The amorphous Mg-Al-Ni composites were prepared by mechanical ball-milling of Mg17Al12 with x wt.%Ni (x=0, 50, 100, 150, 200). The effects of Ni addition and ball-milling parameters on the electrochemicalhydrogen storage properties and microstructures of the prepared composites have been investigated systematically.For the Mg17Al12 ball-milled without Ni powder, its particle size decreases but the crystalstructure does not change even the ball-milling time extending to 120 h, and its discharge capacity is lessthan 15 mAh g_1. The Ni addition is advantageous for the formation of Mg-Al-Ni amorphous structureand for the improvement of the electrochemical characteristics of the composites. With the Ni contentx increasing, the composites exhibit higher degree of amorphorization. Moreover, the discharge capacityof the composite increases from 41.3mAh g_1 (x=50) to 658.2 mAh g_1 (x=200) gradually, and theexchange current density 10 increases from 67.1mA g_1 (x=50) to 263.8mA g_1 (x=200), which is consistentwith the variation of high-rate dischargeability (HRD). The ball-milled Mg17Al12+200 wt.% Nicomposite has the highest cycling discharge capacity in the first 50 cycles.

Nanocrystalline NaAlH4 was directly synthesized by ball milling NaH/Al with TiF3 catalyst underhydrogen pressure of 15-25 bar within 50h. It is found that the synthesized NaAlH4 exhibits a highreversible hydrogen capacity of 4.7wt% with fast reaction kinetics. It can absorb about 3.5 wt %hydrogen even at ambient temperature. The Ti-Al-H active species formed during reactive ballmilling may act as catalyzing agent for hydrogen dissociation/recombination during in situhydrogenation process and subsequent hydriding/dehydriding cycle.

The synergistic effect of metallic Ti and Zr as co-dopants on the reversible hydrogen storage properties of NaAlH4 was investigatedsystematically. Metallic Ti and Zr powders were used directly and separately as dopants, as well as used as co-dopants together in thepreparation of NaAlH4 by hydrogenation of ball-milled mixtures of NaH/Al. The hydriding/dehydriding properties of the composites werethen investigated. It was found that the addition of Ti and Zr powders together as co-dopants on hydriding/dehydriding properties is superiorto doping with Ti or Zr alone. The highest reversible hydrogen capacity of the hydride doped with Ti and Zr together as co-dopants is4.34 wt% at 160℃. The hydriding rate increases with the hydriding pressure increasing from 7.5 to 11.5MPa. The dehydriding kinetics isimproved with the dehydriding temperature increasing from 90 to 130℃, and the dehydriding rate of the composite doped with Ti and Zr asco-dopants is 1.6 and 2.0 times that of the sample doped with Ti and Zr alone at 130℃. Microstructure analysis reveals that the improvementof the hydriding/dehydriding properties of NaH/Al (NaAlH4) can be partially ascribed to the in situ interaction of active titanium-hydride andzirconium-hydrides formed in the ball-milling process and subsequently acted as the catalytic active sites on the surface of hydride matrix. Theeffect of lattice expansion on enthalpy change indicates that the further improved dehydriding property of the composite can also be attributedto a favorable thermodynamic modification of the bulk composite hydride co-doped with Ti-Zr powders. In a word, the synergistic effect onthe improvement of hydriding/dehydriding properties by introducing the addition of Ti–Zr together as co-dopants can be ascribed to both the "superficial catalytic process" and the "favorable thermodynamic modification" of the composite.

Co-free LaNi4.92Sn0.33 alloy samples were prepared by first induction melting and subsequent melt-spinning at different quenching rates, and their microstructure and electrochemical properties were comparatively investigated and compared. The results reveal that the as-cast alloy is of a coarse dendrite structure consisting of two distinct CaCu5 major phases and a scattered minor Sn phase, has high volume expansion and pulverization rates on hydriding and noticeable composition segregation, resulting in a rather poor cycling stability (S200=42.7%). However, the melt-spun alloys prepared at higher quenching rates are highly homogeneous in composition, of the single CaCu5 phase in very fine cellular structure, have lower volume expansion and pulverization rates on hydriding, leading to a noticeably improved cyclic stability (S200=62.5.78%), although their activation rate, initial capacity and high-rate dischargeability are slightly lowered. It is believed that the great improvement in cycling stability of the melt-spun alloys is mainly due to their lower degree of pulverization on hydriding and the higher uniformity in composition, and the relatively lower high-rate dischargeability is mainly due to the decrease in both the electrocatalytic activity and the hydrogen diffusion rate in the alloy bulk

