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

Nickel-oxide composites such as Ni-CO3O4, Ni-MnO2, Ni-Ag2O Ni-Y2O3, 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 10 mol%. Anode gas evolved at every nickel-based composite electrode was composed of N-2, O-2, 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. (c) 2004 Elsevier Ltd. All rights reserved.