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–Co3O4, Ni–MnO2, Ni–Ag2O, Ni–Y2O3, Ni–La2O3, Ni–Nd2O3, and Ni–Sm2O3 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 10mol%. Anode gas evolved at every nickel-based composite electrode was composed of N2, O2, NF3, N2F2, N2F4, and N2O, and its composition was almost the same as that on a nickel sheet electrode. Current efficiency for NF3 formation on Ni–Y2O3 and Ni–La2O3 composite anodes were almost the same as that on the nickel anode, whereas that on Ni–Co3O4 composite anode was very small compared with that on the nickel sheet anode. In the cases of the Ni–Co3O4, the Ni–MnO2, and the Ni–Ag2O composite anodes, the current efficiency for NF3 formation decreased with increase in the oxide concentration in the composite anodes. Current losses caused by nickel dissolution of the Ni–La2O3, the Ni–MnO2, and the Ni–Ag2O composite anodes were small compared with that of the nickel sheet anode, while that of the Ni–Co3O4 composite anode was very large compared with that of the nickel sheet anode. SEM observation revealed that the surfaces of the Ni–La2O3, the Ni–MnO2, and the Ni–Ag2O 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–La2O3 composite anode has the higher electric conductivity, presumably because of presence of both LaF3 and NiO1+x (0≤ x <0.5) in it. [Copyright &y& Elsevier]