Electrodissolution of zinc in a low temperature molten salt and energy conversion applications.

Abstract

The fairly recent development of molten salts with melting points below room temperature has spurred interest in these solutions as electrolytes for high energy density systems. However, relatively little is known about electrode kinetics and transport in low-temperature molten salts. This research project investigated the stoichiometry, transport, and kinetic properties of the zinc dissolution process in mixtures of aluminum chloride and 1-methyl-3-ethylimidazolium chloride and application of this molten salt in energy conversion devices. Transport studies revealed the presence of two limiting current regions in the dissolution process which corresponded to a coordination number of four at low overpotentials and a coordination number of three at high overpotentials. A new mass transfer correlation for rotating cylinder electrodes was found for these high Schmidt number electrolytes. From this correlation, the quantity D$\\mu$/T was calculated to be 3.5 $\imes$ 10$\\sp{-10}$ g cm/s$\\sp2$ K, which corresponds to a chloride ion radius of 2.1 $\imes$ 10$\\sp{-8}$ cm. Kinetic studies revealed that the zinc dissolution rate law is third order with respect to the chloride ion and third order overall. The anodic Tafel slope was found to be 40 mV, and the reaction had an exchange current density of.13 mA/cm$\\sb2$, essentially independent of melt composition. An atomistic mechanism was proposed and shown to be consistent with the kinetics data. Energy conversion studies centered around aluminum and zinc negative electrodes coupled with chlorine and the metal chloride, FeCl$\\sb3$ and CuCl$\\sb2$. Experiments revealed that chlorine gas is quite soluble in these melts, leading to high self-discharge rates which are undesirable. Studies involving metal chlorides revealed the high solubility of these salts in basic melts, which would limit their applications to reserve applications. Experiments conducted with metal chlorides in acidic melt compositions showed the most promise. These cells were able to withst and high discharge rates at voltages greater than 1 volt. Additionally the reversible behavior of these cells will permit their use as secondary batteries. One major shortcoming of this system, the poor utilization of positive reactant, was overcome by the use of high surface area current collectors.