PHOTOELECTRON SPECTROSCOPY STUDY ON THE BONDING CHARACTERISTICS OF Mg, Mg2Ni, Mg2Cu AND THEIR HYDRIDES

Abstract

Magnesium-based alloys are regarded as one kind of the most promising hydrogen storage materials. But they have two evident shortages: their hydrides too stable (working only at high temperatures) and poor hydriding and dehydriding kinetics. In order to improve their hydrogen storage properties, fundamental studies are urgently required. In order to understand the bonding characteristics of magnesium-based alloys and their hydrides, the authors measured the photoelectron spectra of Mg, Mg2Ni, Mg2Cu and their hydrides. The Mg 2p electron binding energy is 49.5 eV for Mg, 53.8 eV for MgH2, 49.4 eV for Mg2Cu, 49.2 eV for Mg2Ni, 49.2 eV for MgCu2, and 50.1 eV for Mg2NiH4. The Mg 2s electron binding energy changes in the same pattern. These values show that a quite large electron transfer from Mg atoms to H atoms occurs when Mg is hydrided to MgH2, and that the Mg-H bonds are rather ionic in nature. In the formation of intermetallicses from Mg and Ni (or Cu), electron transfer occurs from Ni(or Cu) atoms to Mg atoms. In the formation of Mg2NiH4 the increase of Mg 2p (or 2s) electron binding energy is much smaller than that in forming MgH2, and there shows some increase in the Ni 2p electron binding energy, which may suggest that Ni atoms as well as Mg atoms contribute some electrons to H atoms when Mg2Ni is hydrided, and the Mg-H bonds here are not so ionic as in MgH2. From the measured results, a micro-model is proposed to explain the stability and kinetics properties of hydrides of Mg and Mg-intermetallicses Mg2Ni and Mg2Cu.