Mechanism of rapid degradation of nanostructured Mg2Ni hydrogen storage alloy electrode synthesized by mechanical alloying and the effect of mechanically coating with nickel

Nanocrystalline Mg2Ni metal hydride was prepared by mechanical alloying. It had good activation properties and showed a large discharge capacity at room temperature. Nevertheless, it lost 80% of its maximum capacity within ten cycles. The mechanism underlying the rapid, early capacity loss of this material was investigated via various analyses (scanning electron microscopy [SEM], X-ray diffraction. X-ray photoelectron spectroscopy, Auger electron spectroscopy, inductively coupled plasma atomic emission spectroscopy [ICP]. The Mg2Ni phase decomposed to form a Mg(OH)(2) layer at the surface. The thickness of this layer was about 4000 Angstrom after ten cycles. Severe surface cracking during charging/discharging was not revealed by SEM. The amount of the dissolved active materials (Mg, Ni) in the electrolyte was dimunitive. The main cause of the rapid degradation of the electrode was the formation of the passive layer at the surface, which hindered the charge transfer reaction. Mechanical coating with nickel effectively reduced the degradation rate of the electrode. (C) 1999 Elsevier Science S.A. All rights reserved.