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  • 1. Hi2 H 2 FIB/TEM and EPMA Method as Complementary Technique for Investigations on Solid Oxide Electrolycer Cells U. F. Vogt1, D. Wiedenmann1, A. Hauch2, M. Mogensen2 1EMPA, Swiss Federal Laboratories for Materials Testing and Research, Hydrogen & Energy, Dübendorf, Switzerland 2Risø National Laboratory for Sustainable Energy Technical University of Denmark – DTU, Roskilde, Denmark High Temperature Water Electrolyses, Limiting Factors June 9-10, 2009, Karlsruhe High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 2. Hi2 Project objectives H 2 : EPMA and FIB-TEM characterisation of the steam electrode determining chemical and structural changes at the hydrogen electrode determining impurities at the triple-phase boundaries in the active layer of the hydrogen electrode by applying complementary analyzing techniques for integrated investigations applying 2D and 3D characterisation methods U. Vogt - International Conference on Hydrogen Production,ICH2P-09, Oshawa, Canada, May 3-6, 2009
  • 3. SOEC: Cell assembling oxygen electrode / anode T = 600 - 900°C O2 Sr-substituted LSM lanthanum manganite anode Yttria-stabilized + YSZ electrolyte Zirconia - O2- O2- cathode Ni/YSZ H2O (g) H2 BSE image steam electrode / cathode albeite glas sealing: → cell degradation due to Si-poisoning ?? increasing cell resistance decreasing cell efficiency Ma terials Sci ence & Technolog y Dr. Ulrich Vogt - McMaster University, Hamilton, Canada, May 7th 2009
  • 4. Degradation and aging mechanisms in SOECs structural changes chemical changes electron conductive impurities at TPB triple phase boundaries TPB: gas molecules - oxide ions - electrons by evaporation pores & precipitation oxygen conductive blocking of the TPB increase in resistivity the reduction of steam will take place closest to the hydrogen electrode/electrolyte interface Si contamination most likely enters the active hydrogen electrode as gaseous Si(OH)4 (Hauch et al., 2007a). the equilibrium between Si(OH)4 and SiO2 + H2O is shifted towards the formation of silica High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 5. Complementary characterization methods SEM: scanning electron Sr-substituted LSM lanthanum microscopy manganite EPMA: Electron Probe Yttria-stabilized Micro-Analyses YSZ Zirconia large sample areas Ni/YSZ in µm-resolution BSE image TEM: transmission electron pores microscopy Ni FIB: focused ion beam technique YSZ site specific analysis dark field STEM in nm-resolution 500 nm High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 6. EPMA examination: WDS-Spectrometer electron beam analysing crystal advantage of WDS over energy- dispersive spectroscopy (EDS): sample higher (spectral) energy resolution detector lower background (trace elements) lower detection limits (10 ppm) a) EDS b) WDS WDS allows to EDS: Si peak is resolve the superimposed by characteristic Kα1 peaks of the Si Kα1 Si Kα1 peak of Si constituent elements High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 7. Simulated bulk sample EDS of 8YSZ containing 0.1, 1 and 10 mol% SiO2 at (left) and WDS carried out by EPMA (right) on the Ni/YSZ electrode (tested cell) The spectral resolution of the WDS is a magnitude higher than for EDS. The detection limit (Si Kα1-peak) for Si corresponds to a SiO2-content in the range between 0.1 and 1 mol% for the simulated EDS. High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 8. EPMA examination on the µm scale: Element mapping of constituent and contaminant elements at electrolyte - fuel electrode interface of SOECs after operation for 1500 h, 850 °C (Risø cell) Si contaminant found to be agglomerated in the active layer of the stam electrode O2 electrode O2 electrode (LSM-YSZ) electrolyte electrolyte (YSZ) steam electrode steam electrode (Ni-YSZ) O2 electrode electrolyte steam electrode Si virgin reference cell 40 μm High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 9. TEM examination on the nm scale TEM imaging to examine the microstructure at the interface STEM/EDS mapping to detect chemical changes Quantitative analyses of impurities High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 10. Focused Ion Beam preparation of TEM lamellae ~ 20 μm ~ 2 μm PT surface protection layer electrolyte active layer top view the area of interest is selected and protected by a Pt surface protection layer High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 