Tape Calendering Manufacturing Process for Multilayer Thin

Tape Calendering Manufacturing Process for Multilayer Thin-Film
Solid Oxide Fuel Cells
Final Report – Phases I and II September 2000 – April 2004
Dr. Nguyen Minh, Kurt Montgomery
October, 2004
DE-AC26-00NT40705
Hybrid Power Generation Systems, LLC
19310 Pacific Gateway Drive Torrance, CA 90502

DISCLAIMER
“This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor
any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States
Government or any agency thereof.”

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ABSTRACT
This report summarizes the work performed by Hybrid Power Generation Systems, LLC during the Phases I and II under Contract DE-AC26-00NT40705 for the U. S. Department of Energy, National Energy Technology Laboratory (DOE/NETL) entitled “Tape Calendering Manufacturing Process For Multilayer Thin-Film Solid Oxide Fuel Cells”. The main objective of this project was to develop the manufacturing process based on tape calendering for multilayer solid oxide fuel cells (SOFC’s) using the unitized cell design concept and to demonstrate cell performance under specified operating conditions.
Summarized in this report is the development and improvements to multilayer SOFC cells and the unitized cell design. Improvements to the multilayer SOFC cell were made in electrochemical performance, in both the anode and cathode, with cells demonstrating power densities of nearly 0.9 W/cm2 for 650°C operation and other cell configurations showing greater than 1.0 W/cm2 at 75% fuel utilization and 800°C. The unitized cell design was matured through design, analysis and development testing to a point that cell operation at greater than 70% fuel utilization was demonstrated at 800°C. The manufacturing process for both the multilayer cell and unitized cell design were assessed and refined, process maps were developed, forming approaches explored, and nondestructive evaluation (NDE) techniques examined.

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TABLE OF CONTENTS

DISCLAIMER ................................................................................................................................ ii

ABSTRACT .................................................................................................................................. iii

TABLE OF CONTENTS ..............................................................................................................iv

TABLE OF FIGURES ...................................................................................................................v

TABLE OF TABLES.....................................................................................................................xi

EXECUTIVE SUMMARY ...........................................................................................................xii

1. INTRODUCTION..................................................................................................................1

2. EXPERIMENTAL / APPROACH........................................................................................1

3. RESULTS AND DISCUSSION...........................................................................................6

3.1 Multilayer Cell Fabrication and Improvement

6

3.1.1 Cathode Improvements

6

3.1.2 Anode Improvements

25

3.1.3 Cell Mechanical Properties Improvements

31

3.2 Unitized Cell fabrication and design

43

3.2.1 Unitized Cell Design Development

43

3.2.2 Improved Flow Distribution Validation Testing

49

3.3 Manufacturing Process Development

53

3.3.1 Cell Fabrication Process

53

3.3.1.1 Baseline Cell Fabrication Process Verification & Validation

53

3.3.1.2 Non-Destructive Evaluation (NDE)

64

3.3.2 Interconnect Fabrication Process

83

3.3.2.1 Formability of sheet metal interconnects

83

3.3.2.2 Flatness and Variability in Sheet Metal Interconnects

95

3.3.2.3 Metallic Interconnect Assembly Methods

99

3.3.3 Unitized Cell Module Fabrication

109

3.3.3.1 Assembly of Unitized Cell Modules

109

3.3.3.2 Performance Testing

116

4. CONCLUSIONS ...............................................................................................................119

5. REFERENCES .................................................................................................................120

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TABLE OF FIGURES

Figure 1. Fabrication sequence for fabrication of cells. ........................................................2

Figure 2. Schematic of single test test apparatus. .................................................................6

Figure 3. Cell (NJ003-1) performance at 800°C in H2/N2 fuel with a fixed fuel flow rate of 67 cc/min and non-flowing air. ...........................................................................................7

Figure 4. Performance of cell NJ003-1 at different temperatures under hydrogen fuel with a fixed fuel flow rate of 67 cc/min and non-flowing air. ..........................................8

Figure 5. Typical impedance spectra at 800°C at OCV (SPC-3 cathode). ........................9

Figure 6. Effects of DC bias on cathode polarization at 700°C (SPC-3 cathode). .........10

Figure 7. Performance of cell NJ009-2 with a conductive layer on the cathode with a fixed fuel flow rate of 67 cc/min and non-flowing air.....................................................11

Figure 8. Cathode impedance of NJ003-1 (no conductive layer) and NJ009-2 (with conductive layer) at 700°C................................................................................................11

