Studies on the properties and characteristics of the fluorinated AB(5) hydrogen-absorbing electrode alloys

Fluoride layers have been known to contribute to the selective permeability of hydrogen molecules in the gas-solid reaction and provide a protective barrier against impurities such as water vapor, carbon monoxide, air, and others. One of the important roles of the fluorination processes is the removal of the oxide layer from the surface which considerably improves the initial charge/discharge characteristics. However, it has also been found that during fluorination, metallic Ni on the surface dissolves rapidly into the aqueous F- and HF2- ion containing solution (here after, denotes F-solution) and as the result, the electric conductivity is significantly reduced in the fluoride layer. This paper is aimed at investigating the effects of fluorination conditions on the electrochemical properties and characteristics of the AB(5) electrodes for Ni-metal hydride batteries. Special care has been taken to control fluoride formation and its thickness on the particle surface. Ni2+ ions were added to the F-solution in order to protect Ni from being dissolved and lost in the vicinity of the particle surface. The thickness of the fluoride layer depends largely upon the specific surface area to be treated as a function of time. The thickness of the fluoride layer and the state of metallic Ni near the surface region was investigated by EPMA and ICPS and they were correlated with the initial discharge capacity, charge/discharge cycle life, and durability, against a 6 M KOH electrolyte solution.