Enhanced Ionic Conductivity in Ce0.8Sm0.2O1.9: Unique Effect of Calcium Co-doping

Authors


  • P.S.D. thanks the Director of the Central Glass and Ceramic Research Institute, Kolkata, for permission to publish this work. Authors are grateful to Prof. D. D. Sarma, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, also at the Indian Association for the Cultivation of Science, Kolkata, for fruitful discussion and helpful suggestions. P.S.D. performed part of this work under the network program on “Custom Tailored Special Materials” (CMM002) of the Council of Scientific and Industrial Research (CSIR), Govt. of India. K.M. gratefully acknowledges the support of the Department of Science and Technology, Govt. of India, and the Italian Ministry of Foreign Affairs for the Synchrotron Radiation studies. Thanks are due to the beamline scientists at the Circular Polarization Beamline of Elettra, Trieste, Italy, for help with the X-ray absorption studies. S.B. is indebted to CSIR for the award of a Senior Research Fellowship.

Abstract

In order to identify new oxide ion-conducting materials in the ceria family of oxides, the unique effect of co-doping is explored and a novel series of Ce0.8Sm0.2–xCaxO2–δ compositions is identified that have enhanced properties compared to the single-doped Ce0.8Sm0.2O1.9 and Ce0.8Ca0.2O1.9 compositions. Moreover, the superior characteristics of the co-doped Ce0.8Sm0.2–xCaxO2–δ powders prepared by the mixed-fuel process aid in obtaining 98 % dense ceramics upon sintering at 1200 °C for 6 h. Though a linear increase in conductivity is observed by replacing Sm with Ca, the composition with the maximum amount of Ca and the minimum amount of Sm exhibits a significant improvement in properties compared to the rest in the series. The composition Ce0.80Sm0.05Ca0.15O2–δ exhibits a conductivity as high as 1.22 × 10–1 S cm–1 at 700 °C with minimum activation energy (0.56 eV) and a superior chemical stability to reduction compared to any of the hitherto known (CaSm) compositions. The absence of CeIII, confirmed both from X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, strongly suggests that the observed increase in conductivity is solely due to the oxide ion conductivity and not due to the partial electronic contribution arising from the presence of CeIII and CeIV. To conclude, the experimental results on the Ce0.8Sm0.2–xCaxO2–δ series underscore the unique effect of calcium co-doping in identifying a cost-effective new composition, with a remarkably high conductivity and enhanced chemical stability to reduction, for technological applications.

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