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electrolyte interfaces of solid oxide fuel cells
Journal of Power Sources 360 (2017) 399e408
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Correlative tomography at the cathode/electrolyte interfaces of solid oxide fuel cells sz a, Jochen Joos a, Virginia Wilde b, Heike Sto € rmer b, Florian Wankmüller a, Julian Sza e a, * Dagmar Gerthsen b, Ellen Ivers-Tiffe a b
Institute for Applied Materials (IAM-WET), Karlsruhe Institute of Technology (KIT), Adenauerring 20b, 76131, Karlsruhe, Germany Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT), Engesserstraße 7, 76131, Karlsruhe, Germany
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Correlative Tomography reveals 3D Information of SOFC Cathode/Electrolyte Interfaces. Spatial Organization of SrZrO3 Secondary Phase depends on GDC/YSZ Sintering Temperature. Area Specific Resistance depends on GDC/YSZ Sintering Temperature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2017 Received in revised form 23 May 2017 Accepted 2 June 2017
This paper introduces a correlative tomography technique. It visualizes the spatial organization of primary and secondary phases at the interface of La0.58Sr0.4Co0.2Fe0.8O3-d cathode/10 mol% Gadolinia doped Ceria/8 mol% Yttria stabilized Zirconia electrolyte. It uses focused ion beam/scanning electron microscope tomography (FIB/SEM), and combines data sets from Everhart-Thornley and Inlens detector differentiating four primary and two secondary material phases. In addition, grayscale information is correlated to elemental distribution gained by energy dispersive X-ray spectroscopy in a scanning transmission electron microscope. Interdiffusion of GDC into YSZ and SrZrO3 as secondary phases depend (in both amount and spatial organization) on the varied co-sintering temperature of the GDC/YSZ electrolyte. The ion-blocking SrZrO3 forms a continuous layer on top of the temperature-dependent GDC/ YSZ interdiffusion zone (ID) at and below a co-sintering temperature of 1200 C; above it becomes intermittent. 2D FIB/SEM images of primary and secondary phases at 1100, 1200, 1300 and 1400 C were combined with a 3D FIB/SEM reconstruction (1300 C). This reveals that “preferred” oxygen ion transport pathways from the LSCF cathode through GDC and the ID into the YSZ electrolyte only exist in samples sintered above 1200 C. The applied correlative technique expands our understanding of this multiphase cathode/electrolyte interface region. © 2017 Elsevier B.V. All rights reserved.
Keywords: correlative tomography cathode/electrolyte interface secondary phases spatial organization SrZrO3 FIB/SEM tomography solid oxide fuel cell
1. Introduction Mixed ionic-electronic conducting cathodes based on (La,Sr)
* Corresponding author. e). E-mail address: [email protected] (E. Ivers-Tiffe http://dx.doi.org/10.1016/j.jpowsour.2017.06.008 0378-7753/© 2017 Elsevier B.V. All rights reserved.
(Co,Fe)O3-d (LSCF) are widely used because of their superior oxygen reduction kinetics in high-performance intermediate temperature solid oxide fuel cells (SOFCs) [1e6]. Substantial studies have shown that elemental diffusion occurs between LSCF and the common Zirconia-based electrolyte (8 mol% Yttria stabilized Zirconia - YSZ) at temperatures 900 C, and also during cell fabrication. Sr is
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transported via bulk and surface diffusion along LSCF and GDC or via gas diffusion as Sr(OH)2 in a pore space. Zr diffuses from the electrolyte towards the interface. An oxygen ion-blocking layer of SrZrO3 (SZO) forms [1,7e9]. Although a thin diffusion barrier interlayer of Gd0.2Ce0.8O2d (GDC) is applied between the cathode and electrolyte, the formation of an ion-blocking phase is still observed at this interface region [7,10e12]. This is particularly true when the GDC is screenprinted and still porous after co-sintering with an anodesupported, thin-film YSZ electrolyte. Different methods of applying GDC produce less porous or perfectly dense, yet ultrathin GDC layers (methods include physical vapor deposition (PVD) [12e14] or pulsed laser deposition (PLD) [7,15e18]). However, low deposition rates and high costs are unsuitable for mass-production. In this work, we focus on an anode-supported cell (Fig. 1) with a GDC interlayer deposited by screen printing and co-sintered with the YSZ thin-film electrolyte. Interdiffusion between GDC and YSZ leads to the solid solution formation with different compositions. At 800 C, ionic conductivity of the “worst” composition (Ce0.8Zr0.38Gd0.18Y0.07O1.87) is up to 40 to 70 times lower compared to GDC or YSZ [19]. Therefore, the “optimum” co-sintering temperature for such a half-cell needs to be identified before the LSCF cathode can be screen-printed and sintered in a second step at a much lower temperature. Without knowledge of the elemental composition and the spatial organization of both ID and ionblocking SZO phase, manufacturing of high-performance solid oxide fuel cells is not possible. Moreover, understanding the transport of oxygen ions across this complex interface is challenging, because elemental and structural inhomogeneity is expected to take place on more than one length scale. Horita et al. used secondary ion mass spectroscopy (SIMS) to localize SZO formation at the LSCF/GDC/YSZ interface [20]. The authors observed 18O diffusion under cathodic polarization and correlated the 18O peak position with the SZO phase by SEM/EDXS analysis. Elemental distribution maps show a 3e5 mm SZO layer after cathodic polarization at 700e900 C. They proposed an acceleration mechanism of SZO formation under polarization, because the activity of SrO increases gradually with decreasing oxygen potential in the LSC/LSF system [21]. It was emphasized, that a part of the SZO formation already took place during sample fabrication, but no sintering temperature is provided. The solid solution formation between GDC and YSZ was not analyzed. More recently, Matsui et al. published a study of the
Fig. 1. Micrographs of an anode-supported solid oxide fuel cell with an LSCF cathode, GDC interlayer, YSZ electrolyte and Ni/YSZ anode; the screen-printed GDC was cosintered with YSZ at 1300 C. (a) SEM cross section (Inlens, 1.3 kV); (b) SEM cross section of cathode/electrolyte interface (Inlens, 1.3 kV); (c) STEM/EDXS elemental mapping of GDC interlayer, SZO and GDC/YSZ secondary phases, and YSZ electrolyte; FIB polished cross sections: (d) ETD, 4 kV, (e) Inlens, 4 kV, (f) ETD, 1.3 kV, (f) Inlens, 1.3 kV.
microstructural changes within a LSCF/Sm-doped Ceria (SDC)/YSZ system by combining FIB/SEM and transmission electron microscopy [22]. SDC was screen-printed on YSZ and sintered at 1250 C, followed by sintering of the LSCF cathode at 1150 C. FIB/SEM grayscale images using an Energy selective Backscattered (EsB) detector revealed a <1 mm thin, discontinuous SZO layer before discharge operation. This layer increased in volume six-fold upon discharge for 400 h at 1000 C and filled the pores within the SDC interlayer. The authors suggested that SZO was embedded in a Smrich SDC/YSZ solid solution, as indicated by elemental mapping in HAADF-STEM. A change in the grayscale values of this area was observed in FIB/SEM images, but a differentiation between ID, SDC and YSZ was not performed. Consequently, multiscale morphologies existing in LSCF/GDC (or SDC)/YSZ interfaces, as analyzed above by Matsui et al. [22] and shown in Fig. 1, demand a combination of different microscopy techniques for a correlation of elemental composition with grayscale values. They also require a refined approach for sampling and segmenting grayscale values. In this work, we combine (i) FIB/SEM, using multiple combinations of detector type and acceleration voltage for segmentation of material phases from grayscale information at the microscale, and (ii) STEM-EDXS for analyzing the elemental composition of material phases at the nanoscale. In this way, large sample volumes comprising the spatial organization of porous LSCF cathode, GDC interlayer, SZO ion-blocking layer, ID and YSZ electrolyte can be visualized in 2D FIB/SEM images and rendered in 3D FIB/SEM reconstructions. The procedure for this correlative imaging technique is presented in detail in Section 2. Moreover, we vary the co-sintering temperature (1100e1400 C) of screen-printed GDC interlayer on YSZ electrolyte to understand whether this fabrication step has an intricate influence on the volume and distribution of secondary phases. Finally, we monitored the ohmic and cathodic polarization losses using electrochemical impedance spectroscopy on symmetric cells (LSCF/GDC/ YSZ/GDC/LSCF). 2. Experimental 2.1. Sample fabrication, electrochemical and electron microscopy methods Symmetrical cells were manufactured using 8 mol% Yttria stabilized Zirconia (YSZ) electrolyte substrates, 200 mm thick (Itochu, Tokyo, Japan). This was coated on both sides with a screen-printed Gd0.2Ce0.8O2-d (GDC) interlayer, 5.5 mm thick (Forschungszentrum Jülich, Jülich, Germany) and co-sintered at 1100, 1200, 1300 or 1400 C for 3 h. Subsequently, a La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF) cathode layer, 30 mm thick (Forschungszentrum Jülich), was screenprinted on both sides of the GDC and sintered at 1080 C for 3 h. Electrochemical impedance spectroscopy (EIS) on symmetrical cells gives, in our experience, a sensitive measure of the cathode/ electrolyte interface performance. The measurement setup and the applied evaluation of impedance data are thoroughly described by Hayd et al. [23]. The area specific resistance of the cathodic polarization process (ASRcat) and the ohmic resistance (ASRU) were determined using a Solartron 1260 FRA (frequency response analyzer) in a frequency range of 10 mHze1 MHz. The samples were 4-point wire contacted and exposed to stagnant air at 800 C. In the EIS analysis, a low frequency gas diffusion process of the stagnant air in the gas channels of the flow field and in the contacting Au-meshes (~40 mUcm2) is subtracted. The remaining resistance value is divided by two, to account for only one cathode/ electrolyte interface. ASRU contains contributions from the YSZ electrolyte, the ID and the GDC layer. ASRcat contains contributions from the oxygen reduction reaction at the LSCF surface, oxygen ion
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transport in the LSCF bulk, oxygen ion charge transfer at the (multiphase) interfaces and contributions from the SZO layer [24,25]. The sequential FIB milling and SEM imaging were performed by a ZEISS 1540XB CrossBeam workstation (Carl Zeiss AG, Oberkochen, Germany). All samples were first vacuum infiltrated with a twocomponent epoxy resin to fill the pores. This gives higher mechanical stability and improves differentiation between pores and the solid material phases. The samples were then polished and coated with a thin gold-layer to avoid streaking and charging [26,27]. Within the FIB/SEM system, the electron and ion beams are oriented at a coincident angle of 54 , as shown in Fig. 3. This setup offers the possibility to instantly display the FIB-polished area by SEM without moving the stage. The FIB is first used to uncover the area of interest by milling trenches on both sides with a high emitter current of 10 nA. The trenches prevent shadowing effects and deposition of removed material. Before imaging the area of interest with the electron beam, a smaller emitter current of 2 nA (or lower) is used to polish the surface, making it perfectly planar. More details on the setup and the FIB/SEM processing are given by Joos et al. [27,28]. The elemental distribution and the chemical composition of the cathode/electrolyte interface were determined by energy dispersive X-ray spectroscopy (EDXS) in a FEI Osiris ChemiSTEM transmission electron microscope, operated at 200 kV and equipped with four Bruker silicon drift detectors. Composition quantification was carried out with Esprit software. Imaging was performed in the high-angle annular dark-field scanning transmission electron microscopy mode (HAADF-STEM), which yields chemically sensitive images with a high spatial resolution. Electron transparent, crosssectional specimens were prepared by conventional TEM sample preparation (including grinding, dimpling, ion-etching) and by FIBbased techniques.
