| 摘要: | 摘要
本論文採用第一性原理方法從事用於儲氫的新穎多孔材料的設計。此研究中開發了一個可靠的熱力學模型並用於估算儲氫系統適合運作的條件。美國能源部依據為了實現有競爭性的輕型氫動力車的技術要求製定了一套發展儲氫系統的性能目標。為了實現這些目標，我們設計了多種從半矽氧烷或金剛烷延伸出來的骨架材料並探討它們是否可以用於儲氫。
本論文第3章展示了如何由修飾過的苯環以及四面體的倍半矽氧烷籠合成一個合理儲氫系統。我們使用第一原理密度泛函理論計算探討使用過渡金屬裝飾並有硼摻雜的四面體倍半矽氧烷骨架(boron doped tetrahedral silsesquioxane frameworks， B-TSF)在儲氫的應用。硼摻雜可以明顯的增強過渡金屬與B-TSF中的連接分子的相互作用並避免過渡金屬原子組成團簇。同時硼摻雜也可以維持穩定的氫氣吸附能。在由Sc，Ti，與V裝飾的B-TSF中氫分子的平均吸附能為 0.29，0.40 以及 0.69 eV並具有可接受的氫氣吸附質量比容量 (6.9, 5.6 以及4.15 wt%)。透過計算吉布斯自由能我們也估算了B-TSF作為儲氫材料的運作溫度和壓力範圍。透過進一步在材料設計上的修改，我們或許可以對儲氫性能進行微調。
在第４章中，我們經由使用較大的倍半矽氧烷與各種連接分子，延伸儲氫的多孔骨架材料設計。如果使用較長的連接分子，對氫的質量比容量可以提高到7.5 wt%以上。除此之外，通過改變半矽氧烷的大小，體積比容量可增加到70 g/L 以上。此研究將從事摻雜Sc, Ti等過度金屬的多面體低聚倍半矽氧烷 (polyhedral oligomeric silsesquioxane，POSS)骨架結構的設計。在這一段中，我們不只會探討各種POSS結構可儲蓄多少氫，我們也會討論一些關於那些含有金屬裝飾的結構的穩定性問題，例如會導致儲氫容量下降的金屬簇集反應。除此之外，這項研究將展示如何透過組合不同大小的半矽氧烷以及連接分子來調節POSS結構對氫的質量與體積比容量。
我們在第五章探討五種不同含有Sc裝飾由金剛烷延伸出來的骨架材料的吸氫反應。這裡所探討的五種不同骨架材料是由被不同分子結構連結起來的多羥基金剛烷組成的。當骨架材料完全被氫覆蓋時，平均每個H2分子的吸附能在-0.17與 -0.19 eV之間。我們使用一個簡單的熱力學模型來估計在不同溫度與壓力下的五種材料對氫氣吸附的質量與體積比容量。在此研究中，最有潛力的骨架材料使用苯環結構作為結構分子。該骨架被預測在溫度為358 K及氫氣壓強為100 bar時可吸附4.38 wt% 或 39.82 g/L的氫。由於在此章節討論的骨架材料對於氫的相互作用較弱，在一般儲氫材料的操作條件下，這些材料的氫氣覆蓋率明顯的低於材料完全被覆蓋的情況。這個研究結果展現了探討溫度與壓力對吸氫反應的影響的必要性。
關鍵詞
氫氣儲存，多孔骨架，熱力學，質量與體積比容量，密度泛函理論，再生能源
;Abstract
This thesis deals with the design of novel porous materials with respect to target based hydrogen storage using first principle methods. In order to achieve the performance-based targets, the silsesquioxane and adamantane based frameworks are designed and investigated for hydrogen storage application.
The design of a plausible hydrogen storage system based on assembling the modified benzene rings and tetrahedral silsesquioxane cages is demonstrated in Chapter 3. The transition metals (TM) decorated boron doped tetrahedral silsesquioxane frameworks (B-TSF) for application in hydrogen storage are investigated using first-principles density functional theory calculations. Boron substitution substantially enhances the TM binding energy to the linker of B-TSF to suppress metal clustering as well as maintain stable hydrogen adsorption energy to TMs. The average hydrogen adsorption energies in Sc-, Ti-, and V-decorated B-TSF are 0.29, 0.40, and 0.69 eV, respectively, with an acceptable gravimetric density of 6.9, 5.6, and 4.15 wt %. Gibbs free energy calculations are also carried out to estimate the working temperature and pressure ranges for using B-TSF as a hydrogen storage system. Further modifications in the design of the frameworks may allow us to tune the hydrogen storage properties.
In Chapter 4, the porous frameworks composed of larger silsesquioxane cages linked via a variety of TMs decorated boron doped linkers are designed for hydrogen storage. The H2 gravimetric capacity can be improved to more than 7.5 wt% by using longer linkers. On the other hand, the maximum H2 volumetric capacity can be tuned to more than 70 g/L by varying the size of silsesquioxane cages. This study will deal with polyhedral oligomeric silsesquioxane (POSS) frameworks that are doped with TMs such as scandium (Sc) or titanium (Ti). In this section, the discussion will not only on the H2 uptake in various POSS frameworks but also cover some issues on the stability of the metal decorated framework, e.g., the unwanted clustering of the doped metal. Furthermore, this study will demonstrate that the gravimetric and volumetric capacities of POSS frameworks can be tuned by combining silsesquioxane cages and linkers of different sizes.
In Chapter 5, the hydrogen adsorption in five Sc decorated porous, adamantane based frameworks have been investigated. Each of these frameworks consists of polyhydroxy adamantane units that are connected by a different molecular linker. At full coverage the average H2 adsorption energy is between -0.17 and -0.19 eV per H2 molecule. We use a simple thermodynamic model to estimate the gravimetric and volumetric hydrogen uptake as a function of temperature and pressure. The most promising framework considered here is a structure with benzene units as linkers and is predicted to achieve 4.38 wt% or 39.82 g/L H2 uptake at 358 K and 100 bar H2 pressure. The relatively weak framework-H2 interaction leads to the circumstance that at typical operating conditions, the hydrogen uptake still deviates in non-negligible fashion form full coverage. This finding illustrates the necessity to account for the temperature and pressure dependency of the H2 uptake.
Keywords
Hydrogen storage, porous framework, thermodynamics, gravimetric and volumetric capacities, porous frameworks, density functional theory, renewable energy |
