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Stability and Thermodynamic Analysis of Pt and NiO x F y ∕ Ni Reference Electrodes in a Dehydrated Melt of NH4F ∙ 2HF

The stability of platinum and nickel (bare and fluorinated) electrodes was investigated for use as contamination-free reference electrodes in electrolytic production of NF 3 using Ni as an anode in NH 4 F·2HF melt. Though the platinum reference electrode showed stable potentials, thermodynamic analysis revealed that it is inappropriate as a reference electrode in long-term electrolysis using nickel anode because the potential depends on the concentration ratio of Ni 3+ /Ni 2+ . The stability of the bare nickel electrode was poor; however, an anodically and a thermally fluorinated nickel electrode showed excellent stability at 0.073 ± 0.001 V at 373 K with good reproducibility. It is therefore concluded that the fluorinated nickel electrodes are useful as reference electrodes for electrolytic production of NF 3 using Ni as an anode in the NH 4 F·2HF melt. The equilibrium potential (0.073 ± 0.001 V) obtained experimentally did not agree with the theoretically calculated potentials of NiF 2 /Ni (-0.178 V) nor NiO/Ni (0.164 V) at 373 K, and it was speculated that the potential might be determined by nickel oxyfluoride, NiO x F y , formed on the surface of nickel electrode.