Catholyte Studies in Copper Oxide‐Magnesium Thermal Cells

Abstract

The present investigation concludes the study of the performance of copper oxide‐magnesium thermal cells operating at 450°C. The objective of this present study was to investigate the cause of the 0.25V step drop in open‐ circuit voltage that occurred during the latter stages of discharge of these cells.The investigation consisted of two parts: (A) Cells were equipped with a reference electrode between the anode and cathode and discharged. These tests showed that the step drop in the open‐ circuit voltage was due to a sudden drop in the activity of the cuprous ion in the vicinity of the cathode. This two‐order‐of‐magnitude drop in the cuprous ion activity was attributed to the formation of a complex. Other cells were discharged which had additions of either magnesium chloride or lithium oxide in the electrolyte. Additions of lithium oxide brought about the formation of this complex, and additions of magnesium chloride retarded the formation of this complex at the expense of a much higher rate of self‐discharge. (B) Potentiometric titrations were performed to more fully investigate the stoichiometry of the complex. These titrations were performed in solutions of the lithium chloride‐potassium chloride eutectic. When 1.5 moles of lithium oxide were added per mole of cuprous oxide, an end point in the titration occurred. The difference in electromotive force (emf) of a copper electrode placed in a saturated solution of cuprous oxide and one placed in a saturated solution of the cuprous complex was the same as the emf difference that occurred at the voltage step during discharge of complete cells. Additions of magnesium chloride to this solid complex returned the cuprous ion to solution by precipitating the oxide ion (as ) associated with the complex.Based on these and our previous findings, a model is presented that explains essentially all the significant features of cell behavior during discharge. This model suggests that, during the initial stages of discharge, magnesium ions move to the catholyte, where they are precipitated as magnesium oxide. During the latter stages of discharge, lithium ion predominates in the charge neutralization process and creates conditions favorable for the formation of the cuprous complex. This lowers the open‐circuit and operating voltages of these thermal cells.