Title Page

Contents

Abstract 13

Chapter 1. Introduction 15

1.1. Environmental issue and CCUS 15

1.2. Types of CCU 18

1.3. Electrochemical CO₂ reduction 20

1.4. Mechanism of electrochemical reaction 24

1.5. Research strategy 29

1.5.1. Alternative oxidation reaction 29

1.5.2. Electrodeposition with hydrogen evolution reaction 32

1.6. Research objective and outline 34

1.7. References 36

Chapter 2. The Zn-gaseous CO₂ fuel-cell system using a sacrificial Zn anode and a high-surface-area dendritic Ag-Cu cathode 39

2.1. Introduction 39

2.2. Experiments 46

2.3. Results and discussion 49

2.3.1. Characterizations of fabricated electrode 49

2.3.2. Mechanism of the spontaneous CO₂ reduction 55

2.3.3. Performance of the spontaneous CO₂ reduction system 56

2.3.4. Characterization of cathode after reaction 61

2.3.5. Products analysis 63

2.3.6. Catalytic performance 65

2.3.7. Evaluation of economic and energy efficiency 69

2.4. Conclusion 72

2.5. References 73

Chapter 3. Carbon-neutralized direct methanol fuel cell using bi-functional (methanol oxidation/CO₂ reduction) electrodes 78

3.1. Introduction 78

3.2. Experiments 82

3.3. Results and discussion 87

3.3.1. Characterizations of fabricated electrode 87

3.3.2. Mechanism of Carbon-neutralized direct methanol fuel cell 89

3.3.3. Performance of fuel-cell mode 92

3.3.4. Performance of CO₂ reduction mode 96

3.3.5. Comparison of conventional system 100

3.4. Conclusion 104

3.5. References 105

Chapter 4. The development of a gas-feeding CO₂ fuel cell using direct hydrazine oxidation reaction 111

4.1. Introduction 111

4.2. Experiments 115

4.3. Results and discussion 120

4.3.1. System mechanism 120

4.3.2. Characteristic of catalysts 121

4.3.3. System performance in terms of flow rates 126

4.3.4. Water management of cathode 129

4.3.5. System performance in terms of cathode materials 131

4.3.6. Characteristic of electrodes after cell operation 137

4.3.7. System assessment 139

4.3.8. Comparison with development system 141

4.4. Conclusion 144

4.5. References 145

논문요약 149

Chapter 2 8

Table 2-1. The standard reduction potentials for the main products of electrochemical CO₂ reduction 40

Table 2-2. Comparison of slope and exchange-current density (J₀) in Tafel plots of Cu and Ag-Cu electrode 68

Table 2-3. The market price and amounts of reactant, products and electricity 70

Chapter 3 8

Table 3-1. A comparison of the DMFC performance with other studies 95

Table 3-2. The F.E. of products and CO₂ conversion obtained from the two-electrode system with a Pt-Zn anode and a dendritic Pd-Ag cathode for... 103

Chapter 4 8

Table 4-1. Faradaic efficiency of each product and the single-pass CO₂ conversion on Cu and Ag electrode in the N₂H₄/CO₂ fuel cell 135

Table 4-2. The comparison with electrochemical CO₂ reduction using other anodic reaction 140

Table 4-3. The market price and amounts of reactant, products and electricity using Ag cathode 140

Table 4-4. The comparison of the developed two CO₂ fuel-cell systems 142

Chapter 1 9

Figure 1-1. The graph of atmospheric concentration of CO₂ at Mauna Loa Observatory. 16

Figure 1-2. The flowchart of CCUS technology 17

Figure 1-3. the classification of CCU. 19

Figure 1-4. The schematic diagram of conventional electrochemical CO₂ reduction. 22

Figure 1-5. The periodic table with colors and major products. 23

Figure 1-6. Electrochemical reaction mechanism based on electrode potential. 25

Figure 1-7. The pathway of an electrode reaction in an electrochemical cell. 26

Figure 1-8. The standard reduction potential of CO₂ and H₂ at different pH. 30

Figure 1-9. Schematic diagram of a HER and electrodeposition using a hydrogen bubble template. 33

