Hierarchial hydrogen storage in clathrates of Ammonia Borane. Theoretical study

Abstract

A brief overview of the dissertation given in this abstract is divided into five points showing its topicality, objective, goals, scientific novelty, and practical significance. The topicality is reflected in a need for the replacement of the fossil fuels driven economy with economy oriented towards renewable sources of energy, in which hydrogen is used as an energy carrier. This need is dictated by three reasons: (i) ecological problems mostly induced by the carbon dioxide emission; (ii) limitedness of the reserves of hydrocarbons; (iii) political issues related to the localization of hydrocarbons in few places around the globe. In any implementation of hydrogen economy, which is a possible cure for the mentioned issues, the production and storage of hydrogen are the most challenging tasks to solve. The dissertation is focused on the problem of hydrogen storage. For today, none of the known materials meets all the requirements imposed on practical on-board hydrogen storage media. The main idea proposed and explored in this dissertation is the "hierarchical storage of hydrogen". We envisage materials that would offer various means of reversible hydrogen binding. Each level of hydrogen storage would have different characteristics that become advantageous in different circumstances. A material with hierarchical hydrogen storage could be superior in comparison with conventional materials, in which hydrogen is bound at one level only. In particular, we explore materials in which a fraction of hydrogen is physically bound and the remaining part is chemically bound. The physical binding provides hydrogen that is kinetically easily accessible, whereas the chemical binding assures a high overall hydrogen density. We suggest that hydrogen clathrates of a high hydrogen content material, like ammonia borane, could serve as models of hierarchical hydrogen storage. An objective of the dissertation is thus to validate the possibility of storage of molecular hydrogen in clathrates of ammonia borane using methods of theoretical chemistry and materials science. The goals of the dissertation can be formulated as follows: (i) to identify possible structures of clathrates of ammonia borane; (ii) to estimate hydrogen capacity of the clathrates; (iii) to estimate pressure-temperature regimes required for the stabilization of these clathrates. The scientific novelty of the dissertation includes: (i) formulation of the "hierarchical hydrogen storage" concept; (ii) formulation of construction principles for clathrates of ammonia borane and identification of their possible structures; (iii) estimation of hydrogen capacity of the clathrates; (iv) development of a model of clathrates phase equilibria, which is based on the energy of intermolecular host-guest interactions and the entropy of guest molecules enclosed in clathrate cages; (v) an estimation of the pressure-temperature stability zone for these clathrates. The practical significance of the dissertation is in justification of further experimental works on clathrates of ammonia borane, for which the required stabilization pressure and temperature conditions are determined. The work proposed and thoroughly explored hierarchical method of hydrogen storage and resulted in identification of stable cages and periodic structures of possible clathrates of ammonia borane. The most stable extended system of these clathrates was found to be more stable than molecular crystal of ammonia borane at low temperatures. Hydrogen capacity of this hypothetical clathrate structure was estimated to be 21 wt%. To predict the pressure-temperature stability zone of the material a model of clathrate phase equilibria has been formulated and tested on known hydrates. The model showed that clathrates of ammonia borane could be stabilized at ambient pressure when temperature is lowered to 77 K.