Title Page
ABSTRACT
Contents
CHAPTER 1. Introduction 19
1.1. Study Background 21
1.2. Overview of Synthesis Methods for CNTs 24
1.3. The FC-CVD Method for Synthesizing CNT Assemblies 28
1.4. The Roles of FC-CVD Synthesized CNTs in Electrochemistry Applications 31
1.5. Thesis Objectives 35
CHAPTER 2. Exploring the Feasibility of FC-CVD-Synthesized CNT Assemblies as Current Collectors for Energy Storage 37
2.1. Introduction 39
2.2. Experimental Methods 42
2.2.1. Preparation of AC-CNTs and Graphite-CNTs electrodes 42
2.2.2. Preparation of Ru nanoparticles embedded in CNTs electrodes 42
2.2.3. Material characterization 43
2.2.4. Electrochemical measurements 44
2.3. Investigation of FC-CVD-Synthesized CNT Assemblies as Current Collector Alternatives 46
2.3.1. Microstructural characterization of CNT sheet-based electrodes 46
2.3.2. Electrochemical characterization of capacitor-type AC-CNTs 51
2.3.3. Electrochemical characterization of battery-type Graphite-CNTs 55
2.4. Design of Material-Substrate Integrated Electrodes Using Hierarchical Structures of FC-CVD-Synthesized CNT Assemblies 60
2.4.1. The fabrication process of integrated electrode 60
2.4.2. Investigation of the effect of acid-treatment time on CNT sheets 63
2.4.3. Optimization of Ru NPs loading on Ru@a-CNTs 66
2.4.4. Microstructural characterization of Ru@a-CNTs 68
2.4.5. Electrochemical properties of Ru@a-CNTs 72
2.4.6. Application and commercial viability of Ru@a-CNTs-based deformable capacitors 77
2.5. Summary 81
CHAPTER 3. Transition Metal@CNT Sheets as Durable HER Catalysts-Integrated Electrodes for Water Electrolysis 83
3.1. Introduction 85
3.2. Experimental Methods 88
3.2.1. Synthesis of NFS@CNTs composite electrodes 88
3.2.2. Material characterization 88
3.2.3. Electrochemical investigation 89
3.2.4. Configuration of mini-scale AEMWE single-cell 90
3.3. Results & Discussions 91
3.3.1. Morphological and microstructural investigation of NFS@CNTs 91
3.3.2. Structural and compositional characterization of NFS@CNTs 96
3.3.3. Electrochemical analysis: HER catalytic performances of NFS@CNTs 100
3.3.4. Optimizing layered NFS@CNTs for performance enhancement 106
3.4. AEMWEs Single-Cell Configuration 109
3.5. Summary 112
CHAPTER 4. Understanding the Effect of Defects in CNT Assemblies for Applications as COR-Resistive Substrate in OER Environments 113
4.1. Introduction 115
4.2. Experimental Methods 118
4.2.1. Sample preparation 118
4.2.2. Material characterization 118
4.2.3. Electrochemical characterization 119
4.3. Preliminary Investigations and Motivation 120
4.3.1. Investigation of the effect of carbon crystallinity on COR 120
4.3.2. Understanding the role of synthesis parameters in controlling defects in CNT assemblies 124
4.4. Elucidation of the Relationship between Defects and COR in CNT Assemblies 127
4.4.1. Evaluation of COR by theoretical analysis 127
4.4.2. In-depth analysis of the correlation between COR and defect distribution 129
4.5. Validation of COR Reduction through Defect Mitigation 137
4.6. Summary 145
CHAPTER 5. Conclusion and Outlook 146
5.1. Summary of Thesis Objectives and Achievements 146
5.2. Implications of Thesis 148
References 150
요약(국문초록) 169
CURRICULUM VITAE 172
Table 1.1. Synthesis methods for CNTs 27
Table 1.2. The summary of the benefits of FC-CVD-synthesized CNTs for energy storage and conversion 34
Table 2.1. Relative percentage of the chemical bonds based on subpeaks extracted from the deconvoluted C 1s core-level spectra of the CNT sheets treated for different... 65
Figure 2.1. Conceptualization of two types of electrodes to validate the potential of CNT assemblies as novel current collectors for energy storage: 1) to verify their... 41
Figure 2.2. The fabrication process of CNT sheets-based electrodes (Graphite-CNTs and AC-CNTs). 47
Figure 2.3. The FE-SEM images of pristine CNT sheets: (A) surface and (B) cross-sectional morphology. 48
Figure 2.4. The cross-sectional FE-SEM images of electrodes: (A) AC-CNTs, (B) Graphite-CNTs, (C) AC-Al, and (D) Graphite-Cu. 49
Figure 2.5. The surface FE-SEM images of electrodes: (A) AC-CNTs, (B) Graphite- CNTs, (C) AC-Al, and (D) Graphite-Cu. 50
Figure 2.6. Electrochemical characterization of AC-CNT half-cells: (A) CV curves at different scan rates of 1, 2, 5, 10, 20, 50, 100, and 200 mV s¯¹ and (B) potential... 53
Figure 2.7. (A) Nitrogen adsorption-desorption isotherms and (B) distribution of cumulative pore volume as a function of the pore diameter of AC powder, CNT... 54
Figure 2.8. Electrochemical characterization of Graphite-CNT half-cells: (A) CV curves at a scan rate of 0.5 mV s¯¹ and (B) potential profiles at a current density of... 58
Figure 2.9. (A) Nyquist plots of Graphite-CNTs and the conventional electrodes in the frequency range of 100 kHz to 0.01 Hz and (B) Warburg factor (б) obtained from... 59