The microstructure and electrochemical behavior of V2.1TiNi0.4Zr0.06Cr0.152 hydrogen storage electrode alloy have been investigated incomparison with V2.1TiNi0.4Zr0.06 alloy. The results show that V2.1TiNi0.4Zr0.06Cr0.152 alloy consists of a V-based solid solution main phaseand a C14-type Laves secondary phase in the form of three-dimensional network, being similar to V2.1TiNi0.4Zr0.06 alloy, the secondary phaseprecipitates along the grain boundaries of the main phase. As compared with V2.1TiNi0.4Zr0.06 alloy, the unit cell volume of each phase in theV2.1TiNi0.4Zr0.06Cr0.152 alloy contracts. It is found that adding Cr restricts the dissolution of vanadium and titanium into the KOH electrolyte,and improves the corrosion resistance of the alloy, thus the cycling stability after 30 cycles increases from 22.34% (V2.1TiNi0.4Zr0.06) to77.96% (V2.1TiNi0.4Zr0.06Cr0.152). Furthermore, V2.1TiNi0.4Zr0.06Cr0.152 alloy has a better high-rate dischargeability and higher exchangecurrent density compared with V2.1TiNi0.4Zr0.06 alloy, but its maximum discharge capacity decreases.

The hydrogen storage properties and microstructures of Ti-doped NaAlH4 complex hydrides prepared by hydrogenation of ball-milled NaH/Al mixture with x mol% Ti powder (x=0, 4, 6, 10) were nvestigated. It is found that hydrogen as ball-milling atmosphere is better than argon. The reversible hydrogen storage properties improve with increasing Ti content. As the milling time (t) extends from 1 to 24 h, the hydrogen desorption capacity increases first and then decreases, and reaches a maximum capacity of 4.25 wt% at t = 16. The catalytic mechanism for hydrogen storage behavior of Ti-doped NaAlH4 is attributed to the presence of active small TiH1.924 and TiAl particles, which are scattered on the surface of much larger NaAlH4 (NaH/Al) globelets, acting as the catalytic active sites for the complex compound and playing an important catalytic role in the hydriding-dehydriding process.

The phase structures and electrochemical performance ofV2.1TiNi0.5Hf0.05Cox (x=0, 0.113, 0.152 and 0.192) alloys have been investigated. It is found that the addition of Co into the V2.1TiNi0.5Hf0.05 alloy decreases the amount of the V-based solid solution main phase and increases the amount of C14-type secondary phase without any change in the pattern of the three-dimensional network structure. With increasing Co content, the unit cell of the main phase contracts and that of the secondary phase expands. Electrochemical measurements show that the maximum discharge capacity of the Co-added alloys is much less than that of V2.1TiNi0.5Hf0.05 alloy, but their high-rate dischargeability and cycle stability are markedly improved.

The phase structures and electrochemical properties of the V TiNi Hf (x50-0.25) alloys were investigated. It is found that the 2.1 0.5 xaddition of Hf (x$0.05) into the V TiNi alloy changes the secondary phase from the b.c.c. TiNi-based phase to a C14 Laves phase 2.1 0.5with hexagonal structure, in the form of a three-dimensional network along the grain boundaries of the main phase. The amount of theC14 Laves phase increases with increasing Hf content. The discharge capacity of the Hf added alloys remains high. The V TiNi Hf 2.1 0.5 0.05alloy electrode has a discharge capacity of 444 mAh/g (discharge current: 25 mA/g), exceeding that of the V TiNi alloy. Proper Hf 2.1 0.5content (x#0.12) is advantageous for improving the high-rate dischargeability of V TiNi Hf alloys. The presence of Hf always leads 2.1 0.5 xto a degradation in cycle stability of the alloys, mainly due to the increase of the V content in the secondary phase and the increasingtendency to form cracks on the alloy surface in cycling.

The phase structures and electrochemical properties of V2:1TiNix (x=0:1; 0:3; 0:5; 0:7; 0:9) alloys were investigated by thegeneral electrochemical techniques and X-ray powder di#raction, scanning electron microscopy, energy dispersive X-ray spectrometerand emission spectrochemical analysis. Nickel added into V2:1Ti alloy plays a key role in the performance of the newalloys, due to the formation of a continuous TiNi-based secondary phase in the form of a three-dimensional network around themain phase. Though the hydrogen absorbed by the alloy reduces with the increase of nickel content, the actual electrochemicaldischarge capacity and the high-rate discharge capability are improved noticeably. Among the studied alloys, V2:1TiNi0:5 witha good combination of the main phase and the secondary phase has the best overall electrochemical performance.