Hydrogen absorbing properties of ScM₂ Laves phase alloys (M = Fe, Co and Ni)

Hydrogen absorbing properties of ScM₂ (M = Fe, Co and Ni) Laves phase alloys were studied by pressure-composition (PC) isotherm measurements at the temperature range from 293 K to 353 K. Crystal structural change of these alloys during hydrogenation was also studied by in situ X-ray powder diffraction measurements under hydrogen pressure up to 4 MPa. Clear plateaus were observed in all the absorption and desorption isotherms. The enthalpy of formation of ScM₂H_x (M = Fe, Co and Ni) was estimated from the equilibrium plateau pressure observed in the PC isotherm to be -28(1), -30(1) and -16(2), respectively, in units of kJ per mol H₂. In situ X-ray diffraction measurements revealed that the hydrogenation did not produce any change in metal substructure or any indication of amorphization in the ScM₂-H₂ systems (M = Fe, Co and Ni). The cell volume, however, increased by 23.1% (Fe), 21.0% (Co) and 14.5% (Ni) upon hydrogenation.