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High-Resolution Studies on Nanoscaled Ni/YSZ Anodes Julian Szász,† Sascha Seils,‡ Dino Klotz,† Heike Störmer,§ Martin Heilmaier,‡ Dagmar Gerthsen,§ Harumi Yokokawa,∥ and Ellen Ivers-Tiffée*,† †

Institute for Applied Materials - Materials for Electrical and Electronic Engineering (IAM-WET), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany ‡ Institute for Applied Materials (IAM-WK), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany § Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany ∥ Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan ABSTRACT: The performance of Ni/YSZ anodes has been shown to substantially benefit by a reverse current treatment of a solid oxide fuel cell. Within seconds, a solid state reaction builds a nanoscaled layer at the anode/electrolyte interface, thereby increasing the triple-phase boundary density considerably. The reaction mechanism is thoroughly studied using a Ni thin film/YSZ single crystal/Ni/YSZ counter electrode as a model system. The microstructure was analyzed by scanning transmission electron microscopy (STEM) and the chemical composition by atom probe tomography (APT). With these insights, supported by thermodynamic calculations, we propose the following reaction model: The reverse current treatment reduces YSZ congruently with Ni, temporarily forming a Ni-Zr-Y alloy, and Y2O3 precipitates. Afterward, at open circuit voltage conditions, Ni-Zr-Y is reoxidized instantaneously until thermodynamic equilibrium is reached. High-resolution STEM/APT studies disclose a newly formed nanoscaled layer consisting of (i) interconnected Ni with inclusions of 10 nm (YxZr1−x)2O3 precipitates, (ii) a continuous (Y,Zr)-oxide with varying Y:Zr ratio, (iii) unaffected YSZ, and (iv) pores. The results obtained suggest that formation of the nanoscaled layer results from a diffusion-controlled mechanism, with Ni being the fastest species, but also involving Y or Zr, since alterations of the Y:Zr ratio are detected. ionic conductivity.8 Herein, electro-oxidation of H2 is strictly confined to the triple-phase boundary (TPB) between YSZ grains, Ni grains, and fuel gas in the pores (see Figure 1b). The electro-oxidation kinetics and the correlation between TPB density and anode performance was thoroughly studied on model anodes.9−12 Conventional Ni/YSZ anodes take shape within a multistep fabrication, where cosintering at ≥1300 °C ensures densification of the YSZ electrolyte and structural stability of the entire cell architecture. In consequence, Ni and YSZ grains in the anode become microscaled (∼0.6 μm) and TPB density low (2.56 μm−2), as shown in Figure 1a,c; a quantitative analysis by focused ion beam (FIB) tomography is published in the literature.13 Finite element model (FEM) calculations in Figure 2 exemplify the mutual dependence between anode performance (ASRanode) and microstructure characteristics (particle size (ps)), for identical particle sizes of Ni and YSZ.14 Ideally, by reducing the particle size of Ni and YSZ, i.e., from microscaled (ps = 1000 nm) to nanoscaled (ps = 31 nm),

1. INTRODUCTION Fuel cells with a solid oxide electrolyte (SOFC) were initially only studied for applications at high temperatures (above 850 °C) and in large systems (>1 MW). It is, however, wellknown today that the SOFC is suited for many more applications, given its high electrical efficiency, its exceptional fuel flexibility, and its equivalent usability in both fuel cell and electrolysis mode. As SOFC operation temperature has to be significantly lowered, i.e., for mobile applications, or its electrochemical efficiency has to be significantly increased, i.e., for electrolysis mode, electrode performance becomes the key to success for cell designs with thin film electrolytes (with a thickness ranging from 1 to 10 μm).1−4 Nanoscaled cathodes (air electrode) made of mixed ionicelectronic conducting perovskites such as (La,Sr)(Co,Fe)O3−δ or (La,Sr)CoO3−δ provide a significantly enlarged surface area for the oxygen reduction reaction.5 Their performance at 600 °C (expressed as area specific resistance ASR) exceeds microscaled cathodes by a factor of 10−100, depending on microstructure and composition.6,7 The most widely used anodes (fuel electrode) are a blend of metallic Ni with high electronic conductivity, acting as a catalyst for the electrooxidation of H2, and of yttria stabilized ZrO2 (YSZ) with high © 2017 American Chemical Society

Received: January 26, 2017 Revised: May 30, 2017 Published: May 31, 2017 5113

DOI: 10.1021/acs.chemmater.7b00360 Chem. Mater. 2017, 29, 5113−5123

Article

Chemistry of Materials

Figure 1. (a) Scanning electron microscope (SEM) image of the fracture cross section of an anode-supported cell with microscaled anode, consisting of Ni (bright gray), YSZ (dark gray), and pores (black). (b) Scheme of the electro-oxidation of H2 at the TPB of Ni, YSZ, and pore. (c, d) SEM images of the FIB polished cross section with high magnification of the TPB distribution (c) in the microscaled anode and (d) in the nanoscaled Ni/YSZ anode, induced by the reverse current treatment.

