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Title Page

Abstract

Contents

Chapter 1. Introduction 14

1.1. Research Background 14

1.2. Dissertation Structures 19

Chapter 2. Facet-Dependent Activity of Oxide-Derived Cu Nanoparticles for Electrochemical CO₂ Reduction to C₂H₄ 22

2.1. Introduction 22

2.2. Experimental Methods 23

2.3. Results and Discussion 26

2.4. Conclusions 46

Chapter 3. Selective Methanol Production from CO₂ Reduction Reaction (CO2RR) using Composition-Tailored Cu(111) and Cu₂P₂O₇ Hybrid Catalysts 47

3-1. Introduction 47

3-2. Experimental Methods 48

3-3. Results and Discussion 51

3-4. Conclusions 76

Chapter 4. Conclusion 77

REFERENCES 78

List of Tables

Table 1. Faraday efficiency of various Cu₂O catalysts at various applied potentials 32

Table 2. Faraday efficiency of CP and CP discharged catalysts under 0.1M KHCO₃ 59

Table 3. Faraday efficiency of CP and CP discharged catalysts under 0.1M CsHCO₃ 60

Table 4. Long-term faraday efficiency of CP and CP discharged catalysts under 0.1M KHCO₃ electrolyte (LC was only measured.) 63

Table 5. Long-term faraday efficiency of CP and CP discharged catalysts under 0.1M CsHCO₃ electrolyte (LC was only measured.) 64

Table 6. Faraday efficiency of polycrystalline Cu and Cu (111) with and without CP catalyst 67

Table 7. Faraday efficiency of CP and CP discharged catalysts under different purging sources (-1.2V vs RHE) 70

Table 8. Faraday efficiency of CP and CP discharged catalysts under different pH conditions 74

Table 9. Faraday efficiency of CP and CP discharged GDE electrode (-1.0V vs RHE, 1M KHCO₃) 74

Table 10. Various catalysts for producing methanol from electrochemical CO2RR 75

List of Figures

Figure 1. (a) Electrochemical potentials for CO₂ reduction to various products (b) Proper metal catalysts for producing target product (c) Schematic illustration of the mechanism of various metal... 16

Figure 2. Schematic illustration of (a) H-cell and (b) GDE 18

Figure 3. Dissertation structures 21

Figure 4. (a) SEM images of various Cu₂O catalysts (Cube, Flat and Octahedral) (b) TEM image of Cube Cu₂O (c, d) Faraday efficiency and chronoamperometry (CA) of various Cu₂O catalysts for... 27

Figure 5. (a) TEM and (b) SEM image of Cube Cu₂O (c) XRD pattern of various Cu₂O catalysts 28

Figure 6. SEM image of (a) Flat and (b) Octahedral Cu₂O 28

Figure 7. (a, b) Faraday efficiency and partial current density of Cube Cu₂O for CO2RR in 0.1M KHCO₃ at various applied potentials for 1h (c) Cu K-edge in-situ XANES spectra of Cube Cu₂O at -... 30

Figure 8. (a) Current density, (b) faraday efficiency and (c) partial current density of Cube Cu₂O for CO2RR at various applied potentials for 1h 30

Figure 9. Faraday efficiency of (a) Flat and (b) Octahedral Cu₂O for CO2RR at various applied potentials 31

Figure 10. (a) SEM image of Cube Cu₂O for 1h at 20 min intervals 34

Figure 11. (a, b) TEM image of Cube Cu₂O after CO2RR at -1.1V vs RHE for 1 h 34

Figure 12. (a) In-situ Raman spectra of Cube Cu₂O at -1.1V vs RHE (b) Faraday efficiency of Cube Cu₂O at -1.1V vs RHE for 1h at 20 min intervals (c) Scheme of the change the ratio between Cu₂O and... 36

Figure 13. In-situ Raman spectra of (a) Octahedral and (b) Flat Cu₂O at -1.1V vs RHE 36

Figure 14. (a) In situ FT-IR spectra of Cube Cu₂O in the potential range of -1.2V vs RHE to -0.5V vs RHE during CO2RR. CO₂ adsorption process and OCCO formation mechanism. (b) NEB calculation... 38

Figure 15. (a) CO₂ on Cu₂O (100). Reddish brown, red and yellow spheres represent Cu, O and C atoms, respectively. Green lines indicate unit-cell boundaries. The blue circle and X represent the candidate... 38

Figure 16. Hydrogenation processes of CO₂ on Cu (100) surface. Reddish brown, red and white spheres represent Cu, O and H atoms, respectively. H atoms are introduced in the form of H₃O⁺ ions. Initial and... 40

Figure 17. Hydrogenation processes of OCCO on Cu (100) surface. Reddish brown, red, black and white spheres represent Cu, O, C and H atoms, respectively. H atoms are introduced in the form of H₃O⁺... 42

Figure 18. Departure process of C₂H₄ on Cu (100) surface. Reddish brown, black and white spheres represent Cu, C and H atoms, respectively. 43

Figure 19. (a) Position and relative energies of H on Cu (100) surface. Reddish brown and white spheres represent Cu and H atoms, respectively. (b) Hydrogen evolution reaction process on Cu (100) surface.... 45

Figure 20. (a) Schematic illustration for the preparation of the copper and phosphate hybrid catalyst, Graph is about discharge capacity according to cell voltage during discharge process (Battery discharge... 52