Chapter 2 9

Figure 2-1. The schematic diagram of spontaneously operated fuel-cell-type CO₂ reduction system. 45

Figure 2-2. The FESEM images of (a) the dendritic Zn electrode electrodeposited on Ni mesh and (b) the magnified of Zn anode. 50

Figure 2-3. The XRD patterns obtained from the electro-deposited dendritic Zn electrode. 51

Figure 2-4. The FESEM images of (a) the bare Ni mesh and (b) the dendritic Ag-Cu layers electrodeposited on Ni mesh prepared at the high cathodic... 53

Figure 2-5. The XRD patterns obtained from the electro-deposited dendritic Ag-Cu electrode. 54

Figure 2-6. The polarization and power density curves of two-electrode system (a) with a sacrificial Zn anode and a dendritic Cu cathode, and (b) with... 58

Figure 2-7. The polarization and power density curves of static Zn-CO₂ fuel cell using Ag-Cu cathode. 59

Figure 2-8. The time-voltage profiles of Zn-gaseous CO₂ fuel cell at 25 mA/cm². 60

Figure 2-9. The XRD patterns obtained from the electro-deposited dendritic Ag-Cu electrode after the spontaneous CO₂ reduction. 62

Figure 2-10. (a) The FTIR spectrum obtained from the product after the CO₂ reduction in fuel-cell-type electrochemical system with a sacrificial Zn anode... 64

Figure 2-11. Tafel plots of Cu electrode and Ag-Cu electrode for the CO₂ conversion to (a) CO, (b) CH₄, and (c) C₂H₄ product. 67

Figure 2-12. Comparison of F.E. and energy consumption with previously published studies. 71

Chapter 3 11

Figure 3-1. The schematic diagram of the carbon-neutralized direct methanol fuel cell. 81

Figure 3-2. Assembly view of the cell module. 85

Figure 3-3. The FESEM images of (a) the bare Ni mesh and (b) the dendritic Pd-Ag layers electrodeposited on Ni mesh at the high cathodic overpotential of... 88

Figure 3-4. The mechanism of (a) fuel-cell mode and (b) spontaneous CO₂ reduction mode in the carbon-neutralized direct methanol fuel cell. 91

Figure 3-5. (a) The polarization/power density curves and (b) the voltage-time graph at constant current condition in fuel-cell mode. 94

Figure 3-6. (a) The polarization/power density curves and (b) the voltage-time graph at constant current condition in CO₂ reduction mode. 98

Figure 3-7. The FE-SEM image of Pt-Zn bi-functional electrode and EDS element mapping after operation of CO₂ reduction mode. 99

Figure 3-8. The current density-time profiles applied each potential. 102

Chapter 4 12

Figure 4-1. Schematic diagram of aqueous N₂H₄/gas-feeding CO₂ fuel cell. 114

Figure 4-2. Assembly view of the cell module. 118

Figure 4-3. (a) FESEM image and (b) XRD patterns obtained from the Ag NPs cathode. 123

Figure 4-4. (a) FESEM image and (b) XRD patterns obtained from the Pt/C on carbon paper. 124

Figure 4-5. LSV curve for (a) hydrazine oxidation and (b) CO₂ reduction 125

Figure 4-6. The polarization and the power density curve according to (a) flow rate of anolyte and (b) CO₂ at Ag cathode, and (c) flow rate of anolyte and (d)... 128

Figure 4-7. Contact angle of water on Ag cathode (a) before the cell operation and (b)~(e) after the cell operation at 5, 20, 30, and 50 sccm, respectively. 130

Figure 4-8. Comparison of (a) polarization and power density curve and (b) time-voltage profiles of Ag and Cu cathodes. 134

Figure 4-9. Long-term stability test of N₂H₄-CO₂ fuel cell. 136

Figure 4-10. XRD patterns of (a) Cu cathode, (b) Ag cathode and (c) Pt anode before/after operating the N₂H₄-CO₂ fuel cell. 138

Figure 4-11. Comparison of F.E. and energy consumption with previously published studies. 143