Application of potential measurements and sweep voltammetry methods with Ni/NiO/MgO reference electrode for measurement of oxide ions in molten CaCl2-CaO-Ca0

Abstract

Certain pyroprocessing techniques, such as direct oxide reduction (DOR), utilize a molten CaCl₂ as the electrolyte. The DOR process is used to reduce PuO₂ to Pu metal via Ca metal. A measurement of the CaO concentration could be used to know when the reaction has gone to completion. The solubility and stability of the oxide ions in molten CaCl₂ are critical aspects of these techniques. This study investigated electrochemical methods such as open circuit potential, cyclic voltammetry, and normal pulse voltammetry using a Ni/NiO reference electrode and tungsten working and counter electrodes. These methods tracked variations in calcium oxide concentration within molten CaCl₂-Ca. Key findings indicate that the OCP decreases significantly with dissolved Ca in pure CaCl₂ but increases with CaO additions, up to 5.1 wt % CaO. Correlations between oxidation current density and CaO concentration were established, contingent on the use of clean or new W working electrodes for cyclic voltammetry and normal pulse voltammetry methods. The study also identified issues related to surface contamination affecting measurements and established the stability of the Ni/NiO/MgO reference electrode over extended immersion periods, supporting its application in electrochemical monitoring.