Title Page
Contents
LIST OF SYMBOLS 16
LIST OF ABBREVIATIONS 20
LIST OF SUBSCRIPTS 22
ABSTRACT 24
1.0. INTRODUCTION 29
1.1. Fuel Cells 29
1.1.1. Background history of fuel cells 30
1.1.2. Types of Fuel cells 31
1.2. Solid oxide fuel cell (SOFC) 32
1.2.1. Background history and Status of SOFC 33
1.2.2. Principles and operation of SOFC 35
1.2.3. Merits and demerits of SOFC 36
1.2.4. Thermodynamics of SOFC 37
1.2.5. Efficiency of SOFC 40
1.2.6. SOFC materials 42
1.3. Losses associated with SOFC operation 46
1.3.1. Ohmic loss 47
1.3.2. Activation overpotential 48
1.3.3. Concentration overpotential 49
1.3.4. Losses caused by internal leakage current and gas crossover 50
1.4. Electrochemical methods 51
1.4.1. Steady state polarization (SSP) technique 52
1.5. Main Processes in SOFCs 53
1.5.1. Mass transfer in SOFC 53
1.6. Gas transport of uncharged species 54
1.6.1. Gas diffusion in porous media 54
1.6.2. Gas diffusion in porous SOFC electrodes 56
1.7. Problem statement 59
1.8. Research objectives 60
1.8.1. Specific Objectives 60
2. EXPERIMENTAL 62
2.1. Cell components and Properties 62
2.1.1. Cell 62
2.1.2. Cell frame 63
2.1.3. Current collectors 65
2.1.4. Sealant 66
2.1.5. Fuel and oxidant gases 67
2.2. Making of bench-type SOFC single cell 68
2.3. Electrochemical measurement techniques 71
2.3.1. Steady-state polarization (SSP) technique 71
2.3.2. Inert-gas step addition (ISA) method 72
2.3.3. Reactant gas addition (RA) method 77
3. RESULTS AND DISCUSSION 79
3.1. Effect of Gas-Phase Transport Using ISA Method 79
3.1.1. Current-voltage (I-V) behavior 82
3.1.2. Boundary layer effect 85
3.1.3. Overpotential expressions with a gas-phase mass transfer rate 88
3.1.4. Anode ISA results 90
3.1.5. Cathode ISA results 100
3.2. Experimental Analysis of Internal Leakage Current 106
3.2.1. Current-voltage (I-V) behaviour 109
3.2.2. Voltage shift behavior at an open-circuit state 112
3.2.3. Comparison of the voltage shift behaviors for dry and humidified anode gas 113
3.2.4. Determination of internal leakage current using an ISA method 116
3.2.5. Determination of electronic resistivity of the YSZ electrolyte 122
3.3. Effect of Water on the Anodic Overpotential at Low Currents 125
3.3.1. Negligible charge transfer resistance 127
3.3.2. Current-voltage behavior under dry and wet anode gas conditions 127
3.3.3. Current-voltage behavior under different anode gas compositions 132
3.3.4. Current-voltage behavior under different cathode gas compositions 134
3.3.5. Effect of water inducing mass transfer resistance 137
3.3.6. Dry hydrogen fuel 138
3.3.7. Wet hydrogen fuel 144
3.4. Electrode Reaction Properties Using RA Method 146
3.4.1. Reactant gas addition method 150
3.4.2. Anode results 153
3.4.3. Cathode results 167
4. CONCLUSIONS 177
REFERENCE 180
국문요약 197
LIST OF PUBLICATION 201
Table 1. Performance of SOFCs from the latest projects 35
Table 2. Properties of the components of cell I 63
Table 3. Properties of the components of cell II 63
Table 4. Fuel and oxidant utilisation and their corresponding flow rates 67
Table 5. Kinematic viscosity (V, cm²s-1), flow rate (U, cm³ min-1), flow velocity of the gas in channels (U, cm s-1), and Reynolds number (NRe) at 1023 K for an anode of...[이미지참조] 87
Table 6. Kinematic viscosity (V, cm²s-1), flow rate (U, cm³ min-1), flow velocity of the gas in channels (U, cm s-1), and Reynolds number (NRe) at 1023 K for a cathode of...[이미지참조] 87
Table 7. Calculated Cells I and II internal leakage currents at a temperature of 1023 K, E0 of 0.992 V, and at different anode gas flow rates[이미지참조] 121
Table 8. Calculated electronic resistivities of Cells I and II at different anode gas flow rates 124
Table 9. The properties of H₂ and H₂O at 1023 K in the anode fuel gas. 139
Table 10. △VH₂, △VH₂O and ηan values measured using RA and ISA methods at different anode gas utilisations[이미지참조] 166
Table 11. ηO₂,mt,G and △VO₂ values measured using the ISA and RA methods at different cathode gas utilisations[이미지참조] 176
Fig. 1. A schematic diagram of the operating principle of a solid oxide fuel cell 36
Fig. 2. Current-voltage behaviour of a fuel cell showing the theoretical and actual cell voltages 47
Fig. 3. Anode-supported single cells obtained from Kceracell and E&KOA (a) Cell I (b) Cell II 62
Fig. 4. Schematic diagram of the cell frame. (1) Thermocouple port (2) Gas channel (3) Current collector (4) Internal manifold (5) Gas inlet (6) Gas outlet 64
Fig. 5. Image of the (a) anode and (b) cathode cell frames 64
Fig. 6. Images of the (a) silver mesh and (b) Nickel mesh current collectors 65
Fig. 7. Image of glass sealant paste provided by Kceracell 66
Fig. 8. Images of the (a) gas drying unit and (b) bottled desiccant anhydrous 8 mesh 68
Fig. 9. Schematic drawing of the Experimental setup. 1. Mass flow controller; 2. Addition port for the anode; 3. Addition port for the cathode; 4. Humidifier; 5. Heating compressor... 70
Fig. 10. Making of the SOFC bench cell 70
Fig. 11. Images of the (a) DC electronic load, (b) Current connection terminals, (c) voltage sense connection terminals 71
Fig. 12. Schematic drawing of the gas flow routes from the inert gas inlet port to the cell 76
Fig. 13. Schematic depictions of the flow rate changes caused by the added inert gas 77
Fig. 14. Current-voltage behavior at 1023 K and different anode (Uan, flow of H₂ and 3% H₂O) and cathode gas (Uca, air flow) conditions. The unit ccm is cm³ min-1[이미지참조] 84
Fig. 15. Schematic drawing of gas concentration distribution at anode and cathode under polarization 88
Fig. 16. Anodic ISA results with nitrogen addition rates of 300 cm³min-1 at an anode flow rate of 269 cm³min-1 and a cathode flow rate of 622 cm³min-1, 1023 K, 1 atm, and at...[이미지참조] 96
Fig. 17. Effect of the inert gas species on the anodic ISA results with Ui=300 cm³min-1 at uf=0.4, uox=0.4, i=15 A, 1023 K, 1 atm[이미지참조] 97
Fig. 18. Effect of the nitrogen addition rate (Ui) on the anodic ISA results at uf=0.4, i=15 A, 1023 K, 1 atm, (Ui=100, 300 and 500 cm³min-1)[이미지참조] 98
Fig. 19. Voltage behaviour at a fixed nitrogen gas addition flow rate (Ui) of 300 cm³ min-1 and a changing reactant gas flow rates in the form of utilizations (uf). (uox=0.4, i=15...[이미지참조] 98
Fig. 20. Peak heights of 'a' as a function of nitrogen addition rate at various anode utilization conditions, uox=0.4, i=15 A. 1023 K, 1 atm[이미지참조] 99
Fig. 21. Relationship between voltage peak heights of 'a' in Fig. 16 and their shifted utilizations 99
Fig. 22. Cathodic ISA results with nitrogen addition rates of 300 cm³min-1 at a cathodeflow rate of 622 cm³min-1 and an anode flow rate of 269 cm³min-1, 1023 K, 1 atm, and at...[이미지참조] 103
Fig. 23. Effect of the nitrogen addition rate (Ui) on the cathodic ISA results at uf=0.4, i=15 A, 1023 K, and 1 atm (Ui=100, 300, and 500 cm³ min-1)[이미지참조] 104
Fig. 24. Effect of the reactant flow rates (Uf) on the cathodic ISA results at a fixed nitrogen addition rate (Ui) of 300 cm³ min-1 (uf=0.4, i=15 A, 1023 K, 1 atm)[이미지참조] 104
Fig. 25. Peak heights of 'a' as a function of nitrogen addition rate at various cathode utilization conditions, uf=0.4, i=15 A. 1023 K, 1 atm[이미지참조] 105
Fig. 26. Relationship between voltage peak heights of 'a' in Fig. 22 and their shifted utilizations 105
Fig. 27. Current-voltage behaviour of cell I operated at 1023 K using different anode gasflow rates (Uan) and a fixed cathode gas flow rate (Uca). The anode (uf) and cathode (uox)...[이미지참조] 111
Fig. 28. Voltage-shift pattern of Cells (a) I and (b) II with an N₂ gas addition rate of 0.3L min-1 to the anode at 1 atm and 1023 K. (Uan=0.262 L min-1, Uca=0.622 L min-1, and i=0)[이미지참조] 113