Figure 2. Three-dimensional finite element model (FEM), simulating the anode area specific resistance (ASRanode) at 700 °C as a function of particle size (ps) and anode thickness (lanode) for a Ni/YSZ anode with the fractions: porosity 30%, nickel 33%, and YSZ 37%.

the ASRanode is reduced by a factor of ∼30. This goes hand in hand with an increase of TPB density, and shrinks the effective anode thickness (lanode,ef fective) from 15 μm to 400 nm. The transport of electrons out of the active region via Ni grains as well as the fuel gas (exhaust) transport into (out), however, requires a substantially larger anode volume (e.g., the anodesupport) which may, therefore, still be microscaled. The question arises, however, how to increase the TPB density of SOFC anodes at the anode/electrolyte interface by employing suitable nanostructuring strategies? This has been realized by different groups and methods: (i) ex situ and posttreatment using multistep impregnation of the porous microscaled anode structure by aqueous or metal−organic liquids at room temperature,15−19 (ii) in situ and in operando exsolution

of Ni nanoparticles in perovskite based anodes,20,21 or (iii), most simply, by the reverse current treatment (RCT) of an anode-supported cell running at intermediate temperatures. The latter, reported by our group, applied just for seconds a strong cathodic polarization at 700 °C and thereby induces a layer consisting of nanoscaled Ni, YSZ, and pores. This has been shown to lead to a considerable increase of 40% in anode performance, which was studied thoroughly by electrochemical impedance spectroscopy.22 Scanning and transmission electron microscopy ((S)TEM) consolidated the formation of a nanoscaled layer at Ni grains adjacent to the YSZ electrolyte, as shown in Figure 1d. However, the limited efficiency of the energy dispersive X-ray spectroscopy (EDXS) system in our transmission electron microscope at that time yielded data with 5114


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(convergence angle: 12 mrad) and with a collection-angle range between 80 and 160 mrad. In this imaging mode, the image intensity is proportional to the atomic number Za (1.6 ≤ a ≤ 1.9 depending on the collection angle range) for a TEM specimen with constant thickness.27 The element distribution in the nanoscaled layer was determined by EDXS. The FEI Osiris is equipped with four Bruker Silicon Drift detectors for EDXS analyses (FEI ChemiSTEM Technology). The raw EDXS data were background subtracted, and quantification was only performed for the cations, using the Bruker Esprit Software and the standardless Cliff-Lorimer (thin film approximation) procedure. EDXS mappings were acquired with a resolution of 0.8 nm/pixel which yield elements distributions with a high spatial resolution. High-resolution transmission electron microscopy (HR-TEM) images were taken to analyze the crystal structure of nanoscaled grains. Phase determination of the different (Y,Zr)-oxide phases is facilitated by comparing the two-dimensional Fourier transform of small regions of HR-TEM images with simulated diffraction patterns for the crystal structures of the cubic (c-)phase (symmetry group Fm3̅m, space group number 225, lattice parameter a = 0.5147 nm), tetragonal (t-)phase (symmetry group P42/nmc, space group number 137, a = 0.3631 nm, c = 0.5153 nm), and monoclinic (m-)phase (symmetry group P121/c, space group number 14, a = 0.522 nm, b = 0.527 nm, c = 0.538 nm, and angle β = 80.54°). TEM samples were prepared by the lift-out technique using a FEI Strata 400S dual-beam system, which comprises a Ga+-ion column and a scanning electron microscope.28 To reduce preparation artifacts from the FIB thinning, final FIB polishing was performed at low voltage (5 kV). FIB based TEM specimen preparation offers the advantage of precise site-specific thinning of the Ni thin film/nanoscaled layer/YSZ single crystal interface region (with a thickness between 30 and 50 nm). Moreover, TEM specimens with a homogeneous thickness are obtained as a prerequisite for the interpretation of the HAADF-STEM image intensity in terms of material contrast. 2.2. Atom Probe Tomography. Atom probe sample tips were produced by the lift-out method on a FEI Strata dual-beam system with a single gas injection system and a micromanipulator (Omniprobe 200, Oxford). Prior to cutting, a protective Pt layer 200 nm in thickness was applied by ion beam deposition on the region of interest. FIB etching was performed at 30 kV with 260 pA at the beginning to 26 pA in the last step. Additionally, a final milling step for tip shaping was applied at 5 kV and 81 pA to prevent severe damage by Ga+ ions. APT measurements were performed in a Cameca LEAP 4000X HR system in the laser pulsing mode with a UV-laser wavelength of 355 nm. Sample tips were cooled down to 60 K. The evaporation rate was set to be 0.3% with a laser pulse rate and laser energy of 200 kHz and 50 pJ, respectively. Cameca’s IVAS 3.6.10a software was used for APT data reconstruction, visualization, and analysis of several needleshaped sample tips, revealing the same results. The in-depth analysis of one representative tip is shown here. For the reconstruction of the sample, with around 11.5·106 ranged ions, preset values were used and the tip radius was determined by the tip profile. 2.3. Thermodynamic Calculations. Thermodynamic calculations were performed with the commercially available software MALT (MAterials-oriented Little Thermodynamic database) and were evaluated by Yokokawa and co-workers.23,24 Additional thermodynamic data were taken from the literature.29

