MgH₂–ZrN composites for hydrogen generation by hydrolysis

Abstract

Nanostructured MgH₂-ZrN composites with different zirconium nitride contents were produced by mechanical milling in hydrogen. The phase composition of the composites was studied by X-ray powder diffraction. Traces of unreacted magnesium were found in the composites with 5 and 15 wt.% zirconium nitride. Scanning electron microscopy revealed that increase in the ZrN content led to MgH₂ refinement. The processes of hydrogen generation by hydrolysis of synthesized nanostructured MgH₂-ZrN composites were studied. The hydrolysis kinetics was examined in pseudoisothermal conditions by measuring the volume of displaced water. The optimal composition that promoted the maximum specific velocities and volume of hydrogen released was MgH₂-10 wt.% ZrN. The kinetic features peculiar to hydrolysis of the MgH₂-10 wt.% ZrN nanocomposite in MgCl₂ solutions were studied. When the MgCl₂ content increased from 0 to 50 mmol/l, the degree of conversion became twice as high. At the same time, a further increase in the MgCl₂ content to 100 mmol/l led only to a 3% increase in conversion. 500 ml H₂ in pure water and 990 ml H₂ in MgCl₂ solution were generated for 90 min by hydrolysis from 1 g of the MgH₂–10 wt.% ZrN composite. The activation parameters for hydrolysis of this nanocomposite material in water were calculated. The activation energies of freshly prepared and passivated MgH₂ were close: 44 ± 4 and 49 ± 7 kJ/mol, respectively. The degree of conversion increased from 26 to 63% when temperature raised from 40 to 60 °С.