Study On Electrochemical Hydrogen Storage Performances Of Mg-Ni Based Alloys

Hydrogen is an ideal fuel for clean energy in the future because it is lightweight, highly abundant and friendly to environment. The realization of a hydrogen economy depends on the development of suitable hydrogen storage materials. Mg-based hydrogen storage alloys have attracted great attention as negative electrode materials for nickel-metal hydride (Ni-MH) batteries because of their low cost, light weight, rich mineral resources and high theoretical discharge capacity. For example, theoretical gravimetric storage hydrogen capacity of Mg2Ni alloy, assuming the formation of Mg2NiH4, is3.6mass%(equivalent to999mAh/g for the discharge capacity), which is approximately2.7times as large as that of LaNis. However, the practical applications of Mg2Ni-type alloy for Ni-MH batteries are full of challenges because of its sluggish hydriding/dehydriding kinetics, low electrochemical discharge capacity relative to the theoretical value at room temperature and poor cycle stability in alkaline solution.Mechanical alloying (MA) is a solid-state powder processing technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. This technique is capable of decreasing crystal size, introducing numbers of grain boundaries and defects, as well as facilitating the formation of nanostructured and amorphous phase. Therefore, MA has been widely used to prepare Mg2Ni-type alloy possessing favourable electrochemical hydrogen storage properties. Experimental and theoretical studies indicate that partial elements substitution is appropriate for improving electrochemical hydrogen storage properties of Mg2Ni-type alloy. Therefore, in this work, MA and partial elements substitution are combined to synthesize Mg-Ni based hydrogen storage alloys. Mn, Al and Ti have been chosen to substitute Ni or Mg in Mg2Ni-type alloy.Mg2Ni1-xMnx (x=0,0.125,0.25,0.375) electrode alloys are prepared by MA under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A new phase Mg3MnNi2is formed only when x=0.375after48h of milling. This is the first time that the one-step mechanical alloying in argon atmosphere is carried out to synthesize the new Mg3MnNi2phase directly. This new phase belongs to the face-centered cubic lattice (Fd-3m) with the lattice constant a being1.1484nm. The Mg3MnNi2phase could improve the electrochemical hydrogen storage properties of Mg2Ni-type alloy.The substitutional doping of Mn in Mg2Ni phase as well as the electronic structure of Mg3MnNi2phase has been investigated by first principles density functional theory calculations. The possibility of the site of Mn-substitution in Mg2Ni lattice has been confirmed to be Mg(6i)>Mg(6f)>Ni(3d)>Ni(3b) positions. The stability of phases gradually decreases along the sequence pure Mg2Ni phase> Mg3MnNi2phase>Mn-substitution doped Mg2Ni phase, which is consistent with the experimental results.Mg2-xAlxNi (x=0,0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by MA. The microstructures of synthesized alloys are characterized by XRD, SEM and transmission electron microscopy (TEM). XRD analysis results indicate that Al substitution results in the formation of AINi-type solid solution that can interstitially dissolve hydrogen atoms. In contrast, the addition of MWCNTs hardly affects the XRD patterns. SEM observations show that after co-milling with5wt.%MWCNTs, the particle sizes of both Mg2Ni and Mg1.75Al0.25Ni milled alloys are decreased explicitly. The TEM images reveal that ball milling is a good method to cut long MWCNTs into short ones. These MWCNTs aggregate along the boundaries and surfaces of milled alloy particles and play a role of lubricant to weaken the adhesion of alloy particles. The majority of MWCNTs retain their tubular structure after ball milling except a few MWCNTs whose tubular structure is destroyed. Electrochemical measurements indicate that all milled alloys have excellent activation properties. The Mg1.75Al0.25Ni-MWCNTs composite shows the highest discharge capacity due to the synergistic effects of MWCNTs and Al on the electrochemical hydrogen storage properties of Mg2Ni-type alloy. However, the improvement on the electrode cycle stability by adding MWCNTs is unsatisfactory.Mg2-xTixNi (x=0,0.5) electrode alloys have been prepared by MA under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by XRD,SEM and TEM. The effects of substitutional doping of Ti in Mg2Ni phase have been investigated by first principles density functional theory calculations. XRD analysis results indicate that Ti substitution for Mg in Mg2Ni-type alloys results in the formation of TiNi (Pm-3m) and Ni3Ti intermetallics. With the increase of milling time, the TiNi phase captures Ni from Mg2Ni to further form Ni3Ti phase. The calculated results of enthalpy of formation indicate that the most preferable site of Ti substitution in Mg2Ni lattice is Mg(6i) position and the stability of phase gradually decreases along the sequence TiNi3phase> TiNi phase> Mg9Ti3M2(6i)Ni6Ti-doped phase> Mg2Ni phase. SEM observations show that the average particle sizes of Mg2Ni and Mg1.5Ti0.5Ni milled alloys decrease and increase, respectively with increasing the milling time. The TEM analysis results reveal that TiNi and Mg2Ni coexist as nanocrystallites in the Mg1.5Ti0.5Ni alloy milled for20h. Electrochemical measurements indicate that the maximum discharge capacities of Mg2Ni and Mg1.5Ti0.5Ni alloys rise and decline, respectively with the prolongation of milling time. The Mg1.5Ti0.5Ni alloy milled for20h shows the highest discharge capacity among all milled alloys. The capacity retaining rate of Mg1.5Ti0.5Ni milled alloys is better than that of Mg2Ni milled alloys.