The Calcium Anode in Molten Nitrate Electrolytes

Abstract

The electrochemical behavior of the calcium anode in molten nitrates was determined using constant current methods. Investigations were made in both LiNO3 and KNO3 at 350~ and in KNO3-LiNO3 (60-40 mole percent) over a temperature range of 250~176 Differential scanning calorimetry studies were also made on calcium in many nitrate mixtures. No exothermic reactions were detected for calcium in any pure alkali metal nitrate melt; however, chemical reactions were apparent for many calcium-molten nitrate systems which cantalned added halide salts. The rate of the calcium electrode reaction is determined largely by the passivating oxide film formed on calcium in molten nitrates. The addition of halides that promote breaks in the film can greatly improve both the electrode kinetics and the open-circuit potential. With sufficient breakdown of the passivati~g layer, high current densities can be obtained for the calcium anode with very little polarization. The effectiveness of the added halides in reducing polarization generally decreased in the order I-> Br-> C1-> F-. The experimental results can be explained by a passivating layer model involving specific adsorption of anions at the CaO/ solution interface. Molten nitrates are attractive for possible use in thermal batteries due to their favorable physical properties which include low melting points and high electrical conductivities. In addition, alkali metal nitrate melts are stable over a reasonable temperature range, have low volatility, and have a viscosity and surface tension similar to aqueous solutions (1,2). Thermal batteries fulfill unique military applications which require rapid activation and high discharge rates for short time periods (3). Generally, thermal batteries operate at temperatures between 500 ~ and 600~ using the LiC1-KC1 [59-41 mole percent (m/o)] eutectic which melts at 352~ Much lower melting points are possible with nitrate electrolytes; for example, KNO3-LiNO3 (57-43 m/o) melts at 132~ and KNO3-LiNOsNaNO3 (53-30-17 m/o) melts at 120~ (1, 2). Furthermore, problems associated with formation of a calcium-lithium alloy, CaLl2, at the calcium anode in LiC1-KC1

High Rate Discharge of Ca / LiClO4 ‐ LiNO3 Thermal Battery Cells

“…Open-circuit cell potentials close to these calculated values have been observed at 350~ in the presence of chloride ions that attack the passivating film on the anode (8,9). Previous studies showed that Ca/LiNQ-LiC1-KC1 [50-25-25 mole percent (m/o)] thermal battery cells gave promising results when discharged at 10 mA/cm 2 over a temperature range of 250~176 (9).…”

“…At current densities above 20-30 mA/cm~, however, the cell performance deteriorates due to polarization at the anode. The rate of the calcium anode reaction in molten nitrates is determined largely by the passivating film that hinders the passage of calcium ions across the oxide layer (8). Comparisons of measured and calculated cell resistances suggest that the anodic oxide film makes a sizeable contribution to the total cell resistance (9).…”

Achievements, Challenges, and Prospects of Calcium Batteries

“…Since the entropy variation for the oxide formation is negative, the free energy becomes less negative as temperature increases. Therefore, it is not surprising that the first reports on Ca metal anodes in the early 1960s (section ) were on cells operated at high temperatures (>450 °C). − , The electrolyte chemistry was very similar to Ca metal production; molten salts, typically eutectic mixtures of CaCl 2 and CaF 2 , were employed and involved electrowinning and electrorefining processes performed at >850 °C . Later on, the concept of liquid metal electrodes also used higher temperatures, typically operating at >500 °C …”

“…Some attempts to modify the surface of the calcium metal anodes with acetic acid to yield CaCO 3 were reported, despite the chemistry being shown to be extremely complex involving also corrosion with formation of both Ca 2 CrO 4 Cl and KCaCl 3 . Efforts to replace the chlorides by nitrates, with lower melting points, as electrolytes were also reported . A maximum exchange of one electron per mole of calcium was observed with the formation of a passivation film most likely consisting of CaO, hindering any fast kinetics.…”

Reactions Involving Silver Ions and Silver Metal in Molten Nitrates

“…Portions of this work were presented as Paper 586 at the Denver, Colorado, Meeting of the Society, Oct. [11][12][13][14][15][16] 1981, and as Paper 341 at the Detroit, Michigan, Meeting of the Society, Oct. 17-21 1982. Portions of this work were presented as Paper 586 at the Denver, Colorado, Meeting of the Society, Oct. [11][12][13][14][15][16] 1981, and as Paper 341 at the Detroit, Michigan, Meeting of the Society, Oct. 17-21 1982.…”