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Philosophiae Doctor - PhD / Magnesium hydride has long been regarded as a promising candidate for lightweight hydrogen storage applications, owing to reasonably high theoretical capacity (7.6 wt. %). It is burdened by slow absorption/desorption kinetics which has been the target for improvement of many research groups over the years. Nanostructured MgH2 prepared by high energy reactive ball milling (HRBM) of Mg under hydrogen atmosphere with the addition of V or Ti results in modified MgH2 that demonstrates superior hydrogenation/dehydrogenation kinetics without a crippling compromise in storage capacity. Mg – FeV nanocomposites prepared via ball milling of Mg and FeV raw materials demonstrated up to 96.4% of the theoretical storage capacity and comparable kinetics to Mg - V prepared via the same method using pure refined V (which is far costlier than FeV). In both cases, the hydrogenation/dehydrogenation kinetics was much improved than pure Mg alone, as evidenced by faster hydrogenation times. In terms of cyclic stability, Mg – 10FeV demonstrated improvement over pure Mg with final absorption and desorption capacities of 4.93 ± 0.02 wt. % and 4.82 ± 0.02 wt. % respectively over 30 cycles. When compared against Mg – V, Mg – FeV showed slightly inferior improvements, attributed to incomplete hydrogenation of V in the presence of Fe. However, they share similar crystalline BCC, BCT – V2H and FCC - VH phases with the size of less than 10 nm and demonstrated the same behaviour at high temperatures; at temperatures approaching 400 °C, particle sintering became an issue for both nanocomposites resulting in a drop in absorption capacity even in the first cycle. The further inclusion of carbonaceous species showed several effects, one of which was an improvement in hydrogen uptake speed as well as kinetics for the addition of 5 wt. % activated carbon. For the sample with 5 wt. % graphite, the appearance of an initial incubation period of up to 60 minutes was noted, presumably corresponding to the duration of time when the carbon was sheared and crushed before hydrogenation commences.

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Philosophiae Doctor - PhD / The main objective of this study was to advance kinetic performances of formation and decomposition of magnesium hydride by design strategies which include high energy ball milling in hydrogen (HRBM), in combination with the introduction of catalytic/dopant additives. In this regard, the transformation of Mg → MgH2 by high energy reactive ball milling in hydrogen atmosphere (HRBM) of Mg with various additives to yield nanostructured composite hydrogen storage materials was studied using in situ pressure-temperature monitoring that allowed to get time-resolved results about hydrogenation behaviour during HRBM. The as-prepared and re-hydrogenated nanocomposites were characterized using XRD, high-resolution SEM and TEM, as well as measurements of the mean particle size. Dehydrogenation performances of the nanocomposites were studied by DSC / TGA and TDS; and the re-hydrogenation behaviour was investigated using Sieverts volumetric technique.

A candidate hydrogen storing material should have high storage capacity and fast dehydrogenation kinetics. On this basis, magnesium hydride (MgH2) is an outstanding compound with 7.66 wt % storage capacity, despite its slow dehydriding kinetics and high desorption temperature. Therefore in this study, bulk and surface alloys of Mg with improved hydrogen desorption characteristics were investigated. In this respect, formation energies of alloyed bulk MgH2 as well as the adsorption energies on alloyed magnesium (Mg) and MgH2 surface structures were calculated by total energy pseudopotential methods. Furthermore, the effect of substitutionally placed dopants on the dissociation of hydrogen molecule (H2) at the surface of Mg was studied via Molecular Dynamics (MD). The results displayed that 31 out of 32 selected dopants contributed to the decrease in formation energy of MgH2 within a range of ~ 37 kJ/mol-H2 where only Sr did not exhibit any such effect. The most favorable elements in this respect came out to be / P, K, Tl, Si, Sn, Ag, Pb, Au, Na, v Mo, Ge and In. Afterwards, a systematical study within adsorption characteristics of hydrogen on alloyed Mg surfaces (via dynamic calculations) as well as calculations regarding adsorption energies of the impurity elements were performed. Accordingly, Mo and Ni yielded lower adsorption energies / -9.2626 and -5.2995 eV for substitutionally alloyed surfaces, respectively. MD simulations presented that Co is found to have a splitting effect on H2 in 50 fs, where the first hydrogen atom is immediately adsorbed on Mg substrate. Finally, charge density distributions were realized to verify the distinguished effects of most 3d and 4d transition metals in terms of their catalyzer effects.

