Qingan Zhang

Remarkable enhancement in dehydrogenation of MgH2 by a nano-coating of multi-valence Ti-based catalysts

A Ti-based multi-valence catalyst was coated on the surface of ball milled Mg powders (∼1 μm in diameter), aiming to decrease the desorption temperature and increase the kinetics of hydrogen release from MgH2 by its catalytic effect on thermodynamics. The catalysis coating was prepared by the chemical reaction between Mg powders and TiCl3 in THF solution, which is ∼10 nm in thickness and contains multiple valences in the form of Ti (0), TiH2 (+2), TiCl3 (+3) and TiO2 (+4). It is believed that the easier electron transfer among these different Ti valences plays a key role in enhancing the hydrogen recombination for the formation of a hydrogen molecule (e.g.). This recombination is generally regarded as the key barrier for hydrogen desorption of MgH2. Experimentally, temperature-programmed desorption (TPD) and isothermal dehydrogenation analysis demonstrate that the MgH2 – coated Ti based system (denoted as Mg–Ti) has excellent dehydrogenation properties, which can start to release H2 at about 175 °C and release 5 wt% H2 within 15 min at 250 °C. The dehydrogenation reaction entropy (ΔS) of the system is changed from 130.5 J K−1 mol−1 H2 to 136.1 J K−1 mol−1 H2, which reduces the Tplateau to 279 °C at an equilibrium pressure of 1 bar. A new mechanism has been proposed that multiple valence Ti sites act as the intermediate for electron transfers between Mg2+ and H−, which makes the recombination of H2 on Ti (in forms of compounds) surfaces much easier.

(Nd1.5Mg0.5)Ni-7-Based Compounds: Structural and Hydrogen Storage Properties

The structural and hydrogen storage properties of (Nd(1.5)Mg(0.5))Ni(7)-based alloys (i.e., A(2)B(7)-type) with a coexistence of two structures (hexagonal 2H and rhombohedral 3R) are investigated in this study. In both 2H- and 3R-type A(2)B(7) structures, Mg atoms occupy Nd sites of Laves-type AB(2) subunits rather than those of AB(5) subunits because Mg substitution for Nd in the AB(2) subunits more significantly strengthens the ionic bond in the system. An increase in the A-atomic radius or the B-atomic radius stabilizes the 2H structure, but a decrease in the A-atomic radius or the B-atomic radius is favorable for formation of the 3R structure. The 2H-A(2)B(7) and 3R-A(2)B(7) phases in each alloy have quite similar equilibrium pressures upon hydrogen absorption and desorption, which show a linear relationship with the average subunit volume. The hydriding enthalpy for the (Nd(1.5)Mg(0.5))Ni(7) compound is about -29.4 kJ/mol H(2) and becomes more negative with partial substitution of La for Nd and Co/Cu for Ni but less negative with partial substitution of Y for Nd.

Synergetic effects of hydrogenated Mg3La and TiCl3 on the dehydrogenation of LiBH4

To improve the hydrogen storage performance, Mg3La alloy and TiCl3 is ball milled simultaneously with LiBH4 to yield a designed composition. It has been revealed that there is a synergistic effect of Mg3La alloy and TiCl3 on the dehydrogenation of LiBH4, which improves the dehydrogenation performance when compared to adding either hydrogenated Mg3La or TiCl3 alone. 4.3 wt% of H2 can be released within 10 min at 400 °C in the LiBH4 + Mg3La + TiCl3 system. The dehydrogenation activation energy of this system is 52.6 kJ mol−1, which is much lower than that of pristine LiBH4. In addition, the LiBH4 + Mg3La + TiCl3 system preserves fast dehydrogenation kinetics and stable hydrogen storage capacity in the re-/dehydrogenation cycles.

Enhanced hydrogen storage properties of a Mg–Ag alloy with solid dissolution of indium: a comparative study

A comparative study of Mg5.7In0.3Ag and Mg6Ag alloys was conducted to reveal the effects of indium (In) solid solutions on the hydrogen storage properties of Mg-based alloys. Different from the Mg6Ag alloy, the as-cast Mg5.7In0.3Ag alloy was composed of a Mg(In) solid solution and Mg3Ag. However, an initial hydrogen absorption/desorption treatment (i.e., activation) propelled the In atoms in Mg(In) toward Mg3Ag, forming (Mg, In)3Ag in the activated sample. This transformation involving the dissolution of In atoms from Mg into solid Mg3Ag not only greatly improved the thermodynamics of hydrogen desorption but also enhanced its catalytic effect on hydrogen desorption from additional MgH2. The (Mg, In)3Ag–H2 system exhibited altered thermodynamics, as its enthalpy change of the hydrogen desorption was 62.6 kJ mol−1 H2. Moreover, the activation energy of the hydrogen desorption from the Mg5.7In0.3Ag sample was lowered to 78.2 kJ mol−1.

Carbon nanomaterial-assisted morphological tuning for thermodynamic and kinetic destabilization in sodium alanates

In the present work, we develop an effective strategy, i.e., carbon nanomaterial-assisted morphological tuning, for both thermodynamic and kinetic destabilization in complex hydrides based on the interaction between the complex anion and the carbon matrix. The NaAlH4/carbon nanomaterials of graphene nanosheets (GNs), fullerene (C60) and mesoporous carbon (MC) were selected as model systems for illustrating the positive effect of carbon nanomaterial-assisted morphological tuning. It is demonstrated that through the dissolution–recrystallization process, the morphologies of NaAlH4 can be altered from the scale-like continuous structure for the GN-assisted sample, to flower-like structures with diameters ranging from 5 to 10 μm for the C60-assisted sample and to uniform particles with an average diameter of about 2 μm for the MC-assisted sample. Correspondingly, the onset temperature for dehydrogenation of NaAlH4 is reduced to about 188, 185 and 160 °C for the samples assisted with GNs, C60 and MC, respectively, much lower than 210 °C for the pristine sample. A remarkable reduction in activation energy for three-step dehydrogenation is also obtained in NaAlH4/carbon nanomaterial composites relative to the pristine sample, and the improved efficiency of carbon nanomaterials for kinetics is found to be in the order of MC > C60 > GNs. These positive improvements can be attributed to both the particle refinement and interaction between NaAlH4 and the carbon nanomaterial that are in intimate contact with each other, which are not only evidenced by FE-SEM observation, but also supported by 27Al solid-state NMR characterization.

