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

Abstract

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.