Reversible Hydrogen Uptake Research Articles

Effect of Eutectic Melting, Reactive Hydride Composites, and Nanoconfinement on Decomposition and Reversibility of LiBH4–KBH4

Eutectic melting, reactive hydride composites, and nanoconfinement have the potential to improve the reversible hydrogen storage properties in metal borohydrides. We study and compare the combined effect of all three methods on reversible hydrogen release and uptake of the eutectic melting lithium potassium borohydride system, 0.725LiBH4–0.275KBH4, with low melting temperature (Tm = 105 °C). The eutectic mixture and reactive hydride composites of the eutectic mixture with Mg or MgH2 are melt infiltrated into a CO2 activated nanoporous carbon scaffold, and their properties are compared to those of bulk samples. The decomposition of 0.725LiBH4–0.275KBH4 and the reactive hydride composites initiates simultaneously with the melting at 105 °C, but the decomposition remains slow until higher temperatures are reached (T > 300 °C). Eutectic melting appears to improve kinetics of hydrogen release and absorption, while nanoconfinement lowers the main hydrogen release temperature in the first cycle by up to 200 °C. ...

Versatile preparation of graphene-based nanocomposites and their hydrogen adsorption

We report the facile and versatile preparation of graphene-based nanocomposites of magnetite, silver, titania, nickel, and silicon by solid state mechanical mixing. The composites were directly employed for nitrogen and hydrogen gas adsorption studies at 77 K and equilibrium pressure of 1 atm. Under the experimental conditions the chemically synthesised graphene support had a solid state BET surface area of 499 m2/g and a total reversible hydrogen uptake of 0.5 wt.%. Among our prepared nanocomposite samples, the silver nanoparticles promoted the most significant increase in hydrogen uptake per metre squared of material by 30% when compared to the parent graphene sample. However, the total hydrogen uptake by weight percentage in the studied graphenic materials remains low. The hydrogen storage capacity observed for the graphene nanocomposites can be qualitatively accounted for by considering the contributions of the catalyst particles on the hydrogen storage, the contact between catalyst and graphene, and the structural nature of the graphene surface.

Effects of surface thermodynamics on hydrogen isotope exchange kinetics in palladium: Particle and flow models

Palladium is an important material for separation of hydrogen from other gases, separation of hydrogen isotopes, and for hydrogen storage. Its main advantages are its high selectivity and rapid, highly reversible uptake and release of hydrogen at near-ambient temperatures and pressures. Toward a more comprehensive understanding of its behavior, we present particle and continuum multiphysics mathematical models of the coupled reactive transport of hydrogen isotopes in the context of a single palladium sphere, and of flow in a packed palladium hydride bed. The models consider rates of chemical reactions and mass transport within a hydride bed, and incorporate a multistep reaction mechanism involving the metal bulk, metal surface, and gas phases. A unique feature in this formulation is that the chemical reaction model accounts for all absorption, adsorption, and diffusion activation energies. In particular, the adsorption energy is believed to depend strongly on the composition and atom-scale structure of the surface. We perform a parametric study to evaluate the effects of temperature, surface adsorption energy, and hydride particle radius on the isotope exchange kinetics. The models are useful in designing optimal hydride beds operating at various temperatures, with varied hydride particle size, and surface conditions.

Hydrogen sorption properties of compounds based on BCC Ti1 − xV1 − yCr1 + x + y alloys

Among the hydrogen storage materials, BCC structure type materials such as V-based alloys reveal promising characteristics due to there reactivity with hydrogen at ambient temperature. A systematic XRD study was performed in order to obtain a cell parameter-composition map in the BCC domain and attempt a correlation with the sorption properties. However, activation of this type of materials may not be straightforward. The aim of this study was to co-melt a main BCC phase with a minor metallic one acting as activating phase. The synthesis of co-melted materials with a specific microstructure starting from Ti1−xV1−yCr1+x+y (x⩽0.4, y⩽0.3) and Zr7Ni10 alloys was successfully realized. The hydrogen decrepitation of the compound was studied in correlation with its microstructure. Pressure-composition-isotherms were recorded, the absorption and desorption isotherms measured as a function of time (sorption kinetics measurements) show reversible hydrogen uptake up to 2wt.%.

Effect of crystallite size of Al on the reversible hydrogen storage of NaAlH4 and few aspects of catalysts and catalysis

NaAlH4 has been catalyzed by MWCNT, Ce rich mischmetal (Mm), MmNi5 and TiO2 catalysts. The general aspect which relates every catalyst with the hydrogen storage capacity of NaAlH4 system has been verified through XRD analysis. Interesting features like chemical reduction, grain size variation, hydrogenation/dehydrogenation and phase transformation of the catalytic species are noticed. In the case of reversible hydrogen uptake, an interesting relationship exists between the restored hydrogen capacity, crystallite size of Al (desorbed in the dehydrogenation reaction) and the applied hydrogen pressure. Thus, as far as the reversible hydrogen storage in NaAlH4 is concerned, the mysterious role of catalyst seems to be a process of restricting the size of Al in a narrow range. The factors considered for analyzing this claim are discussed in detail.

