ELECTRODEPOSITION OF TANTALUM FROM

A NOVEL LOW TEMPERATURE MOLTEN SALT

 

 

Morio Matsunaga, Masatsugu Morimitsu, Takamitsu Matsuo, and Hiroyuki Tabata

 

Department of Applied Chemistry, Kyushu Institute of Technology

1-1 Sensui, Tobata, Kitakyushu 804-8550, Japan

 

 

We have found that a mixture of anhydrous tantalum(V) chloride with 1-methyl-3-ethlyimidazolium chloride (EMIC) forms an ionic liquid at low temperatures less than 373 K, and have conducted the experiments on the electrochemistry of Ta(V) in this novel low temperature molten salt. In a 40:60 mol% TaCl₅-EMIC melt, the reduction of Ta(V) occurs via two reaction steps at 373 K; one is the reduction of Ta(V) to Ta(IV) involving a reversible one electron transfer, and the other is that of Ta(IV) to Ta(II) with a reversible two electrons transfer. No further reduction of Ta(II) to lower oxidation states of tantalum species has been confirmed, thus tantalum electrodeposition hardly occurs in this melt.

However, Ta(II) can be further reduced when LiF is added into the melt. In the cyclic voltammogram of a 30:60:10 mol% TaCl₅-EMIC-LiF at 373 K, there are two new reduction waves following the wave corresponding to the reduction of Ta(IV) to Ta(II), indicating that the addition of LiF induces further reductions of Ta(II) to lower oxidation states of tantalum species. In fact, when the mole ratio of added LiF is 5 mol% and more, the electrodeposits obtained by constant current electrolyses have been found to be tantalum by XRD and XPS analyses of the electrodeposits.

The electrochemistry of Ta(V) has been also examined in a 55:45 mol% TaCl₅-EMIC melt in order to assess the effect of Lewis acidity of the melt on the reduction process of Ta(V). In this Lewis acidic melt, the reduction sequence of Ta(V) is the same as that in the 40:60 mol% TaCl₅-EMIC melt, except the difference in the reversibility of each reaction step: i.e., both reaction steps of Ta(V) to Ta(IV) and Ta(IV) to Ta(II) have some irreversibility in the acidic melt. Further, the addition of LiF into the melt does not induce the electrodeposition of tantalum, though some cathodic waves corresponding to the reduction of Ta(II) have been seen in the LiF-added melt. Such a difference in the effect of LiF addition may be related to the structure of Ta(V) chlorocomplex depending on the melt acidity.

We will discuss the roles of LiF to the reduction of Ta(V) and the deposition mechanism of tantalum in the LiF-added basic TaCl₅-EMIC melt, with other experimental results on the ionic structure of tantalum species in the melt with and without LiF.