Thermodynamics-based Phase and Structure Engineering of Multi-component Catalysts for Electrochemical Energy Conversion = 전기화학적 에너지 전환 소자용 다중 원소 촉매의 열역학 기반 구조 및 상 제어에 관한 연구

다국어 초록 (Multilingual Abstract)

The electrochemical reaction to produce valuable chemicals such as ethylene (C2H4) and ethanol (C2H5OH) from carbon dioxide (CO2) or hydrogen (H2) from water has been receiving great attention in that it can realize sustainable energy cycle without carbon emission and store intermittent renewable electricity for a long-term. In order to improve the reaction rate and efficiency of the electrocatalysis and implement it in industrial scale, it has been essential to develop an advanced electrocatalyst. Therefore, research methodologies combined with theories and experiments were actively being conducted in this field. Optimal materials in respect of facet or composition were designed through density functional theory (DFT) calculation and artificial intelligence aided machine learning. Fabricating the target materials and revealing the underlying reaction mechanism were reinforced the prediction. In addition, supporting materials such as graphene, carbon fibers, and metal oxide were combined with the active materials to further impart functionality such as electrical conductivity and porosity. Therefore, optimizing the phase structure in such multi-component electrocatalysts determined its performance. However, so far, the phase and structure modification of multi-component nanomaterials have not been fully achieved, and even if it was limitedly possible, processing parameters were found through numerous trials-and-errors. These limitations not only reduced the versatility of materials, but were also inappropriate for mass production, preventing industrial implementation. Currently, few studies have been conducted on the fabrication design methodology that can actually implement the target material compared to the progress of the theoretical catalysts design.
Therefore, in this thesis, we would design the fabrication process of electrocatalysts based on thermodynamic and kinetics rationale, obtaining structural and phase engineering in multi-components system. In this thesis, the fabrication process of nanomaterials was designed through theoretical approaches such as thermodynamics and kinetics, inspired by metallurgy-based steel making process. Through thermodynamic and kinetic considerations, we determined the processing parameters minimizing trial-and-error, but also found specific conditions that could induce or suppress target reactions. Through this strategy, it has been shown that the structure and phase can be successfully controlled to maximize electrocatalytic performance such as activity, selectivity, and stability of multi-component catalysts. This research methodology was successfully applied to the electrocatalysts for hydrogen evolution reaction (HER) catalysts and CO2 reduction reaction (CO2RR), and examples thereof were specifically as follows.
First, the 'Boudouard reaction', which is a thermodynamic equilibrium between C-CO-CO2, was applied to Cu nanoparticles for CO2RR. Severe surface reconstruction such as particle agglomeration, facet reshaping, and dissolution caused performance degradation and disturbed long-term operation. To suppress the surface reconstruction, we confined Cu nanoparticles in carbon layer. Exploiting Boudouard reaction enabled spontaneous formation of carbon layer on Cu nanoparticles, which successfully suppressed surface reconstruction during CO2RR. For the confinement process, formation of CO and carbon deposition was controlled by regulating the oxygen partial pressure (pO2), thus the amount of CO generated accordingly. The pO2 value was determined through thermodynamic product predictions. The Cu nanoparticles confined in carbon shell exhibited improved stability in various aspects such as surface electronics and microstructure compared to the conventional Cu nanoparticles. In addition, the porous carbon shell enabled surface doping of internal Cu particles, thereby further improving the CO2RR selectivity to C2+ products. Cu nanoparticles coated with carbon shells implemented long-term operation of CO2-to-C2H4 electroconversion.
In addition, ternary alloy system consisting of a sacrificial metal, Cu, and reaction environments modifier metal was fabricated, and porous catalysts with controlled microstructure, composition, and atomic structure were produced through removing the sacrificial metal. In order to obtain the highly porous catalysts, eutectic composition and high under cooling was essential. It was shown that the microstructure could be controlled by adjusting the cooling rate. Porous Cu catalysts enabled robust CO2-to-C2+ products electroconversion in neutral electrolyte. When adding the reaction environment modifier metal, atomic distribution was dependent on the intermetallic miscibility, which could be predicted on thermodynamic activity of the solution. Furthermore, non-equilibrium alloy fabrication method, which increased the undercooling, resulted in supersaturation of Cu atoms in Ag lattice. It was revealed sufficient undercooling overcame thermodynamic miscibility. Through this kinetic approach, Ag-Cu alloy catalysts with properly controlled atomic distribution was fabricated, and by controlling the ratio, alloy catalysts capable of selectively producing C2H5OH from CO2 was developed.
As HER catalysts material, thermodynamic-based predictive fabrication was applied to transition metal dichalcogenide (TMD) and phosphide, which had high intrinsic HER activity. The fabrication process for Ni phosphide/carbon nanofiber catalyst was divided into two step through comparing the reaction tendencies of competing reactions. In the first step, the structure where active nanoparticles were hybridized with carbon support was optimized in terms of size, distribution, porosity, etc. by controlling pO2. Controlling pO2 enabled structural modification through regulated combustion of carbon support. And during the second step, the phase of Ni phosphide was modified by controlling the partial pressure of the phosphorus gas. At this time, the target phase was designed to have optimal surface bonding strength with reaction intermediate products through the DFT calculations. The processing parameters to synthesize the target phase was determined by revealing the equilibrium phosphorus pressure from the reaction Gibbs free energy of nickel phosphide. Ni phosphide/carbon nanofibers composite which was modified in respect of number of active sites and intrinsic activity obtained superior HER activity and stability.
Applying the predictive fabrication methodology was conducted to 2-dimensional TMD materials. A process of forming a sulfur vacancy (Vs) in TMDs lattice was implemented through a gas-solid phase reaction between highly reactive CO and chalcogen atoms. Through the DFT calculations, it was found that if Vs was formed in TMDs, transition can be induced to a metallic metastable phase. To implement this reaction experimentally, we designed a calcination process using CO/CO2 mixture gas. At this time, using thermochemical calculation program, the gas and solid products according to the gas ratio and temperature was predicted, and processing parameters to induce Vs formation was determined. We confirmed that the polymorph transition to the metallic 1T occurred, and this Vs formation process was applied to the carbon composite materials which had beneficial characteristics for HER catalysts. In carbon nanofibers composite, TMDs was granted structural controllability like stacking number and growth length. Simultaneous modification in the polymorph and the structure resulted in enhanced activity as HER electrocatalysts. Our Vs formation process had material versatility, enabling wide application to other TMCs. The process was likewise implemented to tungsten disulfide, which exhibited high intrinsic electrical mobility. The cation dependency was considered through calculations of reaction Gibbs free energies of possible reactions. It was revealed that the phase could be controlled in a wide range such as oxide, carbide, sulfide, etc. depending on the calcination temperature and CO fraction. Polymorph transition to the metallic 1T phase was also occurred. Mechanism on the polymorph transition was revealed by DFT calculations.
This thesis suggested that structures and phase of the electrocatalysts can be extensively modified by the reactions that has been exploited in the classical metallurgy. Processing parameters was rationally determined based on thermodynamics and kinetics only with slight considerations for surface energy from nanosizing. In multi-component catalysts such as Cu/carbon, Ni phosphide/carbon, two-dimensional TMDs/carbon composite, and Cu alloy, various competing reaction occurred. Thermodynamic predictive fabrication strategy was effect to determine the processing parameters to induce the target reaction and suppress impurity formation. This synthesis methodology minimized trial-and-error and enabled material-customized fabrication process design to achieve enhanced performance of electrocatalysts. The fabrication process was also suitable for mass production and would serve a key role in implementing the electrocatalysis for carbon-neutral energy cycle.

