Investigation on the application of platinum anode towards direct electrochemical reduction of solid metal oxides in CaCl₂–CaO melt

Molten CaCl₂–CaO system is a suitable electrolyte medium for reducing metal oxides to metal by in-situ electro-generated calcium metal. The graphite anode, which acts as a reactive anode in this melt, leads to several parasitic reactions and decreases the current efficiency. The present study investigates the usage of platinum anode towards the direct oxide electrochemical reduction of ThO₂ and NiO by electro-generated calcium in CaCl₂–1 wt % CaO melt at 900 °C. Primarily, the anodic behavior of platinum electrode was investigated using electrochemical techniques such as linear sweep voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy, cyclic voltammetry and potentiostatic electrolysis in CaCl₂–CaO melt. Platinum exhibited the most anodic polarization potential, positive corrosion potential, and highest oxidation resistance compared to nickel and gold. It also showed a clearly demarcated potential window for oxygen evolution and much better physico-chemical stability compared to gold electrode. Electro-calciothermic reduction experiments conducted with ThO₂ and NiO cathodes using platinum anode demonstrated the feasibility of metallization, and platinum's mass loss rate in these experiments was found to be much less (in the order of ~ 0.016 g cm⁻² h⁻¹). The study showed that platinum could be used as an anode in the electrochemical reduction of solid metal oxides in CaCl₂–1 wt % CaO melt with minimal mass loss.