Innovative strategies to enhance properties of solid hydrogen storage materials based on hydrides

Resumen

Nowadays, the development of renewable energy resources is attracting much more attention in order to reduce economic and environmental problems associated to the use of fossil fuels. However, their production is variable during time and they cannot provide a constant supply of energy. The use of ‘hydrogen’ as a vector of energy could be a solution to this important limitation. In the case of onboard applications, the simplest idea would be to use hydrogen as liquid or gas, but these methods have important disadvantages. In the former case, 30% of the energy would be used in order to maintain the cryogenic state of hydrogen. In the latter case, storage of pressurized hydrogen would require huge tanks at high conditions of pressure. For these reasons, the application of fuel cell using hydrogen as their energy source requires the development of new solid state hydrogen storage materials. Hydrides, which are some of the most promising solid compounds, are limited by thermodynamic and kinetic properties that must be overcome before they are used in a real application. In state of the art, a literature review of different techniques that have been used to improve kinetics and thermodynamic limitations of bulk hydrides is done, mainly focused in nanoconfinement of different (complex) hydrides in a host, technique which preserve the enhanced properties during cycling. It could be concluded that till now there is not any work in which all the targets for onboard hydrogen storage systems set by the US Department of Energy (DoE) are accomplished. Thus, more investigation is needed and this PhD was defined in this context with the objective of developing a new solid state hydrogen storage material based on hydrides which, by the reduction of particle size (chapters 1 and 2), and by the incorporation and stabilization in a support material (chapters 3-5) can improve the kinetics and reversibility of the hydrogen storage and release processes. In chapter 1, micronized Magnesium Acetate was used as a precursor of magnesium hydride (MgH2) and magnesium oxide (MgO). This hydride has been widely studied due to its high hydrogen storage capacity (7.8 wt%) and its low cost. However, high temperatures are required to decompose it (higher than 300 °C), and hydrogen release is slow which make it difficult to use. Thus, nanoengineering was proposed in order to overcome these limitations. In this work, the precursor of MgH2 was micronized by Supercritical Anti Solvent (SAS) process comparing to conventional milling. The influence of concentration of precursor in the initial solution, the temperature of SAS process and CO2 fraction was studied on the particle size of the precursor. Unprocessed magnesium acetate particles of 200 µm were converted into sub-micrometric particles with particle sizes ranging from 300 nm to 700 nm after SAS processing with regular spherical morphology and amorphous crystalline structure. In contrast to this homogeneous product, mechanically milled particles showed similar mean particle sizes, but with irregular morphology, crystalline structure and bimodal particle distribution. After hydrogenation or calcination process, the acetate was converted into MgH2 and MgO, respectively. As consequence of decrease of particle size, hydrogen release kinetics was enhanced due to the reduction of diffusion distances with a direct relationship between the particle size of the precursor and the kinetics of the hydride. In chapter 2, Ethane 1,2 diamineborane (EDAB), a carbon derivative of Ammonia Borane (used in chapters 3 and 4) was used as hydride. EDAB has a high content in hydrogen (10 wt%), which is released below 200 °C in a two-step reaction. Moreover, it is very stable under ambient conditions, which makes it easier and safer to manipulate it in air atmosphere than other candidate hydrides. EDAB was micronized from THF solutions using the same Supercritical Antisolvent (SAS) process. After micronization, prismatic bulk EDAB of about 400 µm with a crystallite size of 100 nm was converted into microspheres of less than 2 µm with a crystal size of 50 nm. In this case, the concentration of the inlet solution, the temperature of SAS process and the fraction of carbon dioxide did not cause an important change on the final properties of EDAB. As result of reduction of particle size, the kinetic of release of hydrogen by thermolysis at 100 °C was significantly enhanced due to the reduction in the diffusion length, reducing the time needed to release hydrogen by a factor of six. Moreover, a suppression of induction time was obtained due to destabilization of the hydride after treatment. However, enhanced properties are not preserved after micronization of the hydride. Because of it, nanoconfinement in a support is