Enhancing Oxygen Evolution Reaction through Control of Catalytic Support and Ionomer Interfaces for AEMWEs = 촉매 지지체 및 이오노머 계면 제어를 통한 음이온 교환막 수전해 산소 발생 반응 향상

국문 초록 (Abstract)

기후 변화로 인한 우려가 증가함에 따라 지속 가능한 수소 생산 기술에 대한 세계적 관심이 크게 증가하고 있다. 특히 음이온 교환막 기반 수전해 장치는 비귀금속 촉매의 사용이 가능하고 알칼리 조건 하에서 운전할 수 있는 장점으로 인해 많은 각광을 받고 있다. 그러나 촉매의 불안정성, 촉매-이오노머 인터페이스의 붕괴, 그리고 해수 환경 구동과 같은 엄격한 조건 하에서 실용성 한꼐 등 여러가지 중요한 도전 과제가 남아있다.
본 논문에서는 전해도 환경 (pH, 전해질 구성, 구조적 안정성 등)을 정밀하게 이해하고 이를 기반으로 한 세부적 전략을 제시하여 음이온 교환막 수전해 시스템에 적용이 가능한 촉매 지지체와 전극 인터페이스 환경을 제어함으로 성능과 내구성을 근본적으로 개선하였다.
2장에서는 촉매 지지체를 통한 산소 발생 반응의 개선을 위해 고결정성의 그래핀화 카본 쉘 및 이차원 물질인 맥신을 활용하였다. 첫째로, 고결정성의 그래핀화 카본 캡슐화 니켈-철 촉매를 통해 양극에서의 전기화학적 부식을 완화시키는 솔루션을 제시하였다. 특히 계산 과학을 통해 정밀한 탄소 캡슐의 산화 안정성에 대해 정리하였으며, 1,100 시간 이상의 안정적인 구동을 보여주었다. 둘째로, 맥신 기반 지지체에 대한 선도적 연구는 Ti 기반 맥신을 선호하는 기존 연구와는 달리 실험적 및 계산적 분석을 통해 몰리브데늄 기반의 맥신 (Mo2CTx)이 산소 발생 반응의 촉매 지지체로 우수함을 최초로 입증하였다. 이 기초적 통찰을 바탕으로 NiFe 층상 이중 수산화물이 성공적으로 합성되어 촉매 성능을 현저히 향상시켰다. 합성 과정 중 몰리브데늄 산화물이 층상 구조에 도핑되어 직접 해수 수전해에 2,000시간 이상 안정적 구동에 핵심 역할을 한다는 사실을 입증하였다.
3장에서는 촉매 표면에 4차 암모늄 기능기를 도입함으로써 촉매층 내에 반응물인 수산화 이온(OH-)의 이동 및 운반을 가속화하는 촉매에 대해 컨셉을 제시하였으며, 실험을 통해 증명하였다. 체계적인 전기화학 및 제타 전위 분석을 통해 최적화된 4차 암모늄 기능기가 촉매의 국부 표면 pH를 현저히 향상시켰고, 이에 대해 산소 발생 반응이 현저히 개선됨을 다양한 실험 방법을 통해 증명하였다.
음이온 교환막 수전해 시스템은 구동 pH에 따라 각기 다른 문제점을 안고 있으며, 이에 대한 구체적 접근 방법을 설계하고 대응하는 전략이 필수적이다. 기존의 고정 관념을 탈피하고 새로운 시각으로 접근하여 시스템에 대한 총괄적 이해를 통해 발전해야만 산업 적용 가능성이 증가할 것이다. 위 학위 논문을 통해 제안된 방법론들은 산소 발생 반응에 적용 가능한 촉매 지지체 설계와 촉매-이오노머 계면 제어를 통해 음이온 교환막 수전해 시스템에 대한 새로운 이해와 기초를 재정립한다.