2.2. Image acquisition and grayscale contrast at 1.3 kV and 4 kV Conventional FIB/SEM tomography generally alternates between milling and image acquisition. First, the focused ion beam removes a layer of a defined thickness. Second, the SEM image is recorded. This procedure is automatically repeated until the volume of interest is achieved [27e29]. As stated by Wilson et al., the accelerating voltage of the primary electrons of the SEM is an important variable for maximizing the contrast between different material phases [30]. In this study, we discovered that a low accelerating voltage of 1.3 kV reveals the best grayscale contrast between the primary phases LSCF, GDC, YSZ, and pores. However, the distinctive features of the secondary phases, the ID and the ion-blocking SZO phase can only be deduced at a higher accelerating voltage of 4 kV. This contrast difference is assumed to be a result of a change in the backscattering coefficient of the electrons, which depends on the accelerating voltage for voltages below 5 kV [31]. Fig. 2 demonstrates the applied correlative imaging technique. (a) FIB/SEM tomography at 1.3 kV results in a 3D representation of the primary material phases: LSCF cathode in light-gray, GDC interlayer in gray and YSZ thin-film electrolyte in dark-gray. (b) Combination of the chemical information received by STEM/EDXS with the grayscale information at 4 kV in the stack of SEM images; this discloses the secondary material phases SZO and ID. (c) Combination of the information (a) and (b), rendering the primary phases (LSCF in blue, GDC in brown, YSZ in yellow) and the secondary material phases (SZO in turquois and GDC/YSZ in green). Fig. 3 demonstrates the sequential acquisition of grayscale
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information in FIB/SEM: automated FIB milling (1) and SEM imaging (Everhart-Thornley and Inlens detector) at an accelerating voltage of 1.3 kV (2), followed by SEM imaging (Everhart-Thornley and Inlens detector) at an accelerating voltage of 4 kV (3), including repeated individual adjustment of the electron beam position (field of view) and parameters like focus, stigmator, brightness and contrast at steps 2 and 3. To preserve the sample at 4 kV, data are only sampled in the smaller area of interest. Generally, the resolution of the SEM images and the milling step distance differ from each other [26,32,33]. Due to the high time expenditure, a FIB milling step distance of 50 nm is chosen for larger volumes, while the SEM pixel size is 25 nm. 2.3. Segmentation and phase assignment The process of segmentation associates the grayscale values of each pixel in the SEM images to the material phases, varying from 0 (black) to 255 (white). Before phase-identification the noise in the grayscale images must be decreased to minimize incorrect allocation. Thus, several filtering and segmentation steps were performed; implemented using the software MATLAB (from the MathWorks, Natick, MA, USA) and the open-source program ImageJ. Filters based on Fast Fourier Transformation and Anisotropic Diffusion (implemented in 3D) and a Median Filter (ImageJ) were applied [34]. The segmentation process was started with the images obtained at an accelerating voltage of 1.3 kV (both EverhartThornley and Inlens detector). Evaluation of phases using thresholding based on grayscale values of the pixels (e.g. Ref. [35]) did not reveal satisfying results. Therefore, a region growing algorithm, developed by our group for Ni-YSZ anodes [34], was successfully applied for phase segmentation. Fig. 4 shows the SEM images at 1.3 kV and 4 kV and corresponding grayscale histograms. The distinct peaks in Fig. 4 (c) are correlated to the primary material phases LSCF, GDC and YSZ. A simplification was unavoidable at this stage: inclusions of brighter contrast within the GDC are identified as Co- and Fe,Gd-oxides [36e38] by STEM/EDXS, but are herein assigned to the GDC phase. Because charging of the sample causes outshining of the pores (Inlens detector), the Everhart-Thornley image (1.3 kV) in Fig. 4 (b) was used for segmentation between pores and primary material phases. The resulting 3D representation appears in Fig. 2 (a). The SEM images at 4 kV in Fig. 4 (a) and (b) are used for the segmentation of secondary phases. However, automated segmentation was impossible, as the grayscale contrast was not high enough (cf. Fig. 4 (e), (f)), and filtering was inefficient. Therefore, the slight differences in grayscale values were differentiated manually using the open source graphics editor GIMP. It should be mentioned that a manual segmentation depends on subjective perception. The human eye, conversely, allows a highly sophisticated determination of material phases, not easily matched in accuracy by automated segmentation. 2.4. Elemental composition The elemental composition of this complex cathode/electrolyte interface region was now analyzed on the nanoscale by HAADFSTEM in combination with EDXS. It was then correlated to the acquired grayscale information [36,37]. Fig. 5 (c) exemplarily shows the GDC-YSZ interface region of a sample sintered at 1250 C. The chemical composition at different positions (1e4) was quantitatively analyzed and is given in Table 1. The elemental compositions at position 2 and 3 are of great interest because here two different secondary phases can be determined at the interface between GDC
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Fig. 2. Schematic representation of the method presented in this work. (a) FIB/SEM tomography (SEM accelerating voltage: 1.3 kV) of the cathode/electrolyte interface allows a 3D representation of the primary material phases La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF), Gd0.2Ce0.8O2-d (GDC) and 8 mol% Yttria stabilized Zirconia (YSZ). (b) Additional correlation of STEM/ EDXS and FIB/SEM images (4 kV) enables determination of the secondary material phases (SZO and ID). (c) Resulting 3D microstructure containing both primary and secondary material phases.
insights are discussed in the following sections. 3.1. Accumulation of secondary phases at different co-sintering temperatures
Fig. 3. Illustration of the advanced operation procedures for 3D reconstruction: tomography cycle; automated alternating FIB milling (step 1) and SEM imaging (Everhart-Thornley and Inlens detector) at a defined accelerating voltage of 1.3 kV (step 2), interrupted by manual SEM imaging (step 3) at an accelerating voltage of 4 kV (the different contrast in grayscale reveals the secondary phases).
