Improved Thermal Stability of Efficient Proton-Conducting Anodic ZrO2-WO3 Nanofilms by Incorporation of Silicon Species

Abstract

Novel proton-conducting amorphous anodic ZrO2-WO3-SiO2 films, 200 nm thick, are prepared by anodizing of sputter-deposited Zr37W47Si16 at 100 V with current decay for 1.8 ks in 0.1 mol dm−3 phosphoric acid electrolyte at 20°C. The resultant anodic films have been characterized using electrochemical impedance spectroscopy, transmission electron microscopy, glow discharge optical emission spectroscopy and Rutherford backscattering spectroscopy. The addition of silicon species to the anodic ZrO2-WO3 film significantly enhanced the thermal stability. Even after thermal treatment at 300°C in dry Ar atmosphere, the anodic ZrO2-WO3-SiO2 films revealed stable proton conductivity in the temperature range of 50–225°C, while the anodic ZrO2-WO3 on the Zr43W57 loses the proton conductivity by annealing at 250°C. The anodic film on the Zr37W47Si16 consisted of two layers, comprising an outer thin ZrO2 layer, free from tungsten and silicon species, and an inner main layer containing all zirconium, tungsten and silicon species. The results in this study suggest that the conductivity deterioration at high annealing temperatures is associated with the diffusion-induced formation of a poorly-conducting layer near the alloy/anodic oxide interface.