Surface Engineered Cobalt based Transition Metal Compound Heterostructures and Montmorillonite Natural Clay for Electrocatalytic Water Splitting Applications = 전기화학적 수분 분해 응용을 위한 표면 엔지니어링된 코발트 기반 전이 금속 화합물 이종구조 및 몬모릴로나이트 천연 점토

다국어 초록 (Multilingual Abstract)

The transition to green energy resources is essential for creating a sustainable environmental future. This requires the widespread adoption of renewable energy sources to reduce carbon footprints. Hydrogen is envisioned as the fuel of the future and can be derived from water through electrocatalytic water-splitting processes if electricity generated from renewable energy is used. The development of an advanced electrocatalyst is essential to expedite the key water-splitting reactions such as the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). We have developed transition metal-based electrocatalysts, including chalcogenides and layered hydroxide compounds, and have demonstrated high efficiency compared to precious metals such as platinum, ruthenium, and palladium. The high electrocatalytic performance of our designed catalyst is attributed to the presence of potential candidates like Cobalt (Co) and iron (Fe), particularly for OER and Copper (Cu), and Nickel (Ni) chalcogenides for HER reactions. The partially filled d-orbitals in these compounds have multiple oxidation states, and hence promote the reversible oxidation and reduction processes, which helps to stabilize H* and OH*/OOH* intermediates adsorption to enhance H2 and O2 generation. Moreover, the formation of cobalt-based heterostructures with these compounds reduces activation energy for intermediate adsorption via Fermi-level alignment at the interface, which regulates the electron transfer dynamics necessary for the redox reactions.
The copper sulfide in the djurleite phase (Cu1.97S) was synthesized using the chemical bath deposition method (CBD) at 60oC (C60) and 120oC (C120) on nickel foam (NF). Further, cobalt was incorporated into the C60 and C120 structure using a hydrothermal method, resulting in C60/Co and C120/Co catalysts. The C60 catalyst demonstrates superior HER performance with a lower overpotential of 164 mV at 10 mA/cm2, due to copper vacancies and nickel sulfide formation. Meanwhile, the enhanced OER kinetics observed in the C60/Co catalyst (overpotential of 240 mV at 10 mA/cm2) result from the cation exchange reaction-induced formation of a hybrid CoS-CoS2 interface. In contrast, the phase was transformed to an unstable Cu2S phase without the CoS-CoS2 interface in C120/Co, leads to inferior HER and OER performance. Though metal sulfide catalysts demonstrate excellent electrochemical performance, they suffer from surface oxidation when in contact with the KOH electrolyte, especially at OER potentials. To address this issue, we have encapsulated CuSCN with a FeCo (FeCo/CuSCN) to inhibit direct contact with KOH electrolyte and then was used as an OER catalyst. Interestingly, the inductively coupled plasma results confirm a substantial amount of lower concentration of metal ions and Sulphur in the FeCo/CuSCN30 (Cu:S 0.37:3.4) than CuSCN30 (Cu:S 2.67:26) catalyst used in OER, due to the passivation effect.
We have developed a novel double CoFe-LDH/NiAl-LDH structure via in-situ hydrothermal synthesis, considering cobalt's advantages in the OER reaction and the synergistic effects of heterostructures. The formation of CoFe-LDH is influenced by the aluminum doping concentration, with Al3+ serving as the rate-determining factor in the LDH formation. The in-situ Raman and depth profile XPS analysis indicate that aluminum doping affects the Co2+ concentration in LDH (Al40 CoFe30 and Al60 CoFe20) and promotes the formation of high-valent CoIII/IV-O active species which play a crucial role in OER reactions. In contrast, catalysts without Al dopant (CoFe30 and CoFe20) predominantly exhibit NiII/III-O species and demonstrate lower catalytic activity. The projected density of states (PDOS) in the aluminum-doped samples is situated very close to the Fermi level, which significantly enhances electron transfer processes. The Mott-Schottky analysis indicates that the energy levels of Al40CoFe30 are favorable for hydrogen evolution reactions (HER) with a potential of 178 mV at 50 mA/cm2. Meanwhile, Al60CoFe20 exhibits an accumulation layer conducive to OER reactions, demonstrating a potential of 200 mV at 50 mA/cm2. This study underscores the importance of manipulating the Fermi-level alignment in layered double hydroxides (LDHs) through strategic metal doping to achieve efficient water-splitting reactions. The montmorillonite K10 clay was modified by calcination at high temperatures (300-900°C) and incorporated with functionalized carbon nanotubes (CNTs). The sample heated at 600°C (K-Mont+CNT600) underwent structural modifications, breaking the hydroxyl (OH) groups and facilitating greater CNT adsorption into its structure. This adsorption transforms the bulk clay into individual sheets, thereby imparting hydrophobic properties and enhancing the electrochemical surface area. The CNTs act as pillars between the clay layers, improving access for electrolyte ions and enhancing electron transport. In contrast, the annealed sample prepared without CNTs (K-Mont) shows significantly lower catalytic activity. The incorporation of CNTs into the modified clay reduces the full cell over potential by 250 mV compared to pristine K-Mont, demonstrating enhanced performance.