a substantially lower signal-to-noise ratio and a reduced sensitivity toward the detection of composition fluctuations, which prevented an in-depth analysis of the solid state reaction mechanism.22 For this reason, we have newly studied the solid state reaction mechanism that leads to the formation of the nanoscaled Ni/YSZ anode using a model system with a welldefined geometry made of a Ni thin film/YSZ single crystal/ Ni/YSZ counter electrode (see the Experimental Methods section). Microstructure and chemical composition of the nanoscaled layer were now analyzed by a FEI Osiris transmission electron microscope with a highly efficient EDXS system and, in addition, with atomic resolution by atom probe tomography (APT) in a Cameca LEAP 4000X HR system (see the Experimental Methods section). With these insights, supported by thermodynamic calculations using the software MALT (MAterials-oriented Little Thermodynamic database), we propose a reaction model for the solid state reaction mechanism between Ni and YSZ, during RCT and afterward, at open circuit voltage (OCV) conditions.23,24

2. EXPERIMENTAL METHODS Experiments were performed on a model system, consisting of a Ni thin film/YSZ single crystal/Ni/YSZ counter electrode. A YSZ single crystal (9.5YSZ − 19 at. % Y stabilized ZrO2, CrysTec GmbH, 500 μm thickness, size 1 cm2) was contacted subsequently on both sides by an anode. First, a NiO/8YSZ paste was screen-printed and dried for 24 h at 70 °C and subsequently sintered at 1300 °C for 3 h in air. NiO was reduced to Ni in a hydrogen atmosphere at 800 °C and then cooled down with a protective gas mixture (0.05 atm H2 in 0.95 atm N2) to achieve a porous Ni/YSZ counter electrode, 15 μm thick. Second, a Ni thin film was deposited by magnetron sputtering on the cleaned and polished side of the YSZ single crystal at 600 °C in an argon atmosphere pArgon = 5.5·10−11 bar (4.13·10−8 Torr). This produced a dense and uniform Ni thin film (thickness: 400 nm). The film thickness and deposition temperature guarantee sufficient stability against Ni agglomeration due to dewetting of the Ni film at elevated temperatures and atmospheric conditions.25 Another benefit is the easier preparation for TEM and APT analysis, since a sufficiently thick Ni thin film remains on top, which protects the nanoscaled structures during preparation by FIB cutting. The setup for electrochemical experiments is a one-gas atmosphere setup and is described elsewhere.26 Inside the chamber, the model system is fixed between two Al2O3 flow fields (area: 1 cm2). A Ni mesh is used to ensure electronic contact across the entire Ni thin film surface for a homogeneous electrical potential distribution during the RCT. The desired fuel gas mixture (0.055 atm H2O in 0.945 atm H2) was achieved by mixing H2 and O2 in a preburner chamber with a total flow rate of 500 sccm. The flow was accurately adjusted by Bronkhorst mass flow controllers, and the correct fuel gas mixture was verified by a Nernst probe close to the model system. All electrochemical experiments followed script controlled conditions (temperature, fuel gas mixture, time) to ensure reproducibility. The strong cathodic polarization during the RCT was applied with an external load (Novocontrol Alpha-AK analyzer and a Novocontrol POT/GAL10V/15A potentiostat) which recorded the current/voltage behavior as a function of time, during the RCT and afterward at OCV conditions. The model system was analyzed “post-test” and only a single RCT was conducted, with predefined parameters (jRCT = −2 A·cm−2, 700 °C). 2.1. Electron Microscopy. The model system was investigated at different stages of preparation using scanning electron microscopy (Carl Zeiss GEMINI 1540XB) to verify thickness and the quality of both, Ni/YSZ counter electrode and Ni thin film. The nanoscaled layer was characterized with a FEI Osiris transmission electron microscope operated at 200 kV. High-angle annular dark-field scanning transmission microscopy (HAADF-STEM) imaging was typically performed under conditions which yield a beam diameter of 0.4 nm

3. RESULTS AND DISCUSSION 3.1. Microstructure. 3.1.1. Transmission Electron Microscopy. The impact of the RCT can be intuitively realized comparing the model system before the RCT (“as prepared” in Figure 3a) and after the RCT has been conducted (“post-test” in Figure 3b). The Ni thin film shrinks from 400 to 200 nm (“post-test”), indicating that the position of the original Ni thin film/YSZ single crystal interface is close to the center of the nanoscaled layer. Figure 3c shows a HAADF-STEM image of a similar region as Figure 3b at higher magnification. Three regions can be clearly distinguished: (i) the dense Ni thin film 5115


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Figure 3. FIB-polished SEM cross-sectional images of the model system: (a) “as prepared”, before the RCT, and (b) “post-test”, after the RCT. (c) HAADF-STEM cross-sectional image of the model system “post-test” after the RCT showing (i) the Ni thin film (light gray), (ii) the nanoscaled layer consisting of Ni (light gray) with dark dots inside, (Y,Zr)-oxide (medium to dark gray), and pores (black), (iii) the YSZ single crystal (dark gray). (d) Small section of a HR-TEM image of the nanoscaled layer.

