Pr2NiO4+δ Cathode Properties and Reaction at Interface with Ceria and Zirconia

Background

La₂NiO_4+δ and Pr₂NiO_4+δ are known as mixed conductors and are candidates for SOFC cathodes [1]. SOFC cathodes are frequently used with a ceria buffer layer to avoid any reaction between the cathode and the zirconia electrolyte. We investigated the properties of Pr₂NiO_4+δ and La₂NiO_4+δcathodes and the influence of the ceria buffer layer on the cathode properties. We also investigated the phase compatibility of those cathodes with ceria (GDC) and zirconia (8YSZ).

Experiments

First, La₂NiO_4+δ and Pr₂NiO_4+δ cathode materials were synthesized by employing a solid-state reaction at 1300^oC. The cathode materials were ball-milled. The mean diameter of the grain of the cathodes was about 1 micron. Each material was in a single phase. The cathode slurries were printed on a Ce_0.8Sm_0.2O_1.9 (SDC) spin coated buffer layer on an 8YSZ (0.2 mm thick) electrolyte. The cathode sintering temperature was 1100^oC. Pt was used for the anode and reference electrodes. Three-terminal AC impedance measurements were conducted before and after the DC current loading processes (106 mA/cm² and then 354 mA/cm²) at 800^oC in air. The phase compatibility of the cathode with ceria or zirconia electrolyte was examined with x-ray diffraction analysis. The cathode and Ce_0.9Gd_0.1O_1.95 (GDC) or 0.92ZrO₂-0.08Y₂O₃ (8YSZ) powders were mixed and sintered under the same conditions as the cathode sintering process (1100^oC for two hours). Then their crystal structures were examined with an x-ray diffractometer.

Results and discussion

The impedance plots for those cathodes at 800^oC are shown in Fig.1.

They were measured after a DC current loading of 354 mA/cm² for 55 hours. The cathodes had a similar grain size, but the Pr₂NiO_4+δ cathode had a smaller interface resistance than the La₂NiO_4+δ cathode. To examine the influence of the reaction between and 8YSZ electrolyte, we also fabricated cells without the SDC buffer layer. The impedance plots for the cathodes without the buffer layer are also shown in Fig.1. The interface resistances for Pr₂NiO_4+δ and La₂NiO_4+δ cathodes without the SDC buffer layer are smaller than those with the SDC buffer layer. This shows that the SDC buffer layer degraded rather than improved the cathode performance. Fig.2 shows the X-ray diffraction patterns of the mixture of Pr₂NiO_4+δ powder and electrolyte powders (GDC or 8YSZ) sintered at 1100^oC. The Pr₂NiO_4+δ reacted completely with the GDC and produced Ce(Pr)O_2-δ or Ce(La)O_2-δ and NiO. This shows that Pr₂NiO_4+δ is highly reactive with cerias including GDC. The reaction products of the mixture of cathode and 8YSZ are also shown in Fig.2. After sintering, Pr₂Zr₂O₇ was formed, but Pr₂NiO_4+δ remained. This shows that the Pr₂NiO_4+δ reacts with zirconias in the cathode sintering process. But the interface between Pr₂NiO_4+δ and zirconias are more stable than that of Pr₂NiO_4+δ and cerias.

These results reveal that Pr₂NiO_4+δ and La₂NiO_4+δ cathodes without an SDC layer outperform those with an SDC layer.

  References

[1] J. C. Grenier, et al. ECS Transactions, Vol. 57 (1) pp. 1771-1779 (2013).