Dalin Sun

Mesoporous metal–organic frameworks: design and applications

Metal–organic frameworks (MOFs), which are constructed from the assembly of organic ligands with metal ions or metal clusters, have high potential applications in the fields of gas storage, separations and catalysis. MOFs involving mesopores are considered to have specific performance in such fields. In this mini review, we are mainly focussing on the recent developments in mesoporous MOFs including the design strategies and their most important applications.

Monodisperse magnesium hydride nanoparticles uniformly self-assembled on graphene.

Monodisperse MgH2 nanoparticles with homogeneous distribution and a high loading percent are developed through hydrogenation-induced self-assembly under the structure-directing role of graphene. Graphene acts not only as a structural support, but also as a space barrier to prevent the growth of MgH2 nanoparticles and as a thermally conductive pathway, leading to outstanding performance.

Mg–TM (TM: Ti, Nb, V, Co, Mo or Ni) core–shell like nanostructures: synthesis, hydrogen storage performance and catalytic mechanism

Magnesium (Mg) was coated by different transition metals (TM: Ti, Nb, V, Co, Mo, or Ni) with a grain size in the nano-scale to form a core (Mg)–shell (TM) like structure by reaction of Mg powder in THF solution with TMClx. The thickness of the TM shell is less than 10 nm. TPD-MS results show the Mg–Ti sample can release hydrogen even under 200 °C. It is experimentally confirmed that the significance of the catalytic effect on dehydrogenation is in the sequence Mg–Ti, Mg–Nb, Mg–Ni, Mg–V, Mg–Co and Mg–Mo. This may be due to the decrease in electro-negativity (χ) from Ti to Mo. However, Ni is a special case with a high catalytic effect in spite of the electro-negativity. It is supposed that the formation of the Mg2Ni compound may play an important role in enhancing the hydrogen de/hydrogenation of the Mg–Ni system. It is also found that the larger the formation enthalpy, the worse the dehydrogenation kinetics.

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.

Electrochemical performances of ZrM2 (M=V, Cr, Mn, Ni) Laves phases and the relation to microstructures and thermodynamical properties

Abstract Zr-based AB 2 (B=V, Cr, Mn) Laves phases are studied for their ability to reversibly store hydrogen and are potential negative electrode materials in Ni–MH batteries. The hydrides formed are too stable for electrochemical use and their thermodynamical properties can be adapted by Ni substitution on the B sublattice. However, measured electrochemical reversible capacities are very low compared to solid–gas results. This phenomenon is related to passivation and surface corrosion in electrolytic medium. To overcome this problem, the role of secondary phases has been studied following two routes: precipitation of controlled amounts of Zr–Ni binary intermetallic compounds or addition of a rare earth (R) element leading to precipitation of RNi compound in the matrix. The consequences on the electrochemical behaviour are discussed in terms of the microstructure of the alloys which lead to composite electrodes by taking advantage of the bulk properties of the Laves phase and the surface properties of the secondary phase(s).

Pseudocapacitance-tuned high-rate and long-term cyclability of NiCo2S4 hexagonal nanosheets prepared by vapor transformation for lithium storage

The high conductivity of bimetallic thiospinel NiCo2S4 endows energy storage devices with very fascinating performance. However, the unsatisfactory rate capability and long-term cyclability of this material series significantly limit their large-scale practical applications such as in electric vehicles and hybrid electric vehicles. Herein, we successfully synthesized NiCo2S4 hexagonal nanosheets with a large lateral dimension of ∼1.35 μm and a thickness of ∼30 nm through a vapor transformation method. The dynamic transformation process of the NiCo2S4 polycrystalline nanosheets from NiCo-hydroxide has been revealed in detail. Originating from their two-dimensional thin-sheet structure with a high aspect ratio, the induced extrinsic capacitive contribution as high as 91% makes them an ideal candidate for high-capacity and high-rate lithium-ion anodes. The NiCo2S4 nanosheets deliver a reversible capacity of 607 mA h g−1 upon 800 cycles at a current density of 2 A g−1. This outstanding long cycle performance sheds light on the structural design of electrode materials for high-rate lithium-ion batteries.

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.

Three-Dimensional Graphene/Single-Walled Carbon Nanotube Aerogel Anchored with SnO2 Nanoparticles for High Performance Lithium Storage

A unique 3D graphene-single walled carbon nanotube (G-SWNT) aerogel anchored with SnO2 nanoparticles (SnO2@G-SWCNT) is fabricated by the hydrothermal self-assembly process. The influences of mass ratio of SWCNT to graphene on structure and electrochemical properties of SnO2@G-SWCNT are investigated systematically. The SnO2@G-SWCNT composites show excellent electrochemical performance in Li-ion batteries; for instance, at a current density of 100 mA g-1, a specific capacity of 758 mAh g-1 was obtained for the SnO2@G-SWCNT with 50% SWCNT in G-SWCNT and the Coulombic efficiency is close to 100% after 200 cycles; even at current density of 1 A g-1, it can still maintain a stable specific capacity of 537 mAh g-1 after 300 cycles. It is believed that the 3D G-SWNT architecture provides a flexible conductive matrix for loading the SnO2, facilitating the electronic and ionic transportation and mitigating the volume variation of the SnO2 during lithiation/delithiation. This work also provides a facile and reasonable strategy to solve the pulverization and agglomeration problem of other transition metal oxides as electrode materials.