Hydrogen storage studies of mesoporous and titanium based materials

Concerns over green house gas emissions and their climate change effects have lead to a concerted effort into environmental friendly technologies. One such emphasis has been on the implementation of the hydrogen economy. There are four major impediments to the implementation of a hydrogen economy: hydrogen production, distribution, storage and conversion. This thesis is focused on exploring the hydrogen storage problem. Hydrogen can be stored by a wide range of methods. One of these methods involves using a secondary material that stores hydrogen by either physisorbing hydrogen onto its surfaces or by reacting with it to form a new compound. Of the wide variety of materials that can interact with hydrogen, three different materials were chosen; (1) nano-structured materials of high surface area; mesoporous silica (MCM-41) and titanate nanotubes, and (2) hydrides of Ti-Mg-Ni alloys. Results of the hydrogen on mesoporous silica (MCM-41) showed 1 wt.% H[subscript]2 to a maximum of 2 wt.% H[subscript]2 for 500 to 1060 m2/g surface area, respectively, at 77 K. Doping these samples with Al or Zn did not make an appreciable difference but rather they reduced the surface area available for hydrogen adsorption. Adorption of hydrogen at room temperature was neglifible (0.1 wt.% up to an equilibrium pressure of 5 MPa). Sodium titanate nanotubes showed hydrogen adsorption that increased with increasing hydrogen pressure at 77 K. Hydrogen adsorption reached 0.4 wt.% at an hydrogen equilibrium pressure of 2.6 MPa. Exchange of sodium ions in the titanate nanotubes with Zn and Li did not have an impact on hydrogen adsorption.However, partial substitution of Na ions for H ions resulted in an increase in hydrogen adsorption from 0.4 wt.% to 0.8 wt.% while decreasing the pressure required for maximum hydrogen uptake from 2.6 MPa to 0.5 MPa at 77 K. Desorption from this sample also showed strong hysteresis indicating hydrogen adsorption into the interlayer spacing of the nanotube wall. Hydrogen adsorption at room temperature was negligible for all samples being below 0.1 wt.%, up to a hydrogen equilibrium pressure of 5 MPa. Ti-Mg-Ni alloys are interest as 11 wt.% hydrogen has been reported in the literature; specifically for Ti53Mg47Ni20. Samples with various stoichiometries of Ti, Mg and Ni were produced via balling and their hydrogen sorption properties examined. Measured hydrogen absorption ranged from 2.5 wt.% to 5.0 wt.%. Measurements were hindered by the high temperature (723 K) used during the activation process. The high temperature ensured decomposition of titanium hydride but resulted in the vaporisation and deposition of magnesium on the sample cell filter. This had the duel effect of reducing the total hydrogen absorption and to sporadically block the sample cell filter. However, in those cases where the hydrogen flow was not impeded, absorption kinetics were measured to be extremely rapid. For example, greater than 95 % of the total hydrogen uptake of 3.7 wt.% for the sample ball-milled in the molar ratio of 65:133:20 (Ti:Mg:Ni) occurred within 60 seconds at room temperature.However, the low equilibrium pressure meant a negligible amount of hydrogen could be desorbed at this temperature. X-ray diffraction revealed that after hydriding, the samples comprised varius mixtures of MgH[subscript]2, TiH[subscript]2 and hydrides of the intermetallic compounds Mg[subscript]2Ni and Ti[subscript]2Ni. The amount of each of these hydride phases changed according the intial starting stoichiometries of each sample

The Mechanochemical synthesis of magnesium hydride nanoparticles

A mechanochemical method was used to synthesise magnesium hydride nanoparticles with an average crystallite size of 6.7 nm. The use of a reaction buffer was employed as a means of particle size control by restricting agglomeration. Increasing the amount of reaction buffer resulted in a decrease in crystallite size, as determined via X-ray diffraction, and a decrease in particle size, evidenced by transmission electron microscopy

