模板法製備規則孔洞碳材料及其能源應用

Template-directed fabrication of ordered porous carbon materials for energy applications

黃俊賢(chun-hsien Huang)

指導教授 ： 董瑞安

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00134

摘要

21世紀人類面臨能源、資源逐漸匱乏及環境污染的困境，其中最大的挑戰之一是開發具有高效率的能源轉換與儲存系統，例如超級電容器，鋰離子電池，染料敏化太陽能電池。自從1999年規則中孔洞碳材料的發現，此類材料的設計與應用創新已為科學研究提供了新的研究思維。為提升孔洞碳材料在能源領域的使用效能，孔洞結構及其功能化的設計已成為近年來規則中孔洞碳材料在能源應用上主要的研究方向。因此，本研究主要的目標是利用軟/硬模板合成法製備階層式規則孔洞碳材料以及規則中孔洞鈦基材料-碳複合材料，同時利用各種表面鑑定技術評估材料的物化特性，以期能將所開發的孔洞碳材料應用於能源貯存及轉換領域。材料合成採用三嵌段高分子F127以及微米級聚苯乙烯小球分別做為中孔洞及大孔洞模板，酚醛樹脂為碳的前驅物；經由溶劑揮發誘導有機-有機(酚醛樹脂-三嵌段高分子)或有機-有機-無機(酚醛樹脂-三嵌段高分子-氧化鈦前驅物)自組裝形成規則中孔洞結構。
研究結果發現本研究所開發的技術可透過雙模板及溶劑揮發自組裝法製備階層式規則孔洞碳材料，其階層規則的孔洞結構包含規則的大孔結構及互相連結的孔洞結構系統(大孔窗口、規則中孔洞以及微孔)。透過金屬Ni奈米顆粒的催化，在1000ºC即可將階層孔洞碳材料部分碳材轉化為石墨化結構(HOPC-Ni-1000-g；g表石墨結構、1000表熱裂解溫度)，同時仍保有三維孔結構以及磁性分離的特性。SEM/TEM及XRD結果發現階層規則的孔洞包含有規則的大孔結構(220 nm)及互相連結的孔洞結構系統(大孔窗口、規則中孔洞(4.7 nm)以及微孔)，此孔洞結構可做為離子緩衝空間以及離子傳輸通道，且由於碳材料中含有金屬鎳，因此具有磁性分離特性(3.8 emu/g)，相當適合作為超級電容器及染料敏化太陽能電池的先進電極材料。在超級電容器的電性測試方面，雖然HOPC-Ni-1000-g的比表面積只有296 m2/g，因此其電容值在掃瞄速率3 mV/s只達73.4 F/g；但在高掃描速率下(200 mV/s)的電容值仍可保有47.1 F/g。並且具有較佳的電解液通透性質(24.8 μF/cm2 at 3 mV/s)以及重覆循環掃描效能(>5400 cycles)。當配合利用微波輔助水熱法製備出的TiO2奈米顆粒作為工作電極，而HOPC-Ni-1000-g為對電極組成染料敏化太陽能電池時，HOPC-Ni-1000-g對電解液(I3-)具有良好的電化學催化特性，且元件在標準光譜(AM 1.5G, 100 mW/cm2)量測下的光電轉換效率可達5.2%，接近使用Pt做為對電極的太陽能電池效率(6.7%)。
另利用軟模板結合溶劑揮發誘導自組裝法，可成功製備出規則中孔洞鈦基材料-碳複合材料，其中鈦基奈米晶體顆粒嵌入在無定形的碳骨架結構中，並往孔道空間成長。本研究發現碳含量(25, 35, and 50 wt%)、熱裂解溫度(450-1200ºC)、以及熱裂解氣氛(N2 及 Ar)對材料的熱穩定性以及鈦基奈米晶體的晶相轉換有相當大的影響。當碳含量低於35 wt%，鈦基奈米晶體會由anatase轉換為rutile，Magneli phases，以及TiN (熱裂解氣氛為: N2)或具有缺陷的TiCx (x< 1，熱裂解氣氛為: Ar)。當碳含量為50 wt%，鈦基奈米晶體會由anatase轉換為TiN (熱裂解氣氛N2)或TiC(熱裂解氣氛Ar)。研究結果也發現，氧化鈦奈米晶體與碳材料結合後，在不同的熱裂解氣氛下經由高溫處理能夠轉變為Magneli、TiN或TiC晶相，但由於晶體的成長也會導致孔洞材料的規則結構被破壞。此規則中孔洞二氧化鈦-碳複合材料具有均一的孔徑大小、高比表面積和孔體積、開放與連續的孔洞結構等性質有助於鋰離子的嵌入/嵌出與電解液的擴散，進而提升鋰離子遷移速率及提高鋰離子電池的效能。研究發現65Ti-35C複合材料(含有65 wt%的二氧化鈦以及35 wt%的碳)對鋰離子電池具有最佳的電化學活性，在0.1C的充放電速率且經過80次的循環掃描後仍具有500 mAh/g的電容值，此外，在充放電速率達到5C時，其電容值可達98 mAh/g。本研究結果明確顯示，階層式規則孔洞碳材料以及規則中孔洞鈦基材料-碳複合材料為相當具有新穎性及發展潛力的孔洞碳材料且可應用做為能源電極材料使用於能源轉換及貯存系統包括鋰離子電池、超級電容器及染料敏化太陽能電池等。這些材料明確的合成方法與物化特性及多功能性，將能提昇規則孔洞碳材料的研究發展能量，將之實際應用於能源貯存及轉換系統。