국문 초록 (Abstract)

전기화학 반응을 이용해 이산화탄소로부터 에틸렌, 에탄올과 같은 가치 있는 화합물을 생산하거나, 물로부터 수소를 생산하는 방식은 탄소배출 없는 에너지 순환을 구현할 수 있을 뿐 아니라 시공간적 불균형이 심한 신재생에너지를 화합물의 상태로 오랫동안 저장할 수 있다는 점에서 큰 주목을 받고 있다. 이 반응의 속도 및 효율을 향상시키고, 산업적으로 구현하기 위해서는 고성능의 전기화학촉매 개발이 필수적이다. 따라서 제일원리와 기계 학습을 통한 이론적 물질 설계부터 나노 소재의 합성 및 반응 원리 규명으로 이어지는 이론과 실험이 결합된 연구 방법론이 촉매분야에서 활발하게 이루어지고 있다. 게다가 전기전도도, 다공성과 같은 기능성을 추가적으로 부여하기 위해 그래핀, 탄소섬유, 산화물 등과 같은 재료들이 활물질과 복합체를 구성하기도 하며, 이처럼 다중 원소로 구성된 전기화학촉매 재료에서 상과 구조를 최적화하는 것은 그 성능을 좌우한다. 하지만 현재까지 다중 원소로 이루어진 나노 소재의 상과 구조를 완벽하게 제어하지 못하고 있는 실정이며, 제한적으로 가능하다고 하더라도 그 공정 변수는 수많은 시행착오를 통해 찾아내고 있다. 이는 재료의 범용성이 떨어뜨릴 뿐 아니라 대량생산에도 부적합하여 산업적 활용을 가로막고 있다. 현재 이론적 접근을 통해 촉매 재료를 설계하는 연구 방법론의 발전 수준에 비해 그 재료를 실제로 구현할 수 있는 공정 설계 방법론은 거의 연구가 진행되지 않았다. 따라서 구조와 상이 최적화된 촉매 재료를 이론적 기반에 의해 예측된 공정에 의해 대량으로 제작하고, 나노스케일에서 그 상과 구조를 정밀하게 제어하는 이러한 연구 방법론은 전기화학 촉매의 실제 적용에 있어 매우 핵심적이다.
본 연구에서는 야금학 기반의 제철 공정 설계기술에서 영감을 받아 나노 소재의 합성 공정을 열역학 및 동역학과 같은 이론적 접근을 통해 설계했다. 이론적 계산을 통해 설정된 공정 변수는 합성 공정에서의 시행착오를 최소화할 뿐 아니라, 특정 반응을 유도 및 억제할 수 있는 조건을 찾아낼 수 있었다. 이를 통해 다중 원소로 이루어진 촉매재료에서 활성도 및 안정성과 같은 성능을 극대화 하기 위해 구조와 상을 성공적으로 제어할 수 있음을 보였다. 다양한 전기화학 반응 들 중 활용도가 높은 이산화탄소 환원 반응 및 물 분해를 통한 수소 발생 반응 촉매 개발에 이러한 연구방법론을 성공적으로 적용했으며 그 예시는 구체적으로 다음과 같다.
먼저, 탄소-일산화탄소-이산화탄소 간 평형 반응인 ‘부두아 반응’을 구리 나노입자 제작 공정에 도입하여 구리나노입자 표면에 자발적으로 탄소 껍질을 형성시킴으로서 이산화탄소 환원 촉매의 성능 저하 원인이었던 표면 재구성을 충분히 억제했다. 열처리 공정 동안 산소 분압과 그에 따른 일산화탄소의 발생량을 제어하여 부두아 반응 정도를 제어할 수 있었으며 이는 열역학 계산을 통해 공정 변수를 설정할 수 있었다. 탄소 껍질이 형성된 구리 나노입자는 대조군의 구리 입자에 비해 표면 전자 구조, 미세구조 등 다양한 측면에서 안정성이 향상되었다. 게다가, 이렇게 형성된 다공성의 탄소 껍질은 내부 구리 입자의 표면 도핑을 가능하게 해 구리 촉매의 이산화탄소 환원 선택도를 더욱 향상 시킬 수 있었다. 탄소 껍질 코팅 된 구리 나노 입자를 통해 전환 효율 및 안정성이 뛰어난 이산화탄소 환원 촉매전극을 구현했다.
그리고 희생금속-구리-조촉매로 이루어진 합금 원소를 제작하고, 희생원소를 제거하는 방식을 통해 미세구조, 조성, 및 원자구조가 제어된 다공성 촉매를 제작했다. 다공성 촉매를 제작하기 위해서는 공융 조성을 설정하는 것이 필수적이었으며, 냉각속도를 조절함으로서 미세구조를 제어할 수 있음을 보였다. 금속 용융액을 냉각하는 과정에서 그 속도를 충분히 빠르게 하면 은과 구리가 열역학적인 혼화성을 넘어 과냉각 되는 현상을 확인했다. 이를 통해 원자 분포가 적절히 제어된 구리-은 합금 촉매를 제작할 수 있었으며 그 비율을 제어함으로서 이산화탄소를 에틸렌과 에탄올 두가지 산물에 대해 선택적으로 합성할 수 있는 촉매를 개발했다.
