Electroreduction of indium tin oxide in a CaF2–NaF–CaO molten salt at the solid oxide membrane anode system

Abstract

The electroreduction of indium tin oxide (ITO) using a solid oxide membrane anode was investigated in a molten salt of CaF2–NaF–CaO at 1150 °C under an inert atmosphere. In the CaF2–NaF blank salt, the first reduction peak did not change from −0.25 V (vs. W pseudo reference electrode (R. E.)) for the addition of up to 2 wt% CaO, but the second reduction peak was observed at −0.55 V (vs. W pseudo R. E.) in 1 wt% CaO, and −0.4 V (vs. W pseudo R. E.) in 2 wt% CaO. Cyclic voltammetry analysis was carried out using ITO as the cathode, and a reduction peak was observed at +0.31 V (vs. W pseudo R. E.). This is a positive value about 0.24 V greater than the theoretical value, probably arising because of the increase in the oxygen ion activity of CaO. The optimum ITO particle size was determined to be 600–1000 μm considering the maximum reduction efficiency dependent on the sedimentation rate, specific melt droplet growth, and reduced InSn film formation. The formation of an InSn alloy with almost the same mass ratio as the initial ITO was confirmed by combining the results of X-ray diffraction and energy dispersive X-ray spectroscopy analysis.