible physical explanation of each process (i.e., single peak) identified in the DRT curves could be easily extrapolated, as indicated in the following. Process P1 decreases with increasing temperature and presents the highest oxygen partial dependency (pO2−0.60). The low frequency process is attributed to the molecular oxygen diffusion within the porous cathode, with the setup and the gold meshes used as current collectors.43,50 The middle frequency process, P2, is thermally activated and pO2 dependent. Clearly, the BSCF10Y cathodes do not present the Gerischer behavior characteristic of MIEC materials with coupled surface-exchange and bulk diffusion. In agreement with our previous publications, this process (P2) caused by the cathode electrochemical reaction appears at a higher frequency ( f ≥ 100 Hz) than the simulated one, with the ALSmodel indicating that faster surface-exchange kinetics take place.28,51 Processes P3 and P4 (higher frequency processes >100 Hz) are thermally activated but show little or no change with oxygen partial pressure variation. They are most likely related to ionic and electronic interfacial resistances. If the cathode has fast oxygen surface-exchange kinetics, the penetration depth of the cathode

becomes smaller, and the other processes close to the electrode− electrolyte interface would then become visible, as suggested by Adler et al.51 The cathodic area specific resistance of BSCF10Y measured in this work was compared to symmetrical cells with BSCF cathodes from 600 to 800 °C under synthetic air in Figure 3a (EIS measurements of our group).28 For all these cathodes, the ASRcat was calculated after simultaneous fitting of the EIS spectra and the DRT curves, and is equal to the polarization resistance losses without the gas diffusion losses (P1). At 700 °C the undoped BSCF cathodes present a lower ASRcat of 0.014 Ω cm2 compared to the 0.022 Ωc m2 of BSCF10Y. These values obtained at initial conditions are consistent with the concentration of oxygen vacancies of the respective cathodes.12 On the other hand, the activation energies of BSCF and BSCF10Y are 0.87 and 0.68 eV, respectively. The higher activation energy and lower performance of the BSCF/GDC symmetrical cells at low temperatures can be associated with the hexagonal phase formation (the presence of secondary phases in the BSCF cathodes is confirmed in the following sections). Moreover, the polarization resistances of the cathodes are highly dependent on the ambient, thermal and time history conditions of the samples. In our recent work, special care was taken to in situ sinter the cathodes in synthetic air in order to minimize external degradation sources, such as the ambient contaminants introduced during the sintering or due to time and temperature history. This fact is clearly confirmed in Table 1 which shows a comparison of the polarization resistance (ASRpol) at 600 °C of BSCF cathodes in symmetrical cells reported in literature. The table is not a complete literature review but gives a tentative idea of the spread of the polarization resistance values, likely due to the above-mentioned degradation sources. The ASRpol spreads from 0.070 Ω cm2 (this work) to 4.530 Ω cm2 at 600 °C. Apart from the nanoscaled-BSCF cathode of Asano et al.39 (0.039 Ω cm2), the polarization resistances of the in situ sintered undoped BSCF (0.050 Ω cm2) and BSCF10Y (0.070 Ω cm2) cathodes calculated as ASRpol = RP1 + RP2 + RP3 + RP4 are among the best performance cathodes operating at low temperatures (i.e., T = 600 °C). SEM cross-section images were taken of the cathode/electrolyte interface of the BSCF10Y and BSCF porous cathodes 1319


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Table 1. Comparison of the Total Polarization Resistance (ASRpol) of the BSCF10Y Cathodes Presented in This Work against Other BSCF Cathodes in Symmetrical Cells Reported in the Literature cathode

electrolyte

sintering T (°C)

ASRpol @ 600 °C (Ω cm2)

ref

10% Y-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.50.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ (Ba0.5Sr0.5)1.03Co0.8Fe0.2O3−δ (Ba0.5Sr0.5)0.97Co0.8Fe0.2O3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ + SDC Ba0.5Sr0.5Co0.8Fe0.2O3−δ + GDC Ba0.5Sr0.5Co0.8Fe0.2O3−δ + LaCoO3 nanoscaled-Ba0.5Sr0.5Co0.8Fe0.2O3−δ nanoscaled-Ba0.2Sr0.8Co0.8Fe0.2O3−δ nanofiber-Ba0.5Sr0.5Co0.8Fe0.2O3−δ nanoscaled-Ba0.5Sr0.5Co0.8Fe0.2O3−δ

