Ionic Conductivity of Gd3+ Doped Cerium Pyrophosphate Electrolytes with Core-Shell Structure

Abstract

In this work, 10% Gd3+-doped cerium pyrophosphates (CGPs) with core-shell structure are synthesized by reacting Ce0.9Gd0.1O2 with H3PO4 in a two-step solid-state slow digestion method. Use of high P/(Ce+Gd) ratio in initial reaction mixture gives a core-shell morphology composed of crystalline CGP core and amorphous phosphate (PmOn) shell. The amorphous phosphate helps in densification of CGP pellets during sintering, without causing the appearance of any impurity. Variation of ionic conductivity of CGPs with temperature is studied in unhumidified and humidified air conditions, and is explained on the basis of microstructure, phosphate content and proton conduction mechanism. The basic dissolution of protons occurs in the crystalline pyrophosphate phase of material bulk at the oxygen vacancies formed due to the aliovalent doping of Gd3+ and acidic dissolution of protons occurs in the amorphous phase due to the hydrolysis of PmOn groups. Ionic conductivities of CGP samples vary in 10−3−10−2 range in 90-230°C range in humidified air and maximum conductivity obtained is 2.91 × 10−2 S cm−1 at 190°C, pH2O = 0.16 atm. The pH2O dependence and long term response (for 450 h) of the ionic conductivity in humidified air is analyzed for potential application as electrolyte in proton-conducting ceramic electrolyte fuel cells.