Study on Mechanism of Film Formation on Nickel Electrode in a Molten NH_4F·2HF and Effect of Water in the Melt on Characteristics of Film

摘要

The mechanism of film formation on the Ni anode in dehydrated and hydrated melts of NH_4　F·2HF and the effect of trace water in NH_4 F·2HF melt on the physico-chemical properties of oxidized layer on the Ni anode were studied at 100°C. The potentiodynamic and potentiostatic polarization behaviors of the Ni anode were investigated to elucidate the anodic processes. The oxide layer is composed of NiF_2 with a small amount of nickel oxides such as NiO and Ni_2O_3 and a highly oxidized nickel fluoride formed on the Ni anode polarized at the potentials higher than 4.5 V vs. H_2, and it grew thicker by the repetition of the alternate formation and degradation with the time of electrolysis. The composition and the thickness of the oxide layer may reflect on the current density on the nickel anode under polarization. The overall anodic current and the current loss caused by nickel dissolution at the potential range higher than 4.75 V were decreased by a trace of water in the melt, presumably due to the formation of dense oxide layer.