Hydrogen storage over alkali metal hydride and alkali metal hydroxide composites

Alkali metal hydroxide and hydride composite systems contain both protic (H bonded with O) and hydridic hydrogen. The interaction of these two types of hydrides produces hydrogen. The enthalpy of dehydrogenation increased with the increase of atomic number of alkali metals, i.e., -23 kJ/mol(H2) for LiOH-LiH, 55.34 kJ/mol(H2) for NaOH-NaH and 222 kJ/mol(H2) for KOH-KH. These thermodynamic calculation results were consistent with our experimental results. H-2 was released from LiOH-LiH system during ball milling. The dehydrogenation temperature of NaOH-NaH system was about 150 degrees C; whereas KOH and KH did not interact with each other during the heating process. Instead, KH decomposed by itself. In these three systems, NaOH-NaH was the only reversible hydrogen storage system, the enthalpy of dehydrogenation was about 55.65 kJ/mol(H2), and the corresponding entropy was ca. 101.23 J/(mol(H2)center dot K), so the temperature for releasing 1.0 bar H-2 was as high as 518 degrees C, showing unfavorable thermodynamic properties. The activation energy for hydrogen desorption of NaOH-NaH was found to be 57.87 kJ/mol, showing good kinetic properties.