Figure 1. Pinhole tester (a) and the plane figure of the upper part (b) in pinhole tester. 27

Figure 2. Scheme of the out-cell humidifier for membrane contamination. 30

Figure 3. Schematic diagram of a single cell configuration used in measurement of hydrogen crossover current. 35

Figure 4. Schematics of the cell configurations with one electrode. (a) anode only mode and (b) cathode only mode. 36

Figure 5. Correlation between the drop of cell performance and the weight increase of the second failed membrane samples. 44

Figure 6. Detected impurities in the second failed membrane samples by XRF. 45

Figure 7. Equivalent electrical circuit (a) representing a fuel cell under an external load IL as well as a shorting resistor Rs and resistance calculation (b) by ohm's law.(이미지참조) 46

Figure 8. Detected Pt in the second failed membrane samples by EPMA. 47

Figure 9. Flow diagram of the analytical methods for finding the degradation source in the early failed membrane. 54

Figure 10. Amount of ionic contaminants on Nafion 112 membranes, which were humidified with city water in the out-cell humidifier for 7 and 14 days. 58

Figure 11. Amount of ionic contaminants on 25 ㎠ MEAs, which were humidified with city water in the out-cell humidifier for 7 and 14 days. 59

Figure 12. Comparison of ionic contaminants on 25 ㎠ MEAs, which were humidified with city water through the in and out-cell humidification for 3 days. 60

Figure 13. Change of the cell voltages during a single cell operation (25 ㎠ MEA) with city water under the constant current of 400 ㎃/㎠ for 170 h. 61

Figure 14. Comparison of a single cell performance (25 ㎠ MEA) before and after cell operation with city water under the constant current of 400 ㎃/㎠ for 170 h. 62

Figure 15. Amount of ionic contaminant in the membrane after a single cell operation (25 ㎠ MEA) with city water under the constant current of 400 ㎃/㎠ for 170 h. 63

Figure 16. Contaminated Ca2+ concentration in Nafion 112 membranes, which were immersed in 37 ㎎/l Ca2+ solution for the different time.(이미지참조) 65

Figure 17. Comparison of the single cell performances of MEAs, which were consisted with the contaminated Nafion 112 membranes to the different level of Ca2+ ions.(이미지참조) 66

Figure 18. Comparison of current densities at 0.6 V. Cells were assembled with contaminated Nafion 112 membranes to the different level of Ca2+.(이미지참조) 67

Figure 19. IECs of the contaminated Nafion 112 membranes as a function of the amount of contaminated Ca2+ ions.(이미지참조) 70

Figure 20. Water-uptake of the contaminated Nafion 112 membrane as a function of the amount of contaminated Ca2+ ions.(이미지참조) 71

Figure 21. Comparison of the single cell performances of MEAs, which were consisted with the fresh Nafion 112 membrane and E-TEK electrodes impregnated with the contaminated ionomer. 72

Figure 22. Chemical structure of Nafion membrane. 73

Figure 23. ATR spectra of the degraded Nafion 117 membranes in 10％ H₂O₂ solution with various Fe2+ concentration.(이미지참조) 76

Figure 24. ATR spectra of the degraded Nafion 117 membranes in 4 ppm Fe2+ solution with various H₂O₂ concentration.(이미지참조) 77

Figure 25. Weight loss of the degraded Nafion 117 membranes as a function of Fe2+ concentration in 10 ％ H₂O₂ solution.(이미지참조) 79

Figure 26. Weight loss of the degraded Nafion 117 membranes as a function of H₂O₂ concentration in 4 ppm Fe2+ solution(이미지참조) 80

Figure 27. Water-uptake of the degraded Nafion 117 membranes as a function of Fe2+ concentration in 10 ％ H₂O₂ solution.(이미지참조) 82

Figure 28. Water-Uptake of the degraded Nafion 117 membrane as a function of H₂O₂ concentration in 4 ppm Fe2+.(이미지참조) 83

Figure 29. IEC of the degraded Nafion 117 membranes as a function of Fe2+ concentration in 10 ％ H₂O₂ solution.(이미지참조) 85

Figure 30. IEC of the degraded Nafion 117 membranes as a function of H₂O₂ concentration in 4 ppm Fe2+ solution.(이미지참조) 86

