[show abstract][hide abstract] ABSTRACT: A series of monometallic borohydrides and borohydride eutectic mixtures have been investigated during thermal ramping by mass spectroscopy, differential scanning calorimetry, and photography. Mixtures of LiBH4-NaBH4, LiBH4-KBH4, LiBH4-Mg(BH4)2, LiBH4-Ca(BH4)2, LiBH4-Mn(BH4)2, NaBH4-KBH4, and LiBH4-NaBH4-KBH4 all displayed melting behaviour below that of the monometallic phases (up to 167 °C lower). Generally, each system behaves differently with respect to their physical behaviour upon melting. The molten phases can exhibit colour changes, bubbling and in some cases frothing, or even liquid-solid phase transitions during hydrogen release. Remarkably, the eutectic melt can also allow for hydrogen release at temperatures lower than that of the individual components. Some systems display decomposition of the borohydride in the solid-state before melting and certain hydrogen release events have also been linked to the adverse reaction of samples with impurities, usually within the starting reagents, and these may also be coupled with bubbling or frothing of the ionic melt.
Physical Chemistry Chemical Physics 10/2013; · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The thermal decomposition of anhydrous Pa3[combining macron] Li2B12H12 was studied in situ by high resolution synchrotron X-ray diffraction. A first-order phase transition can be observed at 355 °C where the unit cell volume expands by ca. 8.7%. The expanded β-Li2B12H12 polymorph simultaneously decomposes to a hydrogen poor γ-Li2B12H12-x phase. Expansion of the unit cell across the discontinuity is consistent with reorientational motion of B12H12(2-) anions, and the presence of a frustrated Li(+) lattice indicating Li ion conduction.
Physical Chemistry Chemical Physics 08/2013; · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The purpose of this study is to compare the thermal and structural stability of single phase Li2B12H12 with the decomposition process of LiBH4. We have utilized differential thermal analysis/thermogravimetry (DTA/TGA) and temperature programmed desorption-mass spectroscopy (TPD-MS) in combination with X-ray diffraction (XRD) and Fourier Transform Infra-Red (FTIR) spectroscopy to study the decomposition products of both LiBH4 and Li2B12H12 up to 600°C, under both vacuum and hydrogen (H2) backpressure. We have synthesized highly pure single phase crystalline anhydrous Li2B12H12 (Pa-3 structure type), and studied its sensitivity to water and the process of deliquescence. Under either vacuum or H2 backpressure, after 250°C, anhydrous Li2B12H12 begins to decompose to a sub-stoichiometric Li2B12H12-x composition, which displays a very broad diffraction halo in the d-spacing range 5.85- 7.00 Å, dependent on the amount of H released. Ageing Pa-3 Li2B12H12 under 450°C/125 bar H2 pressure for 24 hours produces a previously unobserved well-crystallized beta-Li2B12H12 polymorph, and a nanocrystalline gamma-Li2B12H12 polymorph. The isothermal release of hydrogen pressure from LiBH4 along the plateau and above the melting point (Tm = 280°C) initially results in the formation of LiH and gamma-Li2B12H12. The gamma-Li2B12H12 polymorph then decomposes to a sub-stoichiometric Li2B12H12-x composition. The Pa-3 Li2B12H12 phase is not observed during LiBH4 decomposition. Decomposition of LiBH4 under vacuum to 600°C produces LiH and amorphous B with some Li dissolved within it. The lack of an obvious B-Li-B or B-H-B bridging band in the FTIR data for Li2B12H12-x suggests the H poor B12H12-x pseudo-icosahedra remain isolated and are not polymerized. Li2B12H12-x is persistent to at least 600°C under vacuum, with no LiH formation observable, and only a ca. d = 7.00 Å halo remaining. By 650°C, Li2B12H12-x is finally decomposed and amorphous B can be observed, with no LiH reflections. Further studies are required to clarify the structural symmetry of the beta and gamma-Li2B12H12 polymorphs, and sub-stoichiometric Li2B12H12-x.
