Development of electrospun 1D nanofibers for advancing kinetic-to-electrical energy converting self-powered electronics

국문 초록 (Abstract)

전기방사 기술은 전기수력학(EHD; electrohydrodynamics) 기반의 플랫폼 기술로서, 충진 밀도(packing density)를 조절할 수 있는 1차원(1D; one-dimensional) 나노섬유를 제조하며, 광범위한 비표면적과 기계적으로 순응(complient)적인 섬유 간 네트워크를 구현한다. 높은 전기장 내 전기수력학적 조건에서, 맥스웰 응력(electric Maxwell stress)이 점탄성 및 유변학적 반응과 상호경쟁하며 고분자 액적은 테일러 콘(Taylor cone)을 형성한다. 연속적인 제트 방출은 고분자 용액 내 전하 이완(charge relaxation)과 유체역학적 이완(hydrodynamic relaxation) 거동에 의해 지배되며, 인가 전기장이 특성 임계값을 초과하면, 대전된 제트가 방출되어 휘핑(굽힘) 불안정성을 거치며 점차 가늘어지고(즉, 전단 박화), 이후 용매 증발을 통해 고화된다. 이러한 강한 신장 응력(elongational stretching)과 공정 중 전기장 노출은 형성된 섬유 내부에서 사슬 정렬과 비등방성 극성 배열을 추가적으로 촉진하며, 결과적으로 압전 및 마찰전기 전하 이송과 관련된 전기적 물성 향상시킬 수 있다.
본 연구에서는 이러한 나노섬유 플랫폼을 기반으로 세 가지 자가발전(self-powered) 기술을 개발하였다. (1) 전기방사 섬유 구조체에 금속 전기도금을 적용하여 유연한 투명 전도 전극(TCEs; transparent conductive electrodes)을 구현하였으며, 자가 접합(self-junction)된 교차점을 갖는 퍼콜레이션 네트워크를 형성함으로써 굽힘 변형 하에서도 전기적 경로를 안정화하였다. (2) 자가 발전형 정전 필터개발을 위해 마찰음전기(tribonegative) polyvinylidene fluoride (PVDF)/BaTiO3와 kraft lignin 및 tannic acid를 포함하는 마찰양전기(tribopositive) polycaprolactone (PCL) 복합소재를 결합한 마찰전기 멤브레인 페어를 설계하였고, 마찰전기 향상에 기반한 미세먼지(PM; particulate matter) 제거와 항균 기능을 동시에 구현하였다. (3) 유기 할라이드 계면 결정(organohalide interfacial crystal)을 도입하여 분자 간 상호작용을 강화함으로써 극성 결정화를 향상시킨 PVDF 나노섬유 기반 압전 나노발전기(PENG)를 구현하였으며, 저응력 및 초음파 구동 자극 내 신체 이식형 의료기기(IMDs; implantable medical devices) 통합형 에너지 하베스팅 가능성을 제시하였다.
종합적으로, 본 결과는 전기방사 나노섬유가 자가발전형 전기기기 개발분야 내 기능성 전극, 마찰전기 계면, 압전층을 공동 설계할 수 있는 통합적이고 범용적인 플랫폼임을 시사한다.

다국어 초록 (Multilingual Abstract)

Electrospinning is an electrohydrodynamics (EHD)-based platform technology that produces one-dimensional nanofibers with tunable packing density, extensive surface-to-volume ratio, and mechanically compliant fiber-to-fiber networks. In the electrohydrodynamic conditions under a high electric field, the polymer droplet forms a Taylor cone as Maxwell stress competes with viscoelastic and rheological response. Sustained jetting is governed by charge and hydrodynamic relaxation in the polymer solution. When the applied field exceeds a characteristic threshold, an electrified jet is ejected, undergoes whipping (bending) instability, and progressively thins (i.e., shear thinning) before solidifying through solvent evaporation. The intense elongational stress and in situ electric-field exposure further promote chain alignment and anisotropic polar ordering within the resulting fibers, thereby enhancing electrical properties regarding piezoelectric and triboelectric-charge transport.
Using this nanofiber platform, three self-powered technologies are developed in this study. (1) Flexible transparent conductive electrodes (TCEs) are realized by metal electroplating on electrospun scaffolds, forming percolative network with self-junctioned intersections that stabilize electrical pathways under bending deformation. (2) A triboelectric membrane pair is engineered for developing self-powered electrostatic filter by combining tribonegative polyvinylidene fluoride (PVDF)/BaTiO3 and tribopositive polycaprolactone (PCL) composites incorporating kraft lignin and tannic acid, enabling triboelectricity-enhanced particulate matter (PM) removal together with antibacterial activity. (3) A polar-crystallinity-enhanced PVDF nanofiber piezoelectric generator (PENG) is implemented using an organohalide interfacial crystal to reinforce intermolecular interactions and enable IMD-integrated energy-harvesting under low-stress and ultrasound-driven stimulations.
Overall, these results establish electrospun nanofibers as a unified, versatile platform for co-designing functional electrodes, triboelectric interfaces, and piezoelectric layers in the development of self-powered electronic applications.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Self-powered electronic applications 1
• 1.2. Fiber-based multifunctional self-powered electronics 4
• 1.3. Electrohydrodynamics-based 1D nanofibers 6
• Chapter 2. Experimental sections 13
• 2.1. Electroplated metal fiber electrodes 13
• A. Materials 13
• B. Fabrication of Ni microfibers 13
• C. Characterizations 14
• 2.2. Triboelectricity-enhanced PM filter membranes 16
• A. Materials 16
• B. Solution preparation for triboelectric membranes 16
• C. Electrospinning PVDF- and PCL-based nanofibers 18
• D. Antibacterial study 19
• E. Filtration test 20
• F. Characterizations 20
• 2.3. Performance-enhanced PENGs 22
• A. Materials 22
• B. Solution preparation 22
• C. Fabrications of PVDF/DABCO-NH4I3 nanofibers and PENFs 23
• D. DFT calculations 24
• E. Characterizations 25
• Chapter 3. Results and discussion 27
• 3.1. Electroplated metal fiber electrodes 27
• A. Limitations of conventional conductive materials for flexible TCEs 27
• B. Self-junctioned Ni-electroplated microfibers 29
• C. Optoelectronic properties based on percolation conductive network 34
• D. Flexible Ni microfiber-based TCEs for self-powered electronics 40
• 3.2. Triboelectricity-enhanced PM filter membranes 46
• A. Triboelectricity-driven self-powered electrostatic filter 46
• B. Electrospun triboelectric membrane pair 49
• C. Physicochemical analyses of PVDF- and PCL-based membranes 54
• D. KPFM-based surface potential analysis 59
• E. Triboelectric output performance during contact electrification 62
• F. Antibacterial activity of tribopositive membrane 65
• G. PM removal efficiency 67
• 3.3. Performance-enhanced PENGs 74
• A. Polymorphic phase transformation of semicrystalline PVDF 74
• B. Fabrication of PVDF/DABCO-NH4I3 nanofibers 77
• C. Intermolecular interactions between PVDF and DABCO-NH4I3 81
• D. Crystallographic analyses on polar β-phase crystallinity 84
• E. GIWAXS-based crystallographic characterizations 93
• F. Piezoelectric response of single nanofiber 101
• G. Energy-harvesting performance under compressive conditions 104
• H. Energy-harvesting performance for PENG-integrated IMD applications 108
• Chapter 4. Conclusion 114
• References 117
• Appendix Section 140
• Abstract in Korean 150