The effect of Hf content on the phase structures and electrochemical properties of V2.1TiNi0.5Hfx (x=0-0.25) hydrogen storage alloys

The phase structures and electrochemical properties of the V2.1TiNi0.5Hfx (x=0-0.25) alloys were investigated. It is found that the addition of Hf (xgreater than or equal to0.05) into the V2.1TiNi0.5 alloy changes the secondary phase from the b.c.c. TiNi-based phase to a C14 Laves phase with hexagonal structure, in the form of a three-dimensional network along the grain boundaries of the main phase. The amount of the C14 Laves phase increases with increasing Hf content. The discharge capacity of the Hf added alloys remains high. The V2.1TiNi0.5SH0.05 alloy electrode has a discharge capacity of 444 mAh/g (discharge current: 25 mA/g), exceeding that of the V2.1TiNi0.5 alloy. Proper Hf content (xless than or equal to0.12) is advantageous for improving the high-rate dischargeability of V2.1TiNi0.05Hfx alloys. The presence of Hf always leads to a degradation in cycle stability of the alloys, mainly due to the increase of the V content in the secondary phase and the increasing tendency to form cracks on the alloy surface in cycling. (C) 2002 Elsevier Science B.V. All rights reserved.