Synthesis of conducting polymer-intercalated vanadate nanofiber composites and their application to energy storage devices

국문 초록 (Abstract)

암모늄 바나듐산염 나노 섬유는 바나듐 산화물 수용액과 황산 암모늄을 혼합하여 간단한 환류 및 초음파화학적 방법을 통해 합성되었다. 암모늄 바나듐산염의 기반물질인 바나듐 산화물의 층상 구조는 다양한 이온 종 및 유기 종을 바나듐의 결정 구조 내부로 삽입을 가능하게 하기 때문에 에너지 저장 장치의 활성 물질로서 상당한 잠재적 응용 가능성을 가지고 있다. 그러나, 바나듐 산화물의 자체적으로 매우 낮은 전기 전도도는 바나듐 산화물을 전기 화학적 에너지 저장 장치에 응용하는데 있어 제한요소가 되어왔다. 지금까지 이러한 낮은 전기 전도도를 해결하기 위해 바나듐 산화물을 기반으로 하는 암모늄 바나듐산염 나노 구조체에 대한 연구가 진행되어 왔지만, 합성과정에서 계면활성제와 강한 산성 촉매를 첨가제로 사용 할뿐만 아니라 심지어24시간 이상의 고열/고압 공정을 필요로 하는 복잡한 합성 과정 때문에 경제적인 합성 방법에 대한 연구가 필요한 실정이다. 본 연구에서는 매우 간단하고 경제적인 환류법과 초음파화학법을 사용하여 바나듐산염 나노섬유의 결정 격자 내부로 전도성 고분자 폴리 에틸렌 다이옥시 싸이오펜을 층간삽입시켜 복합체를 형성하는 새로운 합성법을 제공한다. 환류법으로 합성된 암모늄 바나듐산염 나노섬유와 전도성 고분자와의 복합체는 염료감응형 태양전지의 상대전극 소재로 적용되었으며 초음파화학법으로 합성된 암모늄 바나듐산염 나노섬유와 전도성 고분자 복합체는 슈도커패시터의 전극 재료로 사용되어 실용화 가능성을 모색했다. 그 결과, 환류법으로 합성된 복합체는 바나듐산염의 결정 층간에 삽입된 전도성 고분자 폴리 에틸렌 다이옥시 싸이오펜에 의해 향상된 전기 전도도 (1.9 × 10-2 S/cm)를 나타냈다. 이 복합체의 광 전환 효율은 백금 상대전극 (6.5 %)과 유사한 6.0 % 보였다. 또한, 초음파화학법으로 합성된 전도성고분자 폴리 에틸렌 다이옥시 싸이오펜과 암모늄 바나듐 산염 나노섬유 복합체를 의사 축전기의 전극 재료로서 적용한 경우, 폴리 에틸렌 다이옥시 싸이오펜이 바나듐 산염의 결정격자사이에 환류법으로 합성된 복합체보다 더 많이 삽입되어 더욱 향상된 전기전도도와 그로 인해 더욱 넓어진 바나듐산염 나노 섬유의 결정 격자 면간 거리가 전해질의 칼륨 이온이 바나듐 산염 나노 파이버의 격자층 사이에서 빠르게 확산되는 것을 가능하게 하여, 의사 축전기의 전하 저장 비용량 및 율특성을 향상시켰다.

다국어 초록 (Multilingual Abstract)

