The effect of Hf content on the phase structures and electrochemical properties of V 2.1TiNi 0.5Hf x ( x=0–0.25) hydrogen storage alloys

Abstract

The phase structures and electrochemical properties of the V 2.1TiNi 0.5Hf x ( x=0–0.25) alloys were investigated. It is found that the addition of Hf ( x≥0.05) into the V 2.1TiNi 0.5 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 V 2.1TiNi 0.5Hf 0.05 alloy electrode has a discharge capacity of 444 mAh/g (discharge current: 25 mA/g), exceeding that of the V 2.1TiNi 0.5 alloy. Proper Hf content ( x≤0.12) is advantageous for improving the high-rate dischargeability of V 2.1TiNi 0.5Hf x 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.