This dissertation addresses the primary challenge of electrocatalyst surface oxidation through the passivation effect and highlights innovative strategies, including cobalt-based heterostructures, aluminum-doped layered double hydroxides, and functionalized clay-carbon nanotube composites, to enhance HER and OER performance. Furthermore, it provides deeper insights into Fermi-level band alignment under water-splitting potentials, which is useful for tuning water redox properties.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Motivation for the research work 1
• 1.1.1 Sustainable energy transition 2
• 1.1.2 Hydrogen as future resource 3
• 1.2 Paths of hydrogen production 4
• 1.2.1 Grey hydrogen 4
• 1.2.2 Blue hydrogen 5
• 1.2.3 Green hydrogen 5
• 1.2.4 Turquoise hydrogen 6
• 1.3 Water splitting process 7
• 1.3.1 Electrocatalytic water splitting 7
• 1.3.2 Photocatalytic water splitting 8
• 1.3.3 Thermochemical water splitting 9
• 1.4 Electrochemical cells and reactions 11
• 1.4.1 Galvanic or voltaic cells 11
• 1.4.2 Electrolytic cells 13
• 1.4.3 Faraday's law 15
• 1.4.4 Standard potential 16
• 1.4.5 Oxidation-reduction mechanism 19
• 1.4.6 Current-potential curve 20
• 1.4.7 Nernst equation 22
• 1.5 Electrode Process 24
• 1.5.1 Faradic process 24
• 1.5.2 Non-faradic process 25
• 1.5.3 Polarization and capacitance 25
• 1.5.4 Electrical double layer 27
• 1.6 Electrode reaction kinetics in Faradic process 31
• 1.6.1 Pathways of electrode reactions 31
• 1.6.1.1 Mass transport 32
• 1.6.1.2 Electron transfer kinetics (at the electrode surface) 32
• 1.6.1.3 Adsorption-desorption kinetics 33
• 1.6.2 Electrode kinetics 33
• 1.6.3 Butler-Volmer equation 36
• 1.7 Interfaces in Electrochemistry 38
• 1.7.1 N-type and P-type semiconductors 38
• 1.7.2 Semiconductor-semiconductor interface 38
• 1.7.3 Metal-semiconductor interface (Schottky contacts) 42
• 1.7.4 Semiconductor-electrolyte interface 44
• 1.8 Water splitting mechanism 46
• 1.8.1 Hydrogen evolution reaction (HER) 48
• 1.8.1.1 Hydrogen reaction mechanism in acid and alkaline electrolyte 48
• 1.8.1.2 Gibb's free adsorption energy of HER intermediates 50
• 1.8.2 Oxygen evolution reaction (OER) 51
• 1.8.2.1 OER reaction mechanism in alkaline and acid electrolyte 51
• 1.8.2.2 Gibb's free adsorption energy of OER intermediates 54
• 1.9 Optimization factors of catalyst surface 55
• 1.9.1 Vacancies and crystal defects 55
• 1.9.2 Surface morphology 55
• 1.9.3 Catalyst active surface area 55
• 1.9.4 Mass loading and bubble effect 56
• 1.9.5 Catalyst-substrate contact 56
• 1.10 Activity evaluation parameters 56
• 1.10.1 Overpotential 56
• 1.10.2 Tafel slope 57
• 1.10.3 Transfer coefficient 58
• 1.10.4 Charge-transfer resistance 59
• 1.10.5 Electrochemical active surface area 60
• 1.10.6 Turn over frequency 60
• 1.10.7 Mass and specific activity 61
• 1.10.8 Stability 61
• 1.10.9 Faradic efficiency 62
• 1.11 Water electrolyzer cell 62
• 1.11.1 Two-electrode cell configuration 62
• 1.11.2 Three-electrode cell configuration 63
• 1.12 Water electrolysis technology 63
• 1.12.1 Alkaline water electrolysis 64
• 1.12.2 Proton exchange membrane (PEM) 64
• 1.12.3 Anion exchange membrane (AEM) 65
• 1.13 Objectives of this research work 66
• 2. Electrocatalyst materials, synthesis methods and characterization techniques 67
• 2.1 D-block elements as electrocatalyst materials 67
• 2.2 Electrocatalyst for HER reactions 67
• 2.2.1 Precious transition metals 68
• 2.2.2 Transition metal chalcogenides 68
• 2.2.3 Transition metal phosphides 69
• 2.2.4 Layered double hydroxides 70
• 2.2.5 Metal alloys, carbides, and nitrides 71
• 2.2.6 Single atom and Carbon-based composites 71
• 2.3 Electrocatalyst for OER reactions 72
• 2.3.1 Precious transition metals 72
• 2.3.2 Transition metal oxides 73
• 2.3.3 Transition metal hydroxides/oxyhydroxides 73
• 2.3.4 Perovskite oxides 74
• 2.3.5 Spinel oxides 75
• 2.3.6 Layered double hydroxides 75
• 2.3.7 Metal-organic and covalent-organic frameworks 76
• 2.4 Montmorillonite earth-clay 77
• 2.5 Synthesis methods 78