Figure 2.10. (A) Schematic of the fabrication procedures of Ru@a-CNTs. CNT sheets were synthesized by using a vertical-type CVD furnace. Ru@a-CNTs were... 62
Figure 2.11. Investigate the effect of acid treatment duration on CNT structure and surface: (A) XPS C 1s spectra with five deconvoluted subpeaks, (B) the water... 64
Figure 2.12. (A) FE-SEM and EDS mapping images, (B) CV curves as a function of scan rate, and (C) specific capacitances of Ru@a-CNTs with different mass... 67
Figure 2.13. FE-SEM images of (A) p-CNTs, (B) a-CNTs, (C) Ru@p-CNTs, and (D) Ru@a-CNTs. EDS mapping images of the surface (E) and cross-section (F) of Ru@a-CNTs. 70
Figure 2.14. (A) XRD patterns of p-CNTs, a-CNTs, Ru@p-CNTs, and Ru@a-CNTs. (B) The XPS C 1s spectra for p-CNTs and a-CNTs and C 1s/Ru 3d spectra for Ru@p-... 71
Figure 2.15. The electrochemical performances of p-CNTs, a-CNTs, Ru@p-CNTs, and Ru@a-CNTs: (A) CV curves at different scan rates of 10-1000 mV s¯¹, (B)... 75
Figure 2.16. XPS Ru 3p core-level spectra for Ru@a-CNTs at 6 points in the given CV curve. The spectra were deconvoluted into three subpeaks indicating Ru, RuO₂,... 76
Figure 2.17. (A) Schematic of quasi-solid-state electrochemical capacitors composed of Ru@a-CNTs and a PVA-H₃PO₄ gel electrolyte. (B) Cyclability of... 79
Figure 2.18. (A) Schematic illustration of six Ru@a-CNTs-based quasi-solid-state electrochemical capacitors connected in series. (B) Voltage profiles with the number... 80
Figure 3.1. The fabrication procedures of Ni@CNTs, NiFe@CNTs, NiS@CNTs, and NFS@CNTs 93
Figure 3.2. The surface FE-SEM images of (A) Ni@CNTs, (B) NiFe@CNTs, (C) NiS@CNTs, and (D) NFS@CNTs. (E) The cross-section morphology and EDS... 94
Figure 3.3. HR-TEM image, SAED pattern, and TEM/EDS images of the NFS catalyst particle in NFS@CNTs. 95
Figure 3.4. XRD patterns of CNT-based electrodes: pristine CNTs, Ni@CNTs, NiFe@CNTs, NiS@CNTs, and NFS@CNTs. 98
Figure 3.5. Sub-peak fitted XPS spectra: (A) survey spectra, (B) Ni 2p, (C) S 2p, and (D) Fe 2p. 99
Figure 3.6. Electrochemical characterizations of Ni@CNTs, NiFe@CNTs, NiS@CNTs, NFS@CNTs: (A) the polarization curves of HER and (B)... 103
Figure 3.7. Electrochemical characterizations of Ni@CNTs, NiFe@CNTs, NiS@CNTs, NFS@CNTs: (A) CV curves within the potential range of ± 0.05 V vs.... 104
Figure 3.8. Demonstration of the superior durability of NFS@CNTs: (A) The multistep chronopotentiometry measurement of NFS@CNTs, NFS@CC, and... 105
Figure 3.9. HER catalytic activity change according to the number of NFS@CNTs electrode layers: (A) polarization curves, (B) Tafel plot, and (C) overpotential and... 108
Figure 3.10. (A) The schematic of mini-scale AEMWE single-cell composed of NFS@CNTs (cathode for HER) and Co₃O₄ (anode for OER). (B) Polarization curves... 111
Figure 4.1. Carbon oxidation reaction and relevant current evaluation (iC,Ox) along with the OER (iOER) of carbon-supported catalyst. [이미지참조] 117
Figure 4.2. The analysis of the COR behavior in an OER environment of four commercial carbon materials (Denka black, Vulcan black, Ketjen black, and... 122
Figure 4.3. Raman spectra of four commercial carbon materials (Denka black, Vulcan black, Ketjen black, and graphite): (A) the measured spectra and (B) the sub-... 123
Figure 4.4. Evaluation of carbon crystallinity change trends according to H₂ flow rate and precursor injection rate control in the synthesis process. 126
Figure 4.5. FE-SEM images of surface morphology of CNTs fabricated under synthesis conditions with different precursor injections (6, 8, 10, 12, and 14 ml h¯¹). 132
Figure 4.6. Analysis of the evolution of COR behavior and surface defect distribution with a precursor injection rate of 8 ml h¯¹ and carrier gas flow rate of... 133
Figure 4.7. Analysis of the evolution of COR behavior and surface defect distribution with a precursor injection rate of 10 ml h¯¹ and carrier gas flow rate of... 134
Figure 4.8. Correlation between ID:IG ratio and area fraction of sub-peaks (D*, D**, and D') representing specific defect and COR charge by synthesis conditions...[이미지참조] 135
Figure 4.9. (A) A plot expressing the relationship between COR charge and Raman crystallinity (ID:IG ratio) of various carbon materials including commercially...[이미지참조] 136
Figure 4.10. Validation of the effectiveness of thermal defunctionalization: (A-D) surface morphologies of before (A, B) and after (C, D) treatment of two types of... 141
Figure 4.11. Changes in Raman spectra and defect distributions of CNT assemblies after defunctionalization: (A) Before and (B) after defunctionalization. (C)... 142
Figure 4.12. The defect distribution change after COR by the defunctionalization of CNTs: (A) pristine CNTs (10-1400 and 10-1800), (B) defunctionalized CNTs (10-... 143
Figure 4.13. Comparison of durability change behavior after OER catalyst introduction on four CNT supports (10-1400, 10-1800, 10-1400-D, and 10-1800-D)... 144