Interface Resistance of an SOFC Cathode with a Pr 1-X Tb x O 2-D Active Layer

Abstract

Introduction Pr6O11 is known to be a mixed conductor [1]. When it is used in the cathode of a solid oxide fuel cell (SOFC), it provides improved cathode performance [2]. However, it has several phase transition points between room temperature and the operating temperature of around 800oC. This phase transition is a drawback of Pr6O11 when it is used in SOFC cathodes, because it can cause mechanical failure in start/stop thermal cycles.The phase transition could be mitigated by doping with other elements. Rare earth elements are candidate dopants. We examined CeO2 as a dopant for Pr6O11. By doping Ce in Pr6O11, some phase transitions were suppressed, but another phase transition with a very large weight change (highly significant oxygen desorption) was created [1].Tb4O7 is a rare earth oxide and it has an oxygen non-stoichiometry. This suggests that this material is a mixed conductor at high temperature. Therefore, we focused on Tb4O7 as a dopant for Pr6O11. We synthesized Pr1-x Tb x O2-d as the active layer of the cathode and investigated the high-temperature characteristics and cathode performance of the resulting cell. Experiments First, Pr1-x Tb x O2-d (x=0-1.0) was synthesized by employing a solid-state reaction at 1300oC. The crystalline phase of each material was examined by using x-ray diffraction analysis. Then some of these materials were ball-milled. Coin cells with a Ce0.9Gd0.1O1.95 (GDC) electrolyte were prepared. A LaNi0.6Fe0.4O3 (LNF) anode was printed on a 2-mm-thick GDC electrolyte and sintered at 1100oC. LNF and Pt were used for the anode and reference electrode, respectively. Then the active layer was screen-printed on the other side and sintered at 1100oC. We used LNF since it has several merits including high electronic conduction and good thermal expansion matching that of the electrolyte. To evaluate the interface resistance, three-terminal AC impedance measurements were conducted before and after the DC current loading processes (106 mA/cm2 and then 354 mA/cm2) at 800oC in air. The high-temperature properties, including phase change and oxygen desorption for the Pr1-x Tb x O2-d were examined by TG-DTA in an airflow between room temperature and 1000oC. Results As-sintered Pr1-x Tb x O2-d (x=0-1.0) samples at room temperature were all in the fluorite phase. The single phase of a solid solution was easily obtained over the whole composition range. Figure 1 shows the TG curves (in a 2oC/min cooling process) of Pr1-x Tb x O2-d (x=0-1.0). Some steep weight changes caused by a phase transition were detected for endmembers of this binary system. However, phase transitions below 800oC disappeared when x in Pr1-x Tb x O2-d was in the 0.3 to 0.6 range. The risk of mechanical failure in thermal cycles can be reduced when a composition in this range is used for the SOFC cathode.Figure 2 shows AC impedance plots for LNF cathodes with an active layer. Plots for an LNF cathode without an active layer are also shown. They show the initial performance of the cathodes. The interface resistance was estimated from each loop diameter and was as large as 1.2 W cm2 for the LNF cathode without an active layer. However, the interface resistance for LNF cathodes with an active layer of Pr1-x Tb x O2-d (x = 0.0, 0.3, 0.5, 1.0) was much smaller than that of LNF. This result shows that a Pr1-x Tb x O2-d active layer improves cathode performance. Both Tb4O7 and Pr6O11 are considered to enhance the cathodic reaction. The interface resistance depends on the active layer composition. The performance of the active layer improves, when the Pr concentration is increased. After current loading, the interface resistance of all the cathodes changed. But the composition dependence of the interface resistance did not change and may be caused by the mixed conduction difference. The Pr4+ ionic radius is larger than that of Tb4+, and so the oxide ion conduction in a Pr rich oxide should be larger. Conclusions We used TG measurements to show that several phase transition points of Pr1-x Tb x O2-d were suppressed when x was between 0.3 and 0.6.The insertion of an active layer of Pr1-x Tb x O2-d into an LNF cathode and GDC electrolyte reduces the cathode interface resistance at 800oC.