Pure and palladium&amp / #8208 / covered 350 nm thick magnesium thin films were deposited on glass substrates via thermal evaporation. In the as&amp / #8208 / deposited state, films were highly textured with Mg (001) parallel to the substrate. Hydrogen loading experiments were carried out in two different conditions / namely isothermal and isochronal. Hydrogenation behaviors of the thin films were followed by twopoint probe electrical resistance and optical transmittance measurements, as well as x&amp / #8208 / ray diffraction studies. Isothermal hydrogenation experiments conducted on Pd&amp / #8208 / covered Mg thin films have revealed that these films can absorb hydrogen at temperatures starting from 333 K, producing MgH2 with a random texture. When the films were heated slowly starting from the room temperature, on the other hand, hydrogenation gives rise to a textured MgH2, where (110) parallel to the substrate with a minor (101) component. Formation of the textured hydride in isochronal loading was discussed within the context of lattice mismatch in Mg to MgH2 transformation. It was further shown that formation of such a textured hydride in Mg thin films minimizes in&amp / #8208 / plane lattice distortion.

In this dissertation, hydrogen storage enhancement in hypercrosslinked polystyrene, effects of single walled carbon nanotubes (SWCNTs) supported ruthenium (Ru) catalyst on the kinetics and ammonia suppression in the LiNH2-MgH2 complex hydride system and the accuracy of hydrogen storage measurements are investigated in detail. High surface area physisorption materials are of interest for room temperature hydrogen storage enhancement by spillover. Six different commercially available hypercrosslinked polystyrenes are screened by considering the specific surface area, average pore size, pore volume, and adsorption enthalpy. MN270 is selected mainly due to its high surface area and narrow pores for investigation of the spillover enhancement at room temperature. Two different platinum (Pt) doped MN270 samples are prepared by wet impregnation (MN270-6wt%Pt) and bridge building technique (MN270-bridged) with an average Pt particle size of 3.9 and 9.9nm, respectively, as obtained from X-ray diffraction analysis. Pt doping altered the surface property of MN270, and reduced the nitrogen and hydrogen uptake at 77 K and 1 atm due to pore blocking. The room temperature hydrogen uptake at 100 atm demonstrated a 10% enhancement for the MN270-bridged (0.36 wt. %) compared to the pristine MN270 (0.32 wt. %), but did not show any enhancement for the MN270-6wt%Pt under the same conditions. The hydrogen uptake of MN270-bridged has little value for practical applications; however, it showed the effectiveness of the bridge building technique. The LiNH2 - MgH2 (2:1.1) complex metal hydride system (Li-Mg-N-H), which is prepared by high energy ball milling, is investigated in terms of the hydrogen ab/desorption kinetics and the concomitant NH3 emission levels. By selecting more intense ball milling parameters, the hydrogen ab/desorption kinetics were improved and the NH3 emission reduced. However, it is shown that NH3 emission cannot be completely eliminated by ball milling. The hydrogen desorption kinetics of the Li-Mg-N-H system is much faster than the absorption kinetics at a specific T and P, but the desorption kinetics degraded considerably over a number of cycles as opposed to the stabilized absorption kinetics. Furthermore, SWCNTs and 20 wt. % Ru doped SWCNTs (SWCNT-20Ru) are utilized as catalysts to study their effects on NH3 emission and kinetics characteristics of the Li-Mg-N-H system. The SWCNT doped sample did not show any kinetics improvement, whereas the SWCNT-20Ru doped sample showed similar kinetics performance as that of the base sample. More importantly, the presence of SWCNT increased the NH3 emission as compared to the base sample. On the other hand, SWCNT-20Ru doping reduced the NH3 emission compared to the SWCNT doping, but did not eliminate it completely. As revealed from the mass spectrometry signals, the SWCNT-20Ru catalyst starts to decompose NH3 at a temperature as low as 200°C. However, an optimal catalyst still needs to be developed by fine tuning the Ru particle size and the SWCNT structural properties to maximize its effectiveness to suppress NH3 release in the Li-Mg-N-H system. The design of a volumetric measurement apparatus is studied by means of an uncertainty analysis to provide guidelines for optimum hydrogen sorption measurements. The reservoir volume should be as small as possible (i.e., 10 cc) to minimize the uncertainty. In addition, the sample mass loading has a profound effect on the uncertainty and the optimum loading is a function of the sample's intrinsic storage capacity. In general, the higher the sample mass loading the lower is the uncertainty, regardless of any other parameters. In cases where the material to be tested is not available in gram quantities, the use of high accuracy pressure and temperature transducers significantly mitigates the uncertainty in the sample's hydrogen uptake. Above all, the thermal equilibration time is an important parameter for high accuracy measurements and needs to be taken into consideration at the start of the measurements. Based on computational analysis, a 5 min wait time is required for achieving thermal equilibrium when the instrument enclosure temperature is different than the ambient temperature.

