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 6.44MeV resonance of H-1(F-19,alpha gamma)O-16 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 hand 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. (C) 2008 Elsevier B.V. All rights reserved.