Kinetic performance of hydrogen generation enhanced by AlCl3 via hydrolysis of MgH2 prepared by hydriding combustion synthesis

Abstract

Magnesium hydride (MgH2) is a very promising hydrogen storage material and it has been paid more and more attention on the application of supplying hydrogen on-board because the theoretical hydrogen yield is up to 1703 mL/g when it reacts with water. However, the hydrolysis reaction is inhibited rapidly by the passivation layer of Mg(OH)2 formed on the surface of MgH2. This paper reports that high purity MgH2 (∼98.7 wt%) can be readily obtained by the process of hydriding combustion synthesis (HCS) and the hydrogen generation via hydrolysis of the as-prepared HCSed MgH2 can be dramatically enhanced by the addition of AlCl3 in hydrolysis solutions. An excellent kinetics of hydrogen generation of 1487 mL/g in 10 min and 1683 mL/g in 17 min at 303 K was achieved for the MgH2-0.5 M AlCl3 system, in which the theoretical hydrogen yield (1685 mL/g) of the HCSed product was nearly reached. The mechanism of the hydrolysis kinetics enhancement was demonstrated by the generation of a large amounts of H+ from the Al3+ hydrolysis and the pitting corrosion (Cl−) of the Mg(OH)2 layer wrapped on the surface of MgH2 even at a low temperature. In addition, the apparent activation energies for the MgH2 hydrolysis in the 0.1 M AlCl3 and 0.5 M AlCl3 solutions are decreased to 34.68 kJ/mol and 21.64 kJ/mol, respectively, being far superior to that of reported in deionized water (58.06 kJ/mol). The results suggest that MgH2 + AlCl3 system may be used as a promising hydrogen generation system in practical application of supplying hydrogen on-board.