Figure 9. SPC-3 cathode in cell NJ009-2 ..............................................................................12

Figure 10. SPC-4 cathode in cell NN33.................................................................................12

Figure 11. Polarization of cell NJ59-1 under hydrogen with a fixed fuel flow rate of 67 cc/min and non-flowing air. ...............................................................................................13
Figure 12. Cell performance at 0.826A/cm² and 70% fuel utilization ................................15

Figure 13. Polarization of cell NP105 under hydrogen with a fixed fuel flow rate of 67 cc/min and non-flowing air. ...............................................................................................16

Figure 14. Microstructure of the SPEX-5 cathode in cell NJ097. ......................................17

Figure 15. Cell NJ097 with experimental cathode, SPCEX-5 with pure hydrogen fuel and a fixed fuel flow rate of 67 cc/min and non-flowing air..........................................18

Figure 16. Cathode Microstructure with Graded Porosity (Cell NJ128). ..........................19

Figure 17. Performance losses in Cell NJ128 and NJ59-1 at 700°C. ..............................19

Figure 18. Polarization of Cell NP117 with hydrogen as fuel and air as oxidant............20

Figure 19. SEM Micrograph of a polished cross section of the cathode of cell NP112-1, electrolyte surface at the bottom of the micrograph. ....................................................21

Figure 20. Polarization of Cell NP126-1 with hydrogen as fuel and air as oxidant. .......22

Figure 21. SEM micrograph of a polished cross section of cathode in cell NP126-1 ....22

Figure 22. Micrograph of anode with graphite fiber pore former (NN33) .........................26

Figure 23. Micrograph of a bilayer made with anode “N” (NJ009)....................................28

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Figure 24. Anode “B” cell performance under different fuel utilization at 800C, and hydrogen as the fuel and air as oxidant..........................................................................29
Figure 25. Cell Performance of NJ205-1 with porous anode “C” and SPC-4 Cathode, hydrogen as fuel and air as oxidant. ...............................................................................30
Figure 26. Performance of cell NJ205-1 at 1.72 A/cm2 load and 75%, 80%, 85% fuel utilizations at 800°C, hydrogen as fuel and air as oxidant...........................................30
Figure 27. Polari zation curve for a cell with anode “C” and an improved cathode SPCEX5 at 800°C (NJ208). ......................................................................................................31
Figure 28. (a) Testing set-up with (b) details of the ring-on-ring loading fixture. A high temperature controlled gas enclosure surrounded the fixture during high temperature testing. ...........................................................................................................33
Figure 29. Characteristic strength of the tested samples. ..................................................35 Figure 30. Tested bilayer samples (a) air, 800°C and (b) reduced, 800°C. .....................36
Figure 31. Measured residual stress in the electrolyte for the bilayer samples. The effect of the mismatch in the coefficient of thermal expansion (CTE) is clearly seen in the results. Where the inverted triangles are for the reduced samples and the circles are for the unreduced sample..............................................................................37
Figure 32. SEM image of the surface of the anode with YSZ powders which are present during sintering. ..................................................................................................................38
Figure 33. Fracture surfaces of reduced bilayers at (a) 25°C and (b) 800°C. The anode surface is at the top and the arrows indicate the location of dense YSZ possibly originating from the powder on the surface shown in Figure 32.................................39
Figure 34. (a) Fracture surface of a reduced bilayer with (b) magnified image of the interface showing zones of local high porosity. .............................................................40
Figure 35. Biaxial flexure strength data for the various mechanical properties improvement stratagies .....................................................................................................41
Figure 36. Performance of a cell with a 3Y-anode composition with pure hydrogen fuel and a fixed fuel flow rate of 67 cc/min and non-flowing air..........................................42
Figure 37. Polarization curve for 3Y-20A baseline anode cell. Cathode was SPC-4. ..42
Figure 38. Schematic of unitized cell. ....................................................................................43
Figure 39. Typical fabrication sequence for unitized cell fabrication and stacking.........44
Figure 40. Diagram of the interconnect structure for cell NO023......................................45
Figure 41. Split Flow design. ...................................................................................................46
Figure 42. Effect of total gas flow rate on the gas flow rate at individual exit holes (1 is nearest the inlet).................................................................................................................48

