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Cyclic Voltammetry of ZrO2 Powder in the Metallic Cavity Electrode in Molten CaCl2

The electrochemical reduction of the insulative ZrO 2 powder in molten CaCl 2 was investigated using the metallic cavity electrode (MCE) in molten CaCl 2 at 850°C. Cyclic voltammograms (CVs) revealed two consecutive reduction peaks corresponding to (i) ZrO 2 to Zr x O (x ≥ 1) and (ii) Zr x O to Zr. The intermediate, Zr x O, was metastable and underwent disproportionation to ZrO 2 and Zr, which was responsible for the detection of Zr metal in the potentiostatic reduction at less negative potentials. In the anodic scan, four main oxidation processes were observed. The relevant reactions were rationalized as the reoxidation of (iii) Zr x O to ZrO 2 , (iv) Zr to ZrO 2 , (v) Zr to ZrCl 2 , and (vi) Zr to ZrCl 4 . The metastable intermediate also contributed to the unique current variations in the anodic potential scans under different conditions. Potentiostatic electrolysis of the ZrO 2 powder in the MCE at the feature potentials of the CVs and analyses of the electrolysis products by scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed the electroreduction mechanism and revealed the localized conversion of the dense aggregates of the submicrometer particles of ZrO 2 to cauliflower-like aggregates of the nanoparticulates of Zr in the early stage of the electroreduction process.