The current efficiency for NF 3 formation was investigated using a Boron-Doped diamond (BDD) anode in the NH 4 F·mHF melts. The current efficiency for NF 3 formation depended on both the current density and the NH 4 F-concentration in the melt and it trended to decrease at the higher current density. Its maximum value on the BDD anode in a NH 4 F·2.0HF melt was 72.4% at 40 mA cm −2 and that in a NH 4 F·1.7HF melt was 46.2% at 100 mA cm −2 . Addition of nickel fluoride to the NH 4 F·mHF melt was effective for increasing the current efficiency for NF 3 formation. This is because Ni 2+ ion may be oxidized on the BDD anode to form highly oxidized nickel compounds such as NiF 3 and/or Ni 2 F 5 on the anode and NiF 6 3− and/or NiF 6 2− ions in the melt, which are strong fluorinating agents. The maximum current efficiency for NF 3 formation on the BDD anode in a NH 4 F·2.0HF melt having nickel ion was 61.9% even at the current density of 1000 mA cm −2 .

The current efficiency for NF 3 formation and the current loss caused by Ni dissolution were investigated in electrolysis of the NH 4 F·2HF melts with and without alkali metal fluorides such as CsF, KF, and LiF. The addition of CsF to the melt was most effective for increasing the NF 3 current efficiency. In contrast, the addition of KF to the melt decreased the current efficiency for NF 3 formation and increased the current loss caused by Ni dissolution. The SEM observation and XRD analysis revealed that the oxidized layer formed on nickel in the melt containing LiF or CsF was composed of NiF 2 with highly oxidized nickel fluoride. On the other hand, the oxidized layer in the melt containing KF was composed of only KNiF 3 , and was very brittle. Therefore, it is concluded that KF is detrimental to the nickel anode and highly oxidized nickel fluoride may relate to the NF 3 current efficiency.፧