YSZ Anodes - Chemistry

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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 Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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Chemistry of Materials

High Resolution Studies on Nanoscaled Ni/YSZ Anodes Julian Szász†, Sascha Seils‡, Dino Klotz, Heike Störmer§, Martin Heilmaier‡, Dagmar Gerthsen§, Harumi Yokokawa#, 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 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. Afterwards, at open circuit voltage conditions, Ni-Zr-Y is re-oxidized 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.

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, well-known 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 ionic-electronic 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 to 100, depending on microstructure and composi-

tion.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 electro-oxidation of H2, and of Yttria stabilized ZrO2 (YSZ) with high 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 1 (b)). 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 co-sintering 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 1 (a) and (c); a quantitative analysis by focused ion beam (FIB) tomography is published in literature.13 Finite element model (FEM) calculations in Figure 2 exemplify the mutual dependence between anode performance (ASRanode)

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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 grey), YSZ (dark grey) and pores (black). (b) Scheme of the electro-oxidation of H2 at the TPB of Ni, YSZ and pore. (c) and (d) SEM image 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.

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), 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,effective) 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, require a substantially larger anode volume (e.g. the anode-support) 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 post treatment using multi-step 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 simple, 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 1 (d). However, the limited efficiency of the energy dispersive X-ray spectroscopy (EDXS) system in our transmission electron microscope at that time yielded data with a substantially lower signal-to-noise ratio and a reduced sensitivity towards the detection of composition fluctuations, which prevented an in-depth analysis of the solid state reaction mechanism.22


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10 area specific resistance ASRanode / Ω·cm²

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1000

T = 700 °C

316 1.45 Ω·cm²

1

100 31

0.1

10 0.046 Ω·cm² 3

0.01

0.001 10-3

lanode,effective ≈ 400 nm 10-2

10-1

$

100

15 µm 101

102

anode thickness lanode / µm 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 %.

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 well-defined geometry made of a Ni thin film / YSZ single crystal / Ni/YSZ counter electrode (see experimental 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 experimental 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 afterwards, at open circuit voltage (OCV) conditions.23,24 2. Experimental Methods Experiments were performed on a model system, consisting of 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 3h in air. NiO was reduced to Ni in hydrogen atmosphere at 800 °C and then cooled down with 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 guarantees 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 flowrate 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 afterwards at OCV conditions. The model system was analyzed



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“post-test” and only a single RCT was conducted, with predefined parameters (jRCT = -2 A∙cm-2, 700 °C). 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 (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 ChemiSTEMTM 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 (HRTEM) 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 twodimensional Fourier transform of small regions of HRTEM images with simulated diffraction patterns for the crystal structures of the cubic (c-)phase (symmetry group Fm3ത݉, 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 P1 21/c, space group number 14, a = 0.522 nm, b = 0.527 nm, c = 0.538 nm and angle β = 80.54 degrees). 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 artefacts 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. 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

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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 needle-shaped 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. Thermodynamic Calculations Thermodynamic calculations were performed with the commercially available software MALT (MAterialsoriented Little Thermodynamic database) and were evaluated by H. Yokokawa and co-workers.23,24 Additional thermodynamic data was taken from literature.29 3. Results & 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 3 (a)) and after the RCT has been conducted (“post-test” in Figure 3 (b)). 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 3 (c) shows a HAADF-STEM image of a similar region as Figure 3 (b) at higher magnification. Three regions can be clearly distinguished: (i) the dense Ni thin film (light grey) with a wavy interface towards the (ii) nanoscaled layer, with a thickness of about 300 nm, (iii) the YSZ single crystal (dark grey). The composition sensitivity of the HAADF-STEM image suggests that the layer is composed of nanoscaled grains of Ni (light grey), (Y,Zr)-oxide (medium to dark grey regions) and pores (black regions). We note that some Ni grains in Figure 3 (c) 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 3 (d)). 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 anodesupported 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 1 (d). It further illus-


