In situ observation and kinetic modeling of the fundamental mechanisms underlying hydrogen sorption in forged Mg–Mg2Ni composites

While Mg–Mg₂Ni composites are promising for hydrogen storage, their implementation is hindered by our incomplete understanding of absorption/desorption kinetics. Here, we combine in situ neutron diffraction with kinetic and microstructural analyses to uncover the sorption mechanism of deuterated hydrogen D₂ in a Mg–Mg₂Ni composite processed by fast forging. Phase transitions upon first absorption are found to be different from subsequent absorptions. The first absorption involves rapid formation of Mg₂NiD_0.3-x followed by simultaneous formation of MgD₂ and Mg₂NiD₄. Kinetic modeling indicates that surface nucleation of the magnesium hydride is rate-limiting. Subsequent absorptions involve two phases, Mg and Mg₂NiD_0.3-x, which promote absorption. Kinetic modeling and microstructure analysis indicate that (1) MgD₂ nucleation occurs at the Mg–Mg₂NiD_0.3-x interface and (2) Mg₂NiD₄ formation is kinetically controlled by deuterium diffusion through the growing Mg₂NiD₄ plate. In all desorptions, deuterium release starts by rapid decomposition of Mg₂NiD₄ into Mg₂NiD_0.3-x, followed by slower MgD₂ decomposition.