The demands on Hydrogen fuel based technologies is ever increasing for substitution or replacing fossil fuel due to superior energy sustainability, national security and reduced greenhouse gas emissions. Currently, the polymer based proton exchange membrane fuel cell (PEMFC), is strongly considered for on-board hydrogen storage vehicles due to low temperature operation, efficiency and low environmental impact. However, the realization of PEMFC vehicles must overcome the portable hydrogen storage barrier. DOE and FreedomCAR technical hydrogen storage targets for the case of solid state hydrides are: (1) volumetric hydrogen density > 0.045 kgH2/L, (2) gravimetric hydrogen density > 6.0 wt%, (3) operating temperature < 150 degrees C, (4) lifetimes of 1000 cycles, and (5) a fast rate of H2 absorption and desorption. To meet these targets, we have focused on lithium borohydride systems; an alkali metal complex hydride with a high theoretical hydrogen capacity of 18 wt.%. It has been shown by Vajo et al. that adding MgH2, improves the cycling capacity of LiBH4. The pressure-composition-isotherms of the destabilized LiBH4 + MgH2 system show an extended plateau pressure around 4-5 bars at 350 degrees C with a good cyclic stability. The mentioned destabilizing mechanism was successfully utilized to synthesize the complex hydride mixture LiBH4 + 1/2MgH2 + Xmol% ZnCl2 catalyst (X=2, 4, 6, 8 and 10) by ball milling process. The added ZnCl2 exhibited some mild catalytic activity which resulted in a decomposition temperature reduction to 270 degrees C. X-ray powder diffraction profiles exhibit LiCl peaks whose intensity increases proportionately with increasing ZnCl2 indicating an interaction between catalyst and hydride system, possibly affecting the total weight percent of desorbed hydrogen. Thermal gravimetric analysis profiles for MgH2 + 5mol% nanoNi and LiBH4 + ZnCl2 + 3mol% nanoNi indicate that small concentrations of nano-nickel acts as an effective catalyst that reduces the mixture desorption temperature to around 225 degrees C and 88 degrees C, respectively. Future work will be focused on thermodynamic equilibrium studies (PCT) on the destabilized complex hydrides.

Vienas iš didžiausių iššūkių su kuriais susiduria kompanijos norėdamos panaudoti vandenilio energetikos technologijas įvairiuose prietaisuose – vandenilio saugojimas. Dabartiniu metu egzistuoja trys technologijos, kurios naudojamos vandenilio saugojimui: suspaustas, kriogeninis vandenilis ir vandenilio saugojimas kompleksiniuose junginiuose. Suspaustas ir atšaldytas vandenilis jau pasiekė savo technologinius limitus. Daugiausiai vilčių dedama į vandenilio saugojimą kompleksinėse anglies nanostruktūrose ir metalų hidriduose. Šis darbas yra susijęs su bandymu sintetinti magnio hidridą, panaudojant magnetroninio garinimo sistemą, garinant magnį vandenilio ir argono reaktyvioje aplinkoje. Magnio hidridas yra vienas iš labiausiai tiriamų metalų hidridų vandenilio saugojimui. Deja, dabartiniu metu nėra sukurta technologiškai paprastų patikimų (greita vandenilio absorbcija ir desorbcija, minimalūs nuostoliai ir t.t) ir ekonomiškai efektyvių magnio hidrido sintezės metodų. Darbe gautos struktūros ištirtos panaudojant paviršiaus profilometrijos, SEM, EDS ir XRD metodus. Gauti rezultatai parodė, kad gautos struktūros yra tik dalinai magnio hidridas. / Hydrogen storage is one of the main challenges related with the use of hydrogen energy technologies in daily activities. Three main technologies for hydrogen storage available today: compressed, krio hydrogen and hydrogen storage in different compounds. Compressed and crio hydrogen almost reached its technological limits. A lot of expectations were related with hydrogen storage in carbon nanostructures and metal hydrides. This work is mainly related with magnesium hydride synthesis using magnetron sputtering in reactive hydrogen and argon atmosphere. Magnesium hydride is one of the most promising material. Unfortunately, there is no technologically simple and reliable methods (fast adsorbion/desorption kinetics, minimal losses and etc.) for magnesium hidride synthesis. The magnesium based structures which were received during the work were analyzed using SEM and EDS, surface profilometry and XRD methods. It is shown that received structure only partially transformed to magnesium hydride.

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