Characterisation of mechanochemically synthesised alane (AlH3) nanoparticles

A mechanochemical synthesis process has been used to synthesise alane (AlH3) nanoparticles. The alane is synthesised via a chemical reaction between lithium alanate (LiAlH4) and aluminium chloride (AlCl3) at room temperature within a ball mill and at 77K within a cryogenic mill. The reaction product formed consists of alane nanoparticles embedded within a lithium chloride (LiCl) by-product phase. The LiCl is washed with a solvent resulting in alane nanoparticles which are separated from the by-product phase but are kinetically stabilised by an amorphous particle surface layer. The synthesis of a particular alane structural phase is largely dependent on the milling conditions and two major phases (α, α′) as well as two minor phases (β, γ) have been identified. Ball milling at room temperature can also provide enough energy to allow alane to release hydrogen gas and form aluminium metal nanoparticles. A comparison between XRD and hydrogen desorption results suggest a non-crystalline AlH3 phase is present in the synthesised samples

Hydrogen Desorption Properties of Bulk and Nanoconfined LiBH4-NaAlH4

Nanoconfinement of 2LiBH4-NaAlH4 into a mesoporous carbon aerogel scaffold with a pore size, BET surface area and total pore volume of Dmax = 30 nm, SBET = 689 m2/g and Vtot = 1.21 mL/g, respectively is investigated. Nanoconfinement of 2LiBH4-NaAlH4 facilitates a reduction in the temperature of the hydrogen release by 132 °C, compared to that of bulk 2LiBH4-NaAlH4 and the onset of hydrogen release is below 100 °C. The reversible hydrogen storage capacity is also significantly improved for the nanoconfined sample, maintaining 83% of the initial hydrogen content after three cycles compared to 47% for that of the bulk sample. During nanoconfinement, LiBH4 and NaAlH4 reacts to form LiAlH4 and NaBH4 and the final dehydrogenation products, obtained at 481 °C are LiH, LiAl, AlB2 and Al. After rehydrogenation of the nanoconfined sample at T = 400 °C and p(H2) = 126 bar, amorphous NaBH4 is recovered along with unreacted LiH, AlB2 and Al and suggests that NaBH4 is the main compound that can reversibly release and uptake hydrogen

Hydrogen storage properties of nanoconfined LiBH4-NaBH4

In this study a eutectic melting composite of 0.62LiBH4-0.38NaBH4 has been infiltrated in two nanoporous resorcinol formaldehyde carbon aerogel scaffolds with similar pore sizes (37 and 38 nm) but different BET surface areas (690 and 2358 m2/g) and pore volumes (1.03 and 2.64 mL/g). This investigation clearly shows decreased temperature of hydrogen desorption, and improved cycling stability during hydrogen release and uptake of bulk 0.62LiBH4-0.38NaBH4 when nanoconfined into carbon nanopores. The hydrogen desorption temperature of bulk 0.62LiBH4-0.38NaBH4 is reduced by ~107 °C with the presence of carbon, although a minor kinetic variation is observed between the two carbon scaffolds. This corresponds to apparent activation energies, EA, of 139 kJ mol-1 (bulk) and 116-118 kJ mol-1 (with carbon aerogel). Bulk 0.62LiBH4-0.38NaBH4 has poor reversibility during continuous hydrogen release and uptake cycling, maintaining 22% H2 capacity after four hydrogen desorptions (1.6 wt.% H2). In contrast, nanoconfinement into the high surface area carbon aerogel scaffold significantly stabilizes the hydrogen storage capacity, maintaining ~70% of the initial capacity after four cycles (4.3 wt.% H2)

New directions for hydrogen storage: Sulphur destabilised sodium aluminium hydride

Aluminium sulphide (Al2S3) is predicted to effectively destabilise sodium aluminium hydride (NaAlH4) in a single-step endothermic hydrogen release reaction. The experimental results show unexpectedly complex desorption processes and a range of new sulphur containing hydrogen storage materials have been observed. The NaAlH4-Al 2S3 system releases a total of 4.9 wt% of H2 that begins below 100°C without the need for a catalyst. Characterisation via temperature programmed desorption, in situ synchrotron powder X-ray diffraction, ex situ x-ray diffraction, ex situ Fourier transform infrared spectroscopy and hydrogen sorption measurements reveal complex decomposition processes that involve multiple new sulphur-containing hydride compounds. The system shows partial H2 reversibility, without the need for a catalyst, with a stable H2 capacity of ~1.6 wt% over 15 cycles in the temperature range of 200°C to 300°C. This absorption capacity is limited by the need for high H2 pressures (>280 bar) to drive the absorption process at the high temperatures required for reasonable absorption kinetics. The large number of new phases discovered in this system suggests that destabilisation of complex hydrides with metal sulphides is a novel but unexplored research avenue for hydrogen storage materials

