Role of Chemical State for Electrocatalysis in Electroreduction of CO2 and Water = 금속 촉매의 화학적 상태가 전기화학적 이산화탄소 전환 반응과 물분해 반응에 끼치는 역할에 관한 연구

국문 초록 (Abstract)

산업혁명 이후 오랜 기간 동안 인류의 화석연료 사용으로 인해 배출되는 이산화탄소의 현재 대기 중 농도는 2014년 400 ppm을 넘어선 이후에도 계속해서 증가하고 있다. 이러한 이산화탄소 농도의 증가는 현재 대중적으로도 전 지구적인 기후변화 문제를 일으키고 있는 것으로 이해되고 있다. 따라서 현재 많은 연구자들이 이산화탄소를 직접적으로 저감하기 위해 탄소 포집 및 저장 또는 유용화 연구에 많은 관심을 가지고 있다. 뿐만 아니라 화석연료의 고갈적 순환을 벗어나 지속 가능한 에너지 순환을 가지는 수소연료의 활용 또한 매우 중요한 연구분야로 주목 받고 있다.
본 학위 논문은 전기화학 반응을 응용하여 위의 문제에 대한 해결책을 논의한다. 특히, 전기화학적 이산화탄소 환원 및 수전해를 통한 수소 생산에 대해 자세히 논의한다. 1장에서는 본 연구의 배경과 연구된 각 전기화학 반응 (이산화탄소 환원, 수소 생성 및 산소 생성 반응) 에 대해 기술하고자 한다.
전기화학적 이산화탄소 환원반응으로 고부가가치 생성물을 만들어내기 위해서는 C-C 결합을 촉진하는 것이 매우 중요하다. C-C 결합 형성은 구리 촉매에서 가장 유망한 활성을 보여주는 것으로 알려져 있다. 때문에, 그 활성의 증가 원인에 대해서 많은 연구자들이 관심을 가지고 있다. 본 논문의 2장에서는 양극산화 된 수산화구리 전극에서 증가된 C-C 결합 형성 능력을 확인하였다. 또한, 이에 대한 원인이 구리의 혼합된 화학적 상태와 연관이 있음을 스침각 X-선 회절 분석, 연 X-선 흡수 분광 분석 및 X-선 광전자 분광 분석을 통해 규명하였다.
3장에서는 전기분무법을 이용하여 30~40nm의 작은 입자크기를 가지는 CuO 촉매를 가스 확산 층 전극에 직접 합성하여 70 % 이상의 매우 높은 C2 생성물 선택도를 얻었다. 그리고 분무량에 따라서 C2와 C1 생성물의 비율을 조절 할 수 있었다. 또한, 반응 중에 구리의 산화 화학종이 존재함을 실시간 경 X-선 흡수 분광 분석을 통해 확인하였다. 동시에, 반응 시간에 따라 촉매의 형태가 나노입자에서 400 nm 크기의 입방체 모양으로 변화함에 따라 C2 생성이 감소함을 확인하였다. 이를 통해 구리 촉매에서 C-C 결합형성은 구리의 화학 종뿐 아니라 촉매 형태 변화에도 깊은 연관이 있음을 규명하였다.
4장에서는 전기화학적 수소 생산 반응에 대한 연구를 논의한다. 산성 환경에서의 물 산화 촉매의 낮은 안정성을 문제를 피하기 위해 염기성 환경에서 수소 생성을 논의한다. 또한, 백금 족 귀금속 촉매의 대체 또한 중요한 연구 내용이다. 이를 위해 Ti 도핑 된 Cu 촉매를 합성 하였고, 이 촉매는 합성과정에서의 산소 결핍 자리를 많이 유도 할수록 성능이 좋아지는 것을 확인하였다. 또한 그 산소 결핍자리는 도핑 된 Ti 원소에 주로 생기며, 그 곳에서 친산소성 높아져 촉매의 물 분자의 흡착을 용의하게 하여 Cu 촉매의 반응성을 높임을 제안했다.
수전해 기반 반응에서, 물산화 반응의 속도는 일반적으로 느리기 때문에, 물산화 반응의 성능을 증가시키는 것은 전체 반응 효율개선에 중요하다. 때문에, 물 산화 촉매의 성능증가에 관한 연구를 5장에서 논의한다. 산성조건에서의 낮은 촉매 안정성과 Ir 과 Ru같은 귀금속 촉매의 사용으로, 염기성 전해질에서 Ni-Fe 기반 촉매의 연구가 주목 받았다. 이 연구에서는, Ni 촉매에 Fe-N-C 지지체를 도입하며 활성점을 분리하였고, 이로써 Ni 에서 Fe로 움직이는 전하이동이 Ni의 높은 원자가를 유지하며 산소 발생 반응의 성능을 높였음을 규명했다.
6장에서는 지금까지 논문에서 논의한 각 반응의 연구들의 향후 전망에 대해 정리했다. 7장에서는 본 논문에서 논의된 연구들의 요약과 결론에 대해 언급하고, 전기 촉매의 화학적 상태 변화와 활성 사이의 연관성을 탐구할 이후의 연구 방향을 정리하며 논문을 마무리 한다.

