Advanced Ti – based AB and AB2 hydride forming materials
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Date
2011
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Publisher
University of the Western Cape
Abstract
Ti – based AB and AB₂ hydride forming materials have shown to be very promising hydrogen storage alloys due to their reasonable reversible hydrogen storage capacity at near ambient conditions, abundance and low cost. However, these materials are not used extensively due to their poor activation performances and poisoning tolerance, resulting insignificant impeding of hydrogen sorption. The overall goal of this project was to develop the knowledge base for solid-state hydrogen storage technology suitable for stationary and special vehicular applications focussing mainly on Ti – based metal hydrides. In order to accomplish this goal, the project had a dual focus which included the synthesis methodology of Ti – based AB and AB₂ materials and the development of new surface engineering solutions, based on electroless plating and chemical vapour deposition on the surface modification of Ti – based metal hydride forming materials using Pd-based catalytic layers. TiFe alloy was synthesised by sintering of the Ti and Fe powders and by arc-melting. Sintered samples revealed three phases: TiFe (major), Ti₄Fe₂O, and β-Ti. Hydrogen absorption showed that the sintered material was almost fully activated after the first vacuum heating (400 °C) when compared to the arc-melted sample requiring several activation cycles. The increase in the hydrogen absorption kinetics of the sintered sample was associated with the influence of the formed hydrogen transfer catalyst, viz. oxygen containing Ti₄Fe₂O₁₋ₓ and β-Ti, which was confirmed by the XRD data from the samples before and after hydrogenation. The introduction of oxygen impurity into TiFe alloy observed in the sintered sample significantly influenced on its PCT performances, due to formation of stable hydrides of the impurity phases, as well as destabilisation of both β-TiFeH and, especially, γ-TiFeH₂. This finally resulted in the decrease of the reversible hydrogen storage capacity of the oxygen-contaminated sample. TiFe alloy was also prepared via induction melting using graphite and alumo-silica crucibles. It was shown that the samples prepared via the graphite crucible produced TiFe alloy as the major phase, whereas the alumo-silica crucible produced Ti₄Fe₂O₁-x and TiFe₂ as the major phases, and TiFe alloy as the minor one. A new method for the production of TiFe – based materials by two-stage reduction of ilmenite (FeTiO₃) using H₂ and CaH₂ as reducing agents was developed. The reversible hydrogen absorption performance of the TiFe – based material prepared via reduction of ilmenite was 0.5 wt. % H, although hydrogen absorption capacity of TiFe reported in the literature should be about 1.8 wt. %. The main reason for this low hydrogen capacity is due to large amount of oxygen present in the as prepared TiFe alloy. Thus to improve the hydrogen absorption of the raw TiFe alloy, it was melted with Zr, Cr, Mn, Ni and Cu to yield an AB₂ alloy. For the as prepared AB₂ alloy, the reversible hydrogen sorption capacity was about 1.3 wt. % H at P=40 bar and >1.8 wt.% at P=150 bar, which is acceptable for stationary applications. Finally, the material was found to be superior as compared to known AB₂-type alloys, as regards to its poisoning tolerance: 10-minutes long exposure of the dehydrogenated material to air results in a slight decrease of the hydrogen absorption capacity, but almost does not reduce the rate of the hydrogenation. Hydrogen storage performance of the TiFe-based materials suffers from difficulties with hydrogenation and sensitivity towards impurities in hydrogen gas, reducing hydrogen uptake rates and decreasing the cycle stability. An efficient solution to this problem is in modification of the material surface by the deposition of metals (including Palladium) capable of catalysing the dissociative chemisorption of hydrogen molecules. In this work, the surface modification of TiFe alloy was performed using autocatalytic deposition using PdCl₂ as the Pd precursor and metal-organic chemical vapour deposition technique (MO CVD), by thermal decomposition of palladium (II) acetylacetonate (Pd[acac]₂) mixed with the powder of the parent alloy. After surface modification of TiFe – based metal hydride materials with Pd, the alloy activation performance improved resulting in the alloy absorbing hydrogen without any activation process. The material also showed to absorb hydrogen after exposure to air, which otherwise proved detrimental.
Description
Doctor Scientiae
Keywords
Scanning electron microscopy, solid-state hydrogen storage technology, Liquid hydrogen, Hydrogen--Storage