Electroreduction of tungsten oxide(VI) in molten salts with added metaphosphate

Abstract

Tungstate ions WO₄ ²⁻ are not electrochemically active in chloride melts. Upon introduction of PO₃ ⁻ ions into the melt, two waves appear in the voltammograms at potentials −0.1— −0.2 V and −0.3— −0.5 V vs. Pb/Pb²⁺ reference electrode. With a PO₃ ⁻ concentration ratio of 0.01<[PO₃ ⁻]/[WO₄ ²⁻]<0.18, the potentiostatic electrolysis product of WO₄ ²⁻ at the above mentioned potentials is metallic tungsten; a NaPO₃ concentration increase for ratios [PO₃ ⁻]/[WO₄ ²⁻]≫ 0.18 results in tungsten phosphide in electrolysis product. Cyclic voltammograms and dependence of half-peak potentials on electrode polarization rate indicate the irreversible character of the electrode process. Electrode process modeling allows us to state that the first wave in the voltammogram of KCl-NaCl-Na₂WO₄-NaPO₃ system corresponds to tungsten oxychlorides discharge while the second wave appears due to the discharge of ditungstate ions. In the voltammograms of Na₂WO₄-NaPO₃ melts, reduction wave was observed at −1.1- −1.2 V potentials. Proportionality of limiting current to NaPO₃ concentration, constancy of I _d/v ^1/2 ratio, and I _d/nFc constant kinetic value equal to (8.3−9.5)×10⁻⁵ cm/s for steady-state wave indicate that electrode process rate is limited by electrochemically active particle diffusion to the electrode. Nascent ditungstate ions become electrochemically active in the overall electrode process. Charge transfer stage is reversible.