10 nm in diameter. The (Y,Zr)-oxide has a speckled appearance (medium to dark gray) which is indicative of composition and/ or strain variations. Figure 4b presents the results of the corresponding EDXS mapping with color coded elemental distributions representing Ni (gray), Y (yellow), and Zr (red). On the basis of the comparison of the HAADF-STEM image and the elemental distribution, the complex structure of the nanoscaled layer can be better recognized. The representative phases are additionally marked by white circles in Figure 4b, indicating (i) metallic Ni, which contains (ii) Y-rich precipitates, and (iii−iv) an interconnected (Y,Zr)-oxide with varying Y:Zr ratio. The variation of Y:Zr ratio is shown in the compositional images in Figure 4c,d from the marked region in Figure 4a. Zr and Y distributions clearly indicate that the (Y,Zr)-oxide contains Zr-rich and Y-rich regions and that the Y-rich precipitates contain small amounts of Zr. The quantitative Zr- and Y-concentration profiles in Figure 4v were extracted from the EDXS mapping. The composition-line profiles show that the Y:Zr ratio varies between 3:97 and 36:64 over a distance of 50 nm (with 1−2 at. % precision). The range of observed Y:Zr ratios by STEM-EDXS and APT (as will be shown in the following section) is well compatible with the results of the HR-TEM based phase analyses. According to the phase diagram,32 we expect the c-phase for Y:Zr ratios ≥ 19:81, the t-phase at Y:Zr ratios between 18:82 and 2.5:97.5, and the m-phase at Y:Zr ratios below 2:98 at 700 °C. Further EDXS analyses of the YSZ single crystal (not shown here) revealed a Y:Zr ratio of 20:80 (which agrees well with the nominal composition of 19:81), thus ensuring that the YSZ single crystal has remained unaltered. Neither metallic Zr nor intermetallic Ni-Zr alloys were found in the analyzed regions; in particular, Ni and Zr were only mutually exclusively detected. We note that the O concentration was also qualitatively analyzed and is high in the Y- and Zr-containing regions. It is, therefore, justified to assume that, after the RCT (“post-test”), all (Y,Zr)-phases are present as oxides besides the metallic Ni phase. 3.1.2. Atom Probe Tomography. APT analysis is an ideal supplement to the STEM analyses when studying alloys and

(light gray) with a wavy interface toward the (ii) nanoscaled layer, with a thickness of about 300 nm, and (iii) the YSZ single crystal (dark gray). The composition sensitivity of the HAADFSTEM image suggests that the layer is composed of nanoscaled grains of Ni (light gray), (Y,Zr)-oxide (medium to dark gray regions), and pores (black regions). We note that some Ni grains in Figure 3c contain dark dots. The nanocrystalline structure of the layer is verified by HR-TEM imaging (see small section of a HR-TEM image in Figure 3d). Ni and different (Y,Zr)-oxide phases can be distinguished by the determination of the local crystal structure. Analyzing the grains in 9 HR-TEM images, we find that the c-phase dominates. However, the t-phase is also present in about 25% of the analyzed grains and one grain in the m-phase was identified. The microstructure appears similar to our previous analysis on anode-supported cells, wherein a nanoscaled Ni/YSZ anode (therein denoted as nanostructured Ni/YSZ interlayer - NSI) was formed between Ni grains which were adjacent to the YSZ electrolyte, as shown exemplarily for one individual Ni/YSZ contact in Figure 1d. It further illustrates the potential of an in operando increase of TPB density to such an extent that the electro-oxidation kinetics of H2 may completely occur within an anode layer thickness of a few hundred nanometers. However, this nanoscaled layer is only productive, if the following requirements are fulfilled: the Ni phase, transporting the electrons (e−), is interconnected from the TPB to the adjacent microscale anode, as well as the YSZ phase, transporting oxygen ions (O2−), is interconnected from the YSZ electrolyte to the TPB. Advantageously, the YSZ phase should consist of YSZ with maximum ionic conductivity, i.e., between 16 and 20 at. % Y.30,31 For all of these reasons, it is of utmost importance to carefully examine the chemical composition and spatial arrangement of the nanoscaled layer by high-resolution STEM combined with energy dispersive X-ray spectroscopy (EDXS) and with atomic resolution by APT, ideally throughout its entire thickness. These analysis techniques are now available and disclose important new features. In Figure 4a, a HAADF-STEM image of the nanoscaled layer is shown at higher magnification. The Ni phase (light gray) contains distinct, dark dots with about 5116


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Figure 4. (a) High magnification HAADF-STEM image of the nanoscaled layer and (b) corresponding EDXS mapping with color coded elemental distribution of the intensity of the Ni (gray), Zr (red), and Y (yellow) signal. (c, d) Compositional images of the Zr and Y signal, respectively. (v) The Zr- and Y-concentration profile indicated by the white arrow in (a)−(d).

