Design and Development of High Entropy Oxysulfide Electrocatalysts for Efficient Anion Exchange Membrane Water Electrolysis : Design and Device-Level Evaluation of Transition Metal Oxide-Derived Phosphate Electrocatalysts for Anion Exchange Membrane Water Electrolysis = 고효율 음이온 교환막 수전해를 위한 고엔트로피 옥시설파이드 전기촉매의 설계 및 개발

국문 초록 (Abstract)

본 학위논문은 음이온 교환막 수전해(AEMWE) 시스템을 위한 지구 풍부 원소 기반 다중 전이금속 옥시설파이드 전기촉매의 합리적 설계, 대량 합성, 및 실질적 적용 가능성에 대해 심층적으로 연구하였다. 화석연료 고갈과 기후 변화 문제가 심화됨에 따라, 지속가능하고 저탄소 기반의 에너지 전환 기술 개발이 전 세계적으로 중요한 과제로 대두되고 있다. 수소는 높은 질량당 에너지 밀도와 친환경적 활용 특성으로 인해 차세대 에너지 캐리어로 주목받고 있으나, 기존의 화석연료 기반 수소 생산 방식은 이산화탄소 배출과 에너지 효율 저하 문제를 동반한다. 이러한 한계를 극복하기 위한 대안으로 전기화학적 수전해 기술이 각광받고 있으나, 실제 상용화를 위해서는 높은 과전압, 느린 반응 속도, 그리고 고가의 귀금속 촉매 의존성이라는 기술적 장벽을 해결해야 한다. 특히 산소 발생 반응(OER)은 다전자 전달이 수반되는 복잡한 반응 메커니즘으로 인해 수전해 시스템의 주요 병목 반응으로 작용하며, 알칼라인 조건에서 효율적인 수소 발생 반응(HER)을 구현하는 것 또한 비귀금속 촉매에서는 여전히 도전 과제로 남아 있다. 제1장에서는 전기화학적 수전해의 열역학적 및 속도론적 기본 원리와 함께 HER 및 OER의 반응 메커니즘을 체계적으로 정리하였다. 또한 과전압, Tafel 기울기, 전기화학적 활성 표면적(ECSA), 전환 빈도(TOF), 파라데이 효율, 장기 안정성 등 촉매 성능 평가 지표를 정리하고, 기존 귀금속 및 비귀금속 기반 전기촉매의 한계를 분석하여 본 연구의 필요성과 방향성을 제시하였다. 제2장에서는 용액 연소 합성법(Solution Combustion Synthesis, SCS)과 화학적 욕조 증착법(Chemical Bath Deposition, CBD)을 결합한 합성 전략을 통해 Fe, Cu, Co, Zn, Ni 기반 다중 전이금속 옥시설파이드 촉매를 제조하였다. 이 공정은 다공성 다금속 산화물 전구체 형성과 선택적 황 도입을 가능하게 하여, 나노볼–나노시트 계층 구조와 풍부한 계면을 갖는 촉매를 구현하였다. 구조 및 표면 분석 결과, 합성된 촉매는 균일한 원소 분포와 조절 가능한 산화 상태를 가지며, 조성에 따른 전기촉매 특성 차이가 명확히 확인되었다. 전기화학적 성능 평가 결과, Cu-FCCZN 촉매는 알칼라인 조건에서 우수한 HER 활성을 나타내었으며, Fe-FCCZN 촉매는 뛰어난 OER 성능과 장기 안정성을 확보하였다. 이러한 성능 향상은 계층적 나노구조에 의한 활성점 증가, 전하 전달 저항 감소, 그리고 다중 전이금속 간 전자 구조 상호작용에 기인한 것으로 분석되었다. 제3장에서는 최적화된 촉매를 적용하여 활성 면적 16 cm2의 AEM 수전해 셀을 제작하고, 고전류 밀도(200 mA cm-2) 조건에서 250시간 이상의 안정적인 구동을 실증하였다. 본 연구는 귀금속을 사용하지 않는 고효율 수전해 촉매의 설계 전략을 제시함과 동시에, 실험실 수준을 넘어 산업적 수소 생산 시스템으로 확장 가능한 기술적 기반을 제공한다는 점에서 학문적·실용적 의의를 지닌다.

