Synthesis and Design of Heterogeneous Cobalt based Derivatives with Unique Nano-structural Morphologies for Electrocatalytic Water Oxidation

다국어 초록 (Multilingual Abstract)

In order to meet the ever-increasing demand for energy, a worldwide transition away from fossil fuels to renewable solar fuels is required. However, the intermittency of local sun radiation mandates a low cost and efficient storage system for solar energy. Using solar-derived electricity to drive the water splitting reaction for generation of dihydrogen (H2 evolution reaction at cathode) and dioxygen (O2 evolution reaction at anode) is one promising method for storing solar energy to fuels. But, the water splitting currently suffers from large energetic barriers on the oxygen side, this necessitate the use of suitable electrocatalyst to minimize energy barrier at anode. The catalyst that generally shows the best trade-off between catalytic activity and stability for oxygen evolution reaction is IrO2. However, the high cost and scarcity of IrO2 hinder their practical applications. Therefore, recently several attempts have been devoted to break these hurdles by developing inexpensive and earth abundant catalysts. This work focuses on developing and understanding ways to promote the catalysis of the oxygen evolution reaction on earth-abundant cobalt metal based catalysts. A combination of electrochemical and surface characterization technique were used to correlate changes in the surface properties of the cobalt-based materials to changes in it catalytic activity towards OER.
Three different synthesis methods were used to generate cobalt-based materials and their catalytic activity were examined for their potential effect on the OER. In the first study (Chapter 3), The OER activity of the Co3O4 was enhanced by coating carbon on the surface of Co3O4 nanorods. In this work a simple method for synthesis of carbon-Co3O4 composite using hydrothermal and subsequent annealing method was developed. In which, metal precipitation and glucose carbonization reactions were combined in-situ. The role of carbon in synthesis of specific shaped Co3O4 was demonstrated by time dependent hydrothermal experiments. The OER results shows that the carbon coating on the surface of Co3O4 nanorods significantly increases the charge transfer rate of the electrode material by improving its conductivity resulting there higher OER performance than bare Co3O4 material. But the prepared composite still showed low stability as well as slightly higher overpotential for OER.
To improve the stability as well as overpotential of Co3O4 for OER, an easy synthesis method for the preparation of novel mesoporous (MP) Co3O4 and NiCo2O4 was developed in the second part of this study (Chapter 4). The method involves a template-free hydrothermal and subsequent annealing method. In which, diethylenetriamine used as a complexing agent, which was able to control crystal growth of corresponding metal hydroxide, leading to the formation of a mesoporous structure. Both, MP-Co3O4 and MP-NiCo2O4 display excellent electrocatalytic activity towards water oxidation with lower onset potential and overpotential (MP-Co3O4 ɳ10 = 302 mV and MP-NiCo2O4 ɳ10 = 322 mV). The improved charge transfer reactions were mainly due to the mesoporous structures of MP-Co3O4 and MP-NiCo2O4. The prepared materials were also tested for methanol electro-oxidation (MOR), in which MP-NiCo2O4 shows excellent catalytic activity for MOR. Furthermore, MP-Co3O4 and MP-NiCo2O4 electrode displays excellent catalytic stability for OER (10 h).
In the next part of the study (Chapter 5), using a similar approach but instead of hydrothermal method, Microwave irradiation method was used followed by air annealing for the synthesis of Cobalt oxide. In which, the well-defined Co3O4 microrods were successfully anchored onto the surface of stainless steel mesh substrate with the assistance of diethylenetriamine using commercially available microwave instrument. Co3O4@SUS possesses outstanding catalytic activity toward water oxidation. In water oxidation, the current density of 10 mA cm–2 was achieved at 298 mV overpotential with a low Tafel slope of 105 mV dec–1. In addition to low overpotential, Co3O4@SUS was stable under conditions of continuous O2 evolution for an extended period (24 h). The results show a highly efficient, scalable, and low-cost method for developing highly active and stable OER electrocatalysts in alkaline solution.
It is known that the conductivity of a metal is generally better than that of its oxides, so in the last part of this study (Chapter 6) a metal alloy was synthesized instead of metal oxide. Consequently, a method for synthesizing highly nanoporous carbon Ni/Co metal alloy composite (SBET = 1442.65 m2 g-1) was developed. Synthesis of materials done by hydrothermal reaction followed by a KOH activation. Chitosan was used as a carbon source. The formation mechanism of well-developed Ni/Co alloy carbon composite was investigated by time dependent KOH activation study. The prepared composites were characterized by various techniques and tested for methanol electro-oxidation, and results shows corresponding high current density and low onset potential for methanol electro-oxidation. The porous architectures of the catalyst provides higher specific surface areas and larger pore volumes that maximized the availability of electron transfer within the nano sized electrocatalyst surface area. The same property also provide better mass transport of reactants to the electrocatalyst. The as-synthesized Ni/Co alloy carbon composite may be useful for OER reactions. In summary, the ability of Co-based materials to act as an efficient catalyst for OER has been demonstrated using different methods to synthesize catalyst with good activity. The co-correlation between synthesis, morphology and performance has clearly been shown.
Keywords: Water oxidation; Electro-catalysis; Oxygen evolution; Diethylenetriamine, Methanol oxidation; Morphology