Journal of the American Chemical Society 04/2013; 135:6930. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: A simple mechanical milling and annealing process has been used to synthesize CaNi5-
based hydrogen storage alloys. Heat treatment at 800 �C under vacuum results in the
formation of a crystalline CaNi5 phase. Secondary phases, including Ca2Ni7 and MoeNi, are
formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in
the unit cell volume of the CaNi5 phase. The hydrogen storage capacity of all substituted
alloys is reduced and the plateau pressures are lower than those of pure CaNi5. Fairly flat
plateau regions are retained for all compositions except the CaNi4.8Mo0.2 composition
where a Ca2Ni7 phase is dominant. The incorporation of Mo also causes slow sorption
kinetics for the CaNi4.9Mo0.1 alloy. CaNi4.9Al0.1 maintains its initial hydrogen absorption
capacity for 20 cycles performed at 85 �C but the other substituted alloys lose their capacity
rapidly, especially the CaNi4.8Mo0.2 composition.
International Journal of Hydrogen Energy 01/2013; 38:2325. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: The hydrogen cycled (H) planetary milled (PM) NaAlH4 + xM (x < 0.1) system (M = 30 nm Ag, 80 nm Al, 2–3 nm C, 30 nm Cr, 25 nm Fe, 30 nm Ni, 25 nm Pd, 65 nm Ti) has been studied by high resolution synchrotron powder X-ray diffraction. Isothermal absorption kinetic isotherms have been measured over the first two H cycles. The PM NaAlH4 + 0.1Ti system has also been studied by high resolution transmission electron microscopy (TEM). 80 nm Al and 2–3 nm C were inactive, and would not allow hydrogen (H) desorption from NaAlH4. 30 nm Cr, 25 nm Fe, 30 nm Ni, and 25 nm Pd showed activity, but with weak kinetics of only ca. 1 wt.% H/hour. The NaAlH4 + 0.1Ti system displays absorption kinetics of ca. 7 wt.% H/hour, comparable to TiCl3 enhanced NaAlH4 after five H cycles. After H cycling the PM NaAlH4 + 0.1Ti system, we observe a body centred tetragonal (bct) χ-TiH2 phase, which displays intense anisotropic peak broadening. The broadening is evident as a massive dislocation density of ca. 1.20 × 1017/m2 in high resolution TEM images of the χ-TiH2 phase. All originally added Ti can be accounted for in the bct χ-TiH2 phase by quantitative phase analysis (QPA) after five H cycles. The PM NaH + Al + 0.02 (Ti-nano-alloy) system shows absorption kinetic rates in the order TiO2 > TiN > TiC > Ti, with rapid hydrogenation kinetics of ca. 23 wt.% H/hour for TiO2 enhanced NaAlH4, equivalent to TiCl3 enhanced NaAlH4. The TiN and TiC are partially reduced by ca. 7 and 22% respectively, and the TiO2 is completely reduced. The location of the reduced Ti cannot be discerned by X-ray diffraction at these minor proportions.
International Journal of Hydrogen Energy 10/2012; 37(20):15175–15186. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have studied the complex decomposition mechanism of cubic γ-Mg(BH4)2 (Ia3̅d, a = 15.7858(1) Å) by in-situ synchrotron X-ray diffraction, temperature-programmed desorption, visual observation of the melt, and Fourier transform infrared (FTIR) spectroscopy. The decomposition and release of hydrogen proceeds through eight distinct steps, including two polymorphic transitions before melting, with a new ε-Mg(BH4)2 phase at ca. 150 °C. After melting, strong changes in sample color from yellow to brown to gray are consistent with the unknown Mg–B–H phase(s) (that diffract with high d-spacing halos) in the sample changing from an average composition of MgB2H5.3 at 325 °C, to MgB2.9H3.2 at 350 °C, and to MgB4.0H3.7 by 450 °C. From 350 to 450 °C, the crystalline Mg proportion increases. No combination of previously assigned anionic BnHm species (including MgB12H12 and Mg(B3H8)2) can account for the average composition of the unknown proportion of the sample. This is supported by FTIR spectra showing an absence of terminal B–H resonances in the 2500 cm–1 region that are present for B12H12 and B3H8 anionic species. Our combined analysis strongly indicates the presence of as yet unidentified Mg–B–H phase(s) in postmelted decomposed Mg(BH4)2 samples.
