Effects of Biochar Post-treatments on Performance Enhancement of Moisture-Diffusion Energy Harvesters = 바이오차 후처리 공정이 수분 확산형 에너지 하베스터 성능 향상에 미치는 영향

국문 초록 (Abstract)

참나무 버섯 재배용 원목(oak mushroom logwood, OML)을 바이오매스 전구체로 사용하여 셀룰로오스-탄소 기반 수분 확산형 에너지 하베스터(MDEH)용 활성 바이오차 전극을 제조하였다. OML은 KOH 활성화 유무 및 서로 다른 탄화 온도에서 탄화한 뒤, 기공 구조와 표면 화학을 조절하기 위해 단시간 건식 밀링과 질산 처리를 수행하였다. 이러한 변화는 원소 분석, 질소 흡·탈착, 입도 분석, 라만 분광법, X-선 광전자 분광법(XPS)을 통해 분석하였다. 그 결과, 약 700 ℃에서 KOH 활성화 후 5분 밀링, 5 M HNO₃ 3시간 처리를 거친 바이오차가 메조공과 산소 함유 작용기의 균형이 가장 우수하였고, 비표면적은 약 1000 m²g-¹에 근접하였다. 최적화 된 바이오차로 제작한 MDEH는 최대 개방전압(Voc) 약333 mV, 단락전류(Isc) 약18.7 μA를 나타냈으며, 24개 소자로 구성된 모듈은100 μF 커패시터를 약20분 만에 3 V로 충전하고 30분 이상 전압을 유지하였다.
두 번째 단계에서는 전기전도도, 표면 상태 및 젖음성을 추가로 제어하기 위해 코발트 혼입과 폴리비닐알코올(polyvinyl alcohol, PVA) 코팅을 적용하였다. 코발트가 혼입된 바이오차는 필름의 전도도를 증가시키고 표면 작용기 구성을 변화시켜 동일한 시험 조건에서 Voc와 Isc를 향상시켰다. PVA 코팅은 필름 두께를 증가시키고 물 침투를 지연시켜 전압이 유지되는 시간을 늘렸다. 코발트 혼입과 PVA 코팅을 모두 적용적용한 최적MDEH는 최대 Voc 572 mV, Isc 39 μA에 도달하였다. 또한 전기적 성능이 향상된 필름 25장을 5직렬 × 5병렬(5-series × 5-parallel) 어레이로 연결하여 누수 감지 모듈을 제작하였으며, 이 모듈은 누수수만으로 약 2V의 전압을 발생시켜 소형 커패시터를 충전하고 LED를 점등하며, 외부 전원 없이 Bluetooth Low Energy(BLE) 비콘을 구동할 수 있었다.
이러한 결과는 OML이 실용적인 자가발전형 센서 전극으로 전환될 수 있음을 보여주며, 밀링, 산화, 코발트 혼입, PVA 코팅과 같은 후처리 공정의 조합이 바이오매스 유래 MDEH의 성능과 응용 범위를 향상시키는 효과적인 수단임을 시사한다.

다국어 초록 (Multilingual Abstract)

