Abstract
Limited knowledge of the thermodynamic and transport properties of refractory materials in the liquid state remains a key challenge limiting their application. Using alternating current (AC) and direct current (DC) techniques, the electrochemical kinetics of oxygen evolution and metal deposition was investigated in a pendant droplet of molten alumina (Al2O3) with three iridium (Ir) electrodes in a thermal imaging furnace. For the first time, the direct electrolytic decomposition of molten Al2O3 to oxygen gas and aluminum (Al) metal (alloyed with Ir) was observed, confirming the ionic nature of molten Al2O3. The decomposition potential of molten Al2O3 was measured with high precision using AC voltammetry, and the results demonstrated remarkable sensitivity to variation in temperature enabling measurement of chemical potential and entropy of Al at the Ir-rich solid-liquid phase boundary for the first time. The results were in remarkably close agreement with the most recent thermodynamic assessment of the Al-Ir system.
Highlights
Experimental observations in this study show typically dispersed metallic particles of Ir in the melt similar to those reported in Ref. 47, but no detectable signs of Ir loss were observed both while operating Ir as an anode during bulk electrolysis or as a RE
Iridium in molten Al2O3 is hereafter considered as an inert electrode under both open circuit and anodic polarization, within the parameters and limits of detection employed in this study
The decomposition voltage of molten Al2O3 was measured with high precision using ACV, and the results demonstrated remarkable sensitivity to temperature enabling measurement of Gibbs energy and entropy without the use of a membrane
Summary
Further scanning toward negative potentials, a current plateau at 77 mA (label A) is observed at around E = 0.8 V. The features of the shapes of the harmonic current responses for both anodic and cathodic faradaic reactions were found to be reproducible, independent of temperature and cycle number. (bottom) with temperature for various electrode configurations and gas atmospheres (error bars represent 1σ uncertainty from multiple scans in both cathodic and anodic directions and lines show fitting according to E = first-order parameter (β1), and β0 + β1 EC∗ was. Electrolysis.—To acquire further insight into the nature of the cathodic and anodic reactions coinciding with the faradaic events observed above, potentiostatic and galvanostatic electrolysis.
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