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A novel electrolytic cell with Pt,O2(air)|ZrO2 used as reference electrode was constructed integrally by using a one-end-closed magnesia partially stabilized zirconia solid electrolyte tube. Various electrochemical measurement techniques such as cyclic voltammetry (CV), chronopotentiometry (CP) and chronoamperometry (CA) were applied, and thermodynamic software Factsage was combined to study the effects of NiO concentration and temperature on electrochemical behavior of Ni ion in SiO2-CaO-MgO-Al2O3 molten slag at temperatures not less than 1673 K. The results indicate that Ni in molten slag is in stable form of Ni2+ instead of Ni3+; reduction of Ni2+ in molten slag is a diffusion control reaction process with a single step; smaller concentration of NiO and higher temperature lead to better reversibility of Ni2+ reduction reaction; with CV, CP and CA electrochemical measurement techniques, diffusion coefficients of Ni2+ at 1723 K are derived, respectively. Further, diffusion activation energy of Ni2+ from data corresponding to CV and CA techniques is derived from Arrhenius equation factored in diffusion coefficient and temperature.

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Production of metallic iron through molten oxide electrolysis using inert electrodes is an alternative route for fast ironmaking without CO2 emissions. The fact that many inorganic oxides melt at ultrahigh temperatures (>1500 K) challenges conventional electro-analytical techniques used in aqueous, organic and molten salt electrolytes. However, in order to design a feasible and effective electrolytic process, it is necessary to best understand the electrochemical properties of iron ions in molten oxide electrolytes. In this work, a magnesia-stabilised zirconia (MSZ) tube with a closed end was used to construct an integrated three-electrode cell with the "MSZ | Pt | O2 (air)" assembly functioning as the solid electrolyte, the reference electrode and also the counter electrode. Electrochemical reduction of iron ions was systematically investigated on an iridium (Ir) wire working electrode in the SiO2-CaO-MgO-Al2O3 molten slag at 1723 K by cyclic voltammetry (CV), square wave voltammetry (SWV), chronopotentiometry (CP) and potentiostatic electrolysis (PE). The results show that the electroreduction of the Fe 2+ ion to Fe on the Ir electrode in the molten slag follows a single twoelectron transfer step, and the rate of the process is diffusion controlled. The peak current on the obtained CVs is proportional to the concentration of the Fe 2+ ion in the molten slag and the square root of scan rate. The diffusion coefficient of Fe 2+ ions in the molten slag containing 5 wt% FeO at 1723 K was derived to be (3.43 ± 0.06)×10 -6 cm 2 s -1 from CP analysis. However, a couple of following processes, i.e. alloy formation on the Ir electrode surface and interdiffusion were found to affect the kinetics of iron deposition. An ECC mechanism is proposed to account for the CV observations. The findings from this work confirm that zirconia-based solid electrolytes can play an important role in electrochemical fundamental research in high temperature molten slag electrolytes.

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