and YSZ. The EDXS data at position 2 reveal a layer of SZO (Sr:Zr ratio of 1:1) and negligible concentrations of other cations. Position 3 discloses an atomic ratio for Ce:Gd:Y:Zr:Sr of 15:23:12:49:1, indicating a markedly Gd- (and Y-) enriched ID, which is in agreement with literature [11,39]. For simplification, this region is treated as one chemically homogeneous phase, with an exemplary ionic conductivity (cf. Table 2). Position 1 confirms the chemical composition of a nearly unaffected GDC interlayer with Ce:Gd:Y:Zr:Sr as of 74:18:2:5:1, whereas position 4 confirms the composition of the YSZ thin-film electrolyte with Ce:Gd:Y:Zr:Sr as of <1:<1:14:84:<1. Transferring this information into color-coded images, Fig. 5 (c) represents (from top to bottom) the GDC interlayer (orange) with pores (black), SZO grains of <1 mm (turquois) meshed with the GDC and ID (green), but not connected to the electrolyte YSZ (yellow). LSCF (blue) is not contained in this section. It is obvious that the appearance of the secondary phases in Fig. 5 (c) agrees well with the spatial organization of the SEM grayscale contrast in Fig. 5 (a) and (b). This coincidence facilitates the transcription of the grayscale information from ETD and Inlens detector at 4 kV into SZO (turquois) and ID (green) (cf. Fig. 5 (d)-(f)). 3. Results and discussion The procedure described in the previous section was then applied to the samples co-sintered at four different temperatures, and furthermore, the elemental information derived by STEM/EDXS from a sample co-sintered at 1250 C was transferred to a correlative 3D tomography from the sample co-sintered at 1300 C. New
Fig. 6 displays the segmented 2D images of the FIB-polished cross-sections after screen-printing GDC on the YSZ electrolyte, co-sintering at 1100 (a), 1200 (b), 1300 (c) and 1400 C (d) and sintering the LSCF cathode in a second step at 1080 C. The porous LSCF cathode (white) is top, and it should be noticed that the GDC interlayer (middle gray) always remains porous, while pore size increases with co-sintering temperature. Beneath the porous GDC interlayer, the turquois areas indicate SZO formation, while the green areas indicate the extension of the Gd- (and Y-) enriched ID passing over into unaffected YSZ (dark gray). The sequence in Fig. 6 (a,b,c,d) gives clear evidence that the amount, proportion and spatial organization of the two secondary phases depend on cosintering temperature. The samples co-sintered at 1100 and 1200 C (Fig. 6 (a) and (b)) are characterized by two continuous layers (each 1 mm) fitted between porous GDC and dense YSZ. The thickness and uneven shape of both secondary phase layers are not significantly altered within this temperature range. It is worth noting at this point that moderate co-sintering promotes the formation of a continuous ion-blocking layer of SZO and a continuous ID. This finding underlines that a moderate co-sintering temperature may not be a preferential treatment of anode-supported solid oxide fuel cells with screen-printed GDC interlayers on YSZ electrolytes. Co-sintering at 1300 C further promotes SZO formation (Fig. 6 (c)), the extension of the SZO layer doubles but becomes intermittent, whereas the Gd- (and Y-) enriched ID extends slightly more into the YSZ electrolyte. The change in spatial organization of SZO is of great importance, as it is expected that every gap in the SZO opens diffusion paths for O2--ions from the cathode to the electrolyte. As already reported by our group [24,36], and shown in Table 3 for the symmetric cells analyzed therein, the area specific polarization resistance ASRcat drops by two orders of magnitude when the co-sintering temperature of the half-cells is raised above 1250 C. (A detailed study on the effects of different co-sintering temperatures on electrochemical performance is not within the framework of this study. We will soon publish a related paper.) This pronounced spreading of SZO was validated by grayscale analysis and elemental correlation, and was exemplarily shown in three additional 2D SEM images (Fig. 6 (e,f,g)). In conclusion, the analyzed image sections are representative. However, co-sintering at the even higher temperature of 1400 C completely changes the picture (Fig. 6 (d)): the commonness of SZO at the analyzed interface region declines drastically.
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Fig. 4. SEM images and corresponding histograms: (a) Inlens detector (I: 1.3 kV, III: 4 kV); (b) Everhart-Thornley detector (II: 1.3 kV, IV: 4 kV); (c) the clearly distinguishable peaks in the histogram allow a segmentation of the primary material phases LSCF, GDC and YSZ; (d) a better contrast between pore and material phase using ETD allows a segmentation of the pore phase; (e) secondary phases which are visible in (a) cannot clearly distinguished in the histogram; (f) secondary phases show a different contrast in grayscale compared to (e).
This positive effect correlates to a drastic increase of the ID thickness, here highlighted in green. A detailed STEM-EDXS study [41] on the sample co-sintered at 1400 C, disclosed a Gd-rich (and Zr-rich) region adjacent to the YSZ electrolyte and a Ce-rich (and Zrpoor) upper region adjacent to the GDC. It is speculated that this dense upper Zr-poor region acts like the initially desired dense GDC interlayer: it separates the porous GDC (Sr is evaporated as Sr(OH)2 into pores) from a Zr-rich region impeding SZO formation. Again, it must be emphasized that this contribution concentrates on visualizing the spatial organization of primary and secondary phases by correlative tomography. Thus, it is restricted to the most important details at the micro- and nanoscales. A more detailed study of the interplay between fabrication procedure, density/porosity and elemental composition at this highly complex and extended interface region is in preparation and will be published shortly by Wilde et al.
3.2. Preferred oxygen transport pathways So far it has been shown that the amount, proportion and spatial organization of the ion-blocking SZO phase is tremendously influenced by the fabrication procedure of each cell, i.e. by the cosintering temperature of the GDC/YSZ interface. We assume that once oxygen ions are incorporated into the LSCF cathode they will (i) either be blocked at a continuous SZO layer, associated with a very high cell resistance, or if possible, (ii) bypass ion-blocking SZO areas and continue through the ID into the YSZ electrolyte, resulting in a markedly lower cell resistance. This would require a sufficient number of “preferred oxygen pathways”, i.e. direct contact between LSCF cathode/GDC interlayer/ID/YSZ electrolyte. As proof of concept, a new routine counting the direct contact between GDC interlayer and ID was implemented in MATLAB and applied to the 2D SEM images already discussed in
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Fig. 5. Original SEM image of the 1300 C sample obtained with (a) the ETD at 4 kV and (b) the Inlens detector at 4 kV. (c) Color-coded assignment of phases in a cross-section of the GDC-YSZ interface (GDC sintering temperature: 1250 C) derived from an EDXS elemental mapping; a line-scan (cf. Table 1) reveals (1) GDC, (2) SZO, (3) ID, and (4) YSZ. (d) Edited SEM image of the 1300 C sample obtained with the ETD at 4 kV; comparing the contrast in grayscale with (c) allows assignment of the SZO phase (turquois). (e) Edited SEM image (Inlens detector at 4 kV); the information of (c) and (d) are used to additionally assign the ID (green) as a further secondary phase.(f) SEM image shown in (a) with assigned secondary phases. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Atomic ratio of the different cations in Fig. 5 (c), shown at different positions of the line-scan. The precision of the composition analysis is ~3 at%. Line-scan position
1 2 3 4
Atomic ratio [%] Ce
Gd
Y
Zr
Sr
74 <1 15 <1
18 <1 23 <1
2 <1 12 14
5 50 49 84
1 49 1 <1
Table 2 Ionic conductivities at 800 C. Material
Ionic conductivity s [S m-1]
GDC SrZrO3 Ce0.8Zr0.38Gd0.18Y0.07O1.87 YSZ
8.70a 1.87 E04b 1.25 E01a 5.40a
a b
[19]. Computed from Ref. [40].
Fig. 6. The number of overlapping pixels, herein abbreviated as GDC/ID, was counted for each row of pixels parallel to the cross section (starting at position 0 nm) and divided by the maximum of possible pixels (width of the cross section, in pixels). So a value of GDC/ID ¼ 1 for the intensity of the contact area would imply a GDC
interlayer in full contact with the ID over the whole cross section. Vice versa, GDC/ID ¼ 0 would imply that no transport path for oxygen ions exists in the analyzed data set. Additionally, the SZO intensity distribution was counted by the number of pixels normalized by the width of the same cross section. The resulting ratios, denominated in Fig. 7 as “Intensity contact area GDC/ID” and as “Intensity SZO distribution”, are plotted as intensity signals along the 2D SEM images, and look very different for co-sintering temperatures between 1100 and 1400 C. At 1100 and 1200 C, the ion-blocking SZO forms a continuous layer in a well confined region, which is a few hundred nm thick (SZO intensity ¼ 1 at a position around 4000 nm) in Fig. 7 (a) and (b). No preferred oxygen transport pathways exist in these samples and the intensity of the contact area of GDC/ID is zero. Fig. 7 (c) depicts the SZO intensity signal at 1300 C: it broadens significantly, but it does not exceed intensity ¼ 0.65. In the same confined region, the contact area GDC/ID now reaches a peak intensity ¼ 0.04, indicating a (small) interface area that is now available for preferred oxygen transport pathways. At 1400 C, depicted in Fig. 7 (d), it is here statistically proven, that the contact area GDC/ID has further increased (peak intensity of 0.18), whereas the intensity of SZO has decreased to 0.18. As already discussed, the drop off in SZO intensity at 1400 C goes hand in hand with a broader extension of the ID, which has split up into a Ce-rich (and Zr-poor) upper and a Gd-rich part adjacent to the YSZ electrolyte [41]. This examination predicts a high cell resistance for a GDC/YSZ
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Fig. 6. Segmented SEM images of the GDC layer and its interfaces to the LSCF cathode and YSZ electrolyte, sorted by increasing GDC sintering temperature (1100e1400 C, (a)e(d)). The turquois shaded areas indicate SZO formation, while the green shaded areas represent ID. (e)e(f) Validation of the microstructure (TSinter,GDC ¼ 1300 C) revealed in (c) at different sample positions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3 Measured area specific polarization resistance, ASRcat, and ohmic resistance, ASRU, of the symmetric cathode half cells as a function of the GDC/YSZ co-sintering temperature, measured at 800 C. TSinter,GDC
ASRU
ASRcat
U cm2
U cm2
0.251 0.258 0.230 0.249
5.329 3.100 0.037 0.007
C
1100 1200 1300 1400
co-sintering temperature of 1100 and 1200 C (oxygen ions are blocked at the continuous SZO layer), a decrease in cell resistance for the 1300 C cell (SZO layer is not continuous, but still spread out) and by far the lowest resistance for the 1400 C cell (SZO layer is full of gaps). Table 3 lists polarization resistance, ASRcat, with coherent dependency on the GDC/YSZ co-sintering temperature, and ohmic resistance, ASRU, with negligible changes. While ASRcat, is in the region of several Ucm2 for a co-firing temperature of 1100 and 1200 C, it drops by about two orders of magnitude from 3.100 Ucm2 at 1200 C to 0.037 Ucm2 at 1300 C, and even lower to 0.007 Ucm2 at 1400 C. In our experience, the size of the last two ASRcat values is common for the polarization contribution of a nominally identical LSCF/GDC/YSZ interface at 750 C in high performance anode-supported cells, when GDC was co-sintered with YSZ at 1300 C. An initial value of 0.015 Ucm2 was quantified by an
adequate equivalent circuit model, as reported by Endler-Schuck et al. [42]. Endler-Schuck monitored the course of the ASRcat of a nominally identical LSCF/GDC with respect to operation temperature (600, 750 and 900 C) and time (up to 1000 h). A thorough analysis of all data sets lead to the outcome that the degradation of the ASRcat increases with decreasing temperature. After 1000 h of operation, the ASRcat at 900 C raised only by 0.012%/h, and the largest part of the degradation was assigned to the Ni/YSZ anode. Kiebach et al. concluded that Sr diffusion leading to SZO formation occurs during firing of the cell. No SZO formation during operation (1600 h under current load) at 700 C was found, verified by SEMEDS, HR-TEM-EDS and TOF-SIMS analysis [43]. This finding supports our assumption that most of the SZO formation takes place during sintering of the LSCF cathode. It is therefore insignificant for the degradation of anode supported cells. Precondition seems to be a sintering of the GDC interlayer/YSZ electrolyte system at high temperatures, in our case 1300 C. More evidence on correlation between SZO distribution, ASRcat and GDC sintering temperature will be published soon in a separate paper. However, it must be noted that processing parameters of anode-supported solid oxide fuel cells are extremely varied among different cell developers, and therefore findings should not be transferred between research groups without reflection. Nevertheless, we will conduct a FIB/SEM grayscale analysis, as described in this work, on anode-supported cells after 1000 h of operation under various conditions in the near future. The spatial organization of SZO (cf. Fig. 6) and the availability of oxygen ion transport pathways (cf. Fig. 7) seem to be
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Fig. 7. Plots of contact area (preferred oxygen transport pathways) between ID and GDC layer in dependence to the position on the cross section. The figures are ordered by GDC sintering temperature. The SZO distribution (number of pixels normalized by the width of the cross section) was added for better illustration of contact area location. Note the different dimension between the axes.