• 2.5.1 Chemical bath deposition 78
• 2.5.2 Hydrothermal method 79
• 2.5.3 Electrodeposition 79
• 2.6 Physicochemical characterization techniques 80
• 2.6.1 X-ray diffraction 80
• 2.6.2 X-ray photoelectron spectroscopy 81
• 2.6.3 Field emission scanning electron microscope 82
• 2.6.4 Transmission electron microscope 83
• 2.6.5 Fourier transform infrared microscopy 83
• 2.6.6 Raman/In-situ Raman spectroscopy 84
• 2.6.7 Branauer-Emmett Teller method 85
• 2.7 Density functional theory 87
• 2.8 Electrochemical evaluation techniques 89
• 2.8.1 Cyclic voltammetry 89
• 2.8.2 Linear sweep voltammetry 91
• 2.8.3 Electrochemical impedance spectroscopy 92
• 2.8.4 Chronopotentiometry/chronoamperometry 94
• 3. Djurleite copper sulfide coupled cobalt sulfide interface for stable and efficient electrocatalyst 96
• 3.1 Introduction 96
• 3.2 Preparation of cobalt incorporated copper sulfide on nickel foam 97
• 3.3 Results and discussions 98
• 3.3.1 Crystallographic studies 98
• 3.3.2 Morphological studies 100
• 3.3.3 Chemical oxidation state 102
• 3.3.4 Electrochemical studies 104
• 3.3.4.1 OER electrochemical studies 104
• 3.3.4.2 HER electrochemical studies 105
• 3.3.4.3 Electrode kinetics 107
• 3.3.4.4 Overall water splitting studies 108
• 3.3.4.5 Post-characterization studies 109
• 3.4 Conclusion 111
• 4. Pentlandite compound-anchored CuSCN as a stable electrocatalyst in highly alkaline solutions 112
• 4.1 Introduction 112
• 4.2 Electrocatalyst preparation 113
• 4.2.1 Synthesis of copper thiocyanate (CuSCN/NF) 113
• 4.2.2 Synthesis of iron cobalt encapsulated copper thiocyanate (FeCo/CuSCN/NF) 114
• 4.3 Results and discussion 115
• 4.3.1 Crystal structure 115
• 4.3.2 Morphological structure 116
• 4.3.3 Surface chemical state 118
• 4.3.4 Chemical structure and bonding 120
• 4.3.5 Electrochemical studies 121
• 4.3.5.1 Oxygen evolution reaction 121
• 4.3.5.2 Hydrogen evolution reaction 123
• 4.3.5.3 Steady-state chronoamperometry studies 124
• 4.3.5.4 Full-cell studies 125
• 4.3.6 Benefits of passivation layer and OER post-characterization results 126
• 4.3.6.1 Crystal structure and morphology 126
• 4.3.6.2 ICP metal dissolution rate 127
• 4.4 Conclusion 128
• 5. Unveiling the aluminum doping effects of in-situ transmogrified dual-LDH heterostructure and its fermi-level alignment to water splitting potentials 129
• 5.1 Introduction 129
• 5.2 Preparation process of dual-layered CoFe-LDH/NiAl-LDH electrocatalyst 130
• 5.3 Results and discussion 131
• 5.3.1 Crystal phase 131
• 5.3.2 Surface heterostructure and morphology 133
• 5.3.3 XPS spectra and depth profile XPS 135
• 5.3.4 Ex-situ Raman spectra 137
• 5.3.5 Electrochemical studies 138
• 5.3.6 Brief investigation of aluminum doping effects in the formation of LDH 142
• 5.3.6.1 Preliminary supporting studies 142
• 5.3.6.2 Depth profile XPS 146
• 5.3.6.3 ICP-OES atomic percentage 147
• 5.3.6.4 In-situ Raman studies 148
• 5.3.6.5 DFT theoretical calculations 149
• 5.3.7 Fermi-level alignment and barrier heights 156
• 5.4 Conclusion 158
• 6. Phase-modified montmorillonite clay with spatially supported carbon nanotubes for sustainable and efficient catalytic reactions 160
• 6.1 Introduction to sustainable earth-based clay material 160
• 6.2 Fabrication process of K10 clay using nickel foam 161
• 6.3 Results and discussion 162
• 6.3.1 TEM microstructure and morphology 162
• 6.3.2 Rietveld refinement and crystal structures 164
• 6.3.3 Crystallite size-lattice strain by Williamson-Hall plot 167
• 6.3.4 Pore size-volume distribution 167
• 6.3.5 TGA thermal studies 168
• 6.3.6 Molecular and chemical-state studies 169
• 6.3.7 Electrochemical studies 171
• 6.3.7.1 OER studies 171
• 6.3.7.2 HER studies 173
• 6.3.7.3 Charge-transfer coefficient 174
• 6.3.8 Overall water splitting performance 176
• 6.3.9 Surface and structural analysis of stability tested samples 177
• 6.3.9.1 Molecular and functional groups 177
• 6.3.9.2 Oxidation-state shifts 179
• 6.4 Conclusion 179
• References 181
• Abstract (Korean) 206
• List of Publications 208
• List of Conferences 209
• Acknowledgments 210