Microstructures of the oxides on the activated AB2 and AB5 metal hydride alloys surface

Abstract The surface oxides of the activated metal hydride alloys used as the negative electrode for nickel–metal hydride battery were studied by both scanning and transmission electron microscope techniques. In transition metal based AB 2 metal hydride alloys, the surface of the powder is covered with oxides as a product of oxidation from the electrolyte and protected by zirconium oxide and vanadium-rich BCC-structured secondary phase. In the rare-earth based AB 5 and A 2 B 7 metal hydride alloys, the surface is decorated with nano-structured needles and larger-scaled rods of hydroxides from the precipitation of rare earth ions after the oxidation by the electrolyte. Further TEM studies show the existence of a buffer oxide layer sandwiched between the inclusion-embedded surface oxide and alloy bulk in both AB 2 and AB 5 alloys. In both cases, the inclusions are found to be metallic nanocrystals mainly composed of Ni and Co as determined by electron energy loss spectroscopy, selective area electron diffraction, transmission electron atomic image, and X-ray energy dispersive spectroscopy. The Co-to-Ni ratio in the inclusion is larger than that in the bulk due to the less corrosive nature of Co. The additions of Co and Al in the AB 2 are found to reduce number of activation cycles needed to generate a surface oxide with proper catalytic capability.