Nanocrystalline magnesium for hydrogen storage

Hydrogen storage in Mg: A most promising material

Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials

Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy because of its prominent advantages such as high energy density (142 MJ kg(-1); ref. 1), great variety of potential sources (for example water, biomass, organic matter), light weight, and low environmental impact (water is the sole combustion product). However, there remains a challenge to produce a material capable of simultaneously optimizing two conflicting criteria--absorbing hydrogen strongly enough to form a stable thermodynamic state, but weakly enough to release it on-demand with a small temperature rise. Many materials under development, including metal-organic frameworks, nanoporous polymers, and other carbon-based materials, physisorb only a small amount of hydrogen (typically 1-2 wt%) at room temperature. Metal hydrides were traditionally thought to be unsuitable materials because of their high bond formation enthalpies (for example MgH(2) has a ΔHf~75 kJ mol(-1)), thus requiring unacceptably high release temperatures resulting in low energy efficiency. However, recent theoretical calculations and metal-catalysed thin-film studies have shown that microstructuring of these materials can enhance the kinetics by decreasing diffusion path lengths for hydrogen and decreasing the required thickness of the poorly permeable hydride layer that forms during absorption. Here, we report the synthesis of an air-stable composite material that consists of metallic Mg nanocrystals (NCs) in a gas-barrier polymer matrix that enables both the storage of a high density of hydrogen (up to 6 wt% of Mg, 4 wt% for the composite) and rapid kinetics (loading in <30 min at 200 °C). Moreover, nanostructuring of the Mg provides rapid storage kinetics without using expensive heavy-metal catalysts.

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Air-stable magnesium nanocomposites provide rapid and high-capacity hydrogen storage without using heavy-metal catalysts

We examined the catalytic effect of nanoparticle 3d-transition metals on hydrogen desorption (HD) properties of MgH(2) prepared by mechanical ball milling method. All the MgH(2) composites prepared by adding a small amount of nanoparticle Fe(nano), Co(nano), Ni(nano), and Cu(nano) metals and by ball milling for 2 h showed much better HD properties than the pure ball-milled MgH(2) itself. In particular, the 2 mol % Ni(nano)-doped MgH(2) composite prepared by soft milling for a short milling time of 15 min under a slow milling revolution speed of 200 rpm shows the most superior hydrogen storage properties: A large amount of hydrogen ( approximately 6.5 wt %) is desorbed in the temperature range from 150 to 250 degrees C at a heating rate of 5 degrees C/min under He gas flow with no partial pressure of hydrogen. The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nanoparticle Ni metals as catalyst homogeneously dispersed on the surface of MgH(2). In addition, it was confirmed that the product revealed good reversible hydriding/dehydriding cycles even at 150 degrees C. The hydrogen desorption kinetics of catalyzed and noncatalyzed MgH(2) could be understood by a modified first-order reaction model, in which the surface condition was taken into account.

Catalytic effect of nanoparticle 3d-transition metals on hydrogen storage properties in magnesium hydride MgH2 prepared by mechanical milling.

The absorption of hydrogen in magnesium (purity, 99.8%–99.94%) was studied in the temperature range 260–425 °C and at pressures from the equilibrium value to 2 MPa above equilibrium. At constant temperature the absorption rate depends on the pressure whereas total absorption is attained in approximately the same time regardless of the pressure. The final composition is very close to stoichiometric MgH2 except at the lowest temperatures and highest pressures investigated when the reaction becomes extremely slow or ceases completely at 80%–90% of the stoichiometric composition. Accurate determination of the relative molar enthalpy gives a value of −70 kJ (mol H2)−1. No hysteresis in the ordinary sense was observed, but although no change in the plateau pressure occurred desorption of from 5%–15% of the remaining hydrogen required almost zero pressure. The actual value appears to be dependent on the material.

Formation and decomposition of magnesium hydride

Experiments on a large number of magnesium powders varying in particle size, morphology, purity and surface oxidation have shown that most of these parameters influence the kinetics of the hydride formation. Although no single step in the reaction will in general be rate determining we have in a previous investigation been able to assign a nucleation and growth model to the initial hydriding of an atomized powder. This powder of nearly spherical particles (d ≈ 90 μm) with a thin oxide coating has also been used in this investigation of the nucleation and growth mechanisms. It is found that the nucleation is rate determining in the initial hydriding only and that the growth takes place entirely by interface migration of hydrogen from the particle surface. The pressure-nucleation relation and impurity effects on the ultimate degree of reaction are discussed.

Elements of hydride formation mechanisms in nearly spherical magnesium powder particles

The reaction of hydrogen with commercially pure magnesium powder (above 99.7%) was investigated in the temperature range 250–400 °C. Hydrogen is readily sorbed above the dissociation pressure. During the initial exposure the magnesium powder sorbs hydrogen slowly below 400 °C but during the second exposure the sorption is fast from about 250 °C and is nearly completed when 400 °C is reached after 10 min; no change in the sorption rate is observed with further cycling. In most experiments the resultant hydride is close to stoichiometric MgH2. Desorption is found to be slower and to require higher temperatures than sorption but is still practicable. Comparisons of powders with particle sizes ranging from less than 37 μm to more than 500 μm indicate that the specific surface area is the rate-determining factor. Scanning electron micrographs show that after sorption the particles become spongy. The fact that the particles do not disintegrate is explained by a sintering process at the working temperatures. Exposure to air does not impair the sorption ability; on the contrary, it appears that surface oxidation plays an important role in the reaction. Some handling problems, e.g. the reaction of the hydride with water vapour in air of normal humidity, were also investigated.

Magnesium for hydrogen storage

Formerly the hydriding of magnesium was considered to be a very slow, incomplete process. During the last ten years, however, it has been demonstrated that magnesium in the form of finely comminuted powder (50 μm or less) readily reacts with hydrogen to form the stoichiometric compound MgH 2 completely. Attempts have been made to describe theoretically the formation of hydride in more or less spherical magnesium particles. However, very few studies have been done on the hydriding of magnesium foils. In our study pure magnesium was rolled to foil thicknesses down to 20 μm. The foils were exposed to hydrogen in the pressure range 1–3 MPa and temperatures between 650 and 700 K. The hydride formation was followed by gravimetry and by microscopy. The effect of foil thickness on the hydride formation is described and the mechanism and kinetics of hydride formation are discussed. It is concluded that the chemical reaction between magnesium and hydrogen is the rate-limiting step for the growth of the nuclei and that the transport processes are fast.

B. Larsen

Papers

Formation and decomposition of magnesium hydride

Elements of hydride formation mechanisms in nearly spherical magnesium powder particles

Magnesium for hydrogen storage

Magnesium for hydrogen storage

The formation of hydride in pure magnesium foils