나노구조체 기반의 고성능 에너지 저장 및 변환에 관한 연구

Abstract

This thesis describes a development of multi-functional yarn based on nanofiber for energy application technology. The first research area in this thesis is the development of nanomembranes and yarn supercapacitor having a high energy storage and power density, characterization of its performance, and a discussion on the theoretical analysis and experiments to improve the energy charge․discharge performance. The second research area is the development of artificial muscles driven by electrochemical stimulation in the air, performance analysis, and a discussion of the potential applications. Finally, the third is to make a fabric using yarns having thermoelectric properties, analyze the performance, and a discussion of the theoretical approach for improving performance and application possibilities. Mechanically robust, transparent, and free-standing hybrid nanomembranes made of densified carbon nanotube sheets that were coated with poly(3,4-ethyleneioxythiophene) (PEDOT) using vapor phase polymerization and their performance as supercapacitor were reported. Average thickness of hybrid nanomembrane containing 85 wt.% PEDOT/1-layer MWNT sheet is ~66 nm and its mechanical strength and modulus of 127 MPa and 18 GPa, respectively, which values surpass other nanomembrane structures. Several limitations of bare MWNT sheet that is hydrophobicity, low capacitance, and collapse in solution were solved by coating with PEDOT. The transmittance of the 85 wt.% PEDOT/1-layer MWNT was 56% (at 550 nm wavelength) and 18% lower than for a 1-layer MWNT sheet (74%). Also, the sheet resistance is 312 Ω/sq, which is 3 times lower than that of a 1-layer MWNT sheet (958 Ω/sq). The 85 wt.% PEDOT/1-layer MWNT hybrid nanomembrane attached on a current collector had gravimetric capacitance of ~62 F/g and 85 wt.% PEDOT/2-layer MWNT hybrid nanomembrane had gravimetric capacitance of ~84 F/g. PEDOT/MWNT hybrid nanomembrane is scrolled into ~20 μm yarn. Plying the biscrolled yarn with a metal wire as a current collector is capable to make weavable, sewable, and knottable yarn that function as high performance electrodes of redox supercapacitors. High mechanical strength (367±113 MPa), modulus (5.9±1.4 GPa) and flexibility of biscrolled yarns enabled fabrication of knotted yarns and braids. Volumetric capacitance of ~147 F/cm3 in liquid electrolyte and ~145 F/cm3 in solid electrolyte at scan rate of 1 V/s. The discharge current of biscrolled yarn supercapacitor linearly increases with voltage scan rate up to ~80 V/s and ~20 V/s for liquid and solid electrolyte, respectively, and the discharge rate capability can be evaluated from the relaxation time constant (τ0), which is ~17 ms (liquid electrolyte) and ~80 ms (solid electrolyte). This means an excellent contact between the metal wire and the yarn supercapacitor and shows a rapid accessibility of the electrolyte ions to yarns. This yarn capacitance shows high cycle life that little depends on sewing (99% after 10,000 cycles). The explanation for such a high power and energy performance, and stable cycle life for two-ply yarn supercapacitors lies in the novel structure of the biscrolled yarn. The biscrolled yarn consists of multilayer of 2-layer MWNT/75 wt% PEDOT nanomembranes provides a continuous pathway for electronic transport from yarn center to yarn surface as well as sufficient porosity through hundreds of layers is favorable structure for electrolyte ions transport at high charge/discharge rate. The yarn supercapacitor had an average power density of 40 W/cm3 and energy density of up to 1.4 mWh/cm3, which are comparable to commercial lithium thin-film battery, activated carbon electrochemical capacitor, and supercapacitor. All-solid-state, electrochemically powered, and tensile