Hydrogen storage in Pd capped thermally grown Mg films: Studies by nuclear resonance reaction analysis

The paper reports the studies on reversible hydrogen storage characteristics of Pd/Mg/Si (glass) films by depth profiling hydrogen using 6.44 MeV resonance of 1H(19F,αγ)16O nuclear reaction. These bilayered coatings wherein Mg films are prepared by thermal evaporation, exhibit excellent hydrogenation-dehydrogenation characteristics. Mg films absorb 6.0-7.0 wt.% hydrogen on hydrogenation under 0.15 MPa hydrogen gas pressure in 348-423 K temperature range for ≤5 h and undergo complete dehydrogenation in 3 h at 373 K under dynamic vacuum. Pd(40 nm)/Mg(250 nm) films perform satisfactorily up to two cycles of hydrogenation (348 K, 4 h) and dehydrogenation (373 K, 3 h), however, complete hydrogen release is affected in the third cycle. Hydrogen depth profiles suggest that hydriding starts at Pd/Mg interface and subsequently proceeds further into the interiors of the films with the formation of hydrogen deficient non-stoichiometric magnesium hydride as an intermediate step. The hydrogenated films exhibit room-temperature stability that depends on the temperature and duration of hydrogenation. The formation of metastable magnesium hydride may be responsible for the observed (de)hydrogenation characteristics of the films.