B. Larsen

Formation and decomposition of magnesium hydride

Abstract 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.

Elements of hydride formation mechanisms in nearly spherical magnesium powder particles

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.

Magnesium for Hydrogen Storage

Abstract A study of the hydrogenation characteristics of fine magnesium powder during repeated cycling has been performed using a high-pressure microbalance facility. No effect was found from the cycling regarding kinetics and storage capacity. The reaction rate of the absorption process was fast at temperatures around 600 K and above, but the reversed reaction showed somewhat slower kinetics around 600 K. At higher temperatures the opposite was found. The enthalpy and entropy change by the hydrogenation, derived from pressure-concentration isotherms, agree fairly well with those reported earlier.

Magnesium for hydrogen storage

Abstract 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.

The formation of hydride in pure magnesium foils

Abstract 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 MgH2 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.

Hydrogen sorption performance of pure magnesium during continued cycling

Abstract Preliminary investigations of the hydrogen absorption - desorption by commercially pure magnesium powder under continuous operation show little or no reduction in hydrogen capacity up to 70 cycles and high temperature exposure exceeding 1200 h. Absorption was studied at 260°–425°C and hydrogen pressures up to 2.0 MPa above equilibrium. Desorption was with a few exceptions done at 400°C at hydrogen pressures below 150 kPa. For practical application the hydrogen exchange may be limited to 75–90% of the complete metal to stoichiometric hydride reaction. A change of the macroscopic structure of the powder into a highly porous, sintered agglomerate did not reduce the hydrogen capacity or the reaction rate. Although this change in structure caused no deterioration of the cycling performance a further development may not be acceptable. For observation over a much larger number of cyclings a fully automated, triple line cycling facility permitting simultaneous testing under different conditions has been constructed.

On the hydrogenation mechanism in magnesium I

Abstract The first time hydriding of spherical magnesium particles covered by a thin oxide layer and sieve-fractionated into narrow size distributions within the range 40–90 μm was followed by microgravimetry. The size distributions of the fractions were determined by semiautomatic image analysis. The hydridings were run at 402°C and 3 MPa hydrogen pressure after heating in helium. A dependence of the rate of hydriding on the heat treatment prior to reaction was observed and it is proposed that the heat treatment causes oxygen atoms to diffuse into the bulk metal and thereby break up the protective oxide layer. Based on the observed hydride propagation in the metal particles, a statistical model for the hydriding of a particle is applied to the hydriding curves for a series of samples. The data are found to be in fine agreement with the proposed model. It is concluded that care must be taken when generalizing results from the hydriding of magnesium powders.

Effect of oxygen on the Mg-H reaction☆

Abstract Two identical samples of magnesium powder (purity, 99.58%) were hydrogenated at approximately 30 bar and 380 °C and dehydrogenated under vacuum at the same temperature about 500 times. The first sample was exposed to pure hydrogen (purity, 99.9999%) and the second was exposed to hydrogen containing 85 ppm O and 8 ppm H2O vapour. In both experiments a moderate overall reduction in the amount of hydrogen absorbed and desorbed was observed. This can be ascribed to a reduced absorption rate with increased number of cycles. The effect of oxygen was negligible, and this was confirmed by a precision absorption measurement performed after the cycling experiment. Despite the decreased absorption rate, which was mainly observed at higher degrees of reaction, little change in the desorption kinetics was observed.

Long-Term Cycling of the Magnesium Hydrogen System

Abstract Magnesium powder with a grain size of approximately 50γm was hydrogenated for 30 min and dehydrogenated the same time at 390°C, 515 times. A moderate loss in hydrogen storage capacity was observed and was ascribed to a measured decrease in reaction kinetics as the cycle number increased. The time for maximum hydrogen absorption was found to depend significantly on cycle number while the time for maximum desorption was found to be virtually independent of cycle number.