| الوصف: |
This thesis describes the preparation and characterisation of potential ‘modular’ solid state hydrogen storage solutions for on-board applications. The systems investigated throughout this work are based on reactions between light weight hydroxides and hydrides. In many senses light metal hydroxides can be seen as attractive candidates for hydrogen storage: they are low cost, present negligible toxicity and it is not possible to poison the fuel cell with decomposition products, unlike in nitrogen or boron containing systems. However, as the dehydrogenation products are the respective oxides, the major drawback of such systems lays in the fact the thermodynamics of rehydrogenation are not favourable for onboard applications. Hence, the system must be considered as a ‘charged module’, where the regeneration is performed ex-situ. Dehydrogenation can be achieved through reaction with light metal hydrides such as LiH or MgH2. A wide range of ‘modular’ release systems can be studied, however the most interesting in terms of theoretical gravimetric capacity, kinetics and thermodynamics within reasonable temperature range (RT - 350°C) use magnesium and lithium hydroxide and their hydrate forms. The present work focuses on the full investigation of three main systems: · Mg(OH)2 – MgH2 system · Mg(OH)2 – LiH system · LiOH(·H2O) – MgH2 system (both anhydrous and monohydrate LiOH were used) Mixtures of hydroxides and hydrides were prepared by manually grinding stoichiometric amounts of the starting materials. Further, nanostructuring the reactants was investigated as a means to control the dehydrogenation reaction and enhance the kinetics and thermodynamics of the process. Nanostructured Mg(OH)2 and LiOH(·H2O) have been successfully obtained using both novel and conventional synthetic routes. Reduction of the particle size of both hydrides was effectively achieved by mechanically milling the bulk materials. As detailed throughout Chapters 3, 4 and 5, promising results were obtained when employing nanosized reactants. The onset temperatures of hydrogen release were decreased and the overall systems performances enhanced. Particularly interesting results were obtained for the LiOH – MgH2 system, which exhibit a dramatic decrease of the onset temperature of H2 release of nearly 100 K when working with milled and nanostructured materials with respect to bulk reagents. All systems were characterised mainly by Powder X-ray diffraction (PXD) and simultaneous thermogravimetric analysis (TG-DTA) mass spectroscopy (MS). TG-DTA2 MS experiments were performed to obtain information on the onset and peak temperature of hydrogen release, weight loss percentage and nature and amount of the gases evolved during the reaction. Ex-situ PXD studies have been performed for each system in order to try and identify any intermediate species forming during the dehydrogenation process and ultimately propose a mechanism of H2 release. Since two fundamentally different types of reaction pathway could be proposed for the Mg(OH)2 – LiH system, powder neutron diffraction (PND) was employed for following the reaction in-situ. Developing a complete model of the dehydrogenation process in terms of mechanistic steps was found to be pivotal in order to understand and enhance such systems further. |
