Sabin Boily

Catalytic effect of transition metals on hydrogen sorption in nanocrystalline ball milled MgH2-Tm (Tm=Ti, V, Mn, Fe and Ni) systems

Abstract Intensive mechanical milling was used to make MgH2–Tm (Tm=3d-transition elements Ti, V, Mn, Fe, Ni) nanocomposite powders. The hydrogen storage properties of these composite powders were evaluated. The five 3d-elements Ti, V, Mn, Fe and Ni showed different catalytic effects on the reaction kinetics of Mg–H system. Desorption was most rapid for MgH2–V, followed by MgH2–Ti, MgH2–Fe, MgH2–Ni and MgH2–Mn at low temperatures. The composites containing Ti exhibited the most rapid absorption kinetics, followed in order by Mg–V, Mg–Fe, Mg–Mn and Mg–Ni. Formation enthalpy and entropy of magnesium hydride were not altered by milling with transition metals, while the activation energy of desorption for magnesium hydride was reduced drastically.

Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride

It has recently been discovered that energetic ball milling of hydrides can improve their hydrogen sorption properties significantly. In this work, we present a systematic study of structural modifications and hydrogen absorption–desorption kinetics of ball-milled magnesium hydride. Structural investigations showed that after only 2 h of milling, a metastable orthorhombic (γ) magnesium hydride phase is formed. A Rietveld analysis of the X-ray diffraction spectrum of the 20 h milled sample gave a proportion of 74 wt.% MgH2, 18 wt.% γ MgH2 and 8 wt.% MgO. The hydrogen capacity and sorption kinetics were measured before and after milling. We found that the sorption kinetics are much faster for the milled sample compared to the unmilled one. This explains the fact that the hydrogen desorption temperature of the ball-milled sample as measured by pressured differential scanning calorimetry (PDSC), is reduced by 64 K compared to the unmilled sample. There is no significant change of the storage capacity upon milling and the absorption plateau pressure does not change. From the desorption curves, the activation energy was deduced. The milling also increased the specific surface area. This was confirmed by SEM micrographs and BET measurements. Possible mechanisms explaining the improved kinetics are presented.

Hydrogen desorption kinetics of a mechanically milled MgH2+5at.%V nanocomposite

Abstract The hydrogen desorption kinetics of mechanically milled MgH 2 +5at.%V nanocomposite were determined under various desorption pressures and temperatures. The reaction rate constant was extracted from the time-dependent desorption curves. The relationships of rate constant with pressure and temperature were established. It was found that the hydrogen desorption at high temperature and under high driving force, is controlled by the interface (Mg/MgH 2 ) motion. When the driving force is small, the early stage of hydrogen desorption is controlled by nucleation and growth and the later stage is controlled by long range hydrogen diffusion. At temperatures below 523 K, the nucleation and growth process dominates the hydrogen desorption. High temperature annealing (673 K) of the nanocomposite results in slower desorption kinetics and increased activation energy of desorption. At high temperatures, the rate-limiting step changes from interface control (before annealing) to surface control (after annealing), while at low temperatures, the rate-limiting step of desorption does not change after annealing.

Hydrogen absorption properties of a mechanically milled Mg–50 wt.% LaNi5 composite

Abstract Mechanical milling was used to make composite Mg–50 wt.% LaNi 5 powders. The structural changes during the milling process and the hydrogen storage properties of the mechanically milled composite were characterized. Mechanical milling leads to a nano-composite, which is not stable upon high temperature (573 K) hydriding and dehydriding cycling. The nano-composite transforms to a new Mg+LaH x +Mg 2 Ni composite, which is stable upon further cycling. The new composite has excellent hydrogen absorption kinetics at low temperatures. The storage capacity reaches 2.5 wt.% in 500 s under 1.5 MPa hydrogen at 302 K. The optimum capacity is 4.1 wt.% at intermediate temperatures (523–573 K). The high absorption rate is explained by the high quantity of phase boundaries and the porous surface structure.

Hydriding behavior of Mg-Al and leached Mg-Al compounds prepared by high-energy ball-milling

The structure and hydrogen absorption properties of Mg:Al alloys prepared by high-energy ball milling were studied over the whole compositional range. These materials were prepared in their as-milled and Al-leached forms. The latter are obtained from the former materials by leaching out Al in a 1 N NaOH solution. The structure of the various alloys was determined by X-ray diffraction. The structure of the material in the hydrided state was also determined in some cases. In the as-milled state, hcp Mg(Al) with a small proportion of Mg17Al12 and fcc Al(Mg) are formed at Mg:Al (90:10) and (20:80) compositions, respectively. At intermediate (58:42) and (37:63) compositions, the intermetallic Mg17Al12 and Mg3Al2 phases are formed, respectively. Following leaching, the Al content of Mg:Al (90:10) and (20:80) varies from 10.4 and 77.0 to 3.0 and 51.0 at.%, respectively. In both cases, noticeable change in the XRD pattern confirms that bulk dissolution of Al has been achieved. There is a two-fold increase in the specific surface area of Mg:Al (90:10) following leaching of Al. In the case of Mg:Al with intermediate compositions, dissolution of Al, if any, does not lead to discernable modification in the structure of the material. The measured hydrogen capacity of the as milled material decreases with Al content, from H/M=1.74 for pure un-milled Mg, to 1.38 for Mg:Al (90:10), and then to 1.05 for Mg:Al (75:25). In each case, there is a further 10–15% decline of the hydrogen absorption capacity after leaching. In the case of Mg:Al (58:42), which basically only contains a nanocrystalline Mg17Al12 intermetallic phase, hydriding leads to the formation of MgH2 and Al. This reaction is totally reversible and Mg17Al12 is recovered upon de-hydriding. In each case, there is an increase in the kinetics of hydrogen absorption and desorption following Al leaching.

