| 摘要: | 氫能源已被國際認可為未來清潔能源的主流。由於氫在燃料電池中與氧氣反應只會產生能量和水。因此，整個反應過程不會排放二氧化碳等溫室氣體，被認為有助於減緩全球暖化的發展。然而，實現氫經濟的一個瓶頸是缺乏安全有效的氫氣儲存方法。與壓力氫氣儲存相比，固態氫儲存被認為是一種安全有效的氫氣儲存方式。
在這項研究中，旨在開發一種具有良好氫氣儲存性能且具有成本效益的材料。已經有文獻指出g-C3N4奈米管在室溫和中等壓力下可能儲存高達5.45 wt.%的氫氣。因此，本研究旨在使用高能量行星球磨機進一步提高g-C3N4奈米管的氫氣儲存能力。在這項工作中，通過將起始材料（三聚氰胺和三聚氰酸）在不同速度（50、70、80、90、100、110和240 rpm）下研磨2或5小時，然後在550°C下煅燒4小時，製備出了g-C3N4奈米管。觀察到所得到的g-C3N4s奈米管的比表面積與研磨轉速成正比變化。此外，還觀察到只有在球磨速度高於80 rpm時才形成管狀的g-C3N4。而這些管的直徑則與球磨機速度成反比。當行星式球磨機的速度增加到240 rpm時，只有在穿透式電子顯微鏡下才能解析出管的尺寸。上述觀察結果表明，使用高能量行星球磨機可以有效地製備具有管狀結構的g-C3N4。樣品的氫氣儲存能力是在室溫下，在最高13 MPa的壓力下通過氫氣PCI測定的。發現在行星式球磨速度為80到110 rpm之間製備的所有樣品的氫氣容量約為1.0 wt.%。然而，對於在行星式球磨中使用最高速度240 rpm製備的g-C3N4，其具有最高的比表面積，為122.2 cm2/g，以及在11.7 MPa壓力下最高的儲氫能力，達到1.724 wt.%。這表明通過高能量行星球磨機的研磨可以改變材料的表面電荷，從而有效增強其氫氣儲存性能。
本次研究中還採用了第二種方法來製備g-C3N4奈米管。在這種自組裝方法中，將三聚氰胺與乙二醇及硝酸混合，形成超分子中間體，然後將中間體在550°C下煅燒2小時。所產生的g-C3N4奈米管具有更完整的管狀結構（樣品標記為CNNT-65）。與前一方法中產生的g-C3N4相比，觀察到CNNT-65的比表面積較低，為11.3 cm2/g。然而，在類似的氫氣充氣壓力下，它展現了與C-110（具有33.6 cm2/g的比表面積）相似的氫氣儲存能力。這可能歸因於更完整的奈米管結構。
我們得出結論，通過上述兩種方法，都可以獲得g-C3N4奈米管。此外，1.724 wt.%的氫氣儲存能力和具經濟效益的起始材料使所產生的g-C3N4在氫氣儲存應用中非常具有潛力。
;Hydrogen energy has been internationally recognized as the mainstream of clean energy for the future. Since hydrogen reacts with oxygen in the fuel cells to produce energy, only water is produced. Thus the entire reaction process does not emit greenhouse gases such as carbon dioxide, which is the main cause of the global warming. However, one of the bottle necks to the realisation of hydrogen economy is the lack of safe and effective way to store hydrogen. As compared to the pressurized hydrogen gas storage, solid-state hydrogen storage is considered to be a safe and effective way to store hydrogen.
In this research, the aim has been to develop a cost-effective material with good hydrogen storage properties. It has been suggested that g-C3N4 nanotubes have the potential to store up to 5.45 wt.% of hydrogen under room temperature and moderate pressure. Hence, this study aims to further enhance the hydrogen storage capacity of g-C3N4 nanotubes using a high-energy planetary ball mill. In this work, g-C3N4 nanotubes were produced by grinding the starting materials (melamine and cyanuric acid) at various speeds, specifically 50, 70, 80, 90, 100, 110, and 240 rpm, for a duration of 2 or 5 hours, followed by calcinating at 550 °C for 4 hours. It was observed that the resulting g-C3N4 nanotubes exhibited a variation in specific surface area proportional to the grinding speed. Additionally, it has been observed that the tubular g-C3N4 only formed when the ball milling speed was above 80 rpm. Furthermore, the diameter of these tubes was inversely proportional to the speed of the ball mill. When the speed of the planetary ball milling went up to 240 rpm, the size of the tube could only be resolved under the transmission electron microscope. The above observations suggest that the use of a high-energy planetary ball mill can effectively produce g-C3N4 with tubular structures. The hydrogen storage capacities of the samples were determined using a PCI at room temperature and under a pressure up to 13 MPa. It was found that all these samples produced under the planetary ball milling speed of 80 to 110 rpm yielded a hydrogen capacity around 1.0 wt.%. However, for the g-C3N4 produced using the highest speed of 240 rpm in the planetary ball milling, it has the highest specific surface area of 122.2 cm2/g and the highest storage capacity of 1.724 wt.% at a pressure of 11.7 MPa. This suggests that grinding through a high-energy planetary ball mill can alter the surface charge of the material, thereby effectively enhancing its hydrogen storage performance.
A second approach to fabricate g-C3N4 nanotubes has been adopted in this work. In this self-assembling method, melamine was mixed with ethylene glycol and nitric acid to form the supramolecular intermediate, and then the intermediate was calcinated at 550 °C for 2 hours. The so-produced g-C3N4 nanotubes have more complete tubular structures (sample labeled as CNNT-65). When compared with g-C3N4 produced in the previous method, it was observed that CNNT-65 which has a lower specific surface area of 11.3 cm2/g. However, it exhibited a similar hydrogen storage capacity to C-110 (with a specific surface area of 33.6 cm2/g), under similar hydrogen charging pressure. This might be attributed to the more complete nanotube structure.
We concluded that with both of the two methods above, g-C3N4 nanotubes could be obtained. Also, the hydrogen storage capacity of 1.724 wt.% and the cost-effective starting materials made the so-produced g-C3N4 very useful for hydrogen storage applications.
Keywords: Hydrogen storage; graphitic carbon nitride; nanotube; microstructure; high-energy ball milling |
