A large number of hydrogen storage alloys have been developed as negative electrode materials for Ni/MH batteries. Their performances differ greatly in terms of specific capacity, activation, rate dischargeability, and cyclic lifetime. There is an apparent trend to concentrate on low cost, light weight, and excellent charge–discharge properties.
On the other hand, in the negative electrode (anode), the active material is a special kind of alloy known as hydrogen storage alloy (or metal hydride (MH) alloy) that is capable to store hydrogen in a reversible way [ 12 ]. During charge, the applied voltage splits water molecules into hydroxide ions and hydrogen protons.
Generally, the electrochemical kinetics of hydrogen storage alloy electrodes is mainly determined by both charge-transfer process on the alloy surface and hydrogen atom diffusion within the bulk of the alloy.
Metal hydrides are regarded as promising candidates for the negative materials of nickel/metal-hydride (Ni/MH) batteries due to their high-energy density, favorable charge and discharge ability, long charge–discharge cyclic life, and environmental compatibility [5, 6, 10 – 16].
The AB 2 hydrogen storage intermetallic compounds have been investigated extensively because of their potential application in high-capacity negative electrodes for Ni=MH batteries. The AB 2 -type alloys mainly form one of two structures, either the cubic C15 structure or the hexagonal C14 structure [70, 71].
Rare-earth-based AB 5 -types compounds such as LaNi 5 with 1.5 wt% of hydrogen absorption capacity are the main anode materials for the Ni-MH batteries, although there are some successes in using the rare-earth-free AB 2 -type alloys (A: hydride-forming elements; B: elements with low affinity with hydrogen) .