J.O. Ström-Olsen

Structure, catalysis and atomic reactions on the nano-scale: a systematic approach to metal hydrides for hydrogen storage

Abstract.We show how reducing structure, catalysis and atomic reactions to the nano-scale may be used in a systematic way to substantially enhance the hydrogenation properties of metal hydrides. We examine, with examples from a wide range of hydrides, the direct impact of nano-scale structure, subsequent improvements in kinetics through nano-scale solid state catalysis, the special properties of nano-composites, and the role played by nano-scale reactions.

Nanocrystalline metal hydrides

Nanocrystalline hydrides are a new class of material in which outstanding hydrogen sorption may be obtained by proper engineering of the microstructure and surface. In this paper the thermodynamics and kinetics of hydride formation in a nanocrystalline alloy are presented. The role of grain size, grain boundaries, internal strain and chemical disorder is discussed, as is the effect of catalyst particles at the surface of the metal. Powders of nanocrystalline alloys, modified with a catalyst, readily absorb hydrogen, with no need for prior activation, in strong contrast to conventional hydrides. The materials show substantially enhanced absorption and desorption kinetics, even at relatively low temperatures. Examples of various nanocrystalline metal hydrides are given, based on Mg, Mg2Ni, FeTi and LaNi5.

Hydrogen absorption in nanocrystalline Mg2Ni formed by mechanical alloying

Abstract High-energy ball milling has been used to produce nanocrystalline Mg2Ni with grain sizes of about 20–30 nm. Results on the hydrogen storage characteristics of the material are presented. Enhancements were found in the kinetics of hydrogen absorption and in the activation pretreatment. Nanocrystalline Mg2Ni readily absorbs hydrogen at temperatures lower than 250 °C. Further improvements (i.e. absorption at room temperature with relatively good kinetics) have been effected by additional surface modification with Pd catalyst.

Synergy of hydrogen sorption in ball-milled hydrides of Mg and Mg2Ni

A remarkable enhancement of hydrogen desorption kinetics has been found for magnesium-based materials, ball-milled in the hydrogenated state. Ball-milling has been used to introduce both strain and structural changes in the hydrides of Mg, Mg2Ni and their mixtures, facilitating desorption of hydrogen and reducing the operational desorption temperature. Moreover, ball-milling of the mixtures of MgH2 and Mg2NiH4 results in a synergetic effect of desorption, allowing the mixture to operate at temperatures of 220°C–240°C, with excellent absorption/desorption kinetics and with total hydrogen capacity exceeding 5 wt.%. This behaviour has been maintained over hydrogenation cycling. It is concluded that mechanically-treated hydrides offer a new opportunity for magnesium-based materials, exploiting the high capacity of magnesium hydride while operating at much lower temperature than conventional MgH2.

Sodium alanates for reversible hydrogen storage

Abstract In this paper we show that sodium alanates may be used for reversible hydrogen storage, with the advantage of having high storage capacity combined with low cost. Both NaAlH 4 and Na 3 AlH 6 have been investigated for this application, and two complementary techniques have been used: improvement of the reaction kinetics by mechanical grinding, and chemical modification of the alloys. By these methods remarkable desorption/absorption kinetics are obtained. Sodium alanates so modified are capable of reversible hydrogen storage at the relatively low temperatures of around 80–140°C, with a capacity of between 2.5 and 3.0 wt.%. The hydrides have an even higher reversible capacity of about 4.5–5 wt.% when operated at temperatures around 150–180°C.

Crystallization characteristics of Ni‐Zr metallic glasses from Ni20Zr80 to Ni70Zr30

We present the results of a crystallization study in glassy Ni‐Zr. The alloys were prepared by melt spinning over the continuous range Ni20Zr80 to Ni70Zr30 including for the first time the region between Ni40Zr60 and Ni60Zr40. As expected we find that the glasses are most stable at and around the eutectic compositions, except that at the extreme Zr‐rich eutectic the glasses are destabilized by the premature precipitation of ω‐Zr. The products of crystallization indicate that the current crystalline phase diagram is incomplete: a new peritectoid phase must be added at the composition Ni2Zr.

The influence of oxygen and other impurities on the crystallization of NiZr2 and related metallic glasses

The role of oxygen and other impurities on the crystallization characteristics of Ni‐Zr glasses near the composition NiZr2, as well as for FeZr2, CoZr2, and NiHf2, has been investigated. For NiZr2 glasses with 1 at. % oxygen, the first crystallization product is the metastable E93 structure with a =1.227 nm instead of the equilibrium C16 structure. A similar effect is found for samples containing ≳3 at. % B. For FeZr2, CoZr2, and NiHf2 the first crystallization product is also E93 structure, even with very small levels of oxygen (≤0.2 at. %). The formation of the E93 structure is always accompanied by an increase in the electrical resistivity, an increase which transmission electron microscopy shows is intrinsic to the phase and unrelated to crystallite size. For Ni36.5Zr63.5 and Ni42Zr58 the crystallization is also accompanied by an increase in electrical resistance and the evolution of a crystal structure similar to the E93 structure in the size of the unit cell and packing fraction but with a different...

Crystallization characteristics of Cu‐Zr metallic glasses from Cu70Zr30 to Cu25Zr75

The crystallization characteristics of glassy Cu‐Zr has been studied by differential scanning calorimetry, x‐ray diffraction, and measurement of electrical resistance and magnetic susceptibility. Our results allow us to remove many discrepancies between previously published data; they also suggest that the equiatomic Cu‐Zr does not exist as a single phase in the crystalline state. We observe parallel changes on crystallization between the magnetic susceptibility and the electrical resistance, which we interpret through changes in the d density of states.

Catalytic effect of Pd on hydrogen absorption in mechanically alloyed Mg2Ni, LaNi5 and FeTi

In this paper we present the effect of palladium catalysis on the surface activity of nanocrystalline materials. Three classic hydrogen-absorbing alloys, Mg2Ni, LaNi5, and FeTi, were prepared in a nanocrystalline state by ball milling and modified by the addition of small amounts of Pd (less than 1 wt.%). This modification gave powders able to absorb hydrogen at room temperature in the as-produced state, with no need for any activation. Hydrogen absorption characteristics for the nanocrystalline Mg2Ni, LaNi5 and FeTi were enhanced: the absorption rates were much faster, even at lower temperatures.

Hydrogenation properties of complex alkali metal hydrides fabricated by mechano-chemical synthesis

Mechano-chemical synthesis was used to produce complex alkali metal hydrides. Various complex hydrides have been synthesized, for example Li3AlH6, Na3AlH6, (Li–Na)3AlH6, (Li–Na–B)3AlH6. Complex alkali metal hydrides fabricated by mechano-chemical processes exhibit very good reactivity in hydriding/dehydriding reactions. They have excellent cyclability and cover a wide range of operational temperatures and pressures.