proposed in order to limit the growth of the hydride during cycling (chapters 3, 4 and 5). In chapter 3, Ammonia Borane (AB) was used as a promising chemical hydride due to its high content in hydrogen (19.6 wt%), low molecular weight (30.7 g/mol), moderate decomposition temperature and stability and safety in handling. However, it has kinetic limitations due to long induction times to disrupt dihydrogen bond and slow release of hydrogen. Moreover, during the decomposition process, the emission of some volatile byproducts such as borazine, diborane or ammonia can be released which could be poisonous for hydrogen fuel cells. In this work, nanoconfinement of AB in a support was proposed as solution to enhance these limitations. Silica aerogel microparticles were used as support to stabilize and encapsulate the hydride. The formation of aerogel microparticles was done combining sol-gel process and supercritical drying using batch and semicontinuous drying apparatus. Silica aerogel microparticles produced by the two drying techniques had a surface area ranging from 400 to 800 m2/g with more than 1 cm3/g of volume of pores and a mean particle diameter ranging from 12 to 30 μm. The influence of shear rate, the amount of catalyst during sol-gel process, the amount of dispersant solvent and hydrophobic surface modification on particle size distribution (PSD) were studied. As result, irregular aerogel particles were obtained for hydrophilic gels, while regular, spherical particles with smooth surfaces were obtained for hydrophobic gels. AB was loaded into silica aerogel microparticles in low concentrations ranging from 1 wt% till 5% wt depending on the surface modification of the aerogel. Impregnation method followed by precipitation of AB using dicloromethane (DCM) was used and then, supercritical batch drying was used in order to obtain dried particles. This low encapsulation efficiency could be explained by the possibility of lost AB during washing with DCM and/or the decomposition of AB during supercritical drying process in presence of silica. By stabilization of AB into silica aerogel microparticles resulted in faster hydrogen release kinetics than unprocessed AB or using conventional mechanical milling. In chapter 4, the concentration of AB homogeneously loaded in the pores of hydrophilic silica aerogel was increased up to 60 wt% due to the innovative process used to produce it and the favorable textural properties of silica aerogels (up to 2 cm3/g). It is the first time that more than 50 wt% of AB has been successfully stabilized in a support. It was done by a novel process, based on a simultaneous aerogel drying and ammonia borane gas antisolvent precipitation using pressurized carbon dioxide at subcritical conditions in order to avoid decomposition process from the previous chapter. The influence of the amount of Ammonia Borane loaded on the aerogel on the thermal and structural properties of the material was analyzed. The resulting material showed faster hydrogen release kinetics without induction time by thermolysis at 80 °C and suppression of volatile subproducts in contrast to bulk AB. This fact was due to a significant reduction in the mean size of the hydride after confinement and the presence of SiOH and SiOSi groups of silica aerogel. Furthermore, by nanoconfination of AB, the morphological properties of the material were preserved after isothermal release of hydrogen avoiding foaming process, which could be favorable properties for a subsequent material regeneration process. In chapter 5, Magnesium Borohydride Mg(BH4)2 was used as a promising hydrogen storage material because of its high hydrogen storage capacity (14,8 wt% H2, 0.112 Kg/L). However, it is still limited by slow hydrogen release kinetics and by the harsh conditions required for reversible hydrogen sorption due to the formation of stable intermediates. Composites made of commercial Mg(BH4)2 and synthesized silica aerogel microparticles were prepared by thermal treatment in hydrogen. As a result, the sorption properties of the prepared composite were improved due to the destabilization of the hydride by silica; not only reducing the decomposition temperature by 60 °C but also enhancing the kinetics of dehydrogenation at 300 °C, that was two times faster in silica composites than in bulk Mg(BH4)2. Additionally, re-hydrogenation of the prepared composite at comparatively mild conditions of 390 °C and 110 bar H2 was done for the first time, achieving a hydrogen storage material with a reversible release of hydrogen up to 6 wt% H2. Results indicate that silica aerogel chemically interacted with Mg(BH4)2, acting as an additive, which could result in different routes with different amounts and types of intermediates, influencing on the kinetics and cyclability of the hydrogen storage material. This research was done at Hydrogen Lab at Pavia (Italy).