Niobium Speciation in NaCl-KCl Based Melts: An Electrochemical and Spectroelectrochemical Study

Abstract

A variety of electrochemical techniques (polarization curves measurements, chronopotentiometry, chronoamperometry, linear, cyclic and square-wave voltammetry, impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. Information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra) methods. The major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl6 3-. Soluble niobium(V) species, NbCl6 -, represented the highest oxidation state of niobium in NaCl-KCl melts. It was shown that oxidation of Nb(IV) ions, NbCl6 2-, proceeds in a single step. Cathodic reduction of niobium(IV)-containing melts on tungsten electrode at -1.55÷-1.6 V (vs. Cl-/Cl2) resulted in formation of niobium metal. Electrochemical and spectroelectrochemical data indicate that intermediate niobium(III) species can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts.