Rate-Controlling Steps for the Hydriding Reaction of the Intermetallic Compound Mg₂Ni.

Mg₂Ni samples were prepared by sintering a pelletized mixture under an argon atmosphere in a stainless steel crucible at 823 K. The XRD pattern of the prepared Mg₂Ni sample showed a well crystallized Mg₂Ni phase. The hydriding and dehydriding properties of the prepared samples were examined at 518-593 K under relatively low hydrogen pressures of 3-7 bar H₂. At 573 K under 7 bar H₂, the activation of the Mg₂Ni sample was completed at the number of cycles of six (n = 6). At n = 7, the hydrided fractions of the sample were 0.53 (1.99 wt% H) at 4.97 min, 0.72 (2.71 wt% H) at 9.52 min, 0.81 (3.05 wt% H) at 31.15 min, and 0.81 (3.05 wt% H) at 60.07 min. The particle sizes of the prepared Mg₂Ni were not homogeneous and the particles had irregular shapes. We analyzed the rate-controlling steps for the hydriding reaction of the intermetallic compound Mg₂Ni by examining the dependences of hydriding rates on hydrogen pressure and temperature in the same reacted fraction ranges. The analyses in the same reacted fraction ranges were done in order to eliminate the influence of the interfacial area on the hydriding rate. When the driving force, which is the difference between the applied hydrogen pressure Po and equilibrium plateau pressure Peq at a given temperature, was low, the nucleation of Mg₂Ni hydride controlled the hydriding rate of Mg₂Ni. After the nucleation of the Mg₂Ni hydride, the rate-controlling step of the hydriding reaction of Mg₂Ni was analyzed to be the forced flow of hydrogen molecules through pores, inter-particle channels, or cracks.