關鍵字

模板法 ； 規則孔洞 ； 碳材料 ； 石墨化 ； 二氧化鈦 ； 超級電容器 ； 鋰離子電池 ； 染料敏化太陽能電池

並列摘要

In response to the needs of modern society and emerging ecological concerns, one of the biggest challenges in 21st century is to develop powerful electrochemical energy conversion and storage devices, such as supercapacitor, lithium-ion battery, and dye-sensitized solar cell. The introduction of ordered mesoporous carbon in the 1999 opens a new chapter in material sciences, and the significant progress made during the past decade in the design and application of ordered mesoporous carbon has provided fresh incentives for further innovations. A noteworthy is the fact that the required disruptive improvement in energy and environmental science has motivated the design of porous architecture and functionality of porous carbons through the aid of suitable templates and by introducing procedures for carbon framework functionality. The major goal of this study is to establish the synthesis protocols, the soft- and hard-templating strategies, in the fabrication of hierarchically-ordered porous carbons with graphitic nanostructures and ordered mesoporous titanium based materials-carbon composites, and to evaluate their physicochemical properties by many characterization methods. The soft- and hard-templating strategies are employed to fabricate porous carbon materials with hierarchically ordered porous structure and ordered mesoporous TiO2-carbon composites. Herein, the amphiphilic triblock copolymer Pluronic F127 and micro-sized polystyrene sphere were used as mesopore and macropore template, respectively. The phenol-formaldehyde resins were used as carbon precursor. The formation of ordered mesoporous structure was relied on organic-organic (PF resin-F127) or organic-organic-inorganic (PF resin-F127-TiO2 precursor) evaporation-induced self-assembly process (EISA).
Three-dimensional, magnetically-separable, and hierarchically ordered porous carbon (HOPC) with designed porous textures has been successfully fabricated by dual-templating method with EISA. In addition, in order to obtain graphitic structure in hierarchically ordered porous carbon, the Ni species was used as catalyst for graphitization. The roles of Ni catalyst and pyrolysis temperature (600-1200ºC) and atmosphere (N2 or H2/N2) in the microstructures of hierarchically ordered porous carbon were elucidated. The synthesized HOPC-Ni-1000-g material (g=graphitic, 1000pyrolysis temperature of 1000°C) exhibits well-crystallized graphitic domains, excellent magnetic properties (3.8 emu/g), and designed porous textures. The designed porous textures of the hierarchically ordered porous carbons are composed of highly ordered, macroporous (220 nm), interconnected porous structures, including macroporous windows, hexagonally ordered mesopores (4.7 nm), and useful micropores. The HOPC with graphitic nanostructure has designed porous texture, serving as an ion-buffering reservoir, an ion-transport channel, and a charge-storage material, and is expected to be advanced an electrode material for high-rate supercapacitor and dye-sensitized solar cells (DSSCs). In supercapacitor, HOPC-Ni-1000-g has a low specific surface area (296 m2/g) and a low gravimetric specific capacitance (73.4 F/g at 3 mV/s), but improved electrical conductivity, better rate performance (47.1 F/g at 200 mV/s), higher electrolyte accessibility (24.8 μF/cm2 at 3 mV/s), and excellent cycling performance (>5400 cycles). In DSSCs, HOPC-Ni-1000-g counter electrode exhibits higher electrocatalytic activity towards I3- reduction. The photovoltaic conversion efficiency of the cell using HOPC-Ni-1000-g counter electrode reaches 5.2 % at one sun (AM 1.5G, 100 mW/cm2) which is close to that (6.7 %) of cell using conventional Pt counter electrode.
