Investigations on the cycle stability and the structure of the MmNi3.6Co0.75Mn0.55Al0.1 hydrogen storage alloy -: I.: Measurements and analysis of the cycle stability and the phase structure

Electrochemical charge-discharge cycling tests have been made on the alloys of composition MmNi(3.6)Co(0.75)Mn(0.55)Al(0.1) prepared with different casting conditions. Casting conditions had a great influence on the cycle stability and the phase structure of the alloy. The cycle stability of the alloy could be improved by quenching the melted alloy using the melt-spinning technique compared with its corresponding as-cast alloy of the same composition. The higher the quenching rate, the more stable the alloy. However, a quenching rate higher than 20 ms(-1) was disadvantageous and unnecessary because it was not effective in further improving the cycle stability of the alloy and led to too large a decrease in discharge capacity. Compared with previous results, the effect of the quenching rate on the cycle stability and the discharge capacity can vary with alloy composition. XRD studies showed that the as-cast alloy contained segregations whose diffraction peaks even appeared in the XRD profiles. To obtain a high-performance alloy not only the quenching rate but also the alloy composition should be optimized when preparing the electrode alloys by melt-spinning. (C) 1999 Published by Elsevier Science S.A. All rights reserved.