一維暨三維奈米結構對於電化學特性及產氫應用的研究

Abstract

由於對石化燃料的依賴噵致近來發生的能源危機，從事發展新穎奈米結構材料在乾淨能源方面之應用變成非常重要的研究。本論文首先利用化學氣相沉積法所製備出垂直陣列之掺氮奈米碳管擁有獨特的微結構和電化學特性，藉由微結構、鍵結、電子轉移行為以及後續白金觸媒的電化學沉積等探討掺氮對於奈米碳管之影響。發現掺氮奈米碳管所形成的表面缺陷，導致近乎逆的電子轉移，並提供白金粒子於奈米碳管表面的成核點以應用於燃料電池。接著研發三種不同的奈米結構，即銅奈米粒子/氧化鋅奈米棒之奈米複合材料、微波活化氧化銅奈米針尖/氧化鋅奈米棒之奈米複合材料以及氧電漿活化氧化銅-氧化鋅之反蛋白石複合結構，以作為微型重組器的觸媒。首先使用銅奈米粒子/氧化鋅奈米棒之奈米複合材料為觸媒可於低重組溫度下(250 ℃)達到高甲醇轉換率(93%)、高氫產率(183 mmol gcat-1 h-1)、低一氧化碳濃度(170-210 ppm)。這可歸因於銅奈米粒子之高表面積和分散性、銅物種的電子結構被修飾以及金屬-承載體間存在強烈的交互作用。接著以微波處理對於氧化銅奈米針尖/氧化鋅奈米棒之奈米複合材料，證明可明顯改善甲醇重組反應之催化性能，此可歸因於氧化銅/氧化鋅界面處有缺陷形成與強交互作用力產生。然後利用氧電漿處理製備出含有高濃度氧空缺之氧化銅-氧化鋅反蛋白石複合結構，以提升甲醇重組反應的催化性能，即可於更低重組溫度下(230 ℃)達到接近完全甲醇轉換率、高氫產率、低一氧化碳濃度與出色的穩定性。最後，利用碳改質之氧化鋅反蛋白石結構以作為光電化學分解水產氫的光電極。藉由加熱氧化鋅和聚苯乙烯蛋白石模板，可以直接在ITO基板上合成出碳嵌入氧化鋅基材的反蛋白石結構。於光子轉換成電流的量測中，相較於純氧化鋅結構在可見光波段，碳改質之氧化鋅反蛋白石結構其光反應表現出顯著的增加。於照明功率密度為100 mW/cm2下，碳改質之氧化鋅反蛋白石結構表現出高光電流密度(1 mA/cm2)與高光子轉換成氫的效率(0.75%)。這些結果顯示出碳改質之氧化鋅反蛋白石結構，具有應用於光電化學分解水的潛力。

There has become of interest in the development of novel and nanostructured materials for the application of clean energy. Part I is focused on “Effects of in-situ Nnitrogen-doping on the microstructure and electrochemical activity of carbon nanotubes”. In this work, the CNx NTs doped with an optimal N concentration resulted in a nearly reversible ET behavior due to uniform and high density of surface defects which are desirable for further nucleation of Pt particles on the surface of CNx NTs to form a composited electrode for electrochemical energy device applications such as fuel cells and capacitors. In Part II, we have developed three kinds of different nanostructures, i.e. Cu nanoparticle (NP)/ZnO nanorod (NR) nanocomposites, microwave-activated CuO nanotip/ZnO NR nanocomposites, and O2 plasma-activated CuO-ZnO inverse opals, for high efficiency of microreformer applications. The fist one, high conversion of methanol (93%), high hydrogen production rate (183 mmol gcat-1 h-1), low CO formation (170-210 ppm), and good stability at a low-reformation temperature of 250 ℃ have been achieved for Cu NP/ZnO NR nanocomposites. The superb catalytic performance of the Cu NP -decorated ZnO NR nanostructures can be attributed to the larger surface area and enhanced dispersion of fine Cu NPs, formation of microstrain, the modification of electronic structure of Cu species, and the existence of strong metal-support interaction (SMSI) effect. The second one, microwave treatment to CuO nanotip/ZnO NR nanocomposites has been demonstrated to remarkably improve the catalytic performance in methanol reforming reaction. Comparative to conventional thermal annealing, microwave treatment significantly enhances the catalytic activity of the catalysts, which might be attributed to defect formation, i.e. microstrain, and strong interaction in CuO/ZnO interface. The third one, we report the use of oxygen vacancies-rich Cu-ZnO inverse opals via O2 plasma treatment to further enhance the catalytic performance of methanol reforming reaction at a low-reaction temperature of only 230 ℃, yielding nearly complete conversion of methanol, ultrahigh hydrogen production rate, ultralow CO formation, and outstanding stability. For the last Part III, we reported the simple synthesis of carbon-modified ZnO inverse opals, and their implementation as photoelectrodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. The in-situ incorporating carbon into the ZnO matrix was synthesized directly on the ITO substrate through annealing of the blends of ZnO and polystyrene-opal template. Incident-photon-to-current-efficiency measurements carried out on PEC cell with carbon-modified ZnO inverse-opal photoanodes demonstrate a significant increase of photoresponse in the visible region compared to pure ZnO structures. Upon illumination at a power density of 100 mW/cm2, carbon-modified ZnO inverse-opals show high photocurrent density of 1 mA/cm2 with photon-to-hydrogen conversion efficiency of 0.75%. These results suggest substantial potential of carbon-modified ZnO inverse opals in PEC water splitting applications.