Figure 21. (a) Digital image of Cu, CP and CP-0.8 catalysts 52

Figure 22. (a-f) Image of CP and discharged CP catalysts (Before washing in D.I water) 53

Figure 23. (a-f) Image of CP and discharged CP catalysts (After washing in D.I water) 53

Figure 24. (a) TEM and b) mapping images of CP catalyst 54

Figure 25. (a) TEM and b) mapping images of CP-0.8 catalyst 54

Figure 26. (a,b) XPS spectra of CP and discharged CP catalysts (CP-1.5, CP-1.0 and CP-0.8) (c) Raman spectra showing the D and G band from CNT in CP and CP-0.8 catalyst (d) IR spectra showing the... 55

Figure 27. Electrochemical CO2RR performance of CP and discharged CP catalysts. (a-c) Current density and Faraday efficiency of CP and discharged CP catalysts (CP, CP-1.5, CP-1.2, CP-1.0 and CP-... 57

Figure 28. Current density of CP and CP-0.8 catalyst in 0.1M CsHCO₃ electrolyte 57

Figure 29. Linear sweep voltammetry (LSV) and partial current density of CP and CP-0.8 catalyst under two electrolytes (0.1M KHCO₃ and 0.1M CsHCO₃) 58

Figure 30. Long-term stability of CP and discharged CP catalysts (a) Current density and (b) Faraday efficiency of CP and CP-0.8 catalyst under 0.1M KHCO₃ and 0.1M CsHCO₃ electrolyte for 12h at 2h... 62

Figure 31. (a) XPS spectra of CP and discharged CP catalysts (CP-1.5, CP-1.0 and CP-0.8) (b) XRD peak of CP and CP-0.8 catalysts before and after CO2RR under -1.2V vs RHE (c) Faraday efficiency... 66

Figure 32. Current density of polycrystalline copper and Cu (111) with and without CP catalyst in 0.1M KHCO₃ electrolyte under -1.2V vs RHE 66

Figure 33. In situ XAFS analysis of CP-1.5 catalysts in 0.1M KHCO₃ electrolyte under -1.2V vs RHE for 1h at 20min intervals 67

Figure 34. (a) Schematic illustration of mechanism for producing methanol from CO2RR (b) Faraday efficiency of CP-0.8 in 0.1M KHCO₃ electrolyte under different atmosphere (CO₂, HCOOH, CO and HCHO) 69

Figure 35. Current density and Faraday efficiency of CP and CP-0.8 in 0.1M KHCO₃ electrolyte under different atmosphere (CO₂, HCOOH, CO and HCHO) 69

Figure 36. (a) Faraday efficiency of CP-0.8 catalyst under different pH conditions (Selected pH is 4, 7, 9, 11 and 14) (b) Schematic illustration of gas diffusion electrode (GDE) system (c) Current density... 72

Figure 37. Current density and Faraday efficiency of CP and CP-0.8 catalyst under different pH conditions (Selected pH is 4, 7, 9, 11 and 14) 73

초록보기

 The electrochemical CO₂ reduction reaction (CO2RR) has received great attention as a promising technology to address global warming and reduce carbon emissions. CO2RR should lower the production of hydrogen, which is a competitive reaction, and make valuable chemicals such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH₃OH), and ethylene (C₂H₄). To produce a target product from CO2RR, designing and selecting the proper metal catalyst is important. Among various metal catalysts, copper is a unique material allowing the production of C1 and C2 products such as CH₃OH and C₂H₄. Herein, we prepared two copper-based catalysts for CO2RR: 1) oxide-derived copper catalyst for ethylene production and 2) copper and phosphate hybrid catalyst for methanol production. First, copper-based catalysts, including Cu and Cu₂O, have been widely utilized for generating C2 products for CO2RR. While Cu (100) and Cu (111) surfaces are known to favor C₂H₄ and CH₄ production, respectively, few studies have researched the selectivity changes of Cu₂O catalysts according to crystal surfaces. We synthesized various Cu₂O catalysts (Cube, Flat, and Octahedral) with different crystal surfaces to enhance C₂H₄ production. We observed that Cube Cu₂O with a (100) crystal surface exhibited a remarkable selectivity of C₂H₄. In-situ analysis and density functional theory (DFT) calculations revealed the gradual reduction of Cu₂O to Cu during CO2RR, indicating the Cu surface is the main active site for promoting the production of C₂H₄. Second, we introduce copper-based compounds combined with phosphate-based materials to improve methanol production. Due to sluggish kinetics, few studies have researched to enhance methanol production. However, phosphate-based materials play a critical role in reducing the energy barrier of COOH, which is a crucial intermediate for methanol production. Based on this information, we developed a Cu2P2O7 (CP) catalyst through a battery discharge process to significantly improve the selectivity for methanol production for CO2RR. The discharged CP catalyst exhibited a higher selectivity for methanol than pristine CP. Furthermore, the discharged CP catalyst demonstrated excellent stability and methanol yield in long-term reactions for 12 hours and under various pH conditions. In-situ studies and DFT calculations confirmed that the coexistence of Cu (111) crystal surface and the phosphate-based compound is important for methanol production, with the main pathway favoring HCOOH over CO. We suggest that these findings contribute valuable insights for the design of novel electrochemical CO2RR systems.

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