Structural characterization and electrochemical hydrogen storage properties of Mg2Ni1−xMnx

 Introduction 

Mg-based (Mg2Ni-type) hydrogen storage alloys are considered to be promising negative electrode materials for nickelmetal hydride (Ni-MH) batteries because of their low cost, light weight, rich mineral resources and high theoretical discharge capacity [1,2]. However, the poor hydriding/dehydriding kinetics and low electrochemical discharge capacity at room temperature [3], as well as severe oxidation in alkaline solution of Mg2Ni-type electrode become the obstacle for the practical use in Ni-MH batteries. 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 [4]. 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 properties [5e7]. Partial elements substitution has been proved to be appropriate for improving electrochemical properties of Mg2Ni-type alloy [8e10]. Theoretical calculation has indicated that partial substitution of alloying element M for Ni in Mg2Ni can modify the enthalpy of formation for hydride, resulting in altering the phase stability [11]. The substitution of Mn for Ni or Mg in Mg2Ni-type alloys has been reported in many literatures. Woo et al. [6] performed the substitution of Mn for Ni in Mg2Ni alloy to improve the electrochemical properties. Yang et al. [12] found that replacement of Ni in Mg2Ni by Mn lowered the decomposition plateau pressure. Jurczyk et al. [13] obtained an enhanced discharge capacity by substituting Mg with Mn in Mg2Ni alloy. Kohno et al. [14] reported that as a result of substitution of Mg with Mn, absorption of hydrogen occurred at lower temperature. Gasiorowski et al. [15] found that the partial substitution of Mg by Mn in Mg2Ni alloy leaded to an increase in discharge capacity at room temperature. It can be seen that different authors used different substitution method. Some used Mn to substitute Ni, while others used Mn to substitute Mg in Mg2Ni. Tsushio et al. [16] proposed that for designing a quaternary alloy system of Mg2Ni by substitution method, the substitution for Ni had to be considered first and then for Mg. However, none of these literatures have reported the formation of the new Mg3MnNi2 phase. Three preparing methods have been reported to synthesize the Mg3MnNi2 alloy. Denys et al. [17] synthesized Mg3MnNi2 intermetallic compound by high-energy ball milling in argon atmosphere with subsequent pressing and sintering. They found that the synthesized Mg3MnNi2 phase exhibited an ideal electrochemical discharge curve and good cyclic stability in comparison with other magnesium alloys. Dobrovolsky et al. [18] obtained Mg3MnNi2 using milling a mixture of Mg, Ni and Mn powders in hydrogen atmosphere followed by thermal decomposition of the MA product. Hsu et al. [2] prepared Mg3MnNi2 alloy by the method integrating conventional melting and isothermal evaporation casting process (IECP). In this work, the method integrating mechanical alloying and partial elements substitution is carried out to produce Mg2Ni-type alloy. The effects of the substitution of Mn for Ni and milling time on microstructures and electrochemical properties of Mg2Ni-type alloy are investigated. It is worthy to note that the new Mg3MnNi2 phase has been synthesized in this research by much simpler one-step technique which is milling elemental powders of Mg, Ni and Mn in argon atmosphere. Therefore, this one-step technique is different from the three-step technique in Ref. [17], the milling in hydrogen atmosphere followed by thermal decomposition in Ref. [18] and the method integrating conventional melting and IECP in Ref. [2]. To the best of our knowledge, this is the first time that the one-step mechanical alloying in argon atmosphere is reported to synthesize the new Mg3MnNi2 phase. The influence of Mg3MnNi2 phase on electrochemical properties of Mg2Ni-type alloy is also studied. Exploring new Mg-based hydrogen storage alloy is significant for developing hydrogen storage technology. Mg3MnNi2 phase, as a new hydrogen storage alloy, has been proved to possess better electrochemical hydrogen storage properties. This work may provide a guide for improving the electrochemical characteristics of Mg2Ni-type alloy, especially the antioxidation property, and developing new Mg-based electrode materials. 

Experimental procedure 

Starting elemental powders of Mg (purity 99.8%, particle size %50 mm, from GoodFellow), Ni (purity 99.5%, particle size %250 mm, from GoodFellow) and Mn (purity 99.3%, particle size e325 mesh, from Alfa) were mixed according to the designed composition Mg2Ni1!xMnx (x ¼ 0, 0.125, 0.25, 0.375). And then the mixtures were poured into the stainless steel vials (volume 50 ml) together with two stainless steel balls (diameter 20 mm) in the glove box filled with argon. The ball to powder weight ratio is 20:1. The MA was carried out under argon atmosphere at room temperature using a planetary high-energy ball mill (Retsch PM 400) at a speed of 400rpm. The milling was performed for 4 h, 8 h, 16 h, 24 h and 48 h respectively. In order to avoid severe cold welding during high-energy milling, for 16 h, 24 h and 48 h of the milling, the ball milling was stopped every 8 h to crush the bulk materials in vials as well as scrape the powder adhered to the balls and the walls of vials in the glove box filled with argon. This also ensured that all elemental powders were fully mixed and the composition was uniform. The milling was interrupted for 30min per hour to dissipate the heat and to reduce the excessive rise of temperature. The structures of the MA alloys with different compositions milled for different periods of time were analyzed by the Bruker D8 Advance X-ray diffractometer with Cu Ka radiation (l ¼ 0.15418 nm) filtered by nickel. The crystallite size and lattice strain were calculated based on the approximation (Eq. (1)) that combines the Wilson formula and Scherrer formula following Williamson-Hall style plot [19]. b ¼ 23tanq þ 0:9l dcosq (1) where b is the full-width at half maximum intensity of a Bragg reflection excluding instrumental broadening, q the Bragg i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 6 7 9 4 e6 8 0 3 6795 angle, l the wavelength of the X-ray radiation, 3 the effective lattice strain and d the average crystallite size. On rearrangement, the Eq. (1) can be written as: bcosq ¼ 23sinq þ 0:9l d (2) the bcosq of each peak was plotted as a function of sinq and a straight line can be obtained by the least squares method with the intercept as 0.9l/d and slope as 23 [4]. From these, the crystallite size d and lattice strain 3 can be calculated. The morphologies of the powdered samples were observed using the JEOL JSM-5800LV Scanning Electron Microscope. Electrochemical charge and discharge properties were investigated using Multi-channel Battery Interface ATLAS 0461. The working electrodes were prepared by pressing (under 80kNcm!2 pressure) mechanically alloyed powder and 10wt% addition of Ni to small pellet form between nickel nets, which acted as the current collector for the electrode. The electrodes were autoeactivated at first three charge/discharge cycles. The electrochemical properties of electrodes were measured in an H-type shape glass cell, using NiOOH/Ni(OH)2 as the counter electrode (with much larger capacity than working electrode) and Hg/HgO as the reference electrode in 6 M KOH electrolyte at room temperature. The working electrode was first charged at 20 mA g!1 until hydrogen liberation and then after 60 min of rest it was discharged at 10 mA g!1 with a cut-off potential set at !0.7V.