The increasing demand for energy supply and a rising population urged to explore more sustainable energy resources. Hydrogen is expected to play an important role based on environmentally clean resources and carrier in the future energy economy. The development of technologies and infrastructures that are required for the hydrogen economy has become an important subject in science and engineering. One of the key issues to be tackled is the development of suitable hydrogen storage materials, which allows a practical and safe usage of hydrogen fuels in transport vehicles.
The design of novel porous materials with respect to target based hydrogen storage using first principle methods is presented in this thesis. The reliable thermodynamic model is developed and used to estimate the proper operating condition of hydrogen storage systems. The performance targets for the hydrogen storage systems were developed by Department of Energy U.S.A. and these targets are based on achieving similar performance and cost levels as competitive light-duty vehicles. 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 (TMs) 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 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. At full coverage, 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 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 has been investigated. Each of these frameworks consists of polyhydroxy adamantane units that are connected by a different molecular linker. In contrast to our previous approach for metal binding sites, a polyhydroxy adamantane units instead of molecular linkers can act as anchor sites for four Sc atom, which in return can bind four H2 molecules per Sc sites at full coverage. 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.