Microstructures of the Activated Si-Containing AB2 Metal Hydride Alloy Surface by Transmission Electron Microscope

The surface microstructure of an activated Si-containing AB2 metal hydride (MH) alloy was investigated by transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS). Regions of the main AB2 and the secondary TiNi (B2 structure) phases directly underneath the surface Zr oxide/hydroxide layers are considered electrochemically inactive. The surface of AB2 is covered, on the atomic scale, by sheets of Ni2O3 with direct access to electrolyte and voids, without the buffer oxide commonly seen in Si-free AB2 alloys. This clean oxide/bulk metal alloy interface is believed to be the main source of the improvements in the low-temperature performance of Si-containing AB2 alloys. Sporadic metallic-Ni clusters can be found in the surface Ni2O3 region. However, the density of these clusters is much lower than the Ni-inclusions found in most typical metal hydride surface oxides. A high density of nano-sized metallic Ni-inclusions (1–3 nm) is found in regions associated with the TiNi secondary phase, i.e., in the surface oxide layer and in the grain boundary, which can also contribute to enhancement of the electrochemical performance.