compared to the STEM-EDXS results. The Y+Zr concentration constantly amounts to 35 at. %, whereas O concentration amounts to 65 at. %, thus suggesting the presence of a (YmZr1−m)O2 phase. This highly localized concentration profile indicates an even higher decomposition of the YSZ single crystal with its nominal Y concentration of 19 at. %; the Y:Zr ratio varies from 0:100 to 65:35. The overall lower Y content can be explained by the presence of (YxZr1−x)2O3 precipitates embedded in the interconnected Ni phase. In summary, the results obtained by both STEM-EDXS and APT suggest that formation of the nanoscaled layer results from a diffusion-controlled mechanism, with Ni being the fastest species, but also involving Y (or Zr) as mobile species, since alterations of the Y:Zr ratio can be detected throughout the analyzed sample tip volume of the nanoscaled layer.35,36 Hence, already a partial decomposition of Y0.19Zr0.81O2 into (YxZr1−x)2O3 precipitates and a continuous (YmZr1−m)O2 phase, in both cases with varying Y:Zr ratios, leads to the assumption that the ionic conductivity of all oxide phases no longer amounts to the maximum value only, but may also be significantly reduced.30 3.2. Thermodynamic Considerations. In the following, the solid state reaction observed at the Ni/YSZ interface in the model system shall be supported by thermodynamic calculations with the software package MALT. The stability of the individual phases Ni, ZrO2, and Y2O3 is initially considered at ambient pressure as a function of temperature (T) and oxygen partial pressure (pO2). Figure 6 shows the phase equilibria between metal and metal oxide for T = 600−850 °C and for pO2 = 10°−10−70 atm. Each solid line represents a borderline;

nanoscaled precipitates less than 10 nm, as the elemental distribution of all atomic species, namely, Ni, Zr, Y, and oxygen, can be quantitatively determined with atomic resolution.33,34 A needle-shaped sample tip of approximately 3 μm in length was prepared from the model system by FIB (see the Experimental Methods section). The analyzed part of the sample tip (Figure 5a, left) contained portions of the Ni thin film, the nanoscaled layer in its entire thickness, and the interface to the YSZ single crystal. The results of APT analyses are shown in Figure 5b−d as an elemental atom mapping of Ni (gray), Zr (red), Y (yellow), and O (blue). Comparison with Figure 4, therefore, specifies the STEM-EDXS findings on the atomic scale: an interconnected Ni phase is present in metallic form (Figure 5d,e), since the O signal is superimposed solely with the Zr and Y signals. Y-rich precipitates with a diameter of approximately 10 nm are embedded into the metallic Ni, as can be seen in the disc-shaped cutouts with 5 nm thickness, respectively. In Figure 5 (ii) and (iii), concentration profiles were carried out in these sample sections, showing that these Y-rich precipitates exhibit a Y+Zr concentration of about 40 at. %, whereby the Y:Zr ratio determined at a total of four positions varies between 100:0 and 50:50. As the O concentration always remains at 60 at. %, this yields a (YxZr1−x)2O3 phase composition. The interconnected (Y,Zr)oxide shown in Figure 5c shows also a varying Y:Zr ratio. A concentration profile (Figure 5 (i)) of the phase, carried out inside a cylinder over a distance of 75 nm (the position is indicated by a black arrow in Figure 5c), yields the O concentration and a spatial elemental distribution of Zr (red), Y (yellow), and Y+Zr (black) with a higher resolution, as 5117


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Figure 5. (a) Needle-shaped sample tip with (left) the part analyzed by APT with a volume of 830 531 nm3. (b)−(d) Elemental atom mappings of (b) Ni (gray), Zr (red), Y (yellow), with disk-shaped cutouts showing Y-rich precipitates embedded in the Ni phase with (ii) and (iii) exemplary concentration profiles at positions, encircled in (e). (c) The (Y,Zr)-oxide (Ni-regions above 40 at. % are cut out) with (i) the concentration profile of Y, Zr, and O inside a cylinder indicated by the black arrow in (c). (d) Ni, O (blue) signal. (e) The interconnected isosurface at 40 at. % Ni.

As mentioned above, the stabilization energy of the solid solution YSZ is neglected. Directly following the beginning of the RCT, the measured cell voltage increases to 2.44 V in the case of an anodesupported cell (ASC), which corresponds to a pO2 of 5.9·10−52 atm, applying the Nernst equation against air.37 In the case of 10YSZ, the cell voltage is already higher than the decomposition voltage, calculated from Weppner’s data (2.3 V at 700 °C against air, equal to a pO2 = 4.7·10−49).38 This can be

above each line, the representative constituents (NiO, ZrO2, and Y2O3) are oxidized. Below each borderline, each species is reduced and appears as metallic Ni, Zr, or Y. Prior to the beginning of the RCT, i.e., at OCV conditions, the pO2 is determined by a Nernst probe operated against air, yielding a value of 4.8·10−24 atm at 700 °C for a fuel gas composition of 0.055 atm H2O in 0.945 atm H2. This value is shown as a black circle in Figure 6. Metallic Ni coexists with both cations present in YSZ in form of their oxides, namely, ZrO2 and Y2O3. 5118


