Quantification of the Impact of Residual H2O on Cathodic Behavior in Molten CaCl2 Electrolysis

The H 2 generated during the electrolysis of H 2 O present in molten CaCl 2 inhibits metallothermic reduction by metallic Ca. The impact of residual H 2 O on the cathodic reaction was estimated by electrolyzing H 2 O-containing CaCl 2 at 1173 K in Ar atmosphere using high-speed imaging, which visualized the H 2 bubbles occurring around the cathode, to analyze the incomplete removal of H 2 O in CaCl 2 . The H 2 gas generation occurred simultaneously with the electrochemical deposition of Ca when the H 2 O content increased. Moreover, we established that the cathodic current density increased with respect to the same potential. The H 2 generation estimated from the Coulombic efficiency was miniscule when the ratio of H 2 generation to the total amount of electricity contributing to the electrochemical reaction was calculated from the directly observed bubble volume. Therefore, synthesis of calcium hydride due to contact with precipitated Ca is highly probable, in addition to the H 2 generation observed in the form of bubbles. Graphical Abstract