Controlled Hydrothermal Synthesis of Complex Mixed Oxides Using Solution Redox Chemistry

ABSTRACTWe present the results of a study of the solvothermal synthesis of mixed-metal cerium-containing oxides all prepared from CeCl3.7H2O at less than 250 ºC in single step reactions. The use of NaBiO3 in the presence of aqueous NaOH yields fluorite solid solutions Ce1-xBixO2-x/2 (x ≤ 0.6), whereas the use of either H2O2 or NaBrO3 as oxidant in the presence of TiF3 yields a Ce(IV) pyrochlore (Na0.33Ce0.67)2Ti2O7. With replacement of a fraction of the Ti reagent by Sn(IV) acetate, tin doping is possible in the pyrochlore. The materials have all been assessed for their use in catalysis by performing temperature programmed reduction (TPR) experiments under dilute hydrogen flow. The cerium-bismuth oxides show large and apparently reversible hydrogen uptake, but in situ powder X-ray diffraction shows that this is accompanied by phase separation into bismuth metal and CeO2 that occurs over 5 or more TPR cycles. In contrast, the cerium (IV) titanate pyrochlore shows reversible reduction at low temperature (150 ºC, after an activation step), which gives the material potential use as a precious metal support for catalysis: such as in the water-gas-shift reaction. Although Sn doping lowers the onset of reduction of the pyrochlore, consistent with an expanded lattice, the materials suffer from collapse to give SnO.

Stability and Reversibility of Lithium Borohydrides Doped by Metal Halides and Hydrides

In an effort to develop reversible metal borohydrides with high hydrogen storage capacities and low dehydriding temperature, doping LiBH4 with various metal halides and hydrides has been conducted. Several metal halides such as TiCl3, TiF3, and ZnF2 effectively reduced the dehydriding temperature through a cation exchange interaction. Some of the halide doped LiBH4 are partially reversible. The LiBH4 + 0.1TiF3 desorbed 3.5 wt % and 8.5 wt % hydrogen at 150 and 450 °C, respectively, with subsequent reabsorption of 6 wt % hydrogen at 500 °C and 70 bar observed. XRD and NMR analysis of the rehydrided samples confirmed the reformation of LiBH4. The existence of the (B12H12)−2 species in dehydrided and rehydrided samples gives insight into the resultant partial reversibility. A number of other halides, MgF2, MgCl2, CaCl2, SrCl2, and FeCl3, did not reduce the dehydriding temperature of LiBH4 significantly. XRD and TGA-RGA analyses indicated that an increasing proportion of halides such as TiCl3, TiF3, and ZnCl2 from 0.1 to 0.5 mol makes lithium borohydrides less stable and volatile. Although the less stable borohydrides such as LiBH4 + 0.5TiCl3, LiBH4 + 0.5TiF3, and LiBH4 + 0.5ZnCl2 release hydrogen at room temperature, they are not reversible due to unrecoverable boron loss caused by diborane emission. In most cases, doping that produced less stable borohydrides also reduced the reversible hydrogen uptake. It was also observed that halide doping changed the melting points and reduced air sensitivity of lithium borohydrides.

Computational Analysis of Amine−Borane Adducts as Potential Hydrogen Storage Materials with Reversible Hydrogen Uptake

Amine-borane adducts are promising compounds for use in hydrogen storage applications, and the efficient catalytic release of hydrogen from these systems has been recently demonstrated. However, if hydrogen storage is to be of practical use, it is necessary that, once hydrogen has been removed from the material, it can be put back into the system to recharge the appliance. In order to develop such systems, we computationally screened a range of amine-borane adducts for their thermodynamic dehydrogenation properties. Structural trends, which lay the foundation for the possible design of amine-borane systems that exhibit reversible dihydrogen uptake, are established. We found that it is mainly the strengths of the dative bonds in both starting materials and products that govern the thermodynamic parameters of the dehydrogenation reactions. Thus, in general, electron-donating groups on nitrogen and electron-withdrawing groups on boron lead to more favorable systems. It is also possible to design promising systems whose thermodynamic parameters are a consequence of different steric strain in starting materials and products.

An investigation of the H2 uptake in Mg–Nb–O ternary phases

Various Mg/Nb/O mixed phases have been prepared by solid state reaction in order to mimic similar compounds formed in the ball milled system composed of MgH2 and Nb2O5. The latter oxide has a role as a promoter of the hydrogen uptake and desorption in the Mg/MgH2 system for hydrogen storage. One mixed phase in particular (Mg3Nb6O11) have shown a reversible uptake of molecular hydrogen, confirmed by thermal desorption experiments followed by mass spectroscopy. Comparison of the behaviour of this phase with NbO shows how a reversible interaction with hydrogen may be linked to the presence of octahedral niobium clusters in both oxides.

