Measurement of the pressure-composition isotherms of high-temperature and low-temperature metal hydrides

Abstract

Many interesting applications are based on the knowledge of the thermodynamic equilibrium of the metal–metal hydride–hydrogen system. Practical applications for this system include hydrogen storage for stationary and vehicular purposes, hydrogen compression, heat pumps, cooling apparatus as well as heat stores. One of the most advantageous metal hydrides for high-temperature heat stores is nickel-doped magnesium, a material with excellent kinetic properties, high hydrogen concentration, low density and high heat of reaction. A disadvantage of magnesium is the minimum reaction temperature of about 423 K. The objective of this research work is to find low-temperature metal hydrides whose reaction enthalpy ΔrH is able to heat up the magnesium, starting from ambient temperature. In this study the results of an isochoric measurement method for determining pressure–composition isotherms (PCI), reaction enthalpies and reaction entropies are presented on the basis of already investigated as well as hitherto unsurveyed metal hydrides. The measuring method considers the real gas behavior in addition to a test reactor, which employs an improved sealing system. Presented in this study are the results of five metal–metal hydride–hydrogen systems in thermodynamic equilibrium. The measuring method has proved to be reliable because of the test reactor's sealing system.