Yuan Ze University 元智大學 Department of Mechanical Engineering. SOFC Science and Technology

Transcription

1 FUEL CELL CENTER Yuan Ze University 元智大學 Department of Mechanical Engineering ---ooo--- SOFC Science and Technology Dr. Guo-bin Jung

2 Outline Chapter 1. Introduction to Solid Oxide Fuel Cell Chapter 2. Electrolytes Chapter 3. Cathode Materials Chapter 4. Anode Materials Chapter 5. Interconnect Chapter 7. Cell/Stacks Chapter 8. Fuels Chapter 9. SOFC System

3 Sealing? One of the challenges in manufacturing planar solid oxide fuel cells (psofcs) commercially available hightemperature alloys is in hermetically sealing the ceramic and metallic components such that the for use in the stack and balance-of-plant, but resulting joint remains rugged and stable over the lifetime of the stack. 3

4 Sealing? 4

5 Sealing? 5

6 Functions : SOFC seals prevent mixing of fuel and oxidant within stack. SOFC seals prevent leaking of fuel and oxidant from stack. SOFC seals electrically isolate cells in stack. SOFC seals may provide mechanical bonding of components. 6

7 Requirements of seals in solid oxide fuel / electrolyzer cells In planar SOFCs / SOECs, gas-tight sealing is required along the edges of the electrodes, electrolyte, and interconnect as well as between individual cells to bond the cell stacks. prevent mixing of gases in the anode and the cathode, and provide electrical insulation to avoid shunting. 7

8 Properties and fabrication requirements of a SOFC / SOEC seal Thermal properties Thermal expansion coefficient at x10^-6/ C¹. Thermally stable for ~5000 h for mobile applications and for ~ 50,000 h for stationary applications at C cell operating temperatures. 8

9 Properties and fabrication requirements of a SOFC / SOEC seal Chemical properties: Resistant to vaporization and compositional change in stringent oxidizing and wet reducing atmospheres at C. Limited or no reaction with other cell components. 9

10 Properties and fabrication requirements of a SOFC / SOEC seal Mechanical properties : Withstand external static and dynamic forces during transportation and operation. Resistant to thermal cycling failure during start-up and shut-down of cell stacks. 10

11 Properties and fabrication requirements of a SOFC / SOEC seal Electrical properties : Electrical resistivity 10^4Ωcm at operating temperature. Electrical resistivity greater than 500 Ωcm between cells and stacks at nominal stack operating condition (0.7 V at ma/cm² ). 11

12 Properties and fabrication requirements of a SOFC / SOEC seal Sealing ability : Sealing load <35 KPa. Withstand differential pressure up to KPa across a cell or stack. Total fuel leakage <1% for the duration of the cell life. 12

13 Properties and fabrication requirements of a SOFC / SOEC seal Fabrication flexibility : Flexible design, low processing cost, and high reliability. 13

14 Notes : Much of SOFC seal work to date is proprietary (design-specific). Limited SOFC seal information in open literature Planar designs typically require multiple seals per stack repeat unit. Several different types of seal might be used per repeat unit. 14

15 Seal designs and materials will largely depend on the cell and stack configurations and contacting surfaces / materials 15

16 Basic Sealing Approaches Rigid, bonded seals Room-temperature analog: Epoxy glue Materials: Glass, glass-ceramic, braze Compressive seals Room-temperature analog: Rubber O-ring, gasket Materials: Mica-based Compliant, bonded seals Room-temperature analog: Rubber glue Materials:?? 16

17 Compliant seal Advantages Low thermal stress Disadvantages Non-wetting to other SOFC/SOEC components. Poor oxidation resistance Hydrogen embrittlement Electrically conductive 17

18 Rigid, Bonded Seals Additional Requirements for Rigid, Bonded Seals (typically glass, glass-ceramic) Thermal expansion match - Ni/YSZ 12 ppm/k - 8-YSZ 10 ppm/k - Doped Ceria ppm/k - Doped Lathanum Gallate 11.5 ppm/k. Sealing temperature - Must be all previous cell/stack fabrication steps. - Higher than the operational temperature. Good wetting during sealing. 18

19 Rigidly bonded seal Advantages Hermetic sealing Tailorable performance by composition design High electrical resistivity Suitable for stationary and mobile applications Flexible in design and fabrication Disadvantages Brittle at low temperatures Poor resistance to thermal cycling Chemical reaction with other cell components 19

20 Potential Glass Systems P2O5 based glasses Low CTE, low strength B2O3 based glasses Volatile, low softening temperatures. SiO2 based glasses Best available glass candidate (?) Alkaline earth aluminosilicate glasses - High electrical resistivity. - High thermal expansion (matching other SOFC stack components), - Rapid crystallization kinetics. 20

21 BaO-Al2O3 SiO2 Seals 21

22 Glass/Metal Chemical Interactions 22

23 Glass Sealing Procedure 23

24 Brazed seals Potential alternative to glass-based seals Involves use of molten filler metal which flows and fills gap between components. Pros: Wetting behavior of molten metal facilitates hermetic sealing Easy to fabricate Properties can be tailored (CTE, Tm ) Cons: Electrically conductive! Few systems compatible with sealing under oxidizing conditions. - Noble metal brazes expensive - Ag relatively inexpensive, but is unstable in dual environment 24

25 Compressive seals Very little reported development work compared to rigid seals. Pros: May provide mechanical ìde-couplingî of adjacent stack components (avoid thermal stress development during fabrication, operation, thermal cycling). Potentially easy to fabricate In simplest form, no viscous/liquid sealing step required. 25

26 Cons: Potential for high leak rates through seal/component interfaces for simple gasket approaches. Few stable, compliant, hermetic candidate materials Load frame required to maintain compressive stress - Adds expense, complexity - Effect of long-term compressive load on dimensional stability of other stack components? 26

27 Compressive seal Advantages Easy replacement of seals in a malfunctioning cell stack. Resistance to thermal cycling Disadvantages Application of external load Complex design and high cost High gas leakage rate Unsuitable for mobile applications Poor stability Electrically conductive 27

28 Basis of compressive seal: Mica 28

29 Concept of hybrid compressive seal Simple mica layer yields excessively high leak rates through interfaces Mica: compliant in 2-D (x-y plane) Metal/glass interlayer: compliant in 3-D; seals off interfaces 29

30 Thermal cycling degradation of hybrid seals 30

31 Alumina Felt Seal Material Specifications Dimensions: 100 mm x 100 mm Thickness: 1 mm Compression: ~20% at 0.06 MPa (8 psi) Density: 0.19 g/cc at 0.06 MPa Organic Binder: ~5% Temperature Max: 1650 C 31

32 High Temperature Sealing Paste Specifications Drying and Curing Instructions: Let dry for 1-4 hours at room temperature Recommended to be cured at 200 C for 2 hours Then at 500 C for 2 hours before use 32

33 Alumina Slurry Specifications Works with: Alumina Felt 100 x 100 x 1 mm 3 Alumina Felt 170 x 170 x 1 mm 3 Alumina Felt Seals for 28 cm 2 Test Manifolds 33

34 Flexible Mica Paper Specifications Dimensions: 100 mm x 100 mm Thickness: 0.5 mm 34