Electrical-Transport Properties of Hydrated and Anhydrous Vanadyl Phosphate in the Temperature Range 20−200 °C

The ac/dc conductivity of hydrated and anhydrous vanadyl phosphate has been measured in the range 20−200 °C in wet and dry nitrogen. Emf and thermoelectric power measurements have also been carried out at room temperature and 180 °C, respectively. At 20 °C and 58% relative humidity, the ac conductivity of a pellet of VOPO4·2H2O is of the order 10-5 S cm-1. Both conductivity and emf measurements indicate that at room temperature VOPO4·2H2O is a mixed protonic−electronic conductor with dominant protonic component (t H + ≈ 0.9). The presence of mixed conduction has allowed to use VOPO4·2H2O as an electrode able to exchange reversibly protons with a solid-state protonic conductor. Proton conduction in hydrated vanadyl phosphate decreases with increasing temperature, due to the loss of intercalated water, and vanishes at 70 °C concomitantly with the formation of VOPO4·H2O. The activation energy for electronic conduction of anhydrous VOPO4 in a dry nitrogen atmosphere is 11.9 kcal/mol between 100 and 200 °C. In this temperature range the electronic conductivity is enhanced by the presence of water chemisorbed on surface sites. Possible mechanisms accounting for this effect are suggested.