다국어 초록 (Multilingual Abstract)

Due to the human use of fossil fuels for a long period after the Industrial Revolution, the current atmospheric concentration of CO2 emitted has continued to increase even after exceeding 400 ppm in 2014. This increase in CO2 concentration is currently widely understood as causing a global climate change problem. Therefore, many researchers have a lot of interest in carbon capture and storage or utilization research in order to directly reduce CO2. In addition, the use of hydrogen fuel that has a sustainable cycle outside the finite cycle of fossil fuels is also attracting attention as important research field.
This thesis is approached as a solution to the above problem by applying electrochemical reaction. In particular, the electrochemical CO2 reduction and hydrogen production through water electrolysis are discussed in detail. In chapter 1, the background of this study and each electrochemical reaction (CO2 reduction, hydrogen evolution and oxygen evolution reaction) studied are described.
In order to produce a value-added products through electrochemical CO2 reduction, it is very important to promote C-C bonding. The C-C bond formation is known to show the most promising activity in Cu catalysts. Consequently, many researchers are interested in the cause of the increase in C-C coupling activity. In Chapter 2, the increased C-C bond formation ability in anodized copper hydroxide electrode was observed. The cause of this increase in performance was investigated to be related to the mixed chemical state of Cu through grazing incidence X-ray diffraction (GI-XRD), soft X-ray absorption and X-ray photoelectron spectroscopy (XAS and XPS) analysis.
In Chapter 3, a CuO catalyst having a small particle size of 30 to 40 nm was synthesized directly on the gas diffusion layer electrode using the electrospray method to obtain high C2 product selectivity. Also, the ratio of C2 and C1 products could be adjusted according to the spray amount. The presence of oxidizing species of Cu during the reaction was found through in situ/operando X-ray absorption spectroscopy. Simultaneously, the C2+ production decreased was observed as the catalyst morphology change form nanoparticle to a cube shape of 400 nm size according to the reaction time. Therefore, this chapter suggested that the formation of C-C bonds in copper catalysts is closely related not only to the chemical species of Cu, but also to the morphology of the catalyst.
In the fourth chapter, we discusses the study of electrochemical hydrogen evolution reaction. In order to avoid the problem of water oxidation catalyst low stability in acid media, the hydrogen production conducted in basic media. In addition, replacement of the Pt group noble metal catalyst is also important research content. A Ti-doped Cu catalyst was synthesized, and the performance of this catalyst improved as more oxygen vacancy site in the synthesis process. The oxygen vacancy site mainly occurs in doped Ti element, which the oxophilicity increases, making the adsorption of water easier. It is proposed that the increased oxygen binding energy is the cause of the activity enhance of catalyst.
In the water based electrolysis reaction, the kinetics of the water oxidation is generally sluggish. By increasing the water oxidation activity, the overall reaction efficiency can also be improved. Thus, the study on the performance increase of water oxidation catalyst is discussed in the Chapter 5. Due to the low stability of the catalysts in an acidic media and the use of Ir and Ru noble metals, Ni-Fe based catalysts receive attention in a basic electrolyte. In this study, the active sites were separated by introducing a Fe-N-C support into the Ni catalyst. Through this, we found that the charge transfer occurring form Ni to Fe enhance oxygen evolution performance by maintaining a high valency state of Ni.
The Chapter 6 refer to the future perspective of the studies of each reaction discussed in this thesis so far. In chapter 7, the summary and conclusions of the studies discussed in this dissertation are mentioned. In addition, this thesis concludes with summary of future research direction to explore the correlation between the chemical state change and activity of electrocatlayst.

