1 Energy and Materials, Department of Chemistry, Technical University of Denmark2 Department of Chemistry, Technical University of Denmark3 unknown4 Technical University of Denmark5 Department of Energy Conversion and Storage, Technical University of Denmark
The electrochemical behavior of tantalum in the form of K2TaF7 in an LiF-NaF-KF eutectic melt has been studied by linear voltammetry in the temperature range of 560 to 815-degrees-C with and without additions of Na2O. An amperometric titration has been performed by measuring the heights of the cathodic and the anodic peaks. It was shown that at a molar ratio Na2O/K2TaF7 = 1 the predominating complex in the melt is TaOF5(2-), whereas with an Na2O/K2TaF7 molar ratio of 2 it is TaO2F(x)(x-1-), probably in the form of TaO2F4(3-). Increase in the Na2O/K2TaF7 molar ratio in excess of two leads to a decrease of tantalum concentration in the melt, and precipitation of KTaO3 occurs. Both the fluoro complex and the monooxofluoro complex were reduced to metal in a single five-electron step. The fluoro complexes, in the temperature range 625 to 815-degrees-C with potential scan rates 0.5 V . s-1 they discharge irreversibly. Monooxofluoro complexes discharge irreversibly at all temperatures and scan rates studied. The diffusion coefficient of the tantalum fluoro complex depends on the temperature as log D = -2.55 - 2044/T with an activation energy of 39.1 kJ . mol-1. For the tantalum monooxofluoro complex the dependence is log D = -2.35 - 2293/T with an activation energy of 43.9 kJ . mol-1.
Electrochemical Society. Journal, 1994, Vol 141, Issue 11, p. 2982-2988