11. Focused Ion Beam preparation of TEM lamellae ~ 20 μm PT surface ~ 2 μm protection layer boxes are cut out by sputtering the material with a highly focussed Ga-ion beam before and behind the protected area top view electrolyte active layer High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 12. Focused ion beam preparation of SOEC cells the lamella is thinned down to 200 - 250 nm 200 – 250 nm electrolyte active layer a window is cut (~100 nm) to further thin the most interesting sample details top view electrolyte active layer High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 13. Focused ion beam preparation of SOEC cells inclined view a window is cut (~100 nm) to further thin the most interesting sample details electrolyte active layer High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 14. FIB/TEM investigation of SOECs at electrolyte/fuel electrode interface window dark-field STEM image 5μm showing the microstructure of the interface active layer electrolyte High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 15. FIB/TEM investigation of SOECs at electrolyte/fuel electrode interface: pore YSZ YSZ pore Ni Ni YSZ pore EDS mappings of the constituent elements and of Si contaminations pore between Ni-YSZ particles YSZ dark-field STEM image 500 nm SiO2/Al2O3 = 7 High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 16. Conclusions Complementary microscopy and microanalysis techniques are highly recommendable, for comprehensive chemical and structural characterisation of SOC from micro- to nano-scale EDS / WDS bulk sample chemical analysis enables qualitative and semi- quantitative information on the micro-scale of the porous electrodes TEM lamellae were successfully prepared by the focused ion beam technique to provide site specific TEM lamellae of the inhomogeneous and porous hydrogen electrode and interface towards the dense electrolyte of solid oxide cells. Thin film TEM EDS measurements enabled quantitative information especially on the nano-scale for the electrolyte/hydrogen interface of solid oxide cells. SiO2 rich and Al2O3 containing impurities have been detected and they were found to block triple phase boundaries of the hydrogen electrode High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe 16 Ma terials Sci ence & Technolog y
  • 17. The project was Acknowledgement founded by: Sixth Framework Programme 6.1.ii, Sustainable Energy Systems Project Nr.: 503765 Acronym: Hi2H2 www.Hi2H2.com High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe 17 Ma terials Sci ence & Technolog y
  • 18. Phase Size Distribution - Validation of methodic reproducibility Sources of uncertainty: I. „Manual segmentation“ by thresholding development of automated segmentation technique II. At what image size do we get representative analysis? comparative study with different imaging sizes and magnifications I Segmentation: Grain boundaries in BSE images are not sharp but they rather represent transition zones with a diffuse gray-scale gradient. The width of these transition zones is depending on imaging parameters (kV, pixel resolution) and material properties (density and density contrast, interface-geometry). e- e- BSE BSE image analysis A 3kv B A B B‘ 5kv 15kv B‘‘ B‘‘‘ Influence of interface geometry Pixel size 9 x 9nm Influence of kV 18.4µm 6 µm B11: Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe High 10kx_a Z II [9nm] Ma terials Sci ence & Technolog y
  • 19. Validation of PSD: I) reproducible segmentation procedure Conventional Segmentation: „Finding“ threshold which separates phases Trying to find the median line in the transition zone New procedure, fully automated, input= typical gray scale of three phases, size of structural elements (strel, standard values, f of kV and Pixel resolution) A1. starting with conservative thresholding by using typical grayscale values binarization of „certain regions“ for ni, pore and CGO (no noise filtering) A2: removing apparent Ni-rims with opening-filter B: redistribution of „non-assigned“ pixels by dilation of „certain regions“, size of „strel“ according to interaction volume (due to dilation also noise is removed) C: Result = 4phase-map with 4 discrete grayscale values (pore=0, Ni=100, CGO=200, unassigned=255) input for PSD analysis Automated procedure = reproducible result A. Conservative Thresholding B. Redistribution of ‚non-assigned‘ Pixels Fourth phase=non-assigned pixels, which e- BSE represent image imperfections (<1%) Ni Ni + Pore + CGO Ni CGO Pore CGO Pore Redistribution by dilation 6 µm of „certain phases“ Pore C. Result = 4phase-map CGO 18.4µm 6 µm 6 µm B11: 10kx_a Z II [9nm] High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 20. Validation of PSD: I) reproducible segmentation procedure Overlay of Ni-outline Overlay of pore-outline Overlay of CGO-outline Results from automated segmentation: „4phase“-image with dicrete gray-levels Overlay of Rest-outline Pore = 0 ‚black‘ Ni = 100 CGO = 200 Rest = 255 ‚white‘ 6 µm B11: 10kx_a Z II Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe High [9nm] Ma terials Sci ence & Technolog y
