Abstract: The effects of annealing treatment (1 173-1 273 K, 3 h) on the microstructures and electrochemical performances of Co-free MlNi_4.0Al_0.3Si_0.1Fe_0.6 hydrogen storage alloys were investigated. XRD analyses indicate that both as-cast and annealed alloys consist of a CaCu₅-type structure single phase, but the crystallizability and homogeneity of the annealed alloys are improved. The optical microscope and EDS analyses show that the as-cast alloy has a crassi dendrite structure with noticeable composition segregation, while the alloy annealed at 1 173 K has a more conspicuous dendrite structure and a more homogenous composition, and the microstructures of the alloys annealed at 1 223-1 273 K change to an equiaxed structure. The electrochemical tests indicate that the annealed alloys, as compared with as-cast alloy, show a higher discharge capacity, much better cycling stability and lower high-rate dischargeability. It is found that the decrease in both exchange current density and diffusion coefficient of hydrogen in alloy bulk is the main reason for the degraded high-rate dischargeability (HRD) of annealed alloys.

Key words: hydrogen storage electrode alloy; Co-free alloy; annealing treatment; microstructure; electrochemical property