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Figure 43. Percentage of total mass flow versus axial distance for various design iterations. .............................................................................................................................49
Figure 44. Polarization curves of cell NO-209 at 800°C......................................................50
Figure 45. Power density vs fuel utilization at 0.7 V and 800°C. .......................................50 Figure 46. Cell NO-215 polarization curves at 800°C under different fuel flow rates. ....51
Figure 47. Cell NO-215 power density vs. fuel utilization at different temperatures. .....52
Figure 48. Pareto chart of key manufacturing parameters. ................................................54
Figure 49. Fishbone of the baseline process variation in SOFC manufacturing. Impact was assigned based on expert judgment, and was used to guide prioritization of areas to be investigated. ...................................................................................................55
Figure 50. A small section of the data sheet collected on bilayer tape manufacturing..56
Figure 51. Descriptive statistics of bilayer tape thickness for 118 bilayer cell tapes that remained from the manufacture of 126 cells. ................................................................61
Figure 52. Descriptive statistics on the bilayer cell tape weight based on 118 bilayer cell tapes that remained from the manufacture of 126 cells...............................................62
Figure 53. Flatness run chart for the first 30 fired bilayers. The red line denotes the mean flatness......................................................................................................................63
Figure 54 X-ray digital radiography of three tapes in various stages of processing, TN4719 = electrolyte tape, TN 4758 = bilayer tape (electrolyte, anode 1, and anode 2) with the electrolyte surface up, and TN4782 = anode 2 tape. The electrolyte tape and anode 2 tapes are representative of the tapes used to fabrication the bilayer tape shown. ............................................................................................................66
Figure 55. IR and X-ray images of the sintered bilayer sample NDE-9 are shown. The upper left panel is an IR image of the front surface whereas the lower left panel is the back surface. The X-ray image is shown in the right panel. Note: this cell was broken and only half of the cell is seen in the image and the sample was imaged with the electrolyte surface up..........................................................................................67
Figure 56. SOFC sample NDE-5 is shown under IR (left) and X-ray (right)....................68
Figure 57: The flatness and surface smoothness of the sample NDE-5 is shown as a function of the position on a diameter. ............................................................................69
Figure 58 X-ray images of the SOFC plate (NDE-3) are shown in reverse video (i.e. white is low density, and black indicates high density). The right panels where taken at higher magnification. ..........................................................................................70
Figure 59. Two ultra-sonic images of sample NDE-3 are shown in the panels above. The left panel shows a surface wave image while the right panel shows an impulse reverberation image...........................................................................................................70

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Figure 60. SEM image is of the defect cross –section with a higher magnification image shown on the right. .............................................................................................................71
Figure 61. Two images of SOFC (NDE-7) are shown. The left panel is an IR-image and the right panel is an X-ray radiograph. ....................................................................72
Figure 62. X-ray radiograph of bilayer tape (NDE-S2) with the calibration standards in the left side of the frame (left). The right panel shows a enlarged region of the cell (NDE-S2) illustrating defects or non-uniformities at the 600 µm (15 mil) level. Black points, (circled in red) identify voids, while white points (circled in green), indicates regions of high density. .....................................................................................................74
Figure 63. X-ray radiograph of bilayer tape (NDE-S5) with the aluminum calibration standards in the left side of the frame.............................................................................75
Figure 64. X-ray radiographs of bilayers NDE-S2 (left) and NDE-S5 (right) after sintering. Both images are shown rendered in the identical gray scale. Shown on the inset of each image is a re-normalized gray scale image of the apparent “missing” edges of the bilayers. .......................................................................................76
Figure 65. X-ray radiographs of NDE-S2 before (left) and after (right) cathode application. Two circles creating an annulus have been drawn on the fired bilayer image to give an approximate indication of where the cathode was deposited.......77
Figure 66. X-ray images of sample NDE-S2 before (left) and after (right) cathode firing. ...............................................................................................................................................78
Figure 67. Photograph and wide latitude X-ray radiograph (right) of the sample NDE-S5 after the cathode has been fired. .....................................................................................79
Figure 68. Radiographs of samples NDE-S2 (left) and NDE-S5 (right) are shown above....................................................................................................................................79
Figure 69. Fixed flow polarization curve for NDE cells S4, S7 and S9. Tests were performed with 64% hydrogen and balance nitrogen for fuel and at 800°C. ............80
Figure 70. Polarization curves for 3 NDE cells S4, S7 and S9 under fixed fuel utilizations. All data was taken at 800°C under 64% hydrogen balance nitrogen for fuel and 20% air utilization. ...............................................................................................81
Figure 71. Engineering stress strain curves from 0.025” thick stainless sheets. ............84
Figure 72. Sample dimensions used for forming limit curve generation. .........................86
Figure 73. Samples prior to testing........................................................................................86
Figure 74. Limiting strains measured on 0.025” thick stainless steel. ..............................87
Figure 75. Method to shift forming limit curves between various sheet thicknesses. ....88
Figure 76. Forming limit curves for (a) 0.025” thick stock; (b) 0.010” thick stock; (c) 0.005” thick stock. ..............................................................................................................88

