Porous Mg formation upon dehydrogenation of MgH2 thin films

The hydrogenation and dehydrogenation of a thin film of Mg with a Pd cap layer was measured using neutron reflectometry. Upon hydrogenation, (at 373 K and 0.2 MPa H2), the Mg film swelled in the surface normal direction by an amount roughly equal to the difference in volume between MgH2 and Mg. After dehydrogenation (at 343–423 K), the Mg film returned to a composition of Mg but retained the swelled thickness by incorporating voids. The presence of the voids is confirmed by SEM micrographs. The voids may explain some of the changes in absorption kinetics after full cycling of Mg films.