GDC GDC GDC GDC GDC SDC SDC SDC GDC SDC SDC SDC SDC SDC SDC GDC SDC GDC SDC GDC GDC

900 (in situ) 900 (in situ) 900 950 950 950 1000 1000 1000 1000 1000 1000 1000 1000 1000 1100 950 700 900 950 1050

0.070 0.050 0.100 0.468 4.530 0.072 0.055−0.071 0.097 0.150 0.099 0.093 0.107 0.068 0.107 0.064 0.175 0.210 0.039 0.190 0.094 2.220

this work 28 29 30 31 32 3 33 34 35 36 37 36 37 35 38 32 39 40 30 31

after the impedance measurements. Figure 3b shows the BSCF10Y/GDC interface after the electrochemical characterization detailed above. The BSCF10Y porous layers show a uniform thickness of 35 μm. A higher magnification of the electrode (Figure 3d) shows the particle surface and its smooth morphology. In contrast, the undoped BSCF cathode (Figure 3c,e) shows nanosized precipitates on the surface, indicating the presence of secondary phases.8,11 In agreement with that, the XRD pattern of the BSCF10Y cathodes shows a single cubic phase, while the XRD pattern of the BSCF cathodes shows both cubic and hexagonal phases, after the electrochemical measurements (Figure S2). These results also agree with previous studies conducted on ceramic bulk samples under ambient air, which reported that the cubic phase of the Y-doped BSCF has a larger temperature stability range than that of BSCF, due to the higher ionic monovalent radius of Y3+ (cation mismatch) which suppresses the Co-valence change, modifying the overall charge balance.8,12,13 3.2. Determination of k* by Coupling EIS and FIB-SEM Tomography. A representative volume (V = 9606 μm3, voxel size 30 nm) and the microstructure parameters of a porous BSCF10Y cathode were obtained by a combination of FIB, SEM, and data processing. The reconstructed electrode shown in Figure 4a was sintered at T = 900 °C for 60 h to simulate the same sintering conditions of the electrochemically characterized electrodes. The 3D volume was obtained after alignment, by stacking and processing the 2D SEM images (sequentially collected by FIB milling an infiltrated, polished, cross-section sample). The microstructure parameters were determined by selfwritten algorithms in MATLAB (The MathWorks, Natik, MA): • Porosity fraction (ε), dividing the voxels assigned as pore by the total voxel number • Particle size (dBSCF10Y), using the Euclidean distance transform calculation • Surface area (a), by the marching cube algorithm • Tortuosity of the solid-phase (τBSCF10Y), solving the transport equation More details on the algorithms can be found in former works.45−47 Table 2 shows the calculated microstructure

Figure 4. (a) Total 3D-reconstructed volume of a BSCF10Y cathode sintered at 900 °C for 60 h and (b) connectivity of the pore phase (in gray). The red parts represent the isolated voxels, and the violet parts represent the voxels of unknown status.

parameters of the BSCF10Y cathode: a surface area of 1.26 μm−1, an average particle size of 1.18 μm, and a porosity of 39.6%. The microstructure of the BSCF10Y cathode is coarser than the previously reconstructed LSCF and BSCF cathodes.28,52 It could be attributed on one hand to the higher sintering activity of the BSCF10Y electrodes compared to that of LSCF53,54 and, on the other hand, to the longer sintering time (t = 60 h) used to achieve adequate cathode/electrolyte attachment. Figure 4b shows the connectivity of the pore phase. The connected parts (in gray) refer to the pore voxels that percolate 1320


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higher number of oxygen vacancies.12 The k* dependency of pO2−0.5 for BSCF (compared to pO2−0.2 for BSCF10Y) confirms that the concentration of oxygen vacancies plays a prime role in the determination of the k-rate.53,59 However, undoped BSCF forms secondary phases in the measured temperature range, which causes degradation of its oxygen transport properties over time. This assumption was confirmed by long-term electrical conductivity relaxation measurements performed in our group with both BSCF and BSCF 10Y dense ceramic bulks.14 3.3. Influence of CO2-Containing Atmospheres. The influence of CO2-containing atmospheres (i.e., 1% and 3% vol CO2 in synthetic air, which are 25−75 times higher than the CO2 content in ambient air) was measured at T = 700 °C on in situ sintered symmetrical cells with BSCF and BSCF10Y porous electrodes. To describe the processes that occur in more detail, the DRT curves of the corresponding impedance spectra were obtained. Figure 6a,b showed the DRT curves of BSCF/GDC/ BSCF and BSCF10Y/GDC/BSCF10Y cells, respectively. The data was collected before and after the addition of 1% vol CO2 for 50 h at T = 700 °C (EIS spectra were acquired every 1 h). For both cells the addition of 1% vol CO2 revealed an increase of the P2 process, attributed to the cathodic electrochemical reaction. As stated above, the electrodes are surface-exchange controlled, and thus, a degradation of P2 implies a degradation of the oxygen surface-exchange coefficient. Accordingly, the frequency of P2 is shifted to lower values ( f ∼ 100 Hz) after CO2 is introduced. This indicates slower surface-exchange kinetics. As expected, the adsorption of CO2 molecules on the surface of the perovskites competes directly with O2 adsorption; they occupy the active sites and degrade the electrodes’ k values. All other processes remain constant. More interestingly, the magnitude of P2 for undoped BSCF is larger than that for BSCF10Y in equivalent test conditions, indicating a higher degradation of the oxygen-exchange kinetics of BSCF. In general terms, the reactivity of a cation with an acidic gas molecule such CO2 can be explained by the Lewis acid−base theory. The higher the acidity of a cation is, the less likely the adsorption of acid molecules to its surface.60 Taking into account that Y3+ occupies both A- and B-sites in BSCF10Y,16 it seems plausible that Y-doping reduces the degradation of the electrochemical properties in CO2-containing atmospheres. The higher tolerance of BSCF10Y could be explained by both the smaller ionic radius of yttrium and its higher acidity (compared to the other A-site cations61). Figure 6c,d shows the DRT curves of both cells before and after 50 h of 1% vol CO2. They also show reversibility

Table 2. Microstructural Parameters of a BSCF10Y Cathode Sintered at 900 °C for 60 h Obtained by FIB-SEM Tomography and Data Processing BSCF10Y volume porosity fraction particle size surface area solid tortuosity

3

V (μm ) ε (%) dBSCF10Y (μm) a (μm−1) τBSCF10Y

9606 39.6 1.18 1.26 1.88

with the gas channel. The isolated voxels within the volume are shown in red and the unknown parts in violet, which refer to isolated voxels that intersect one of the outer boundaries and therefore have unknown connectivity. The percentage of the isolated pore phase is 0.12%, and the connectivity of the solidphase is up to 99.98%. Even though the porosity of 39.6% is lower than in state-of-the-art LSCF cathodes of 44.6%,52 most of the pores within the BSCF10Y electrodes are connected. The oxygen transport parameters of the MIEC cathodes can be determined by combining impedance spectroscopy data and the microstructure parameters determined by FIB-SEM tomography.52,55,56 The following equation can be used to extract the k value for cathodes solely controlled by surface-exchange kinetics, e.g., thin films, high surface area, or nondegraded electrodes:28,56,57 k* =

RT 1 1 * 4F 2 R chemcmc alcat

(1)

