Vanadium hydrides have been studied for practical use in chemical heat pumps, hydrogen compressors, and isotope separations because of their large hydrogen storage capacity (H/M ϭ 2). 1,2 In addition, these alloys have advantages such as strong resistance to pulverization during cycling, high-rate diffusion, and rapid activation. 3 However, since vanadium monohydride is generally too stable to desorb hydrogen, 1 the usable hydrogen storage capacity is only a half of the maximum capacity.Titanium-vanadium alloys, which form a single-phase solid solution over the whole fractions, are also notable candidates of a new hydrogen reservoir because of their lower costs than V and their large hydrogen storage capacity. 4 However, owing to the slow desorption rate of hydrogen from Ti-V solid solution hydrides, they are not suitable for application by themselves without a catalyst for hydrogen desorption. Ni is well known to become the catalyst for facilitating the hydrogen absorption and desorption, and it is substituted for constituent elements or added to the alloys using several methods. 5-9 Furthermore, Tsukahara et al. reported in a series of works 5,10-12 that the TiV 3 Ni 0.56 alloy had a three-dimensional network of two phases, the V-based body-centered cubic (bcc) solid solution main phase as a hydrogen reservoir, and the TiNi-based secondary phase as an electrocatalyst and a current collector, and suggested that the formation of the network structure led to the improvement of charge-discharge characteristics.TiV 1.4 alloy forms a hydride with composition of TiV 1.4 H 4.6 13 which corresponds to the calculated discharge capacity of 1036 mAh g Ϫ1 . It is larger than the calculated discharge capacity (852 mAh g Ϫ1 ) of TiV 4 H 8 which has already been investigated. Therefore, the TiV 1.4 alloy has potential as a new negative electrode material for use in nickel-metal hydride batteries. If this is the case, the partial substitution of the alloy components by Ni should be significant. However, to date there is nothing in the literature on the electrochemical and structural characteristics of the TiV 1.4 and TiV 1.4Ϫx Ni x (0 Յ x Յ 1) electrodes. On the contrary, we have consistently found in our preliminary work that the TiV 0.9 Ni 0.5 alloy electrode showed the highest discharge capacity among TiV 1.4Ϫx Ni x (0 Յ x Յ 1) electrodes. In this work, first, the structural characterization of the TiV 0.9 Ni 0.5 electrode was carried out in detail using X-ray diffraction (XRD) and electron probe microanalysis (EPMA) in order to clarify the reason why it showed high discharge capacity. Second, on the basis of this result, new Ti-V-Ni alloys with compositions of the phases separated from the TiV 0.9 Ni 0.5 alloy, which was composed of two phases, were prepared and characterized electrochemically and structurally, and as a result, the TiV 2.1 Ni 0.3 alloy was, at the first time, found to be a promising negative electrode material with high hydrogen storage capacity.Experimental The Ti-V-Ni alloys were prepared by arc-melting o...