Ammonium vanadate nanofiber (AVNF) based on vanadium oxides was synthesized through facile reflux and sonochemical methods by mixing of ammonium persulfate in solution of aqueous vanadium oxide. Layered structure of vanadium oxides have considerable potential application as an active material for energy storage devices, because they allow the intercalation of various ionic species and organic species to the crystal structure. However, the very low electrical conductivity of vanadium oxide has been a limiting factor regarding its application to electrochemical devices. In addition, when applying vanadium oxide as an electrode material for energy storage devices, the layered structure of vanadium oxide was collapsed by the repeated charge/discharge cycles, and thus the charge storage capacitance of the energy storage devices was decreased.
To address the low electrical conductivity and structural instability of vanadium oxide, studies have been conducted on vanadium oxide-based synthesis of AVNF and their composites. These vanadate nanofibers have been synthesized by various methods including the surfactant-assisted solution method, hydrothermal method, sol-gel method, and electrochemical method in terms of synthetic methodology. However, these synthetic methods require the addition of surfactants and catalysts as well as a high-temperature/high-pressure process that is longer than 24 hours. Moreover, there has been little research on vanadate-conducting polymer composites, and there has been no application of these composites to energy storage devices to the best of our knowledge. Therefore, research should be conducted on economical experimental methods for synthesizing AVNFs having a one-dimensional structure and their composites as well as their applications to energy storage devices. In this study, we present a novel synthetic method for forming PEDOT / AVNF composites by intercalating poly (3,4-ethylene dioxythiophene) (PEDOT) into a vanadate nanofiber interlayer using a very simple and economical reflux and sonochemical method. Also, the fiber formation mechanism of AVNF was studied by spectroscopic, microscopic, thermal gravimetrical, and crystallographic analysis techniques in detail. The reflux method-based ammonium vanadate nanofiber (R-AVNF) and its composite (R-EAVNF) were applied as materials for counter electrodes (CEs) of dye sensitized solar cells (DSSCs). In addition, the sonochemical method-based ammonium vanadate nanofiber (S-AVNF) and its composite (S-EAVNF) were applied as electrode materials for pseudocapacitors to explore the feasibility of practical application. As a results, the R-EAVNF had an improved electrical conductivity (1.9 × 10-2 S/cm) due to the intercalated PEDOT. The power conversion efficiency of the R-EAVNF composite material was 6.0%, which was similar to that of the Pt CE (6.5%). In addition, when the S-EAVNF was applied as an electrode material for pseudocapacitors, the crystal lattice distance of the vanadate nanofiber expanded by the PEDOT intercalation allowed the K+ ions of the electrolyte to rapidly diffuse between the lattice layers of the vanadate nanofiber, improving the charge storage capacitance and rate capability of the pseudocapacitor.

목차 (Table of Contents)

• CHAPTER 1. Introduction: Overview 1
• CHAPTER 2. Synthesis of ammonium vanadate nanofibers by using reflux in aqueous V2O5 solution with ammonium persulfate 26
• 2.1. Introduction 26
• 2.2. Experimental Procedure 27
• 2.2.1. Materials 27
• 2.2.2 Preparation of ammonium vanadate nanofiber 27
• 2.2.3 Characterization 28
• 2.3. Results and discussion 29
• 2.3.1. Synthetic transition of V2O5 29
• 2.3.2. Intercalation process of ammonium cations 35
• 2.3.3. Analysis of ammonium cation and water molecules in interlayers of vanadate nanofibers 36
• 2.3.4. Structural analysis of nanofiber with morphological transformation 44
• 2.3.5. Crystal structure analysis and chemical components of nanofiber 52
• 2.3.6. Electrical conductivity analysis 57
• 2.4. Summary 62
• CHAPTER 3. Synthesis of PEDOT/ammonium vanadate nanofiber composites for counter electrode of dye-sensitized solar cells 64
• 3.1. Introduction 64
• 3.2. Experimental Procedure 66
• 3.2.1. Materials 66
• 3.2.2. Preparation of composite samples 66
• 3.2.3. Fabrication of DSSCs 67
• 3.2.4. Characterization 72
• 3.3. Results and discussions 74
• 3.3.1. Crystal structure analysis and chemical components of composite nanofiber 74
• 3.3.2. The morphology analysis 85
• 3.3.3. The electrical conductivity analysis 89
• 3.3.4. The electrochemical analysis 91
• 3.3.5. Application of DSSCs 95
• 3.4. Summary 98
• CHAPTER 4. Synthesis of conducting polymer-intercalated vanadate nanofiber composites using a sonochemical method for high performance pseudocapacitor applications 100
• 4.1. Introduction 100
• 4.2. Experimental Procedure 105
• 4.2.1. Materials 105
• 4.2.2. Preparation of S-AVNF and S-EAVNF by the sonochemical method 105
• 4.2.3. Characterization 108
• 4.2.4. Preparation of electrodes 109
• 4.2.5. Preparation of asymmetric devices 110
• 4.2.6. Electrochemical measurements 111
• 4.2.7. Calculation of electrochemical performance 111
• 4.3. Results and Discussions 113
• 4.3.1. Comparison of sonochemical and reflux methods 113
• 4.3.2. Electrochemical performance 128
• 4.4. Summary 156
• CHAPTER 5. Conclusions 157
• References 162
• Summary in Korean 171
• 감사의 글 173