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Figure 6. Phase equilibria at ambient pressure as a function of temperature: (solid lines) for the Ni-O, Zr-O, and Y-O systems. Calculated oxygen partial pressure (pO2) at the Ni/YSZ interface: (black circle) at OCV conditions (pO2 = 4.8·10−24 atm) and (green square) during the RCT, when ZrO2 is reduced to Zr (below pO2 = 1.58·10−49 atm, indicated by the green arrow). Figure 7. Thermodynamic equilibria at ambient pressure and 700 °C of the Ni-Zr-Y-O system: calculated (a) at OCV conditions (pO2 = 1·10−24 atm), (b) during the RCT (pO2 = 1.58·10−49 atm). All phases appear in the solid state.

understood as the voltage necessary to break up Coulomb forces between cations (Zr4+) and anions (O2−) along with severe structural changes. Weppner reported further that, at this point, the electronic and the ionic partial conductivities of YSZ attain comparable values.39 However, in the case of the model system, the exact pO2 cannot be calculated with the Nernst equation against air because both electrodes are exposed to the same hydrogen fuel gas composition. We assume, therefore, a pO2 during RCT below ∼10−49 atm for the model system, which is marked in Figure 6 by the green square and arrow. Hence, the decomposition voltage of ZrO2 is reached and most likely ZrO2 is reduced to metallic Zr, whereas Y2O3 still remains in the oxidized state. This statement will be discussed later in relation to the solid state reaction mechanism. In the following, the stability discussion is continued considering possible interactions between the individual phases, thereby taking into account that the YSZ single crystal is in direct contact with the metallic Ni thin film in the model system. Figure 7 shows the chemical potential diagram of the Ni-Zr-Y-O system at OCV conditions, and during the RCT at 700 °C. At OCV conditions, the Ni, ZrO2, and Y2O3 phases are all stable (see Figure 7a), in accordance with Figure 6. The solid solutions of YSZ in its cubic and the tetragonal phases are not shown in Figure 7a because the present set of the thermodynamic data for the Y2O3-ZrO2 system indicates that ZrO2 and the Y4Zr3O12 phase are in equilibrium. However, in realistic environments existing in fuel cells, a decomposition of

YSZ into ZrO2 (monoclinic) and Y4Zr3O12 phase is not observed. Instead, the transformation into the coexisting cubic and tetragonal phases takes place, leading to the conductivity degradation at low temperatures.40 Although the diagram in Figure 7 does not reveal such transformation effects, we will consider this effect in the following, if necessary. During the RCT, the thermodynamic equilibria of the initial phases strongly change, as shown in Figure 7b at pO2 = 1.58·10−49 atm. The oxide phases Y2O3 and Y4Zr3O12 are still present, but below this specific pO2, Zr is stable solely in its metallic phase. This has already been assumed in our former publication,22 based on TEM analyses, and is now verified by STEM-EDXS and APT analysis with the model system. However, a pO2 below 10−49.2 atm is only confined to a small volume adjacent to the Ni thin film (e.g., the thickness of the nanoscaled layer), and most of the YSZ single crystal remains unaffected as an oxide phase. However, at the Ni/YSZ interface, and therefore with increased Ni activity, MALT calculations predict several stable binary intermetallic compounds, namely, NiZr2, NiZr, Ni10Zr, Ni21Zr8, Ni3Zr, Ni7Zr2, and Ni5Zr. It is important to note that, according to MALT calculations, Zr and Y may be solved in a Ni (fcc) metal. Still, the formation of a ternary Ni-Zr-Y alloy cannot be treated here by MALT as 5119


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Figure 8. (top) Current/voltage behavior during strong cathodic polarization (RCT), and afterward at OCV conditions (OCV-1) and (OCV-2). (a-1, b-1, and c-1) Schemes of the corresponding electrochemical potential profiles across the model system (Ni thin film/YSZ single crystal/Ni/ YSZ counter electrode). (a-2, b-2) The proposed reaction model and proposed microstructure for the solid state reaction mechanism that induced (c-2) the nanoscaled layer. Note that the microstructure in (c-2) is analyzed “post-test” in Figures 3−5.

