As-received MgH2 Research Articles

Synergy of elemental Fe and Ti promoting low temperature hydrogen sorption cycling of magnesium

We studied the catalytic effects of Titanium, Iron and FeTi intermetallic on the desorption kinetics of magnesium hydride. In order to separate the catalytic effects of each element from additional synergistic and alloying effects, Mg–Ti and Mg–Fe mixtures were studied as a baseline for Mg–Fe–Ti elemental and Mg–(FeTi) intermetallic composites. Sub-micron dimensions for MgH 2 particles and excellent nanoscale catalyst dispersion was achieved by high-energy ball-milling as confirmed by analytical electron microscopy techniques. The composites containing Fe shows desorption temperature of 170 K lower than as-received MgH 2 powder, which makes it suitable to be cycled at relatively low temperature of 523 K. Furthermore, the low cycling temperature prevents the formation of Mg 2FeH 6. In sorption cycling tests, Mg-10% Ti and Mg-10% (FeTi), after about 5 activation cycles, show fast desorption kinetics initially, but the kinetics also degrade faster than for all other composites, eventually slowing down by a factor of 7 and 4, respectively. The ternary Mg–Fe–Ti composite shows best performance. With the highest BET surface area of 40 m 2/g, it also shows much less degradation during cycling. This is attributed to titanium hydride acting as a size control agent preventing agglomeration of particles; while Fe works as a very strong catalyst with uniform and nanoscale dispersion on the surface of MgH 2 particles.

Chemical vapor synthesis of Mg–Ti nanopowder mixture as a hydrogen storage material

Abstract Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen contents. In this work, the hydrogen release and uptake properties of Mg/5%Ti nanopowder mixture prepared by a chemical vapor synthesis (CVS) process were investigated. Samples were made in a CVS reactor, in which reactant powders were fed into evaporator placed inside the reactor by means of specially designed powder feeders. The produced Mg/5%Ti was hydrogenated in an autoclave under 10.3 MPa pressure and 150 °C for 12 h. Thermogravimetric analysis (TGA) showed that 5.2 wt% of hydrogen began to be released from 190 °C while temperature was increased at a heating rate of 5 °C/min up to 350 °C under an argon flow. This onset temperature of Mg–Ti nanopowder dehydrogenation was much lower than that of MgH2 alone, which is 381 °C. In addition, the activation energy of dehydrogenation was 104 kJ/mol, which is much smaller than that of as-received MgH2 (153 kJ/mol).

Hydrogen storage properties of the Mg–Ti–H system prepared by high-energy–high-pressure reactive milling

Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen content. In the present work, we investigated the hydrogen release/uptake properties of the Mg–Ti–H system. Samples were prepared from the mixtures of MgH 2 and TiH 2 in molar ratios of 7:1 and 4:1 using a high-energy-high-pressure (HEHP) mechanical ball-milling method under 13.8 MPa hydrogen pressure. Thermogravimetric analysis (TGA) showed that a relatively large amount of hydrogen (5.91 and 4.82 wt.%, respectively, for the above two samples) was released between 126 and 313 °C while temperature was increased at a heating rate of 5 °C min −1 under an argon flow. The onset dehydrogenation temperature of these mixtures, which is 126 °C, is much lower than that of MgH 2 alone, which is 381 °C. The activation energy of dehydrogenation was 71 kJ mol −1, which is much smaller than that of as-received MgH 2 (153 kJ mol −1) or as-milled MgH 2 (96 kJ mol −1). Furthermore, the hydrogen capacity and the dehydrogenation temperature remained largely unchanged over five dehydrogenation and rehydrogenation cycles.

Structural Stability and Dehydrogenation of (MgH<SUB>2</SUB>+Al, Nb) Powder Mixtures during Mechanical Alloying

High-energy ball milling of MgH 2 with appropriate alloying elements is reported to improve the hydrogenation kinetics of MgH 2 but the relevant information on dehydrogenation is scarce. Here, a systematic study was carried out to clarify the effects of two key alloying elements, Nb and Al, on the microstructure and the dehydrogenation behaviour of the MgH 2 . Intensive mechanical alloying was carried out to synthesise (MgH 2 + M) powder mixtures (M=Nb, Al). XRD Rietveld analysis revealed the formation of a new bcc phase in the (MgH 2 +Nb) mixture and a (Al, Mg) solid solution in the (MgH 2 +Al) mixture. The amount of the newly formed phase in each case increased with milling time, while the level of Nb or Al decreases. SEM analysis of the milled powders showed the existence of nano-particles within 20 hours of milling. Thermogravimetry (TG) results showed that the mechanically alloyed (MgH 2 +Nb) mixture released about 3.9 mass% H 2 and the milled (MgH 2 + Al) about 5.4 mass% H 2 at 300°C within 10 minutes, compared with only 1.5 mass% of the milled MgH 2 powder and 1.0 mass% of the as-received MgH 2 under the same conditions.