A thesis submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering Johannesburg, 2016 / The thesis investigated the effect of partial substitution of Cr or Ti with 2-6 at.% Fe, or 0.05-0.10 at.% Rh/Pd on the structure, hardness, corrosion behaviour and hydrogen storage characteristics of an arc-melted Ti35V40Cr25 at.% alloy. The effects of an annealing and a quenching heat treatment on the properties were also investigated. Melting of the eight alloys was done in a water-cooled, copper-hearth arc melting furnace under an argon atmosphere. Each of the eight ingots was cut into three: one as the as-cast sample and the other two separately quartz-sealed and loaded in two batches in a heat treatment oven and heated to 1000 °C for 1 hour. The first set of quartz tubes were immediately removed and broken in cold water to quench the alloy, hence locking the microstructure. The second batch was loaded into the furnace, heated to 1000 °C for 1 hour and then slowly furnace-cooled. The alloys (as-cast and heat treated) were characterised for phase identification using optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) with Electron Diffraction X-ray Spectroscopy (EDS) using an Oxford system. Thermo-Calc software was used to model the phases using the Solid Solution 4 and Titanium 3 Databases. The hardness values (under a 2 kg load) of all samples were recorded. Potentiodynamic corrosion tests were performed in 6M KOH at 25 °C, and Tafel curves were recorded from -1.4V to -0.2V with a scanning rate of 1mV/sec. A Sievert’s apparatus was used for pressure composition temperature (PCT) measurements at 30, 60 and 90 °C. All the alloys contained a primary bcc (V) phase. The secondary phases were a combination of αTi, Ti(Cr,V)2 Laves phases (C14, C15 or C36) and a minor ωTi phase. The cell volume of the primary (V) phase decreased with addition of Fe and 0.05 Rh but increased with 0.1 Rh and Pd. The hardness of the base alloy increased with additions of Fe and 0.10 at.% Pd, but decreased with additions of Rh and 0.05 at.% Pd. Additions of Rh, Pd and 2 at.% Fe decreased the corrosion rate, while additions of 5 and 6 at.% Fe increased the corrosion rate. The reversible hydrogen storage capacity (RHSC) of the base alloy, otherwise known as useful capacity, was enhanced with addition of Pd and Rh, but decreased with Fe addition. Both annealing and quenching increased the hardness of the 0.05 at.% Rh and all the Fe containing alloys. Heat treatment decreased the hardness of the base alloy, both Pd alloys and v the 0.10 at.% Rh samples. Quenching decreased the hardness of the 0.10 at.% Rh and both Pd-containing alloys. The corrosion rate of the 0, 5 and 6 at.% Fe, 0.05 at.% Rh and the Pd-containing alloys decreased after annealing.at.% FeThe rate increased after annealing the 2 at.% Fe and 0.10 at,% Rh samples. The as-cast sample containing 2 at.% Fe had the lowest corrosion rate (0.0004 mm/y) and the quenched 6 at.% Fe was the least corrosion resistant sample with a corrosion rate of 0.037 mm/y. The quenched 5% Fe alloy had the highest hardness (460 MPa), while the annealed 0.10 at.% Rh sample had the lowest (388 MPa). The quenched 0.05 at.% Pd sample had the highest RHSC (2.28 wt%) while the lowest RHSC of 0.44 wt% was observed in the as-cast 2 at.% Fe sample. Annealing improved the RHSC of all samples except the base Ti35V40Cr25 and 6 at.% Fe alloys, while quenching was detrimental to RHSC of all the samples but the 2 at.% Fe, 0.05 at.% Pd and 0.10 at.% Rh alloys. Increasing the addition of palladium from 0.05 to 0.10 at.% Pd showed no significant improvement on RHSC of the base alloy, thus addition of 0.05 at.% Pd would be sufficient. The RHSC of the annealed 0.05 Rh alloy (2.25 wt% H) was close to the value of the 0.10 at.% Pd, so rhodium could be considered as an alternative to the quenched 0.05 at.% Pd. The RHSC was 1.56, 0.44, 0.75 and 0.68 wt% for 0, 2, 5 and 6 at.% Fe as-cast alloys respectively. Although the 2 at.% Fe alloy had the lowest RHSC, it could find its application as electrode in 6M KOH solution electrolyte because of its low corrosion rate. / MT2016

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2008. / Committee Chair: Mei-Yin Chou; Committee Member: Erbil, Ahmet; Committee Member: First, Phillip; Committee Member: Landman, Uzi; Committee Member: Wang, Xiao-Qian.