The Journal of Physical Chemistry C 06/2012; 116(29):15231. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Increased reliance on solar energy conversion technologies will necessarily constitute a major plank of any forward global energy supply strategy. It is possible that solar photovoltaic (PV) technology and concentrating solar thermal (CST) power technology will play roughly equal, but complementary roles by 2050. The ability to increase reliance on CST power technology during this period, however, will be constrained by a number of factors: the large plant sizes dictated by economies of scale, the high associated transmission infrastructure investment cost, and the limitations of current thermal energy storage technologies. Thus, solar technology's main midterm role is seen to be as hybrid solar thermal power plant. The development of low-cost, high-temperature, high-energy density thermal energy storage systems is needed to enable CST plants to be dispatchable and accelerate the deployment of this technology. Thermochemical storage has the best potential to achieve these energy storage requirements and a brief overview of thermochemical energy storage options for CST plants points to high-temperature metal-hydride thermochemical heat energy storage systems. Hydrogen storage systems offer the highest energy storage capacity per volume and are therefore the most likely candidates for achieving the goal of fully dispatchable CST plants. A number of high-temperature metal-hydride thermochemical solar energy storage systems have been proposed and a small number of these systems are currently being investigated and developed. A key component of this work is matching the thermochemical metal-hydride system with a suitable “low-temperature” hydrogen storage material to produce systems that are self-regulating. A summary of the development status of these systems suggests that, despite the technical challenges associated with high-temperature thermochemical energy storage systems, their potential advantages are now seeing development occurring. Although in-
the early stages, their commercialisation could be fast tracked.
Proceedings of the IEEE 03/2012; · 6.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: CaNi5ebased alloys have been synthesized by mechanical alloying followed by isothermal
annealing. The formation of the CaNi5 structure occurred when the milled powders were
heated at 800 �C under vacuum for 3 h. The abundance of CaNi5 phase in the alloys ranges
from 60 to 70 wt.%. Replacement of Zr into the Ca site reduces the unit cell volume of CaNi5
whilst replacement of Cr into the Ni site slightly increases the unit cell volume. The
hydrogen storage capacity of all substituted alloys is decreased and the hydrogen sorption
plateau regions are narrowed compared to those of pure CaNi5. Substitution of Zr into the
Ca site extinguishes the flat plateau region unlike replacement of Cr into the Ni site where
a flat plateau is maintained. The reaction enthalpy DH for both absorption and desorption
are directly proportional to the unit cell volume of the alloys. The hydrogen storage
capacity of all alloys rapidly decays for the first 50 cycles at 85 �C followed by a more
gradual decline after 50 further cycles. The hydrogen storage capacity of the alloys after 200
cycles is in the range of 65-75% of the initial capacity.
International Journal of Hydrogen Energy 02/2012; 37:7586. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: The MgH2 þ 0.02Ti-additive system (additives ¼ 35 nm Ti, 50 nm TiB2, 40 nm TiC, <5 nm
TiN, 10 � 40 nm TiO2) has been studied by high-resolution synchrotron X-ray diffraction,
after planetary milling and hydrogen (H) cycling. TiB2 and TiN nanoparticles were synthesised
mechanochemically whilst other additives were commercially available. The
absorption kinetics and temperature programmed desorption (TPD) profiles have been
determined, and compared to the benchmark system MgH2 + 0.01Nb2O5 (20 nm). TiC and
TiN retain their structures after milling and H cycling. The TiB2 reflections appear
compressed in d-spacing, suggesting Mg/Ti exchange has occurred in the TiB2 structure.
TiO2 is reduced, commensurate with the formation of MgO, however, the Ti is not evident
anywhere in the diffraction pattern. The 35 nm Ti initially forms an fcc Mg47.5Ti52.5 phase
during milling, which then phase separates and hydrides to TiH2 and MgH2. At 300 �C, the
MgH2 + 0.02 (Ti, TiB2, TiC, TiN, TiO2) samples display equivalent absorption kinetics, which
are slightly faster than the MgH2 + 0.01Nb2O5 (20 nm) benchmark. All samples are
contaminated with MgO from the use of a ZrO2 vial, and display rapid absorption to ca. 90%
of capacity within 20 s at 300 �C. TPD profiles of all samples show peak decreases compared
to the pure MgH2 milled sample, with many peak profiles displaying bi-modal splitting. TPD
measurements on two separate instruments demonstrate that on a 30 min milling time
scale, all samples are highly inhomogenous, and samplings from the exact same batch of
milled MgH2 + 0.02Ti-additive can display differences in TPD profiles of up to 30 �C in peak
maxima. The most efficient Ti based additive cannot be discerned on this basis, and milling
times [ 30 min are necessary to obtain homogenous samples, which may lead to artefactual
benefits, such as reduction in diffusion distances by powder grinding or formation
of dense microstructure. For the hydrogen cycled MgH2 + 0.01Nb2O5 system, we observe
a face centred cubic Mg/Nb exchanged Mg0.165Nb0.835O phase, which accounts for ca. 60% of
the originally added Nb atoms.