수소발생 전극용 촉매재료로서는 활성도가 높다고 알려진 전이금속 칼코젠화합물과 인화합물에 우리의 연구 방법론을 적용해보았다. 수소발생전극용 니켈 인화합물/탄소나노섬유 촉매에서는 열역학적 고려를 통해 합성과정을 2단계로 나누었고, 1단계에서는 산소분압을 제어함으로서 활물질-담체 간 구조를 입자크기, 분포, 다공성 등의 측면에서 최적화했다. 그리고 2단계 열처리과정에서 인 기체의 분압을 조절함으로서 니켈 인화합물의 상을 제어했다. 이때 목표 상은 제일 원리 계산을 통해 반응 중간산물과의 표면 결합력을 최적으로 갖도록 설계되었으며, 이 상을 합성하기 위한 공정 변수는 니켈 인화합물의 반응 자유에너지를 계산함으로서 찾아내었다. 이를 통해 활성도 및 안정성이 뛰어난 니켈인화합물/탄소나노섬유 촉매를 개발할 수 있었다.
그리고 반응성이 큰 일산화탄소와 칼코젠 원소 간의 기상-고상 반응을 통해 이차원 전이금속 칼코젠화합물 격자 내에 빈자리결함을 형성하고, 준안정상으로의 전이를 유도하는 공정을 최초로 개발했다. 제일 원리 계산을 통해 전이금속 칼코젠화합물 내에 빈자리 결함이 형성되면 금속성을 띄는 준안정상으로 전이를 유도할 수 있음을 알아냈고, 이를 실험적으로 구현하기 위해 일산화탄소/이산화탄소 혼합기체를 이용한 열처리 공정을 설계했다. 이때 열역학적 계산 프로그램을 통해 혼합기체 비율에 따른 기체 및 고체 안정상을 예측할 수 있었고, 특정 반응이 진행되기 위한 공정변수를 설정할 수 있었다. 준안정상으로의 전이가 일어남을 실험적으로 확인했고, 이 공정을 탄소나노섬유와의 복합재료에 적용하여 상과 구조가 제어된 촉매를 제작할 수 있었다. 구조가 제어된 준안정상의 전이금속 칼코젠화합물/탄소나노섬유는 수소발생전극촉매로서 우수한 활성을 보였다. 또한 일산화탄소가 매개된 상전이 공정을 다른 전이금속 칼코젠 화합물에 적용하여 마찬가지의 상전이를 실험적으로 구현했고, 전이금속에 따른 서로 다른 반응경향성을 깁스 반응에너지를 통해 예측할 수 있었다. 온도 및 일산화탄소 분압과 같은 공정 변수를 확장하여 재료의 상을 산화물, 탄화물, 황화물 등 넓은 범위에서 제어가능함을 보였다.
본 연구에서는 구리/탄소나노섬유, 니켈 인화합물/탄소나노섬유, 2차원 전이금속화합물/탄소나노섬유, 다공성 구리 합금 입자와 같은 다중 원소 촉매에서 열역학 및 동역학적으로 예측된 변수에 의해 구조와 상이 다양하게 제어될 수 있음을 보였다. 이러한 합성 방법론은 시행착오를 최소화하고, 전기화학 촉매가 최적의 성능을 구현할 수 있도록 맞춤형으로 공정 변수를 설정할 수 있게 한다. 게다가 적용된 촉매 제작 공정은 대량생산에 적합한 것으로 전기화학반응을 기반으로 한 탄소 중립적 에너지 순환을 구현하는데 핵심적 역할을 할 것이다.

