Thickness dependence of proton conductivity of anodic ZrO2–WO3–SiO2 nanofilms

Abstract

Amorphous ZrO2–WO3–SiO2 nanofilms are simply prepared by anodizing of sputter-deposited Zr37W47Si16 alloy at several formation voltages for 1.8ks in 0.1moldm−3 phosphoric acid electrolyte at 20°C. Efficient proton conductivity was observed after thermal treatment at 250°C with the conductivity enhanced by reducing the film thickness. The conductivity is enhanced more than one order of magnitude by reducing the thickness from 300 to 140nm. The anodic oxide films consist of two layers, comprising a thin outer ZrO2 layer free from silicon and tungsten species and an inner main layer containing all zirconium, silicon and tungsten species. The thickness-dependent conductivity of the anodic ZrO2–WO3–SiO2 films is associated with the conductivity of the outer ZrO2 layer, which increases exponentially with reducing the film thickness. The area-specific resistivity of 0.14Ωcm2, which is below the minimum requirement (0.2Ωcm2) for a practical electrolyte membrane in commercial fuel cells, is achieved at a temperature at 225°C for 100nm-thick anodic ZrO2–WO3–SiO2 films.