Improved thermal stability of efficient proton-conducting anodic ZrO ₂-WO₃ nanofilms by incorporation of silicon species

Novel proton-conducting amorphous anodic ZrO₂-WO ₃-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 20C. 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 300C in dry Ar atmosphere, the anodic ZrO₂-WO₃-SiO ₂ films revealed stable proton conductivity in the temperature range of 50-225C, while the anodic ZrO₂-WO₃ on the Zr ₄₃W₅₇ loses the proton conductivity by annealing at 250C. 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 alloyanodic oxide interface.