Oak mushroom logwood (OML) was used as a biomass precursor to prepare activated biochar electrodes for cellulose–carbon moisture-diffusion energy harvesters (MDEHs). OML was carbonized at different temperatures with and without KOH activation, followed by short dry milling and nitric acid treatment, to tune pore structure and surface chemistry. Elemental analysis, nitrogen adsorption–desorption, particle size analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to track these changes. A biochar prepared at around 700 °C with KOH activation, 5 min milling, and 5 M HNO₃ treatment for 3 h showed a good balance of mesopores and oxygen-containing groups, with a specific surface area close to 1000 m² g⁻¹. MDEHs made from this optimized biochar showed a Voc up to about 333 mV and an Isc of about 18.7 μA. A 24-unit module charged a 100 μF capacitor to about 3 V in roughly 20 min and maintained the voltage for more than 30 min.
In the second stage, cobalt mixing and polyvinyl alcohol (PVA) coating were applied to further control electrical conductivity, surface state, and wetting behavior. Cobalt-mixed biochar increased film conductivity and modified surface functional groups, leading to higher Voc and Isc under the same test conditions. PVA coating increased the film thickness and slowed water penetration, extending the time during which the voltage was maintained. The best MDEH based on Co-mixed and PVA-coated films reached a maximum Voc of 572 mV and Isc of 39 μA. Several improved films were then connected in 5-series × 5-parallel arrays to build a leak-detection module that generated a 2 V from leaked water, charged small capacitors, lit an LED, and drove a Bluetooth Low Energy (BLE) beacon without any external power supply.
These results show that waste OML can be converted into practical self-powered sensor electrodes, and that combined post-treatments such as milling, acid oxidation, cobalt mixing, and PVA coating are effective tools for improving the performance and application range of biomass-derived MDEHs.

목차 (Table of Contents)

• Abstract I
• Contents III
• List of Tables VI
• List of Figures VII
• List of Abbreviations X
• 1. Introduction 1
• 1.1. Global energy and environmental challenges 1
• 1.2. Water based power generation 2
• 1.3. Cellulose–carbon moisture-diffusion energy harvester (MDEH) 4
• 1.4. Biomass-derived biochar 8
• 1.5. Objectives 9
• 2. Literature review 11
• 2.1. Fundamentals of electric double layers and streaming potentials 11
• 2.1.1. Electric double layer formation at solid-liquid interfaces 11
• 2.1.2. Moisture-diffusion and voltage generation in porous media 12
• 2.2. Carbon materials for moisture-diffusion energy harvesters 13
• 3. Materials and methods 17
• 3.1.Preparation of OML biochar with and without KOH activation 17
• 3.2. Dry milling and nitric acid treatment of OML biochar 20
• 3.3. Cobalt mixing protocols for BC–Co samples 21
• 3.4. MDEH film fabrication and PVA coating protocol 22
• 3.5. Physicochemical characterization 23
• 3.5.1 Elemental analysis, TGA, and nitrogen adsorption–desorption 23
• 3.5.2 Particle size analysis and zeta potential / conductivity 24
• 3.5.3 XRD, Raman spectroscopy, XPS, and SEM analysis 25
• 3.6. Electrical performance tests of MDEH 27
• 3.6.1. Voc and Isc measurements 27
• 3.6.2 Size-dependent MDEH performance 28
• 3.6.3 Analog sensor test and Capacitor discharging experiments 29
• 3.6.4 Leak-detection sensor module design and testing 30
• 4. Results and discussion 32
• 4.1. Physicochemical optimization of OML-derived biochar for MDEH 32
• 4.1.1. Effect of KOH activation and carbonization temperature 32
• 4.1.2. Effect of milling time on structure and texture 36
• 4.1.2.1. Textural and particle-size evolution of milled OML-BC 36
• 4.1.2.2. Structural analysis by XRD and Raman of milling OML-BC 40
• 4.1.3. ffect of nitric acid treatment on porosity and surface chemistry 42
• 4.1.4. MDEH performance and correlation analysis 46
• 4.1.5. Demonstration of self-powered sensor operation and capacitor charging behavior 50
• 4.2. Effects of cobalt mixing and PVA coating on MDEH performance 54
• 4.2.1. SEM–EDS evidence of dispersed Cobalt on BC 54
• 4.2.2. Physicochemical properties of Cobalt mixing and PVA coating 57
• 4.2.3. Cobalt mixing effects on porosity and BET surface area 62
• 4.2.4. Cross-sectional architecture and PVA-coating effects in MDEH films 65
• 4.2.5. Cobalt/PVA MDEH performance, size effect, and endurance 69
• 4.2.6. MDEH-Co20-PVA2–driven Bluetooth leak-alarm module 73
• 5. Conclusion 75
• References 77