Switchable mirrors based on nickel–magnesium films

An electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin, magnesium-rich Ni–Mg films prepared on glass substrates by co-sputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on cathodic polarization in alkaline electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extraction, and to protect the metal surface against oxidation.

Simple formal kinetics for the reversible uptake of molecular hydrogen by [Ni-Fe] hydrogenase from Desulfovibrio gigas.

Enzymatic electrocatalysis, triggered and monitored by means of cyclic voltammetry, enabled us to achieve quantitative analysis of the kinetics of the hydrogenase catalyzed process, in the 7.8-10.0 pH range, in the presence of an electrochemically generated redox mediator. The quantitative analysis can be carried out by use of a quite simple SRC model. The simplicity of the SRC model is compatible with the existence of multiple redox microstates, which can be combined in a potential adjustable triangular mechanism consisting of three catalytic cycles, which are formally identical from the kinetic point of view. The steps involved in the kinetic control of the reversible process are H2 uptake or production at the Ni-Fe catalytic site and the intermolecular electron transfer between the mediator and the distal [4Fe-4S] cluster. The related rate constants have been determined. For the two accompanying intramolecular electron transfers which proceed at equilibrium, the equilibrium constants were found to be in very good agreement with previously published data.

Reversible Hydrogen Uptake in Carbon-Based Materials

ABSTRACTSeveral approaches for achieving reversible hydrogen uptake by carbon are considered, including intercalation, adsorption by a graphite surface, hydrogenation of fullerenes, and the filling of carbon nanotubes. Most scenarios suggest that it is difficult to achieve an atomic uptake [H/C] ratio exceeding unity. Evidence for H2 uptake by various carbon materials is reviewed.

Irreversible hydrogen uptake on Mo2N catalyst

Volumetric H2-uptake measurements on an Mo2N (79 m2g−1) sample reduced at 673 K have been carried out and the uptake isotherms in the temperature range of 308–623 K have been determined. Both the total and reversible hydrogen uptake increased with the uptake temperature. The irreversible hydrogen uptake increased abruptly when the uptake temperature was raised up to 423 K. The maximum of irreversible hydrogen uptake was measured at 473 K. The HIR/Mo ratio calculated from the uptakes obtained in the temperature range of 308–623 K varies in the range of 0.0010–0.0202. One possible mechanism for hydrogen adsorption is proposed to be heterolytic dissociation on Mo-N paris, in which the molybdenum atoms are in unsaturated coordination.

Reversible uptake of hydrogen and nitrogen at cobalt in the solid state. Influence of the counter anion on the formation of classical dihydride vs. nonclassical .eta.2-dihydrogen forms of [(PP3)CoH2

Reversible uptake of hydrogen and nitrogen at cobalt in the solid state. Influence of the counter anion on the formation of classical dihydride vs. nonclassical .eta.2-dihydrogen forms of [(PP3)CoH2

Microvoids and defect chemistry at the SiSiO 2 interface studied by positron annihilation depth profiling

A low energy positron beam (few keV) has been used to study device quality dry thermal SiO 2 layers (∼ 50 nm thick) on Si(100). By tuning the beam energy over the range 0–10 keV to vary implantation depth, and by accounting for positron diffusion as well as annihilation in SiO 2 and Si, depth profiles of material properties are obtained which sense material, defect, and electric field properties. The unusual observation of ortho-positronium 3γ-decay at the interface demonstrates that microvoids > 1 nm in size are present in these oxides, apparently an intrinsic consequence of the thermal oxidation process. The microvoid signal increases with annealing, possibly due to initiation of the interfacial oxide decomposition reaction. Reversible hydrogen uptake and desorption are also observed, illustrating the interface sensitivity of positron studies.

Studies of hydride formation in Ti 1−xZr xMn 2

Hydriding characteristics are presented for compounds in Ti 1−xZr xMn 2 such as amount of hydrogen take up, equilibrium pressures as a function of temperature and derived thermodynamic parameters (ΔH, ΔS, ΔG). Large reversible hydrogen uptake at ambient conditions is observed in a certain region of composition.

Reversible uptake of hydrogen by hydrocarbons chemisorbed on palladium

The reversible uptake of hydrogen by a hydrocarbon residue chemisorbed on silica-supported palladium was quantitatively studied by ir spectroscopy. Adsorption isotherms of hydrogen in the range 20–80 °C showed θ ∞ √p and yielded an isosteric heat of adsorption of 12.5 kcal mole −1 which was independent of coverage in the range studied, viz., 0.2 < θ < 0.9. The exchange of the adsorbed hydrocarbon with deuterium and the reverse exchange with hydrogen at 40–80 °C was shown to follow first-order kinetics. The activation energy was 18 kcal mole −1 and a kinetic isotope effect of ~3 increased the rate of hydrocarbon exchange compared with deuterocarbon exchange. The thermodynamic and kinetic studies were related and a potential energy diagram was constructed to describe the energetics of adsorption of hydrogen.