RCT. Applying a strong cathodic polarization of jRCT = −2 A·cm−2, the voltage increases over 10 ms from OCV to approximately 6.5 V and then decreases exponentially down to a constant value of 2.5 V. The voltage jump up to 6.5 V is caused by Ohmic losses (ηohmic) within the 500 μm thick YSZ single crystal and the polarization losses (ηcounter electrode, ηNi thin film) in the Ni/YSZ counter electrode and the Ni thin film, respectively. In order to maintain charge carrier transport (O2− ions and e−) at a level of jRCT from the Ni thin film to the Ni/YSZ counter electrode, H2O is also, at least initially, catalytically reduced to H2 and O at the Ni thin film surface; the latter is then dissolved in the Ni phase and electrochemically reduced to O2− ions at the interface between the Ni thin film and the YSZ single crystal. We assume that a pO2 below 1.58·10−49 atm is only reached in the vicinity of this interface and causes strong alterations in a confined volume. YSZ is reduced congruently with Ni, for the most part, resulting in a fairly large amount of metallic Ni, Zr, and Y2O3. The O2− ions migrate via oxygen vacancy sites (V•• O ) to the Ni/YSZ counter electrode where they react with H2 to form H2O. The oxygen vacancies form clusters in the affected region, resulting in pores.

the Y concentration of technical alloys usually tends to be very low and thermodynamic data are incomplete. Wagner found Ni5Zr as an intermetallic compound at the Ni/YSZ interface in a similar polarization experiment which had been cooled down while maintaining the applied voltage.41 Combining results from microstructural investigations and thermodynamic calculations, we conclude that the formation of a Ni-Zr-Y alloy is the predominant mechanism during the RCT, but during the subsequent OCV phase, reoxidation to (YxZr1−x)2O3 precipitates and an interconnected (YmZr1−m)O2 phase is very likely to occur. 3.3. Reaction Model. On the basis of the results given above as well as detailed parametric studies previously published (variation of temperature, RCT duration, current density jRCT, fuel gas mixture) on ASC’s, as well as in model systems, an attempt is made in the following to elaborate a reaction model which describes the solid state reaction mechanism at the Ni/YSZ interface during the RCT.26,42,43 Figure 8 shows the current/voltage behavior of the model system during strong cathodic polarization (RCT), and after switching back to OCV conditions (OCV-1 and OCV-2). 5120


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Chemistry of Materials Ni diffuses out of the Ni thin film into the affected region and reacts with Zr (and to some extent with Y), forming a Ni-Zr-Y alloy. Moreover, a part of the Y-component in the Ni-Zr-Y alloy is reoxidized with oxygen or oxygen ions, reacts with unaffected YSZ, and forms Y-rich YSZ (YmZr1−m)O2 (Figure 8 (a-2)). This is due to a reduction−dissolution−reoxidation mechanism of the Y-component taking place in order to establish the equilibrium concentration of phases. A layer is temporarily formed between the Ni thin film and the YSZ single crystal, consisting of Ni-Zr-Y alloy, Y2O3, unaffected YSZ, Y-rich YSZ, and pores. The corresponding electrochemical potential profiles of η(O2−), η(e−), and μ(O) are shown in Figure 8 (a-1). As mentioned before, the electronic and ionic conductivities are comparable at the interface between the Ni thin film and the YSZ single crystal.41 OCV-1. Upon switching off the strong cathodic polarization jRCT, the voltage drops from 2.5 V to approximately 0.95 V within 10 ms and remains constant for about 10 s. Herein, the reoxidation of the Ni-Zr-Y alloy proceeds until a thermodynamic equilibrium is reached. The voltage of 0.95 V persisting for 10 s is assigned to the difference in oxygen potential difference between the Ni-Zr-Y alloy/YSZ interface and the fuel gas phase, as discussed in the literature (c.f. Figure 8 (b-1)).44,45 The oxygen potential of the Ni-Zr-Y alloy/YSZ interface is governed by the Zr-component which has to be reoxidized. The equilibrium is established via O2− ions supplied by the YSZ and e− in the Ni. Obviously, in this model experiment, it took about 10 s for the interface to be in equilibrium with the fuel gas.46 It should be noted here that reoxidation kinetics influence pore formation and depend, in general, on sample geometry.46,47 In the model system, reoxidation occurs rather slow, because of a rather long O2− ion diffusion pathway through the 500 μm thick YSZ single crystal and impeded fuel gas access of the dense Ni thin film. The Y and Zr solubility in the Ni-Zr-Y alloy decreases at first; then, Zr is reoxidized on the temporarily existing Y2O3 precipitates, changing their composition into (YxZr1−x)2O3 and, to a larger extent, on the Y-rich YSZ phase (YmZr1−m)O2. Since a large part of the Y has segregated from the YSZ during reverse current treatment in the form of (YxZr1−x)2O3 precipitates in the Ni, the freshly formed (YmZr1−m)O2 phase is Zr-rich and does not form the c-, but rather the t-phase.40,48 Please note that, at 700 °C, cubic and tetragonal phases coexist, meaning that no kinetic barrier for the nucleation and growth of tetragonal phases exists. OCV-2. Here, the thermodynamic equilibrium is established, the voltage drops to 0 V, and the model system shows the final microstructure that is analyzed “post-test” in Figure 3 and the chemical composition that is analyzed in Figures 4 and 5. According to these results (see Figure 8 (c-2)), the nanoscaled layer is composed of the following: i. an interconnected Ni phase with (YxZr1−x)2O3 precipitates (size 10 nm) embedded in ii. an oxide with a varying Y:Zr ratio: • Zr-rich YSZ (t- and m-phases), • Y-rich YSZ (c-phase), both categorized as (YmZr1−m)O2 including iii. unaffected YSZ (cubic, Y:Zr ratio of 19:81) and iv. pores. Comparing the results presented here with findings of Klotz et al. confirms the assumption that, during RCT (Figure 8 (a-2)), Ni diffuses toward the affected YSZ region,

