Proton Conduction in In[sup 3+]-Doped SnP[sub 2]O[sub 7] at Intermediate Temperatures

Abstract

SnP2 O7 -based proton conductors were characterized by Fourier transform infrared spectroscopy (FTIR), temperature-programmed desorption (TPD), X-ray diffraction (XRD), and electrochemical techniques. Undoped SnP2 O7 showed overall conductivities greater than 10-2 S cm-1 in the temperature range of 75-300°C. The proton transport numbers of this material at 250°C under various conditions were estimated, based on the ratio of the electromotive force of the galvanic cells to the theoretical values, to be 0.97-0.99 in humidified H2 and 0.89-0.92 under fuel cell conditions. Partial substitution of In3+ for Sn4+ led to an increase in the proton conductivity (from 5.56× 10-2 to 1.95× 10-1 S cm-1 at 250°C, for example). FTIR and TPD measurements revealed that the effects of doping on the proton conductivity could be attributed to an increase in the proton concentration in the bulk Sn1-x Inx P2 O7. The deficiency of P2 O7 ions in the Sn1-x Inx P2 O7 bulk decreased the proton conductivity by several orders of magnitude, which was explained as due to a decrease in the proton mobility rather than the proton concentration. The mechanism of proton incorporation and conduction is examined and discussed in detail. © 2006 The Electrochemical Society. All rights reserved.