A synthesis method for cobalt doped carbon aerogels with high surface area and their hydrogen storage properties

Carbon aerogels doped with nanoscaled Co particles were prepared by first coating activated carbon aerogels using a wet-thin layer coating process. The resulting metal-doped carbon aerogels had a higher surface area (1667 m2 g-1) and larger micropore volume (0.6 cm3 g-1) than metal-doped carbon aerogels synthesised using other methods suggesting their usefulness in catalytic applications. The hydrogen adsorption behaviour of cobalt doped carbon aerogel was evaluated, displaying a high w4.38 wt.% H2 uptake under 4.6 MPa at -196 C. The hydrogen uptake capacity with respect to unit surface area was greater than for pure carbon aerogel and resulted in 1.3 H2 (wt. %) per 500 m2 g-1. However, the total hydrogen uptake was slightly reduced as compared to pure carbon aerogel due to a small reduction in surface area associated with cobalt doping. The improved adsorption per unit surface area suggests that there is a stronger interaction between the hydrogen molecules and the cobalt doped carbon aerogel than for pure carbon aerogel

Impact of soft contact lens edge design and mid-peripheral lens shape on the epithelium and its indentation with lens mobility

Purpose. To evaluate the influence of soft contact lens midperipheral shape profile and edge design on the apparent epithelial thickness and indentation of the ocular surface with lens movement. Methods. Four soft contact lens designs comprising of two different plano midperipheral shape profiles and two edge designs (chiseled and knife edge) of silicone-hydrogel material were examined in 26 subjects aged 24.7 ± 4.6 years, each worn bilaterally in randomized order. Lens movement was imaged enface on insertion, at 2 and 4 hours with a high-speed, high-resolution camera simultaneous to the cross-section of the edge of the contact lens interaction with the ocular surface captured using optical coherence tomography (OCT) nasally, temporally, and inferiorly. Optical imaging distortions were individually corrected for by imaging the apparent distortion of a glass slide surface by the removed lens. Results. Apparent epithelial thickness varied with edge position (P < 0.001). When distortion was corrected for, epithelial indentation decreased with time after insertion (P = 0.010), changed after a blink (P < 0.001), and varied with position on the lens edge (P < 0.001), with the latter being affected by midperipheral lens shape profile and edge design. Horizontal and vertical lens movement did not change with time postinsertion. Vertical motion was affected by midperipheral lens shape profile (P < 0.001) and edge design (P < 0.001). Lens movement was associated with physiologic epithelium thickness for lens midperipheral shape profile and edge designs. Conclusions. Dynamic OCT coupled with high-resolution video demonstrated that soft contact lens movement and image-corrected ocular surface indentation were influenced by both lens edge design and midperipheral lens shape profiles

Novel synthesis of porous aluminium and its application in hydrogen storage

A novel approach for confining LiBH4 within a porous aluminium scaffold was applied in order to enhance its hydrogen storage properties, relative to conventional techniques for confining complex hydrides. The porous aluminium scaffold was fabricated by sintering NaAlH4, which was in the form of a dense pellet, under dynamic vacuum. The final product was a porous aluminium scaffold with the Na and H2 having been removed from the initial pellet. This technique contributed to achieving highly dispersed LiBH4 particles that were also destabilised by the presence of the aluminium scaffold. In this study, the effectiveness of this novel fabrication method of confined/destabilised LiBH4 was extensively investigated, which aimed to simultaneously improve the hydrogen release at lower temperature and the kinetics of the system. These properties were compared with the properties of other confined LiBH4 samples found in the literature. As-synthesised samples were characterised using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Nitrogen Adsorption measurements. The hydrogen storage capacity of all samples was analysed using temperature programmed desorption in order to provide a comprehensive survey of their hydrogen desorption properties. The porous aluminium scaffold has a wide pore size distribution with most of the porosity due to pores larger than 50 nm. Despite this the onset hydrogen desorption temperature (Tdes) of the LiBH4 infiltrated into the porous aluminium scaffold was 200 °C lower than that of bulk LiBH4 and 100 °C lower than that of nanosized LiBH4. Partial cycling could be achieved below the melting point of LiBH4 but the kinetics of hydrogen release decreased with cycle number