Magnesium hydride is a promising hydrogen source because of its high mass density of hydrogen, 15.2%, when it is hydrolyzed; MgH 2 + 2H 2 O = Mg(OH) 2 + 2H 2 + 277 kJ. However, a magnesium hydroxide, Mg(OH) 2 , layer forms rapidly on the surface of the unreacted MgH 2 as the pH increases, hindering further reaction. The purpose of this study is to find acids that could effectively accelerate the reaction by using a chemical equilibrium analysis where the relationships of pH to concentration of ionized Mg were calculated. For the best performing acid, the calculated and measured relationships were compared, and the effects of acid concentration on hydrogen release were measured. The analysis revealed that citric acid and ethylenediamine-tetraacetic acid were good buffering agents. The calculated and measured relationships between pH and concentration of ionized Mg were in good accord. Hydrogen release improved considerably in a relatively dilute citric acid solution instead of pure distilled water. The maximum amount of hydrogen generated was 1.7  10 3 cm 3 ·g -1 at STP after 30 min. We estimated the exact concentration of citric acid solution for complete MgH 2 hydrolysis by a chemical equilibrium analysis method.

Abstract:This paper describes the ultrasonic irradiation on the hydrolysis of magnesium hydride to enhance hydrogen generation; the effects of the ultrasonic frequency and the sample size on the hydrogen generation were mainly examined. In the experiments, three MgH 2 particle and nanofiber samples were soaked in distilled water and ultrasonically irradiated at frequencies of 28, 45, and 100 kHz. Then, the amount of hydrogen generated was measured. We found that the low frequency of ultrasonic irradiation and the relatively small sample size accelerated the hydrolysis reaction MgH 2 + 2H 2 O = Mg(OH) 2 + 2H 2 + 277 kJ. In particular, the MgH 2 nanofibers exhibited the maximum hydrogen storage capacity of 14.4 mass% at room temperature at a frequency of 28 kHz (ultrasound irradiation). The results also experimentally validated that a combination of ultrasonic irradiation and MgH 2 hydrolysis is considerably effective for efficiently generating hydrogen without heating and adding any agent.