Electrochemical Studies of Tantalum in Fluorochloroaluminate Melts at 200–450°C

Abstract

The electrochemistry of tantalum(V) species in sodium fluorochloroaluminate melts (10 mole percent NaF) has been investigated in the temperature range of 200 to 450~ using cyclic, normal pulse, and square wave voltammetries, exhaustive electrolysis, Raman and electronic spectroscopies, and x-ray diffraction methods. The electrochemical behavior of tantalum(V) is strongly dependent on temperature. Three main reduction waves are observed at a temperature of 300~ or higher. The first and second reduction waves merge into one wave at temperatures below 300~ The first reduction wave is associated with the reduction of tantalum(V) to tantalum(IV) Species followed by adimerization reaction which occurs very slowly at lower temperatures. The second reduction wave is believed to be the reduction of the tantalum(IV) dimer, Ta~ +, to a tantalum(III) species (probably Ta~+). The tantalum(III) species decomposes resulting in the formation of the cluster, Ta6CI~. The last reduction wave is assigned to the reduction of the trivalent tantalum species to a divalent tantalum species, which is highly unstable and decomposes to form the tantalum cluster, Ta~CI~L and metallic tantalum. The clusters are slowly reduced to metallic tantalum.The electrochemistry of tantalum is complicated by the existence of various compounds with different oxidation states, such as Ta 5 § Ta 4., Ta 3 § Ta~ ~ § Ta~ 4 § Ta ~ 1-4 in A1C13-NaC1 melts. 5' 8 McCarley et al. 2'3 reported that anhydrous low-valent tantalum halides can be synthesized by the reduction of tantalum(V) halides with aluminum metal at. appropriate temperatures. The electrolytic reduction and oxidation of tantalum and other refractory metal species in molten halide salts, 713 organic solvents, ~4 and room temperature me]ts~ have received considerable attention, since these metals generally have very high melting points and high corrosion resistance.We are interested in the electrochemistry and electroplating of refractory metals such as Nb, Ta, and W in alkali chloroaluminate and fluorochloroalumina-te melts. von Barner et al. 5 have recently reported electrochemical and spectroscopic studies of tantalum species in A1C13-NaC1 melts at 160-300~ Tantalum(V) forms two different species, TaCI~ and TaC15, in basic (A1ClJNaC1 mole ratio <1) and moderately acidic A1C13-NaC1 melts/' In addition, TaOC1; is formed in basic melts in the presence of small amounts of oxide ions. 5 The reduction of tantalum(V) in an acidic A1C13-NaC1 [51-49 mole percent (m/o)] melt at 175~ is believed to follow the sequence; Ta ~+ + e-= Ta 4., 2 Ta 4+ = Ta~ +, Ta~ § + 2e-= Ta~ +, 5 Ta~ + = Ta~ 4+ + 4 Ta% This reduction leads to the formation of a tantalum cluster. Formation of metallic tantalum was not observed in the electrolysis at 175~ in the sodium chloroaluminate melts.McCurry ~7 investigated the electrochemical behavior of tantalum(V) in A1C13-NaC1 melts saturated with NaCI, noted A1C13-NaCI~at melts, as a function of the oxide concentration in these melts. These studies resulted in a voltammetric method empl...

Electrochemical and Simultaneous Spectroscopic Study of Reduction Mechanism and Electronic Conduction During Electrodeposition of Tantalum in Molten Alkali Chlorides

“…Ta6C1A2). Such species have been shown to be byproducts during electrolysis in fluorochloroaluminate melts [22] and have been stabilized in both molten salt {8, 231 and aqueous [23] soiutions. However, the strongest Raman band measured in aqueous solutions for (Ta6ClI2)X2 (X = C1, Br, I) clusters is the symmetric metalmetal vibration whose frequency for all three halides is between 203-199cm-' [24].…”

TEM Studies of Carbon Coated LiFePO4 After Charge Discharge Cycling

“…Despite its high theoretical capacity of 170 mAh/g a major obstacle to the commercialization of LiFePO 4 is its poor electronic conductivity that limits achievable discharge rates (1). Reduction in particle size (4), carbon coating (5)(6)(7) and addition of small metallic particles (8) are among the approaches used to enhance the electrochemical performance of LiFePO 4 . We successfully employed carbon coating with pyromellitic acid and ferrocene to enhance the rate capability of electrodes based on LiFePO 4 (9).…”