Chemical equilibrium analysis for hydrolysis of magnesium hydride to generate hydrogen

Magnesium hydride is a promising hydrogen source because of its high mass density of hydrogen, 15.2%, when it is hydrolyzed; MgH2 + 2H2O = Mg(OH)2 + 2H2 + 277 kJ. However, a magnesium hydroxide, Mg(OH)2, layer forms rapidly on the surface of the unreacted MgH2 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 × 103 cm3 g−1 at STP after 30 min. We estimated the exact concentration of citric acid solution for complete MgH2 hydrolysis by a chemical equilibrium analysis method.