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trates 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 should the YSZ phase consist of YSZ with maximum ionic conductivity, i.e. between 16 and 20 at.% Y.30,31 For all 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 4 (a), a HAADF-STEM image of the nanoscaled layer is shown at higher magnification. The Ni phase (light grey) contains distinct, dark dots with about 10 nm in diameter. The (Y,Zr)-oxide has a speckled appearance (medium to dark grey) which is indicative of composition and/or strain variations. Figure 4 (b) presents the results of the corresponding EDXS mapping with color coded elemental distributions representing Ni (grey), Y (yellow) and Zr (red). Based on the comparison of the HAADF-STEM image and the elemental distribution, the complex structure of the na-

noscaled layer can be better recognized. The representative phases are additionally marked by white circles in Figure 4 (b) 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 image Figure 4 (c) and (d) from the marked region in Figure 4 (a). Zr and Y distributions clearly indicate that the (Y,Zr)-oxide contains Zrrich and Y-rich regions and that the Y-rich precipitates contain small amounts of Zr. The quantitative Zr- and Yconcentration profiles in Figure 4 (v) 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 o 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.

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



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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 (grey), Zr (red) and Y (yellow) signal. (c) and (d) compositional image of the Zr and Y signal, respectively. (v) the Zr- and Y-concentration profile indicated by the white arrow in (a-d).

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 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 needleshaped sample tip of approximately 3 µm length was prepared from the model system by FIB (see experimental section). The analyzed part of the sample tip (Figure 5 (a),

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 5 (b),(c) and (d) as an elemental atom mapping of Ni (grey), Zr (red), Y (yellow) and O (blue). Comparison with Figure 4 therefore specifies the STEMEDXS findings on the atomic scale: an interconnected Ni phase is present in metallic form (Figure 5 (d) and (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.


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Figure 5. (a) Needle-shaped sample tip with (left) the part analyzed by APT with a volume of 830531 nm³. (b), (c) and (d) elemental atom mappings of (b) Ni (grey), 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 iso-surface at 40 at.% Ni.


Chemistry of Materials 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 5 (c) 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 5 (c)) yields the O concentration and a spatial elemental distribution of Zr (red), Y (yellow) and Y+Zr (black) with a higher resolution, as 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

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 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 But in 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 solid state reaction mechanism.

pO2 / atm

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2Ni + O2 ↔ 2NiO Ni

10-20

OCV

10-30

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 und 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 = 100…10-70 atm. Each solid line represents a borderline, 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 co-exists with both cations present in YSZ in form of their oxides, namely ZrO2 and Y2O3. 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 anode-supported cell (ASC) which corresponds to a pO2 of 5.9∙10-52 atm, applying the Nernst equation against air.37 In case of 10YSZ, the cell voltage is already higher than the

Zr + O2 ↔ ZrO2

10-40

Zr

RCT 10-50

Y 4Y + 3O2 ↔ 2Y2O3

10-60

10-70 600

650

700

750

800

T / °C

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 = -24 4.8∙10 atm) and (green square) during the RCT, when ZrO2 -49 is reduced to Zr (below pO2 = 1.58·10 atm, indicated by the green arrow).

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,


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ZrO2 and Y2O3 phases are all stable (see Figure 7 (a)), in accordance with Figure 6. The solid solutions of YSZ in its cubic and the tetragonal phases are not shown in the Figure 7 (a) 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 co-existing 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.

a)

OCV (pO2 = 10-24 atm)

µ(Zr) / RT

-30

-20

µ(Ni) / RT -10

0

ZrO2

3.3. Reaction Model Ni Y2O3

-80

-62

b)

RCT (pO2 = 1.58·10-49 atm) -30

-20

Y4Zr3O12

-20 Y2O3

-66

-70

µ(Ni) / RT -10

Zr

-30

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. But 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 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 re-oxidation to (YxZr1-x)2O3 precipitates and an interconnected (YmZr1-m)O2 phase is very likely to occur.

