Voltammetric Study on the Behaviour of Refractory Metals in ((BMP)(TFSA)) Ionic Liquid.

Abstract

Electrochemical studies of tantalum fluoride and zirconium fluoride in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([BMP][TFSA]) ionic liquid were carried out in presence of lithium fluoride using cyclic voltammetry technique. The electrochemical behaviour of Ta(V) and Zr(IV) on gold and platinum substrates was investigated in the temperature range from R.T. to 200 °C at different values of scan rate and different potential windows. Special attention was paid to the mechanism of cathodic reduction. Cyclic voltammetries exhibit well-defined peaks attributed to the formation of metallic deposits on the substrates. The knowledge of electrode and chemical reactions based on the experimental results of the refractory metals considered allow us to propose a possible reaction path for the oxidation/reduction processes. The mechanisms of electrochemical reduction of Tantalum(V) depends strongly on the composition of the bath; using TaF5 as metal salt, two cathodic peaks due to Ta(V)/Ta(III) and Ta(III)/Ta reduction were observed. The cyclic voltammograms recorded at different values of scan rate have shown that the Ta(III)/Ta reduction process is mainly controlled by diffusion of the electroactive species to the electrode surface. It has been proposed that the mechanism for reduction of zirconium ions occurs in a one step process exchanging four electrons and controlled by the zirconium ions diffusion in the ionic liquid. The electrochemistry of refractory metals (IVB, VB and VIB groups) has been widely studied in molten salts to optimize the cathodic deposition process of these metals. Due to their properties, such as high strength and high corrosion resistance, refractory metals are widely used in electronics, optics, sensors, automotive, nuclear and aerospace industries. Conventional aqueous media cannot be always used as electrolytes due to the narrow electrochemical windows, low thermal stability and evaporation (Simka et al., 2009). The mechanism of the electrodeposition of refractory metals is not yet fully elucidated. Senderoff and Mellors (1966) developed a general process for the electrodeposition of eight of the nine refractory metals of groups IVB, VB and VIB as dense coherent deposits by using a solution of the refractory metal in a molten alkali-fluoride eutectic mixture from 680 °C to 800 °C. They concluded that an irreversible metal-producing step is a necessary (though not sufficient) condition for the deposition of coherent deposits from molten salts. The mechanical properties of refractory metals, such as the excellent corrosion resistance and the nuclear properties of some, make them particular interesting as coatings. Inman and White (1978) have summarized in a review the electrochemistry of refractory metals in molten salts electrolytes. The authors have reported that the recovery of a primary metal is ensured by using, as solvent, a melt having a much larger decomposition voltage than that of the solute and an inert cathode; controlling the electrodeposition of metal by rate processes other than mass transfer, will lead to coherent deposits. Few years later, Girginov et al. (1995) have presented a selective review of the anodic and cathodic processes of Ti, Zr, Nb and Ta from molten salts-based electrolytes in order to investigate the optimization of cathodic deposition of these metals highlighting that the two main types of electrolyte employed in the electrodeposition of refractory metals were chloride and fluoride based. The Ta(V) reduction has been studied in a wide range of melts, such as chlorine or fluorine melts. In FLINAK-K2TaF7 DOI: 10.3303/CET1441017