X-ray Absorption Spectroscopy and X-ray Diffraction Studies of the Thermal and Li-Driven Electrochemical Dehydrogenation of Nanocrystalline Complex Hydrides Mg2MHx (M = Co, Ni)

The conversion reaction of Mg2MHx (M = Co, Ni) complex hydrides with lithium has been investigated by in and ex situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). The structural properties of the involved products are compared to those obtained by thermal decomposition as well as to the results of previous studies on the Mg2FeH6 hydride. For all complex hydrides, M–H bonding is clearly evidenced by XAS. Whatever the decomposition reaction (thermal or electrochemical), Mg–M alloys (MgCo and Mg2Ni) are obtained for Mg2CoH5 and Mg2NiH4, respectively, whereas hydrogen depletion from Mg2FeH6 leads to decomposition into elemental Mg and Fe. A significant disorder is observed for all M-containing phases, which increases as Fe < MgCo < Mg2Ni; affects mainly the transition-metal sites; and is more pronounced for the electrochemical route. The consequences of these structural properties on the expected reversibility of the conversion reaction of Mg-based complex hydrides with lithium are discussed.