vital for understanding the effect of GDC/YSZ co-sintering temperature. 3.3. Correlated 3D tomography As outlined in Section 2, correlative 3D tomography was applied to the most challenging distribution of primary and secondary phases, represented in the sample with a GDC/YSZ co-sintering temperature of 1300 C. A volume of 10.7 14.7 1.4 mm3 was investigated, applying a resolution of 25 25 50 nm3 (cf. Fig. 8 (a)). Each single primary and secondary phase is represented as an individual material phase and can be separately visualized using the software Blender. The colors yellow, green, turquois, brown, blue and dark gray were assigned to the material phases YSZ electrolyte, ID, SZO, GDC interlayer, LSCF cathode and pore phase, respectively. Fig. 8 (b) shows that the porosity within LSCF and GDC is high, but the porosity suddenly drops off at the surface of the SZO layer, meaning that no pores are in direct contact with the ID. This indicates that SZO is formed at the interface of the ID: i. Sr is transported via bulk and surface diffusion along LSCF and GDC or via gas diffusion as Sr(OH)2 in a pore space. ii. Zr diffuses from the YSZ electrolyte through the ID.
iii. Zr reacts with Sr to SZO. Fig. 8 (c) shows the spreading of SZO grains into the GDC interlayer, and the top down view in Fig. 8 (d, e) indicates that SZO has pores, which are filled by GDC or by GDC/YSZ solid solution, and thereby enable oxygen ion transport from the LSCF cathode via these ion-conducting phases to the YSZ electrolyte. More details on the correlation between cell manufacturing parameters, SZO distribution (continuous or intermittent), and the functionality of the GDC layer are discussed elsewhere [24,36,37,44,45].
4. Conclusions Correlative tomography was applied to analyze and visualize the elemental and spatial distribution of performance limiting secondary phases which develop at the cathode/electrolyte interface of solid oxide fuel cells, composed of a Lanthanum Strontium Cobalt Ferrite (LSCF) and Yttria stabilized Zirconia (YSZ). It was shown, that a chemical reaction leading to SrZrO3 (SZO) is not prevented, but its amount and spatial distribution are influenced by an interlayer of screen-printed Gd0.2Ce0.8O2-d (GDC), when co-sintered with the YSZ electrolyte (from 1100, 1200, 1300e1400 C). Key factors assumed are thickness and lateral composition of the solid
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Fig. 8. Visualization of reconstructed 3D structure (GDC sintering temperature of 1300 C). (a) Representation of the whole structure (10.7 14.7 1.4 mm3) showing the gain in information achieved by the presented method. (b) LSCF, pore phase, SZO, ID and YSZ, revealing direct contact between pore phase and SZO. The contact between SZO and the pores shows that a Sr gas diffusion through the pores during cathode sintering leads to SZO formation at the interface. There is no contact between pore phase and ID or YSZ revealing that SZO is formed during sintering developing a layer between pore and ID. (c) SZO, ID and YSZ: pronounced spreading of the uncontinuous SZO. (d) Top down view of part of the SZO phase showing the gaps in between the structure. (e) Top down view of part of the ID: dimpled structure as a result of spread SZO distribution.
solution formed between GDC and YSZ. Visualization of elemental composition and quantitative analysis of secondary phase distribution were correlated with electrochemical performance via impedance analysis of symmetric cells (LSCF/GDC/YSZ/GDC/LSCF). The introduced method is based on FIB/SEM analysis and supported by high resolution compositional mappings obtained by STEM/EDXS. In FIB/SEM mode, four data sets from the EverhartThornley and Inlens detector at acceleration voltages of 1.3 kV and 4.0 kV delivered a broader spectrum of grayscale information and allowed a skillful segmentation into four primary and two secondary material phases. In addition, the grayscale information was correlated to the elemental distribution gained by STEM/EDXS and rendered to four color-coded 2D SEM images of samples cosintered at 1100e1400 C and one 3D reconstruction volume of 10.7 14.7 1.4 mm3. This showed the spatial organization of all primary and secondary phases of the sample co-sintered at 1300 C. As a result, it is possible to deduce a linkage between the microstructure and the resistance of the samples. A continuous SZO layer causes a high cell resistance of several Ucm2 (blocked oxygen ion transport from the cathode to the electrolyte), whereas an intermittent SZO layer (GDC co-sintering temperature of 1300 C) leads to a resistance drop by two orders of magnitude. Since further SZO decrease (GDC co-sintering temperature of 1400 C) improves the performance of the cathode/electrolyte interface only slightly, it is proposed that the accessibility of oxygen ion transport pathways across the complex cathode/electrolyte interface is crucial for cell performance. The spatial organization of SZO and the availability of oxygen ion transport pathways from the cathode to the electrolyte are vital for understanding the effect of GDC/YSZ co-sintering temperature on the performance of an anode-supported solid oxide fuel cell with mixed conducting LSCF cathode.
The area specific polarization resistance ASRcat of this specific cathode/electrolyte interface drops by two orders of magnitude, when the GDC/YSZ co-sintering temperature of half-cells is raised well above 1250 C. Transferability of our findings to anode-supported cells from other suppliers is, in general, possible, but restricted due to varying processing parameters. Acknowledgements The authors would like to thank Dr. Norbert H. Menzler from Forschungszentrum Jülich for providing the GDC and LSCF paste and cell samples investigated in this work. Sincere thanks are given to J. Packham for proofreading the manuscript. This work was supported by the European Commission [grant agreement: 278257]; the Friedrich-und-Elisabeth-BOYSEN-Stiftung and by the Deutsche Forschungsgemeinschaft (DFG) [IV 14/16-1, RA 306/17-1]. Further support by the DFG through the Center for Functional Nanostructures (CFN) is gratefully acknowledged. References [1] H. Yokokawa, H. Tu, B. Iwanschitz, A. Mai, J. Power Sources 182 (2008) 400e412. [2] L. Kindermann, D. Das, H. Nickel, K. Hilpert, Solid State Ionics 89 (1996) 215e220. [3] A. Mai, V.A.C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stoever, Solid State Ionics 176 (2005) 1341e1350. € ver, J. Power Sources 156 [4] F. Tietz, V.A.C. Haanappel, A. Mai, J. Mertens, D. Sto (2006) 20e22. [5] M.Y. Oh, A. Unemoto, K. Amezawa, T. Kawada, J. Electrochem. Soc. 159 (2012) F659eF664. [6] E. Perry Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27e34. [7] F. Wang, M. Nishi, M.E. Brito, H. Kishimoto, K. Yamaji, H. Yokokawa, T. Horita, J. Power Sources 258 (2014) 281e289. [8] S. Badwal, Solid State Ionics 52 (1992) 23e32.