<p>In a time of global environmental problems due to overuse of fossil fuels, and a subsequent depletion of the supplies, hydrogen is considered as one of the most important renewable future fuels for use in clean energy systems with zero greenhouse-gas emission. Hydrogen storage is the main issue that needs to be solved before the technology can be implemented into key areas such as transport. The high energy density, good stability and reversibility of metal hydrides make them appealing as hydrogen storage materials. In this thesis research on synthesis and hydrogen absorption properties for intermetallic compounds based on scandium and aluminium is reported. The compounds were synthesized by arc melting or induction melting and exposed to hydrogen in a high pressure furnace. Desorption investigations were performed by thermal desorption spectroscopy. The samples were analyzed by x-ray powder diffraction and electron microscopy. ScAlNi, crystallizing in the MgZn2-type structure (space group: P63/mmc; a = 5.1434(1) Å, c = 8.1820(2) Å), was found to absorb hydrogen by two different mechanisms at different temperature regions. At ~120 °C hydrogen was absorbed by solid solution formation with estimated compositions up to ScAlNiH0.5. At ~500 °C hydrogen was absorbed by disproportionation of ScAlNi into ScH2 and AlNi. The reaction was found to be fully reversible due to destabilization effects which lowered the decomposition temperature of ScH2 by ~460 °C.</p>

First principles computation can be used to investigate an design materials in ways that can not be achieved with experimental means. We show how computations can be used to rapidly capture the essential physics that determines the useful properties in different applications. Some applications for predicting crystal structure, thermodynamic and kinetic properties, and phase stability are discussed. This first principles tool set will be demonstrated with applications from rechargeable batteries and hydrogen storage materials. / Singapore-MIT Alliance (SMA)

In a time of global environmental problems due to overuse of fossil fuels, and a subsequent depletion of the supplies, hydrogen is considered as one of the most important renewable future fuels for use in clean energy systems with zero greenhouse-gas emission. Hydrogen storage is the main issue that needs to be solved before the technology can be implemented into key areas such as transport. The high energy density, good stability and reversibility of metal hydrides make them appealing as hydrogen storage materials. In this thesis research on synthesis and hydrogen absorption properties for intermetallic compounds based on scandium and aluminium is reported. The compounds were synthesized by arc melting or induction melting and exposed to hydrogen in a high pressure furnace. Desorption investigations were performed by thermal desorption spectroscopy. The samples were analyzed by x-ray powder diffraction and electron microscopy. ScAlNi, crystallizing in the MgZn2-type structure (space group: P63/mmc; a = 5.1434(1) Å, c = 8.1820(2) Å), was found to absorb hydrogen by two different mechanisms at different temperature regions. At ~120 °C hydrogen was absorbed by solid solution formation with estimated compositions up to ScAlNiH0.5. At ~500 °C hydrogen was absorbed by disproportionation of ScAlNi into ScH2 and AlNi. The reaction was found to be fully reversible due to destabilization effects which lowered the decomposition temperature of ScH2 by ~460 °C.

<p>Hydrogen is abundant, uniformly distributed throughout the Earth's surface and its oxidation product (water) is environmentally benign. Owing to these features, it is considered as an ideal synthetic fuel for a new world energetic matrix (renewable, secure and environmentally friendly) that could allow a sustainable future development. However, for this prospect to become a reality, efficient ways to produce, transport and store hydrogen still need to be developed. In the present thesis, theoretical studies of a number of potential hydrogen storage materials have been performed using density functional theory. In NaAlH₄ doped with 3d transition metals (TM), the hypothesis of the formation of Ti-Al intermetallic alloy as the main catalytic mechanism for the hydrogen sorption reaction is supported. The gateway hypothesis for the catalysis mechanism in TM-doped MgH₂ is confirmed through the investigation of MgH₂ nano-clusters. Thermodynamics of Li-Mg-N-H systems are analyzed with good agreement between theory and experiments. Besides chemical hydrides, the metal-organic frameworks (MOFs) have also been investigated. Li-decorated MOF-5 is demonstrated to possess enhanced hydrogen gas uptake properties with a theoretically predicted storage capacity of 2 wt% at 300 K and low pressure.</p><p>The metal-hydrogen systems undergo many structural and electronic phase transitions induced by changes in pressure and/or temperature and/or H-concentration. It is important both from a fundamental and applied viewpoint to understand the underlying physics of these phenomena. Here, the pressure-induced structural phase transformations of NaBH₄ and ErH₃ were investigated. In the latter, an electronic transition is shown to accompany the structural modification. The electronic and optical properties of the low and high-pressure phases of crystalline MgH₂ were calculated. The temperature-induced order-disorder transition in Li₂NH is demonstrated to be triggered by Li sub-lattice melting. This result may contribute to a better understanding of the important solid-solid hydrogen storage reactions that involve this compound. </p>

