McGrath, Alexander John
(2025)
Synthesis and characterisation of metal alloys for hydrogen storage and related applications.
PhD thesis, University of Nottingham.
Abstract
This thesis investigates metal alloys for solid-state hydrogen storage, specifically the AB2 family, focusing on the methods used to synthesise hydrogen storage alloys, characterisation of the microstructures present, and the
hydrogen storage properties relevant for stationary (energy storage and hydrogen compression) and mobile (light-duty passenger vehicles) applications. Throughout the thesis, the relationship between the processes used to synthesise a metal alloy, the observed microstructures and their effect on the measured hydrogen storage properties plays a critical role, and is frequently assessed and described. Metal alloys offer a safer, and more efficient mode of hydrogen storage compared to well-established storage methods, such as compressed gas or liquid hydrogen. They typically operate at lower pressures (1-50 bar) and provide high energy densities (95-160 kg H2/m3), however the main issue surrounding the use of metal alloys is the low gravimetric capacity that is usable in moderate conditions (1-2 wt.% at ca. 30◦C). Of the metal alloy families studied for solid-state hydrogen storage, the AB2 family offers the best hydrogen storage properties in moderate conditions (20-30 ◦C), such as gravimetric storage capacity (1.5-2 wt.%) and plateau pressure (5-50 bar). Many substitutions are available to fine-tune alloy properties, which could enable their use for a range of applications.
An in-depth understanding of the binary Ti-Cr AB2 system was developed, primarily to provide a basis for future elemental substitutions into the alloy composition, but to also improve upon the properties reported for AB2 compounds based upon the Ti-Cr system. Alloys that contained more Ti (TiCr1.55) were found to absorb a greater amount of hydrogen (1.50 wt.% at 253 K) with faster absorption kinetics, and had a lower plateau pressure (9.4 bar) and slope factor (2.58). This was due to the substitution of Ti into B-sites in the Laves phase, which increased the unit cell size, allowing more hydrogen to be stored at lower pressures.
Annealing of a Ti-Cr composition (TiCr1.64) produced a single C15 Laves phase whilst significantly reducing the presence of minor Ti and Cr single phases, but also reduced the hydrogen storage capacity, due to the increased amount of oxygen present after annealing, as well as the diffusion of residual Cr in the alloy into the C15 Laves phase. The properties of binary Ti-Cr AB2 alloys, such as a gravimetric storage capacity of 1.5 wt.% at 253 K for the TiCr1.55 alloy, show that the alloys could be used in low-temperature
applications, such as refrigerated transport.
Substitutions into the Ti-Cr AB2 system, including Zr, Mn, Fe and V were carried out to produce multicomponent alloys that would have improved hydrogen storage properties in moderate conditions (25-30 ◦C). These substitutions produced alloys with a higher gravimetric storage capacity at
room temperature (1.71 wt.% for a TiZrCrMnFe AB2 composition at 298 K), as well as lower values of plateau pressure and slope. The kinetics of absorption and the activation for multicomponent alloys were hindered, which was due to the use of elements in the alloy composition that oxidise easily
(Mn, Fe, V). The properties of the TiZrCrMnFe alloys improved upon the commercially available hydrogen storage alloy Hydralloy C5, where a higher gravimetric storage capacity was achieved without the use of V, which is more expensive than all of the constituent elements of the TiZrCrMnFe alloys. The multicomponent alloys reported were found to be suitable for supplying hydrogen to fuel cells, as well as hydrogen compression, where the multicomponent alloy could be used in the first stage of compression.
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