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Figure 77. Typical corner dimensions that may pose limits on stamped interconnect design...................................................................................................................................89
Figure 78. Flow/current collector features that may be stamped in sheet metal interconnects.......................................................................................................................89
Figure 79. Axisymmetric finite element simulations for determination of RROOT .............90 Figure 80. Calculated major strains for (a) draw depth of 1mm and (b) draw depth of
2mm......................................................................................................................................91
Figure 81. Forming limit diagrams of strains in inside (left) and outside (right) corners for 0.025” sheet. Draw depth = 0.250”. ..........................................................................92
Figure 82. Forming limit diagrams of strains in inside (left) and outside (right) corners for 0.010” sheet. For Draw depth = 0.250”....................................................................92
Figure 83. Deformed element plot for forming of 0.005” thick stock into a corner with radius of 0.050”...................................................................................................................93
Figure 84. Schematic of the 1.5” stamped cup test vehicle. ..............................................94
Figure 85. Photograph of a 1.5” stamped cup test vehicle, 0.025” thick metal sheet....94
Figure 86. Components of 6” test vehicle, from left to right: Stamped interconnect “pan”; manifold spacer sheet; anode current collector perforated sheet; cathode current collector support sheet; cathode current collector perforated sheet; 4” tape calendared cell....................................................................................................................95
Figure 87. Schematic representation of interconnect non-flatness. (a) Ideal situation; cell and interconnect are flat. (b) Local non-uniformity in current collector surface (c) Global curvature in current collector. Contact resistance varies with bond paste thickness and contact pressure. ......................................................................................96
Figure 88. 115 mm (4.53”) square sheet of 0.127 mm (0.005”) thick stainless steel sheet was stamped with an array of dimples. Dimple diameter = 3.5 mm (0.138”); spacing = 4 mm (0.154”); height = 0.5 mm (0.197”).....................................................97
Figure 89. Flatness measurements on prototype interconnect. ........................................97
Figure 90. Height profiles for four 6” interconnects. ............................................................98
Figure 91. Spring back simulation in cup drawing of a 2” ID cup, to a depth of 0.2”. ...98
Figure 92. Springback in 1.5” cups.........................................................................................99
Figure 93. Resistance projection welding process schematic. ........................................101
Figure 94. Laser edge weld (small spot) of two 0.025” thick sheets...............................103
Figure 95. Laser edge weld (large spot)..............................................................................103
Figure 96. Laser lap weld of two 0.025” thick sheets........................................................104

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Figure 97. Laser lap weld of two 0.025” thick sheets that resulted in a full penetration through the first sheet. .....................................................................................................105
Figure 98. Laser edge weld of 0.005” thick stock. .............................................................106
Figure 99. E-beam edge weld of 0.025” thick sheets........................................................106
Figure 100. E-beam lap weld of 0.025” thick sheets. ........................................................107
Figure 101. Photograph of samples after laser edge welding of 0.025” material with glass sealed cell. ..............................................................................................................108
Figure 102. Photograph of samples after E-beam welding of 0.025” material with glass sealed cell..........................................................................................................................108
Figure 103. Schematic of planar SOFC stack, cell module, and components..............111
Figure 104. Design choices that govern stack assembly pathways. ..............................112
Figure 105. Assembly operations in building a stack........................................................112
Figure 106. Sequence #1 All at once – pre-assemble all metallic interconnect components along with cell support. .............................................................................114
Figure 107. Sequence #2a: Close anode side interconnect, seal cell before stack, assemble and manifold. ..................................................................................................114
Figure 108. Sequence #2b: Preseal cell, close interconnect, assemble and manifold. .............................................................................................................................................115
Figure 109. Six inch test vehicle cross-section schematic. ..............................................116
Figure 110. Stamped, laser welded 6” interconnects. Cathode interconnect (left); Anode interconnect (right). .............................................................................................117
Figure 111. Curvature measurements on cell and interconnects used in test Oly001. .............................................................................................................................................117
Figure 112. Performance curve from test Oly001..............................................................118
Figure 113. Performance curve from test Oly002..............................................................119

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