International Journal of Hydrogen Energy 11/2011; 37:4227. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: Isotopic Mg11B2 has been deuterated at 400 °C and 800 bar, with the production of β-Mg(11BD4)2 observed by in situ neutron diffraction. A natural MgB2 sample has been deuterated under similar conditions and studied ex situ by high resolution X-ray synchrotron diffraction. In both cases, quantitative phase analysis (QPA) indicates a ca. 43% yield of the high temperature (β) phase, with the rest of the sample composed of unreacted MgB2 and Mg or MgD2. A joint refinement of the neutron and X-ray synchrotron data has been performed, yielding a final β-Mg(11BD4)2 structure in space group Fddd, with new D positions. Anisotropically broadened (odd, odd, odd) reflections are attributed to microstructural features, rather than antiphase boundaries. QPA of the isotopic sample indicates ca. 10% of B atoms are in a noncrystalline state. A broad feature is evident in the ex situ X-ray synchrotron data, covering a wide d-spacing range from ca. 3.80–5.45 Å, consistent with the formation of amorphous Mg(BD4)2 and amorphous B. For both samples, macroscopic fusing occurs, forming an extremely hard phase with a glassy black appearance, which is hydrogen impermeable and inhibits further formation of β-Mg(BH4)2. The fused surface regions of the sample have been studied by transmission (TEM) and scanning (SEM) electron microscopy. TEM studies show amorphous regions on the surface, consistent with amorphous B, and a Mg–B–O–H phase.
[show abstract][hide abstract] ABSTRACT: Magnesium silicide (Mg2Si) has attracted interest as a hydrogen storage material due to favorable thermodynamics (ΔHdesorption = 36 kJ/mol H2) for room temperature operation. To date, direct hydriding of Mg2Si under hydrogen gas to form MgH2 and Si has only been attempted at low pressure and has been hindered by poor kinetics of absorption. In this paper we study the dehydrogenation reaction with in-situ neutron powder diffraction and present results of our attempts to hydrogenate Mg2Si under both hydrogen and deuterium gas up to temperatures of 350 °C and pressures of 1850 bar. Even under these extreme absorption conditions Mg2Si does not absorb any measureable quantity of hydrogen or deuterium.Highlights► Poor hydrogen desorption kinetics from MgH2 & Si below 250 °C. ► Hydrogen absorption of Mg2Si kinetically limited and unfeasible below 1850 bar up to 350 °C. ► Hydrogenation of Mg2Si at low pressure and temperature will only be possible with highly effective catalysts.
International Journal of Hydrogen Energy 08/2011; 36(17):10779-10786. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: The incorporation of nanoscale Co particles (with sizes from a few nanometres) into porous carbon aerogels (CAs) was investigated. Elemental maps of the nanoscale metal particles embedded within CA were obtained using energy filtered transmission electron microscopy. The microstructure of Co doped carbon aerogels was further investigated using small angle X-ray scattering and nitrogen adsorption at 77 K. The isosteric heat of adsorption (QstQst) was investigated as a function of hydrogen uptake at temperatures from 77 K to 110 K over the pressure range of 0–0.25 MPa. The isosteric heat of adsorption at low H2 concentration for Co doped CA (9.0 kJ mol−1) was found to be higher than for pure CA (5.8 kJ mol−1).
International Journal of Hydrogen Energy 08/2011; 36(17):10855–10860. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H2, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H2 and −99.8 ± 0.2 J/mol H2/K for absorption and 25.4 ± 0.1 kJ/mol H2 and 108.5 ± 0.2 J/mol H2/K for desorption, indicating the presence of a significant hysteresis effect.