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Glossary ASR: Area Specific Resistance EDXS: Energy Dispersive X-ray Spectroscopy ETD: Everhart-Thornley Detector FIB: Focused Ion Beam GDC: Gadolinium (III) Oxide (Gd2O3) doped Ceria: Gd0.2Ce0.8O2-d HAADF: High-Angle Annular Dark-Field ID: GDC/YSZ Interdiffusion Layer LSCF: La0.58Sr0.4Co0.2Fe0.8O3-d PLD: Pulsed Laser Deposition PVD: Physical Vapor Deposition SEM: Scanning Electron Microscope SIMS: Secondary Ion Mass Spectroscopy SOFC: Solid Oxide Fuel Cell STEM: Scanning Transmission Electron Microscope SZO: Strontium Zirconate (SrZrO3) YSZ: Yttrium (III) Oxide (Y2O3) stabilized Zirconia: Y0.16Zr0.84O2-d
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Correlative tomography at the cathode/electrolyte interfaces of solid oxide fuel cells sz a, Jochen Joos a, Virginia Wilde b, Heike Sto € rmer b, Florian Wankmüller a, Julian Sza e a, * Dagmar Gerthsen b, Ellen Ivers-Tiffe a b
Institute for Applied Materials (IAM-WET), Karlsruhe Institute of Technology (KIT), Adenauerring 20b, 76131, Karlsruhe, Germany Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT), Engesserstraße 7, 76131, Karlsruhe, Germany
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Correlative Tomography reveals 3D Information of SOFC Cathode/Electrolyte Interfaces. Spatial Organization of SrZrO3 Secondary Phase depends on GDC/YSZ Sintering Temperature. Area Specific Resistance depends on GDC/YSZ Sintering Temperature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2017 Received in revised form 23 May 2017 Accepted 2 June 2017
This paper introduces a correlative tomography technique. It visualizes the spatial organization of primary and secondary phases at the interface of La0.58Sr0.4Co0.2Fe0.8O3-d cathode/10 mol% Gadolinia doped Ceria/8 mol% Yttria stabilized Zirconia electrolyte. It uses focused ion beam/scanning electron microscope tomography (FIB/SEM), and combines data sets from Everhart-Thornley and Inlens detector differentiating four primary and two secondary material phases. In addition, grayscale information is correlated to elemental distribution gained by energy dispersive X-ray spectroscopy in a scanning transmission electron microscope. Interdiffusion of GDC into YSZ and SrZrO3 as secondary phases depend (in both amount and spatial organization) on the varied co-sintering temperature of the GDC/YSZ electrolyte. The ion-blocking SrZrO3 forms a continuous layer on top of the temperature-dependent GDC/ YSZ interdiffusion zone (ID) at and below a co-sintering temperature of 1200 C; above it becomes intermittent. 2D FIB/SEM images of primary and secondary phases at 1100, 1200, 1300 and 1400 C were combined with a 3D FIB/SEM reconstruction (1300 C). This reveals that “preferred” oxygen ion transport pathways from the LSCF cathode through GDC and the ID into the YSZ electrolyte only exist in samples sintered above 1200 C. The applied correlative technique expands our understanding of this multiphase cathode/electrolyte interface region. © 2017 Elsevier B.V. All rights reserved.
Keywords: correlative tomography cathode/electrolyte interface secondary phases spatial organization SrZrO3 FIB/SEM tomography solid oxide fuel cell
1. Introduction Mixed ionic-electronic conducting cathodes based on (La,Sr)
* Corresponding author. e). E-mail address: [email protected] (E. Ivers-Tiffe http://dx.doi.org/10.1016/j.jpowsour.2017.06.008 0378-7753/© 2017 Elsevier B.V. All rights reserved.
(Co,Fe)O3-d (LSCF) are widely used because of their superior oxygen reduction kinetics in high-performance intermediate temperature solid oxide fuel cells (SOFCs) [1e6]. Substantial studies have shown that elemental diffusion occurs between LSCF and the common Zirconia-based electrolyte (8 mol% Yttria stabilized Zirconia - YSZ) at temperatures 900 C, and also during cell fabrication. Sr is
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transported via bulk and surface diffusion along LSCF and GDC or via gas diffusion as Sr(OH)2 in a pore space. Zr diffuses from the electrolyte towards the interface. An oxygen ion-blocking layer of SrZrO3 (SZO) forms [1,7e9]. Although a thin diffusion barrier interlayer of Gd0.2Ce0.8O2d (GDC) is applied between the cathode and electrolyte, the formation of an ion-blocking phase is still observed at this interface region [7,10e12]. This is particularly true when the GDC is screenprinted and still porous after co-sintering with an anodesupported, thin-film YSZ electrolyte. Different methods of applying GDC produce less porous or perfectly dense, yet ultrathin GDC layers (methods include physical vapor deposition (PVD) [12e14] or pulsed laser deposition (PLD) [7,15e18]). However, low deposition rates and high costs are unsuitable for mass-production. In this work, we focus on an anode-supported cell (Fig. 1) with a GDC interlayer deposited by screen printing and co-sintered with the YSZ thin-film electrolyte. Interdiffusion between GDC and YSZ leads to the solid solution formation with different compositions. At 800 C, ionic conductivity of the “worst” composition (Ce0.8Zr0.38Gd0.18Y0.07O1.87) is up to 40 to 70 times lower compared to GDC or YSZ [19]. Therefore, the “optimum” co-sintering temperature for such a half-cell needs to be identified before the LSCF cathode can be screen-printed and sintered in a second step at a much lower temperature. Without knowledge of the elemental composition and the spatial organization of both ID and ionblocking SZO phase, manufacturing of high-performance solid oxide fuel cells is not possible. Moreover, understanding the transport of oxygen ions across this complex interface is challenging, because elemental and structural inhomogeneity is expected to take place on more than one length scale. Horita et al. used secondary ion mass spectroscopy (SIMS) to localize SZO formation at the LSCF/GDC/YSZ interface [20]. The authors observed 18O diffusion under cathodic polarization and correlated the 18O peak position with the SZO phase by SEM/EDXS analysis. Elemental distribution maps show a 3e5 mm SZO layer after cathodic polarization at 700e900 C. They proposed an acceleration mechanism of SZO formation under polarization, because the activity of SrO increases gradually with decreasing oxygen potential in the LSC/LSF system [21]. It was emphasized, that a part of the SZO formation already took place during sample fabrication, but no sintering temperature is provided. The solid solution formation between GDC and YSZ was not analyzed. More recently, Matsui et al. published a study of the
Fig. 1. Micrographs of an anode-supported solid oxide fuel cell with an LSCF cathode, GDC interlayer, YSZ electrolyte and Ni/YSZ anode; the screen-printed GDC was cosintered with YSZ at 1300 C. (a) SEM cross section (Inlens, 1.3 kV); (b) SEM cross section of cathode/electrolyte interface (Inlens, 1.3 kV); (c) STEM/EDXS elemental mapping of GDC interlayer, SZO and GDC/YSZ secondary phases, and YSZ electrolyte; FIB polished cross sections: (d) ETD, 4 kV, (e) Inlens, 4 kV, (f) ETD, 1.3 kV, (f) Inlens, 1.3 kV.
microstructural changes within a LSCF/Sm-doped Ceria (SDC)/YSZ system by combining FIB/SEM and transmission electron microscopy [22]. SDC was screen-printed on YSZ and sintered at 1250 C, followed by sintering of the LSCF cathode at 1150 C. FIB/SEM grayscale images using an Energy selective Backscattered (EsB) detector revealed a <1 mm thin, discontinuous SZO layer before discharge operation. This layer increased in volume six-fold upon discharge for 400 h at 1000 C and filled the pores within the SDC interlayer. The authors suggested that SZO was embedded in a Smrich SDC/YSZ solid solution, as indicated by elemental mapping in HAADF-STEM. A change in the grayscale values of this area was observed in FIB/SEM images, but a differentiation between ID, SDC and YSZ was not performed. Consequently, multiscale morphologies existing in LSCF/GDC (or SDC)/YSZ interfaces, as analyzed above by Matsui et al. [22] and shown in Fig. 1, demand a combination of different microscopy techniques for a correlation of elemental composition with grayscale values. They also require a refined approach for sampling and segmenting grayscale values. In this work, we combine (i) FIB/SEM, using multiple combinations of detector type and acceleration voltage for segmentation of material phases from grayscale information at the microscale, and (ii) STEM-EDXS for analyzing the elemental composition of material phases at the nanoscale. In this way, large sample volumes comprising the spatial organization of porous LSCF cathode, GDC interlayer, SZO ion-blocking layer, ID and YSZ electrolyte can be visualized in 2D FIB/SEM images and rendered in 3D FIB/SEM reconstructions. The procedure for this correlative imaging technique is presented in detail in Section 2. Moreover, we vary the co-sintering temperature (1100e1400 C) of screen-printed GDC interlayer on YSZ electrolyte to understand whether this fabrication step has an intricate influence on the volume and distribution of secondary phases. Finally, we monitored the ohmic and cathodic polarization losses using electrochemical impedance spectroscopy on symmetric cells (LSCF/GDC/ YSZ/GDC/LSCF). 2. Experimental 2.1. Sample fabrication, electrochemical and electron microscopy methods Symmetrical cells were manufactured using 8 mol% Yttria stabilized Zirconia (YSZ) electrolyte substrates, 200 mm thick (Itochu, Tokyo, Japan). This was coated on both sides with a screen-printed Gd0.2Ce0.8O2-d (GDC) interlayer, 5.5 mm thick (Forschungszentrum Jülich, Jülich, Germany) and co-sintered at 1100, 1200, 1300 or 1400 C for 3 h. Subsequently, a La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF) cathode layer, 30 mm thick (Forschungszentrum Jülich), was screenprinted on both sides of the GDC and sintered at 1080 C for 3 h. Electrochemical impedance spectroscopy (EIS) on symmetrical cells gives, in our experience, a sensitive measure of the cathode/ electrolyte interface performance. The measurement setup and the applied evaluation of impedance data are thoroughly described by Hayd et al. [23]. The area specific resistance of the cathodic polarization process (ASRcat) and the ohmic resistance (ASRU) were determined using a Solartron 1260 FRA (frequency response analyzer) in a frequency range of 10 mHze1 MHz. The samples were 4-point wire contacted and exposed to stagnant air at 800 C. In the EIS analysis, a low frequency gas diffusion process of the stagnant air in the gas channels of the flow field and in the contacting Au-meshes (~40 mUcm2) is subtracted. The remaining resistance value is divided by two, to account for only one cathode/ electrolyte interface. ASRU contains contributions from the YSZ electrolyte, the ID and the GDC layer. ASRcat contains contributions from the oxygen reduction reaction at the LSCF surface, oxygen ion
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transport in the LSCF bulk, oxygen ion charge transfer at the (multiphase) interfaces and contributions from the SZO layer [24,25]. The sequential FIB milling and SEM imaging were performed by a ZEISS 1540XB CrossBeam workstation (Carl Zeiss AG, Oberkochen, Germany). All samples were first vacuum infiltrated with a twocomponent epoxy resin to fill the pores. This gives higher mechanical stability and improves differentiation between pores and the solid material phases. The samples were then polished and coated with a thin gold-layer to avoid streaking and charging [26,27]. Within the FIB/SEM system, the electron and ion beams are oriented at a coincident angle of 54 , as shown in Fig. 3. This setup offers the possibility to instantly display the FIB-polished area by SEM without moving the stage. The FIB is first used to uncover the area of interest by milling trenches on both sides with a high emitter current of 10 nA. The trenches prevent shadowing effects and deposition of removed material. Before imaging the area of interest with the electron beam, a smaller emitter current of 2 nA (or lower) is used to polish the surface, making it perfectly planar. More details on the setup and the FIB/SEM processing are given by Joos et al. [27,28]. The elemental distribution and the chemical composition of the cathode/electrolyte interface were determined by energy dispersive X-ray spectroscopy (EDXS) in a FEI Osiris ChemiSTEM transmission electron microscope, operated at 200 kV and equipped with four Bruker silicon drift detectors. Composition quantification was carried out with Esprit software. Imaging was performed in the high-angle annular dark-field scanning transmission electron microscopy mode (HAADF-STEM), which yields chemically sensitive images with a high spatial resolution. Electron transparent, crosssectional specimens were prepared by conventional TEM sample preparation (including grinding, dimpling, ion-etching) and by FIBbased techniques.