목차 (Table of Contents)

• Abstract i
• Table of Contents iv
• List of Tables viii
• List of Figures ix
• Chapter 1. Introduction 1
• 1.1. Electrochemical energy conversion for sustainable future 1
• 1.2. Phase and structure control issue of multi-elements catalysts 5
• 1.3. Electrocatalytic CO2 reduction 8
• 1.3.1. Techno-economic analysis on CO2 reduction 8
• 1.3.2. Current state and issue 12
• 1.4. Electrocatalytic water splitting 15
• 1.5. Objective of the thesis 17
• 1.6. Organization of the thesis 20
• Chapter 2. Theoretical Background 21
• 2.1. Thermodynamics for predictive synthesis 21
• 2.1.1. Phase diagram 22
• 2.1.2. Ellingham diagram 25
• 2.1.3. Boudouard reaction 28
• 2.2. Electrocatalytic CO2 reduction 30
• 2.2.1. Reduction mechanism on electrocatalysts 30
• 2.2.2. Surface reconstruction mechanism 35
• 2.2.3. Efficient production of liquid fuels 41
• 2.2.4. Electrolyzer type 51
• 2.3. Transition metal compounds for water splitting 55
• Chapter 3. Formation of Quasi-graphitic Carbon Shell on Cu Nanoparticles for Surface Stabilization during Electrocatalytic CO2 Reduction 58
• 3.1. Introduction 58
• 3.2. Experiments 60
• 3.2.1. Material preparation 60
• 3.2.2. Characterization 62
• 3.2.3. Electrocatalytic reduction of CO2 62
• 3.2.4. Thermodynamic and DFT calculations 65
• 3.3. Boudouard reaction design for C shell formation 72
• 3.3.1. Thermodynamic consideration 72
• 3.3.2. Study on quasi-graphitic C shell 79
• 3.4. Incorporation of p-block elements in Cu surface 92
• 3.5. CO2RR activity and stability investigation 103
• 3.6. DFT theoretical calculations for mechanism study 107
• 3.7. Summary 117
• Chapter 4. Metallurgical Ternary Alloy Fabrication for Efficient CO2-to-Ethanol Electroconversion 118
• 4.1. Introduction 118
• 4.2. Metallurgical considerations on alloy fabrication 122
• 4.3. Binary alloy design for CO2-to-C2+ products conversion 129
• 4.4. Addition of third metal to the alloy for ethanol production 139
• 4.5. CO2 reduction performance evaluation 152
• 4.6. Summary 156
• Chapter 5. Phase and Structure Engineering of Ni Phosphide/Carbon Nanofibers for Hydrogen Evolution Reaction Electrocatalysts 157
• 5.1. Introduction 157
• 5.2. Experiments 160
• 5.2.1. DFT calculations 160
• 5.2.2. Thermodynamic calculations 160
• 5.2.3. Catalysts fabrication 161
• 5.2.4. Electrochemical catalytic performance evaluation 162
• 5.3. DFT calculations for setting the target phase 166
• 5.4. Thermodynamic fabrication process design 170
• 5.5. Fabrication of Ni phosphide/carbon nanofibers 174
• 5.5.1. Structure modification of Ni/carbon nanofibers 174
• 5.5.2. Phase control of Ni phosphide 178
• 5.6. Electrocatalytic performance 184
• 5.6.1. Evaluation of electrochemical performance 184
• 5.6.2. Mechanism study on HER of Ni phosphide 187
• 5.7. Summary 189
• Chapter 6. S Vacancy-induced Polymorph Control of Molybdenum Disulfide for Hydrogen Evolution Reaction Electrocatalysts 190
• 6.1. Introduction 190
• 6.2. Experiments 193
• 6.2.1. DFT calculations 193
• 6.2.2. Thermodynamic calculations 193
• 6.2.3. Materials fabrication and calcination 194
• 6.2.4. Material characterization 194
• 6.2.5. Electrocatalytic performance measurement 195
• 6.3. Study on effect of S vacancy on polymorph transition 196
• 6.4. Thermodynamic fabrication process design 202
• 6.5. Polymorph and structure control of MoS2 207
• 6.6. Electrocatalytic performance of 1T-MoS2 219
• 6.7. Summary 222
• Chapter 7. Polymorph and Phase Engineering of Transition Metal Dichalcogenides via CO-mediated Reactions 223
• 7.1. Introduction 223
• 7.2. Experiments 226
• 7.2.1. Thermodynamic calculations 226
• 7.2.2. Materials synthesis and characterization 226
• 7.2.3. Theoretical mechanism study 227
• 7.3. Phase engineering of TMDs via gas-solid reaction 228
• 7.3.1. Thermodynamic prediction and products analysis 228
• 7.3.2. Investigation on polymorph conversion 238
• 7.4. Mechanism study on polymorph conversion of TMDs 242
• 7.5. Summary 250
• Chapter 8. Conclusion 251
• 8.1. Summary of results 251
• 8.2. Future works and suggested research 253
• References 255
• Abstract (In Korean) 271
• Curriculum Vitae 275

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