Figure 1.1. Reaction pathway for HER and OER. 16

Figure 1.2. OER mechanism. (a) conventional AEM mechanism. (b) LOM mechanism. 19

Figure 2.1. Synthesis and morphological analysis of the multiple-oxidation-state NiFe catalyst. (a) Schematic of the synthesis procedure for MOS NiFe. The red, orange, green, blue, gray, and white... 26

Figure 2.2. Transmission electron microscopy and element mapping images of FeS₂. 27

Figure 2.3. Transmission electron microscopy and element mapping images of NiFeS₂. 27

Figure 2.4. X-ray diffraction patterns of FeS₂ and NiFeS₂. 28

Figure 2.5. Transmission electron microscopy-energy-dispersive X-ray spectrum of NiFeS₂ corresponding to Figure 2.3. Inset: weights and atomic ratios of Ni, Fe, and S. 28

Figure 2.6. High-resolution transmission electron microscopy image of NiFeS₂ in multiple-oxidation-state NiFe catalyst. Magnified image shows the lattice fringe of NiFeS₂. 29

Figure 2.7. High-resolution transmission electron microscopy image of Ni NPs in multiple-oxidation-state NiFe catalyst. 29

Figure 2.8. Oxidation state analysis of the multiple-oxidation-state NiFe catalyst and the intermediate products. High-resolution X-ray photoelectron spectroscopy spectra of (a) Fe 2p, (b) Ni 2p, and (c) S... 30

Figure 2.9. Raman spectra of FeS₂ and NiFeS₂. 31

Figure 2.10. High-resolution X-ray photoelectron spectroscopy spectrum of Ni 2p for Ni anchored on the carbon support. 31

Figure 2.11. Raman spectra of NiFeS₂ (before carbonization) and the multiple-oxidation-state NiFe catalyst (after carbonization). 32

Figure 2.12. Electrocatalysis of the oxygen evolution reaction. (a) OER polarization curves, (b) Tafel plots, and (c) summary of the overpotentials at 10 mA cm⁻² and Tafel slopes for FeS₂, NiFeS₂, the... 33

Figure 2.13. Oxygen evolution reaction performance of Ni anchored on the carbon support. 33

Figure 2.14. Cyclic voltammetry (CV) curves at different scan rates for FeS₂, NiFeS₂, and the multiple-oxidation-state NiFe catalyst (MOS NiFe). The anodic and cathodic current densitie... 35

Figure 2.15. Polarization curves normalized to electrochemical active surface area for FeS₂, NiFeS₂, and the multiple-oxidation-state NiFe catalyst. 36

Figure 2.16. (a) High-resolution transmission electron microscopy (HR-TEM) image of the multiple- oxidation-state NiFe catalyst (MOS NiFe). (b) Magnified HR-TEM image taken from the marked region... 36

Figure 2.17. High-resolution X-ray photoelectron spectroscopy spectra of Fe 2p and Ni 2p for the multiple-oxidation-state NiFe catalyst (MOS NiFe) after the chronopotentiometry stability test operated... 37

Figure 2.18. Analysis of the oxygen evolution reaction mechanism via density functional theory simulations. (a) Gibbs free energy plot for FeS₂ (black line), NiFeS₂ (blue line), and the multiple-... 38

Figure 2.19. Optimized structures of each intermediate on the surface layer of (a) FeS₂, (b) NiFeS₂, and (c) the multiple-oxidation-state NiFe catalyst (MOS NiFe). The colors indicate the... 39

Figure 3.1. Morphological and elemental characterizations of LSC/LMO heterostructure. (a) Schematic of LSC/LMO synthesis process including chemical oxidation and ball milling steps. (b) TEM image of... 50

Figure 3.2. EELS spectrum and corresponding chemical map of Mo-M and O-K edges for LSC/LMO. 52

Figure 3.3. OER polarization curves of LSC/LMO with different weight ratios. 53

Figure 3.4. Nyquist plots of LSC/LMO for OER with various weight ratios. Inset: Equivalent electrical circuit model used for fitting the Nyquist complex-plane impedance plot. 54

Figure 3.5. Cyclic voltammetry (CV) curve of various weight ratios of LSC/LMO. Each CV curve in the double-layer capacitance region is measured at scan rates from 20 to 160 mV s⁻¹ with a 20 mV s⁻¹ interval. 54

Figure 3.6. (a) Double-layer capacitance values (Cdl) and (b) ECSA values of LSC/LMO with different weight ratios.[이미지참조] 55

Figure 3.7. Electrochemical OER performance of LSC/LMO. (a) OER polarization curves of LSC/LMO, IrO₂, LSC, and LMO recorded in O₂-saturated 1 M KOH. (b) Tafel slopes of different... 56

Figure 3.8. High-resolution XPS spectra of Co 2p and Mo 3d of LSC/LMO after the chronoamperometric stability test at 100 mA cm⁻² for 200 h. 56

Figure 3.9. XRD spectrum of LSC/LMO after the chronoamperometric stability test at 100 mA cm⁻² for 200 h. 57

Figure 3.10. TEM and HR-TEM images of LSC/LMO after the chronoamperometric stability test at 100 mA cm⁻² for 200 h. Red and yellow boxes correspond to the domain of LSC and Mo nanograin, respectively. 57

Figure 3.11. Polarization curves for the overall water-splitting. The inset image shows the two electrodes system for the overall water-splitting. 57