2.2. Image acquisition and grayscale contrast at 1.3 kV and 4 kV Conventional FIB/SEM tomography generally alternates between milling and image acquisition. First, the focused ion beam removes a layer of a defined thickness. Second, the SEM image is recorded. This procedure is automatically repeated until the volume of interest is achieved [27e29]. As stated by Wilson et al., the accelerating voltage of the primary electrons of the SEM is an important variable for maximizing the contrast between different material phases [30]. In this study, we discovered that a low accelerating voltage of 1.3 kV reveals the best grayscale contrast between the primary phases LSCF, GDC, YSZ, and pores. However, the distinctive features of the secondary phases, the ID and the ion-blocking SZO phase can only be deduced at a higher accelerating voltage of 4 kV. This contrast difference is assumed to be a result of a change in the backscattering coefficient of the electrons, which depends on the accelerating voltage for voltages below 5 kV [31]. Fig. 2 demonstrates the applied correlative imaging technique. (a) FIB/SEM tomography at 1.3 kV results in a 3D representation of the primary material phases: LSCF cathode in light-gray, GDC interlayer in gray and YSZ thin-film electrolyte in dark-gray. (b) Combination of the chemical information received by STEM/EDXS with the grayscale information at 4 kV in the stack of SEM images; this discloses the secondary material phases SZO and ID. (c) Combination of the information (a) and (b), rendering the primary phases (LSCF in blue, GDC in brown, YSZ in yellow) and the secondary material phases (SZO in turquois and GDC/YSZ in green). Fig. 3 demonstrates the sequential acquisition of grayscale
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information in FIB/SEM: automated FIB milling (1) and SEM imaging (Everhart-Thornley and Inlens detector) at an accelerating voltage of 1.3 kV (2), followed by SEM imaging (Everhart-Thornley and Inlens detector) at an accelerating voltage of 4 kV (3), including repeated individual adjustment of the electron beam position (field of view) and parameters like focus, stigmator, brightness and contrast at steps 2 and 3. To preserve the sample at 4 kV, data are only sampled in the smaller area of interest. Generally, the resolution of the SEM images and the milling step distance differ from each other [26,32,33]. Due to the high time expenditure, a FIB milling step distance of 50 nm is chosen for larger volumes, while the SEM pixel size is 25 nm. 2.3. Segmentation and phase assignment The process of segmentation associates the grayscale values of each pixel in the SEM images to the material phases, varying from 0 (black) to 255 (white). Before phase-identification the noise in the grayscale images must be decreased to minimize incorrect allocation. Thus, several filtering and segmentation steps were performed; implemented using the software MATLAB (from the MathWorks, Natick, MA, USA) and the open-source program ImageJ. Filters based on Fast Fourier Transformation and Anisotropic Diffusion (implemented in 3D) and a Median Filter (ImageJ) were applied [34]. The segmentation process was started with the images obtained at an accelerating voltage of 1.3 kV (both EverhartThornley and Inlens detector). Evaluation of phases using thresholding based on grayscale values of the pixels (e.g. Ref. [35]) did not reveal satisfying results. Therefore, a region growing algorithm, developed by our group for Ni-YSZ anodes [34], was successfully applied for phase segmentation. Fig. 4 shows the SEM images at 1.3 kV and 4 kV and corresponding grayscale histograms. The distinct peaks in Fig. 4 (c) are correlated to the primary material phases LSCF, GDC and YSZ. A simplification was unavoidable at this stage: inclusions of brighter contrast within the GDC are identified as Co- and Fe,Gd-oxides [36e38] by STEM/EDXS, but are herein assigned to the GDC phase. Because charging of the sample causes outshining of the pores (Inlens detector), the Everhart-Thornley image (1.3 kV) in Fig. 4 (b) was used for segmentation between pores and primary material phases. The resulting 3D representation appears in Fig. 2 (a). The SEM images at 4 kV in Fig. 4 (a) and (b) are used for the segmentation of secondary phases. However, automated segmentation was impossible, as the grayscale contrast was not high enough (cf. Fig. 4 (e), (f)), and filtering was inefficient. Therefore, the slight differences in grayscale values were differentiated manually using the open source graphics editor GIMP. It should be mentioned that a manual segmentation depends on subjective perception. The human eye, conversely, allows a highly sophisticated determination of material phases, not easily matched in accuracy by automated segmentation. 2.4. Elemental composition The elemental composition of this complex cathode/electrolyte interface region was now analyzed on the nanoscale by HAADFSTEM in combination with EDXS. It was then correlated to the acquired grayscale information [36,37]. Fig. 5 (c) exemplarily shows the GDC-YSZ interface region of a sample sintered at 1250 C. The chemical composition at different positions (1e4) was quantitatively analyzed and is given in Table 1. The elemental compositions at position 2 and 3 are of great interest because here two different secondary phases can be determined at the interface between GDC
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Fig. 2. Schematic representation of the method presented in this work. (a) FIB/SEM tomography (SEM accelerating voltage: 1.3 kV) of the cathode/electrolyte interface allows a 3D representation of the primary material phases La0.58Sr0.4Co0.2Fe0.8O3-d (LSCF), Gd0.2Ce0.8O2-d (GDC) and 8 mol% Yttria stabilized Zirconia (YSZ). (b) Additional correlation of STEM/ EDXS and FIB/SEM images (4 kV) enables determination of the secondary material phases (SZO and ID). (c) Resulting 3D microstructure containing both primary and secondary material phases.
insights are discussed in the following sections. 3.1. Accumulation of secondary phases at different co-sintering temperatures
Fig. 3. Illustration of the advanced operation procedures for 3D reconstruction: tomography cycle; automated alternating FIB milling (step 1) and SEM imaging (Everhart-Thornley and Inlens detector) at a defined accelerating voltage of 1.3 kV (step 2), interrupted by manual SEM imaging (step 3) at an accelerating voltage of 4 kV (the different contrast in grayscale reveals the secondary phases).