국문 초록 (Abstract)

지속적인 에너지 수요를 만족하기 위하여 화석연료로부터 재생가능한 태양 연료로의 전환에 대한 연구가 필요하다. 그러나 지역적 태양광 방사의 차이로 인하여 저비용, 고효율 태양에너지 저장시스템을 얻는 데는 어려움이 있다. 여기서 태양에너지를 연료로 저장하는 한가지 기대되는 방법은, 태양에너지를 이용한 물분해로 양극에서 수소를, 음극에서 산소를 얻는 것이다. 그러나, 물분해는 현재 산소 부위에서 막대한 에너지 장벽을 지니며 음극에서 매우 적절한 전기촉매의 개발을 요구한다. 현재 산소발생반응(OER: Oxygen Evolution Reaction)용으로 촉매활성 및 안정성을 가장 구현하는 촉매는 IrO2 이다. 그러나 이 촉매는 고비용 및 희소성으로 실제적 적용에 한계가 있다. 그러므로 많은 시도들은 저비용으로 흔한 코발트 금속기반 촉매에 초점을 맞춘다. 여기서 전기화학적/표면 특성법을 사용하여 코발트계 소재들의 표면 물성과 OER 촉매활성의 변화의 관계들을 조사한다.
본 연구에서는 3가지 다른 제조방법을 사용하여 코발트계 소재를 개발하였으며 그들의 OER에 대한 촉매활성 능력을 조사하였다. 처음 연구에서는, Co3O4 의 OER 활성이, 나노막대의 표면 상에 탄소를 코팅함으로써 증진되었다. 여기서 수열 합성 및 열처리법을 사용하여 단순하게 탄소-Co3O4 복합체를 합성할 수 있었다. 특별히 금속 침전 및 글루코오스 탄화 반응이 포함되었다. 특정한 형상의 Co3O4 의 합성시 탄소의 역할은, 시간별로 열수화과정에서 구현될 수 있었다. OER결과로서, 그 전기전도도를 증신시키게 되어 전극물질의 전하 전달속도를 크게 증진시킨다는 것을 확인하였다. 그러나 합성된 복합체는 안정성이 여전히 낮았고 OER의 과포텐샬이 다소 더 높아졌다.
To improve the stability as well as overpotential of OER용 Co3O4 f의 안정성 및 과포텐샬을 증진시키기 위하여 신규 메조기공의 (MP) Co3O4 및 NiCo2O4 을 개발하게 되었다. 여기서는 복합화 소제로서 디에틸렌트리아민을 사용하였으며 여기서 대응하는 금속 하드록사이드의 결정 성장을 제어할 수 있었고 이로써 메조기공 구조 성형이 가능하였다. MP-Co3O4 와 MP-NiCo2O4 는 보다 낮은 시작 포텐샬 및 과포텐샬 (MP-Co3O4 ɳ10 = 302 mV, MP-NiCo2O4 ɳ10 = 322 mV)을 지니며 우수한 OER 전기촉매 활성을 보여주었다. 