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

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

Thermodynamic and kinetic properties of calcium hydride

Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH2), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH2. In this work, a literature review of the wide-ranging thermodynamic properties of CaH2 is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH2. The thermodynamic values of hydrogen release from both molten and solid CaH2 were determined as ΔHdes (molten CaH2) = 216 ± 10 kJ mol−1.H2, ΔSdes (molten CaH2) = 177 ± 9 J K−1 mol−1.H2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH2 of 947 ± 65 °C. Similarly, in the solid-state: ΔHdes (solid CaH2) = 172 ± 12 kJ mol−1.H2, ΔSdes (solid CaH2) = 144 ± 10 J K−1 mol−1.H2. Moreover, the activation energy of hydrogen release from CaH2 was also calculated using DSC analysis as Ea = 203 ± 12 kJ mol−1. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material

Barium carbonate and barium titanate for ultra-high temperature thermochemical energy storage

The significance of energy storage should not be underestimated in enabling the growth of renewables on the path towards decarbonisation. In this research, a novel ultra-high temperature reactive carbonate composite, 2BaCO3:TiO2, is introduced. Upon heating, the composite initially forms a mixture of BaCO3:BaTiO3, which on further heating reacts to form Ba2TiO4 and CO2 in a reversible thermochemical reaction. The enthalpy and entropy of the carbonation reaction involving Ba2TiO4 were determined manometrically to be ∆H = 295 ± 9 kJ∙mol−1 of CO2 and ∆S = 214 ± 7 J∙K−1∙mol−1 of CO2, respectively. The CO2 cycling capacity of the composite was evaluated using a Sieverts apparatus and thermogravimetric analysis, and sintering was identified as a potential cause of capacity loss. The addition of nickel was employed to mitigate the effect of sintering, resulting in a stable reversible capacity of up to 50 % of the theoretical maximum. The composite's cyclic capacity retention, low cost, and high energy storage density make it a promising candidate for energy storage applications at ≈ 1100 °C, although improvement to the cyclic capacity would lead to a more favourable application potential

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

Thermochemical energy storage in SrCO3 composites with SrTiO3 or SrZrO3

Thermochemical energy storage offers a cost-effective and efficient approach for storing thermal energy at high temperature (∼1100 °C) for concentrated solar power and large-scale long duration energy storage. SrCO3 is a potential candidate as a thermal energy storage material due to its high energy density of 205 kJ/mol of CO2 during reversible CO2 release and absorption. However, it loses cyclic capacity rapidly due to sintering. This study determined that the cyclic capacity of SrCO3 was enhanced by the addition of either reactive SrTiO3 or inert SrZrO3, where the molar ratios of SrCO3 to SrZrO3 were varied from 1:0.125 to 1:1. Thermogravimetric analysis over 15 CO2 sorption cycles demonstrated that both materials retained ∼80 % of their maximum cyclic capacity on the milligram scale. Repeated measurements using gram scale samples revealed a decrease in maximum capacity to 11 % using a sample of SrCO3 – 0.5 SrZrO3 over 53 cycles, while the use of SrTiO3 additives allowed for the retention of 80 % maximum capacity over 55 cycles. These findings highlight the potential of reactive additives in enhancing the performance of thermochemical energy storage systems, while providing valuable insights for the development of cost-effective materials

Ammonia-induced precipitation of zirconyl chloride and zirconyl-yttrium chloride solutions under industrially relevant conditions

The influence of concentration and added chloride salts on the solution speciation of zirconyl chloride solutions, and the precipitate formed upon addition of aqueous ammonia, has been investigated. Crystalline zirconium oxychloride octahydrate samples available on an industrial scale were investigated using ICP-OES, XRD and SEM. The samples had a remarkably consistent level of the trace elements and LOI and contained approximately 2 wt.% hafnium. Zirconyl chloride solutions at industrially relevant concentrations of 0.81 and 1.62 M were studied by small angle X-ray scattering, and the particle radii were found to be unchanged within experimental error. Yttrium–zirconium mixed solutions relevant to the Solid Oxide Fuel Cell market (containing 3, 5, 8 and 10 mole% yttrium) were also investigated, and it was found that the added yttrium did not significantly change the particle radii or particle-particle distances.Solutions at the same concentrations were then precipitated using a continuous double jet precipitation apparatus with aqueous ammonia as the base. Using DLS it was found that the zirconyl chloride solution at higher concentration yielded a larger precipitated particle size (1.0 ± 0.1 μm, 4.2 ± 0.1 μm). The yttrium–zirconium mixed solutions were found to give a consistent increase in particle size with increasing yttrium levels (2.0 ± 0.1 μm, 3.7 ± 0.2 μm, 4.5 ± 0.1 μm and 4.9 ± 0.2 μm). To investigate if the growth effect was most influenced by the cations or the increasing chloride concentrations, sample solutions containing mixtures of caesium chloride/zirconyl chloride and calcium chloride/zirconyl chloride with the same concentration of added chloride anions as that of the 8 mole% yttrium–zirconium sample were precipitated. The increased particle size was found to be most dependent on the type of cation and did not appear to be as significantly dependent on the concentration of chloride ions

Novel solvates M(BH4)3S(CH3)2 and properties of halide-free M(BH4)3 (M = Y or Gd)

Rare earth metal borohydrides have been proposed as materials for solid-state hydrogen storage because of their reasonably low temperature of decomposition. New synthesis methods, which provide halide-free yttrium and gadolinium borohydride, are presented using dimethyl sulfide and new solvates as intermediates. The solvates M(BH4)3S(CH3)2 (M = Y or Gd) are transformed to a-Y(BH4)3 or Gd(BH4)3 at ~140 °C as verified by thermal analysis. The monoclinic structure of Y(BH4)3S(CH3)2, space group P21/c, a = 5.52621(8), b = 22.3255(3), c = 8.0626(1) Å and ß = 100.408(1)°, is solved from synchrotron radiation powder X-ray diffraction data and consists of buckled layers of slightly distorted octahedrons of yttrium atoms coordinated to five borohydride groups and one dimethyl sulfide group. Significant hydrogen loss is observed from Y(BH4)3 below 300 °C and rehydrogenation at 300 °C and p(H2) = 1550 bar does not result in the reformation of Y(BH4)3, but instead yields YH3. Moreover, composites systems Y(BH4)3–LiBH4 1 : 1 and Y(BH4)3–LiCl 1 : 1 prepared from as-synthesised Y(BH4)3 are shown to melt at 190 and 220 °C, respectively

Hydrogenation properties of lithium and sodium hydride – closo-borate, [B10H10]2− and [B12H12]2−, composites

© 2018 the Owner Societies. The hydrogen absorption properties of metal closo-borate/metal hydride composites, M2B10H10-8MH and M2B12H12-10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H2) = 400 bar and by ex situ hydrogen absorption measurements at p(H2) = 526 to 998 bar. The in situ experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH4) are formed. On the contrary, the M2B12H12-10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H2) = 537 to 970 bar.11B MAS NMR of the M2B10H10-8MH composites reveal that the molar ratio of LiBH4or NaBH4and the remaining B species is 1:0.63 and 1:0.21, respectively. Solution and solid-state11B NMR spectra reveal new intermediates with a B:H ratio close to 1:1. Our results indicate that the M2B10H10(M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B12H12]2-composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage