Ca sub(3)Co sub(4)O sub(9) is one of the most promising thermoelectric oxide materials at high temperature. Its structure consists of two misfit layers: the CaO--CoO--CaO rocksalt-type (RS) layer and ...the CdI sub(2)-type CoO sub(2) layer. In this paper, we reported the synthesis of single phase Ca sub(3)Co sub(4-x)M sub(x)O sub(9) (where M=Fe, Cr, and Ga). Thermoelectric measurements showed the variation of thermoelectric properties depending on the charge state of the substitutional elements. We showed the direct evidence for the location of the substitutional elements in the system using an X-ray absorption spectroscopy technique. The extended X-ray absorption fine structure spectra were fitted with the models. Also, the X-ray absorption near edge structure spectra were compared with the simulated spectra from the first principle calculation. The data analysis show that for Fe and Cr substitution, the Fe and Cr atoms are more likely to be located in the RS layer rather than in the CoO sub(2) as presumed previously. For the Ga case, Ga atoms accommodate both sites; still the majority stays in the RS layer. This is supported by the calculation of the total energy of the system, which showed that the total energy is lower when the substituted elements were located in the RS layer.
Hydrogen adsorption energies were investigated in three different types of iso-reticular Metal Organic Framework-16, Zn-/Mg-/Ca-MOF16, decorated with either Li, Na, or K. Concerning the binding ...strengths of the alkali metals, the density functional theory calculations reveal that Li is bound strongest to the host framework, followed by K and Na. Decoration with Li also results in the highest hydrogen adsorption energies among the studied alkali metals. Furthermore, Zn-MOF16 exhibits the highest hydrogen adsorption energies near the metal oxide cluster, while hydrogen binding strengths at organic linker sites do not differ substantially between Zn-/Mg-/Ca-MOF16. Based on these results, we conclude that for Metal Organic Framework-16, Li-decorated Zn-MOF16 appears to be the optimal choice for hydrogen storage among the nine combinations.
This thesis deals with first-principles calculations based on density functional theory to investigate hydrogen storage related properties in various high-surface area materials and the ground state ...crystal structures in alkaline earth dicarbide systems. High-surface area materials have been shown to be very promising for hydrogen storage applications owing to them containing numerous hydrogen adsorption sites and good kinetics for adsorption/desorption. However, one disadvantage of these materials is their very weak interaction with adsorbed hydrogen molecules. Hence, for any feasible applications, the hydrogen interaction energy of these materials must be enhanced. In metal organic frameworks, approaches for improving the hydrogen interaction energy are opening the metal oxide cluster and decorating hydrogen attracting metals, e.g. Li, at the adsorption sites of the host. In covalent organic framework-1, the effects of the H2-H2 interaction are also found to play a significant role for enhancing the hydrogen adsorption energy. Moreover, ab initio molecular dynamics simulations reveal that hydrogen molecules can be trapped in the host material due to the blockage from adjacent adsorbed hydrogen molecules. In light metal hydride systems, hydrogen ions play two different roles, namely they can behave as "promoter" and "inhibitor" of Li diffusion in lithium imide and lithium amide, respectively. By studying thermodynamics of Li+ and proton diffusions in the mixture between lithium amide and lithium hydride, it was found that Li+ and proton diffusions inside lithium amide are more favorable than those between lithium amide and lithium hydride. Finally, our results show that the ground state configuration of BeC2 and MgC2 consists of five-membered carbon rings connected through a carbon atom forming an infinitely repeated chain surrounded by Be/Mg ions, whereas the stable crystal structure of the CaC2, SrC2 and BaC2 is the chain type structure, commonly found in the alkaline earth dicarbide systems.
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 712
Materials which possess a high lithium ion conductivity are very attractive for battery and fuel cell applications. Hydrogenation of the fast-ion conductor lithium nitride (Li3N) leads to the ...formation of lithium imide (Li2NH) and subsequently of lithium amide (LiNH2). Using ab initio molecular dynamics simulations, we carried out a comparative study of the Li diffusion in these three systems. The results demonstrate that hydrogen can work as both promoter and inhibitor of Li mobility, with the lowest transition temperature to the superionic state occurring in Li2NH. Furthermore, we show that the creation of Li vacancies strongly affects Li diffusion in Li3N, but not so in Li2NH. Finally, we explain our findings with the help of a simple model.
We studied five alkaline earth dicarbide systems MAEC2 (where M-AE = Be-Ba) by using ab initio random structure search. For BeC2 and MgC2, the lowest energy and dynamically stable configuration ...consists of five-membered carbon rings connected to each other via an individual arbon atom, stabilized through the donation of electrons from the surrounding alkaline earth ions. For CaC2, SrC2, and BaC2, our study shows that the chain crystal structure is more stable than the predicted structure due to strains induced by the increasing size of alkaline earth metal ions. The reaction energies of the typical synthesis pathway are comparable to those calculated for the experimental reaction of the known chain-type structure. Finally, the proposed structure should be optically distinguishable due to a significantly narrower band gap.