다국어 초록 (Multilingual Abstract)

Over recent decades, global interest in sustainable hydrogen production technologies has significantly grown due to climate change concerns, especially related to the decarbonization of industrial sectors. Among various hydrogen production methods, Anion exchange membrane water electrolyzers (AEMWEs) have emerged as promising candidates due to their ability to utilize non-platinum group metal catalysts and operate under alkaline conditions. However, critical challenges remain, including catalyst instability, degradation of electrode interfaces, and limited practical applicability under harsh conditions such as seawater electrolysis.
In this dissertation, I present systematic engineering strategies that integrate precise understanding of the electrolyzer operating environment—such as local pH, electrolyte composition, and structural stability—to fundamentally improve both the performance and durability of catalyst supports and electrode interfaces in AEMWEs.
First, highly crystalline graphitic carbon-encapsulated NiFe catalysts were introduced as a robust solution to mitigate the electrochemical corrosion typically experienced at the anode. Comprehensive electrochemical studies, combined with density functional theory (DFT) analyses, revealed exceptional stability attributed to precise carbon encapsulation, demonstrated by stable operation over 1,100 hours under highly alkaline conditions.
Next, a pioneering investigation into MXene-based supports demonstrated that electrical conductivity alone does not predict catalyst suitability. Contrary to prevalent assumptions favoring Ti-based MXenes, experimental and computational analyses established Mo2CTx MXene as a superior OER support material, despite its comparatively lower electrical conductivity. Based on this foundational insight, a NiFe layered double hydroxide (LDH) was successfully integrated onto Mo2CTx MXene, significantly enhancing catalytic performance. An unexpected yet beneficial oxidation of Mo sites within MXene introduced MoOx- species that doped the catalyst surface, providing a protective barrier against chloride ion intrusion. Consequently, this catalyst demonstrated remarkable stability and performance in direct seawater electrolysis.
Additionally, the introduction of quaternary ammonium (QA) functional groups onto catalyst surfaces strategically improved the local OH⁻ ion transport at the catalyst-ionomer interfaces, particularly under conditions prone to local ion starvation. Through systematic electrochemical and zeta-potential analyses, optimized QA functionalization significantly enhanced local pH and enhanced OER activity and stability.
Beyond catalyst-specific improvements, this dissertation also explored various degradation pathways at the membrane electrode assembly (MEA) level, including catalytic support corrosion, catalytic active metals’ oxidation state shift, and crystal structure change. Diagnostic methodologies such as electrochemical overpotential breakdown analysis and ex-situ characterization techniques were systematically utilized to identify and address these degradation issues, enabling targeted interventions and informed material improvements. Additionally, in-situ X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) analyses were specifically conducted to elucidate structural and chemical transformations of MXene-supported catalysts under realistic OER conditions, providing crucial insights for enhancing their stability and activity.
Overall, the novel methodologies presented here—spanning innovative catalyst support designs, MXene-based material developments, and sophisticated catalyst-ionomer interface engineering—provide robust foundations for advancing AEMWEs towards commercial-scale sustainable hydrogen production.

목차 (Table of Contents)