actuators using a spinnable carbon nanotube (MWNT) sheet were demonstrated. The torsional and tensile actuation is achieved by reversible yarn volume changes driven by electrolyte ion influx/release during electrochemical charge/discharge. Torsional stroke of 53o/mm for a low applied voltage (5 V) and tensile stroke of 1.3% at 2.5 V were obtained without the use of an additional complex three-electrode electrochemical setup. The tensile actuator maintained its contraction following charging and subsequent disconnection from the power supply because it is own supercapacitor (~91.5% of the muscle contraction was maintained for one hour following discontinuation of the applied voltage). Finally, lightweight, wearable, and inexpensive thermoelectric (TE)-yarn-based textile are developed. We demonstrate a successful attempt to fabricate weaved textiles based on p-, and n-type (antimony telluride, and bismuth telluride, respectively) semiconductor highly flexible yarns by twisting the deposited these materials on the highly aligned PAN-electrospun sheet. The crystallinity of semiconductor materials is improved by thermal annealing and the polymer-based electrospun sheet with intrinsic low thermal conductivity as a template is highly related to energy harvest performance. In this thesis, we suggested three different textile structures such as zigzag-stitch, garter stitch, and plain weave and analyze the thermoelectric performance from these textiles. The generated maximum power per area of zigzag-stitch, garter-stitch, and plain weave had ~0.11 W/m2, ~0.09 W/m2, and ~0.62 W/m2, respectively, at the same temperature gradient of 55 oC. Also, the generated maximum power per couple of zigzag-stitch, garter-stitch, and plain weave had ~0.24 μW, ~0.21 μW, and 1.01 μW, respectively, at the same temperature gradient of 55 oC. Specially, the plain weave is stable structure to thermal, then the maximum power per area of 8.56 W/m2 and power per couple of 14.1 μW were obtained at the temperature gradient of 200 oC. More generally, use of multifunctional biscrolled yarn could widely applicable as power sources for portable, miniaturized electronic devices such as micro-robot, wearable electronic textiles, and implantable medical devices. |본 논문은 에너지 응용 기술을 위한 나노섬유 기반의 다기능 섬유의 개발을 목표로 하고 있다. 첫번째 연구분야는 높은 에너지 저장 및 전력밀도를 갖는 나노멤브레인 및 실형태 슈퍼커패시터의 개발, 그것의 성능 및 특성 분석, 에너지 충․방전 성능을 향상시키기 위한 이론적 해석과 실험에 대한 논의이다. 두번째 연구분야는 앞서 개발한 실형태의 슈퍼커패시터를 이용하여 공기중에서 전기화학적인 자극에 의해 구동하는 인공근육의 개발, 엑츄에이션 성능 분석 및 응용 가능성에 대한 논의이다. 마지막으로 세번째 연구분야는 열전특성을 갖는 실을 제조하여 직물을 제작한 뒤, 이것의 성능을 분석하고, 성능 향상을 위한 이론적 접근과 응용 가능성에 대한 논의로 구성된다. 탄소나노튜브(CNT) 시트 위에 전도성 고분자를 기상중합법을 이용하여 코팅한 기계적 강도가 우수하고, 독립적으로 서있을 수 있는 (free-standing) 하이브리드 나노멤브레인을 제조하였다. 1장의 탄소나노튜브 시트 위에 전도성 고분자(PEDOT)를 85중량비(wt.%)로 코팅한 하이브리드 나노멤브레인의 평균두께는 66 nm 이고, 기계적 강도 및 탄성계수는 각각 127 MPa, 18 GPa으로 기존의 고강도 나노멤브레인들이 갖는 기계적 강도 및 탄성계수를 능가한다. 하이브리드 나노멤브레인은 기존의 탄소나노튜브 시트가 갖는 소수성, 낮은 에너지 저장 능력, 용액내에서의 구조무너짐 등을 친수성을 지닌 전도성 고분자(PEDOT)를 코팅함으로써 해결했다. 또한, 1장의 탄소나노튜브 시트/85중량비(wt.%) 나노멤브레인의 550 nm 파장에서의 투명도는 56%이고 면저항은 312 Ω/sq이다. 이는 1장의 탄소나노튜브 시트의 투명도 (74%)보다 18% 감소한 값이지만 면저항 (958 Ω/sq)은 1장의 탄소나노튜브 시트보다 3 배 이상 향상시킨 것이다. 전도성 고분자(PEDOT)의 우수한 전기화학적 충전용량(capacitance) 성능으로 인해 1장의 탄소나노튜브 시트/85중량비(wt.%) 나노멤브레인은 10 mV/s의 스캔 속도에서 62 F/g의 단위질량당 에너지 충전용량을 갖고, 2장의 탄소나노튜브 시트/85중량비(wt.%) 나노멤브레인은 같은 조건에서 84 F/g의 값을 갖는다. 하이브리드 나노멤브레인을 꼬은 뒤, 금속 와이어를 같이 플라이하여 제조한 실 형태의 슈퍼커패시터는 가벼우며, 직물제조가 가능하다. 제조한 실의 인장강도는 367±113 MPa 이고, 탄성계수는 5.9±1.4 GPa 으로써 이는 구부리고 매듭을 짓거나 다양한 직조가 가능한 범위의 기계적 물성이다. 2-플라이 슈퍼커패시터는 스캔속도가 1 V/s 에 대해 부피로 정량화된 에너지 충전용량이 액체 전해질에서 147 F/cm3이고, 고체전해질에서 145 F/cm3 이었다. 액체와 고체 전해질에서 스캔속도를 각각 80 V/s및 20 V/s까지 적용했을 때까지 에너지 충전용량을 잘 유지하였고, 매우 낮은 이완시간상수(=τ0) (액체(80 ms), 고체(17 ms)) 값을 가졌다. 이는 금속 와이어와 실 형태의 슈펴커패시터 사이의 접촉이 우수함을 의미하며, 전해질 이온의 빠른 접근 가능성을 보여준다. 