and YSZ. The EDXS data at position 2 reveal a layer of SZO (Sr:Zr ratio of 1:1) and negligible concentrations of other cations. Position 3 discloses an atomic ratio for Ce:Gd:Y:Zr:Sr of 15:23:12:49:1, indicating a markedly Gd- (and Y-) enriched ID, which is in agreement with literature [11,39]. For simplification, this region is treated as one chemically homogeneous phase, with an exemplary ionic conductivity (cf. Table 2). Position 1 confirms the chemical composition of a nearly unaffected GDC interlayer with Ce:Gd:Y:Zr:Sr as of 74:18:2:5:1, whereas position 4 confirms the composition of the YSZ thin-film electrolyte with Ce:Gd:Y:Zr:Sr as of <1:<1:14:84:<1. Transferring this information into color-coded images, Fig. 5 (c) represents (from top to bottom) the GDC interlayer (orange) with pores (black), SZO grains of <1 mm (turquois) meshed with the GDC and ID (green), but not connected to the electrolyte YSZ (yellow). LSCF (blue) is not contained in this section. It is obvious that the appearance of the secondary phases in Fig. 5 (c) agrees well with the spatial organization of the SEM grayscale contrast in Fig. 5 (a) and (b). This coincidence facilitates the transcription of the grayscale information from ETD and Inlens detector at 4 kV into SZO (turquois) and ID (green) (cf. Fig. 5 (d)-(f)). 3. Results and discussion The procedure described in the previous section was then applied to the samples co-sintered at four different temperatures, and furthermore, the elemental information derived by STEM/EDXS from a sample co-sintered at 1250 C was transferred to a correlative 3D tomography from the sample co-sintered at 1300 C. New
Fig. 6 displays the segmented 2D images of the FIB-polished cross-sections after screen-printing GDC on the YSZ electrolyte, co-sintering at 1100 (a), 1200 (b), 1300 (c) and 1400 C (d) and sintering the LSCF cathode in a second step at 1080 C. The porous LSCF cathode (white) is top, and it should be noticed that the GDC interlayer (middle gray) always remains porous, while pore size increases with co-sintering temperature. Beneath the porous GDC interlayer, the turquois areas indicate SZO formation, while the green areas indicate the extension of the Gd- (and Y-) enriched ID passing over into unaffected YSZ (dark gray). The sequence in Fig. 6 (a,b,c,d) gives clear evidence that the amount, proportion and spatial organization of the two secondary phases depend on cosintering temperature. The samples co-sintered at 1100 and 1200 C (Fig. 6 (a) and (b)) are characterized by two continuous layers (each 1 mm) fitted between porous GDC and dense YSZ. The thickness and uneven shape of both secondary phase layers are not significantly altered within this temperature range. It is worth noting at this point that moderate co-sintering promotes the formation of a continuous ion-blocking layer of SZO and a continuous ID. This finding underlines that a moderate co-sintering temperature may not be a preferential treatment of anode-supported solid oxide fuel cells with screen-printed GDC interlayers on YSZ electrolytes. Co-sintering at 1300 C further promotes SZO formation (Fig. 6 (c)), the extension of the SZO layer doubles but becomes intermittent, whereas the Gd- (and Y-) enriched ID extends slightly more into the YSZ electrolyte. The change in spatial organization of SZO is of great importance, as it is expected that every gap in the SZO opens diffusion paths for O2--ions from the cathode to the electrolyte. As already reported by our group [24,36], and shown in Table 3 for the symmetric cells analyzed therein, the area specific polarization resistance ASRcat drops by two orders of magnitude when the co-sintering temperature of the half-cells is raised above 1250 C. (A detailed study on the effects of different co-sintering temperatures on electrochemical performance is not within the framework of this study. We will soon publish a related paper.) This pronounced spreading of SZO was validated by grayscale analysis and elemental correlation, and was exemplarily shown in three additional 2D SEM images (Fig. 6 (e,f,g)). In conclusion, the analyzed image sections are representative. However, co-sintering at the even higher temperature of 1400 C completely changes the picture (Fig. 6 (d)): the commonness of SZO at the analyzed interface region declines drastically.
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Fig. 4. SEM images and corresponding histograms: (a) Inlens detector (I: 1.3 kV, III: 4 kV); (b) Everhart-Thornley detector (II: 1.3 kV, IV: 4 kV); (c) the clearly distinguishable peaks in the histogram allow a segmentation of the primary material phases LSCF, GDC and YSZ; (d) a better contrast between pore and material phase using ETD allows a segmentation of the pore phase; (e) secondary phases which are visible in (a) cannot clearly distinguished in the histogram; (f) secondary phases show a different contrast in grayscale compared to (e).
This positive effect correlates to a drastic increase of the ID thickness, here highlighted in green. A detailed STEM-EDXS study [41] on the sample co-sintered at 1400 C, disclosed a Gd-rich (and Zr-rich) region adjacent to the YSZ electrolyte and a Ce-rich (and Zrpoor) upper region adjacent to the GDC. It is speculated that this dense upper Zr-poor region acts like the initially desired dense GDC interlayer: it separates the porous GDC (Sr is evaporated as Sr(OH)2 into pores) from a Zr-rich region impeding SZO formation. Again, it must be emphasized that this contribution concentrates on visualizing the spatial organization of primary and secondary phases by correlative tomography. Thus, it is restricted to the most important details at the micro- and nanoscales. A more detailed study of the interplay between fabrication procedure, density/porosity and elemental composition at this highly complex and extended interface region is in preparation and will be published shortly by Wilde et al.
3.2. Preferred oxygen transport pathways So far it has been shown that the amount, proportion and spatial organization of the ion-blocking SZO phase is tremendously influenced by the fabrication procedure of each cell, i.e. by the cosintering temperature of the GDC/YSZ interface. We assume that once oxygen ions are incorporated into the LSCF cathode they will (i) either be blocked at a continuous SZO layer, associated with a very high cell resistance, or if possible, (ii) bypass ion-blocking SZO areas and continue through the ID into the YSZ electrolyte, resulting in a markedly lower cell resistance. This would require a sufficient number of “preferred oxygen pathways”, i.e. direct contact between LSCF cathode/GDC interlayer/ID/YSZ electrolyte. As proof of concept, a new routine counting the direct contact between GDC interlayer and ID was implemented in MATLAB and applied to the 2D SEM images already discussed in
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Fig. 5. Original SEM image of the 1300 C sample obtained with (a) the ETD at 4 kV and (b) the Inlens detector at 4 kV. (c) Color-coded assignment of phases in a cross-section of the GDC-YSZ interface (GDC sintering temperature: 1250 C) derived from an EDXS elemental mapping; a line-scan (cf. Table 1) reveals (1) GDC, (2) SZO, (3) ID, and (4) YSZ. (d) Edited SEM image of the 1300 C sample obtained with the ETD at 4 kV; comparing the contrast in grayscale with (c) allows assignment of the SZO phase (turquois). (e) Edited SEM image (Inlens detector at 4 kV); the information of (c) and (d) are used to additionally assign the ID (green) as a further secondary phase.(f) SEM image shown in (a) with assigned secondary phases. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Atomic ratio of the different cations in Fig. 5 (c), shown at different positions of the line-scan. The precision of the composition analysis is ~3 at%. Line-scan position
1 2 3 4
Atomic ratio [%] Ce
Gd
Y
Zr
Sr
74 <1 15 <1
18 <1 23 <1
2 <1 12 14
5 50 49 84
1 49 1 <1
Table 2 Ionic conductivities at 800 C. Material
Ionic conductivity s [S m-1]
GDC SrZrO3 Ce0.8Zr0.38Gd0.18Y0.07O1.87 YSZ
8.70a 1.87 E04b 1.25 E01a 5.40a
a b
[19]. Computed from Ref. [40].
Fig. 6. The number of overlapping pixels, herein abbreviated as GDC/ID, was counted for each row of pixels parallel to the cross section (starting at position 0 nm) and divided by the maximum of possible pixels (width of the cross section, in pixels). So a value of GDC/ID ¼ 1 for the intensity of the contact area would imply a GDC
interlayer in full contact with the ID over the whole cross section. Vice versa, GDC/ID ¼ 0 would imply that no transport path for oxygen ions exists in the analyzed data set. Additionally, the SZO intensity distribution was counted by the number of pixels normalized by the width of the same cross section. The resulting ratios, denominated in Fig. 7 as “Intensity contact area GDC/ID” and as “Intensity SZO distribution”, are plotted as intensity signals along the 2D SEM images, and look very different for co-sintering temperatures between 1100 and 1400 C. At 1100 and 1200 C, the ion-blocking SZO forms a continuous layer in a well confined region, which is a few hundred nm thick (SZO intensity ¼ 1 at a position around 4000 nm) in Fig. 7 (a) and (b). No preferred oxygen transport pathways exist in these samples and the intensity of the contact area of GDC/ID is zero. Fig. 7 (c) depicts the SZO intensity signal at 1300 C: it broadens significantly, but it does not exceed intensity ¼ 0.65. In the same confined region, the contact area GDC/ID now reaches a peak intensity ¼ 0.04, indicating a (small) interface area that is now available for preferred oxygen transport pathways. At 1400 C, depicted in Fig. 7 (d), it is here statistically proven, that the contact area GDC/ID has further increased (peak intensity of 0.18), whereas the intensity of SZO has decreased to 0.18. As already discussed, the drop off in SZO intensity at 1400 C goes hand in hand with a broader extension of the ID, which has split up into a Ce-rich (and Zr-poor) upper and a Gd-rich part adjacent to the YSZ electrolyte [41]. This examination predicts a high cell resistance for a GDC/YSZ
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Fig. 6. Segmented SEM images of the GDC layer and its interfaces to the LSCF cathode and YSZ electrolyte, sorted by increasing GDC sintering temperature (1100e1400 C, (a)e(d)). The turquois shaded areas indicate SZO formation, while the green shaded areas represent ID. (e)e(f) Validation of the microstructure (TSinter,GDC ¼ 1300 C) revealed in (c) at different sample positions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3 Measured area specific polarization resistance, ASRcat, and ohmic resistance, ASRU, of the symmetric cathode half cells as a function of the GDC/YSZ co-sintering temperature, measured at 800 C. TSinter,GDC
ASRU
ASRcat
U cm2
U cm2
0.251 0.258 0.230 0.249
5.329 3.100 0.037 0.007
C
1100 1200 1300 1400
co-sintering temperature of 1100 and 1200 C (oxygen ions are blocked at the continuous SZO layer), a decrease in cell resistance for the 1300 C cell (SZO layer is not continuous, but still spread out) and by far the lowest resistance for the 1400 C cell (SZO layer is full of gaps). Table 3 lists polarization resistance, ASRcat, with coherent dependency on the GDC/YSZ co-sintering temperature, and ohmic resistance, ASRU, with negligible changes. While ASRcat, is in the region of several Ucm2 for a co-firing temperature of 1100 and 1200 C, it drops by about two orders of magnitude from 3.100 Ucm2 at 1200 C to 0.037 Ucm2 at 1300 C, and even lower to 0.007 Ucm2 at 1400 C. In our experience, the size of the last two ASRcat values is common for the polarization contribution of a nominally identical LSCF/GDC/YSZ interface at 750 C in high performance anode-supported cells, when GDC was co-sintered with YSZ at 1300 C. An initial value of 0.015 Ucm2 was quantified by an
adequate equivalent circuit model, as reported by Endler-Schuck et al. [42]. Endler-Schuck monitored the course of the ASRcat of a nominally identical LSCF/GDC with respect to operation temperature (600, 750 and 900 C) and time (up to 1000 h). A thorough analysis of all data sets lead to the outcome that the degradation of the ASRcat increases with decreasing temperature. After 1000 h of operation, the ASRcat at 900 C raised only by 0.012%/h, and the largest part of the degradation was assigned to the Ni/YSZ anode. Kiebach et al. concluded that Sr diffusion leading to SZO formation occurs during firing of the cell. No SZO formation during operation (1600 h under current load) at 700 C was found, verified by SEMEDS, HR-TEM-EDS and TOF-SIMS analysis [43]. This finding supports our assumption that most of the SZO formation takes place during sintering of the LSCF cathode. It is therefore insignificant for the degradation of anode supported cells. Precondition seems to be a sintering of the GDC interlayer/YSZ electrolyte system at high temperatures, in our case 1300 C. More evidence on correlation between SZO distribution, ASRcat and GDC sintering temperature will be published soon in a separate paper. However, it must be noted that processing parameters of anode-supported solid oxide fuel cells are extremely varied among different cell developers, and therefore findings should not be transferred between research groups without reflection. Nevertheless, we will conduct a FIB/SEM grayscale analysis, as described in this work, on anode-supported cells after 1000 h of operation under various conditions in the near future. The spatial organization of SZO (cf. Fig. 6) and the availability of oxygen ion transport pathways (cf. Fig. 7) seem to be
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Fig. 7. Plots of contact area (preferred oxygen transport pathways) between ID and GDC layer in dependence to the position on the cross section. The figures are ordered by GDC sintering temperature. The SZO distribution (number of pixels normalized by the width of the cross section) was added for better illustration of contact area location. Note the different dimension between the axes.
