Studies on cobalt-free AB5-type hydrogen storage alloys

Cobalt-fret AB(5)-type hydrogen storage alloys were examined for lowering MH raw materials cost. The hydrogen absorption-desorption and electrode properties of these alloys were investigated. For comparison, a cobalt-containing AB(5)-type hydrogen storage alloy was also examined. Electrochemical measurements showed that cobalt-containing La0.6Ce0.4Ni3.77Co0.6Mn0.36Al0.27 alloy had a maximum capacity of 295 mAh/g at a rate of 0.2 C. The capacity decay is 26.5% after 300 cycles at 1.0C rate with 100% depth of discharge (DOD). The Cobalt-free La0.5Ce0.4Ti0.1Ni3.77Mn0.36Al0.27(CuFeCr)(0.6) alloy showed very good cycling stability, although the initial capacity (266 mAh/g at 0.2C rate) was lower than that of the cobalt-containing alloy. The capacity decay of the cobalt-free alloy was 23.1% after 300 cycles under the same rate and DOD. The high cycling stability of the cobalt-free alloy was mainly due to a secondary phase present as a three-dimensional network since it played a key role of microencapsulating the main phase grains. The alloy microstructure and chemical composition were examined by scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA). In addition, the mechanism of the hydriding/dehydriding reaction on alloy electrodes was also investigated and is discussed using the electrochemical impedance spectroscopy (EIS) technique. (C) 1999 Elsevier Science S.A. All rights reserved.