Here, R is the gas constant, T the temperature, F the Faraday constant, Rchem the chemical resistance obtained in the fitting (2ASRP2 = Rchem), cmc the lattice site concentration calculated from XRD measurements at each specific T and pO2,58 a the surface area, and l*cat the electrochemical active thickness. As discussed before for cathodes with fast exchange kinetics, this length should be in the range of the penetration depth (defined in ALS-model) or thinner.28 Figure 5a shows tentative k* values for the BSCF10Y electrodes at T = 600−900 °C in synthetic air and Figure 5b in an oxygen partial pressure from 0.02 to 0.21 atm at T = 700 °C. The k* values were calculated assuming lcat * is equal to the particle size. The k* values of in situ sintered BSCF cathodes calculated by the same methodology are shown as comparison (for more information please refer to our recent publication28). Compared to BSCF10Y, BSCF presents a higher surface-exchange coefficient under all measured conditions, which is in agreement with its

Figure 5. Surface-exchange coefficient (k*) of BSCF10Y and undoped BSCF porous cathodes surface-exchange controlled at (a) T = 600−900 °C under synthetic air and at (b) pO2 = 0.02−0.21 atm at T = 700 °C. The k* values were calculated by coupling EIS and FIB-SEM tomography data * to be equal to the particle size. assuming lcat 1321


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Figure 6. DRT curves at T = 700 °C of a BSCF/GDC/BSCF and a BSCF10Y/GDC/BSCF10Y cell: (a, b) before and after the addition of 1% vol CO2 for 50 h, and (c, d) initial (under synthetic air), and after 50 h degradation in 1% vol CO2, and showing reversibility (3 h after switching off the CO2).

Figure 7. Relative change of the polarization resistance ASRpol/ASR0 of symmetrical cells with BSCF and BSCF10Y electrodes over time at T = 700 °C in (a) 1% vol CO2 and (b) 3% vol CO2 and recovery after CO2 is switched off. SEM images of the (c, d) BSCF and (e,f) BSCF10Y electrodes after the electrochemical characterization (50 h in 1% vol CO2 and 3% vol CO2 atmospheres and recovery at T = 700 °C, respectively).

show a lower relative ASR increase of 1.7 and 8 times after 50 h in 1 and 3% vol CO2-containing atmospheres, respectively. The CO2 degradation has a remarkably different initial effect in each cathode. The slope (equivalent to the degradation rate) during the first 10 h is 0.131 and 0.025 for BSCF and BSCF10Y in 1% vol CO2 and 0.422 and 0.129 for BSCF and BSCF10Y in 3% vol CO2, respectively. The oxygen surface-exchange process is degraded; we identified two possible causes that could be acting independently or in combination. It could be due to the adsorption of CO2 onto the active surface sites of the perovskites (the adsorption process is reflected in the fast initial degradation rates) and/or the formation of carbonates (the poisoning effect is more pronounced for BSCF and, obviously, for higher CO2 concentrations).

(3 h after switching off the CO2). Once the CO2 is switched off, the amplitude of P2 decreases, and the frequency increases. This confirms that the physical origin of the process is related to the oxygen reduction reaction and that the cathodes are surfaceexchange controlled. The magnitude of P2 and, thus, the surfaceexchange coefficient are not completely recovered; either carbonates could have formed, or a small fraction of CO2 could have remained adsorbed to the surface. Figure 7 shows the relative change of the total polarization losses after introducing 1% vol and 3% vol CO2 for both BSCF/ GDC and BSCF10Y/GDC symmetrical cells at 700 °C. The relative ASR of the BSCF cells increases by 2.8 times after 50 h in 1% vol CO2 and by 12 times after 50 h in 3% vol CO2. The BSCF10Y cells 1322


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Figure 8. Distribution of cations obtained by STEM-EDXS within grains of a porous (a) undoped BSCF and porous (b) BSCF10Y cathode after initial sintering at 900 °C for 60 h in ambient air.

