Comparative study of electrochemical performances of the as-melt Mg20Ni10-xMx (M = None, Cu, Co, Mn; x=0, 4) alloys applied to Ni/metal hydride (MH) battery

The partial substitution of M (M = Cu, Co, Mn) for Ni has been performed in order to ameliorate the electrochemical hydrogen storage performances of Mg2Ni-type electrode alloys. The melt spinning technology was used to prepare the Mg20Ni10-xMx (M = None, Cu, Co, Mn; x = 0, 4) electrode alloys. The impacts of the melt spinning and the M (M = Cu, Co, Mn) substitution on the structures and electrochemical hydrogen storage characteristics of the alloys were investigated systemically. The analysis of XRD and TEM reveals that the as-spun (M = None, Cu) alloys display an entire nanocrystalline structure, whereas the as-spun (M = Co, Mn) alloys hold a nanocrystalline/amorphous structure, indicating that the substitution of M (M = Co, Mn) for Ni facilitates the glass formation in the Mg2Ni-type alloys. Besides, all the as-cast alloys have a major phase Mg2Ni, whereas the M (M = Co, Mn) substitution brings on the formation of some secondary phases, MgCo2 and Mg for the (M = Co) alloy, and MnNi and Mg for the (M = Mn) alloy. Based upon the electrochemical measurements, an evident impact engendered by melt spinning on the electrochemical performances of the alloys appears. The cycle stability of the alloys augments monotonously with the growing of the spinning rate. The discharge capacity and high rate discharge ability (HRD), however, act differently. Melt spinning enhances the cycle stability of the (M = Co, Mn) alloys dramatically, but exerts an adverse impact on the (M = None, Cu) alloys. The high rate discharge ability (HRD) of all the alloys grow considerably with the rising of the spinning rate except for (M = Mn) alloy, whose HRD has a maximum value with the variation of the spinning rate. Furthermore, the substitution of M (M = Cu, Co, Mn) for Ni enhances the electrochemical performances of the as-cast and spun Mg2Ni alloys evidently, but the M (M = Mn) substitution gives rise to a decline of HRD when the spinning rate reaches 20 m/s. (C) 2012 Elsevier B. V. All rights reserved.