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. H₂ was released from LiOH-LiH system during ball milling. The dehydrogenation temperature of NaOH-NaH system was about 150 °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·K), so the temperature for releasing 1.0 bar H₂ was as high as 518 °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.