Deterioration of Laves phase alloy electrode during cycling

A study is made of the deterioration mechanism of a Laves phase alloy electrode without addition of Ni powder by means of electrochemical impedance spectra (EIS), X-ray diffraction (XRD), inductively coupled plasma (ICP) and X-ray photoelectron spectroscopy (XPS). The XRD analysis shows that after cycling, the alloy bulk still has the capability to store hydrogen. The deterioration mechanism is considered to be mainly a surface process. In particular, particle pulverization (increased specific surface area and reduced the particle size) is the main reason for electrode deterioration. Dissolution of V and Mn increases gradually during cycling. On the other hand, particle pulverization increases further the dissolution rates of V and Mn, which are proportional to the specific surface area due to the generation of a new, fresh surface. In addition, it is demonstrated by XPS that the depth of oxide layers of Zr and Mn increases with cycling and results in an increase in the reaction impedance and a decrease in the surface electronic conductivity. Therefore, a change in the structure of the particle surface layer, due to particle pulverization and dissolution of V and Mn, contributes to the deterioration of the Laves phase electrode.