이는 전적으로 메조기공으로 인한 것이었다. 이들 소재는 메탄올 전기산화 (MOR)에도 적용되었으며 여기서는 MP-NiCo2O4 이 우수한 촉매활성을 보였다. 더욱이, MP-Co3O4 와 MP-NiCo2O4 전극은 OER에 대한 우수한 촉매 안정성도 구현하였다 (10시간).
다른 한가지 방법으로 수열반응 대신에 마이크로파 조사법을 사용하고 산화열처리법을 이용하여 산화코발트를 합성하였다. Co3O4 마이크로막대는, 디에틸렌트리아민을 이용하여 스테인레스 스틸망 표면 상에 성공적으로 코팅시킬 수 있었다. 이와 같이 제조된Co3O4@SUS는 물의 산화 반응에 현저한 촉매활성 효과를 구현하였다. 10 mA cm–2 의 전류밀도를 298 mV 과포텐샬에서, 105 mV dec–1의 낮은 Tafel 기울기로 구현할 수 있었다. 낮은 과포텐샬과 함께, Co3O4@SUS는 24시간의 긴 시간동안 연속적인 산소발생 조건하에서 안정하였다. 이 결과는 알칼리 용액에서 높은 활성 및 안정된 OER 전기촉매를 개발할 수 있는 아주 고효율, 저가의 방법론을 제시하였다.
금속의 전기전도도가 금속산화물보다 일반적으로 더 좋다는 것으로 알려져 있다. 따라서 본 연구에서는 금속산화물 대신 금속합금을 합성하여 사용하고자 하였다. 결과적으로 아주 높은 나노기공의 탄소 Ni/Co 금속 합금 (SBET = 1442.65 m2 g-1)을 개발하였다. 이 물질은 수열반응 및 KOH 활성화 반응으로부터 합성되었다. 탄소원으로는 키토산을 사용하였다. Ni/Co 합금 탄소 복합체의 합성 메커니즘은 시간 의존 KOH 활성화 연구에 의하여 조사되었다. 이들 소재들은 메탄올 전기산화용으로 테스트되었으며 그 결과는, 높은 전류밀도 및 낮은 시작 포텐샬을 보여주었다. 이 촉매의 나노기공 구조체는, 보다 높은 비표면적 및 큰 기공부피를 제공한다. 이것은 나노 크기의 전기촉매 표면적 내 전자 이동성을 극대화할 수 있게 한다. 합성된 Ni/Co 합금 탄소 복합체는OER 반응에 융용할 수 있다. 끝으로, 코발드계 물질들이 OEF용 고효율 촉매로서 사용될 수 있음을 여러 다른 복합촉매 및 제조방법을 통하여 구현했으며 이들이 좋은 촉매 활성을 지님을 확인하였다.

키워드: 물 산화, 전기촉매, 산소 발생, 디에틸렌트리아민, 메탄올 산화, 형상

목차 (Table of Contents)

