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The electrochemical hydrogenation properties of two boron-free crystalline Co powder samples were investigated. They exhibited almost the same maximum discharge capacities with 2-h charging, 415 and 428 mAh/g ͑corresponding to CoH 0.91 and CoH 0.93 ͒ for samples A and B, respectively. The high discharge capacity of Co powder was attributed to hydrogenation of Co and phase transition between the phases of hexagonal close-packed and face centered cubic.Hydrogen can be stored in various forms such as high pressure gas, metal hydrides, adsorption on carbon materials, liquid hydrogen, etc. Many types of hydrogen storage alloy have been developed in the past few decades, including AB-type intermetallic compounds, 1 AB 2 -type Laves phase alloys, 2 Mg-based alloys, 3 and AB 5 -type rare-earth alloys. 4 For battery application, lower plateau pressure and good kinetics are required. For AB 2 -type alloys, although the theoretical discharge capacities are higher than those of AB 5 -type alloys, they are more difficult to activate to be used as a negative electrode material in batteries. For Mg-based alloys, their high discharge capacities are attractive, but the low cycling stability and poor kinetics are not suitable for practical application. The AB 5 -type hydrogen storage alloys are more often used in commercial batteries because of their suitable plateau pressure and fast activation. However, the discharge capacities of AB 5 -type alloys are decreased by pulverization and oxidation of the alloys. Willems 5 and Chartouni et al. 6 pointed out that the cycle life of electrode could be improved by partial substitution of Ni by Co that lowers the hardness of the alloys. Mixing hydrogen storage alloy with Co powder might result in formation of a conductive layer of precipitate which ensures good electrical contact of the alloy powder, in addition to a slight increase in electrochemical capacity due to the reaction of cobalt in the charge and discharge cycling. 7 Durairajan et al. pointed out that the discharge capacity of pure Co was only 50 mAh/g, 8 that might be due to the faradaic reaction of Co/Co͑OH͒ 2 .Recently, it has been reported that amorphous Co nanoparticles prepared by reduction with NaBH 4 can absorb a large amount of hydrogen. 9-12 When ultrafine Co-B amorphous alloy particles were tested by electrochemical charging/discharging, the reversible maximum discharge capacity could be as high as 300 mAh/g. 12 In the cyclic voltammetry ͑CV͒ test, Mitov et al. proposed that the cathodic and anodic peaks are due to electrochemical adsorption and desorption of hydrogen, respectively. The residual B in the sample may play an important role by exchanging position with hydrogen atom in the metallic host lattice. 9 Therefore, it would be possible to use ultrafine Co-B amorphous powder as a new hydrogen storage material.During the charge process the negative electrode undergoes reduction reaction, pure cobalt cannot be reduced to other form because it is in the zero valence. Therefore, the decomposition of water might be pr...

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Alloy nanoparticles of Pd x Ni 100-x (x ) 20, 30, 70, and 80) were prepared by the coprecipitation method. The surface segregation of nanoparticles, driven by the surface energy, strain energy, and reduction potential, was studied systematically. The surface composition, surface species, and alloy structure of as-prepared Pd x Ni 100-x and heated Pd 30 Ni 70 alloy nanoparticles was characterized by ESCA, TPR, and XRD, respectively. The structure of as-prepared Pd x Ni 100-x alloy nanoparticles is not homogeneous and changes with the alloy content x. The surface is Pd enriched for as-prepared Pd 80 Ni 20 alloy due to the strain energy effect. In contrast, the surface is Ni enriched for as-prepared Pd x Ni 100-x alloy with x ) 20, 30, and 70 due to the reduction potential effect. In addition, a combinatory process of surface segregation and homogenization during the heat treatment was observed on the surface of Pd 30 Ni 70 alloy nanoparticles after heating at T > 620 K.

The combinational modification of the morphology, alloying, and support for Pt catalysts has been optimized towards the oxygen reduction reaction. Graphene-supported PtPd nanorods have lower unfilled Pt d-states than carbon-supported Pt nanoparticles (Pt/C) and their specific and mass activities after the accelerated durability test are about 6.5 and 2.7 times higher than those of Pt/C, attributed to the synergistic electronic modification effect and graphene-metal interaction.