International Journal of Hydrogen Energy 07/2011; 36(13):7587-7593. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: The NaH + Al + 0.02CeCl3 system has been studied by high-resolution X-ray synchrotron diffraction and transmission electron microscopy (TEM), after planetary milling under hydrogen and hydrogen (H) cycling. Isothermal absorption kinetics were determined at 150 °C, and compared with the NaH + Al + 0.02TiCl3 system, indicating that CeCl3 and TiCl3 are equally effective additives, with CeCl3 preferred on the basis of hydrogen storage capacity. After milling, AlCe contains 100% of the Ce. After the first H absorption, we observe two Al1−xCex phases. The first, AlCe, contains ca. 60% of the originally added Ce atoms. The AlCe phase observed after milling and H cycling is chemically disordered, with complete exchange between the Al and Ce sublattices occurring, yielding zero intensity in ordering reflections such as (100). In the absorbed state after H cycling, the remaining 40% of Ce atoms are contained in a cubic Al1−xCex phase not previously observed in the Al-Ce phase diagram. Indexing yields a primitive cubic unit cell of dimension 7.7111 Å, in space group P23. Lineshape analysis indicates the AlCe and unknown cubic Al1−xCex phases are ca. 35 nm and 30 nm in dimension respectively. High resolution TEM imaging confirms that both Al1−xCex phases are embedded on the NaAlH4 surface, and localised energy dispersive X-ray spectroscopy (EDS) indicates a ca. 2:1 Al:Ce ratio for the unknown cubic Al1−xCex phase.
International Journal of Hydrogen Energy 07/2011; 36(14):8403–8411. · 3.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: Ball-milled sodium amide and magnesium hydride (NaNH2:MgH2 = 1:1 molar ratio) desorbs 3.3 wt % of hydrogen between 70 and 335 °C with three desorption events. X-ray diffraction indicates that the hydrogen desorption is associated with two unidentified magnesium-containing phases. Fourier transform infrared spectroscopy indicates that these two phases correspond to an imide and nitride, respectively. Analysis of the desorption products shows a large excess of NaH and MgH2 and that optimizing the starting reagents will increase both the kinetics and the amount of hydrogen desorbed from the system below 165 °C.
[show abstract][hide abstract] ABSTRACT: Magnesium hydride nanoparticles were synthesized within a carbon aerogel (CA) scaffold using a dibutylmagnesium precursor. The synthesis reaction was tracked using small-angle X-ray scattering (SAXS) to analyze the structural evolution during MgH2 formation. The CA/MgH2 composite was also investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM) to provide a better representation of the physical system. The CA has a large quantity of 2 nm pores as shown by nitrogen adsorption data. Both SAXS and TEM investigations confirm that MgH2 does form within the 2 nm pores but XRD proves that there is also a significant quantity of larger MgH2 particles within the system. Variations between hydrogen desorption isotherms from the CA/MgH2 composite and bulk MgH2 are detected that are indicative of changes in the decomposition properties of the small fraction of 2 nm MgH2 nanoparticles within the CA/MgH2 composite, changes which match theoretical predictions.
[show abstract][hide abstract] ABSTRACT: Organic aerogels were derived from acetic acid catalysed resorcinol and furfural and then dried directly in supercritical carbon dioxide without the use of a solvent exchange process. These aerogels were further carbonised in nitrogen and activated in CO2 in order to obtain their corresponding carbon aerogels. The carbon aerogels prepared by this method had a greater proportion of micropores in addition to a much shorter preparation time (on the order of days) than those prepared by other studies. The effect of different drying techniques on the microstructure of the wet gels was investigated by nitrogen adsorption at cryogenic liquid nitrogen temperature. Nitrogen adsorption at 77 K allowed the determination of surface areas and pore volumes, further analysed by the Dubinin–Radushkevich model and density functional theory model. The surface area and micropore volume of carbon aerogels prepared by this method increased by 19% and 12%, and accordingly, hydrogen uptake capacity was increased by 10% from 4.9 ± 0.2 wt.% to 5.4 ± 0.3 wt.% at 4.6 MPa and 77 K.Graphical abstractResearch highlights▶ Dry directly without a solvent exchange process. ▶ Reduce gelation time using acetic acid as a catalyst. ▶ Increased microporosity enhances of the binding energy between hydrogen and carbon adsorbent. ▶ Increased microporosity enhances the binding energy between hydrogen and carbon adsorbent.
Journal of Supercritical Fluids The 01/2011; 55(3):1115-1117. · 2.73 Impact Factor