직물로 제작한 뒤, 10,000 순환 횟수 동안에도 우수한 성능 유지특성을 보였다. 슈퍼커패시터의 우수한 에너지 저장 및 전력 밀도 성능은 전자와 전해질 이온의 이동을 효과적으로 제공할 수 있는 실 구조로부터 기인한다. 나노멤브레인(두께: ~100 nm)이 꼬여 생긴 수 백겹의 층에는 다양한 크기의 기공이 존재하고, 이는 충•방전 동안 전해질 이온의 효과적인 이동을 가능하게 한다. 부피로 정량화한 실 슈퍼커패시터의 에너지 밀도는 1.4 mWh/cm3 이고 평균전력밀도는 40 W/cm3 으로 기존의 상업적인 리튬 박막 배터리, 활성탄소 커패시터, 슈퍼커패시터 등과 견줄 정도의 우수한 성능을 보여주었다. 탄소나노튜브 숲(CNT forest)로부터 당김에 의해 만들어진 탄소나노튜브 에어로젤 시트를 꼬아 전기화학적 자극에 의해 공기중에서 비틀림 (torsion) 또는 인장 (tensile) 구동을 하는 엑츄에이터를 제시했다. 충•방전 동안 전해질 내 일정 부피를 갖는 이온들이 실 내부로 이동하여 실 내부의 부피 팽창을 유도하여 비틀림 또는 인장 구동을 하는 원리이다. 복잡한 3-전극 시스템을 사용하지 않고, 낮은 전압에서 (사각파, 5 V, 50% duty cycle) 53o/mm 의 비틀림 성능을 갖고, 2.5 V의 자극에 대해서 1.2%의 인장 구동성능을 보여주었다. 탄소나노튜브로 제조한 엑츄에이터는 동시에 이온들을 충전할 수 있는 슈퍼커패시터의 특징을 갖기 때문에, 일정 자극에 의해 수축한 인공근육에 가해준 외부 파워를 끊고 난 후에도 1시간동안이나 91.5% 수축한 상태를 유지하였다. 마지막으로 가볍고, 저렴한 열전특성을 지닌 실을 제조하여 이들을 서로 엮어 만든 직물형태의 유연한 고성능 열전소자를 개발하였다. 이를 위해 전기방사(electrospinning) 기법을 이용하여 정렬된 고분자 나노섬유 시트를 제작하고 상온에서 열전특성이 우수한 세라믹 계열의 p-, n- 유형의 물질(p유형: antimony telluride, Sb2Te3, n유형: bismuth telluride, Bi2Te3)을 증착기를 이용하여 코팅한 뒤 꼬아 실을 제조하였다. 열처리를 통해 세라믹 물질의 결정화도 (crystallinity)를 향상시켰으며 주형(template)으로 사용한 고분자 나노섬유가 지닌 물질 고유의 낮은 열 전달률은 열전소자의 에너지 생성 성능과 밀접한 관련이 있다. 본 논문에서 우리는 세가지의 다른 직물 구조(지그재그, 가터, 그리고 평직물)를 제시하였고, 이로부터 열전성능을 분석하였다. 직물의 두께방향으로 ~55 oC의 온도차이를 주게 되면, 지그재그, 가터, 그리고 평직물에서 각각 0.11 W/m2, 0.09 W/m2, 0.62 W/m2의 전기에너지를 생성하였고, 단위 단위커플당 생성되는 전기에너지는 0.24 μW, 0.21 μW, 1.01 μW 였다. 특히 평직물 구조는 열적으로 매우 안정하여 200 oC가 넘는 온도차이에도 안정적으로 8.56 W/m2, 단위커플당14.1 μW의 전기에너지를 생성하였다. 나노 또는 마이크로 크기의 소재를 다기능성 섬유로 제조하여 소형 로봇이나 착용가능한 전자직물 또는 이식가능한 의료용 장치 등의 소형 전자장치에 응용될 수 있는 휴대가능하고 가벼운 전원장치 혹은 에너지 집적장치로 광범위하게 사용될 수 있다.; This thesis describes a development of multi-functional yarn based on nanofiber for energy application technology. The first research area in this thesis is the development of nanomembranes and yarn supercapacitor having a high energy storage and power density, characterization of its performance, and a discussion on the theoretical analysis and experiments to improve the energy charge․discharge performance. The second research area is the development of artificial muscles driven by electrochemical stimulation in the air, performance analysis, and a discussion of the potential applications. Finally, the third is to make a fabric using yarns having thermoelectric properties, analyze the performance, and a discussion of the theoretical approach for improving performance and application possibilities. Mechanically robust, transparent, and free-standing hybrid nanomembranes made of densified carbon nanotube sheets that were coated with poly(3,4-ethyleneioxythiophene) (PEDOT) using vapor phase polymerization and their performance as supercapacitor were reported. Average thickness of hybrid nanomembrane containing 85 wt.% PEDOT/1-layer MWNT sheet is ~66 nm and its mechanical strength and modulus of 127 MPa and 18 GPa, respectively, which values surpass other nanomembrane structures. Several limitations of bare MWNT sheet that is hydrophobicity, low capacitance, and collapse in solution were solved by coating with PEDOT. The transmittance of the 85 wt.% PEDOT/1-layer MWNT was 56% (at 550 nm wavelength) and 18% lower than for a 1-layer MWNT sheet (74%). Also, the sheet resistance is 312 Ω/sq, which is 3 times lower than that of a 1-layer MWNT sheet (958 Ω/sq). The 85 wt.% PEDOT/1-layer MWNT hybrid nanomembrane attached on a current collector had gravimetric capacitance of ~62 F/g and 85 wt.% PEDOT/2-layer MWNT hybrid nanomembrane had gravimetric capacitance of ~84 F/g. PEDOT/MWNT hybrid nanomembrane is scrolled into ~20 μm yarn. Plying the biscrolled yarn with a metal wire as a current collector is capable to make weavable, sewable, and knottable yarn that function as high performance electrodes of redox supercapacitors. High mechanical strength (367±113 MPa), modulus (5.9±1.4 GPa) and flexibility of biscrolled yarns enabled fabrication of knotted yarns and braids. Volumetric capacitance of ~147 F/cm3 in liquid electrolyte and ~145 F/cm3 in solid electrolyte at scan rate of 1 V/s. The discharge current of biscrolled yarn supercapacitor linearly increases with voltage scan rate up to ~80 V/s and ~20 V/s for liquid and solid electrolyte, respectively, and the discharge rate capability can be evaluated from the relaxation time constant (τ0), which is ~17 ms (liquid electrolyte) and ~80 ms (solid electrolyte). This means an excellent contact between the metal wire and the yarn supercapacitor and shows a rapid accessibility of the electrolyte ions to yarns. This yarn capacitance shows high cycle life that little depends on sewing (99% after 10,000 cycles). The explanation for such a high power and energy performance, and stable cycle life for two-ply yarn supercapacitors lies in the novel structure of the biscrolled yarn. The biscrolled yarn consists of multilayer of 2-layer MWNT/75 wt% PEDOT nanomembranes provides a continuous pathway for electronic transport from yarn center to yarn surface as well as sufficient porosity through hundreds of layers is favorable structure for electrolyte ions transport at high charge/discharge rate. The yarn supercapacitor had an average power density of 40 W/cm3 and energy density of up to 1.4 mWh/cm3, which are comparable to commercial lithium thin-film battery, activated carbon electrochemical capacitor, and supercapacitor. All-solid-state, electrochemically powered, and tensile actuators using a spinnable carbon nanotube (MWNT) sheet were demonstrated. The torsional and tensile actuation is achieved by reversible yarn volume changes driven by electrolyte ion influx/release during electrochemical charge/discharge. Torsional stroke of 53o/mm for a low applied voltage (5 V) and tensile stroke of 1.3% at 2.5 V were obtained without the use of an additional complex three-electrode electrochemical setup. The tensile actuator maintained its contraction following charging and subsequent disconnection from the power supply because it is own supercapacitor (~91.5% of the muscle contraction was maintained for one hour following discontinuation of the applied voltage). Finally, lightweight, wearable, and inexpensive thermoelectric (TE)-yarn-based textile are developed. We demonstrate a successful attempt to fabricate weaved textiles based on p-, and n-type (antimony telluride, and bismuth telluride, respectively) semiconductor highly flexible yarns by twisting the deposited these materials on the highly aligned PAN-electrospun sheet. The crystallinity of semiconductor materials is improved by thermal annealing and the polymer-based electrospun sheet with intrinsic low thermal conductivity as a template is highly related to energy harvest performance. In this thesis, we suggested three different textile structures such as zigzag-stitch, garter stitch, and plain weave and analyze the thermoelectric performance from these textiles. The generated maximum power per area of zigzag-stitch, garter-stitch, and plain weave had ~0.11 W/m2, ~0.09 W/m2, and ~0.62 W/m2, respectively, at the same temperature gradient of 55 oC. Also, the generated maximum power per couple of zigzag-stitch, garter-stitch, and plain weave had ~0.24 μW, ~0.21 μW, and 1.01 μW, respectively, at the same temperature gradient of 55 oC. Specially, the plain weave is stable structure to thermal, then the maximum power per area of 8.56 W/m2 and power per couple of 14.1 μW were obtained at the temperature gradient of 200 oC. More generally, use of multifunctional biscrolled yarn could widely applicable as power sources for portable, miniaturized electronic devices such as micro-robot, wearable electronic textiles, and implantable medical devices.