Highly improved electrochemical performances of the nanocrystalline and amorphous Mg2Ni-type alloys by substituting Ni with M (M = Cu, Co, Mn)

Abstract

The element Ni in the Mg2Ni alloy is partially substituted by M (M = Cu, Co, Mn) in order to ameliorate the electrochemical hydrogen storage performances of Mg2Ni-type electrode alloys. The nanocrystalline and amorphous Mg20Ni10-x M x (M = None, Cu, Co, Mn; x = 0-4) alloys were prepared by melt spinning. The effects of the M (M = Cu, Co, Mn) content on the structures and electrochemical hydrogen storage characteristics of the as-cast and spun alloys were comparatively studied. The analyses by XRD, SEM and HRTEM reveal that all the as-cast alloys have a major phase of Mg2Ni but 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. Besides, the as-spun (M = None, Cu) alloys display an entirely nanocrystalline structure, whereas the as-spun (M = Co, Mn) alloys hold a nanocrystalline/amorphous structure, suggesting that the substitution of M (M = Co, Mn) for Ni facilitates the glass formation in the Mg2Ni-type alloys. The electrochemical measurements indicate that the variation of M (M = Cu, Co, Mn) content engenders an obvious effect on the electrochemical performances of the as-cast and spun alloys. To be specific, the cyclic stabilities of the alloys augment monotonously with increasing M (M = Cu, Co, Mn) content, and the capacity retaining rate (S20) is in an order of (M = Cu) > (M = Co) > (M = Mn) > (M = None) for x≤1 but changes to (M = Co) > (M = Mn) > (M = Cu) > (M = None) for x≥2. The discharge capacities of the as-cast and spun alloys always grow with the rising of M (M = Co, Mn) content but first mount up and then go down with increasing M (M = Cu) content. Whatever the M content is, the discharge capacities are in sequence: (M = Co) > (M = Mn) > (M = Cu) > (M = None). The high rate discharge abilities (HRDs) of all the alloys grow clearly with rising M (M = Cu, Co) content except for (M = Mn) alloy, whose HRD has a maximum value with varying M (M = Mn) content. Furthermore, for the as-cast alloys, the HRD is in order of (M = Co) > (M = Mn) > (M = Cu) > (M = None), while for the as-spun (20 m·s-1) alloys, it changes from (M = Co) > (M = Mn) > (M = Cu) > (M = None) for x = 1 to (M = Cu) > (M = Co) > (M = None) > (M = Mn) for x = 4.