The ordered mesoporous titanium based materials (anatase, rutile, Magneli phases, TiN and TiC)-carbon composites (Ti-C) have directly fabricated by supramolecular self-assembly with in-situ crystallization process. The titanium based materials are embedded into the frameworks of carbonaceous matrix. Importantly, the carbon content (25, 35, and 50 wt%), pyrolysis temperature (450-1200ºC), and pyrolysis atmosphere (N2 and Ar) have significant effects on the thermal stability, crystalline phase and crystallinity of Ti-based materials. The crystalline phase changes from anatase, rutile, Magneli phases, and then to TiN (pyrolysis in N2) or defect carbide TiCx (x< 1, pyrolysis in Ar) as the carbon content in nanocomposites is lower than 35 wt%; the crystalline phase of Ti-C composites at 50 wt % carbon content changes directly from anatase to TiN (pyrolysis in N2) or TiC (pyrolysis in Ar). Magneli products, TiN, or TiC materials were formed as the carbothermal reduction of TiO2 at high pyrolysis temperature, unfortunately, the composites lose the ordered mesostructures. The results allow us to elucidate the microstructural changes of titanium based materials inside the ordered mesoporous carbon matrices and open an avenue to the design and synthesis of cooperatively functional ordered mesoporous nanomaterials-carbon composites. In addition, it is advantageous to use ordered mesoporous TiO2-carbon composites as electrode materials for rechargeable Li-ion battery. A series of Ti-C composites with various weight percentages of carbon (25, 35, and 50 wt%) pyrolyzed at 600°C were utilized to evaluate the Li-ion storage performance. The 65Ti-35C material, containing 65 wt% TiO2 and 35 wt% carbon, shows a high capacity of 500 mAhg-1 at 0.1 C after 80 cycles. Moreover, it exhibits a good cyclability and rate capability. The reversible capacity remains at 98 mAh/g at a high rate of 5.0 C, and then recoveries to 520 mAh/g at 0.1 C after 105 cycles.
In conclusion, the materials fabricated in this study are attractive materials and ideal candidates for manifold applications. The versatility and feasibility of these materials have been demonstrated, especially their application to energy storage and conversion. Much effort has to be devoted to systematic studies on the relationship between physicochemical properties of these materials and their performances in energy conversion and storage so that information on the fabrication strategies, properties, and potential applications of these materials can be obtained to stimulate further developments in this fascinating area.

並列關鍵字

Template method ； Ordered porous ； Carbon material ； Graphitization ； Titanium dioxide ； Supercapacitor ； Lithium-ion battery ； Dye-sensitized solar cell

參考文獻

[1] Bottani, E. J. and Tascón, J. M. D., Adsorption by carbons, 1st ed., Elsevier, Amsterdam ; Boston ; London, 2008.

[2] Liang, C. D., Li, Z. J. and Dai, S., Mesoporous carbon materials: Synthesis and modification. Angewandte Chemie-International Edition 2008, 47, (20), 3696-3717.

[3] Inagaki, M., Orikasa, H. and Morishita, T., Morphology and pore control in carbon materials via templating. RSC Advances 2011, 1, (9), 1620-1640.

[4] Xia, Y., Yang, Z. and Mokaya, R., Templated nanoscale porous carbons. Nanoscale 2010, 2, (5), 639-659.

[5] Lu, A. H., Zhao, D. Y. and Wang, Y., in Nanocasting: A versatile strategy for creating nanostructured porous materials, Cambridge : Royal Society of Chemistry, 2010.