• Contents
• Abstract i
• Contents v
• List of Tables and Schemes x
• List of Figures xi
• Chapter 1. Introduction 1
• 1.1. Electrochemical water splitting 1
• 1.1.1. Oxygen evolution reaction (OER) 4
• 1.1.2. Anion exchange membrane water electrolyzer (AEMWE) 7
• 1.2. Membrane electrode assembly (MEA) 9
• 1.2.1. Components of catalyst layer: Support 10
• 1.2.2. Components of catalyst layer: Ionomer 11
• 1.2.3. Catalyst-ionomer interfaces 14
• 1.3. Research objectives 14
• 1.4. Reference 16
• 2.
• Chapter 2. Catalytic support 17
• 2.1. Engineering durable anion exchange membrane water electrolyzers through suppressed electrochemical corrosion of a NiFe-graphitic carbon shell anode catalyst 17
• 2.1.1. Introduction 17
• 2.1.2. Experimental 24
• 2.1.2.1. Materials 24
• 2.1.2.2. Synthesis of NiFe/C, bare NiFe, and NiFe+XC-72 24
• 2.1.2.3. Synthesis of TMA-70 25
• 2.1.2.4. Characterizations of materials 26
• 2.1.2.5. Electrode preparation and measurement for electrocatalytic testing 27
• 2.1.2.6. Electrode preparation and measurement for MEA testing 28
• 2.1.2.7. DFT calculations 30
• 2.1.3. Results and discussion 33
• 2.1.3.1. Research conceptualization 33
• 2.1.3.2. Synthesis and characterization of highly crystalline carbon encapsulated NiFe (NiFe/C) catalysts 36
• 2.1.3.3. OER performance of NiFe/C catalysts via RDE measurements 42
• 2.1.3.4. AEMWE performance of NiFe/C catalysts and theoretical validation of catalytic activity 48
• 2.1.3.5. Assessing the long-term durability of NiFe/C catalysts via AEMWE operation and theoretical investigation of carbon corrosion with various topological defects 55
• 2.1.3.6. Post characterization of NiFe/C catalyst 60
• 2.1.3.7. The impact of topological defects on electrochemical carbon corrosion and description of potential corrosion pathways 64
• 2.1.4. Conclusion 66
• 2.1.5. Reference 68
• 2.2. Unveiling the role of catalytically active MXene supports in enhancing the performance and durability of cobalt oxygen evolution reaction catalysts for anion exchange membrane water electrolyzers 76
• 2.2.1. Introduction 76
• 2.2.2. Experimental 80
• 2.2.2.1. Materials 80
• 2.2.2.2. Synthesis of MXenes 80
• 2.2.2.3. Synthesis of Co@MXenes 81
• 2.2.2.4. Synthesis of TMA-70 82
• 2.2.2.5. Characterizations of materials 82
• 2.2.2.6. RDE experiments 84
• 2.2.2.7. Electrochemical testing 85
• 2.2.2.8. Calculations of the H2 Faradaic efficiency (FE), H2 production rate, and energy conversion efficiency (ECE) 86
• 2.2.3. Results and discussion 89
• 2.2.3.1. Design rationale for Co@MXenes catalyst 89
• 2.2.3.2. Characterization of pristine MXenes and Co@MXenes 92
• 2.2.3.3. Electrochemical properties 99
• 2.2.3.4. Electrochemical stability analysis and in-situ XAS 103
• 2.2.4. Conclusion 108
• 2.2.5. Reference 109
• 2.3. Synergistic effect of NiFe layered double hydroxide structure supported by MXene for direct seawater electrolysis oxygen evolution reaction 113
• 2.3.1. Introduction 113
• 2.3.2. Experimental 117
• 2.3.2.1. Materials 117
• 2.3.2.2. Synthesis of MXene 117
• 2.3.2.3. Synthesis of NiFe@MXene 118
• 2.3.2.4. Synthesis of TMA-70 119
• 2.3.2.5. Characterization of materials 120
• 2.3.2.6. Three electrode configurations and preparations 121
• 2.3.2.7. Electrochemical testing 122
• 2.3.3. Results and Discussion 125
• 2.3.3.1. Characterizations of NiFeMo@MXene 125
• 2.3.3.2. Electronic and local crystal structure 129
• 2.3.3.3. Electrochemical analysis using a three-electrode system 132
• 2.3.3.4. Electrochemical analysis using an MEA system 135
• 2.3.3.5. In-situ XAS characterization 139
• 2.3.3.6. DFT calculations 143
• 2.3.4. Conclusion 145
• 2.3.5. Reference 146
• 3.
• Chapter 3. Engineering catalyst-ionomer interfaces 148
• 3.1. Maximizing local pH through ammonium surface functionalization of oxygen evolution reaction catalysts 148
• 3.1.1. Introduction 148
• 3.1.2. Experimental 153
• 3.1.2.1. Materials 153
• 3.1.2.2. Synthesis of 1,1-dimethylpiperidinium iodide 153
• 3.1.2.3. Synthesis of QA-functionalized catalyst 154
• 3.1.2.4. Synthesis of TMA-70 155
• 3.1.2.5. Materials characterizations 155
• 3.1.3. Results and Discussion 156
• 3.1.3.1. Design rationale for QA-functionalized catalyst 156
• 3.1.3.2. Characterizations of catalyst as is and QA-functionalized catalyst 160
• 3.1.3.3. Zeta potential as a function of QA coverage 163
• 3.1.3.4. Electrochemical analysis using a three-electrode system 166
• 3.1.3.5. Electrochemical analysis using an MEA system 169
• 3.1.4. Conclusion 172
• 3.1.5. Reference 173
• 4.
• Chapter 4. Summary 176
• 4.1. Crystalline carbon-encapsulated NiFe catalysts (Chapter 2.1) 176
• 4.2. MXene-supported transition metal OER catalysts (Chpater 2.2-2.3) 177
• 4.3. QA surface-functionalization (Chapter 3.1) 179
• 4.4. Reference 181
• 국문 요약 182