Thermodynamic and kinetic characterization of the catalysed LiBH4 - MgH2 system

The LiBH4 – MgH2 system is of particular interest among the reactive hydride composites due to its high gravimetric capacity and the full reversibility of the sorption reactions. With the aim to realize a hydrogen storage tank based on this material, a full physico-chemical characterization of the 2:1 molar ratio composition has been undertaken, in order to obtain data fundamental for the simulation of the sorption processes and the design of the system. In particular, the reaction enthalpy, entropy and activation energy for all the sorption steps have been evaluated by PCT and coupled manometric – calorimetric measurements. Optical microscopy has been used to confirm the evolution of the different physico-chemical processes involving liquid and gas phases. Concerning the response of the system to the exothermal absorption and endothermal desorption reaction, the thermal conductivity of the composite in the charged and discharged state has been measured by the transient source method as a function of the temperature and the density, modified by compaction, of the samples. The effect of the density on the sorption properties and the cycling of the materials has been explored by kinetic measurements and scanning electron microscope investigations up to 20 full charging/discharging cycles on pellets compacted at pressure as high as 900 MPa. A strong effect has been noticed on the number of the so-called activation cycles and on the absorption kinetic performance, while no decrepitation and disaggregation effects have been observed, in spite of the presence of the borohydride liquid phase.