RISS 검색 - 학위논문 상세보기

다국어 초록 (Multilingual Abstract)

Fossil fuels are the world's most widely used energy source, but this source is limited and will be depleted over time. Attention is therefore being paid to renewable energy sources to reduce the dependence on fossil fuels. Energy storage and energy conversion technology play an essential role in the renewable energy sector. In particular, proton exchange membrane water electrolyzers (PEMWEs) and proton exchange membrane fuel cells (PEMFCs) are examples of energy storage and energy conversion technologies. Although these technologies offer several advantages over other technologies of renewable energy sectors, there are still some challenges that prevent the commercialization of PEMWE and PEMFC technologies. For PEMWEs, the main roadblock is the anodic oxygen evolution reaction (OER) catalyst, whereas for PEMFCs, it is the cathodic oxygen reduction reaction (ORR) catalyst. Electrocatalysts for the OER/ORR are an important area where many breakthroughs are needed to improve the slow kinetics of the OER/ORR and reduce the amount of precious metal loading. In this light, the aim of the present research is to develop electrocatalysts for the OER/ORR with promising activity and durability.
In terms of the OER catalyst in a PEMWE, a series of boron carbide-supported iridium catalysts were prepared via the wet impregnation method using NaBH4 as a reducing agent. Boron carbide has good electrical conductivity and corrosion resistivity. Physical and electrochemical properties of the catalysts were controlled by changing the synthetic reduction temperature (30 °C–100 °C) and iridium content (10 wt%-60 wt%) on the boron carbide support. The prepared Ir/B4C catalyst is the most promising catalyst for the OER and was synthesized at 100°C reduction temperature. In addition, at 40% loading, Ir/B4C showed maximum OER catalytic performance. The 40%-Ir/B4C catalyst outperformed all synthesized catalysts as well as two commercial catalysts in both activity and durability. The improved performance of 40%-Ir/B4C can be correlated to three key factors: i) high electrochemical surface area, (ii) better electrical conductivity, and (iii) high concentration of Ir(III) on the surface. Controlling the synthetic reduction temperature and iridium content on the B4C support was found to help develop the interaction between iridium and B4C. This metal-support interaction prevents the oxidative dissolution and aggregation of iridium species. In a single cell test, Ir/B4C-40% showed outstanding cell performance of 1.61 V at 1.0 A/cm2 at 0.5 mg/cm2 loading. Using this catalyst in the anode of a PEMWE, the precious catalyst metal loading can be reduced by more than six orders. The Ir/B4C-40% catalyst also showed outstanding durability, e.g. only a small voltage increase of 11 mV during the durability test in MEA performance after operation for 48 hours at a constant current density of 2.0 A/cm2.
In terms of the ORR catalyst in a PEMFC, a durable carbon-based electrocatalyst support was synthesized. Platelet-type carbon nanofiber (PCNF) and Vulcan carbon black (CB) were coated in a uniform and discrete manner with silica, followed by platinum deposition on it. Accelerated degradation testing of the silica-coated catalysts showed higher durability than non-silica-coated catalysts under potential cycling. The silica coating can reduce carbon corrosion during the potential cycling between 1.0 and 1.5 V by i) blocking the carbon support from direct contact with the oxygen source and ii) preventing the effect of oxygen spillover from the platinum to carbon. The results suggest the silica coating on a carbon support is an effective strategy to improve the durability of Pt-based electrocatalysts under potential cycling.