목차 (Table of Contents)

• TABLE OF CONTENTS
• Abstract ····································································· i
• Table of Contents ························································ vii
• List of Figure ·······························································xi
• List of Table ·····························································xviii
• Chapters
• 1. Research background and introduction································· 1
• 1.1. Necessity for sustainable energy cycle······························· 2
• 1.2. Electrochemical CO2 reduction reaction····························· 7
• 1.2.1. Schematic of the conventional H-type cell·················· 7
• 1.2.2. Challenges of electrochemical CO2 reduction············· 10
• 1.2.3. Key factor of C-C coupling in electrocatalysis············ 14
• 1.3. Hydrogen evolution reaction········································· 16
• 1.3.1. Fundamental of reduction of water·························· 16
• 1.3.2. Challenges of HER and strategies··························· 19
• 1.4. Oxygen evolution reaction··········································· 20
• 1.4.1. Fundamental of water oxidation····························· 20
• 1.4.2. Challenges of OER and strategies··························· 23
• 1.5. Chapter Summary····················································· 24
• 1.6. Reference······························································· 27
• 2. Correlation between catalytic chemical state and ability of C-C coupling in anodized Cu(OH)2 catalyst································· 34
• 2.1. Introduction···························································· 35
• 2.2. Experimental··························································· 39
• 2.2.1. Cu elecrocatalyst preparation································ 39
• 2.2.2. Electrochemical CO2 reduction reaction measurement··· 40
• 2.2.3. Product analysis of CO2 reduction reaction················ 41
• 2.2.4. Material Characterization···································· 42
• 2.2.5. X-ray spectroscopic analysis of the Cu electrocatalysts·· 43
• 2.3. Result and discussion················································· 44
• 2.4. Conclusion····························································· 73
• 2.5. Reference······························································· 74
• 3. Eelctrochemical CO2 reduction products ratio controlled in CuO catalyst······································································· 78
• 3.1. Introduction···························································· 79
• 3.2. Experimental section················································· 83
• 3.2.1. One-step synthesis of CuO catalyst by electrospray pyrolysis······················································· 83
• 3.2.2. Electrochemical measurement of CO2 reduction reaction························································· 84
• 3.2.3. Analysis of CO2 reduction reaction products·············· 85
• 3.2.4. Material characterizations···································· 86
• 3.2.5. Measurement of in-situ/operando XAS···················· 87
• 3.3. Result and discussion················································· 88
• 3.4. Conclusions··························································· 123
• 3.5. Reference····························································· 125
• 4. Association between oxygen vacancy and increased activity in Cu-Ti catalyst for alkaline HER················································ 129
• 4.1. Introduction··························································· 130
• 4.2. Experimental························································· 132
• 4.2.1. Ti dopped Cu electrocatalyst synthesis by aerosol calcination···················································· 132
• 4.2.2. Electrochemical measurement····························· 133
• 4.2.3. Material characterizations·································· 134
• 4.2.4. Soft X-ray absorption spectroscopy analysis············· 134
• 4.3. Results and discussions·············································· 135
• 4.4. Conclusion···························································· 149
• 4.5. Reference····························································· 150
• 5. Identification of active site and increased intrinsic activity in Ni-Fe OER catalyst······························································· 152
• 5.1. Introduction··························································· 153
• 5.2. Experimental························································· 156
• 5.2.1. Preparation of carbon-based powder supports··········· 156
• 5.2.2. Working electrode preparation for water oxidation······ 156
• 5.2.3. Electrochemical measurements···························· 157
• 5.2.4. Material characterization··································· 158
• 5.3. Results and Discussion·············································· 159
• 5.4. Conclusion···························································· 190
• 5.5. Reference····························································· 192
• 6. Perspective································································· 196
• 6.1. Electrochemical CO2 reduction···································· 197
• 6.2. Water electrolysis···················································· 205
• 6.3. Reference····························································· 208
• 7. Conclusion and future works·········································· 210
• Bibliography···································································· 215
• Acknowledgement