Fig. 29. Voltage-shift patterns of Cell I with an N₂ gas addition rate of 0.3 L min-1 to an anode consisting of (a) dry H₂ gas and (b) 3% H₂O + H₂ (pressure=1 atm, temperature...[이미지참조] 115
Fig. 30. △Vo,an as a function of the nitrogen addition rate at various anode gas flow rates,(Uca=622 cm³ min-1, i=0, 1023 K, and 1 atm)[이미지참조] 117
Fig. 31. Relationship between α(=UH2/UH2+UN2) and ß(=EXP(2F(E0-△Vo,an-EOCV,m+RT/2Fln PH2√PO2)/RT) for (a) Cell I and (b) Cell II operated at 1023 K under different anode gas flow rates (Uan)[이미지참조] 119
Fig. 32. Relationship between α and △Vo,an for (a) Cell I and (b) Cell II under different anode gas flow rates (Uan)[이미지참조] 123
Fig. 33. Current-voltage behaviour of the cell operated at 1023 K under different anodegas (van) and cathode gas (vca) flow rates. The anode (uf) and cathode (uox) utilisations...[이미지참조] 129
Fig. 34. Current-voltage behaviour of a cell operated at 1023 K under different anode gas compositions and a fixed cathode gas composition. Cathode: air; anode: H₂ and 50% N₂... 133
Fig. 35. Current-voltage behaviour of a cell operated at 1023 K under different anode gas(van) and cathode gas (vca) flow rates. Anode: H₂; cathode: (a) air and (b) pure oxygen[이미지참조] 135
Fig. 36. Current-voltage behaviour of a cell operated at 1023 K under different anode gas(van) and cathode gas (vca) flow rates. Anode: 3% H₂O + H₂; cathode: (a) air and (b) pure oxygen[이미지참조] 136
Fig. 37. The calculated mass transfer resistance of H₂O inducing overpotential (ηmt,H₂O) at dry H₂ fuel with the relations of Eqs. (71,72). The utilisation is based on H₂...[이미지참조] 140
Fig. 38. The calculated mass transfer resistance of H₂ inducing overpotential (ηmtmt,H₂) at the different gas utilisations in the anode. Utilisation is at a current condition of 15 A[이미지참조] 141
Fig. 39. The effect of water-inducing overpotential (ηmt,H₂O) on the I-V curve at low currents. The arrows indicate that the water-inducing mass transfer resistance (Rmt,H₂O)[이미지참조] 144
Fig. 40. The calculated ηmt,H₂O at the 3% H₂O + H₂ fuel with respect to the applied current[이미지참조] 145
Fig. 41. Schematic representation of voltage shift behaviours when adding a reactant gas species A to an electrode of a SOFC at open-circuit and polarisation states 151
Fig. 42. Anodic RA measurement results with an H₂ addition rate of 0.3 L min-1 to the anode of a cell operated at 1023 K at 1 atm, anode H₂ + 3% H₂O flow rate of 0.269 L...[이미지참조] 154
Fig. 43. △EH₂ and △VP,H₂ behaviour with respect to H₂ addition rates at 1023 K, 1 atm, anode H₂ + 3% H₂O flow rate of 0.269 L min-1 (uf=0.4 at 150 mA cm-2), and a...[이미지참조] 156
Fig. 44. Relationship between △VH₂ and H₂ addition rates at different anode fuel (H₂ + 3% H₂O) utilisations (temperature=1023 K, pressure=1 atm, and cathode air feed rate...[이미지참조] 160
Fig. 45. Steady-state polarisation behaviour at different anode H₂O concentrations at 1023K, 1 atm, H₂ gas flow rate of 0.261 L min-1, and cathode air flow rate of 0.622 L min-1[이미지참조] 162
Fig. 46. △VH₂O behaviour as a function of H₂O concentration at various anode fuel utilisations at 1023 K, 1 atm, cathode air flow rate of 0.622 L min-1. (a) △VH₂O with the...[이미지참조] 165
Fig. 47. Cathodic RA measurement results with an O₂ addition rate of 0.3 L min-1 to the cathode of a cell operated at 1023 K, 1 atm, and an anode H₂ + 3% H₂O flow rate of 0.269...[이미지참조] 168
Fig. 48. △EO₂ and △VP,O₂ behaviour with respect to O₂ addition rates at 1023 K, 1 atm, anode H₂ + 3% H₂O flow rate of 0.269 L min-1 (uf=0.4 at 150 mA cm-2), and a cathode...[이미지참조] 170
Fig. 49. Schematic drawing of gas and solid phase concentration distributions of oxygen species at the cathode 172
Fig. 50. Relationship between △VO₂ and O₂ addition rates at different cathode utilisations(temperature=1023 K, pressure=1 atm, and anode H₂ + 3% H₂O feed rate=0.269 L min-1).[이미지참조] 174