Chemical Degradation of Seal Materials in PEM Fuel Cell Environment

1. Chemical Degradation of Seal Materials in PEM Fuel Cell Environment J. Tan, Y. J. Chao, Woo-Kum Lee, CS Smith, JW Van Zee and CT Williams NSF-I/UCRC for Fuel Cells College of Engineering & Information Technology University of South Carolina, USA

2. Cracking Virgin silicon gasket Background • GM found that their silicone gaskets are failing after 2,000 hours (by cracking and increasing roughness). • Outlet water from a stack has a pH of 3-4. • 1-2 ppm HF was found in the outlet water from the stack. • Will seal integrity be one of the major next technological barriers in commercializing Fuel Cells ?

3. Ambient Gasket or Seal Pressure Temperature Chemicals Gasket/seal as a structural member • Characteristics of gasket/seal : • Under compression, exposed to chemicals, high temperature, pressure, cyclic conditions, etc. • Loss of functionality : by cracking and /or stress relaxation • Cracking : due to corrosion under compression (Chemical stability) • Stress Relaxation : material degradation… loss its sealing ability (mechanical stability) • Leachants :detrimental sometimes (chemical stability)

4. Objective: Develop design guidelines for gasket materials in fuel cell stacks • Studies: Investigate the long term mechanical and chemical stability/durability of PEM fuel cell gasket materials • Scope of work: • Testing commercially available elastomeric seal materials • Identifying leachants into the soaking solutions • FEM analysis of seal profiles to understand the mechanical stresses in test specimens and seals during operation • Investigating the degradation mechanisms • Investigating changes in mechanical properties as a function of stress state and soak environment • Developing Accelerated Durability Testing (ADT) methodologies, and confirming similar failure mechanisms (ADT vs. fuel cell)

5. Flow Chart of Studies : Current and intended -

6. PROJECT ACCOMPLISHMENTS • Bend Strip Tests for commercially available elastomeric seal materials (Silicone S, Silicone G, EPDM and Fluoroelastomer T) • Constant load compressive test on gasket samples • ATR-FTIR test to elucidate the material surface chemical degradation • XPS test to analyze the degradation mechanisms for gasket materials • FEA modeling for the bend strip specimens –stress state • Atomic adsorption spectrometry analysis to identify leachants into the soaking solutions

7. Summary of bend experiments performed Silicone S Silicone G EPDM Fluoroelastomer Bending Angle 180° , 120°, 90°, 0° 80°C 60°C Simulated Solution ADT Solution Simulated Solution ADT Solution 4x4x2x2=64

8. Two Solutions used in the Tests • Regular solution - similar to that in PEM FC 12 ppm H2SO4, 1.8 ppm HF and reagent grade water • ADT solution – for short term accelerated testing 1M H2SO4, 10 ppm HF and reagent grade water

9. Gasket sample Fuel cell solution Fixture for 120° bend Polypropylene bottle Experimental setup for bend tests Bottles are placed in an oven with the desired temperature Specimens dimensions: 4cm long x 1 cm wide Fixture for 90° bend Fixture for 180° bend

10. 10 m 10 m 10 m 10 m 10 m 10 m 10 m 10 m Optical micrographs for Silicone S samples with bend angle of 120° exposed to ADT solution at 60°C, 500× (showing the degradation of the surface) Without exposure 6 weeks exposure 10 weeks exposure 12 weeks exposure 14 weeks exposure 17 weeks exposure 23 weeks exposure 19 weeks exposure

11. Sample Results - Gasket Degradation in ADT solution (120o bending , 80 oC vs. 60 oC) Failed at 4 w Started cracking at 3 w Delta weight (%) Started cracking at 3 w Started cracking at 14 w Started cracking at 6 weeks for Silicone G at 60°C Failed at 6 w Time of exposure (weeks) Silicone S and Silicone G degraded; but not EPDM and Fluoroelastomer T

12. 1010 10 1080 7 1260 1150 5 1210 868 3 1420 1 0 Wavenumber (cm-1) ATR-FTIR results for Silicone G before and after exposure to ADT solution at 80°C (showing little chemical degradation) Absorbance Little Change – degradation primarily from stress (--bend tests)

13. 1020 0.32 1150 0.30 0 1090 0.28 1210 0.26 1260 0.24 0.22 0.20 0.18 866 0.16 1 0.14 Absorbance 0.12 0.10 3 0.08 0.06 5 0.04 0.02 7 0.00 10 -0.02 -0.04 1400 1300 1200 1100 1000 900 Wavenumbers (cm-1) Wavenumber (cm-1) ATR-FTIR results for Silicone S before and after exposure to ADT solution at 80°C (showing chemical degradation) vibration of  (Si-O-Si) stretching (Si-CH3) bending mode (Si-OH) stretching vibration Absorbance Degradation from change of chemistry (and stress)

14. X-ray Photoelectron Spectroscopy (XPS) was used to obtain qualitative and quantitative information on the surface of the four gasket materials before and after exposure to ADT solution at 80C and 60C. Surface atomic concentration of each element and ratios of atomic concentrations of O and C to Si for Silicone S sample before and after exposures to ADT solution at 80C

15. no exposure XPS Survey Spectra Silicone S sample before and after exposure to ADT solution at 80C 1 week exposure

16. CH3 OH Fuel cell environment O Si O O Si O + CO2 Oxidation CH3 CH3 C decreased and O increased Possible oxidation mechanism of Si-CH3 groups

17. no exposure XPS (High Resolution) Silicone S sample before and after exposure to ADT solution at 80C 1 week exposure

18. CH3 CH3 Fuel cell environment O Si O O Si OH H+ CH3 CH3 Possible mechanism of Chain Scission in the Backbone - A intensity decreased (backbone chain scission) - B intensity increased

19. Leachants detection Calcium (mg/l) from 120bend tests in ADT solution Calcium values(mg/l) Time of exposure (weeks)

20. Conclusions and Future work • Chemical Stability Optical microscopy, FTIR and XPS tests to reveal the attack of the chemicals to the materials. Silicone materials are not good. • Mechanical Stability – Ductility, hardness, constitutive law, cracking • Degradation of Sealing Capability of gasket materials Stress relaxation tests of gaskets in fuel cell solutions • FEA modeling whenever needed • Selection and fabrication of new seal materials for FC applications

21. Thank you for your attention !Questions ?