Impedance Spectroscopic Investigation of the Reduction Mechanism of Tantalumhalides During Electrodeposition in Molten Salts

Abstract

The mechanism of electrochemical reduction of TaCl5 dissolved in molten alkalihalides or their mixtures has been investigated by impedance spectroscopy in the frequency range 10 mHz≤ω≤100 kHz at various cathodic and anodic potentials and temperatures up to 1100 K. From a quantitative interpretation of the diffusion controlled impedance various reduction steps are resolved to better than ±10 mV. This is demonstrated here for a few selected examples. Besides this, the most important observation reported here for the first time is that electronic contributions can become dominant. Only with appropriate consideration of the electronic conduction a quantitative description of the impedance spectra over the whole potential range is found.

Electronic absorption spectra of reduction products of pentavalent niobium and tantalum in different alkali chloride and oxychloride melts

“…This is strongly supported by separate transport measurements which indicate significant electronic conductivity contributions when reducing TaCls in alkali halide melts. 19 We are aware that this suggestion made here for the first time for an ionic liquid will rise several questions. A crucial problem is how to envisage the local microscopic structure which favors this intervalence charge transfer in the liquid.…”

“…At the same time this intervalence charge transfer may explain the observed enhanced electronic conductivities. 19 An interpretation of the tantalum spectra by d-d transitions alone seems to be excluded, although contributions may be possible. In the niobium spectra the situation seems to be opposite, intervalence charge transfer not prevailing.…”