基於凝膠之熱電、太陽能產水、光熱電轉換之機制與應用

Abstract

熱電材料能將熱能轉換成電能，對於能量轉換系統中的可攜式設備或柔性裝置等等領域是相當重要的。本論文藉由太陽光，設計出以凝膠作為材料能在熱電以外同時能夠能量轉換的功能例如水蒸發或光熱電的能量轉換的應用裝置。本論文將介紹水蒸發、熱電以及光熱電的原理機制。 在第一篇研究中，置備了一種結合海水淨化和熱電的裝置。該裝置透過將熱電水凝膠置於光熱水凝膠之上，利用光熱轉換產生溫度梯度。光熱水凝膠以聚乙烯醇 (PVA) 作為基材，因其低成本和易加工性。為了同時提高太陽能驅動的水蒸發和光熱轉換效率，將球磨的三氧化二鈦 (Ti2O3) 奈米顆粒和奈米碳管 (CNT) 均勻分散到PVA 基質中。水蒸發速率能夠達到高達3.22 kg m−2 h−1。PVA 也為熱電水凝膠的基質，浸泡在 K3[Fe(CN)6]/K4[Fe(CN)6] 氧化還原對溶液中，以利用熱化學電池效應 (TGC) 使得能夠透過熱端和冷端的氧化還原反應產生電壓。所得到的熱電性能為1.48 mV K−1 的離子塞貝克係數 (Si) 和串聯九個熱電水凝膠後的功率密度能來到9.6 mW m−2。這種雙功能裝置能夠同時有效地淨化海水並發電。戶外測試顯示，熱電水凝膠的日產水量為9.2 kg m−2，並能穩定產生130 mV 的電壓。這種方法推進了水淨化和能源生產技術，為再生能源的創新應用開闢了新途徑。 在第二篇研究中，設計出一種雙層光熱電 (PTE)以聚乙烯醇 (PVA) 為基材的水凝膠。水凝膠的上層用奈米碳管 (CNT) 和還原氧化石墨烯 (rGO)，而下層則由純 PVA 水凝膠組成，並調整雙層水凝膠的 DMSO:H2O 溶劑比例。這些方法整體提升了光熱轉換效率，降低了熱導率，並優化了熱電性能。將雙層水凝膠浸泡在 K3[Fe(CN)6]/K4[Fe(CN)6] 氧化還原溶液中，透過調整 DMSO:H2O 比例來優化熱電性能，同時保持低熱導率 (0.42 W m−1 K−1) 並提高了離子塞貝克係數 (1.78 mV K−1)。而雙層結構增加了溫度梯度。在模擬太陽光下，光熱電發電機 (PTEG) 表現出最大溫差為 11.5 °C 的、175 mV 的電壓產生和 300 mW m−2 的最大功率密度，並在戶外測試中展現出優異的長期穩定性。這些特性突顯了其作為太陽能驅動裝置的巨大潛力。

Thermoelectric materials, capable of converting thermal energy into electrical energy, are vital for energy conversion systems, particularly in portable electronics and flexible devices. This thesis presents the design of innovative devices based on hydrogels to harness solar energy for functionalities beyond traditional thermoelectricity, encompassing applications like water evaporation and photothermoelectric conversion. This thesis will introduce the fundamental principles and mechanisms behind water evaporation, thermoelectrics, and photothermoelectrics. In the first research (Chapter 3), we developed a combined system for seawater purification and thermoelectric power generation. This device operates by placing thermoelectric hydrogels on top of photothermal hydrogel, where the photothermal layer generates temperature gradient through photothermal conversion. We used polyvinyl alcohol (PVA) as the matrix for photothermal hydrogel, chosen for its low cost and ease of processing. To enhance both solar-driven water evaporation and photothermal conversion efficiency by uniformly dispersed ball-milled dititanium trioxide (Ti2O3) nanoparticles and carbon nanotubes (CNT) within the PVA matrix. This design enabled an impressive water evaporation rate of up to 3.22 kg m-2 h-1. PVA are also the matrix of thermoelectric hydrogels, which were immersed in K3[Fe(CN)6]/K4[Fe(CN)6] redox pair solution to leverage the thermogalvanic effect. This setup allowed for voltage generation through redox reactions occurring at both the hot and cold ends. The device exhibited excellent thermoelectric properties, characterized by an ionic Seebeck coefficient (Si) of 1.48 mV K-1 and a power density of 9.6 mW m-2 when nine thermoelectric hydrogels connected in series. This dual-functional device effectively purifies seawater and generates electricity. Outdoor tests further confirmed its performance, showing a daily water production of 9.2 kg m-2 and a stable voltage output of 130 mV. This integrated approach advances both water purification and energy generation technologies, paving new ways for applications in renewable energy. In the second research (Chapter 4), a bilayer photothermoelectric (PTE) hydrogel system was designed using polyvinyl alcohol (PVA) matrix. The upper layer of this hydrogel contains carbon nanotubes (CNT) and reduced graphene oxide (rGO), while the bottom layer consisted of pure PVA hydrogel. We systematically adjusted the DMSO:H2O solvent ratio of the entire bilayer hydrogel. These modifications collectively enhanced photothermal conversion efficiency, reduced thermal conductivity, and optimized thermoelectric performance. By immersing the bilayer hydrogel in K3[Fe(CN)6]/K4[Fe(CN)6] redox solution and tuning the DMSO:H2O ratio, we optimized its thermoelectric properties while maintaining a low thermal conductivity of 0.42 W m-1 K-1 and boosting the ionic Seebeck coefficient to 1.78 mV K-1. The bilayer structure significantly amplified the temperature gradient. Under simulated sunlight, the photothermoelectric generator (PTEG) achieved a maximum temperature difference of 11.5 °C, a voltage generation of 175 mV, and a peak power density of 300 mW m-2. Importantly, it demonstrated excellent long-term stability during outdoor testing. These compelling characteristics underscore its substantial potential as a solar-powered device.