Mn2+-Doped CeP2O7 Composite Electrolytes for Application in Low Temperature Proton-Conducting Ceramic Electrolyte Fuel Cells

Abstract

The proton conducting Ce1-xMnxP2O7 (x = 0.05, 0.075, 0.1, 0.125 and 0.15) composite electrolytes were synthesized by two-step slow digestion method with different P/(Ce+Mn) molar ratio. X-ray diffraction (XRD) patterns show that powders as-calcined at 300°C contain pyrophosphate as the only crystalline phase, but sintering at 400°C leads to the formation of a composite with additional crystalline phases of proton conducting metaphosphate, polyphosphate and orthophosphate. The microstructure of sintered pellets of Ce1-xMnxP2O7 (CMP) was analyzed by scanning electron microscopy (SEM). The CMP samples with high phosphate content become denser on sintering. The variation of ionic conductivity with temperature is studied in unhumidified and humidified air for the potential application of CMPs as electrolytes in proton-conducting ceramic electrolyte fuel cells (PCFCs). Among various CMP samples, Ce0.9Mn0.1P2O7 with P/(Ce+Mn) = 2.7 shows maximum conductivity of 6.54 × 10−6 S cm−1 at 450°C in unhumidified air and 1.78 × 10−2 S cm−1 at 170°C in humidified air with water vapor pressure (pH2O) of 0.12 atm. The ionic conductivity of CMPs increases with the increasing pH2O and Ce0.9Mn0.1P2O7 shows maximum conductivity of 2.24 × 10−2 S cm−1 at 170°C in pH2O = 0.16 atm.