Tungsten oxide bronzes with alkali metals

Abstract

Single-phase samples of tungsten bronzes M _x WO₃ (M = K⁺, Rb⁺, Cs⁺) are prepared by solid-state synthesis. The reversibility of the M_0.33WO₃/M⁺-solid electrolyte interface is studied subject to the alkali metal nature and humidity over a wide temperature interval. The exchange current density at 24°C and 58%-relative humidity is 3.6 × 10⁻⁴ A/cm² for the Rb_0.33WO₃/Rb⁺-solid electrolyte interface; 2.2 × 10⁻⁴ A/cm² for the Cs_0.33WO₃/Cs⁺-solid electrolyte interface; and 1.3 × 10⁻⁴ A/cm² for the K_0.33WO₃/K⁺-solid electrolyte interface. A correlation between the reversibility of the bronze|solid electrolyte interface, which is characterized by the exchange current density, and the rate of potential equilibration in sensor systems, where the bronze is a reference electrode, is revealed. Ionic component of the conductivity of the synthesized tungsten oxide bronzes is measured at a background of the predominant electronic conductivity. The ionic conductivity is three orders of magnitude lower than the electronic conductivity; it decreases in the series Rb_0.33WO₃ > Cs_0.33WO₃ > K_0.33WO₃, amounting to 2.3 × 10⁻², 2.1 × 10⁻³, and 2 × 10⁻⁴ S cm⁻¹, respectively. The working capacity of the M_0.3WO₃ bronzes as reference electrodes in sensor systems for carbon dioxide detection is evaluated. The plots of the cell potential vs. the CO₂ concentration in the electrochemical cells are linear, their slopes (59 ± 1 mV/decade) are characteristic for one-electron process. The fastest response to changes in the CO₂ concentration was obtained with the sensor system that used Rb_0.33WO₃ as reference electrode.