shows a high density of small precipitates (also present on the BSCF10Y cathodes, but in a lower concentration). After the electrochemical characterization in 3% CO2 and the recovery (Figure 7b), there is an obvious degradation of the BSCF cathode surface, while the BSCF10Y surface has only a small concentration of precipitates. These small precipitates are absent after measurements without CO2. STEM-EDXS analysis was performed on grains from porous BSCF and BSCF10Y electrodes after annealing at T = 900 °C for 60 h in ambient air. This allowed a determination of the cation distribution and occurrence of secondary phases at initial conditions. The corresponding EDXS mappings for BSCF and BSCF10Y are given in Figure 8a,b, respectively. Element distribution maps of single cations and combined maps for Ba and Co facilitate the detection of secondary phases. The element maps show large regions with the cubic phase for both BSCF and BSCF10Y cathodes. This behavior is expected, and is due to the high sintering temperature (T = 900 °C) where the hexagonal phase does not occur. Nevertheless, the undoped BSCF cathodes also contain Co-oxide and BCO precipitates which act as nucleation centers for the formation of the plate-like structures (lamellar phase composed of stacking BCO, cubic and hexagonal phases) and the hexagonal phase at lower temperatures.8 The BSCF10Y cathode does not contain BCO or Co3O4 precipitates; it is only composed of the cubic BSCF phase. Finally, STEM-EDXS elemental maps were taken of grains from porous BSCF and BSCF10Y cathodes after exposure to 1% vol CO2 for 50 h at T = 700 °C (without recovery). Figure 9a

After the CO2 is switched off, the polarization losses show different behaviors for each cathode. The performance, determined as polarization resistance, of BSCF does not fully recover. The polarization resistance of the BSCF10Y cathodes tends to reach the initial values, and allows a longer recovery time after a high CO2 concentration (3% vol), showing that the polarization losses remain at a constant value. The different behaviors of both BSCF and BSCF10Y cathodes involve the following aspects: (i) In CO2-containing atmospheres (1 and 3% vol CO2), the lower increase of the ASR ratio for BSCF10Y indicates a lower degradation of the oxygen surface-exchange kinetics compared to undoped BSCF. (ii) The performance of the electrodes fails to fully recover after the CO2 is switched off because some CO2 molecules remain adsorbed at the surface (or carbonates are formed). (iii) The ASR of the BSCF increases after the CO2 switch-off, most probably due to the formation of the hexagonal and other secondary phases. (iv) BSCF10Y shows a constant ASR value after the CO2 switch-off. In contrast, the polarization resistance of BSCF increases. Y-doping strongly reduces or suppresses the well-known secondary phase formation of BSCF. SEM images of the cathodes after the electrochemical characterization in 1 and in 3% CO2-containing atmospheres are shown for the BSCF cathodes (Figure 7c,d) and for the BSCF10Y cathodes (Figures 7e,f). After experiments in 1% CO2 for 50 h and the recovery (shown in Figure 7a), the surface of BSCF 1323


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Figure 9. STEM-EDXS element distribution maps of a (a) undoped BSCF cathode particle after exposure to 1 vol % CO2 for 50 h at T = 700 °C. Regions with Ba, Sr, and C enrichments at the edges are marked. Regions containing the BCO phase can be recognized by Sr depletion in the Sr map. (b) Representative BSCF10Y cathode particle after exposure to 1 vol % CO2 for 50 h at T = 700 °C. The regions with Ba, Sr, and C enrichments at the surface are marked, and they are strongly reduced compared to undoped BSCF. Besides that, no other secondary phases are detected.

prepared, which shows several BSCF grains in the vicinity of the GDC substrate. Several secondary phases are present (as expected in undoped BSCF after high temperature treatment at 700 °C). The hexagonal phase can be identified by its Fe depletion (cf. Fe map). The BCO phase is characterized by Sr depletion (cf. small black arrows in Sr map). In other sample regions Co3O4 precipitates were detected. However, the carbonate phase which was detected at the particle surface edges after experiments in 1 vol % CO2 for 50 h is no longer present after recovery in synthetic air. This is in good accordance with the electrochemical characterization results, which show recovery of the ASR of the electrodes after CO2 switch-off. To summarize, this work shows that 10% Y-doped BSCF overcomes and/or improves the main drawbacks of BSCF. It strongly decreases or suppresses the formation of secondary phases, and it decreases the performance degradation in CO2-containing atmospheres.

