| 摘要: | 由於氫氣具有高能量密度，因此被視為具潛力的再生能源，但如何以安全且有效的方式儲存氫氣，仍是實現氫能經濟的一大挑戰。使用氫儲存材料的固態儲氫提供了解決此挑戰的可能方法，因為與壓縮氣態或液態氫相比，這些材料可提供非常高的體積能量密度。氫儲存材料通常以兩種截然不同的方式吸附氫氣：物理吸附（physisorption），氫分子透過范德瓦爾斯力吸附於材料表面。在較低溫度下，熱運動減弱，使這些弱作用力能更有效地將氫分子固定於表面，此現象常見於多數多孔材料與奈米材料中；另一種吸氫方式是化學吸附（chemisorption），氫分子解離成氫原子，並與主體原子形成鍵結，這種機制常見於多數氫儲存合金中，氫以氫化物形式存在。
石墨相氮化碳（graphitic carbon nitride,g-C3N4）奈米管作為氫儲存材料相當有趣，因為 g-C3N4 同時具有物理吸附與化學吸附的特性。因此，本研究的目標是從兩個關鍵面向探討 g-C3N4 的儲氫性能：首先，分析金屬摻雜對其吸附行為的影響；其次，評估不同溫度與壓力條件對其儲氫容量的影響。
在研究的第一部分，g-C3N4 是透過水熱法製備，從 SEM 影像可觀察到 g-C3N4 呈現管狀結構。針對 g-C3N4 進行了不同金屬摻雜方法，主要是鎳與鎂的熱還原摻雜。經 5 wt.% 及 10 wt.% 鎳摻雜後，氫吸附量大量上升。經 5 wt.% 與 10 wt.% 鎂摻雜後，氫吸附量大量上升。為了增強結構以避免塌陷、提升儲氫容量並縮短吸附時間，本研究進一步進行共摻雜 (co-doping)，以總摻雜量 10 wt.%，分別採用 3:1、1:1、1:3 (Mg:Ni) 比例進行。透過 SEM 的 EDX 分析確認金屬均勻摻雜，鎳與鎂均勻分布於 g-C₃N₄ 表面。
本研究的第二部分著重於物理吸附。由於范德瓦爾斯作用，壓力與溫度是影響氫儲存容量的兩大主要因素。根據勒沙特列原理 (Le Chatelier’s Principle) 與吸附等溫線，壓力與溫度對氣體吸附扮演關鍵角色。當壓力增加，氣相中氫分子濃度上升，導致材料表面附近的分子密度增加。在壓力從 3.7 MPa 提升至 10 MPa 時，氫吸附量大量上升。當溫度上升，氫儲存容量下降；反之，溫度下降時，氫儲存容量上升。在較低溫度下，氫分子的熱運動減弱，使弱范德瓦爾斯力能更有效地將其固定。本研究分別於四種溫度下進行測試，分別為 313 K、 298 K、277 K 以及 223 K。
;Hydrogen is a potential renewable energy source due to its high energy density, but storing hydrogen in a safe and effective manner remains a major challenge for realizing a hydrogen economy. Solid-state hydrogen storage using hydrogen storage materials offers a possible solution to this challenge, as these materials provide a much higher volumetric energy density compared to compressed gas or liquid hydrogen. Hydrogen storage materials typically absorb hydrogen in two distinct ways. The first is physisorption, where hydrogen molecules are adsorbed on the surface via van der Waals interactions. At lower temperatures, thermal motion is reduced, allowing these weak interactions to more effectively retain hydrogen molecules on the surface; this mechanism is commonly found in most porous and nanostructured materials. The second is chemisorption, in which hydrogen molecules are dissociated into hydrogen atoms that form bonds with host atoms. This mechanism is adopted by most hydrogen storage alloys, where hydrogen exists in the form of metal hydrides.
Graphitic carbon nitride (g-C3N4) nanotubes are of particular interest as hydrogen storage materials because g-C3N4 exhibits characteristics of both physisorption and chemisorption. Therefore, the objective of this work is to investigate the hydrogen storage performance of g-C3N4 under two key aspects: first, by examining the effects of metal doping on its adsorption behavior; and second, by evaluating the influence of different temperature and pressure conditions on its storage capacity.
In the first part of this research, g-C3N4 was prepared using a hydrothermal method. SEM images revealed that g-C3N4 possesses a tubular structure. Different metal doping methods were employed via thermal reduction to incorporate nickel and magnesium into g-C3N4. After doping with 5 wt.% and 10 wt.% nickel, hydrogen uptake increased a lot, respectively. Similarly, after doping with 5 wt.% and 10 wt.% magnesium, hydrogen uptake increased a lot, respectively. To strengthen the structure, prevent its collapse, increase hydrogen storage capacity, and shorten adsorption time, co-doping was also investigated. By co-doping magnesium and nickel at ratios of 3:1, 1:1, and 1:3 (Mg: Ni) with a total doping amount of 10 wt.%, hydrogen uptakes increased a lot, respectively, were obtained. To ensure uniform metal doping, EDX mapping of SEM confirmed that nickel and magnesium were homogeneously distributed on the surface of g-C3N4.
In the second part of this work, the focus was on physisorption. Pressure and temperature are two main factors affecting hydrogen storage capacity due to their influence on van der Waals interactions. According to Le Chatelier’s principle and adsorption isotherms, both parameters play crucial roles in gas adsorption. As pressure increases, the concentration of hydrogen molecules in the gas phase rises, leading to a higher molecular density near the material surface. When the pressure increased from 3.7 MPa to 10 MPa, hydrogen uptake rose a lot. Conversely, as temperature increased, hydrogen storage capacity decreased, while lowering the temperature enhanced the capacity. At lower temperatures, the thermal motion of hydrogen molecules is reduced, allowing weak van der Waals forces to retain them more effectively. In this study, experiments were conducted at four temperatures: 313 K, 298 K, 277 K, and 223 K. |