Self-Printing on Graphitic Nanosheets with Metal Borohydride Nanodots for Hydrogen Storage.

Although the synthesis of borohydride nanostructures is sufficiently established for advancement of hydrogen storage, obtaining ultrasmall (sub-10 nm) metal borohydride nanocrystals with excellent dispersibility is extremely challenging because of their high surface energy, exceedingly strong reducibility/hydrophilicity and complicated composition. Here, we demonstrate a mechanical-force-driven self-printing process that enables monodispersed (~6 nm) NaBH4 nanodots to uniformly anchor onto freshly-exfoliated graphitic nanosheets (GNs). Both mechanical-forces and borohydride interaction with GNs stimulate NaBH4 clusters intercalation/absorption into the graphite interlayers acting as a ‘pen’ for writing, which is accomplished by exfoliating GNs with the ‘printed’ borohydrides. These nano-NaBH4@GNs exhibit favorable thermodynamics (decrease in ∆H of ~45%), rapid kinetics (a greater than six-fold increase) and stable de-/re-hydrogenation that retains a high capacity (up to ~5 wt% for NaBH4) compared with those of micro-NaBH4. Our results are helpful in the scalable fabrication of zero-dimensional complex hydrides on two-dimensional supports with enhanced hydrogen storage for potential applications.

Hydrogen-induced magnesium–zirconium interfacial coupling: enabling fast hydrogen sorption at lower temperatures

The implementation of magnesium (Mg) as a hydrogen-storage medium has long been restricted because of its rather sluggish hydrogen sorption at high temperatures. Here, we report a method for using hydrogen-induced Mg–Zr interfacial coupling to manipulate the migration of hydrogen atoms and thus tune their uptake and release in a micrometer-sized Mg-rich composite. The associated Mg–Zr–H interfaces were assembled in situ by high-pressure ball milling and isothermal treatment of MgH2 and Zr powders under a hydrogen atmosphere. The interfaces gradually disintegrated upon MgH2 desorption but also recovered their original compositions upon absorption while the ZrH2 originating from Zr hydrogenation remained completely unchanged. Compared to pure MgH2, the hydrogen sorption of the Mg–Zr–H composite was thus shown to be dramatically faster at lower temperatures, whereby it not only absorbed hydrogen close to saturation at 100 °C within 2 h, while the pure Mg did not absorb hydrogen at all, but also started to release hydrogen at ∼235 °C with a reduction in the activation energy of desorption by ∼40 kJ mol−1. These remarkable enhancements cannot be explained by the decrease in the size of the MgH2 grains alone but are most likely due to the introduction of Mg–Zr–H interfaces and large fractions of defects that provide channels for facile hydrogen dissociation and migration into the Mg/MgH2 matrix.

Destabilization of LiBH4 dehydrogenation through H+–H− interactions by cooperating with alkali metal hydroxides

Destabilization by the alkali metal hydroxides LiOH, NaOH, and KOH in the solid-state dehydrogenation of LiBH4 is reported. 6.5 wt% of hydrogen was liberated within 10 minutes at 250 °C. Destabilization originated from the interaction between H+ in [OH]− and H− in [BH4]−. A larger Paulings electronegativity of the alkali metal (Li > Na > K) led to a greater acidity of the proton donor [OH]− site, and thus enhanced destabilization. The temperature of the predominant dehydrogenation was reduced to 207, 221, and 230 °C, for ball milled LiBH4–LiOH, 2LiBH4–NaOH, and 2LiBH4–KOH, respectively. The LiBH4: LiOH stoichiometry greatly affected the destabilization, by providing differing reaction pathways in LiBH4-xLiOH (x = 1, 1.36, 4). The incremental increase in the LiOH content of LiBH4-xLiOH increased the dehydrogenation rate, but the temperature increased from 207 °C (x = 1) to 250 °C (x = 4). 4.1 and 6.5 wt% of hydrogen was liberated within 10 minutes by LiBH4–LiOH and LiBH4–4LiOH, respectively. The incremental increase in dehydrogenation temperature was attributed to differing [BH4]−⋯[OH]− interactions, formed by the differing stoichiometric ratios.

Facile self-assembly of light metal borohydrides with controllable nanostructures

Herein, a self-assembly strategy for the realization of diverse light metal complex borohydride nanoparticles (NPs) with sphere, polygon and hollow geometries is presented. Under ambient conditions, the particle sizes of the LiBH4 NPs formed can easily be controlled by varying the concentration which enables hydrogen gas release at ∼72 °C.

The superior desorption properties of MgCl2-added ammonia borane compared to MgF2-added systems—the unexpected role of MgCl2 interacting with [NH3] units

An uncommon dehydrogenation mechanism for metal chloride-added ammonia borane (NH3BH3, AB) systems revealed that MgCl2 interacts with the [NH3] units in AB, analogous to the amine complex of Mg(NH3)xCl2, thus not only resulting in a remarkable decrease in the hydrogen desorption temperature but also effectively suppressing undesirable volatile by-products, particularly ammonia gas.