다국어 초록 (Multilingual Abstract)

This dissertation investigates the rational design, scalable synthesis, and practical implementation of earth-abundant multi-transition-metal oxysulfide electrocatalysts for high-performance anion exchange membrane water electrolysis (AEMWE). With the accelerating depletion of fossil fuels and intensifying concerns over climate change, the development of sustainable and low-carbon energy conversion technologies has become a global priority. Among various alternatives, hydrogen is widely regarded as a next-generation energy carrier due to its high gravimetric energy density and environmentally benign combustion. Nevertheless, conventional hydrogen production routes, particularly fossil-fuel-based steam reforming, are intrinsically associated with substantial carbon dioxide emissions and energy inefficiencies, limiting their long-term sustainability. Electrochemical water splitting has emerged as a promising strategy for green hydrogen production when coupled with renewable electricity sources. Despite its conceptual simplicity, large-scale commercialization of water electrolysis remains hindered by sluggish reaction kinetics, high overpotentials, and the heavy reliance on scarce and expensive noble-metal-based electrocatalysts such as Pt, Ir, and Ru. In particular, the oxygen evolution reaction (OER), involving a complex four-electron transfer process, constitutes the major kinetic bottleneck, while achieving efficient hydrogen evolution reaction (HER) performance under alkaline conditions remains challenging for non-noble metal systems. Therefore, the development of cost-effective, durable, and highly active electrocatalysts based on earth-abundant elements is essential for advancing next-generation water electrolysis technologies. Chapter 1 provides a comprehensive overview of the fundamental principles governing electrochemical water splitting, including thermodynamic requirements, reaction kinetics, and electrode-electrolyte interfacial phenomena. The mechanistic pathways of HER and OER are systematically discussed, highlighting key descriptors such as hydrogen adsorption free energy, oxygen intermediate stabilization, and electronic structure modulation. Additionally, critical performance evaluation metric–overpotential, Tafel slope, electrochemically active surface area, turnover frequency, Faradaic efficiency, and long-term durability–are introduced as standardized benchmarks for catalyst assessment. This chapter also reviews recent advances in transition-metal-based electrocatalysts and identifies unresolved challenges related to activity enhancement, stability degradation, and scalability, thereby defining the scientific motivation and scope of this thesis. In Chapter 2, a scalable and versatile synthesis strategy combining solution combustion synthesis (SCS) and chemical bath deposition (CBD) is developed to fabricate multi-transition-metal oxysulfide electrocatalysts composed of Fe, Cu, Co, Zn, and Ni (FCCZN). The SCS process enables rapid formation of porous multi-metal oxide precursors with abundant defects and high surface area, while subsequent CBD introduces sulfur species in a controlled manner to generate compositionally tunable oxysulfide. This two-step approach results in hierarchical nanoball–nanosheet architectures with well-defined heterointerfaces, which effectively promote strong metal–metal interactions, optimize electronic structures, and facilitate charge and mass transport. Comprehensive structural and surface characterizations using X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and elemental mapping confirm the successful formation of multiphase oxysulfide structures with homogeneous elemental distribution and tunable oxidation states. Electrochemical investigations reveal pronounced composition-dependent catalytic behavior. Specifically, the Cu-enriched oxysulfide (Cu-FCCZN) exhibits exceptional HER activity in alkaline media, delivering an ultralow overpotential of 70 mV at 10 mA cm-2 and maintaining stable operation for over 100 h at 200 mA cm-2. In contrast, the Fe-enriched oxysulfide (Fe-FCCZN) demonstrates superior OER performance, requiring only 145 mV at 10 mA cm-2 with excellent durability. These enhancements are attributed to the synergistic effects of hierarchical morphology, increased electrochemically active surface area, reduced charge-transfer resistance, and favorable electronic modulation induced by multi-metal interactions. Chapter 3 focuses on translating laboratory-scale catalyst performance into practical device-level validation. A large-area anion exchange membrane water electrolyzer with an active area of 16 cm2 is fabricated using Cu-FCCZN as the cathodic HER catalyst and Fe-FCCZN as the anodic OER catalyst. The assembled AEMWE exhibits stable operation at a high current density of 200 mA cm-2 for 250 h in 1 M KOH, maintaining a low and stable cell voltage. Furthermore, the electrolyzer demonstrates robust performance across a wide range of electrolyte concentrations, underscoring its adaptability to industrially relevant operating conditions. Overall, this dissertation establishes a rational and scalable design framework for multi-metal oxysulfide electrocatalysts, elucidating critical structure-activity durability relationships and demonstrating their practical feasibility in advanced AEM water electrolysis systems. The findings not only contribute fundamental insights into multi-metal catalyst engineering but also provide a viable pathway toward cost-effective and sustainable hydrogen production, thereby advancing the development of next-generation green energy technologies.

목차 (Table of Contents)

• Ⅰ. CHAPTER1 12
• 1.1 Significance of electrochemical water splitting for energy conversion systems 12
• 1.2 Fundamentals of electrochemical water splitting 13
• 1.3 Principle of electrochemical water splitting 15
• 1.4 Parameters for evaluating HER and OER performance 21
• 1.5 Electrocatalysts for water splitting 26
• 1.6 Statement of the problem 27
• Ⅱ. CHAPTER2 34
• 2.1 Introduction 34
• 2.2 Experimental section 37
• 2.3 Result and discussion 43
• 2.4 Conclusion 88