Effect of TiH2 and Mg2Ni additives on the hydrogen storage properties of magnesium hydride

A study was done to determine the effect of TiH2 and Mg2Ni additives on the hydrogen sorption behavior of MgH2. A series of mixtures were made in which MgH2 was ball milled with various amounts of TiH2, Mg2Ni or combinations of both. Temperature programmed desorption (TPD) analysis showed that the addition of increasing amounts of TiH2 to MgH2 resulted in a progressive reduction of the onset temperature of MgH2 by as much as 70 °C for a mixture containing 50 mol% TiH2. However the hydrogen yield also decreased from a high of 7.5 wt% for pure MgH2 to a low of 2.4 wt% for the mixture containing 50 mol% TiH2. The rates of H2 reaction with MgH2 increased with increasing amounts of TiH2. Pressure composition isotherms were also made for each of the mixtures at several temperatures and van't Hoff plots were constructed. The results showed that the enthalpies for hydrogen absorption systematically decreased with increasing amounts of TiH2, from a high of 76 kJ/mole H2 for pure MgH2 to a low of 65 kJ/mole H2 for a mixture containing 50 mol% TiH2. This indicated that thermodynamic stability can be reduced by TiH2 additions. When Mg2Ni was added to these mixtures, the desorption temperatures were reduced dramatically. The addition of 6 or 10 mol% of Mg2Ni to MgH2 lowered the onset temperature from 330 to 190 °C with a hydrogen yield of 6 wt%. The addition of both TiH2 and Mg2Ni catalysts did not result in any further reduction in the onset temperatures. However, the mixture of both catalysts was more effective at increasing reaction rates than either of them alone.