Title Page
Abstract
Contents
List of Abbreviations 11
Chapter 1. Introduction 12
1.1. Background 12
1.2. Electrochemical Reduction of CO₂ 13
1.3. Density Functional Theory (DFT) 16
1.4. Computational Hydrogen Electrode (CHE) 17
Chapter 2. Descriptors based screening of catalysts for electrochemical reduction of CO₂ 19
2.1. Introduction 19
2.2. Computational Methods 21
2.3. Results and Discussion 24
2.3.1. Structure and Stability 24
2.3.2. Activation of CO₂ 30
2.3.3. CO₂RR Mechanistic Analysis 35
2.3.4. Selectivity Evaluation of CO₂RR Catalysts 53
Chapter 3. Conclusion 56
REFERENCES 57
Table 1.1. Sources of CO₂ with accompanying worldwide emission and concentration. Data from Our world in data. 12
Table 1.2. Selectivity of metal electrodes towards different CO₂ products including methane (CH₄), ethylene (C₂H₄), ethanol (EtOH), carbon-monoxide (CO), and formic acid (HCOOH) at applied... 15
Table 2.1. TM binding energies (Eb) for GaN monolayer surface. The Eb is provided for three different defect types, including Ga single vacancy (Ga-SV), Nitrogen single vacancy (N-SV), and gallium-...[이미지참조] 25
Table 2.2. Thermodynamical and Electrochemical stabilities (Udiss) of (TM)SAs embedded at Ga single vacancy (Ga-SV) site. U⁰diss and eN represents the standard dissolution potential of bulk TMs and the...[이미지참조] 28
Table 2.3. Geometrical parameters of CO₂ molecule after adsorption over various single atom catalysts (SACs). Adsorption energy of CO₂ (Eads[*CO₂]), angle between O=C=O (φoco), distance from...[이미지참조] 32
Table 2.4. Bader charge analysis of CO₂ adsorbed systems. TM denotes the transition metal, and the CO₂ adsorbate atoms are represented by C, 1O and 2O. The net charge transfer from surface to adsorbed... 34
Table 2.5. DFT calculated electronic energies, zero-point energies and entropy contributions for the gaseous molecules at T=298 K. 36
Table 2.6. Zero-point energies and entropy contributions to the adsorbate free energy at T=298 K. 36
Table 2.7. Adsorption Gibb's free energies (△G) of first protonation step (*COOH or *OCHO) for chemisorbed systems. 38
Table 2.8. Adsorption Gibbs free energies of elementary reaction steps of CO₂RR over Os and Ir-SAC. 50
Figure 1.1. Overview of CO₂ electroreduction products, along with chemical reactions and corresponding equilibrium potentials. 13
Figure 2.1. Top and side views of (TM)SA embedded into a) Ga-SV, b) N-SV, and c) Ga/N-DV defect sites. The distance from TM to nearest neighbor (dTM-Ga/N) surface coordinating atoms are highlighted...[이미지참조] 23
Figure 2.2. (a) Geometric structures of (TM)SAs embedded into various defect sites including Ga-SV, N-SV, and Ga/N-DV sites. (b) TM binding energies (Eb) for each defect type surface. The more negative...[이미지참조] 26
Figure 2.3. (a) Representative structure of GaN monolayer surface with TM embedded at Ga-SV defect site. (b) Thermodynamic stabilities (Estab), where the blue color in energy scale bar represents the...[이미지참조] 27
Figure 2.4. Snapshots of the ab initio molecular dynamics simulation and corresponding energy and temperature profiles for (a) Os-SAC and (b) Ir-SAC, respectively. The temperature is controlled at 500... 29
Figure 2.5. Schematic illustration of possible CO₂ adsorption configurations and optimized CO₂ adsorbed geometries of all the considered systems. The golden filled line represents the physisorbed... 31
Figure 2.6. The charge density difference plots of CO₂ chemisorbed over various TM-SACs showing a significant amount of charge transfer takes place from the surface to adsorbed CO₂ molecule. The... 33
Figure 2.7. Possible reaction pathways for CO₂ reduction to various C₁ (HCOOH, CO, CH₃OH, and CH₄) products (Color code: transition metal, golden; Ga, light green; N, blue; C, gray; O, red; H, cyan). 35
Figure 2.8. Free energy diagram for CO₂ reduction to HCOOH on (a) Mn-SAC and (b) Rh-SAC. The blue and red curves represent the free energy change for reaction intermediates at zero and applied... 39
Figure 2.9. Limiting potentials (UL(s)) of Ti/V/Cr/Mn/Zr/Nb/Mo/Ru/Rh/Hf/Ta/W and Re embedded SACs for HCOOH formation. Free energy profiles for CO₂ reduction to HCOOH over different TM-...[이미지참조] 40
Figure 2.10. Free energy diagram for CO₂ reduction to *CO on (a) Os-SAC and (b) Ir-SAC. The potential limiting step (PLS) is marked with solid black line. The insets depict the optimized structures... 42
Figure 2.11. Free energy profiles of CO₂ reduction to CH₃OH on Os-SAC at zero and applied potential via different reduction pathways (P1 through P4). Solid black lines indicate the potential-limiting step... 44
Figure 2.12. Free energy profiles of CO₂ reduction to CH₃OH over Ir-SAC at zero and applied potential via different reduction pathways (P1 through P4). Solid black lines indicate the potential-limiting step... 45
Figure 2.13. Free energy profiles of CO₂ reduction to CH₄ on Os-SAC at zero and applied potential via different reduction pathways (R1 through R6). Solid black lines indicate the potential-limiting step... 47
Figure 2.14. Free energy profiles of CO₂ reduction to CH₄ on Ir-SAC at zero and applied potential via different reduction pathways (R1 through R6). Solid black lines indicate the potential-limiting step... 48
Figure 2.15. Optimized configurations of various adsorbates over Os-SAC. (Color code: Os, light pink; Ga, light green; N, blue; C, gray; O, red; H, cyan). 49
Figure 2.16. Optimized configurations of various adsorbates over Ir-SAC. (Color code: Ir, golden- brown; Ga, light green; N, blue; C, gray; O, red; H, cyan). 49
Figure 2.17. Kinetic barriers calculated through nudge elastic band method at the potential limiting steps of (a) Mn and (b) Rh-SACs for CO₂ reduction reaction. (IS) initial state, (TS) transition state and... 52
Figure 2.18. Kinetic barriers calculated through nudge elastic band method at the potential limiting steps of (a) Os and (b) Ir-SACs for CO₂ reduction reaction. (IS) initial state, (TS) transition state and... 52
Figure 2.19. Activity and selectivity of the considered TM-SACs for CO₂ reduction reaction. The difference in CO₂ and H₂ limiting potentials [UL(CO₂)−UL(H₂)] vs. UL(CO₂). The colored area shows...[이미지참조] 54
Figure 2.20. a) Free energy diagram of HER. The inset shows the H-adsorbed optimized geometries of Os-SAC and Ir-SAC b) Faradaic efficiencies of the catalysts for CO₂RR with respect to its competitive... 55