Hydrogen is abundant, uniformly distributed throughout the Earth's surface and its oxidation product (water) is environmentally benign. Owing to these features, it is considered as an ideal synthetic fuel for a new world energetic matrix (renewable, secure and environmentally friendly) that could allow a sustainable future development. However, for this prospect to become a reality, efficient ways to produce, transport and store hydrogen still need to be developed. In the present thesis, theoretical studies of a number of potential hydrogen storage materials have been performed using density functional theory. In NaAlH4 doped with 3d transition metals (TM), the hypothesis of the formation of Ti-Al intermetallic alloy as the main catalytic mechanism for the hydrogen sorption reaction is supported. The gateway hypothesis for the catalysis mechanism in TM-doped MgH2 is confirmed through the investigation of MgH2 nano-clusters. Thermodynamics of Li-Mg-N-H systems are analyzed with good agreement between theory and experiments. Besides chemical hydrides, the metal-organic frameworks (MOFs) have also been investigated. Li-decorated MOF-5 is demonstrated to possess enhanced hydrogen gas uptake properties with a theoretically predicted storage capacity of 2 wt% at 300 K and low pressure. The metal-hydrogen systems undergo many structural and electronic phase transitions induced by changes in pressure and/or temperature and/or H-concentration. It is important both from a fundamental and applied viewpoint to understand the underlying physics of these phenomena. Here, the pressure-induced structural phase transformations of NaBH4 and ErH3 were investigated. In the latter, an electronic transition is shown to accompany the structural modification. The electronic and optical properties of the low and high-pressure phases of crystalline MgH2 were calculated. The temperature-induced order-disorder transition in Li2NH is demonstrated to be triggered by Li sub-lattice melting. This result may contribute to a better understanding of the important solid-solid hydrogen storage reactions that involve this compound.

Ni - MH batteries have superior properties which are long cycle life, low maintenance, high power, light weight, good thermal performance and configurable design. Hydrogen storage alloys play a dominant role in power service life of a Ni - MH battery and determining the electrochemical properties of the battery. LaNi5, belonging to the CaCu5 crystal structure type, satisfy many of the properties. The most important property of LaNi5 is fast hydrogen kinetics. Recently, CaNi5, belonging to same crystal type, has taken some attention due to its low cost, higher hydrogen storage capacity, good kinetic properties. However, the main restriction of its use is its very low cycle life. The aim of the study is to obtain a more stable structure providing higher cycle life by the addition of different alloying elements. In this study, the effect of sixteen alloying elements (Mn, Sm, Sn, Al, Y, Cu, Si, Zn, Cr, Mg, Fe, Dy, V, Ti, Hf and Er) on cycle life was investigated. Sm, Y, Dy, Ti, Hf and Er were added for replacement of Ca and Mn, Sn, Al, Cu, Si, Zn, Cr, Mg, Fe and V were added for replacement of Ni. Alloys were produced by vacuum casting and heat treating followed by ball milling. The cells assembled, using the produced active materials as anode, which were cycled for charging and discharging. As a result, replacement of Ca with Hf, Ti, Dy and Er, and replacement of Ni with Si and Mn were observed to show better cycle durability rather than pure CaNi5.

Density functional theory has been used to investigate cathode materials for rechargeable batteries, carbon nanotube interactions with catalyst particles and transition metal catalyzed hydrogen release in magnesium hydride nanoclusters. An effort has been made to the understand structural and electrochemical properties of lithium iron silicate (Li2FeSiO4) and its manganese-doped analogue. Starting from the X-ray measurements, the crystal structure of Li2FeSiO4 was refined, and several metastable phases of partially delithiated Li2FeSiO4 were identified. There are signs that manganese doping leads to structural instability and that lithium extraction beyond 50% capacity only occurs at impractically high potentials in the new material. The chemical interaction energies of single-walled carbon nanotubes and nanoclusters were calculated. It is found that the interaction needs to be strong enough to compete with the energy gained by detaching the nanotubes and forming closed ends with carbon caps. This represents a new criterion for determining catalyst metal suitability. The stability of isolated carbon nanotube fragments were also studied, and it is argued that chirality selection during growth is best achieved by exploiting the much wider energy span of open-ended carbon nanotube fragments. Magnesium hydride nanoclusters were doped with transition metals Ti, V, Fe, and Ni. The resulting changes in hydrogen desorption energies from the surface were calculated, and the associated changes in the cluster structures reveal that the transition metals not only lower the desorption energy of hydrogen, but also seem to work as proposed in the gateway hypothesis of transition metal catalysis.

In this thesis, we use first-principles calculations to study the structural, electronic, and thermal properties of several complex hydrides. We investigate structural and electronic properties of Na-Li alanates. Although Na alanate can reversibly store H with Ti catalyst, its weight capacity needs to be improved. This can be accomplished by partial replacement of Na with lighter elements. We explore the structures of possible Na-Li alloy alanates, and study their phase stability. We also study the structural and thermal properties of Li/Mg/Li-Mg Amides/Imides. Current experimental results give a disordered model about the structure of Li-Mg Imide, in which the positions of Li and Mg are not specified. In addition the model gives a controversial composition stoichiometry. We try to resolve this controversy by searching for low-energy ordered phases. In the last part, we study the structural, energetic, and electronic properties of the La-Mg-Pd-H system. This quaternary system is another example of hydrogenation-induced metal-nonmetal transition without major reconstruction of metal host structure, and it is also with partial reversible H capacity. Experiment gives partially disordered H occupancy on two Wyckoff positions. Our calculation explains the structural and bonding characteristics observed in experiment.