Electrochemical Characterization of Oxygen-Ion-Reactive Thin Films Fabricated via Nano-Engineering = 나노 공학 공정을 통해 제작된 산소 이온 친화 박막의 전기화학 분석

국문 초록 (Abstract)

무분별한 화석 연료 사용에 의하여 기후 변화에 따른 환경 문제가 지속적으로 제기되고 있다. 이와 같은 문제를 해결하기 위한 방안으로 신재생 에너지원을 이용하는 움직임이 지속되는 가운데, 수소는 청정에너지원으로 연료전지를 통해 전기에너지로 변환이 가능하다. 연료전지를 상용화하기 위하여 성능 및 내구성을 향상시키는 노력이 지속되어왔다. 본 학위논문에서는 스퍼터링 기법을 이용한 나노 공학 공정을 바탕으로 고체산화물 연료전지의 성능 및 내구성 향상을 목표로 수행된 세가지 연구 방향을 소개하며, 산소 환원 반응 향상을 위한 공정 최적화 및 전기화학 결과에 관해 설명하고자 한다.

다국어 초록 (Multilingual Abstract)

The demand for renewable energy sources has grown considerably in response to global warming, which has led to increasingly severe issues such as climate change and natural disasters. Among various renewable energy carriers, hydrogen has been valued for its high energy density and ease of transport. Accordingly, recent years have witnessed growing research interest in both hydrogen production and utilization. In fuel cells, the energy of hydrogen is converted into electricity, and the fuel cells are classified according to their electrolyte material and the type of ions they transport. In particular, solid oxide fuel cells (SOFCs), which conduct oxygen ions through a ceramic electrolyte, operate at elevated temperatures (800–1000 °C) to maintain sufficient ionic conductivity. However, such high temperatures often introduce thermal degradation and impose strict constraints on material selection. To address these limitations, efforts have been made to lower the operating temperature to the intermediate range of 300–500 °C. While this shift helps alleviate thermal stresses, it also compromises SOFC performance owing to temperature-dependent characteristics. To counteract this drawback, considerable research has focused on tuning SOFC components through thin-film fabrication techniques. Accordingly, this study presents three nano-engineering strategies aimed at enhancing either the performance or thermal stability of SOFCs: the incorporation of cermet cathodes, surface overlayers, and electrolyte interlayers. The first strategy explored in this study involves the use of a cermet structure, a composite of ceramic and metal materials. This approach was designed to increase the density of active sites for the oxygen reduction reaction (ORR) through a co-sputtering process. In SOFCs, the triple-phase boundary (TPB) serves as the electrochemical reaction site for both the ORR and hydrogen oxidation reaction and is defined as the physical interface where the catalyst, electrolyte, and reactive gas converge. Increasing the TPB density accelerates electrochemical reactions, thereby improving overall cell performance. To achieve this, SOFC cathodes were fabricated by co-sputtering electrolyte and catalyst materials. Specifically, the cermet cathodes were produced by adjusting the deposition powers of the component materials, which revealed that the elemental ratios were critical in determining SOFC performance. Fuel cells incorporating cermet cathodes with unbalanced ratios exhibited reduced power densities, whereas those with an optimized composition delivered notably enhanced performance. The second strategy involves depositing an overlayer on the cathode to enhance the thermal stability of SOFCs. Cathodes are typically engineered with a nanoporous morphology to maximize surface area; however, sustained exposure to high operating temperatures often leads to thermal agglomeration. This process reduces the cathode surface area and promotes the growth of large grains and a denser structure, ultimately degrading cell performance. To mitigate this degradation, samaria-doped ceria (SDC), a material with high thermal resistance, was deposited onto an Ag cathode via a sputtering process. Cell performance and stability were evaluated before and after the deposition of the SDC overlayer. When the SDC layer interacted with the Ag cathode, the TPB density increased, leading to improved performance compared to the fuel cell incorporating the pristine Ag cathode. Moreover, the SDC overlayer helped maintain performance during extended operation. These results demonstrate that coating ultrathin SDC layers provides an effective means of preserving the nanoporous structure of Ag cathodes. The final strategy for enhancing SOFC performance involves inserting an interlayer between the cathode and electrolyte. Reduced oxygen ions are predominantly incorporated into the electrolyte through grain boundaries. Therefore, increasing the density of these boundaries can further improve oxygen ion incorporation kinetics. A nano-granular yttria- stabilized zirconia (YSZ) interlayer was fabricated using a metallic ZrY sputtering target, with argon and oxygen serving as the carrier and reactive gases, respectively. To promote the formation of a nanogranular morphology and increase grain boundary density, nitrogen gas was also introduced during the sputtering process. A potential concern in this case is the reaction of nitrogen with ZrY to form nitrides; however, oxygen exhibits a higher affinity for ZrY than nitrogen. X-ray photoelectron spectroscopy confirmed that nitrogen was absent from the fabricated interlayers, indicating that the resulting films were composed of oxide-phase YSZ. Fuel cells incorporating YSZ interlayers exhibited substantially improved performance, with power output increasing by up to 1.8-fold compared to the cell incorporating the pristine Pt cathode. This performance enhancement stemmed from reduced activation losses due to increased grain boundary density at the electrolyte–cathode interface. Each of the above strategies contributed to either enhanced SOFC performance or improved stability by increasing reaction site density, improving thermal resistance, or facilitating oxygen ion incorporation. These findings may extend beyond SOFCs to other technologies that benefit from nanoscale surface engineering.

목차 (Table of Contents)

• Chapter 1. INTRODUCTION
• 1-1. Thesis Overview 1
• 1-2. Outline 3
• 1-3. References 3
• Chapter 2. Overview of Solid Oxide Fuel Cells
• 2-1. Mechanism of Solid Oxide Fuel Cells 5
• 2-2. Performance and Losses of Fuel Cells 7
• 2-3. Thin Film Fabrication Process for Solid Oxide Fuel Cells 12
• 2-4. References 14
• Chapter 3. Fabricating Cermet Cathodes to Increase Reaction Site Density
• 3-1. Introduction to the Strategy 15
• 3-2. Fabrication and Analysis of Cermet Cathodes 16
• 3-3. Conclusions 25
• 3-4. References 26
• Chapter 4. Fabricating Overlayers on Cathode Surfaces to Enhance Thermal Stability
• 4-1. Introduction to the Strategy 27
• 4-2. Fabrication and Analysis of Thermally Resistive Thin Films 29
• 4-3. Conclusions 39
• 4-4. References 40
• Chapter 5. Deposition of an Interlayer on the Electrolyte Surface to Improve Surface
• Reaction Kinetics
• 5-1. Introduction to the Strategy 42
• 5-2. Fabrication and Analysis of Grain-Controlled Interlayers 43
• 5-3. Conclusion 54
• 5-4. References 55
• Chapter 6. Summary of Thesis 57
• Abstract in Korean 59