  • 21. Validation of PSD: I) reproducible segmentation procedure New procedure (Matlab): automated, stable, reproducible Original grayscale image (BSE) Results from automated segmentation 6 µm 6 µm 18.4µm 18.4µm Pore = 0 ‚black‘ Ni = 100 CGO = 200 Rest = 255 ‚white‘ B11: 10kx_a Z II [9nm] High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 22. A6 O_555°C A6 O_655°C A5 O_755°C A5 O_855°C A4 O_950°C A4 O_1050°C A4 O_1140°C_0h Anode 2 Anode 1 30 µm 30 µm image analysis Ni coarsening due to NiO reduction vs. temperatures Pores Ni CGO A4-A6, Material O-220306-2 / TS=1200°C / reduction at T=x°C / 0 hours A4-A6, Material O-220306-2 / TS=1200°C / reduction at T=x°C / 0 hours A4-A6, Material O-220306-2 / TS=1200°C / reduction at T=x°C / 0 hours Pores in Anode 2 Ni and Anode 2 CGO in Anode 1 and Anode 2 Imagewidth 30 µm [15nm res] Imagewidth 30 µm [15nm res] Imagewidth 30 µm [15nm res] 60 60 60 Anode 2 50 50 50 1140°C 1140°C 1140°C 1050°C 1050°C 1050°C 40 40 40 950°C 950°C 950°C 855°C 855°C 855°C vol [%] 755°C vol [%] vol [%] 755°C 755°C 30 655°C 30 655°C 30 655°C 555°C 555°C 555°C 20 20 20 10 10 10 0 0 0 100 200 300 400 500 600 700 800 0 0 100 200 300 400 500 600 700 800 0 50 100 150 200 250 300 350 400 radius [nm] radius [nm] radius [nm] slight coarsening of pores coarsening of CGO above 900°C strong Ni coarsening above 900°C in Anode above 900°C High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 23. Conclusions of image analysis validation -Segmentation technique automated method = reproducible, accurate results -Imaging technique: Compromise between good material contrast at high kv and good resolution (small depth penetration) at low kv must be obtained Good results with ESEM are obtained at 5kV, spot 3 Note: At low kV sample preparation becomes more important (scratches) -Image size (window, Pixel res, averaging) for reproducibility image size width is the most important factor for good statistics of anodes, the image width should be >>20µm or alternatively, multiple small images should be analysed (averaged) pixel resolution is not really important as long as it is sufficient to resolve the smallest objects of interest (i.e. 50-100 nm particles) i.e. Pixel size should be < 30nm - comparison „old“ vs „new“: impact of pixel resolution for extreme binning becomes apparent at pixel res. of 75 nm, when small particles can not be detected any more High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 24. Particle Nr density NV / Discrete Size Distribution: Sample B11, Anode 2, Ni and CGO 3 D imaging by FIB tomography 5.0E+09 CGO after splitting with k=0.6 (PB corr) 2“ ne on 4.5E+09 de Zo siti “ e1 no e size distributions c an particle oriented fa „A d 4.0E+09 Current- Tr no of primary particles ter „A = connected network collector In 3.5E+09 [Particles / mm^3] 3.0E+09 Ni after splitting with k=0.6 (PB corr) 2.5E+09 2.0E+09 Electro- lyte 1.5E+09 Ni without splitting (PB corr) 1.0E+09 5.0E+08 CGO without splitting (PB corr) 0.0E+00 10 100 1000 10000 Diameter [nm] Volume fraction VV / Discrete Size Distribution: Sample B11, Anode 2, Ni and CGO phases 0.40 Ni after splitting with k=0.6 (PB corr) 0.35 volume related 0.30 CGO after splitting with k=0.6 (PB corr) 24 µ m 0.25 [µm^3 / µm^3 0.20 Ionic transport in CGO 0.15 Electronic transfer in Ni 0.10 Gas transport in Pores 0.05 Ni without splitting (PB corr) CGO without splitting (PB corr) 0.00 all 3 phases are connected 100 1000 10000 100000 Diameter [nm] High Temperature Water Electrolyses, Limiting Factors, June 9-10, 2009, Karlsruhe Ma terials Sci ence & Technolog y
  • 25. Conditions Acknowledgement resolution: 12 nm Dr. Lorenz Holzer 1500 x 1500 pixel 1500 pictures oxel size: 12 nm edge length 24 µm sample width 2 days of FIB cutting Ma terials Sci ence & Technolog y