Figure 3.12. Structural and chemical state characterizations of the LSC/LMO. (a) XRD patterns of LSC/LMO, LSC, and LMO, which illustrate the crystalline structure of LSC/LMO constitutes mixed... 59

Figure 3.13. Rietveld XRD refinement patterns of LSC/LMO, LSC, and LMO. 60

Figure 3.14. UV-Vis-NIR spectra of LSC/LMO, LMO, and 2H-MoSe₂ indicating metallic features of LSC/LMO and LMO. 60

Figure 3.15. TGA profile of LSC/LMO without and with exposure wet-air environment. 61

Figure 3.16. Pore size distribution of LSC/LMO calculated from N₂ adsorption isotherms. 62

Figure 3.17. High-resolution Co 2p XPS core-level spectrum for LSC. 63

Figure 3.18. High-resolution XPS spectrum of Mo 3d peak for LMO. 64

Figure 3.19. Non-phase shifted Fourier transform (FT) EXAFS spectra of (a) Co K-edge of LSC/LMO, LSC, and Co foil and (b) Mo K-edge of LSC/LMO, LMO, and Mo foil. 65

Figure 3.20. High-resolution O 1s XPS core-level spectra for (a) LSC and (b) LSC/LMO. 66

Figure 3.21. Atomic-level understanding of enhanced OER activity in LSC/LMO. (a) LSC/LMO heterostructure computed using AIMD simulations; blue, pink, dark-red, green, purple, and red spheres... 67

Figure 3.22. Free energy diagrams for (a) LSC and (b) LSC/LMO in adsorbate evolution mechanism. 68

Figure 4.1. Preferred crystal facet engineering of LSC via template-mediated growth approach. (a) Schematic of the synthesis process for template-mediated growth approach. (b,d) HR-TEM images of... 77

Figure 4.2. Transmission electron microscopy energy-dispersive X-ray spectroscopy mapping images and corresponding spectrum of MoReS₂. Inset: atomic ratios of Mo, Re, and S. 78

Figure 4.3. Transmission electron microscopy energy-dispersive X-ray spectroscopy mapping images and corresponding spectrum of La₀.₅Sr₀.₅CoO₃. Inset: atomic ratios of La, Sr, Co, and O. 78

Figure 4.4. Transmission electron microscopy energy-dispersive X-ray spectroscopy mapping images and corresponding spectrum of MoReS₂/LSC. Inset: atomic ratios of La, Sr, Co, O, Mo, Re, and S. 79

Figure 4.5. Raman spectra of MoS₂, ReS₂, and MoReS₂. 80

Figure 4.6. X-ray diffraction patterns of MoS₂/LSC and ReS₂/LSC. 81

Figure 4.7. Scanning electron microscopy (SEM) images of MoS₂, ReS₂, and MoReS₂. 82

Figure 4.8. Scanning electron microscopy (SEM) images of MoS₂/LSC, ReS₂/LSC, and MoReS₂/LSC. Orange regions represent the exposed TMDs growth template. 82

Figure 4.9. Electronic structure modulation of LSC by TMDs template. Schematic describing the atomic structure and electron transfer for (a) MoReS₂/LSC and (b) ReS₂/LSC. The purple, silver, yellow,... 84

Figure 4.10. (a) High-resolution Re 4f XPS spectra for ReS₂ and ReS₂/LSC. High-resolution (b) Mo 3d and (c) Re 4f XPS spectra for MoReS₂ and MoReS₂/LSC. 85

Figure 4.11. Density of states for Re 5d bands in (a) ReS₂ and ReS₂/LSC, (b) MoReS₂ and MoReS₂/LSC, and (c) Mo 4d bands in MoReS₂ and MoReS₂/LSC. The red and blue dashed lines indicate the d band... 85

Figure 4.12. OER activity of pure LSC and in situ grown LSC on TMDs. (a) OER polarization curves and (b) Tafel plots for LSC, MoS₂/LSC, ReS₂/LSC, MoReS₂/LSC, and IrO₂. (c) Nyquist plot, (d)... 87

Figure 4.13. Schematic describing the atomic structure and electron transfer for MoS₂/LSC. The purple, yellow, dark-green, green, blue, and red spheres represent Mo, S, La, Sr, Co, and O atoms, respectively. 89

Figure 4.14. High-resolution Co 2p XPS spectrum and corresponding concentration ratio of Co⁴⁺/Co³⁺ for MoS₂/LSC. 89

Figure 4.15. Raman spectrum of MoS₂/LSC. 89

Figure 4.16. Cyclic voltammetry curves at different scan rates for LSC, MoS₂/LSC, ReS₂/LSC, and MoReS₂/LSC. The anodic and cathodic current densities were obtained at a potential of 0.3 V. 91

Figure 4.17. TEM image of MoReS₂/LSC after chronopotentiometry stability test operated at constant current density of 20 mA cm⁻² for up to 200 h. 91

Figure 4.18. XRD pattern of MoReS₂/LSC after chronopotentiometry stability test operated at constant current density of 20 mA cm⁻² for up to 200 h. 92

Figure 4.19. High-resolution Co 2p XPS spectrum and corresponding concentration ratio of Co⁴⁺/Co³⁺ for MoReS₂/LSC after chronopotentiometry stability test operated at constant current density of 20 mA... 92

Figure 4.20. Computational analysis for OER performance in LSC (110) and (100) surfaces. Free energy diagrams for OER on (a) (110) and (b) (100) LSC surfaces. The dark-green, green, blue, and red... 94