Y4Zr3O12

-70

µ(Zr) / RT

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


-74

0

NiZr2 NiZr

Ni10Zr Ni21Zr8 Ni3Zr Ni7Zr2 Ni5Zr

-40

-23

-25

-21

Figure 7. Thermodynamic equilibria at ambient pressure and 700 °C of the Ni-Zr-Y-O system: calculated (a) at OCV condi-24 tions (pO2 = 1·10 atm), (b) during the RCT (pO2 = -49 1.58·10 atm). All phases appear in the solid state.

During the RCT, the thermodynamic equilibria of the initial phases strongly change, as shown in Figure 7 (b) 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

Based on 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). RCT: Applying a strong cathodic polarization of jRCT = -2 A∙cm-2, the voltage increases over 10 milliseconds 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 Ni thin film and 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 (ܸை∙∙ ) 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. Ni diffuses out



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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 re-oxidized 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 Ni thin film and 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 Ni thin film and YSZ single crystal.41

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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 Figure 4 and 5. According to these results (see Figure 8 (c-2)), the nanoscaled layer is composed of: 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)

OCV-1: Upon switching off the strong cathodic polarization jRCT, the voltage drops from 2.5 V to approximately 0.95 V within 10 milliseconds and remains constant for about 10 seconds. Herein, the re-oxidation of the Ni-Zr-Y alloy proceeds until a thermodynamic equilibrium is reached. The voltage of 0.95 V persisting for 10 seconds 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 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 seconds for the interface to be in equilibrium with the fuel gas.46 It should be noted here, that re-oxidation kinetics influence pore formation and depend, in general, on sample geometry.46,47 In the model system, re-oxidation 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 re-oxidized 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 co-exist, meaning that no kinetic barrier for the nucleation and growth of tetragonal phases exists.

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 towards 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 interconnected phase, thus enabling the transport of electrons from all TPB points towards the anode-support. At the same time, the Zr contained in the Ni-Zr-Y alloy (and, to a lesser extent, also Y) is re-oxidized and reacts either with the Y2O3 precipitates embedded in the Ni, forming (YxZr1-x)2O3, or 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.


voltage / V

6.5

voltage current density

2.5 -2.0 1.0

RCT

0 0

OCV-1 10

a-1)

OCV-2 20

b-1)

Ni thin film

OCV-2

OCV-1

YSZ single crystal

0 time / sec

c-1)

RCT Ni/YSZ counter electrode air 2η(e-) ηohmic

NiO Ni

η(O2-)

ηcounter electrode

V

voltage / V

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


current density / A·cm-2

Page 11 of 16

η(O2-)

η(O2-)

2η(e-)

2η(e-) µ(O)

ηNi thin film µ(O)

µ(O) ZrO2 Zr

not drawn to scale

Y2O3 Y

a-2)

b-2)

Ni-Zr-Y formation

c-2)

Ni-Zr-Y re-oxidation

Nanoscaled Ni/YSZ

Ni thin film e-

Ni-Zr-Y

Zr

Y YSZ

O2-

VO•• O2-

Ni

pore

Y-rich YSZ (YmZr1-m)O2

Y2O3

e(YxZr1-x)2O3

O2-

e-

Zr-rich YSZ (YmZr1-m)O2

Figure 8. (top) Current/voltage behavior during strong cathodic polarization (RCT), and afterwards at OCV conditions (OCV-1) and (OCV-2). (a-1, b-1 and c-1) Scheme 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 Figure 3, 4 and 5.



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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 multi-phase YSZ matrix in any case. Recalling the total conductivity of YSZ as a function of Y dopant concentration, 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 1 (d)). 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:

Page 12 of 16

AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected] (E.I.)

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

Notes The authors declare no competing financial interest.

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.

• 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. • At open circuit voltage conditions, the affected region is re-oxidized 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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Chemistry of Materials (19)

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YSZ Anodes - Chemistry

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