title: Sorption properties in lightweight hydrogen storage materials for portable power applications
Creators: name: Bravo Diaz, Laura
Publisher: name: University of Glasgow
Year: 2018
Subjects: subject: hydrogen storage, solid state materials, portable power pack, magnesium hydride, ammonia borane, activated carbon, polymer electrolyte membrane, fuel cells,

description: Modern society increasingly depends on reliable and secure energy supplies for economic growth and social prosperity. Thus, it is crucial to implement a low-carbon energy carrier based on renewable energy sources to ensure energy security and tackle climate change. Hydrogen (H2) is undoubtedly one of the most promising energy carriers to achieve a low-carbon energy future scenario. However, before the hydrogen economy can become completely viable, the safe and compact storage of H2 is an issue that must be overcome.
This thesis concentrates on the development of potential “modular” solid state H2 storage solutions for portable power applications. A wide range of potential H2 storage materials was investigated with the aim of providing an improved performance in the form of a low desorption onset temperature, fast desorption kinetics and a high H2 gravimetric capacity. This research work focused on the study of light metal hydride – hydroxide systems, in particular the nanostructured MgH2-Mg(OH)2 system, and ammonia borane (AB) composites, specifically AB within a porous carbon-based matrix composites.
The nanostructured MgH2-Mg(OH)2 “modular” H2 release system was investigated as a candidate exothermic filler material combined with an industrial MgH2 matrix to produce a novel solid state H2 storage hybrid tank. It was postulated that the heat of the reaction of the exothermic filler material could initiate and propagate a reaction in the matrix hydride and additionally contribute to the H2 yield. Detailed information about the thermodynamic and kinetic behaviour of the MgH2-Mg(OH)2 system, under operational conditions, was obtained. 
The thermal decomposition of this system was found to be a two-step process, associated with two H2 releases, resulting from: 1) almost simultaneous decomposition of Mg(OH)2 and hydrolysis of MgH2 at 616 K (exothermic event) and 2) decomposition of unreacted MgH2 at 743 K (endothermic event). The formation of a MgO layer on the unreacted MgH2 resulting from the previous hydrolysis was found to retard the H2 release. The formation of MgH2-MgO core-shell structures was investigated and confirmed by kinetic measurements, ex-situ Scanning Electron Microscopy / Energy Dispersive X-ray Spectroscopy (SEM/EDX) analysis and ex-situ Powder X-ray Diffraction (PXD) experiments. Kinetics measurements performed under operational conditions proved the H2 release of the system to be very slow (≈ 20 hours at 573 K). The mechanism for H2 evolution of this system was elucidated by in-situ Powder Neutron Diffraction (PND) performed at the Institut Laue-Langevin (ILL) in Grenoble, confirming the observations by thermal analysis methods and ex-situ PXD experiments. 
The use of additives (graphite and silicon carbide) was investigated to enhance the kinetic and thermodynamic properties in the system. The incorporation of SiC proved to be successful in improving the H2 release of the first step. However, no further kinetic improvements were observed by incorporating additives. Besides, the H2 capacity was slightly reduced by the introduction of 10 wt. % of C/SiC and traces of water were released alongside H2.
AB-based nanocomposites and nanoconfined samples were also investigated with the aim of synthesising novel solid-state H2 storage materials with enhanced desorption properties. Highly ordered mesoporous carbons (FDU-25, CGY-1), activated carbons (AX21, Sigma AC, MAST Carbon TE7), and graphene (Angstron, Alfa), were employed to prepare nanocomposites (via ball milling or solution impregnation) in different ratios. A double-solution impregnated composite with a 2:3 weight ratio of AB to activated carbon (AC) showed the best performance with a dehydrogenation onset of 353 K and the suppression of borazine and boron-based by-products. The use of an external NiCl2 filter absorbed any released gaseous ammonia and no by-products were detected with a mass spectrometer sensitivity of 100 ppb. The nanoconfinement of AB in AC hosts was investigated by simultaneous Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) at the Elettra synchrotron in Trieste. The results confirm that the nanoconfinement of ammonia borane was successfully induced and central to the performance improvements of the H2 storage material.
To underpin the validity of the results and allow a quantitative comparison of the performance of these new developed materials with previously assessed systems, the reproducibility and repeatability of the measurements was ensured by means of intra and inter-laboratory comparisons. This was accomplished by using the facilities at the European Commission Joint Research Centre (JRC), Energy Storage Unit in Petten (The Netherlands) and the laboratories of the School of Chemistry in University of Glasgow (UK).