목차 (Table of Contents)

Contents
Chapter 1. Introduction 23
1.1. Energy outlook 23
1.2. Hydrogen energy- A Promising Technology for Energy Storage 24

Contents
Chapter 1. Introduction 23
1.1. Energy outlook 23
1.2. Hydrogen energy- A Promising Technology for Energy Storage 24
1.2.1. Electrocatalyzed OER 28
1.2.1.1. Recent studies for OER 30
1.2.1.2. Recent trends for supported OER electrocatalyst 31
1.3. Fuel cells- A Promising Technology for Electrical Energy Conversion 32
1.3.1. Electrocatalyzed ORR 36
1.3.1.1. Degradation mechanism of Pt/C electrocatalyst 37
1.3.1.2. Research trends of enhancing the durability of carbon support for Pt-based electrocatalyst 38
1.4. Objective and structure of this dissertation 39
1.5. References 42
Chapter 2. Preparation of boron carbide supported iridium nanoclusters for oxygen evolution reaction 49
2.1. Introduction 49
2.2. Experimental section 51
2.2.1. Preparation of Ir/B4C 51
2.2.2. Material characterization 52
2.3. Results and discussion 53
2.4. Conclusion 62
2.5. References 64
Chapter 3. Enhancing the activity and durability of iridium electrocatalyst supported on boron carbide by tuning the chemical state of iridium for oxygen evolution reaction 69
3.1. Introduction 69
3.2. Experimental 73
3.2.1. Synthesis of Ir/B4C 73
3.2.2. Characterization of the catalysts 74
3.2.3. Electrochemical performance of the catalysts. 75
3.2.4. PEMWE single cell test 76
3.3. Results and discussion 77
3.3.1. Structural characterization of the catalysts 77
3.3.2. Electrocatalytic activity for OER and surface chemistry 82
3.3.3. Durability studies for the OER 91
3.3.4. PEMWE performances of Ir/B4C-100ºC and commercial catalysts 99
3.4. Conclusion 101
3.5. References 103
Chapter 4. Boron carbide-supported iridium catalyst of high activity and stability in proton exchange membrane water electrolyzer with low iridium loading 110
4.1. Introduction 110
4.2. Experimental 113
4.2.1. Materials 113
4.2.2. Preparation of catalysts 114
4.2.3. Characterization of the catalysts 114
4.2.4. Electrochemical measurements of the catalysts 115
4.2.5. PEMWE single cell test 116
4.3. Results and discussion 117
4.3.1. Structural characterization of the catalysts 117
4.3.2. Electrocatalytic activity for OER and surface chemistry 126
4.3.3. Durability studies for the OER 132
4.3.4. PEMWE performances 139
4.4. Conclusion 142
4.5. References 145
Chapter 5. Durability Enhancement of a Pt/C Electrocatalyst using Silica-Coated Carbon Nanofiber as a Corrosion-Resistant Support 151
5.1. Introduction 151
5.2. Experimental 154
5.2.1. Preparation of Silica-Coated PCNF (PCNF-SiO2) and Pt/PCNF-SiO2 154
5.2.2. Characterization of the prepared PCNF-SiO2 and Pt/PCNF-SiO2 155
5.2.3. Electrochemical Analysis 156
5.3. Result and Discussion 157
5.3.1. Coating of silica on CNF: Controlling the content, thickness, and shape of silica 157
5.3.2. Preparation and characterization of Pt/PCNF-SiO2 161
5.3.3. Durability improvement of Pt/PCNF by SiO2 coating on PCNF 165
5.4. Conclusion 172
5.5. References 174
Chapter 6. Enhanced Durability of Pt/C Catalyst by Coating Carbon Black with Silica for Oxygen Reduction Reaction 181
6.1. Introduction 181
6.2. Experimental 184
6.2.1. Preparation of a silica-coated CB supported Pt catalyst (Pt/CB-SiO2) 184
6.2.2. Physical characterization of the prepared CB-SiO2 and Pt/CB-SiO2 186
6.2.3. Electrochemical analysis 186
6.3. Results and discussion 187
6.3.1. Preparation of Pt/CB-SiO2 187
6.3.2. Electrochemical activities of the catalysts for ORR 192
6.3.3. Carbon support durability of the catalysts under potential cycling 194
6.3.4. Platinum metal durability of the catalysts under potential cycling 200
6.4. Conclusion 202
6.5. References 204

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