and forms a layer containing the elements Ni, Zr, and Y, herein denoted as a Ni-Zr-Y alloy.22 However, it is now backed by thermodynamic calculations (MALT) and considerations that the Y-component is temporarily present predominantly as finely dispersed Y2O3 within the Ni-Zr-Y alloy or reacts with unaffected YSZ to form Y-rich YSZ (YmZr1−m)O2, respectively. The existence of a multicomponent system in the reduced state inevitably results in reactions differing from our former reaction model as soon as the RCT is finished. The composition of the nanoscaled Ni/YSZ anode is complex, which is now demonstrated by using high-resolution STEM-EDXS and APT on an experimentally more easily accessible model system. During OCV-1 and OCV-2 (Figure 8 (b-2) and (b-3)), Ni is present as an interconnected phase, thus enabling the transport of electrons from all TPB points toward the anode support. At the same time, the Zr contained in the Ni-Zr-Y alloy (and, to a lesser extent, also Y) is reoxidized and reacts either with the Y2O3 precipitates embedded in the Ni, forming (YxZr1−x)2O3, or with Zr-rich YSZ (YmZr1−m)O2.22 As a result, there is a very significant difference regarding the ionic-conducting phase in the nanoscaled layer: it is not single-phase YSZ, but a (YmZr1−m)O2 phase (with a spatially strongly varying Y:Zr ratio) which is present in the c-, t-, and m-phases. We have to bear in mind that the model system is made of 9.5YSZ instead of 8YSZ used in the anode-supported cell. The diffusivity of O2− ions in YSZ, which we formerly assumed to be even slightly higher, decreases in this multiphase YSZ matrix in any case. Recalling the total conductivity of YSZ as a function of Y dopant concentration, this would lead to a conductivity degradation at worst by a factor of 10.30 On the other hand, the nanoscaled layer shrinks the diffusion length of O2− ions by a factor of 50 (see Figure 2), and enlarges the TPB density substantially (see Figure 1d). All in all, the complex solid state reaction mechanism has become quite well illuminated, and a RCT, forming within seconds such a nanoscaled layer in Ni/YSZ anodes, has been shown to lead to a considerable benefit in performance. Naturally, future studies should emphasize on performance issues at large time scales.

4. CONCLUSION This work shows for the first time an in-depth analysis of the microstructure and of the chemical composition of a nanoscaled Ni/YSZ anode that can be generated by a simple reverse current treatment. As previously reported by Klotz et al., this in operando process leads to an increase in anode efficiency by up to 40% in a solid oxide fuel cell.22 We have newly studied the solid state reaction mechanism that builds within seconds the nanoscaled layer at the original anode/electrolyte interface. A Ni thin film/YSZ single crystal/Ni/YSZ counter electrode was thoroughly analyzed as a model system by high-resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT). Backed by thermodynamic calculations using the software MALT, a reaction model is proposed: • During the reverse current treatment, the YSZ is congruently reduced with Ni, forming a Ni-Zr-Y alloy, while most of Y precipitates temporarily as Y2O3, unaffected YSZ remains, and pores form. • The fast propagating reaction from the Ni thin film into the YSZ single crystal is explained by the high diffusivity of metal atoms in the Ni-Zr-Y alloy. 5121


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Chemistry of Materials • At open circuit voltage conditions, the affected region is reoxidized instantaneously until thermodynamic equilibrium is reached. • Thereafter, the nanoscaled layer consists of (i) interconnected Ni enclosing some (ii) (YxZr1−x)2O3 precipitates, (iii) a (Y,Zr)-oxide matrix with strongly varying Y:Zr ratio, (iv) unaffected YSZ, and (v) pores. The results obtained suggest that the formation of the nanoscaled layer results from a diffusion-controlled mechanism, with Ni being the fastest species, but also involving Y or Zr as mobile species, since alterations of the Y:Zr ratio are detected. Due to a lack of alternative high performance anodes, Ni/YSZ is still the benchmark material for SOFC, which highlights the importance of this approach. With this outcome, other research groups should also be able to fabricate high efficient nanoscaled Ni/YSZ anodes, and ideally in any SOFC design.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ellen Ivers-Tiffée: 0000-0002-8183-2460 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Barbara Scherrer at Technion (Haifa, Israel) for her continuous interest and helpful discussions on APT and Dr. Radian Popescu (LEM, KIT) for the analysis of the HRTEM images. The Karlsruhe Nano Micro Facility (KNMF, www.kit.edu/knmf) at KIT is acknowledged for provision of access to APT instruments at their laboratories. Sincere thanks are given to Stefan Wagner for translation and proofreading and to André Weber, both KIT, for the valuable discussions.

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