Effect of oxide in nickel-based composite anodes on current efficiency for NF3 formation and current loss caused by nickel dissolution in molten NH4F·2HF

Abstract Nickel-oxide composites such as Ni–Co 3 O 4 , Ni–MnO 2 , Ni–Ag 2 O, Ni–Y 2 O 3 , Ni–La 2 O 3 , Ni–Nd 2 O 3 , and Ni–Sm 2 O 3 systems were prepared from mixture of nickel and oxide powders by hot isostatic pressing (HIP). Ratios of oxide in the nickel-oxide composite were 2, 5, and 10 mol%. Anode gas evolved at every nickel-based composite electrode was composed of N 2 , O 2 , NF 3 , N 2 F 2 , N 2 F 4 , and N 2 O, and its composition was almost the same as that on a nickel sheet electrode. Current efficiency for NF 3 formation on Ni–Y 2 O 3 and Ni–La 2 O 3 composite anodes were almost the same as that on the nickel anode, whereas that on Ni–Co 3 O 4 composite anode was very small compared with that on the nickel sheet anode. In the cases of the Ni–Co 3 O 4 , the Ni–MnO 2 , and the Ni–Ag 2 O composite anodes, the current efficiency for NF 3 formation decreased with increase in the oxide concentration in the composite anodes. Current losses caused by nickel dissolution of the Ni–La 2 O 3 , the Ni–MnO 2 , and the Ni–Ag 2 O composite anodes were small compared with that of the nickel sheet anode, while that of the Ni–Co 3 O 4 composite anode was very large compared with that of the nickel sheet anode. SEM observation revealed that the surfaces of the Ni–La 2 O 3 , the Ni–MnO 2 , and the Ni–Ag 2 O composite anodes after electrolysis were covered with a dense oxidized layer with a few pores and/or some defects. Anode potential during electrolysis indicated that the oxidized layer formed on the Ni–La 2 O 3 composite anode has the higher electric conductivity, presumably because of presence of both LaF 3 and NiO 1+ x (0 ≤  x