shows an undoped BSCF electrode particle which contains plateshaped BCO precipitates as secondary phase (characterized by a depletion of Sr; cf. Sr map). Besides that, analyses of other particles also showed the presence of hexagonal BSCF (characterized by a depletion of Fe and elevated concentration of Co) as well as Co-oxide precipitates after high temperature treatment at 700 °C (see also ref 8). However, there are very clear regions at the edges of the particles with strong Ba, Sr, and C enrichment (marked by white lines in Figure 9) which can be attributed to the formation of (BaxSr1−x)CO3. Quantification of the Ba-content x yields values between 0.42 and 0.44, which correspond to previous results on BSCF after CO2 exposure.23 For comparison, element distribution maps of a representative BSCF10Y electrode particle after exposure to 1% vol CO2 for 50 h at T = 700 °C are shown in Figure 9b. The interiors of the particles only consist of the cubic BSCF phase without any other secondary phase. Additionally, only a few small regions with Ba, Sr, and C enrichments are found at the surface of the BSCF10Y particles, indicating the formation of a much smaller volume fraction of carbonates after being subjected to the same highly concentrated CO2 atmosphere and, hence, the markedly enhanced tolerance of BSCF10Y to CO2-containing atmospheres. Figure 10 shows STEM-EDXS element distribution maps of a porous, undoped BSCF cathode after exposure to 1 vol % CO2 for 50 h, and recovery in synthetic air at T = 700 °C. In this particular case a regular cross-section TEM specimen could be

4. CONCLUSIONS In this work the chemical, microstructural, and electrochemical properties of 10% Y-doped BSCF as porous SOFC cathode layers have been studied for the first time. Our study shows methods to highly ameliorate the two main drawbacks that have previously hampered the use of BSCF as a cathode material in SOFC or SOEC applications: (a) the formation of secondary phases is strongly reduced or suppressed, and (b) degradation 1324


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Figure 10. STEM-EDXS element distribution maps of a porous undoped BSCF cathode after exposure to 1 vol % CO2 for 50 h and recovery in synthetic air at T = 700 °C. The hexagonal phase is characterized by Fe depletion (cf. Fe map). The Sr-depleted BCO phase is marked by small black arrows in the Sr map.

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ACKNOWLEDGMENTS The authors thankfully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (DFG) through projects IV 14/21-1 and GE 841/24. Sincere thanks are given to J. Packham for proofreading the manuscript. Special thanks are given to S. van den Hazel for powder preparation and to L.-S. Unger and Dr. S. F. Wagner for the helpful discussion on BSCF based membrane materials.

in highly concentrated CO2-containing atmospheres is severely reduced. The BSCF10Y cathodes were in situ sintered and immediately afterward electrochemically characterized as porous electrodes in symmetrical cells. Analysis of impedance data by the method of distribution of relaxation times (DRT) identified and quantified four processes related to gas diffusion, surface-exchange, interfacial ionic loss, and electronic loss. The BSCF10Y cathodes showed an area specific resistance of 49.9 mΩ cm2 at 600 °C and 0.21 atm, among the lowest ever reported ASR for micrometerscaled cathodes. The calculated high k* values are most likely due to the nondegraded nature of the in situ sintered electrodes. BSCF10Y cathodes in 1 and 3% vol CO2 (which is 25−75 times higher than the CO2 content in ambient air) at 700 °C show a clearly improved oxygen reduction reaction over time compared to that of undoped BSCF. SEM, XRD, and STEM-EDXS mappings proved that the presence of secondary phases in the BSCF10Y cathodes was strongly reduced or even suppressed compared to undoped BSCF. Also, the segregated volume fraction of carbonates is greatly reduced after 50 h in 1% CO2. The improved stability of the cubic phase in BSCF10Y compared to BSCF is linked to the monovalent agent and higher radius of the Y-dopant, while the improved stability in CO2-containing atmospheres is attributed to the higher acidity of Y3+ compared to the A-site cations and the high concentration of the Y-dopant, which is located on both the A- and B-sites of the perovskite.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00028. Electrochemical response and XRD data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Laura Almar: 0000-0001-5103-3812 Ellen Ivers-Tiffée: 0000-0002-3123-6994 Notes

The authors declare no competing financial interest. 1325


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