Measurement and Thermodynamic Analysis of NiF 2 /Ni Electrode Potential in a Dehydrated Melt of NH 4 F·2HF

A Ni reference electrode was prepared and its potential was also determined in a NH4F•2HF melt at 100{degree sign}C. The value of potential on the Ni electrode was unstable for 30 days. On the other hand, potentials on Ni electrodes treated with anodic oxidation at 10 mA/cm2 for 3 days and thermal fluorination with NF3 under 1 atm at 300{degree sign}C for 1 hour became stable after shorter time. The former was 0.075 V and the latter was 0.072 V against hydrogen evolution potential (V vs. H2). The value of potential was almost the same in every measurement. SEM observation and XPS analysis revealed that both pre-treated Ni electrodes were covered with the dense layer composed of NiF2 with a small amount of nickel oxide and/or nickel oxyfluoride. The potential of the NiF2/Ni electrode at 100{degree sign}C calculated from the thermodynamic data is - 0.178 V vs. H2. This value of potential was different from the observed one. On the other hand, the potential of the NiO/Ni electrode calculated from thermodynamic data is 0.164 V vs. H2. As the observed value of potential on the Ni electrode is between both potentials of the NiF2/Ni and the NiO/Ni electrodes, its potential may be determined by nickel oxyfluoride, NiOxFy, in the surface layer of the Ni electrode. Therefore, it is concluded that a Ni electrode covered with the oxidized layer mainly composed of NiF2 is useful as a reference electrode for electrochemical measurement and electrolytic production of NF3 using a nickel anode.