Phase Composition and Long-Term Conductivity of Acceptor Doped Ce(PO<sub>3</sub>)<sub>4</sub> and CeP<sub>2</sub>O<sub>7</sub> with Variable P/Metal Ratio and of CeP<sub>2</sub>O<sub>7</sub>-KH<sub>2</sub>PO<sub>4</sub> Composite
1 Department of Energy Conversion and Storage, Technical University of Denmark2 Fundamental Electrochemistry, Department of Energy Conversion and Storage, Technical University of Denmark
The thermal evolution of the phase composition of CeP2O7 and Ce(PO3)4 with 10 mol% Y and Gd doping, respectively, was examined by in-situ powder X-ray diffraction and thermogravimetry with in-line mass spectroscopy. The phase composition depends critically on the P to metal ratio, the annealing temperature, humidity and time. CeP2O7 and Ce(PO3)4 were completely decomposed to CePO4 following a 1100 h long conductivity test at 155°C. The conductivity of 10 mol% Gd doped Ce(PO3)4 (synthesized with P:(Ce + Gd) = 5.0) reaches a value of 6.4·10−2 S·cm−1 at 150°C under wet conditions (pH2O = 0.2 atm). The conductivity of 10 mol% Y doped CeP2O7 (synthesized with P:(Ce + Y) = 3.1) was 1.9·10−2 S·cm−1 under the same conditions. Long term conductivity measurements are reported here for the first time and the effect of repeated hydration-dehydration cycles on the conductivity is examined. Exsolution of PmOn and increase of the highly hygroscopic amorphous secondary phase significantly affects the conducting properties. KH2PO4 was observed to re-crystallize and form amorphous potassium phosphate at temperatures above 100°C in the 10 mol% Y doped CeP2O7:KH2PO4 composite (synthesized with P:(Ce + Y) = 3.1) resulting in a conductivity value of 2.6·10−2 S·cm−1 at 150°C and pH2O = 0.2 atm.
Journal of the Electrochemical Society, 2013, Vol 160, Issue 8