• Contents Page No.
• Table of Contents……………………………………………………………………………..i
• List of Figures……………………………………………………………………...…….......vi
• List of Tables…………………………………………………………………………..........xii
• List of Schemes………………………………………………………………………….…..xii
• Glossary of Abbreviations and Symbols……………………………………………...…..xiii
• Thesis Abstract………………………………………………...………………………....…xv
• Chapter 1. Introduction 1
• 1.1. Background of study 1
• 1.2. Importance of OER in various applications 6
• 1.2.1. Metal-air batteries 6
• 1.2.2. Water splitting 7
• 1.2.3. Fuel cell 9
• 1.3. Objectives 10
• Chapter 2. Basic of oxygen evolution reaction and related literature review 12
• 2.1. Overall water splitting 12
• 2.2. Mechanism of OER 14
• 2.3. Key performance evaluating parameters of OER 16
• 2.3.1. Electrode 16
• 2.3.2. Electrolyte 17
• 2.3.3. Onset/overpotential 18
• 2.3.4. Tafel slope 19
• 2.4.Literature review 20
• 2.4.1. Precious metal based catalyst 20
• 2.4.2. Earth-abundant transition metals 22
• 2.4.2.1. OER with spinels 22
• 2.4.2.2. Cobalt oxide based materials 22
• 2.5. Strategies to enhance spinel activity towards OER 23
• 2.5.1. Controlling the composition and phase 23
• 2.5.2. Designing micro/nanostructure 24
• 2.5.3. Loading on a conductive matrix 25
• 2.5.4. Active sites for OER 27
• Chapter 3. Environment friendly hydrothermal synthesis of carbon-Co3O4 nanorods composite as an efficient catalyst for oxygen evolution reaction 31
• 3.1. Introduction 32
• 3.2. Experimental 34
• 3.2.1. Chemicals 34
• 3.2.2. Preparation of C-Co3O4-nanorods and Co3O4-microplates 34
• 3.2.3. Characterization 35
• 3.2.4. Electrochemical measurements 36
• 3.2.5. The thermodynamic potential for OER 37
• 3.3. Results and discussion 37
• 3.3.1. Formation mechanism of C-Co3O4-nanorods and Co3O4-microplates 37
• 3.3.2. Structural studies 40
• 3.3.3. Morphological studies 43
• 3.3.4. Electrochemical studies 49
• 3.4. Conclusions 59
• Chapter 4. Diethylenetriamine assisted synthesis of mesoporous Co and Ni-Co spinel oxides as an electrocatalysts for water and methanol oxidation 60
• 4.1. Introduction 61
• 4.2. Experimental section 64
• 4.2.1. Preparation of MP-NiCo2O4 64
• 4.2.2. Preparation of MP-NiO 64
• 4.2.3. Preparation of MP-Co3O4 65
• 4.2.4. Materials characterization 65
• 4.2.5. Electrochemical Measurements 66
• 4.3. Results and discussion 67
• 4.3.1. Proposed synthesis route of MP-Co3O4 and MP-NiCo2O4 67
• 4.3.2. Structural and morphological Studies 68
• 4.4. Electrochemical studies 81
• 4.4.1. Oxygen evolution reaction 81
• 4.4.2. Methanol electro-oxidation reaction 86
• 4.5. Conclusion 94
• Chapter 5. Microwave assisted synthesis of stainless steel mesh-supported Co3O4 micro-rod array as highly efficient catalyst for electrochemical water oxidation 96
• 5.1. Introduction 97
• 5.2. Experimental section 100
• 5.2.1. Preparation of Co3O4@SUS 100
• 5.2.2. Preparation of Co3O4 layered nanosheets (LNS) 101
• 5.2.3. Materials Characterization. 101
• 5.2.4. Electrochemical Measurements. 102
• 5.2.5. Loading of catalyst 102
• 5.2.6. The thermodynamic potential for OER 103
• 5.3. Results and discussion 104
• 5.3.1. Preparation and formation mechanism of Co3O4@SUS and Co3O4-LNS. 104
• 5.3.2. Characterizations of Co3O4@SUS and Co3O4-LNS. 109
• 5.3.3. Electrochemical tests. 122
• 5.4. Conclusions 128
• Chapter 6. Chitosan derived microporous carbon decorated with Ni-Co nano flower synthesis using KOH activation as an electrode material for methanol oxidation 129
• 6.1. Introduction 130
• 6.2. Experimental section 133
• 6.2.1. Chemicals 133
• 6.2.2. Synthesis of material 133
• 6.2.3. Measurement techniques for structural characterization 134
• 6.2.4. Electrochemical measurement 135
• 6.3. Results and discussion 136
• 6.3.1. Electrochemical study 148
• 6.4. Conclusion 156
• References………………………………………………………………………………….157
• Publications………………………………………………………………………………...178
• Korean abstract…………………………………………………………………………....181