Mechanism and Kinetics of the Phase Formation and Dissolution of Na_xWO₃ on a Pt Electrode in a Na₂WO₄–WO₃ Melt

A comprehensive study concerning the phase formation mechanism and growth/dissolution kinetics of sodium tungsten bronze crystals during the electrolysis of a 0.8Na₂WO₄–0.2WO₃ melt was carried out. The regularities of deposit formation on a Pt(111) working electrode were investigated experimentally using cyclic voltammetry, chronoamperometry, scanning electron microscopy, and X-ray diffraction analysis. Models have been developed to calculate the current response during the formation, growth and dissolution of a two-phase deposit consisting of Na_xWO₃ and metallic tungsten or two oxide tungsten bronzes with different sodium content. These models consider mass transfer to the electrode and nuclei; chemical and electrochemical reactions with the participation of polytungstate ions, Na⁺, Na⁰, and O²⁻; as well as the ohmic drop effect. The approach was proposed to describe the dissolution of an Na_xWO₃ crystal with a nonuniform sodium distribution. The fitting of cyclic voltammograms was performed using the Levenberg–Marquardt algorithm. The Na_xWO₃ formation/growth/dissolution mechanism was determined. Concentration profiles and diffusion coefficients of [W_nO_3n]⁻, reaction rate constants, number density of nuclei, and time dependencies of crystal size were calculated. The proposed approaches and models can be used in other systems for the cyclic voltammogram analysis and study of the mechanism and kinetics of electrode processes complicated by phase formation; parallel and sequential electrochemical and chemical reactions; as well as the formation of a deposit characterized by a nonuniform phase and/or chemical composition.