Figure 31. IEC of the degraded Nafion 117 membranes as a function of degradation time in low concentration of radical solution (1 ％ H₂O₂, 2 ppm Fe2+).(이미지참조) 87

Figure 32. Stress-strain behaviors of the degraded Nafion 117 membranes as a function of H₂O₂ concentration in 4ppm Fe2+ solution.(이미지참조) 89

Figure 33. SEM image of the degraded Nafion 117 membrane in 30 ％ H₂O₂ , 4 ppm Fe2+ solution.(이미지참조) 90

Figure 34. In situ limiting hydrogen crossover current of the fresh Nafion 117 membrane. 180 ml/min of humidified hydrogen and nitrogen was supplied to the cell at room temperature and atmospheric pressure. 91

Figure 35. Comparison of hydrogen crossover currents of the fresh and degraded Nafion 117 membranes as a function of temperature at 0.4 V. 92

Figure 36. Comparison of MEA performances with the fresh and degraded Nafion 117 membranes in 30 ％ H₂O₂, 4 ppm Fe2+ solution for 24 and 48 h. 94

Figure 37. CV scans of various concentration of H₂O₂ in 4 N H₂SO₄ saturated with air. 97

Figure 38. H₂O₂ flux measured in effluent water from two different cells, which were operated at different RH (％) conditions in anode only and cathode only modes respectively. 98

Figure 39. OCV changes of a single cell with 25 ㎠ MEA, which was operated at 80℃ under OCV for 130 h. Dry hydrogen (93 ml/min) and humidified air (65 ％ RH, 296 ml/min) were supplied to anode and cathode respectively. 101

Figure 40. Comparison of the single cell performances with 25 ㎠ MEA before and after degradation test. Cell was operated at 80℃ with dry hydrogen and humidified air (65 ％ RH) under OCV condition for 130 h.... 102

Figure 41. Comparison of hydrogen crossover current of the single cell with 25 ㎠ MEA before and after degradation test under OCV and dry gas condition at anode. 103

Figure 42. Cathode fluoride emission rate (FER) during cell operation under OCV and dry gas condition at anode for 144 h. 104

Figure 43. OCV changes of a single cell with 25 ㎠ MEA, which was operated at 80℃ under OCV for 130 h. Humidified hydrogen (65 ％ RH, 93 ml/min) and dry air (296 ml/min) were supplied to anode and cathode respectively. 110

Figure 44. Comparison of the single cell performances with 25 cm² MEA before and after degradation test. Cell was operated at 80℃ with humidified hydrogen (65 ％ RH) and dry air under OCV condition for 130 h.... 111

Figure 45. Comparison of hydrogen crossover current of the single cell with 25 ㎠ MEA before and after degradation test under OCV and dry gas condition at cathode. 112

Figure 46. Cross-sectional TEM images of the membrane degraded under OCV/low humidity condition (14 ％ RH) at anode. (a) TEM image of the whole membrane area, (b) TEM image of the membrane near the cathode side, (c) TEM image of the... 113

Figure 47. Cathode fluoride emission rate (FER) during 510 h operation under OCV at 70℃, 100 ％ RH at anode and 0 ％ RH at cathode. 116

Figure 48. OCV changes of a single cell with 25 ㎠ MEA, which was operated at 70℃ under OCV for 510 h. Humidified hydrogen (100 ％ RH, 93 ml/min) and dry air (0 ％ RH, 296 ml/min) were supplied to anode and cathode respectively. 117

Figure 49. Comparison of hydrogen crossover current of the single cell with 25 ㎠ MEA before and after degradation test under OCV and dry gas condition at cathode for 510 h. 118

Figure 50. Comparison of the single cell performances with 25 ㎠ MEA before and after degradation test. Cell was operated at 70℃ with humidified hydrogen (100 ％ RH) and dry air under OCV condition for 510 h.... 119

Figure 51. Comparison of OCV voltage according to the anode RH during cell operation at 70℃ for 144 h under OCV and dry gas condition at cathode. 120

Figure 52. Voltage changes of the single cell with 25 ㎠ MEA, which was operated at 80℃ with dry hydrogen and humidified air (65 ％ RH) under low current density at 10 ㎃/㎠ for 200 h. 123

Figure 53. Comparison of the single cell performance before and after degradation test. Cell was operated at 80℃ with dry hydrogen and humidified air (65 ％ RH) under low current density at 10 ㎃/㎠ for 200 h.... 124

Figure 54. Comparison of hydrogen crossover current of the single cell with 25 ㎠ MEA before and after degradation test under low current density at 10 ㎃/㎠ for 200 h. 125

Figure 55. Cathode fluoride emission rate (FER) during cell operation under low current density at 10 ㎃/㎠ for 200 h. 126

Figure 56. Comparison of the decrease of cell voltage at each current density. Cells were operated under the different current densities at 80℃ with dry hydrogen and humidified air (65 ％ RH) for 144 h. 128

Figure 57. Comparison of FER loadings after 144 h operation at the different current densities. Cells were operated at 80℃ with dry hydrogen and humidified air (65 ％ RH) for 144 h. 129

Figure 58. FERs from the electrode and non-electrode sides. (a) Cell was operated in cathode only mode at 90℃ for the different time (10~30 h). Humidified (30％RH) air and hydrogen was supplied to the electrode and the non-electrode sides respectively.... 132

Figure 59. Amount of Pt dissolved/deposited in Nafion 112 membrane after cell operation. Cell was operated in cathode only mode at 90℃ for the different time (10~30 h) with humidified air for electrode side and hydrogen for non-electrode side... 133

Figure 60. Correlation between FER and the amount of Pt dissolved/deposited in Nafion 112 membrane after cell operation. Cell was operated in cathode only mode at 90℃ for the different time (10~30 h) with humidified air at the electrode side and ... 134

Figure 61. Change of FERs from the electrode and the non-electrode sides. Cell was operated in anode only mode at 90℃ with humidified (30 ％ RH) hydrogen and air, which were switched repeatedly. 136

Figure 62. Cross-sectional TEM images of the membrane sliced with ultramicrotome, which were obtained after cell operation. Cell was operated in anode only mode at 90℃ with humidified (30 ％ RH) hydrogen and air, which were switched repeatedly. 137

Figure 63. UV spectra of the solutions, in which the fresh and the Pt dissolved membranes were immersed for 12 days 1.33 M NaCl. 140

Figure 64. Schematics of the cell configuration for Pt dissolution/reduction (a) and Pt dissolution (b) in the membrane. 141

Figure 65. FERs obtained from two different cells, in which the stage of Pt reduction was included (a) and excluded (b). 142

Figure 66. Cell configuration to measure the radical formation, in which the electrode was positioned inside the membrane. 143

Figure 67. Comparison of ESR spectra of the fresh and the degraded electrodes. 144

Figure 68. Carbon radical formation mechanism. 145

Figure 69. OCV changes during cell operation under OCV at 90℃ for 144 h. Dry hydrogen and humidified oxygen gas (65 ％ RH) were supplied to anode and cathode respectively. 148

Figure 70. Hydrogen crossover current of the cell, which was operated under OCV at 90℃ for 144 h. Dry hydrogen and humidified oxygen gas (65 ％ RH) were supplied to anode and cathode respectively. 149

Figure 71. Nyquist plots of before and after cell operation under OCV at 90℃ for 144 h. Dry hydrogen and humidified oxygen gas (65 ％ RH) were supplied to anode and cathode respectively. 150

Figure 72. Comparison of cell performances before and after cell operation under OCV at 90℃ for 144 h. Dry hydrogen and humidified oxygen gas (65 ％ RH) were supplied to anode and cathode respectively. 151

Figure 73. SEM images of pinholes (a) formed in membrane and the change of membrane thickness after cell operation under OCV at 90℃ for 144 h. Dry hydrogen and humidified oxygen gas humidified (65 ％ RH) were supplied to anode and cathode respectively. 152

Figure 74. SEM/EDS spectrums of membrane after cell operation under OCV at 90 ℃ for 144 h. Dry hydrogen and humidified oxygen gas humidified (65 ％ RH) were supplied to anode and cathode respectively. 153

Figure 75. Cross-sectional TEM images of the membrane degraded under OCV with dry hydrogen and humidified oxygen gas (RH 65％) for 144 h. (a) TEM image of the whole membrane area, (b) TEM image of the membrane near the cathode, (c) TEM... 154

Figure 76. Schematic drawing of Pt dissolution and deposition in the membrane and decomposition mechanism of membrane by radical generated on Pt deposition. 155