Hydrogenation behavior of a C14 Laves phase under ultra-high hydrogen pressure.

For high pressure metal hydride (MH) compressor applications, it is important to have a thorough understanding of the hydrogenation properties of MH forming compounds under high pressure and temperature conditions, which are still little studied. Here, a Ti 0.90 V 0.30 Mn 1.00 Ni 0.80 compound with MgZn 2 -type AB 2 structure has been investigated by Sieverts' method over a wide pressure and temperature range (up to 100 MPa and between −80 and 120 °C). This provided clear experimental evidence of the downward curvature of the van't Hoff plot in the high-pressure region, which arises from the non-ideal behavior of hydrogen. In this paper, we also show that the high-sorption pressures can be well estimated by the van't Hoff equation using low-pressure data. Ultimately, this study was combined with synchrotron radiation X-ray diffraction (SR-XRD) performed in an effort to monitor the structural evolution of the compound in situ under ultra-high hydrogen pressure. We found that even under extreme conditions of 2 GPa and 200 °C, both the main AB 2 phase and the secondary CsCl-type AB phase were fully hydrogenated and their structures were conserved. The existence of a critical temperature for the AB 2 -H 2 system was demonstrated from the evolution of the SR-XRD patterns recorded for several temperatures during the desorption process. [Display omitted] • Pressure-composition isotherms were measured up to 100 MPa using Sieverts' method. • High sorption pressures can be estimated from low pressure data using fugacity. • Curvature of the van't Hoff plot was observed experimentally at high pressures. • Hydrogen desorption isotherms were plotted from in situ synchrotron data. • Critical temperature features were observed in continuous unloading experiments. [ABSTRACT FROM AUTHOR]