Proton Conduction in Thermally Treated Antimonic Acid Samples

Abstract

The high proton conduction of antimonic acid was found to be maintained even after thermal treatments at 513, 593, 773 and 893 K. The conductivities of the thermally treated samples were comparable to that of untreated antimonic acid, i. e., about 10^-1S·m^-1, at room temperature when the relative humidity was high (Fig.1). The number of protonic transference was confirmed to be unity for each sample (Fig.2). Instrumental analyses by using TG, X-rays and IR showed that antimonic acid loses water and a part of oxygen in the temperature range 513-893 K. (Figs.3 and 4), where the crystal structure changes continuously into that of Sb₈O₃ (Figs.6 and 7). The ion exchange capacity of each sample was found to coincide with the amount of its constitution water (Figs.8 and Table 1). It is emphasized that the skeleton structure of antimonic acid was maintained even after the treatment at 893K. The high conductivities, observed at high relative humidities regardless of the treatment temperature, suggest the importance of physisorbed water. The isotherms of water adsorption (Fig.9) indicated that the conductivity increased drastically with an increase in water adsorption only when the surface coverage (θ) of water exceeded unity (Fig.10). The proton conduction in Grotthus mechanism is considered to be feasible at θ>1 (Fig.11).