| dc.description.abstract | 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 | en_US |