Study of the high temperature characteristics of hydrogen storage alloys

In this work, the phase structure of as-cast and melt-spun (MmY)1 (NiCoMnAl)5 alloys (the content of yttrium is 0-2.5 wt.%) and their electrochemical properties were studied with regard to discharge capacity at different temperatures (30-80 deg. C) and cycling life at 30 deg. C. It is found that the substitution of yttrium increase the electrochemical capacity of the compounds and decrease the difference in capacity between as-cast and as-quenched compounds at 30 deg. C. When increasing the yttrium concentration from 0 to 2.5 wt.%, the cycling life of both the as-cast and the melt-spun compounds deteriorated, although the latter have a slightly longer cycle life than the former. The remarkable feature of the alloys obtained by yttrium substitution is the improvement of the high temperature electrochemical properties. It shows that the stability of the hydrides is increased. Compared with the as-cast alloys, the melt-spun ribbons have higher electrochemical charge/discharge capacity in the range of 40-80 deg. C, especially in the temperature range 70-80 deg. C. In addition, the electrochemical capacities of the melt-spun alloys showed more recovery than the as-cast alloys after charging/discharging at high temperature. This is ascribed to the better oxidation resistance. The analysis of the potential curves illustrate that the improvement of the charging/discharging efficiency for the melt-spun ribbons with yttrium substitution is ascribed to a flatter and lower hydride-formation potential as well as to a higher hydrogen evolution potential. XRD indicates that less lattice strain and defects and more anisotropy existed in melt-spun alloys. This could be beneficial to the improvement of the cycling durability and anti-corrosion properties