Magnesium for Hydrogen Storage

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.