Abstract
Although Mg based hydrogen storage materials absorb hydrogen immediately under appropriate thermodynamic conditions, an obvious incubation period exists for hydrogen desorption. This fact implies that the mechanisms of hydrogen absorption and desorption are different from each other. During hydrogen desorption process, nucleation and growth of Mg are occurring in MgH2. It is considered that free surface is the only nucleation site of Mg in hydrogen desorption process. The nucleation frequency per unit specific surface area should be constant under a fixed temperature and H2 partial pressure. Therefore, it is estimated that the incubation period is proportional to the inverse of specific surface area.
The relationship between incubation period and specific surface area of MgH2 was investigated in order to understand the mechanism of incubation period on magnesium-hydrogen system. MgH2 with several kinds of specific surface area were manufactured by changing milling time of a planetary ball mill. A primary Gibbs free energy for dehydrogenation was conducted as −1.94 kJ mol−1 by controlling H2 pressure for measuring incubation period. The results revealed that the incubation period was proportional to the inverse of specific surface area.