Recent developments in the applications of nanocrystalline materials to hydrogen technologies

The paper discusses the application of nanocrystalline alloys as hydrogen storage materials and as electrocatalysts for solid polymer electrolyte fuel cells. After reviewing some of the requirements of metal hydrides for hydrogen fueled vehicles, the paper presents new results on the structure and hydrogen sorption properties of high storage capacity ball-milled magnesium hydride. The great advantages of milling the hydride instead of the pure metals for producing novel nanostructures with high surface area and for improving hydrogen sorption kinetics are presented. In the second part of the paper, the same technique has been extended to the milling of carbides and chlorides and coupled to a lixiviation process to produce new electrocatalysts for polymer electrolyte fuel cells. This new technology offers the possibility of producing nanoparticles with metastable structures whose specific surface area is much larger than that of any nanocrystalline powders made by conventional ball milling techniques. Pt-based nanoparticles were fabricated and tested as anode in fuel cells under pure and CO-contaminated hydrogen feedstreams.

Hydrogen storage in mechanically milled Mg–LaNi5 and MgH2–LaNi5 composites

Magnesium and magnesium hydride were mechanically milled with LaNi5 to make a Mg–Ni–La ternary alloy for hydrogen storage. Mechanical milling of MgH2+LaNi5 or milling of Mg+LaNi5 followed by a full hydrogenation leads to a composite of MgH2+LaH3+Mg2Ni. Upon hydrogen absorption/desorption cycling, a mixture of Mg+LaH3+Mg2Ni phases is obtained in both cases, but with different powder sizes. The powder size is greatly reduced by using MgH2 instead of Mg in the milling process. The reduction in powder size gives faster absorption kinetics, and slower desorption kinetics. Adding both Ni and La to Mg-based alloys produces a synergetic effect on the hydrogen absorption/desorption. The ternary Mg–Ni–La alloy showed much better absorption and desorption kinetics than the binary alloys Mg–La and Mg–Ni. Lanthanum hydride has strong catalytic effects on absorption of Mg, but weak effects on desorption. Mg2Ni has better catalytic effect than lanthanum hydride at temperatures above 373 K.

Direct synthesis of Mg2FeH6 by mechanical alloying

Abstract The hydride Mg 2 FeH 6 was synthesized by high-energy ball milling of MgH 2 and Fe under argon atmosphere without subsequent sintering. After 60 h of milling, 56% wt. of Mg 2 FeH 6 was synthesized. This yield was deduced from Rieltveld analysis of the X-ray powder measurements and confirmed by pressured differential scanning calorimeter (PDSC). Hydrogen capacity measurements indicated that the loss of capacity with cycling is minimal.

Synthesis of Na3AlH6 and Na2LiAlH6 by mechanical alloying

The direct synthesis of Na3AlH6 and Na2LiAlH6 by energetic mechanical alloying of stoichiometric mixtures of NaH, LiH and NaAlH4 was investigated. Upon milling, the mixture 2NaH+NaAlH4 transforms into the high temperature phase β-Na3AlH6. Similarly, the ball-milling of the mixture NaH+LiH+NaAlH4 produces Na2LiAlH6. The nature of phases was confirmed by X-ray diffraction and pressurized differential calorimetry.

Hydrogen storage properties of nanocrystalline Mg1.9Ti0.1Ni made by mechanical alloying

The mechanical alloying technique was used to make nanocrystalline Mg2Ni and Mg1.9Ti0.1Ni powders. Their hydrogen storage properties were characterized and compared with that of polycrystalline Mg2Ni made by casting. It was found that the ball milled Mg2Ni and Mg1.9Ti0.1Ni can absorb hydrogen at 473 K in the first cycle rapidly without prior activation. The nanocrystalline Mg1.9Ti0.1Ni showed the best kinetics, followed in order by mechanically alloyed Mg2Ni and cast Mg2Ni. Mg1.9Ti0.1Ni absorbs more than 3 wt% H2 in 2000 s at 423 K while the mechanically alloyed and the cast Mg2Ni do not form hydride at this temperature. Partial substitution of Mg by Ti reduced the activation energy of desorption from 69 kJ mol−1 for nanocrystalline Mg2Ni to 59 kJ mol−1 for Mg1.9Ti0.1Ni.