Development of Sm-/Pr-doped ceria materials for electrolyte applications in Solid Oxide Cells

Kurzfassung

Doped ceria materials have been receiving scientific interest concerning their application in energy conversion devices, particularly in solid oxide cells. Development of new materials for solid oxide cell applications is necessary to overcome issues related to degradation, efficiency, costs and sustainability. In this work, samarium and praseodymium doped ceria materials are investigated as a diffusion barrier layer in yttria stabilized zirconia (YSZ)/doped ceria bilayered electrolyte solid oxide fuel cells. Different compositions, including samarium doped ceria Ce0.8Sm0.2O2-d (SDC20), samarium and praseodymium co-doped ceria Ce0.8Sm0.1Pr0.1O2-d (SPDC10) and praseodymium doped ceria Ce0.8Pr0.2O2-d (PDC20) fine powders were synthesized by the citrate sol-gel method and calcined at 500 °C. Material characterization techniques were used to study the characteristics of the samples, such as the crystal structure, particle size, porosity and specific surface area. The produced powders were associated to the fluorite-type phase and displayed an average particle size below 20 nm and a specific surface area above 37 m2·g-1. The increase of praseodymium concentration leaded to higher porosity of the produced diffusion barrier layers. AT the same time, it leaded to larger specific surface areas and smaller particle sizes of the produced powders. Pellets were prepared from the precursor powders and XPS studies showed the existence of a single oxidation state Sm3+ and both Pr3+/Pr4+ and Ce3+/Ce4+ oxidation states in these samples. Inks containing the doped ceria powders were applied through screen printing on commercial half-cells, which consisted of a NiO-YSZ substrate, a NiO-YSZ fuel-electrode, and an YSZ electrolyte layer. Finally, La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) air-electrode ink was hand-brushed and sintered. When sintered at 1080 °C, the air-electrode delivered better performance. Commercial Ce0.8Sm0.2O2-d (SDC20) powder was used as benchmark in a reference cell (C_SDC20). The performance of the cells was evaluated in a SOFC test bench and OCV values and current-voltage characteristics curves were collected. At 750 °C, the SPDC10-based cell reached the highest performance with a current density of 0.67 A·cm-2 at 0.7 V. At the same temperature and potential difference, SDC20-based cell reached 0.57 A·cm-2 being close to the value of 0.60 A·cm-2 for the C_SDC20-based cell. PDC20-based cell displayed the worst result, reaching only 0.34 A·cm-2 for the same parameters. It indicates that Pr doping ratio may have a significant influence on the transport properties of these diffusion barrier layers. After SEM and EDX analysis of cross-sections of the samples the doped ceria layers were found to have high porosity. This can explain the relatively low performance and the observed Sr diffusion from the air-electrode to the YSZ/doped ceria interface, which decreases the performance of the cell. Nonetheless, reasonable performance values were obtained, supporting Sm and Pr co-doped ceria materials should be more studied on the development of new high-performing materials for electrolyte applications.