vital for understanding the effect of GDC/YSZ co-sintering temperature. 3.3. Correlated 3D tomography As outlined in Section 2, correlative 3D tomography was applied to the most challenging distribution of primary and secondary phases, represented in the sample with a GDC/YSZ co-sintering temperature of 1300 C. A volume of 10.7 14.7 1.4 mm3 was investigated, applying a resolution of 25 25 50 nm3 (cf. Fig. 8 (a)). Each single primary and secondary phase is represented as an individual material phase and can be separately visualized using the software Blender. The colors yellow, green, turquois, brown, blue and dark gray were assigned to the material phases YSZ electrolyte, ID, SZO, GDC interlayer, LSCF cathode and pore phase, respectively. Fig. 8 (b) shows that the porosity within LSCF and GDC is high, but the porosity suddenly drops off at the surface of the SZO layer, meaning that no pores are in direct contact with the ID. This indicates that SZO is formed at the interface of the ID: i. Sr is transported via bulk and surface diffusion along LSCF and GDC or via gas diffusion as Sr(OH)2 in a pore space. ii. Zr diffuses from the YSZ electrolyte through the ID.
iii. Zr reacts with Sr to SZO. Fig. 8 (c) shows the spreading of SZO grains into the GDC interlayer, and the top down view in Fig. 8 (d, e) indicates that SZO has pores, which are filled by GDC or by GDC/YSZ solid solution, and thereby enable oxygen ion transport from the LSCF cathode via these ion-conducting phases to the YSZ electrolyte. More details on the correlation between cell manufacturing parameters, SZO distribution (continuous or intermittent), and the functionality of the GDC layer are discussed elsewhere [24,36,37,44,45].
4. Conclusions Correlative tomography was applied to analyze and visualize the elemental and spatial distribution of performance limiting secondary phases which develop at the cathode/electrolyte interface of solid oxide fuel cells, composed of a Lanthanum Strontium Cobalt Ferrite (LSCF) and Yttria stabilized Zirconia (YSZ). It was shown, that a chemical reaction leading to SrZrO3 (SZO) is not prevented, but its amount and spatial distribution are influenced by an interlayer of screen-printed Gd0.2Ce0.8O2-d (GDC), when co-sintered with the YSZ electrolyte (from 1100, 1200, 1300e1400 C). Key factors assumed are thickness and lateral composition of the solid
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Fig. 8. Visualization of reconstructed 3D structure (GDC sintering temperature of 1300 C). (a) Representation of the whole structure (10.7 14.7 1.4 mm3) showing the gain in information achieved by the presented method. (b) LSCF, pore phase, SZO, ID and YSZ, revealing direct contact between pore phase and SZO. The contact between SZO and the pores shows that a Sr gas diffusion through the pores during cathode sintering leads to SZO formation at the interface. There is no contact between pore phase and ID or YSZ revealing that SZO is formed during sintering developing a layer between pore and ID. (c) SZO, ID and YSZ: pronounced spreading of the uncontinuous SZO. (d) Top down view of part of the SZO phase showing the gaps in between the structure. (e) Top down view of part of the ID: dimpled structure as a result of spread SZO distribution.
solution formed between GDC and YSZ. Visualization of elemental composition and quantitative analysis of secondary phase distribution were correlated with electrochemical performance via impedance analysis of symmetric cells (LSCF/GDC/YSZ/GDC/LSCF). The introduced method is based on FIB/SEM analysis and supported by high resolution compositional mappings obtained by STEM/EDXS. In FIB/SEM mode, four data sets from the EverhartThornley and Inlens detector at acceleration voltages of 1.3 kV and 4.0 kV delivered a broader spectrum of grayscale information and allowed a skillful segmentation into four primary and two secondary material phases. In addition, the grayscale information was correlated to the elemental distribution gained by STEM/EDXS and rendered to four color-coded 2D SEM images of samples cosintered at 1100e1400 C and one 3D reconstruction volume of 10.7 14.7 1.4 mm3. This showed the spatial organization of all primary and secondary phases of the sample co-sintered at 1300 C. As a result, it is possible to deduce a linkage between the microstructure and the resistance of the samples. A continuous SZO layer causes a high cell resistance of several Ucm2 (blocked oxygen ion transport from the cathode to the electrolyte), whereas an intermittent SZO layer (GDC co-sintering temperature of 1300 C) leads to a resistance drop by two orders of magnitude. Since further SZO decrease (GDC co-sintering temperature of 1400 C) improves the performance of the cathode/electrolyte interface only slightly, it is proposed that the accessibility of oxygen ion transport pathways across the complex cathode/electrolyte interface is crucial for cell performance. The spatial organization of SZO and the availability of oxygen ion transport pathways from the cathode to the electrolyte are vital for understanding the effect of GDC/YSZ co-sintering temperature on the performance of an anode-supported solid oxide fuel cell with mixed conducting LSCF cathode.
The area specific polarization resistance ASRcat of this specific cathode/electrolyte interface drops by two orders of magnitude, when the GDC/YSZ co-sintering temperature of half-cells is raised well above 1250 C. Transferability of our findings to anode-supported cells from other suppliers is, in general, possible, but restricted due to varying processing parameters. Acknowledgements The authors would like to thank Dr. Norbert H. Menzler from Forschungszentrum Jülich for providing the GDC and LSCF paste and cell samples investigated in this work. Sincere thanks are given to J. Packham for proofreading the manuscript. This work was supported by the European Commission [grant agreement: 278257]; the Friedrich-und-Elisabeth-BOYSEN-Stiftung and by the Deutsche Forschungsgemeinschaft (DFG) [IV 14/16-1, RA 306/17-1]. Further support by the DFG through the Center for Functional Nanostructures (CFN) is gratefully acknowledged. References [1] H. Yokokawa, H. Tu, B. Iwanschitz, A. Mai, J. Power Sources 182 (2008) 400e412. [2] L. Kindermann, D. Das, H. Nickel, K. Hilpert, Solid State Ionics 89 (1996) 215e220. [3] A. Mai, V.A.C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stoever, Solid State Ionics 176 (2005) 1341e1350. € ver, J. Power Sources 156 [4] F. Tietz, V.A.C. Haanappel, A. Mai, J. Mertens, D. Sto (2006) 20e22. [5] M.Y. Oh, A. Unemoto, K. Amezawa, T. Kawada, J. Electrochem. Soc. 159 (2012) F659eF664. [6] E. Perry Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27e34. [7] F. Wang, M. Nishi, M.E. Brito, H. Kishimoto, K. Yamaji, H. Yokokawa, T. Horita, J. Power Sources 258 (2014) 281e289. [8] S. Badwal, Solid State Ionics 52 (1992) 23e32.
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Glossary ASR: Area Specific Resistance EDXS: Energy Dispersive X-ray Spectroscopy ETD: Everhart-Thornley Detector FIB: Focused Ion Beam GDC: Gadolinium (III) Oxide (Gd2O3) doped Ceria: Gd0.2Ce0.8O2-d HAADF: High-Angle Annular Dark-Field ID: GDC/YSZ Interdiffusion Layer LSCF: La0.58Sr0.4Co0.2Fe0.8O3-d PLD: Pulsed Laser Deposition PVD: Physical Vapor Deposition SEM: Scanning Electron Microscope SIMS: Secondary Ion Mass Spectroscopy SOFC: Solid Oxide Fuel Cell STEM: Scanning Transmission Electron Microscope SZO: Strontium Zirconate (SrZrO3) YSZ: Yttrium (III) Oxide (Y2O3) stabilized Zirconia: Y0.16Zr0.84O2-d